Advanced Applications of Ionic Liquids 0323999212, 9780323999212

Table of contents :
Advanced Applications of Ionic Liquids
Copyright
List of contributors
Preface
Dedication
Contents
About the editors
1 Progressions in ionic liquid-based electrochemical research
1.1 Introduction
1.2 Physical properties of ionic liquids
1.2.1 Conductivity
1.2.2 Viscosity
1.2.3 Electrochemical potential window
1.3 Electrochemical properties
1.4 Applications of ionic liquids in electrochemistry
1.4.1 Electrochemical sensors
1.4.2 Electrodeposition
1.4.3 Electroredox
1.4.4 Electrochemical biosensors
1.4.5 Applications of ionic liquids in Li-ion batteries
1.4.6 Applications of ionic liquids for supercapacitors
1.4.7 Applications of ionic liquids in electropolymerization
1.5 Conclusion
References
2 Recapitulation on the separation and purification of biomolecules using ionic liquid-based aqueous biphasic systems
2.1 Introduction
2.2 Applications of ionic liquids-based aqueous biphasic system in separation and purification of biomolecules
2.2.1 Amino acids
2.2.2 Proteins
2.2.3 Enzymes
2.2.4 Nucleic acids
2.3 Conclusion
Acknowledgments
Nomenclature
Abbreviations
Ionic Liquids and Good Buffers
Proteins
Enzymes
Salts
Acid
References
3 Current trends and applications of ionic liquids in electrochemical devices
3.1 Introduction
3.1.1 History of ionic liquids in electrochemical devices
3.2 Ionic liquids in energy storage devices and conversion materials
3.3 Ionic liquid in energy sustainability and CO2 sequestration
3.4 Ionic liquids as a novel electrolyte medium for advanced electrochemical devices
3.5 Ionic liquids’ electrochemical sensing properties
3.6 Applications of room-temperature ionic liquids
3.6.1 Electrochemical applications of room-temperature ionic liquids
3.6.2 Room-temperature ionic liquid as a nonfaradaic biosensing component
3.6.3 Room-temperature ionic liquids in electrochemical gas sensoring
3.7 Ammonium, pyrrolidinium, phosphonium, and sulfonium-based ionic liquids and electrochemical properties
3.8 Current and future prospects
3.8.1 Ionic liquids as electrolytes
3.8.2 Ionic liquids as lubricants and hydraulic fluids
3.8.3 Ionic liquids as chemical production processes
3.8.4 Ionic liquids as hydrogen storage
3.9 Conclusions
References
4 Green chemistry of ionic liquids in surface electrochemistry
4.1 Introduction
4.1.1 Important characteristics of electrochemical reactions
4.1.1.1 Electrochemical current and potential
4.1.1.2 Electrochemical interfaces
4.1.1.3 Models of electrochemical electron transfer
4.1.2 Electrochemistry at the molecular scale
4.1.2.1 Surface structure
4.1.2.2 Bonding of ions
4.1.2.3 Bonding of water
4.1.2.4 Experimental aspects of current/voltage properties
4.1.3 Ionic liquids properties pertinent to surface electrochemistry
4.2 Role of ionic liquids in surface electrochemistry
4.2.1 Carbon ionic liquid electrode
4.2.1.1 Direct electrochemistry of hemoglobin
4.2.1.2 Determination of various substances
4.2.2 Quartz crystal microbalance
4.2.3 Chemical warfare agent
4.2.4 Electrochemical oxidation
4.3 Conclusions
References
5 An evolution in electrochemical and chemical synthesis applications in prospects of ionic liquids
5.1 Introduction
5.2 Electrochemical oxidation reactions using room-temperature ionic liquids
5.2.1 Oxidative self-coupling reaction
5.2.2 Shono oxidation of carbamates
5.2.3 Oxidation of alcohols
5.2.4 Bromination reaction
5.3 Electrochemical reduction reactions using room-temperature ionic liquid
5.3.1 Electroreductive coupling of organic halides
5.3.2 Pinacol coupling reaction
5.3.3 Electrochemical reduction of carbon dioxide gas
5.3.4 Electrocarboxylation reaction
5.3.5 Synthesis of aryl zinc compounds
5.3.6 Electrochemical reductive coupling to form 1,6-diketone
5.3.7 Electrochemical reduction of benzoyl chloride
5.3.8 Organocatalysis using electrogenerated bases
5.4 Electrochemical polymerization reactions using room-temperature ionic liquids
5.5 Electrochemical partial fluorination using room-temperature ionic liquids
5.5.1 Anodic fluorination of dithioacetals
5.5.2 Electrochemical fluorination utilizing mediators
5.5.3 Fluorination of methyl adamantane-1-carboxylate electrochemically
5.6 Other electrochemical reactions using room-temperature ionic liquids
5.6.1 Electrogenerated N-heterocyclic carbenes
5.6.1.1 Synthesis of β-lactams
5.6.1.2 Henry reaction
5.6.1.3 Benzoin condensation
5.6.1.4 Stetter reaction
5.6.1.5 Staudinger reaction
5.6.1.6 Preparation of γ-butyrolactones
5.6.1.7 Esterification reaction
5.6.1.8 Transesterification
5.6.1.9 Oxidative esterification of aromatic aldehydes
5.6.1.10 Preparation of N-acyloxazolidin-2-ones
5.6.1.11 N-Functionalisation of benzoxazolones
5.6.2 Functionalisation of nitroaromatic compounds
5.6.3 Epoxidation reaction using room-temperature ionic liquids
5.7 Conclusions
Abbreviations
References
6 Recent changes in the synthesis of ionic liquids based on inorganic nanocomposites and their applications
6.1 Introduction
6.1.1 Inorganic nanocomposite materials—an overview
6.1.2 Development of inorganic nanocomposite materials synthesis
6.1.3 Role of ionic liquid in the synthesis of inorganic nanocomposite
6.1.4 Application-based importance of ionic liquids in inorganic nanocomposite
6.2 Synthesis of inorganic nanocomposite materials using ionic liquid
6.2.1 Sol-gel method
6.2.2 Hydrothermal method
6.2.3 Microemulsion method
6.2.4 Precipitation and co-precipitation method
6.2.5 Rays mediated method
6.2.5.1 Photochemical method
6.2.5.2 Photocatalytic deposition
6.2.5.3 Sonochemical method
6.2.6 Electrochemical method
6.3 How organic-inorganic is different from inorganic nanocomposites?
6.4 Recent advancements and advantages of inorganic nanocomposites with ionic liquids
6.4.1 Storage of heat energy
6.4.1.1 Advantages
6.4.2 Electrolytic support
6.4.2.1 Advantages
6.4.3 Solvents improvement
6.4.3.1 Advantages
6.4.4 Analytics and purity
6.4.4.1 Advantages
6.4.5 Additives
6.4.5.1 Advantages
6.5 Current applications and their future perspective
6.5.1 Biomedical
6.5.2 Environmental science
6.5.2.1 Water treatment
6.5.2.2 Soil treatment
6.5.2.3 Air pollution treatment
6.5.3 Nuclear science
6.5.4 Food science
6.5.5 Energy storage and transfer
6.5.6 Catalysis
6.5.7 Lubricants
6.5.8 Sensors
6.5.9 Electrochemistry
6.6 Reaction mechanism of ionic liquids-based synthesized nanocomposite materials
6.7 Conclusions
Abbreviations
Author contributions
Conflicts of interest
References
7 Ionic liquids as green and efficient corrosion-protective materials for metals and alloys
7.1 Introduction
7.1.1 Effect of corrosion
7.1.2 Causes of corrosion
7.1.3 Techniques of corrosion protection
7.1.4 Ionic liquids as green corrosion protectors
7.1.5 Applications of ionic liquids
7.1.6 Classification of ionic liquids
7.2 Ionic liquids as corrosion protector for metals and alloy
7.2.1 Ionic liquids as corrosion protector for iron and alloy
7.2.2 Ionic liquids as corrosion protector for Al
7.2.3 Ionic liquids as corrosion protector for Cu and Zn
7.3 Corrosion protection mechanism
7.4 Conclusions and future perspectives
References
8 Ionic liquids as valuable assets in extraction techniques
8.1 Introduction
8.2 Ionic liquids
8.3 Ionic liquids for the extraction of natural products from the plants
8.3.1 Ultrasonic-assisted ionic liquid approach for the extraction of natural products
8.3.2 Microwave-assisted ionic liquid approach for the extraction of natural products
8.3.3 Reactive dissolution of biomass in ionic liquids for the extraction of natural products
8.4 Ionic liquids in extraction of pharmaceuticals from biological and environmental samples
8.5 Ionic liquids for the extraction of contaminants from wastewater
8.5.1 Extraction of toxic metal ions
8.5.2 Extraction of organic pollutants
8.6 Ionic liquids for the extraction of soil contaminants and soil organic matter
8.6.1 Extraction of soil contaminants
8.6.1.1 Extraction of soil organic pollutants
8.6.1.2 Extraction of soil heavy metal ions
8.6.2 Extractions of soil organic matter
8.7 Extraction of rare earth metals
8.8 Ionic liquids for the extraction of food contaminants
8.9 Applications of ionic liquids
8.10 Conclusion and future prospective
Acknowledgments
References
9 An involvement of ionic liquids and other small molecules as promising corrosion inhibitors in recent advancement of tech...
9.1 Consequences of corrosion
9.2 Economic effects
9.3 Methods to control corrosion
9.3.1 Material selection
9.3.2 Coating
9.3.2.1 Metallic coating
9.3.2.2 Organic coating
9.3.2.3 Inorganic coatings
9.4 Inhibitors
9.5 Anodization
9.6 Cathodic protection
9.7 Structure of electrical double layer
9.8 Influence of temperature on the action of Inhibitors
9.9 Corrosion inhibition—an inevitable arena of research
9.10 Importance of ionic liquids (ILs)
9.11 Corrosion is a costly problem to the world
9.12 Ionic liquids as promising coating agents and inhibitors
9.13 Other corrosion inhibitors
9.14 Conclusion
References
10 Role of ionic liquids in bioactive compounds extractions and applications
10.1 Introduction
10.2 Bioactive compound extraction from biomass
10.2.1 Ionic liquid-based liquid–liquid extractions
10.2.1.1 Liquid–liquid extraction with hydrophobic ionic liquids
10.2.1.2 Ionic liquid-based aqueous biphasic systems
10.2.2 Ionic liquid-based solid–liquid extractions
10.2.2.1 Simple/basic ionic liquid-based solid–liquid extractions
10.2.2.2 Microwave-assisted extractions
10.2.2.3 Ultrasonic-assisted extractions
10.2.2.4 More complex/rigid solid-liquid extractions
10.2.2.4.1 Ultrasonic/microwave-assisted extractions
10.2.2.4.2 Ultrahigh pressure extraction
10.2.2.4.3 Negative-pressure cavitation extraction
10.2.2.4.4 Microwave homogeneous liquid–liquid microextraction
10.2.3 Solid–phase extractions
10.2.4 Backward (or back)-extractions
10.3 Applications of ionic liquids
10.3.1 Green solvents—a gentle suspension of biomass
10.3.2 High-purity, inflammable electrolytes for battery and supercapacitor applications
10.3.3 Antistatic agents
10.3.3.1 Liquid antistatic agents
10.3.4 Intrinsically safe high-temperature cooling
10.3.5 Further more advanced applications
10.3.5.1 Air conditioning
10.3.5.2 Hydrogen storage
10.3.5.3 Chemical production processes
10.4 Conclusions and future prospects
Acknowledgments
References
11 Developments in gas sensing applications before and after ionic liquids
11.1 Introduction
11.2 Layout of the chapter
11.3 Electrochemical gas sensors
11.3.1 Electrochemical oxygen gas sensors
11.3.2 Electrochemical ammonia gas sensors
11.3.3 Electrochemical nitrogen oxide gas sensors
11.3.4 Electrochemical volatile organic compounds gas sensors
11.3.5 Electrochemical carbon dioxide gas sensor
11.3.6 Electrochemical methane and oxygen dual gas sensor
11.3.7 Electrochemical hydrogen sulfide carbon nanotube-modified electrode gas sensor
11.4 Optical gas sensors
11.4.1 Optical oxygen gas sensors
11.4.2 Optical carbon dioxide gas sensors
11.4.3 Optical ammonia gas sensors
11.4.4 Optical volatile organic compound gas sensors
11.5 Piezoelectric gas sensors
11.5.1 Quartz-crystal microbalance gas sensors
11.5.2 Surface acoustic wave–based gas sensors
11.5.3 Piezoresistive-based gas sensors
11.6 Other forms of gas sensors
11.6.1 Semiconductor metal-oxide gas sensors
11.6.2 Carbon ionic liquid composite gas sensors
11.6.3 Gated ionic liquid gas sensors
11.7 Conclusions
References
12 Ionic liquids: a tool for CO2 capture and reduced emission
12.1 Introduction
12.2 Aqueous amines used in postcombustion
12.3 Ionic liquids as solvents for CO2 capture
12.3.1 Ionic liquids in the absorption process for CO2 capture
12.3.1.1 Ionic liquids as physical absorbents
12.3.1.2 Ionic liquids as chemical absorbents
12.3.2 Ionic liquids in the adsorption process for CO2 capture
12.3.3 Ionic liquids in membranes process for CO2 capture
12.3.3.1 Supported ionic liquid membrane for CO2 separation
12.3.3.2 Polyionic liquids membranes for CO2 separation
12.3.3.3 Ionic liquid composite membranes for CO2 separation
12.4 Regeneration of CO2 from ionic liquids
12.5 Designing ionic liquids for CO2 capture
12.6 Conclusions
Acknowledgments
Abbreviations
References
13 Applications of ionic liquids in fuel cells and supercapacitors
13.1 Introduction
13.2 The bonding in ionic liquids
13.3 Ionic liquids: evolution
13.4 Ionic liquids in fuel cells
13.5 Ionic liquids in supercapacitors
13.6 Conclusion
13.7 Future scope
References
14 Role of polymeric ionic liquids in rechargeable batteries
14.1 Introduction
14.2 Classification of ionic liquids based on their chemical structure
14.2.1 Protic ionic liquids as electrolytes for lithium-ion battery
14.2.2 Aprotic ionic liquids as electrolytes for lithium-ion battery
14.3 Introduction to Li batteries
14.4 Basics of ionic liquids
14.5 Organic and inorganic ionic liquids in electrical storage systems
14.6 Ionic liquid-based polymers electrolytes historical background
14.7 Polymeric ionic liquids for rechargeable lithium-ion batteries
14.7.1 Emerging of ionic liquid–based polymer electrolyte
14.8 Li/Na-ion battery electrolyte
14.9 Polymer-electrolytes classification
14.9.1 Electrolytes based on dry solid polymer
14.9.2 Electrolytes based on plasticized polymer
14.9.3 Electrolytes based on gel polymer
14.9.4 Electrolytes based on composite polymer
14.10 Ionic liquid-based gel polymer electrolytes application in lithium batteries
14.11 Low melting point alkaline salts in lithium batteries
14.12 Conclusion
Abbreviations
References
15 Progress in optoelectronic applications of ionic liquids
15.1 Introduction
15.2 Principle and structure of dye-sensitized solar cell
15.3 Role of ionic liquids as an electrolyte in dye-sensitized solar cells
15.3.1 Role of poly ionic liquids as solid or quasi-solid electrolyte in dye-sensitized solar cell
15.3.2 Iodine-free PIL as an electrolyte in dye-sensitized solar cells
15.4 Challenges and future prospects
References
16 Role of ionic liquids and their future alternative toward protein chemistry
16.1 Introduction
16.2 Antibacterial and antitumor activities of ionic liquids
16.3 Protein instability and its influencing factors as well as analytical monitoring
16.4 Effect of alkyl chain length of cations of ionic liquids on the stability of proteins
16.5 Effect of cations and anion of ionic liquids on the stability of proteins
16.6 Effect of hydrophobicity of ionic liquids on the stability of proteins
16.7 Effect of viscosity of ionic liquids on the stability of proteins
16.8 Protein folding in ionic liquids
16.9 Enzymes with ionic liquids
16.10 Application of ionic liquids as biocatalysis
16.11 Ionic liquids do not inactivate enzymes like polar organic solvents
16.12 Increased stability of enzymes in ionic liquids
16.13 Cytotoxicity of ionic liquids
16.14 What are neoteric solvents?
16.15 Role of deep eutectic solvents on protein chemistry
16.16 Conclusion
References
17 Ionic liquids in metrological analysis and applications
17.1 Introduction
17.2 Wide-ranging ionic liquids
17.3 Protic and aprotic ionic liquids
17.4 Physicochemical properties of ionic liquids defining metrological parameters
17.4.1 Miscibility and solubility
17.4.2 Solvation with repulsion
17.4.3 Vapor pressure
17.4.4 Density and viscosity
17.5 Electrochemical constancy and conductivity
17.6 Ionic liquids-centered devices
17.7 Configuration of ionic liquids in biosensors
17.8 Aspects of ionic liquids as promoters in biodiesel fabrication
17.9 Affinity attributed to ionic liquids in nanomaterials
17.10 The implication of green diluents in space mechanics
17.11 Space energy
17.12 Compost properties
17.13 Life support techniques
17.14 Hypergolic solutions
17.15 Space emollients
17.16 Lunular fluid-glass contract
17.17 Conclusion
References
18 Antibacterial properties of silver nanoparticles synthesized in ionic liquids
18.1 Introduction
18.1.1 Metal nanoparticles and ionic liquids
18.1.2 Silver as antibacterial agent: historical background
18.2 Silver nanoparticles’ antimicrobial properties and activities
18.3 Discussions and final remarks
Declaration of competing interest
Acknowledgment
References
19 Progressive function of ionic liquids in polymer chemistry
19.1 Introduction
19.2 Ionic liquid
19.3 Structure of ionic liquid
19.4 Some important advantages and characteristics of ionic liquid
19.5 Common methods of making ionic liquids
19.5.1 Anion exchange
19.5.2 Alkylation
19.5.3 Preparation of ionic liquid special performance
19.6 The function of ionic liquid in polymer
19.7 Polymer-doped ionic liquid
19.8 Polymerization of vinyl monomer in ionic liquid
19.9 Polymerizable ionic liquid
19.10 Adsorbed and covalently linked ionic liquids
19.11 Microwave absorbing ionic liquid polymer
19.12 Ionic liquid-polymer composite
19.13 Summary
Conflict of interests
References
20 Potential hazards of ionic liquids: a word of caution
20.1 Introduction
20.2 Environmental concerns of ionic liquids
20.2.1 Cytotoxic action of ionic liquids
20.2.2 Enzymatic interaction of ionic liquids
20.2.3 Phytotoxicity of ionic liquids
20.2.4 Toxicity of ionic liquids to microorganisms
20.3 Factors affecting the toxicity of ionic liquids
20.3.1 Effect of the chemical composition of ionic liquids
20.3.1.1 Cations
20.3.1.2 Alkyl chain length
20.3.1.3 Anions
20.3.2 Effect of environmental factors
20.3.2.1 Dissolved organic matter
20.3.2.2 Salinity
20.4 Fate and transfer of ionic liquids to the environment
20.4.1 Adsorption of ionic liquids
20.4.2 Biodegradation of ionic liquids
20.4.3 Chemical degradation of ionic liquids
20.5 Conclusion and future perspectives
Conflict of interest
References
Index

Citation preview

ADVANCED APPLICATIONS OF IONIC LIQUIDS

ADVANCED APPLICATIONS OF IONIC LIQUIDS Edited by

JAMAL AKHTER SIDDIQUE Marie Curie fellow (List-B), SASPRO-2, Slovak Academy of Sciences, Bratislava, Slovakia

SHAHID PERVEZ ANSARI Department of Applied Chemistry, Zakir Husain College of Engineering and Technology, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India

AFTAB ASLAM PARWAZ KHAN Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia

ABDULLAH M. ASIRI Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2023 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-99921-2 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Susan Dennis Acquisitions Editor: Gabriela D. Capille Editorial Project Manager: Zsereena Rose Mampusti Production Project Manager: Rashmi Manoharan Cover Designer: Greg Harris Typeset by MPS Limited, Chennai, India

List of contributors Maroof Ali Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Fahmeena Asmat Aligarh Muslim University, Aligarh, Uttar Pradesh, India Elham Avirdi Department of Chemistry, Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Natural and Agricultural Sciences, North-West University, Mmabatho, South Africa Indra Bahadur Department of Chemistry, Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Natural and Agricultural Sciences, North-West University, Mmabatho, South Africa Moghal Zubair Khalid Baig Department of Advanced Organic Materials Engineering, Chungnam National University, Daejeon, Republic of Korea Shaista Bano Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Vijaykumar S. Bhamare Department of Chemistry and Centre for Nanoscience and Nanotechnology, KLS Gogte Institute of Technology, Udyambag, Belagavi, Karnataka, India Mansi Chaudhary Department of Chemistry, Atma Ram Sanatan Dharma College, University of Delhi, New Delhi, India; Amity Institute of Applied Sciences, Amity University, Noida, India Chien-Yen Chen Department of Earth and Environmental Sciences, National Chung Cheng University, Min-Hsiung, Chiayi County, Taiwan Indrajit Das Department of Chemistry, Indian Institute of Technology Madras, Chennai, India Saima Farooq Department of Biological Sciences and Chemistry, College of Arts & Sciences, University of Nizwa, Nizwa, Oman Panmei Gaijon Department of Chemistry, Kirori Mal College, University of Delhi, New Delhi, India Geetu Gambhir Department of Chemistry, Acharya Narendra Dev College, University of Delhi, New Delhi, India Ramesh L. Gardas Department of Chemistry, Indian Institute of Technology Madras, Chennai, India Umaima Gazal Chemistry Department, Raja Bahadur Venkata Rama Reddy Women’s College, Affiliated to Osmania University, Hyderabad, Telengana, India Sudipta Ghosh Department of Chemistry, Kirori Mal College, University of Delhi, New Delhi, India

xv

xvi

List of contributors

Manjunath S. Hanagadakar Department of Chemistry, S.J.P.N. Trust’s Hirasugar Institute of Technology, Nidasoshi, Karnataka, India Abul Hasnat Department of Chemistry, Shahjahanpur, Uttar Pradesh, India

Gandhi

Faiz-E-Aam

College,

Seyyed Emad Hooshmand Department of Medical Nanotechnology, Faculty of Advanced Technologies in Medicine, Iran University of Medical Sciences, Tehran, Iran Sushma P. Ijardar Department of Chemistry, Veer Narmad South Gujarat University, Surat, India Mohd. Imran Department of Physics, University), Delhi, New Delhi, India

Jamia

Millia

Islamia

(Central

Md Rabiul Islam Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Shagufta Jabin Department of Chemistry, Faculty of Engineering, Manav Rachna International Institute of Research and Studies, Faridabad, Haryana, India Pallavi Jain Faculty of Engineering and Technology, Department of Chemistry, SRM Institute of Science and Technology, NCR Campus, Ghaziabad, Uttar Pradesh, India Arun Kant Department of Chemistry, Kirori Mal College, University of Delhi, New Delhi, India Lebogang Maureen Katata-Seru Department of Chemistry, Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Natural and Agricultural Sciences, North-West University, Mmabatho, South Africa Abbul Bashar Khan Delhi, India

Department of Chemistry, Jamia Millia Islamia, New

Aftab Aslam Parwaz Khan Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia Imran Khan Department of Chemistry, College of Science, Sultan Qaboos University, Muscat, Oman; CICECO—Aveiro Institute of Materials, Chemistry Department, University of Aveiro, Aveiro, Portugal Jamal Ahmad Khan Applied Sciences and Humanities section, University Polytechnic, Zakir Husain College of Engineering and Technology, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Mohd Arham Khan Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Prasanna S. Koujalagi Department of Chemistry and Centre for Nanoscience and Nanotechnology, KLS Gogte Institute of Technology, Udyambag, Belagavi, Karnataka, India Raviraj M. Kulkarni Department of Chemistry and Centre for Nanoscience and Nanotechnology, KLS Gogte Institute of Technology, Udyambag, Belagavi, Karnataka, India

xvii

List of contributors

Kamlesh Kumari India

Department of Zoology, University of Delhi, New Delhi,

Anita Kumari Yadav Department of Chemistry, Rajdhani College, University of Delhi, New Delhi, India Sandeep R. Kurundawade Department of Chemistry, KLE Technological University, Hubballi, Karnataka, India Moonyong Lee School of Chemical Engineering, Yeungnam University, Gyeongsan, Republic of Korea Jyoti Prakash Maity Department of Earth and Environmental Sciences, National Chung Cheng University, Min-Hsiung, Chiayi County, Taiwan; Department of Chemistry, School of Applied Sciences, KIIT Deemed to be University, Bhubaneswar, Odisha, India Ramesh S. Malladi Department of Chemistry, BLDEA’s V. P. Dr. P. G. Halakatti College of Engineering and Technology, Vijaypur, Karnataka, India Ayaz Mohd Applied Biotechnology Department, University of Technology and Applied Sciences, Sur, Sultanate of Oman A.

Moheman Department of Chemistry, Shahjahanpur, Uttar Pradesh, India

Gandhi

Faiz-E-Aam

College,

Zakira Naureen Department of Biological Sciences and Chemistry, College of Arts & Sciences, University of Nizwa, Nizwa, Oman Alam Nawaz School of Chemical Gyeongsan, Republic of Korea

Engineering,

Yeungnam

University,

Emmanuel A. Oke Department of Chemistry, Veer Narmad South Gujarat University, Surat, India Shweta Pal Department of Chemistry, Atma Ram Sanatan Dharma College, University of Delhi, New Delhi, India; Amity Institute of Applied Sciences, Amity University, Noida, India K. Rama Swami Department of Chemistry, Indian Institute of Technology Madras, Chennai, India Raju Kumar Sharma Department of Chemistry and Biochemistry, National Chung Cheng University, Min-Hsiung, Chiayi County, Taiwan; Department of Earth and Environmental Sciences, National Chung Cheng University, Min-Hsiung, Chiayi County, Taiwan Jamal Akhter Siddique Marie Curie fellow (List-B), SASPRO-2, Slovak Academy of Sciences, Bratislava, Slovakia M. Ramananda Singh Department of Chemistry, Kirori Mal College, University of Delhi, New Delhi, India Prashant Singh Amity Institute of Applied Sciences, Amity University, Noida, India; Department of Chemistry, Atma Ram Sanatan Dharma College, University of Delhi, New Delhi, India Shailendra Kumar Singh Department of Chemistry, Hans Raj College, University of Delhi, New Delhi, India

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List of contributors

Md Palashuddin Sk Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Young-A Son Department of Advanced Organic Materials Engineering, Chungnam National University, Daejeon, Republic of Korea Mohmmad Umiad

Aligarh Muslim University, Aligarh, Uttar Pradesh, India

Mohd Amil Usmani Department of Chemistry, Gandhi Faiz-E-Aam College, Shahjahanpur, Uttar Pradesh, India Rajender S. Varma Regional Centre of Advanced Technologies and Materials, Czech Advanced Technology and Research Institute, Palacky´ University in Olomouc, Olomouc, Czech Republic Sana Zafar Department of Applied Sciences and Humanities, Jamia Millia Islamia (Central University), Delhi, New Delhi, India

Preface Alkylammonium nitrates and chloroaluminates found by the combination of aluminum chloride and quaternary heterocyclic cations provide the origin of ionic liquids. Walden (1914) invented the ionic liquid using alkylammonium nitrate. However, their moisture sensitivity limited their potential application in various fields. It was Wilkes who discovered moisture-stable ionic liquids in the 1990s. Other anions, such as tetrafluoroborate or hexafluorophosphate, were used instead of aluminum chloride. Since the turn of the century, ion liquids (ILs) have attracted considerable attention. We may find different definitions of ILs at different places, but the general definition is that these are salts with melting points below 100 C. The flexibility and broadness of such a definition allows for considerable manipulation. The term “designer solvent” refers to these as well. A vast range of applications are being explored with ILs, as we will discuss in this book. The unique properties of ILs have recently been reported across many different research areas by scientists around the world. Development of ILs continues to grow as more applications are discovered for ILs. This has resulted in rapid growth of the field. The number of papers published each year on ILs has doubled in the past few years, and more importantly, researchers are now experimenting with ILs and discovering how they can benefit their research. In this book, we will present the concepts and recent advances in ionic liquid applications. Our goal is to inspire further curiosity and enthusiasm for these exciting and unique materials in the readers and to inspire them to explore those further. Jamal Akhter Siddique Shahid Pervez Ansari Aftab Aslam Parwaz Khan Abdullah M. Asiri

xxiii

Dedication

The role of teachers in our life definitely started from our home, school, colleges, and universities time to time; this is an endless interaction throughout life. A great teacher is warm, accessible, enthusiastic, and caring. This person is approachable, not only to students but to everyone in and out of the campus. This is the teacher to whom students know they can go with any problems or concerns or even share a funny story. “This work is truly dedicated to our all beloved teachers whose teachings continuously help us in illuminating our academic and moral goals in life.” Thanks to all teachers; Jamal A Siddique Shahid P Ansari Aftab AP Khan Abdullah M Asiri

Contents

List of contributors About the editors Preface

xv xix xxiii

1 Catalysis and electrochemistry 1. Progressions in ionic liquid-based electrochemical research

3

MD RABIUL ISLAM, MOHD ARHAM KHAN, MAROOF ALI AND MD PALASHUDDIN SK

1.1 Introduction 1.2 Physical properties of ionic liquids 1.3 Electrochemical properties 1.4 Applications of ionic liquids in electrochemistry 1.5 Conclusion References

3 4 5 5 17 17

2. Recapitulation on the separation and purification of biomolecules using ionic liquid-based aqueous biphasic systems

23

EMMANUEL A. OKE AND SUSHMA P. IJARDAR

2.1 Introduction 2.2 Applications of ionic liquids-based aqueous biphasic system in separation and purification of biomolecules 2.3 Conclusion Acknowledgments Nomenclature References

23 26 51 52 52 57

3. Current trends and applications of ionic liquids in electrochemical devices

63

AYAZ MOHD, SHAISTA BANO, JAMAL AKHTER SIDDIQUE AND AFTAB ASLAM PARWAZ KHAN

3.1 Introduction 3.2 Ionic liquids in energy storage devices and conversion materials 3.3 Ionic liquid in energy sustainability and CO2 sequestration

vii

63 66 67

viii

Contents

3.4 Ionic liquids as a novel electrolyte medium for advanced electrochemical devices 3.5 Ionic liquids’ electrochemical sensing properties 3.6 Applications of room-temperature ionic liquids 3.7 Ammonium, pyrrolidinium, phosphonium, and sulfonium-based ionic liquids and electrochemical properties 3.8 Current and future prospects 3.9 Conclusions References

4. Green chemistry of ionic liquids in surface electrochemistry

68 68 70 73 78 80 82

89

ABBUL BASHAR KHAN

4.1 Introduction 4.2 Role of ionic liquids in surface electrochemistry 4.3 Conclusions References

5. An evolution in electrochemical and chemical synthesis applications in prospects of ionic liquids

89 94 107 107

113

VIJAYKUMAR S. BHAMARE AND RAVIRAJ M. KULKARNI

5.1 5.2 5.3 5.4

Introduction Electrochemical oxidation reactions using room-temperature ionic liquids Electrochemical reduction reactions using room-temperature ionic liquid Electrochemical polymerization reactions using room-temperature ionic liquids 5.5 Electrochemical partial fluorination using room-temperature ionic liquids 5.6 Other electrochemical reactions using room-temperature ionic liquids 5.7 Conclusions Abbreviations References

6. Recent changes in the synthesis of ionic liquids based on inorganic nanocomposites and their applications

113 115 120 131 133 137 143 145 146

155

RAJU KUMAR SHARMA, JAMAL AKHTER SIDDIQUE, CHIEN-YEN CHEN AND JYOTI PRAKASH MAITY

6.1 6.2 6.3 6.4

Introduction Synthesis of inorganic nanocomposite materials using ionic liquid How organic-inorganic is different from inorganic nanocomposites? Recent advancements and advantages of inorganic nanocomposites with ionic liquids 6.5 Current applications and their future perspective 6.6 Reaction mechanism of ionic liquids-based synthesized nanocomposite materials

155 159 164 165 167 172

Contents

6.7 Conclusions Abbreviations Author contributions Conflicts of interest References

ix 174 175 175 176 176

7. Ionic liquids as green and efficient corrosion-protective materials for metals and alloys

185

MOHD AMIL USMANI, IMRAN KHAN, ABUL HASNAT AND A. MOHEMAN

7.1 Introduction 7.2 Ionic liquids as corrosion protector for metals and alloy 7.3 Corrosion protection mechanism 7.4 Conclusions and future perspectives References

185 188 193 193 194

2 Separation technology 8. Ionic liquids as valuable assets in extraction techniques

199

JAMAL AHMAD KHAN AND SHAGUFTA JABIN

8.1 8.2 8.3 8.4

Introduction Ionic liquids Ionic liquids for the extraction of natural products from the plants Ionic liquids in extraction of pharmaceuticals from biological and environmental samples 8.5 Ionic liquids for the extraction of contaminants from wastewater 8.6 Ionic liquids for the extraction of soil contaminants and soil organic matter 8.7 Extraction of rare earth metals 8.8 Ionic liquids for the extraction of food contaminants 8.9 Applications of ionic liquids 8.10 Conclusion and future prospective Acknowledgments References

9. An involvement of ionic liquids and other small molecules as promising corrosion inhibitors in recent advancement of technologies in chemical industries

199 200 202 204 205 207 209 209 210 210 211 211

223

SHWETA PAL, MANSI CHAUDHARY, PALLAVI JAIN, PRASHANT SINGH, ANITA KUMARI YADAV, SHAILENDRA KUMAR SINGH AND INDRA BAHADUR

9.1 Consequences of corrosion 9.2 Economic effects 9.3 Methods to control corrosion

224 224 226

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Contents

9.4 Inhibitors 9.5 Anodization 9.6 Cathodic protection 9.7 Structure of electrical double layer 9.8 Influence of temperature on the action of Inhibitors 9.9 Corrosion inhibition—an inevitable arena of research 9.10 Importance of ionic liquids (ILs) 9.11 Corrosion is a costly problem to the world 9.12 Ionic liquids as promising coating agents and inhibitors 9.13 Other corrosion inhibitors 9.14 Conclusion References

10. Role of ionic liquids in bioactive compounds extractions and applications

227 227 228 228 229 232 233 233 234 237 239 240

247

ALAM NAWAZ, MOGHAL ZUBAIR KHALID BAIG, MOHMMAD UMIAD, FAHMEENA ASMAT, YOUNG-A SON AND MOONYONG LEE

10.1 Introduction 10.2 Bioactive compound extraction from biomass 10.3 Applications of ionic liquids 10.4 Conclusions and future prospects Acknowledgments References

247 254 271 278 278 278

3 Sensors and biosensors 11. Developments in gas sensing applications before and after ionic liquids

287

VIJAYKUMAR S. BHAMARE AND RAVIRAJ M. KULKARNI

11.1 Introduction 11.2 Layout of the chapter 11.3 Electrochemical gas sensors 11.4 Optical gas sensors 11.5 Piezoelectric gas sensors 11.6 Other forms of gas sensors 11.7 Conclusions References

12. Ionic liquids: a tool for CO2 capture and reduced emission

287 290 290 307 311 313 316 318

327

INDRAJIT DAS, K. RAMA SWAMI AND RAMESH L. GARDAS

12.1 Introduction 12.2 Aqueous amines used in postcombustion

327 330

Contents

12.3 Ionic liquids as solvents for CO2 capture 12.4 Regeneration of CO2 from ionic liquids 12.5 Designing ionic liquids for CO2 capture 12.6 Conclusions Acknowledgments Abbreviations References

xi 331 340 340 342 343 343 344

4 Electronic applications 13. Applications of ionic liquids in fuel cells and supercapacitors

353

SANDEEP R. KURUNDAWADE, RAMESH S. MALLADI, PRASANNA S. KOUJALAGI AND RAVIRAJ M. KULKARNI

13.1 Introduction 13.2 The bonding in ionic liquids 13.3 Ionic liquids: evolution 13.4 Ionic liquids in fuel cells 13.5 Ionic liquids in supercapacitors 13.6 Conclusion 13.7 Future scope References

14. Role of polymeric ionic liquids in rechargeable batteries

353 354 354 355 360 361 362 362

365

MANJUNATH S. HANAGADAKAR, RAVIRAJ M. KULKARNI AND RAMESH S. MALLADI

14.1 Introduction 14.2 Classification of ionic liquids based on their chemical structure 14.3 Introduction to Li batteries 14.4 Basics of ionic liquids 14.5 Organic and inorganic ionic liquids in electrical storage systems 14.6 Ionic liquid-based polymers electrolytes historical background 14.7 Polymeric ionic liquids for rechargeable lithium-ion batteries 14.8 Li/Na-ion battery electrolyte 14.9 Polymer-electrolytes classification 14.10 Ionic liquid-based gel polymer electrolytes application in lithium batteries 14.11 Low melting point alkaline salts in lithium batteries 14.12 Conclusion Abbreviations References

15. Progress in optoelectronic applications of ionic liquids

365 366 369 371 372 372 373 378 378 380 381 383 384 384

391

SANA ZAFAR AND MOHD. IMRAN

15.1 Introduction 15.2 Principle and structure of dye-sensitized solar cell

391 393

xii

Contents

15.3 Role of ionic liquids as an electrolyte in dye-sensitized solar cells 15.4 Challenges and future prospects References

394 408 409

5 Miscellaneous applications 16. Role of ionic liquids and their future alternative toward protein chemistry

417

MANSI CHAUDHARY, SHWETA PAL, KAMLESH KUMARI, INDRA BAHADUR, GEETU GAMBHIR AND PRASHANT SINGH

16.1 Introduction 16.2 Antibacterial and antitumor activities of ionic liquids 16.3 Protein instability and its influencing factors as well as analytical monitoring 16.4 Effect of alkyl chain length of cations of ionic liquids on the stability of proteins 16.5 Effect of cations and anion of ionic liquids on the stability of proteins 16.6 Effect of hydrophobicity of ionic liquids on the stability of proteins 16.7 Effect of viscosity of ionic liquids on the stability of proteins 16.8 Protein folding in ionic liquids 16.9 Enzymes with ionic liquids 16.10 Application of ionic liquids as biocatalysis 16.11 Ionic liquids do not inactivate enzymes like polar organic solvents 16.12 Increased stability of enzymes in ionic liquids 16.13 Cytotoxicity of ionic liquids 16.14 What are neoteric solvents? 16.15 Role of deep eutectic solvents on protein chemistry 16.16 Conclusion References

17. Ionic liquids in metrological analysis and applications

417 420 421 423 423 424 424 425 426 427 429 429 431 432 433 435 436

443

UMAIMA GAZAL

17.1 17.2 17.3 17.4

Introduction Wide-ranging ionic liquids Protic and aprotic ionic liquids Physicochemical properties of ionic liquids defining metrological parameters 17.5 Electrochemical constancy and conductivity 17.6 Ionic liquids-centered devices 17.7 Configuration of ionic liquids in biosensors 17.8 Aspects of ionic liquids as promoters in biodiesel fabrication 17.9 Affinity attributed to ionic liquids in nanomaterials 17.10 The implication of green diluents in space mechanics

443 444 445 447 449 449 450 450 451 452

Contents

17.11 Space energy 17.12 Compost properties 17.13 Life support techniques 17.14 Hypergolic solutions 17.15 Space emollients 17.16 Lunular fluid-glass contract 17.17 Conclusion References

xiii 453 454 454 454 455 456 456 457

18. Antibacterial properties of silver nanoparticles synthesized in ionic liquids

465

ELHAM AVIRDI, SEYYED EMAD HOOSHMAND, INDRA BAHADUR, LEBOGANG MAUREEN KATATA-SERU AND RAJENDER S. VARMA

18.1 Introduction 18.2 Silver nanoparticles’ antimicrobial properties and activities 18.3 Discussions and final remarks Declaration of competing interest Acknowledgment References

465 468 473 474 474 474

6 Future applications and studies 19. Progressive function of ionic liquids in polymer chemistry

479

PANMEI GAIJON, ARUN KANT, SUDIPTA GHOSH AND M. RAMANANDA SINGH

19.1 Introduction 19.2 Ionic liquid 19.3 Structure of ionic liquid 19.4 Some important advantages and characteristics of ionic liquid 19.5 Common methods of making ionic liquids 19.6 The function of ionic liquid in polymer 19.7 Polymer-doped ionic liquid 19.8 Polymerization of vinyl monomer in ionic liquid 19.9 Polymerizable ionic liquid 19.10 Adsorbed and covalently linked ionic liquids 19.11 Microwave absorbing ionic liquid polymer 19.12 Ionic liquid-polymer composite 19.13 Summary Conflict of interests References

479 481 481 481 482 484 484 485 486 488 488 489 490 491 491

20. Potential hazards of ionic liquids: a word of caution

497

SAIMA FAROOQ AND ZAKIRA NAUREEN

20.1 Introduction 20.2 Environmental concerns of ionic liquids

497 501

xiv

Contents

20.3 Factors affecting the toxicity of ionic liquids 20.4 Fate and transfer of ionic liquids to the environment 20.5 Conclusion and future perspectives Conflict of interest References

506 509 513 514 514

Index

523

About the editors Dr. Jamal Akhter Siddique working as an associate professor in a deemed University, Delhi-NCR, is a multidisciplinary researcher in his career, has successfully completed his postdoctoral research tenures around the globe in renowned universities and institutes, he held positions as Post-Doc, Gwangju Institute of Science and Technology (GIST), Post-Doc, World Class University, (WCU), South Korea, Post-Doc, University Technology Malaysia (UTM), Post-Doc, Czech Technical University (CTU), Researcher, Czech University of Life Sciences (CZU), Visiting Researcher, The Sheffield University (TUOS), United Kingdom. He started his research journey in the year 2007 at the Department of Chemistry, Aligarh Muslim University, and earned his Ph.D. in “Physico-chemical studies of biological/biochemical systems.” He worked in different fields of chemistry like polymers, polymer nanoparticles, nano-bio materials, graphene synthesis, hydrogen storage, biodegradable implants, adsorption of Cr metal and its simulation, MSWMFly ash, encapsulation of radioactive waste materials, and many more during his postdoctoral research tenure and published number of a research article in reputed journals. Apart from direct involvement in research, he also worked with multiple journals as an editorial member, guest editor, and asst. editor for special issues entitled “Nanostructure Materials as a Promising Route for Efficient Renewable Energy Production, Storage, and Conversion”, Journal of Nanomaterials, a peerreviewed, published by Hindawi Publication, (London, U.K) [ISSN: 1687-4110], “Academic Research for Multidiscipline” in International Journal of Science Technology and Society, a peer-reviewed journal [ISSN:2330-7412], and the Journal of Medical Imaging and Health Informatics, ISSN: 2156-7018: American Scientific Publishers respectively, etc. Dr. Shahid Pervez Ansari is currently working as an assistant professor in the Department of Applied Chemistry, Zakir Husain College of Engineering and Technology, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India. Prior to joining AMU, he has also worked as a researcher at the Umm Al Qura University, Makkah, Kingdom of Saudi Arabia. He coauthored one textbook “Faiz’s Polymer Chemistry: A Problem Solving Approach”. He has contributed several chapters in a number of books published by good publications

xix

xx

About the editors

such as Elsevier, Springer, and Wiley. He has also published several research papers in the field of conducting polymers, nanocomposites, and photocatalysis in the journals of high repute. He has also been granted one patent in India. Dr. Ansari obtained his Ph.D. in applied chemistry in 2011 and, presently, he is engaged in teaching of M.Sc. (polymer science and technology) and B.Tech. students. Dr. Aftab Aslam Parwaz Khan is currently working as an associate professor in the Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia. He obtained his Ph.D. in chemistry from Aligarh Muslim University, Aligarh, India. He has secured 5 books, 25 chapters, and more than 150 research papers. He is a potential reviewer of several reputed international journals, including Nature, ACS, RSC, WILEY, Elsevier, Springer’s, Bentham, IOP, and Frontiers publishers. He has been serving as an editorial board member of many reputed international journals since 2013. More than 30 international conferences/workshops and 25 research projects have been completed. His research encompasses all aspects of polymer nanomaterials and catalyst synthesis, properties, as well as application in photo catalyst, chemical sensing, biosensing, environmental remediation of pollution, drug delivery system for mechanistic and interaction studies using a wide range of spectroscopic techniques with thermodynamic parameters. Prof. Abdullah M. Asiri is the head of the Chemistry Department at King Abdulaziz University since October 2009 and he is the founder and the director of the Center of Excellence for Advanced Materials Research since 2010 till date. He is a professor of organic photochemistry. He graduated from King Abdulaziz University with B.Sc. in chemistry in 1990 and a Ph.D from the University of Wales, College of Cardiff, United Kingdom in 1995. His research interest covers color chemistry, synthesis of novel photochromic and thermochromic systems, synthesis of novel coloring matters and dyeing of textiles, materials chemistry, nanochemistry and nanotechnology, polymers, and plastics. Prof. Asiri is the principal supervisors of more than 20 M.Sc. and six Ph.D theses; he is the main author of 10 books of different chemistry disciplines. Prof. Asiri is the editor-in-chief of Journal of King Abdulaziz University—Science. A major achievement of Prof. Asiri is the discovery of tribochromic compounds, a class of compounds which change from slightly or colorless to deep colored when subjected to small pressure or when grind. This discovery was introduced to the scientific community as a new terminology published by IUPAC in 2000. This discovery was awarded a patent from European Patent office and from the UK patent.

About the editors

xxi

Prof. Asiri involved in many committees at the KAU level and also on the national level, he took a major role in the advanced materials committee working for KACST to identify the National Plan for Science and Technology in 2007. Prof. Asiri played a major role in advancing the chemistry education and research in KAU, he has been awarded the best researcher from KAU for the past 5 years. He also awarded the Young Scientist award from the Saudi Chemical Society in 2009, and also the first prize for the distinction in science from the Saudi Chemical Society in 2012. He also received a recognition certificate from the American Chemical society (Gulf region Chapter) for the advancement of chemical science in the Kingdome. Also he received a Scopus Certificate for the most published scientist in Saudi Arabia in chemistry in 2008. He is also a member of the editorial board of various journals of international repute. He is the vice president of Saudi Chemical Society (Western Province Branch). He holds 10 US patents, more than 1000 publications in international journals, 50 book chapters, and 30 books. Editors’ affiliations Dr. Jamal Akhter Siddique Marie Curie fellow (List-B), SASPRO-2, Slovak Academy of Sciences, Bratislava, Slovakia Dr. Shahid Pervez Ansari Department of Applied Chemistry, Zakir Husain College of Engineering and Technology, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India Dr. Aftab Aslam Parwaz Khan Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia Prof. Abdullah M. Asiri Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia; Chemistry Department, King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia

C H A P T E R

1 Progressions in ionic liquidbased electrochemical research Md Rabiul Islam, Mohd Arham Khan, Maroof Ali and Md Palashuddin Sk Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India

1.1 Introduction Ionic liquids (ILs) have aroused unprecedented interest in the research community. ILs are a promising and sustainable alternative to traditional organic solvents because they are recyclable and have no negative (or negligible) impact on the atmosphere. ILs are considered “green solvents” and serves as the excellent electrolyte in electrochemistry research [1
3]. They are favorable reaction medium candidates for chemical syntheses due to their exceptional properties, such as excellent solvating potential [1], high electrochemical stability, high thermal stability [2], negligible vapor pressure, nonflammability, and their tunable properties by appropriate cation and anion choices [3]. Due to their exciting properties, a broad window has opened in which ILs have various electrochemical applications. Therefore industrial applications of ILs are increasing rapidly, especially in the electrochemical section. ILs have been widely used in electrocatalysis, electrosynthesis, electrodeposition, electrochemical capacitors, lithium batteries, and electrochemical reactions [4]. Presently, ILs have been becoming popular in electrochemical technology [5
7]. The use of ILs in electrocatalysis and other fields of applied electrochemistry has opened up new possibilities [8
10]. Electrocatalysis is a form of catalysis that accelerates electrochemical reactions on the surface of the electrode [11]. We focused on

Advanced Applications of Ionic Liquids DOI: https://doi.org/10.1016/B978-0-323-99921-2.00019-7

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© 2023 Elsevier Inc. All rights reserved.

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1. Progressions in ionic liquid-based electrochemical research

FIGURE 1.1 Various potential uses of ionic liquids in the field of electrochemical research.

recent advancements of ILs in electrocatalytic applications such as electrochemical sensor [12,13], electrodeposition [14,15], electroredox [16
18], electrochemical biosensor [19
28], use in Li-ion batteries (LIBs) [29
32], supercapacitor (SC) [33
41], and electropolymerization [42
44] in this chapter (Fig. 1.1).

1.2 Physical properties of ionic liquids 1.2.1 Conductivity ILs provide several benefits over typical organic solvents, such as inherent conductivity. ILs are entirely made up of ions, and therefore it has high conductivity. Interestingly, ionic mobility is reduction of ILs due to the massive component of ions present in ILs, which results in

1. Catalysis and electrochemistry

1.4 Applications of ionic liquids in electrochemistry

5

poorer conductivity. However, variations in the chemical structure of ILs alter the conductivity of ILs [45]. ILs with a higher viscosity have poorer conductivity, and the conductivity of ILs depends on the temperature proportionally.

1.2.2 Viscosity Solvent viscosity plays a significant role in electrochemical applications as viscosity determines the “diffusion-controlled mechanism” in chemical reactions [46]. Also, the large viscosity of ILs has a substantial contribution to electrostatic effects [45,46]. Both the cations and anions of ILs have an impact on the viscosity of ILs. The rise in viscosity is dependent on the alkyl chain of ILs.

1.2.3 Electrochemical potential window The redox flexibility of ILs is controlled significantly by the ionic component (cations and anions) of ILs. A wide potential window ability of ILs makes them suitable as electrochemical solvents. The potential window range of ILs is slightly greater than that of the organic electrolytes. ILs have shown typical electrochemical windows of 4.5
7 V (e.g., [bmim][BF4] having a larger electrochemical window of 7 V) [47,48].

1.3 Electrochemical properties The use of traditional molten salts as solvents in electrochemical applications is severely limited due to their high melting temperatures. ILs are generally used as a reaction medium for electrochemical reactions due to their broad electrochemical window. For example, conducting polymers are synthesized by an electrochemical polymerization reaction in the presence of ILs. In addition, ILs have been widely employed in various electrochemical devices as electrolytes. ILs have special features such as nonflammability, strong ionic conductivity, electrochemical stability, and thermal stability. Therefore ILs often use in electrochemical sensors and actuators, photovoltaics, capacitors, batteries, and fuel cells [49
53].

1.4 Applications of ionic liquids in electrochemistry 1.4.1 Electrochemical sensors ILs are generally used as an electrode material because they can facilitate direct electron transfer [12]. ILs have the ability to reduce

1. Catalysis and electrochemistry

6

1. Progressions in ionic liquid-based electrochemical research

FIGURE 1.2 Schematic representation of ClPrNTf2 IL-based electrochemical sensor. (A and B) Fabrication of GCE electrode with ClPrNTf2 IL, (C) I
V responses of the electrode with analytes, and (D) detection and absorption mechanism of 3-methoxy phenol on the fabricated electrode. Source: Reproduced with permission from M.M. Rahman, H.M. Marwania, A.A. Alshehrib, H.A. Albar, J. Bisquertc, A.M. Asiria, Room temperature stable ClPrNTf2 ionic liquid utilizing for chemical sensor development, J. Organomet. Chem. 811 (2016) 74
80.

overpotential while simultaneously increasing response pace, sensitivity, and selectivity. Rahman et al. [13] demonstrated a simple procedure that produced [ClPr]NTf2 ILs at ambient temperature (Fig. 1.2). [ClPr]NTf2 with conductive coating binders on glassy carbon electrodes (GCEs) produced electrochemical phenol sensors with excellent selectivity and sensitivity. The constructed sensors showed an excellent detection limit of 0.022 6 0.002 nM for selectively sensing 3-methoxy phenol. This innovative technique offers a well-organized path for developing effective chemical sensors for environmental contaminants and health care areas on a large scale. A new electrochemical sensor was developed using ILs with 3D porous graphene-carbon nanotube denoted as CNT-IL/GP. It displays outstanding sensing properties in terms of sensitivity and selectivity in electrochemical glucose detection [54]. In another work, an electrochemical sensor was developed by an IL-mediated electrochemiluminescence signal for the direct detection of gaseous ammonia. This sensor shows high sensitivity and selectivity [55]. Further, an electrochemical sensor was developed by using graphene/IL with the imprinted polymer of 2,6-diaminopyridine to detect diamino pyridine derivatives [56].

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1.4 Applications of ionic liquids in electrochemistry

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1.4.2 Electrodeposition Electrodeposition is an essential technique widely utilized in industry and esthetic anticorrosion for wear-resistant coatings [14]. Generally, green ILs solvents act as essential electrolytes for the electrodeposition of metals to minimize the conventional disadvantages of other electrodeposition techniques. Suryanto et al. [14] demonstrated the electrodeposition of silver onto metals and metal oxide substrate using protic ILs. The results revealed that the electrodeposition is achieved by a three-dimensional growth process controlled by progressive nucleation and diffusion. Silver nanoparticle as electrocatalysts on electrode exhibits outstanding catalytic activity in the oxygen reduction process. Further, the study concludes that protic ILs might be an alternate electrolyte for metal electrodes and nanostructured electrocatalysts. Motobayashi group has developed a new electrodeposition method of cobalt (Co) in the presence of IL, which produces interfacial multilayer of ions [3]. It has been observed that this method generates high overpotential (HOP). HOP helps to reorganize the interfacial multilayer structure. Fig. 1.3 represents the mechanism of reorganizing the interfacial multilayer structure. In this mechanism, the anionic first layer is converted to the cationic first layer via excess anions (Fig. 1.3A). Interestingly, the interfacial structure B is obtained with high free energy than structure A or C (Fig. 1.3B and C).

FIGURE 1.3 (A) Multilayer interfacial structure of Co(TFSA)2 and [C3mpyr][TFSA] IL solution on gold electrode (A: anionic first layer, B: excess anions, and C: cation first layer), (B) representation of free energy plot for more positive potential, and (C) for more negative potential. Source: Reproduced with permission from K. Motobayashi, Y. Shibamura, K. Ikeda, Origin of a high overpotential of Co electrodeposition in a room-temperature ionic liquid, J. Phys. Chem. Lett. 11 (2020) 8697
8702.

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1. Progressions in ionic liquid-based electrochemical research

IL/W microemulsion –

Magnetic Mesoporous Catalysts

Working electrode

Electrochemical growth

FIGURE 1.4 An electrodeposition technique was developed for the fabrication of mesoporous films of Pt alloys using IL/water microemulsions. Source: Reproduced with permission from A. Serra`, E. Go´mez, I. V. Golosovsky, J. Nogue´s, E. Valle´s, Effective ionic-liquid microemulsion based electrodeposition of mesoporous Co-Pt films for methanol oxidation catalysis in alkaline media, J. Mater. Chem. A 4 (2016) 7805
7814.

A technique was developed based on IL-water microemulsions (IL/W) for electrodeposition using mesoporous platinum (Pt) deficient alloys films (Co3Pt and CoPt3) [15]. The electrodeposition technique using ILs is demonstrated in Fig. 1.4. In the IL/W systems the electrolytic aqueous solution promotes deposition efficiency. These alloys have high endurance in alkaline/acidic medium and preserved their shape. These alloybased catalysts are very much effective for methanol electrooxidation in an alkaline medium. A new phosphorylated metal oxide-based catalyst was developed using ILs by electrodeposition technique for water oxidation reaction [57]. The MnOx catalyst surface contains a 1:2 ratio of phosphorous and manganese. Therefore it shows both phosphate and oxide characteristics. The catalyst is highly stable for water oxidation reaction compared to the previously reported MnO2 catalyst. In another work, ILs-based electrodeposition of aluminum (Al) was demonstrated where functional aluminum layers were effectively deposited on low-carbon steel using an IL AlCl3/[Emim]Cl [58]. This technique is effective for the electrodeposition of aluminum as it improves electrochemical performances. Recently, 1-methyl-3-octadecylimidazolium bromide IL was employed as a dispersing agent for the electrophoretic deposition of multiwalled CNTs (MWCNTs) on anodized aluminum oxide [59]. Further, IL-based electrodeposition of transition metals such as Ag, Cu, Zn was also reported [60]. In 4,4-bipyridinium ILs-based electrodeposition technique, ILs work as an excellent solvent for dissolving metal salts through metal coordination [61]. Also, ILs-based electrodeposition method was used for cathodic electrochemical deposition of CuI at room temperature [62].

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1.4 Applications of ionic liquids in electrochemistry

9

1.4.3 Electroredox Fontaine et al. developed biredox IL electrolytes (Fig. 1.5) where perfluorosulfonate bearing anthraquinone (3) and 4-hydroxy-TEMPO (type 1 and 2) have anion and the cation functionality and both forms ion pairs (IP1 and IP2) [16]. Also, they conducted extensive studies (experimental and theoretical) to determine the role of redox moiety (contain bulky ions) in electron and mass transfers in electrochemical devices. These types of biredox IL electrolytes, having task-specific ions and redox moieties, show ample opportunities in electrochemical devices. According to the Marcus
Hush theory, the size of solvated redox species does not affect the electron transfer at the electrode surface. Fig. 1.6 demonstrates the formation of graphene intercalated ferrocene nanocatalyst (rGO-[bmim][FeCl4] IL). This IL-induced synthesis of nanocatalyst is the first reported technique for the preparation of

FIGURE 1.5 Demonstration of the synthesis method of biredox IL. Biredox IL contains redox moieties such as cationic and anionic species (denoted as IP1 and IP2). (AQ 5 anthraquinone moiety, T 5 TEMPO moiety). Source: Reproduced with permission from E. Mourada, L. Coustana, S.A. Freunberger, A. Mehd, A. Viouxa, F. Favier, et al., Biredox ionic liquids: electrochemical investigation and impact of ion size on electron transfer, Electrochim. Acta 206 (2016) 513
523.

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1. Progressions in ionic liquid-based electrochemical research

FIGURE 1.6 Schematic representation of intercalation of [bmim][FeCl4] IL and rGO via chemical reactions. (A) Reduction step of GO, (B) intercalation of rGO-bmimFeCL4, (C) formation step of chlorinated mesoporous rGO-FeIL, (D) representation of stabilization step rGO-FeIL layer via (E). Source: Reproduced with permission from S. Sonkaria, H.T. Kim, S.Y. Kim, N. Kumari, Y.G. Kim, V. Khare, et al., Ionic liquid-induced synthesis of a graphene intercalated ferrocene nanocatalyst and its environmental application, Appl. Catal. B 182 (2016) 326
335.

ferrocene assembly in ILs [17]. The electroredox and magnetic properties of nanocatalyst were applied for environmental application. Recently, high-temperature electrolyte polybenzimidazole was developed for application in membrane fuel cells [18]. The performance of the Li-air battery was studied using IL electrolyte. It was found that the IL has the ability to control the efficiency of Li-air battery [63]. The study of electrochemical stability of C60 thin film indicates the central role of [C4mpyrr] [Tf2N] IL electrolyte in enhancing the stability of C60 thin film on Au(111) electrode [64]. Fu et al. enhanced the efficiency of inverted polymer solar cells using self-assembled BenMeImCl IL [65]. The low-temperature fabrication of BenMeIm-Cl IL is one of the advantages of this work. However, the simple synthesis method and environment-friendly nature of BenMeIm-Cl IL add exciting features in solar cell research. Chen et al. [66] explored the sulfur solubility in ILs for the preparation Co9S8 and FeSx thin films. The study paves the new direction for electrodeposition sulfur-based compounds and captures sulfur contamination from the environment.

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11

1.4.4 Electrochemical biosensors Electrochemical biosensor is a device (links to an electrode transducer) where biological molecules are detected electrochemically through a molecular sensing mechanism. The sensing response produces an electrical signal at the electrode from the various biological events. Various recent reports suggest that biomolecules and enzymes are compatible with ILs. Silk, an appealing biomaterial with high mechanical characteristics and biocompatibility, was shown to be miscible with 1-butyl-3-methylimidazolium chloride [bmim][Cl]. Normal cell proliferation and differentiation were supported by the patterned films developed from silk fibroin dissolved in [bmim][Cl] IL [19]. Recently, higher enzyme stability was observed in ILs than in organic solvents [20]. Also, ILs have the ability to stabilize proteins efficiently at high temperatures [21]. Laszlo group experimentally revealed that the peroxidase activities of hemin and microperoxidase-11 in IL were significantly greater than the organic solvents of similar polarity [20]. They showed that the efficiency of the peroxidase activity of hemin proportionally depends on the quantity of IL [20]. The electrochemical behavior of hemin in IL may tune by the ligand field strength and surface adsorption events at the working electrode. Therefore a variety of heme proteins in [bmim] [PF6] IL were studied for electrochemical activities [22]. Heme proteins were encapsulated in agarose hydrogel films in this study. The quasireversible electron transport between heme proteins and electrode (made up of glassy carbon) in the presence of [bmim][PF6] IL was characterized by cyclic voltammetry technique for myoglobin, hemoglobin, cytochrome c, and horseradish peroxidase. Choi et al. developed MWCNT/IL (IL such as [bmim][BF4], [bmim] [PF6], or [bmim][NTf2]) bucky gel electrode for the biosensing of organophosphate nerve agents. In this work, organophosphorus hydrolase (OPH) was immobilized on electrodes. The composite electrode exhibits higher stability and high sensitivity [23]. ILs improve the dispersion of MWCNTs, leading to the formation of a 3D-network structure. ILs have strong compatibility with OPH and provide a faster electron-transfer reaction at the electrode interface. MWCNT/IL-based electrode material is beneficial in manufacturing electrochemical biosensing layers, and the reasons are given as follows: 1. Intrinsic properties (mechanical, electrical, and thermal) of MWCNT are preserved during electrochemical events. 2. Improved ionic conductivity of ILs. 3. ILs are environment-friendly green solvent. 4. Strong interaction between MWCNT and ILs. Xia et al. reported a molecularly imprinted electrochemical biosensor for determining bovine serum albumin (BSA) (Fig. 1.7). They experimented

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1. Progressions in ionic liquid-based electrochemical research

FIGURE 1.7 Preparation method of the molecular imprinted electrochemical sensor is shown using schematic representation. Source: Reproduced with permission from J.F. Xia, X. Y. Cao, Z.H. Wang, M. Yang, F.F. Zhang, B. Lu, et al., Molecularly imprinted electrochemical biosensor based on chitosan/ionic liquid-graphene composites modified electrode for determination of bovine serum albumin, Sens. Actuators B Chem. 225 (2016) 305
311.

with using a modifying GCE made up of chitosan/IL graphene (GR) composites [24]. The synergistic effect of each component enhances the electrochemical response and sensitivity for the detection of BSA. The sensor has a high selectivity, good repeatability, stability, and adequate recovery, indicating future clinical applications. Rahimi et al. developed a new catalase-based biosensor for the detection of hydrogen peroxide [25]. The catalase-based biosensor is made up of amine-functionalized MWCNT and [bmim][BF4] IL. Interestingly, amine-functionalized IL improves the dispersibility of graphene through covalent attachment, and graphene/IL has been used for the fabrication of electrochemical biosensors due to the unique characteristics of graphene/IL [26
28]. For example, graphene/IL-based electrochemical biosensors were employed to detect nicotinamide adenine dinucleotide hydrogen and the detection of guanine [28]. High-stability and highdispersibility properties improve the electrochemical performance of graphene/IL in electroanalytical/electrochemical biosensing applications for the detection of targeted analytes.

1.4.5 Applications of ionic liquids in Li-ion batteries Alternative green energy sources have been widely investigated for the advanced energy storage technologies for minimizing environmental

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13

pollution, such as the greenhouse effect. LIBs, having high energy density, are used as energy source/storage devices in electronics devices [such as portable/wearable electronics devices, hybrid cars, aircraft, energy storage systems (ESSs) grids, and other daily life applications] [29,30]. Further, LIBs are anticipated to be the future energy production source and possibly capable of producing more than half of the world’s required energy [30]. Metal oxides are commonly utilized as the cathode in LIBs, whereas graphite is commonly used as the anode. In this regard, huge efforts have been made in recent years to create high-capacity anodes and high-voltage cathodes to enhance the energy density of LIBs. The electrolyte (i.e., organic bicarbonates, e.g., ethyl methyl carbonate) is one of the most important components for LIBs [31]. These electrolytes have numerous benefits, such as high ionic conductivity and capacity, thermal instability, volatility, and flammability. However, it has safety concerns for LIBs. ILs may be one of the promising alternative electrolytes for the next generation of LIBs. ILs may replace flammable electrolytes and increase battery safety like electrolyte flammability, leakage, and thermal instability. ILs provide rapid ion conduction and electrochemical stability with a broad potential window in LIBs. Extensive research on dicationic room temperature 1,10 -(5,14-dioxo-4,6,13,15-tetraazaoctadecane-1,18-diyl) bis(3-(sec-butyl)-1H-imidazol-3-ium) bis((trifluoromethyl)-sulfonyl) imide IL for LIBs has been going on for decades all over the globe to overcome the above-mentioned problems of LIBs [2,32]. It shows good cycling performance with greater discharge capacity. Additionally, Li(NixCoyMnz)O2 was used as a cathode (high specific capacity and environmentally benign material) and graphite as an active anode material for the fabrication of LIBs. Recently, the Placke group used Pyr14TFSI-LiTFSI ILs-based electrolyte in “dual-graphite” LIB cells [67]. They found ILs provide excellent electrochemical stability and allow highly reversible cation (Li1) and anion (TFSI2) intercalation/de-intercalation on “dual-graphite” electrode. The electrolyte serves as an ion source and a channel for ion transport between the anode and cathode in LIBs. The cell performance was investigated using electrolytes made up of Pyr1,4TFSI IL with various quantities of LiTFSI. The result indicates that the electrolyte’s ionic conductivity was decreased considerably as the mole fraction of LiTFSI increased [68]. However, present LIBs technology is still insufficient for the constantly rising need for high performance, high energy-power capacity, safe, and low-cost energy sources. There is still scope to work on advanced LIBs and ILs-based electrolytes to achieve scalability, sustainability, manufacturability, and recyclability for next-generation LIBs. Future research should focus on the operation of LIBs at a wide range

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1. Progressions in ionic liquid-based electrochemical research

of temperatures, wide electrochemical potential windows (EPWs), and have a long life cycle.

1.4.6 Applications of ionic liquids for supercapacitors An SC, also known as an ultracapacitor or electronic double-layer capacitor, is an electrochemical energy storage device. It stores electronic energy through the simple formation of a double layer in the electrolyte or electrode. Recently, IL-based SCs are developed where ILs uses as an electrolyte. SCs exhibit quick power production, fast charge-discharge capabilities, and a longer lifetime than batteries. Therefore it has been used as an ESS [33]. SCs are utilized as a renewable energy source in various applications such as wearable electronics, electric buses, laptops, mobile phones, aircraft, and different diagnostic equipment in the medical system. Notably, the energy density (Ea) is one of the indispensable factors for promising applications of SCs. However, the low Ea of SCs restricts their applications. SC’s energy density is proportional to the device’s voltage window (V) and specific capacitance (C) (Ea 5 1/2 CV2) [34]. Energy density is related to the square of the potential window (V2), so efficient SCs can be made by creating new high-capacity electrode materials and/or expanding the EPW. The electrochemical stability of the electrolyte is employed to determine the EPW of SC’s devices [35]. The aqueous electrolytes in EPW can rarely exceed 1.0 V as water molecules are breakdown during electrochemical events [36]. However, the cell voltage is obtained 2.7 V for organic electrolytes (such as propylene carbonate and acetonitrile). These organic solvents are toxic, inflammable, and not safe [37]. ILs are considered promising electrolytes of SCs. ILs have many merits such as discrete anion
cation combination, negligible volatility, nonflammability, wide potential window (up to 4.5 V), and high ionic conductivity compared to aqueous/organic electrolytes [38]. Also, ILs are safe electrolytes and use in a wide range of temperatures in SCs. Balducci et al. used pyrrolidinium-based IL as the electrolyte to obtain a broad potential window of 3.5 V [39]. However, the high viscosity and poor ionic conductivity of pyrrolidinium-based IL at ambient temperature limit SCs operation below 50 C temperature [39]. It was found that a suitable nanostructured combination of CNT or graphene-based electrodes and a mixture of ILs electrolytes in SCs could significantly increase the operating temperature range and ionic conductivity [39]. Simon group has demonstrated that the capacitive energy storage of SCs could be enhanced with temperature from 50 C to 100 C using tetraethylammonium tetrafluoroborate (TEA-BF4) ILs electrolytes, shown

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15

FIGURE 1.8 The plot demonstrates the temperature enhancement from 250 C to 100 C for SCs using IL electrolytes. Traditional electrolytes’ propylene carbonate works in the temperature range of 230 C to 80 C. Source: Reproduced with permission from R. Lin, P.L. Taberna, S. Fantini, V. Presser, C.R. Pe´rez, F. Malbosc, et al., Capacitive energy storage from 50 C to 100 C using an ionic liquid electrolyte, J. Phys. Chem. Lett. 2 (2011) 2396
2401.

in Fig. 1.8 [40]. Further, the Simon group has developed SCs with improved temperature range and a widened voltage window using onion-like carbon and vertically aligned CNT array electrodes. Due to high ionic conductivity and comparatively lower viscosity of nonamphilic ILs, several research groups have been focusing on nonamphilic IL electrolytes based on imidazolium and pyrrolidinium for SCs applications. Pyrrolidinium-based ILs have broad EPWs, and imidazolium-based ILs exhibit higher ionic conductivity. Recently, surface-active ILs (SAILs) with self-assembly amphiphilic structures were employed for electric double-layer capacitors [41]. In this work, ion distribution is dominated by the Van der Waals interactions of nonpolar surfactant tails. Therefore SAILs exhibit unusual interfacial ion distributions and show excellent capacitive performance at electrode surfaces. Because of the asymmetric nature of electrolyte ions, most ILs generate layered structure on the surface of planar electrodes, which may alter the electrode’s polarity. ILs can able to form a monolayer of ions and multilayerions inside the nanoporous electrode. Small pores have a greater energy density at lower EPWs, whereas larger pores have a higher energy density at higher EPWs. Hence, amphiphile ILs are suitable as high energy density electrolytes for future generation SCs applications [41].

1.4.7 Applications of ionic liquids in electropolymerization Electrosynthetic conducting polymer has gained interest for electronic device applications such as sensors, light-emitting diodes, and solar cells. The electrochemical polymerization method is cost-effective and easy to

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1. Progressions in ionic liquid-based electrochemical research

FIGURE 1.9 Morphology of electrodeposited polypyrrole polymer films. Morphology of films was studied using AFM and SEM for PPy-bmim-PF6 (A, D), PPy-emim-TFSA (B, E), and bmp-TFSA (C, F). Source: Reproduced with permission from L. Viaua, J. Y. Hihna, S. Lakarda, V. Moutarliera, V. Flaudb, B. Lakarda, Full characterization of polypyrrole thin films electrosynthesized in room temperature ionic liquids, water or acetonitrile, Electrochim. Acta 137 (2014) 298
310.

handle. This method is considered a “green method” as ILs are used as a solvent to avoid toxic elements. Viaua et al. have investigated the formation of polypyrrole film by an electropolymerization reaction in three different ILs (bmimPF6, emimTFSA, and bmpTFSA) at room temperature (Fig. 1.9) [42]. The experimental result reveals that the properties of polymer films were controlled by the viscosity of the ILs solvents used. Therefore the viscosity of the ILs solvent plays a vital role in the formation of polymer films. Ho group has employed electropolymerization technique to obtain poly(3,4ethylenedioxythiophene) composite films doping with various ILs on ITO glasses for dye-sensitized solar cells [43]. In this work, imidazolium cations IL with varied alkyl chains and anions were studied. Interestingly, doping of ILs in film enhances the power conversion efficiencies in the dye-sensitized solar cells. In another work, the monodisperse poly ILs particles were produced for the application of highperformance anhydrous polyelectrolyte-based smart electrorheological materials [44]. The study revealed that IL particles have a substantial electrorheological impact in the dry state, depending on the size of the cation/anion of IL. Further, electropolymerization technique was used for the double electrosynthesis of 3-((4S)-benzyl-2-oxooxazolidin-3-carbonyl)-heptane-2,6-dione using [emim][BF4]. In this report, IL serves as a solvent/electrolyte system [69].

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References

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1.5 Conclusion Significant progress has been observed in the field of electrochemistry using ILs. In this chapter, we have attempted to demonstrate the impact/current advances of IL electrolytes for energy storage applications, particularly for LIBs, SCs, and electrochemical sensors. It has been observed that ILs are the ideal option as next-generation electrolytes in energy storage devices due to their large EPW, low volatility, and high thermal and electrochemical stability. ILs have the ability to work at a wide temperature range. Therefore ILs are using in hybrid electric cars. ILs have attracted attention in electrochemistry due to their CO2 capturing ability. In comparison to commercial organic electrolytes, ILs have low power performance. Therefore extensive research is still required to improve the ILs electrolyte performance.

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2000. [69] L. Palombi, A study on designing a paired electrolysis for electro-induced Michael addition using tetrafluoroborate based ionic liquid as electrolysis medium and precatalyst in a divided cell, Electrochim. Acta 56 (2011) 7442
7445.

1. Catalysis and electrochemistry

C H A P T E R

2 Recapitulation on the separation and purification of biomolecules using ionic liquid-based aqueous biphasic systems Emmanuel A. Oke and Sushma P. Ijardar Department of Chemistry, Veer Narmad South Gujarat University, Surat, India

2.1 Introduction The extraction, separation, and purification of targeted materials from synthetic or natural complex mixtures have been topics of great interest and require tremendous research efforts from the scientific community. The design of safe, economic, and environmentally friendly separation and purification methods for compounds has been paramount in the chemical industry. The choice of suitable method depends upon various factors: (1) requirement of less energy for equilibrium between two phases (2) viscosity of both the phases, (3) quick selectivity for targeted material (4) no alteration in activity, stability, and structure of bioactive compounds [1]. Liquid-liquid extraction (LLE) is a simple and frequently used separation and purification method in the industry [2]. LLE is preferred over other methods used due to certain advantages like higher yield and purity, good selectivity, and cost efficiency [3]. The conventional LLE systems consist of two immiscible phases: (1) water (polar) and (2) volatile organic solvent (non-polar). The non-biodegradable nature of highly volatile organic solvent produces negative effects on the environment and is

Advanced Applications of Ionic Liquids DOI: https://doi.org/10.1016/B978-0-323-99921-2.00007-0

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© 2023 Elsevier Inc. All rights reserved.

24

2. Recapitulation on the separation and purification of biomolecules

also found to harm the activity and functionality of enzymes, proteins, nucleic acid, etc. [4]. In 1896, Martinus Beijerinck [5] observed two immiscible aqueous phases when starch and gelatin were dissolved in water at a certain concentration. In continuous development towards a greener approach to the separation method, Albertson revisited the concept of the coexistence of two water-rich phases, that is, aqueous biphasic system (ABS), and utilized it for the extraction of biomolecules [6]. ABS comprises water-rich phases of two components (polymer-salt, polymer-polymer, or salt-salt) that exist together at specific concentrations and temperatures. Initially, a single-phase system appears at a low concentration of both components. As the concentration of both components increases, a single phase is spontaneously divided into two phases, forming a biphasic system at a particular concentration. Apart from the ease in formation, ABS has distinct advantages over traditional LLE: (1) green credential increases due to the presence of a higher amount of water (80% w/w) (2) lower surface tension between two aqueous phases (3) the interface allow the passage of water and small molecules between two phases which affect the separation process. Due to the above-discussed advantages, ABS has been found their application in various industries; from chemical synthesis to biotechnology and the food industry, etc. [7]. ABS made up of polymer 1 inorganic salts have been extensively studied for the separation and purification of metals, dyes, and biomolecules [8 11]. The aqueous phase of the hydrophobic polymer and inorganic salt offer a small difference in polarity, and increasing the viscosity of the polymer phase after the partition of targeted materials restricts the application of polymer-based ABS [12]. The new solvent, ionic liquids (ILs) are a distinct class of molten salts that specifically consist of two unsymmetrical organic and inorganic ions held together by weak forces of interaction. ILs are popularly known as green solvents for negligible vapor pressure at ambient temperature and are considered a suitable alternative to volatile organic compound (VOC) utilized by the chemical industries for less generation of waste and pollution. They also displayed some sets of unique properties like higher stability, selectivity, large electrochemical window, low melting point, mild environment for biomolecules, recycling nature, and reusability. They have been successfully applied as solvents in chemical manufacturing processes, separation processes, catalysis, extraction, batteries, lubricant, and energy applications [13 18]. The designer capability of ILs plays an important role in their application in the separation and purification method. The nature of anion and cation govern the mutual solubility in water. By changing cationanion combinations, ILs can be customized with any desired set of physicochemical properties like density, viscosity surface tension, and

1. Catalysis and electrochemistry

2.1 Introduction

25

polarity. ILs containing low polarizable anions have limited solubility in water and facilitate the formation of two phases. The solubility of ILs can be further enhanced by incorporating a long alkyl chain cation. Thus, the combined effect of cation and anion manipulates the aqueous solubility of both components for ABS formation [19,20]. To overcome the drawbacks of polymer-salt ABS, ILs were introduced as one of the components of ABS by Roger et al. [21]. They have studied the possibilities of ABS formation between an aqueous solution of imidazolium-based ILs and potassium salts [22]. Following their investigation, a huge amount of research work has been reported for ILBased ABS. ABS formation was reported using imidazolium, ammonium, cholonium, phosphonium, pyrrolidinium, and pyridinium-based ILs with various inorganic salts such as KCl, K3PO4, K2HPO4, K2HPO4KHPO4, K2CO3, KOH, Na2CO3, NaCl, NaOH, Na2CO3, Na2SO4, NaHSO4, NaHCO3, Na2HPO4, Na2H2PO4, (NH4)2SO4, Al2(SO4)3, CaCl2 and MgCl2. Besides inorganic salts, organic salts, amino acids, polymer, carbohydrates, and surfactants were studied as phase separation promoters [19,20]. According to Fig. 2.1, a huge upsurge was recorded from 2016 to 2020 for deeper understanding and wide applications for new combinations of ILs and phase-forming agents [23]. IL-based ABS were reported for the separation or purification of metal, pesticides, dyes, pharmaceuticals, and biomolecules (Fig. 2.2). It can be seen that the separation of biomolecules is most widely studied

FIGURE 2.1 Number of publications reported for applications of IL-based ABS from 2000 to 2021.

1. Catalysis and electrochemistry

26

2. Recapitulation on the separation and purification of biomolecules

FIGURE 2.2 Applications of IL-based ABS in separation and purification of materials.

using IL-based ABS, as they provide the most robust tool for the recovery of products in biotechnology. The main reason is that the biotechnological products are usually prepared on large scale in dilute concentration. In this chapter, we have researched the applications of ILs-based ABS in the separation and purification of amino acids, nucleic acids, proteins, and enzymes. The summary of IL-Based ABS used for the separation of biomolecules is listed in Tables 2.1 2.4 respectively.

2.2 Applications of ionic liquids-based aqueous biphasic system in separation and purification of biomolecules 2.2.1 Amino acids Amino acids are one of the most important building units for human health. ABS has been preferred over the other classical methods (ionexchange chromatography, liquid membrane, solvent extraction, and flotation) that have been utilized in the extraction and purification of amino acids. The extraction of L-tryptophan was extensively investigated by various IL-Based ABS. Fig. 2.3 represents the partition coefficients (K) for L-tryptophan by different combinations of IL-ABS. At the earliest period, ABS consisted of imidazolium-based ILs, and K3PO4 was deployed for the extraction of essential amino acid, L-tryptophan. The partitioning behavior of amino acids was found to rely on hydrophobicity, electrostatic attraction, π π stacking interactions, solubility, nature of the analyte, and affinity for the phases involved [24,25]. Neves et al. [26] revealed that imidazolium-based ILs with a hydroxyl group, aromatic side chain, and double bond were found to promote the migration of amino acids into the IL-rich phase whereas the reverse trend

1. Catalysis and electrochemistry

TABLE 2.1 List of IL-based ABS for the extraction and purification of amino acids. ABS system ILs

Salts

Amino acids

References

[C2MIM][Cl]; [C2MIM][CH3CO2]; [C2MIM][C1SO4]; [C2MIM][C2SO4]; [C2MIM][CF3SO3]; [C4MIM][Cl]; [C4MIM][Br]; [C4MIM][C1SO3]; [C4MIM][N(CN)2]; [C4MIM][TFA];[C4MIM][CF3SO3]

K3PO4

L-tryptophan

[24]

[P1444][C1SO4]

K3PO4

L-tryptophan

[25]

[AMIM][Cl]; [OHC2MIM][Cl]; [C4MIM][Cl]; [C7H7MIM] [Cl]; [C1IM][Cl];[IM][Cl]

K3PO4;

L-tryptophan

[26]

[CnMIM][C1CO2] (n 5 4, 6, 8)

K3PO4

L-tryptophan

[27]

[C4MIM][CF3SO3]; [C4MIM][SCN]; [C4MIM][Cl]; [C4MPIP][Cl]; [C4MPYR][Cl]; [C4MIM][N(CN)2]; [P4444] [Cl]; [N4444][Cl]

K3C6H5O7

L-tryptophan

[28]

[C4MIM][CF3SO3]

D-maltitol, D-(1)-glucose, D-(1)-mannose, D- (-)-fructose, sucrose, xylitol, D-sorbitol, D-(1)-xylose

L-tryptophan

[29]

[C4MIM][Br]

K3C6H5O7

L-tryptophan, Lphenylalanine, L-tyrosine, L-leucine, L-valine

[30]

[CnMIM][BF4](n 5 2, 3, 4 and 6); [CnMIM][Br] (n 5 3, 4 and 6)

NPTAB

L-tryptophan

[31]

[C4MIM][BF4];[C4MIM][CF3SO3]

Tricine, HEPES, TES

L-tryptophan Lphenylalanine

[32]

[BTMA][Cl];[BTBA][Cl]

K3PO4, K2HPO4, K2CO3 KOH

Tryptophan

[33] (Continued)

TABLE 2.1 (Continued) ABS system ILs

Salts

Amino acids

References

[C6MIM][N(CN)2]

C6H5Na3O7

DNP-glycine, DNP-Lalanine, DNP-L-valine DNP-L-leucine

[34]

[N1112OH][ALA]

K3PO4

DNP-alanine, DNP-valine, DNP-leucine DNP-glycine

[35]

[N4444][C3CO2]; [N4444][C4CO2]; [N4444][C5CO2]; [N4444] [C6CO2]; [N4444] [C7CO2]; [N4444][C9CO2]

K3PO4

Tryptophan

[36]

[N1112OH][CnCO2] (n 5 1 7)

K2HPO4

L-tryptophan, Lphenylalanine, L-tyrosine, L-dopa

[37]

ILs

Polymer

Salts

Amino acid

References

[IM][Cl]; [AMIM][Cl]; [C1IM][Cl]; [C2MIM] [Cl]; [OHC2MIM][Cl]

PEG 600

Na2SO4

L-tryptophan

[38]

[C4MIM][Br]

PEG 600/ PEG 400

K3C6H5O7

L-tyrosine, L-tryptophan

[39,40]

[C2MIM][Cl]; [C4MIM][Cl]; [C6MIM][Cl]; [C8MIM][Cl]; [C10MIM][Cl]; [C4MIM] [CF3SO3]; [C4MIM][C1CO2]

PEG 600

NaH2PO4;

L-phenylalanine

[41]

[C4MIM][Cl]; [C4MPYR][Cl]; [C4MPIP][Cl]; [P4444][Cl]; [N4444][Cl]

PEG 400

K3C6H5O7

L-tryptophan, Lphenylalanine L-tyrosine

[42]

[C4MIM][Cl]; [C4MPYR][Cl]; [C4MPIP][Cl]; [P4444][Cl]; [N4444][Cl]

PEG 400

C6H5K3O7/C6H8O7

L-tryptophan, Lphenylalanine L-tyrosine

[43]

TABLE 2.2 List of IL-based ABS used for the separation and purification of proteins. ABS IL

Salts

Proteins

References

[C4MIM][Cl]

K2HPO4

BSA

[44]

[AMM 110]

K2HPO4/KH2PO4

BSA, lysozyme, myoglobin, trypsin

[45]

[C4MIM][Br]; [C6MIM][Br]; [C8MIM][Br]

K2HPO4

BSA, trypsin, cytochrome c, γ globulins

[46]

[C4MIM][Br]; [C6MIM][Br]; [C8MIM][Br]

C6H5K3O7,

Cytochrome C

[47]

[DMEA][C2CO2]; [DMEA][C3CO2]; [DMEA][C4CO2]; [DMEA][C5CO2]; [DEEA][C2CO2]; [DEEA][C3CO2]; [DEEA][C4CO2]; [DEEA][C5CO2]

K2HPO4

BSA

[48]

[BE];[N1112OH][Cl]

K2HPO4, PEG 600

BSA

[49]

[N4444][Cl]; [P4444][Br]; [P4444][Cl]; [Pi(444)1][TOS]; [P1444][C1SO4]

C6H5K3O7/C6H8O7

BSA

[50]

[N1112OH][LYS]; [N1112OH][ALA] [N1112OH][GLY]; [N1112OH][SER]

PPG400

BSA, L-trypsin

[51]

[N1112OH][L-ALA]

PEGDME250, PPG 400

BSA

[52]

[N1112OH][L-HIS]; [N1112OH][L-ARG]; [N1112OH][L-PRO]; [N1112OH][LVAL]

PPG 400

BSA

[53]

[N1112OH][C2CO3]; [N1112OH][BIT]; [N1112OH][DHP]; [N1112OH][DHCit]; [N1112OH][C2CO2]; [N1112OH][C3CO2]; [N1112OH][C1CO2]; [N1112OH] [C1CO3]

PPG 400

BSA

[54]

[N4444][Tricine]; [N4444][TES]; and [N4444][HEPES]

K3C6H5O7

BSA

[55] (Continued)

TABLE 2.2 (Continued) ABS IL

Salts

Proteins

References

[N1112OH][Cl]; [N1112OH][Tricine]; [N1112OH][MES]; [N1112OH][TES]; [N1112OH][HEPES]; TES HEPESC12H22O11

PPG 400

BSA

[56]

[N1112OH][MES]; [N1112OH][CHES]; [N1112OH][Tricine]; [N1112OH] [CHES]; [N1112OH][HEPES]

PPG 400

IgY

[57]

[N1112OH][IA]; [N1112OH][C3C]; [N1112OH][GEN]; [N1112OH][D-GAL]; [N1112OH][ABI]; [N1112OH][PYR]; [N1112OH] [L-ASC]; [N1112OH][QUI]

PPG 400

IgY

[58]

[N1112OH][BIT];[N1112OH][C2CO3]; [N1112OH][DHCit]; [N1112OH][DHP]; [N1112OH][Cl];[N1112OH][C2CO2]; [N1112OH][C1CO3]; [N1112OH][C1CO2]; [N1112OH][CAF];[N1112OH][SYR]; [N1112OH][VAN]; [N1112OH] [GAL]; [N1112OH][C3CO2]

PPG 400

IgG

[59]

IL

Polymer

Salts

Protein

References

[N1112OH][GLY]

PPG 400

NaCl, MgCl2, Na2SO4

Lysozyme, BSA

[60]

[C2MIM][Br]; [C4MIM][Br]; [C4MIM][Cl]; [C4MIM][TOS]; [C4MIM][N (CN)2] [N2222][Br]; [N1111][Br]; [N3333][Br]; [N4444][Br];[P4444][Br]

PEGs (200,400,600, 1000, 2000, 4000,6000, 8000)

C6H5K3O7/ C6H8O7

IgG

[61]

[C2MIM][Cl]; [C4MIM][Cl]; [C6MIM][Cl]; [C4MIM][Br]; [C4MIM] [C1CO2]; [C4MIM][N(CN)2]; [C4MPYR][Cl]; [C4MPY][Cl]

PEG 3350

(NH4)2SO4

Myoglobin

[62]

TABLE 2.3 List of IL-based ABS used for the separation and purification of enzymes. ABS IL

Salts

Enzymes

References

[C2MIM][C2SO4]

K2CO3

Thermomyces lanuginosus lipase

[63]

[C2MIM][C4SO4]

(NH4)2SO4

Candida antarctica lipase

[64]

[N1112OH][GLY]

Triton-X 100, Triton-X 102

Thermus thermophilus HB27 lipase Halomonas sp. LM1C lipase Candida rugosa lipase

[65]

[C2MIM][Cl]; [C4MIM][Cl]; [C6MIM] [Cl]; [C7MIM][Cl]; [C8MIM][Cl]; [C7H7MIM][Cl]; [C4MIM][CF3SO3]; [C4MIM] [N(CN)2]; [C4MIM][C1SO3]; [C4MPYRR][Cl]; [C4MPYR][Cl]; [C4MPIP][Cl]; [C8MPYR][N(CN)2]

KH2PO4/K2HPO4

Candida antarctica lipase B

[66]

[C4MIM][N(CN)2]; [C4MPYR][Cl]; [C4MIM][Cl]; [C8MIM] [Cl]

KH2PO4/K2HPO4

Bacillus sp. ITP-001 lipase

[67]

[N1112OH][Cl]; [N1112OH][DHCit]; [N1112OH][Bit]

THF

Bacillus sp. ITP-001 lipase

[68]

[N4444][MOPSO]; [P4444][MOPSO]; [N4444][BES]; [P4444][BES]; [N4444][TAPSO]; [P4444][TAPSO]

K3C6H5O7

Burkholderia cepacia ST8 lipase

[69]

[N4444][MOPSO]; [P4444][MOPSO]; [N4444][BES]; [P4444][BES]; [N4444][TAPSO]; [P4444][TAPSO]; [N1112OH][MOPSO]; [N1112OH][BES]; [N1112OH][TAPSO]; [N1112OH] [MOPSO]; [N1112OH][BES]; [N1112OH][TAPSO];

(NH4)2SO4, PPG 400, BEPEGran-PPG

Burkholderia cepacia ST8 lipase

[70]

[N1112OH][BES]

PEG 400

Burkholderia cepacia ST8 lipase

[71]

[C8MIM][SAC]

CH3COOH/CH3COONa

α-Amylase

[72] (Continued)

TABLE 2.3 (Continued) ABS IL

Salts

Enzymes

References

[P4444][TAPSO]; [P4444][MOPSO]; [P4444][EPPSO] [P4444] [BICINE];

Na2SO4

α-Chymotrypsin

[73]

[P4444][TAPS]; [P4444][MOPS]; [P4444][EPPS]; [P4444][BICINE]

NaNO3

α-Chymotrypsin

[74]

[AMM110]

K2HPO4/KH2PO4

Lactobacillus brevis alcohol dehydrogenase. Escherichia coli alcohol dehydrogenase

[75]

Iolilyte 221 PG

K2HPO4/NaH2PO4

Rubisco

[76]

[N1112OH][DHP]; Iolilyte 221 PG

PEG 400, K3C6H5O7

Rubisco

[77]

[C2MIM][BF4]; [C2MIM][BF4]

Na3C6H5O7, Na2CO3

Bacillus cereus cyclodextrin glycosyltransferase

[78]

[C4MIM][Cl]; [C4MIM][Br]

K2HPO4

Papain

[79]

[N1112OH][C0CO2]; [N1112OH][C1CO2]; [N1112OH][C2CO2]; [N1112OH][C3CO2]; [N1112OH][C1CO3]; [N1112OH][C2CO3]; [N1112OH][C7O2]; [N1112OH][C2O4]; [N1112OH][C6O7]

PPG 400

BSA, Papain, Trypsin and Lysozyme

[80]

[C4MIM][Cl]

K2HPO4

Horseradish Peroxidase

[81]

[C2MIM][C1CO2]

K2HPO4

Superoxide dismutases

[82]

[N1112OH][Cl]; [N1112OH][DHP]

Tergitol NP-10

Aspergillus flavipes FP-500 Pectinase

[83]

[C4MIM][BF4]

NaH2PO4

Wheat Esterase

[84]

IL

Polymer

Salts

Enzymes

References

[CnMIM][Cl] n 5 2, 4, 6 and 8

PEGs [1500, 4000, 6000, 800]

KH2PO4/ K2HPO4

Candida antarctica lipase B

[85]

[C4MIM][C1CO2]

PEGs (2000, 4000,6000)

K2HPO4

α-Amylase

[86]

[N2222][BF4]; [N4444][Br]; [N2222][Br]; [N1111][Br]; [N2222][Cl]

PEG 4000

NaH2PO4

Papain

[87]

[N1111][Br]; [N2222][Br]; [N4444][Br]; [N2222][Cl]; [N2222] [C1CO2]; [N4444][C1CO2]

PEG 400

(NH4)2SO4

Papain

[88]

[C4mim][Br]; [C4mim][Cl]; [N2222][Br]; [N3333][Br]; [N4444] [Br]; [P4444][Cl]; [P4444][Br]; [C4MPIP][Cl]; [C4MPYRR][Cl]; [C4MPYR][Cl]; [N1112OH][Cl]; [N1112OH][C1CO2]; [N1112OH] [GLY]; [N1112OH][DHP]; [N1112OH][DHCit]; [N1112OH][BIT]

PEG 400, PPG 400

K2HPO4, C6H5K3O7/ C6H8O7, C6H5K3O7,

Laccase

[89]

TABLE 2.4 List of IL-based ABS used for the separation and purification of nucleic acids. ABS IL

Salts

Nucleic acids

References

[C4MIM][BF4]

KH2PO4

DNA

[90]

[C2MIM][C1CO2];[C2MIM][(C1)2PO4]; [C2MIM][C0CO2]; [C2MIM][N(CN)2]; [C2MIM][SCN]; [C2MIM][(C4)2PO4]; [AMIM][Cl];[DBU][C0CO2]; [N1112OH] [C0CO2];[N1112OH][C1CO2]; [N1112OH][C3CO2]; [N1112OH][C5CO2]; [N1112OH] [C7CO2]; [N1112OH][C9CO2]; [N1112OH][C11CO2]; [N1112OH][C2CO3]; [N1112OH] [DTP]; [N1112OH][DHP]; [N1112OH][(C4)2PO4]; [N1112OH][TMPP];[N1112OH][BEP]

NaH2PO4/Na2HPO4

DNA

[91]

[C2MIM][C1CO2]; [C2MIM][(C1)2PO4]; [N1112OH][C0CO2]; [N1112OH][C1CO2]; [N1112OH][C3CO2]; [N1112OH][C5CO2]; [N1112OH][C7CO2]; [N1112OH][C9CO2]; [N1112OH][C11CO2]; [N1112OH][C2CO3]; [N1112OH][DHP]; [N1112OH][(C4)2PO4]; [N1112OH][TMPP]; [C2MIM][Cl]; [C4MIM][Cl]; [C6MIM][Cl]; [GUAN][C0CO2]; [GUAN][C1CO2]; [GUAN][C3CO2]; [GUAN][C5CO2]; [GUAN][C7CO2]; [GUAN][C9CO2]

NaH2PO4/Na2HPO4

DNA

[92]

[BE][C0CO2];[BE][C1CO2]; [BE][C2CO2]; [BE][C3CO2]; [N4444][Br]

Na2CO3, K2HPO4, NaH2PO4, C6H5 Na3O7, PPG 400,

DNA

[93]

[N1112OH][C2CO2]; [N1112OH][C3CO2]; [N1112OH][C4CO2]; [N1112OH][C5CO2]; [N1112OH][C6CO2]

K2CO3

Thymine, adenine, guanine, cytosine

[94]

2.2 Applications of ionic liquids-based aqueous biphasic system

35

FIGURE 2.3 Comparison of K values for L-Tryptophan using IL-ABS.

was noticed in the case of unsubstituted and mono substituted ILs. The presence of Cl2 or Br2 and N(CN)22 anions have a better capacity to extract L-tryptophan according to the Hofmeister series. Louros et al. [25] utilized ABS constituted by tributyl(methyl) phosphonium methylsulfate [P1444][C1SO4] and K3PO4 to study the partitioning behavior of Ltryptophan. The ABS comprising of phosphonium ILs are denser than water, hence, separated conveniently after the partitioning process. They are more thermally stable in comparison to imidazolium-based counterparts and do not possess acidic protons [25]. Wang and his coworkers [27] examined the contribution of the alkyl chain length of ILs on the partitioning of L-tryptophan in ABS composed of 1-hexyl-3-imidazolium acetate, [C6MIM][C1CO2], and 1-ocyl-3imidazolium acetate, [C8MIM][C1CO2]. The former IL gave the highest partitioning of tryptophan due to the self-aggregation of the IL that interacts with L-tryptophan [27]. In another study, ABS made up of imidazolium, phosphonium, and ammonium-based ILs and K3C6H5O7 were examined for the extraction of L-tryptophan [28]. The phosphonium and ammoniumbased ILs showed a higher capacity for the partitioning of L-tryptophan than imidazolium ILs. In comparison, ABS constituted by imidazoliumbased ILs 1 K3C6H5O7 showed lower partition for L-tryptophan compared to the one constituted by imidazolium IL 1 K3PO4-based ABS [26,28]. Freire and coworkers determined the partition behavior of L-tryptophan using 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [C4MIM] [CF3SO3] and various carbohydrates (i.e., D-maltitol, D-(1)-mannose, D-(1)-glucose, D-(2)-fructose, sucrose, xylitol, D-sorbitol and D-(1)-xylose)

1. Catalysis and electrochemistry

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2. Recapitulation on the separation and purification of biomolecules

[29]. The ABS composed of IL-carbohydrate were reported to have more efficiency in comparison to polyethylene glycol (PEG)-polysaccharide ABS. The presence of [C4MIM][CF3SO3] in PEG-carbohydrate can lead to the enhancement of the ABS. Although, only monosaccharides and disaccharides showed the maximum extraction capacity with [C4MIM][CF3SO3]. The extraction capacity was reported not to depend on the type of carbohydrate utilized because of no interactive force between the carbohydrate and L-tryptophan. Though, hydrogen bonding, as well as π π interaction, showed varying impacts on the separation of L-tryptophan [29]. Zafarani-Moattar and his coworkers [30] reported the capacity of ABS constituted by 1-butyl-3-methylimidazolium bromide, [C4MIM][Br] with K3C6H5O7 for the extraction of L-tryptophan, L-tyrosine, Lphenylalanine, L-valine, and L-leucine. These amino acids were studied at their isoelectric point (pH 5 6). The K values of the investigated amino acids were reported in order of L-valine , L-leucine , Ltryptophan , L-phenylalanine , L-tyrosine. The aromatic amino acids: L-phenylalanine, L-tryptophan, and L-tyrosine contain the π system in imidazolium cation. On the other hand, hydrophobicity was responsible for the extraction of L-leucine and L-valine. Comparatively, L-leucine has one extra CH2 in its alkyl chain than L-valine making L-leucine a little hydrophilic and less affinity for the hydrophilic IL-rich phase. Wei et al. [31] studied the extraction of L-tryptophan in [CnMIM] [BF4] (n 5 2, 3, 4 and 6) and [CnMIM][Br] (n 5 3, 4 and 6) ILs and cationic surfactant 3-p-nonylphenoxy-2-hydroxypropyl trimethylammonium bromide (NPTAB) as the phase separating agent. The L-tryptophan preferentially migrated towards the IL-rich phase and ILs containing [BF4] displayed the best extraction efficiencies compared with the [Br] containing ILs. The extraction efficiencies decreased with an increase in the alkyl chain of the cation. Luis and collaborators [32] employed ABS composed of organic biological buffers, also known as Good’s Buffers (GBs) and ILs for the extraction of L-phenylalanine and L-tryptophan. [C4MIM][CF3SO3] and [C4MIM][BF4] formed ABS with N-tris(hydroxymethyl)methyl glycine (TRICINE), N-tris (hydroxymethyl)methyl-2-aminoethanesulfonic acid (TES) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) GBs. In a single step, 22.4% 100% extraction rates were obtained for both amino acids. The extraction rate for L-tryptophan was reported to be highest for ABS [C4MIM][BF4] 1 TRICINE and lowest for [C4MIM][BF4] 1 TES. Similarly, L-phenylalanine was reported to be in the order: [C4MIM][CF3SO3] 1 HEPES , [C4MIM][BF4] 1 TRICINE , [C4MIM][BF4] 1 HEPES C [C4MIM][BF4] 1 TES. For all examined GB-based ABS, amino acids preferentially migrated toward the GB-rich phase (more hydrophilic) unlike in the case of the previously discussed studies [32].

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Pereira and collaborators [38] investigated the effect of IL on PEG 600 1 Na2SO4 ABS and explained the results in terms of the nature of cation and anion and functional groups on the alkyl side chain. ILs were used as adjuvants in the ABS formation were: Imidazolium chloride, [IM][Cl]; 1-methylimidazolium chloride, [C1IM][Cl], 1-ethyl-3-methylimidazolium chloride, [C2MIM][Cl]; 1-hydroxyethyl-3-methylimidazolium chloride, [OHC2MIM][Cl] and 1-allyl-3-methylimidazolium chloride, [AMIM][Cl]. ILs having a higher alkyl chain and substituent on the C2 position were found to promote the migration of amino acids toward the PEG-rich phase. The addition of ILs as adjuvants to ABS made up of PEG-polymer can result in higher extraction efficiency. Hamzehzadeh et al. [39] examined the role of [C4MIM][Br] on the partitioning behavior of L-tyrosine in ABS made up of PEG 600 1 K3C6H5O7. Interestingly, the efficiency of the ABS was increased when [C4MIM][Br] was used as adjuvants. Hamzehzadeh and Vasiresh [40] also studied the partitioning behavior of L-tryptophan using ABS constituted by PEG 400, K3C6H5O7, and [C4MIM][Br] as an adjuvant and observed that the presence of [C4MIM][Br] improved the extraction of L-tryptophan twice by enhancing its migration towards the PEG-rich phase. Yang et al. [41] studied the applicability of [CnMIM][Cl] (n 5 4, 6, 8,10), [C4MIM][CF3SO3], and [C4MIM][C1CO2] (1-butyl-3-methylimidazolium acetate) as adjuvants in PEG 600 1 NaH2PO4 ABS and for the extraction of L-phenylalanine. The K value for L-phenylalanine in the ABS was found to increase with chain length of the ILs (i.e., [C4MIM] [Cl] to [C8MIM][Cl]). The K values for L-phenylalanine were found to be highest (5.403) in [C10MIM][Cl]. To further understand the effect of ILs in polymer-salt ABS, Sousa et al. [95] investigated the partitioning of L-tryptophan, Lphenylalanine, and L-tyrosine using PEG 400 1 K3C6H5O7 ABS. The chloride-based ILs: [C4MIM][Cl], [C4MPIP][Cl], [C4MPRY][Cl], [N4444] [Cl] and [P4444][Cl] were added as adjuvants. The K values for L-tryptophan and L-phenylalanine (except tyrosine) were found to be relatively low when 5% ILs were employed. The higher K value for tyrosine was attributed to its hydrophilic and charged nature at pH 7 and major interactions with the Cl2 by hydrogen bonding and electrostatic interactions [42,95]. Priyanka et al. [33] studied ABS constituted by benzyltrimethylammonium chloride, [BTMA][Cl] and benzyltributylammonium chloride, [BTBA][Cl] with K3PO4, K2HPO4, K2CO3 and KOH for the separation of L-tryptophan. Both systems composed of [BTM][Cl] and [BTBA][Cl] 1 K3PO4 displayed highest K values for L-tryptophan. The effect of salts on the partitioning of L-tryptophan was: K3PO4 . K2HPO4 . K2CO3 . KOH. The higher K values were attributed to the presence of benzene rings in ILs that facilitated the π π stacking interactions between the IL

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2. Recapitulation on the separation and purification of biomolecules

and the tryptophan aromatic rings. Additionally, N H π strong interactions between the pyrrole group of tryptophan and benzyl aromatic group contributed to higher K values. Gomez and Macedo [34] estimated the partition coefficients of N-(2,4Dinitrophenyl)-glycine (DNP-glycine); N-(2,4-Dinitrophenyl)-L-alanine (DNP-L-alanine); N-(2,4-Dinitrophenyl)-L-valine (DNP-L-valine) and N(2,4-Dinitrophenyl)-L-leucine (DNP-L-leucine) amino acids by employing ABS constituted by 1-hexyl-3-methylimidazolium dicyanamide ([C6MIM] [N(CN)2]) and sodium citrate (C6H5Na3O7). The maximum K values obtained for DNP-leucine and four targeted compounds were in the range of 567 2232. Similarly, the partition behavior of DNP-alanine, DNPleucine, DNP-valine, and DNP-glycine was also examined by Requejo et al. [35]. ABS used was composed of cholinium alaninate IL ([N1112OH] [ALA]) and two inorganic salts (K3PO4 and K2HPO4). A similar trend [34] was reported that the highest K values were obtained for DNP-L-leucine and the lowest for DNP-glycine in two IL-Based ABS investigated. The K values of amino acids obtained [34] using [C6MIM][N(CN)2] 1 C6H5Na3O7 were greater than the ones obtained by Requejo et al. [35]. ABS comprised of tetrabutylammonium butanoate, [N4444][C3CO2]; tetrabutylammonium pentanoate, [N4444][C4CO2]; tetrabutylammonium hexanoate, [N4444][C5CO2]; tetrabutylammonium heptanoate, [N4444] [C6CO2]; tetrabutylammonium octanoate, [N4444][C7CO2]; and tetrabutylammonium decanoate, [N4444][C9CO2] in combination with K3PO4 were used for the extraction of tryptophan by Basaiahgari and Gardas [36]. The extraction ability was observed to decreas as the anion chain length of investigated ILs increases. The K values for tryptophan were found in the range of 2 18; indicating poor affinity of the tetrabutylammonium carboxylate based-ILs for partitioning of tryptophan. Belchior et al. [37] studied the odd-even effect of ABS constituted by cholinium carboxylate-based ILs [N1112OH][CnCO2] with n 5 1 7 (consisting of anions with odd and even alkyl chain length), and K2HPO4 for the extraction of L-tryptophan, L-tyrosine, L-phenylalanine and L-dopa. The extraction efficiencies of the targeted amino acids were reported to be between the ranges of 66.8% 99.9%. The high extraction efficiency was linked to the greater salting-out capability of K2HPO4 as well as the strong interactions between the IL and the investigated amino acids. Slightly higher K values were obtained for ABS involving ILs with odd alkyl chains of anion whereas the reverse is also true [37]. The ternary system composed of PEG 400, water and C6H5K3O7: C6H8O7 (potassium citrate:citric acid) buffer at pH 7.0 and the effect of 5 wt.% of several ILs as additives ([C4MIM][Cl], [C4MPIP][Cl], [C4MPRY][Cl], [N4444][Cl] and [P4444][Cl]) on ternary systems (i.e., quaternary ABS containing PEG 400, water, citrate buffer and ILs at pH 7.0) have been reported by Rita de Cassia et al. [43]. The partitioning

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behavior of L-tyrosine, L-tryptophan, L-phenylalanine and was evaluated in these quaternary systems. The addition of 5% ILs in PEG saltbased ABS had modified the characteristics of the coexisting phases, and enhanced the partitioning of the studied biomolecules when compared to IL-salt ABS. The hydrophobicity of the phase afforded by the presence of IL plays the dominant role in controlling the partitioning of the hydrophobic biomolecules in quaternary systems.

2.2.2 Proteins Proteins are the most important molecules for living organisms, as they control many fundamental activities like metabolism, expression of genes, and cell signaling. The traditional route for protein separation involves precipitation, ion exchange, electrophoresis, and chromatography. However, these methods have been proved to be costly, lengthy, and nonscalable. The use of volatile organic solvents in the above-listed methods causes denaturation and deactivation of proteins. Proteins have been employed in therapeutic, biotechnological, and diagnostic, hence, it is important to use the purest form of protein. Pure hydrophobic ILs were reported for the extraction of proteins from an aqueous medium. The little addition of water in ILs further enhances the stability of forming microemulsion [96,97]. Later on, the separation and purification of proteins by IL-Based ABS have been reported extensively in the literature [44 63]. Firstly, Du et al. studied 1-butyl-3-methylimidazolium chloride [C4MIM][Cl] 1 K2HPO4 ABS for the direct extraction of bovine serum albumin (BSA) from human urine. The proteins shifted to the IL-rich phase and all other components remained in the salt-containing phase. They confirmed the retention of biological and structural properties of protein using spectroscopic properties [44]. Dreyer et al. [45] reported the fundamental explanation of the main forces responsible for the separation of four proteins (lysozyme, myoglobin, trypsin, and BSA) into ABS composed of the Ammoeng110 (IL’s cations containing oligoethyleneglycol units) and K2HPO4/KH2PO4. The partitioning of protein is a complex phenomenon that depends upon many factors like size, conformation, and surface structure of a protein, interactions between proteins and inorganic salts as well as with ILs and pH of the system. The gel electrophoresis concluded the presence of electrostatic interactions between cation of ILs and amino acid residue which governs the partitioning of all the four proteins in the IL-phase. The negatively charged BSA was partitioned into the IL phase due to electrostatic interaction with the cation of the Ammoeng. The positively charged trypsin and lysozyme interacted with anions of salts H2PO42 and HPO422. At higher pH, more HPO422 interacted with the positive surface of the protein and facilitated greater partition.

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ABS composed of [C4MIM][Br]; 1-hexyl-3-methylimidazolium bromide, [C6MIM][Br]; and 1-ocyl-3-methylimidazolium bromide, [C8MIM] [Br] with K2HPO4 were investigated by Pei et al. [46]. The ABS was explored for the extraction of proteins: BSA, cytochrome c (Cyt-c), trypsin, and γ globulins. In a single step, the extraction efficiencies of the studied ABS were found between 75% and 100%. The extraction decreased with increasing pH between 7 and 12. The extraction efficiencies of proteins depend upon temperature and alkyl chain length of cation of the ILs. The reported thermodynamic study was supported by the dominance of hydrophobic interactions. However, the authors concluded that the salting-out effect and electrostatic interactions equally contributed to the partitioning of proteins. The spectroscopic measurements were taken to verify the conformation of the extracted proteins into IL rich phase. It was found that 80% 90% activity of trypsin in ILs was maintained in IL rich phase. Lu et al. [47] have reported extraction of cytochrome Cyt-C in imidazolium ILs-based ABS. Various factors like the size of alkyl chain length of ILs cation, pH, the concentration of ILs, salt, and temperature on the extraction efficiency of Cyt-C were reported. The partition of Cyt-C in ABS was explained based on thermodynamic parameters ΔGT* (free energy change), ΔHT* (change in enthalpy) and ΔST* (change in entropy). As explained by Pei et al. [46], the partitioning of Cyt-C was facilitated by the combined effects of electrostatic and hydrophobic interactions. The extraction efficiency of the studied ABS for Cyt-C in a single step was 94%. The UV Vis and circular dichroism (CD) spectra confirmed no change in the structure of Cyt-C; as no strong interactions were observed between the Cyt-C and ILs. A hydroxyl ammonium-based ILs: N,N-dimethylethanolamine propanoate, [DMEA][C2CO2]; N,N-dimethylethanolamine butanoate, [DMEA][C3CO2]; N,N-dimethylethanolamine pentanoate, [DMEA] [C4CO2]; N,N-dimethylethanolamine hexanoate, [DMEA][C5CO2]; diethyl ethylene diamine propanoate, [DEEA][C2CO2]; diethyl ethylene diamine butanoate, [DEEA][C3CO2]; diethyl ethylene diamine pentanoate, [DEEA][C4CO2]; and diethyl ethylene diamine hexanoate, [DEEA] [C5CO2] have been designed and synthesized. IL-Based ABS composed of N,N-dimethylethanolamine propanoate ([DMEA][C2CO2]) were studied to extract proteins [48]. Under optimal conditions, extraction efficiency could be increased to 99.50%. The formation of the proteins remains unaltered even after being transferred to the IL-rich phase Hydrogen bonding interaction, aggregation phenomenon and saltingout effect played important parts in the extraction. ABS made up of betaine ([BE]) or cholinium chloride ([N1112OH][Cl]) and K2HPO4 or PEG 600 were studied for the extraction of BSA, trypsin, lysozyme, and ovalbumin [49]. The efficiency of the ABS was discussed

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in terms of the amount of IL, K2HPO4, and temperature. The K values for BSA in [BE] 1 K2HPO4and [N1112OH][Cl] 1 K2HPO4 were 7.1 and 4.8 respectively. The least K value was observed in ABS composed of [BE] or [N1112OH][Cl] 1 PEG 600. More than 90% of the extraction efficiency of BSA could be obtained in [BE]/K2HPO4. Pereira et al. [50] have combined phosphonium- or ammonium-based ILs, namely: [N4444][Cl]; tetrabutylphosphonium bromide, [P4444][Br]; [P4444][Cl]; tri(isobutyl)methylphosphonium tosylate, [Pi(444)1][TOS]; and [P1444][C1SO4] with C6H5K3O7/C6H8O7 buffer solution. These ABS have been evaluated for the extraction of BSA. The results showed that BSA was extracted 100% into the IL-rich phase in three-step extraction for [P4444][Br]. The BSA was precipitated or denaturated in [P4444][Br] based ABS. The extracted BSA into the IL-rich phase was recovered by dialysis. Fourier-transform infrared spectroscopy (FT-IR) and size-exclusion high-performance liquid chromatography (SEHPLC) were used to confirm the structure of BSA. The extraction of BSA achieved in the studied ABS was higher than those obtained using polymer-salt-based ABS reported earlier. Importantly, a small amount of biodegradable citrate salt and amount of both ammonium and phosphonium-based ILs used in ABS was used. Another report on the ABS formation between cholinium lysinate, [N1112OH][LYS]; cholinium alaninate [N1112OH][ALA]; cholinium glycinate, [N1112OH][GLY]; and cholinium serinate [N1112OH][SER] with PPG 400 was presented by Song et al. [51]. They have used IL anion of different hydrophobicity and acid-base character to check the effect of pH change on ABS. The K values for BSA were obtained between 6 and 11 in the studied ABS. The maximum recovery of 90% has been obtained for BSA using [N1112OH][LYS], [N1112OH][SER], and [N1112OH][GLY] based ABS. The two model proteins, BSA and trypsin were transferred into the IL-rich phase containing less hydrophobic anion and the pH of the system was higher than the isoelectric point of the IL. Thus, the transfer of desired proteins was possible by manipulating the pH of the system and the anion’s hydrophobicity of IL. Zafarani-Moattar et al. [52] have investigated green ABS by combining cholinium L-alaninate, [N1112OH][L-ALA] with PEG dimethyl ether 250 (PEGDME250)/PPG400. It was observed that the phase forming capability of ABS dependent on the temperature, type of polymer, and change in hydrophobicity with temperature. Entropy change was reported as the main driving force for ABS formation. The different ABS have different efficiency to extract BSA which is transferred into the ILrich phase (KBSA . 1). ABS composed of [N1112OH][L-ALA] and PPG 400 as well as PEGDME250 showed maximum extraction efficiency for BSA. However, ABS composed of PEGDME250 was considered a better option due to the ease of recyclability of the polymer.

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Zafarani-Moattar et al. [53] have investigated another greener option using cholinium amino acid-based IL [Ch][AA]-IL with PPG 400 for ABS. The studied [N1112OH][AA]-IL were: cholinium L-histidine, [N1112OH][L-HIS]; cholinium L-arginine, [N1112OH][L-ARG]; cholinium Lproline, [N1112OH][L-PRO]; and cholinium L-valine, [N1112OH][L-VAL]. The partition of BSA in the studied ABS depends upon the hydrophobicity of the amino acid present in the structure of IL: [N1112OH][LARG] . [N1112OH][L-HIS] . [N1112OH][L-PRO] . [N1112OH][L-ARG]. Apart from hydrophobicity of anion, other factors which are responsible for the partitioning of BSA were the salting-out effect and π π interactions between [N1112OH][AA]-IL and BSA. The EE BSA% was greater than 50% for the studied ABS. Similarly, the novel cholinium-based IL, namely: [N1112OH][C2CO3]; cholinium bitartarate, [N1112OH][BIT]; cholinium dihydrogenphosphate, [N1112OH][DHP]; cholinium dihydrogencitrate, [N1112OH][DHCit]; [N1112OH][C2CO2]; [N1112OH][C3CO2]; [N1112OH][C1CO2] [N1112OH] [C1CO3], and PPG400 were studied by Quental et al. to evaluate the extraction of BSA [54]. The results also explained the presence of hydrogen bonding and dispersive forces between BSA and IL that governed the extraction mechanism. ABS containing [N1112OH][BIT], [N1112OH] [DHCit], [N1112OH][C3CO2], [N1112OH][C2CO2], [N1112OH][C1CO3], [N1112OH][C1CO2] displayed maximum (more than 92%) recoveries. Using a different approach, Song et al. [60] have used inorganic salts: sodium chloride (NaCl), sodium sulfate (Na2SO4), and magnesium chloride (MgCl2) as adjuvants in choline glycinate ([Ch][Gly]) and PPG400 based ABS. and examined for partitioning behavior of lysozyme and BSA. The presence of favorable interactions between ions of salts added and proteins enhanced the separation of proteins into the IL phase. The interactions were further dependent on the isoelectric point of the protein, charge density of ions, and pH of the ABS. It was reported that common salt could produce different partitioning of different proteins for selected ABS Thus, the partition of proteins can be controlled using various inorganic salt as adjuvants. Taha et al. [55] introduced a new series of GB-IL. GBs anions TRICINE, TES, HEPES, MES [2-(N-morpholino) ethanesulfonate] and CHES [2-(cyclohexylamino) ethanesulfonate] were combined with [C2MIM]; tetramethylammonium, [N1111]; tetraethylammonium, [N2222]; and [N4444] cations. The GB-IL was found to form ABS with Na2SO4 and K3C6H5O7. ABS composed of [N4444][TRICINE], [N4444][TES], [N4444]-[HEPES] together with K3C6H5O7 were investigated for the extraction of BSA. These ABS were able to extract BSA in IL rich phase with 100% efficiency in a single step. GB-IL containing cholinium cations and GBs anions have selfbuffering properties in the biological pH range, and their polarity can

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be manipulated using different chemical structures of ions. The extractive ability of ABS formed by PPG 400 and several GB-IL for BSA was evaluated. ABS formed by PPG400 and [N1112OH][Cl] as well as GB-IL ([N1112OH][TRICINE], [N1112OH][MES], [N1112OH][TES], [N1112OH] [HEPES], TES and HEPES) and sucrose (C12H22O11) were also studied for comparison. The BSA migrated into the GB-IL-rich phase with 100% extraction efficiencies in a single step [56]. A new biocompatible and biodegradable ABS made up of GB-IL (cholinium-based GB-IL) and PPG400 was used for purification and separation of immunoglobulins which are regarded as Y-shape proteins (IgY) obtained from egg yolk [57]. GB-IL investigated were cholinium 2-(N-morpholino)ethanesulfonate, [N1112OH][MES]; cholinium 2-(cyclohexylamino) ethanesulfonate, [N1112OH][CHES]; cholinium N-[tris(hydroxymethyl) methyl]glycinate, [N1112OH][Tricine]; cholinium 2-(cyclohexylamino)ethanesulfonate, [N1112OH][CHES]; and cholinium 2-[4-(2-hydroxyethyl)piperazin1-yl]ethanesulfonate, [N1112OH][HEPES]. The maximum extraction efficiency was observed between 80% and 94% for the studied ABS due to Van der Waals and hydrogen bonding. These novel ABS showed promising results due to their biocompatibility and maintaining the integrity of the protein. Several biodegradable IL were synthesized from cholinium ions obtained from plant natural acids reported by Mondal et al. [58]. The synthesized IL: cholinium indole-3-acetate ([N1112OH][IA]), cholinium coumarin-3-carboxylate ([N1112OH][C3C]), cholinium gentisate ([N1112OH] [GEN]), cholinium D-galacturonate ([N1112OH][D-GAL]), cholinium abietate ([N1112OH][ABI]), cholinium pyruvate ([N1112OH][PYR]), cholinium L-ascorbate ([N1112OH] [L-ASC]), cholinium quinate ([N1112OH][QUI]), and [N1112OH][C1CO3] were combined with PPG400 for ABS formation. These ABS were first demonstrated for extraction of Immunoglobulins G (IgG) and also later for the purification of IgG from rabbit serum. The three IL [N1112OH][ABI], [N1112OH][C3C] and [N1112OH][GEN] could not form ABS with the PPG 400. Most of the ABS showed 100% extraction of pure IgG into the IL-rich phase retaining their native structure. However, IgG extraction was reduced to 85% in the case of rabbit serum. The purity of IgG was increased up to 58% in comparison to the serum sample. ABS containing [N1112OH][QUI], [N1112OH][ASC], [N1112OH][C1CO3], [N1112OH][PYR], and [N1112OH][GAL] demonstrated maximum extraction efficiency and between 45% and 85% of recoveries were recorded for IgG. The extraction of IgG from rabbit serum using cholinium-based IL and PPG 400 ABS was performed by Ramalho et al. [59]. The cholinium-based IL containing 13 different anions was tested and [N1112OH][DHP] was found to have the highest capacity to form ABS with the PPG 400. IgG was transferred to the IL-rich phase after phase

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separation, due to strong interactions between the IgG and the IL. Extraction efficiency by the two best performing ABS: [N1112OH][C1CO3] and cholinium vanillate, [N1112OH][VAN] was found around 100% and recovery yield was recorded to be greater than 80%. Under optimized conditions, the highest purity achieved for IgG was . 49%. IL-Based ABS were successfully investigated for the extraction of various biomolecules. The use of a high concentration of IL increases the cost of the procedure. The cost of ABS can be reduced by using IL as an additive in traditional polymer-based ABS. The addition of IL can control the polarity range of PEG as well as salt phases. However, IL as an additive does not always produce a positive effect on the extraction of biomolecules. The mechanism and various factors regulating the extraction of targeted material using IL-assisted PEG-based ABS have been addressed by many research groups. The addition of IL as adjuvants enabled modification of the properties of both aqueous phases of polymer and salt, thus, enhancing the extraction efficiency of ABS. However, the effect of the addition of IL depends upon the type of IL, polymer, and salt. The polymer-salt systems assisted by 5% IL was reported for the extraction of pure IgG by Ferreira and coworkers [61]. 1-ethyl-3methylimidazoliumbromide, [C2MIM][Br]; [C4MIM][Br]; [C4MIM][Cl];1butyl-3-methylimidazolium tosylate, [C4MIM][TOS]; [C4MIM][N(CN)2]; [C4MIM][C1CO2]; tetrabutylammonium bromide, [N4444][Br]; and [P4444] [Br] tetrapropylammonium bromide, [N3333][Br]; tetraethylammonium bromide, [N2222][Br]; tetramethylammonium bromide, [N1111][Br]; were used as adjuvants in PEG citrate based ABS. The structural of IgG remained intact during the extraction and also in presence of IL, except [P4444][Br] and [N4444][Br]. The selected ABS were used for the extraction of IgG directly from rabbit serum samples. The purity and yield of IgG were increased in range of 21% 26% and 42% 47% respectively. The effect of IL on PEG 3350 1 (NH4)2SO4-based ABS has been investigated by Marchel et al. [62]. The chosen ILs (as adjuvants) were [C2MIM][Cl], [C4MIM][Cl], [C6MIM][Cl], [C4MIM][Br], [C4MIM] [C1CO2], [C4MIM][N(CN)2], [C4MPYR][Cl] and [C4MPY][Cl]. The affinity of IL anion with water and their hydrogen bond basicity were responsible for phase separation in ABS. When 5% IL was added to the PEG-salt system, the extraction efficiencies for myoglobin were found to be in the range of 3.35% 62.68% in the PEG-rich phase. Due to the acidic pH, and characteristics of the acetate, the maximum KMB was obtained with the use of [C4MIM][C1CO2]. The partitioning of myoglobin into the PEG-rich phase increases with an increase in the concentration of IL to 7.5%. The obtained results indicated that the partitioning behavior of myoglobin was influenced by pH and the hydrophilicity of the phases in equilibrium.

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2.2.3 Enzymes Enzymes have found their use in waste management industries, drugs and pharmaceuticals, as well as paper and food industry. Interestingly, lipases have gotten extensive attention from the researchers where a series of ABS involving ILs were applied for their extraction as well as purification [63 71,85]. Deive and collaborators [63] utilized IL-Based ABS composed of [C2MIM][C2SO4] and K2CO3 for the extraction of Thermomyces lanuginosus lipase (TlL). The analyte preferentially migrated towards the IL-rich phase with intact bio-catalytic activity. After optimization, a 99% recovery rate of the TIL was achieved successfully. In another study, Deive et al. [64] employed ABS constituted by 1ethyl-3-methylimidazolium butyl sulfate, [C2MIM][C4SO4], and (NH4)2SO4 for the separation of Candida Antarctica lipase from aqueous media. In this investigation, Candida Antarctica lipase was reported to migrate preferentially towards the IL-rich phase and 99% extraction efficiency was noted. These results revealed that ABS composed of [C2MIM][C4SO4]/(NH4)2SO4 is a better candidate for the extraction of lipase [64] compared to [C2MIM][C2SO4] 1 K2CO3 [63]. This same research group [65] reported ABS composed of [N1112OH] [GLY] and Triton-X 100/102 for the extraction of three lipase enzymes of different origins from aqueous solutions. The lipase enzymes extracted were thermophilic lipase (TtL) obtained from Thermus thermophilus HB27 and halophilic lipase (HspL) from Halomonas sp. LM1C and a commercial lipase (CrL) were obtained from the mesophilic yeast called Candida rugosa. The extraction efficiencies were found between the ranges of 87 99, 45 81, and 77 98 for CrL, HspL, and TtL, respectively [65]. Ventura et al. [66] also conducted an investigation involving pyridinium, pyrrolidinium, piperidinium, and imidazolium-based IL with KH2PO4/K2HPO4 buffer solution for the extraction of Candida antarctica lipase B (CaL B). The recovery efficiencies obtained for the targeted analyte were in the range of 95.9% 99.8% obtained from the salt-rich phase. However, the highest purification factor (PF) of 2.6 was obtained with [C8MIM][Cl]. Impressed by the results obtained, Ventura and collaborators [67] also employed another set of ABS involving several ILs for the purification and extraction of extracellular lipolytic enzyme generated by Bacillus sp. ITP-001. In this study, the IL-based ABS used comprised of [C4MIM][Cl], [C8MIM][Cl], [C4MIM][N(CN)2] and [C4MPYR][Cl] together with KH2PO4/K2HPO4 buffer solution. The enzyme was observed to migrate preferentially towards the salt-rich phase. Nevertheless higher PF of the enzyme was obtained between 26 and 51 and recovery efficiencies were also reported to be higher in the range of 91% 96%. The same research group [68] further investigated IL-based ABS constituted by choline-based IL: [N1112OH][Cl], [N1112OH][BIT], and [N1112OH]

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[DHCit] in combination with tetrahydrofuran (THF) for the purification of lipase derived from Bacillus sp.ITP-001. The targeted enzyme showed a preference for the IL-rich phase as reported by the authors. Also, ILBased ABS composed of [N1112OH][BIT] was found to give the highest PF of 136.8. The overall trend of all the systems investigated for the enzyme purification was: [N1112OH][Bit] . [N1112OH][DHCit] . [N1112OH][Cl] [68]. ABS constituted by various PEGs of different molecular weights (1500, 4000, 6000, and 8000) and K2HPO4/KH2PO4 in combination with imidazolium-based ILs ([CnMIM][Cl], n 5 2, 4, 6 and 8) as additives were examined for the recovery and purification of CaL B from fermentation broth. The PF of 245 was obtained when IL was used as adjuvants [85], far higher than those obtained with IL-based ABS (PF  51) [68,85]. The highest recovery percentage (94%) of the enzyme was obtained with ABS involving PEG 1500 and [C6MIM][Cl] [85]. Three studies by Lee et al. [69 71] examined the possibilities of utilizing GB-based IL for the recovery and purification of lipase produced by Burkholderiacepacia ST8 using various GB-IL-ABS. In their first study, GB-IL ABS composed of tetrabutylammonium 2-hydroxy-3morpholinopropanesulfonicacid, [N4444][MOPSO]; tetrabutylphosphonium 2-hydroxy-3-morpholinopropanesulfonicacid, [P4444][MOPSO]; tetrabutylammonium 2-[bis(2-hydroxyethyl)amino]ethanesulfonicacid, [N4444][BES]; tetrabutylphosphonium 2-[bis(2-hydroxyethyl)amino]ethanesulfonicacid, [P4444] [BES]; tetrabutylammonium N-[tris(hydroxymethyl)methyl]-3-amino-2hydroxypropanesulfonic acid, [N4444][TAPSO]; tetrabutylphosphonium N[tris(hydroxymethyl)methyl]-3-amino-2-hydroxypropanesulfonic acid [P4444] [TAPSO] with K3C6H5O7 were employed for purifying lipase enzyme obtained from Burkholderiacepacia ST8 [69]. Around 1.7 3.0 fold higher enzymatic activities was recorded in GB-IL phases than in the salt-rich phase. Lee et al. [70] also investigated several GB-IL ABS formed from tetrabutylammonium, tetrabutylphosphonium, and cholinium cations involving GB anions (MOPSO, BES, and TAPSO) in combination with either PPG 400 or (NH4)2SO4 for purifying extracellular lipase obtained through the submerged fermentation by Burkholderia cepacia ST8 microorganism. The researchers reported that lipase displayed its preference for the GBIL phase in the purification systems comprised of GB-IL 1 salt and polymer. The PF of 22.4 and recovery percentage of 94% of lipase were obtained by employing neutral pH-controlled [N4444][BES] 1 (NH4)2SO4 based ABS. Lee et al. [71] further used GB-IL ABS composed of [N1112OH] [BES]-GB and PEG 400 for the recovery and purification of Burkholderia cepacia ST8 lipase. The high recovery rate of 99.3% and PF of 18 were obtained using studied ABS. Considering the neutral pH of this system ([N1112OH][BES] 1 PEG 400), it is suitable for the purification of lipase. Apart from lipases, other enzymes of high significance have been recovered and purified from different sources using ABS comprising of ILs.

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For instance, the partitioning and purification of α-amylase by employing different ABS composed of IL [72,86]. Vahidnia et al. [86] studied the partitioning behavior of α-amylase by employing ABS composed of [C4MIM] [C1CO2] and K2HPO4 with various PEGs (i.e., PEG 2000, PEG 4000, and PEG 6000) as adjuvants. The authors observed α-amylase preference for the IL-rich phase. ABS containing PEG 2000 resulted in the highest K value (24.1) for the α-amylase while PEG 6000 gave the least K value (19.8) [86]. Tonova and Bogdanov [72] investigated IL-Based ABS involving 1-octyl-3methylimidazolium saccharinate ([C8MIM][SAC]) and acetate buffer (CH3COOH/CH3COONa) for the extraction and purification of α-amylase. Greater than 96% of the α-amylase was recovered from the IL-Based ABS at equilibrium (isoelectric point). Gupta research group [73,74] has investigated the extraction of α-chymotrypsin (α-CT), a protease using GB-ILABS. The ABS is composed of tetrabutylammonium- and phosphonium-based IL-GB in combination with Na2SO4. In their first study [73], GB-IL employed were [P4444][TAPSO], [P4444][MOPSO], [P4444][EPPSO] (tetraphosphonium 4(2-hydroxyethyl)-1-piperazinepropanesulfonic acid), and [P4444] [BICINE] (tetrabutylammonium N,N-bis(2-hydroxyethyl)-glycine) with Na2SO4. All the ABS resulted in 100% extraction efficiencies in the ILrich phase, except [P4444][BICINE] GB-IL. The same research group [74] obtained similar results with NaNO3 in combination with all the IL utilized in [73]. Dreyer and Kragl [75] examined the purification of alcohol dehydrogenases obtained from Lactobacillus brevis and Escherichia coli using IL-Based ABS constituted by [AMM110] IL and K2HPO4/KH2PO4 at pH 7. Both enzymes were noted to display a preference for the ILrich phase using the Box-Wilton technique. Desai and collaborators [76] utilized ABS constituted by Iolilyte 221PG IL and K2HPO4/NaH2PO4 for the partitioning of an enzyme called Rubsico (or Ribulose-1,5-bisphosphate carboxylase/oxygenase). The K values were estimated to be 3 4 times greater than the PEGbased ABS. At the concentration of less than 10% (w/w) of the Iolilyte 221PG aqueous solution, Rubsico enzyme became stabilized, hence, appropriate choice of IL and compositions are pertinent. In another study by Ruiz et al. [77], ABS composed of [N1112OH][DHP]/PEG 400, Iolilyte 221 PG/K3C6H5O7, and PEG 400/K3C6H5O7 were reported for the separation of Rubisco enzyme from a microalgae biorefinery. The K values and recovery percentages of the enzyme were reported in the ranges of 2.6 3.6 and 72% 80%, respectively. Ng et al. [78] examined the purification of Cyclodextrin glycosyltransferase enzyme released by Bacillus cereus from fermentation broth by employing ABS constituted by [C2MIM][BF4] (1-ethyl-3-methylimidazolium tetrafluoroborate) IL as well as C6H5Na3O7 and also Na2CO3 (sodium carbonate). In [C2MIM][BF4]/Na2CO3, the Cyclodextrin

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glycosyltransferase enzyme was selectively distributed into the IL-rich phase with a PF of 13.86. The introduction of 3% (w/w) NaCl into the systems was used to promote enzyme partitioning into the IL-rich phase. The authors [79,87,88] employed ABS containing different ILs for the extraction and partitioning of papain enzyme. ABS involving [C4MIM] [Cl]/K2HPO4 and [C4MIM][Br]/K2HPO4 were reported for the partitioning of papain [79]. The rise in the concentration of IL at a pH near the pI of the targeted analyte resulted in 80% 98% of extraction efficiencies after a step of extraction. Yu et al. [87,88] employed ammonium-based IL [N2222][BF4] (tetraethylammonium tetrafluoroborate), [N4444][Br], [N2222][Br], [N1111][Br] and [N2222][Cl] (tetraethylammonium chloride) as adjuvants for the extraction and purification of crude papain by ABS containing PEG 4000 and NaH2PO4. The addition of 3 wt.% [N2222][BF4] as an adjuvant to the ABS containing PEG 4000/ NaH2PO4 resulted in the highest PF (3.177) for papain [87]. For optimized ABS made up of PEG 4000-NaH2PO4 using [N2222][BF4] as an adjuvant, the PF of the crude papain (obtained from payaya latex) was increased from 3.517 to 12.04. Yu and Zhang [83] proceeded to investigate the impact of another set of various ammonium-based IL ([N1111] [Br], [N2222][Br], [N4444][Br], [N2222][Cl], [N2222][C1CO2] and [N4444] [C1CO2]) as adjuvants on the separation and purification of crude papain enzyme obtained from Carica payaya latex using ABS constituted by PEG 400 and (NH4)2SO4. The addition of [N1111][Br] to the ABS containing PEG 400/(NH4)2SO4 resulted in the highest PF value (13.51) compared with PF values obtained for [N2222][Br] and [N4444][Br]. Biodegradable, thermo-sensitive, and non-toxic ABS comprising several ILs have been examined by Li et al. [80]. The cholinium-based IL: [N1112OH] [C0CO2] (Cholinium formate), [N1112OH][C1CO2], [N1112OH][C2CO2], [N1112OH][C3CO2], [N1112OH][C1CO3], [N1112OH][C2CO3], [N1112OH][C7O2] (cholinium benzoate), [N1112OH][C2O4] (cholinium oxalate) and [N1112OH] [C6O7] (cholinium citrate) were combined with polypropylene glycol 446 (PPG 400) and evaluated for the purification of BSA, papain, trypsin and lysozyme. The results obtained demonstrated that no phase separation was noted with [N1112OH][C7O2] and PPG 400. The combined force of coloumbic and hydrophilic interactions were responsible for the partitioning of the protein/enzymes into the IL phase. The extraction efficiencies of ABS obtained ranged from 86% to 99.9% after optimization. The order of extraction of proteins/enzymes follows their sizes, that is, greater partition for smaller protein/enzymes: lysozyme . papain . trypsin . BSA. Cao and collaborators [81] employed IL-Based ABS composed of [C4MIM][Cl]/K2HPO4 for the partitioning of the Horseradish Peroxidase (HRP) enzyme. The extraction efficiency of around 80% and enzymatic activity of more than 90% were observed after optimization

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in IL rich phase. Simental-Martınez and collaborators [82] explored the ability of ABS composed of [C2MIM][C1CO2] IL and K2HPO4/KH2PO4 as well as other ABS for the partitioning of Superoxide dismutases (SOD) produced from Kluyveromyces marxianus microorganism. SOD was noted to display a high preference for the IL-rich phase. Approximately, 91% recovery yield of the SOD was obtained. However, PEG 3350/potassium phosphate ABS demonstrated better performance than [C2MIM][C1CO2]-K2HPO4/KH2PO4 with regards to enzymespecific activity, SOD recovery, and purification efficiency. Furthermore, the food industry has been looking forward to natural additives to improve food quality. In this regard, the utilization of enzymes as additives could be the solution to many of the quality challenges confronted in the food industry. Wolf-Marquez and collaborators [83] studied the extraction and purification of pectinase derived from Aspergillus flavipes FP-500 using ABS composed of cholinium-based IL: [N1112OH][Cl] and [N1112OH][DHP] with non-ionic surfactant Tergitol NP-10. Pectinase was noted to partition preferentially towards the phase containing IL [84]. The extraction percentage of pectinase was found to be greater than 90%. Additionally, pectinase activity was found to increase by A. flavipes when it was scaled up to 5 L from 1 L. Jian et al. [89] examined the capacity of ABS constituted by [C4MIM][BF4] IL and NaH2PO4 at pH 4.8 for the partitioning and purification of wheat esterase enzyme. It was shown that wheat esterase distributed into the phase containing hydrophilic IL [84]. Enzymes possess magnificent catalytic features, effectiveness, biocompatibility, and selectivity. Based on these characteristics, they can be regarded as excellent biological catalysts. Most importantly, laccases are multi-Cu oxidases with outstanding potential for an array of uses; both in the environment and in biotechnology [89]. Several industrial processes have utilized laccases over the past few decades based on the reports made by some authors [98,99]. However, one of the major limitations remains the high cost associated with the downstream processing needed in obtaining highly purified, stable and active laccases [100]. As a result of this, the authors [84] recently examined the efficiency of a series of ABS composed of imidazolium-, pyridinium-, pyrrolidinium-, piperidinium-, tetraalkylphosphonium-, and tetraalkylammonium based ILs for the extraction and purification of laccases enzyme. In addition, authors [84] engaged these ILs to form ABS with salts (C6H5K3O7/C6H8O7, C6H5K3O7, and K2HPO4), polymers (PPG 400 and PEG 400). ILs were also employed as adjuvants in polymer-based systems at pH 8. Laccase was found to show a preference for the phase of the ABS with the most hydrophilic nature regardless of the systems involved. Out of all the examined ILs, cholinium-based ILs proved to be the best candidates for the extraction of laccase and they availed the highest enzyme activity values [89].

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2.2.4 Nucleic acids Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are collectively referred to as nucleic acids. They are macromolecules consisting of repeating units known as nucleotides. Nucleic acids are responsible for so many principal chemical reactions and as well as genetic information that are taking place within a living system. For the first time, Huang and Huang [90] studied the ability of ABS involving [C4MIM][BF4] IL and KH2PO4 for the extraction of DNA derived from yeast. The ultrasonicassisted technique was used for the leaching of the DNA from the yeast cell. The best pH for the extraction of DNA by using the earlier mentioned system was found to be approximately 9. The results obtained demonstrated that IL has a good impact on the extraction of DNA from yeast. In the quest to fabricate a more efficient system for the extraction of genetic material from food products, the Bica research group published two articles [91,92] on the extraction of DNA from maize and meat samples using several IL. IL that were examined for this purpose were imidazolium-, cholinium-, and quanidinium-based as well as others in combination with NaH2PO4/Na2HPO4. The developed systems demonstrated great ability in extracting DNA within 5 minutes and 25 for maize and meats, respectively at room temperature. However, pure IL containing [NTf2]2, [PF6]2 anions, and those with longer alkyl side chains, that is, ([CnMIM]1, n . 8) performed poorly for the extraction of DNA from food products investigated owing to their greater hydrophobic nature [91]. Considering the quality of DNA, effectiveness, and probably environmental friendliness, [N1112OH][C0CO2], [N1112OH][C5CO2], and [C2MIM] [(C1)2PO4] (1-ethyl-3-methylimidazolium dimethyl phosphate) were found to yield the best results regardless of the food products involved. Xu et al. [93] investigated the partition behavior of DNA amidst protein and other contaminants from a bovine whole blood sample using systems constituted by IL and deep eutectic solvent (DES) for the first time. The authors derived IL/DES from betaine/carboxylic acid and betaine/carbohydrates respectively. The ABS betaine formate, [BE] [C0CO2]; betaine acetate, [BE][C1CO2]; betaine propanoate, [BE][C2CO2]; and betaine butanoate [BE][C3CO2] in conjunction with PPG 400, tetrabutylammonium bromide ([N4444][Br]) DES, and various inorganic/ organic salts (Na2CO3, K2HPO4, NaH2PO4, and C6H5Na3O7) were employed for the extraction of DNA. They observed the preferential movement of the DNA towards the phase containing IL, and extraction efficiency above 99% was achieved for the DNA in a single step. The DNA tends to possess a negative charge at a point when pH . pI. At this stage, it shows a higher affinity for the phase comprising of ILs as a result of increased electrostatic attraction between the DNA and the IL. Hence, the distribution of the DNA in the IL-rich bottom phase.

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Recently, Almeida et al. [94] used ABS composed of low toxicity and biodegradable cholinium-based ILs involving [N1112OH][C2CO2], [N1112OH][C3CO2], [N1112OH][C4CO2], [N1112OH] [C5CO2] and [N1112OH] [C6CO2] in combination with K2CO3 for the extraction of the four nitrogenous bases (i.e., thymine, adenine, guanine, and cytosine) present in the DNA. In all the investigated ABS, all the compounds of interest were noted to display a preference for the phase containing IL. The authors obtained extraction rates between 81% and 97% in a single step. However, based on the pH of 11.8 and 12.4 for the top and the bottom phases of the ABS, authors [94] observed that the investigated nitrogenous bases suffered speciation. Thymine, adenine, and cytosine possess a 1 charge, whereas guanine has a 2 charge, and ought to interact with the cholinium cation strongly. No significant differences were noticed in the extraction capacities of ABS comprising of different IL, which were meant to exhibit impact via different cationic 2 anionic interactions.

2.3 Conclusion This chapter gives up-to-date and adequate information on the application of ABS involving ILs with the main focus on biomolecules. It also comprises details on the types of IL and phase forming agents utilized for ABS formation, the role of IL, factors affecting ABS, and possible mechanisms of extraction on targeted biomolecules. The literature reveals that IL-ABS has proven to be a promising separation process for biomolecules. ABS performance greatly depends on temperature, nature of ILs/ salting-out agent, and pH of the solution. ABS involving choline-based ILs combined with polymer and organic salts was considered a promising eco-friendly system. The efficiency of the conventional polymer-based ABS can be manipulated by the addition of a small amount of suitable ILs. For many IL-based ABS, the activity and primary structure of proteins remain intact after being partitioned into the ILs phase. Even though dedicated efforts have been made by researchers, a few challenges need to be analyzed in detail for commercial-scale execution of ABS comprising of ILs. The development of a thermodynamic model is needed for choosing the best combination for designing IL-ABS. A detailed study on the driving forces for the partition of biomolecules into the IL phase is also required. This study will act as guidelines for designing task-specific IL-Based ABS. The most common challenges like reusability of IL and separation of biomolecules (keeping their activity unaltered) from the IL phase should be further explored in detail. Continuous research has been going on to resolve the above-listed drawbacks to make IL-based ABS economical. Even after the few

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drawbacks mentioned earlier, IL-based ABS can be regarded as a promising method for the separation and purification of biomolecules.

Acknowledgments Emmanuel A. Oke acknowledges the financial assistance of Africa Scholarship-MEA under the scheme of the Indian Council for Cultural Relations (2019 20).

Nomenclature Abbreviations ΔGT* ΔHT* ΔST* ABS BBD-RSM CD DNA FT-IR GBs GB-IL GB-IL-ABS ILs K LLE PEG PEGs PF pI PPG RNA SDS-PAGE SEHPLC TLL UV Vis

free energy change enthalpy change entropy change aqueous biphasic system Box-Behnken design coupled with response surface methodology circular dichroism deoxyribonucleic acid Fourier-transform infrared spectroscopy good buffers good buffer ionic liquids good buffer ionic liquids aqueous biphasic systems ionic liquids partition coefficient liquid-liquid extraction polyethylene glycol polyethylene glycols purification factor isoelectric point polypropylene glycol ribonucleic acid sodium dodecyl sulfate-polyacryamide gel electrophoresis size exclusion high-performance liquid chromatography tie line length ultraviolet-visible spectroscopy

Ionic Liquids and Good Buffers [C2MIM][C1CO2] [C4MIM][C1CO2] [C6MIM][C1CO2] [C8MIM][C1CO2] [C2MIM][C1SO4] [C2MIM][C2SO4] [C2MIM][C4SO4] [C4MIM][C1SO3] [P1444][C1SO4] [C4MIM][CF3SO3] [C2MIM][N(CN)2] [C4MIM][N(CN)2]

1-ethyl-3-methylimidazolium acetate 1-butyl-3-methylimidazolium acetate 1-hexyl-3-immidazolium acetate 1-ocyl-3-immidazolium acetate 1-ethyl-3-methylimidazolium methylsulfate 1-ethyl-3-methylimidazolium ethylsulfate 1-ethyl-3-methylimidazolium butylsulfate 1-ethyl-3-methylimidazolium methanesulfonate tributyl(methyl)-phosphonium methylsulfate 1-butyl-3-methylimidazolium trifluoromethanesulfonate 1-ethyl-3- methylimidazolium dicyanamide 1-butyl-3-methylimidazolium dicyanamide

1. Catalysis and electrochemistry

Nomenclature

[C6MIM][N(CN)2] [C4MIM][TFA] [C2MIM][SCN] [C4MIM][SCN] [IM][Cl] [C1IM][Cl] [OHC2MIM][Cl] [AMIM][Cl] [C7H7MIM][Cl] [C2MIM][Cl] [C4MIM][Cl] [C6MIM][Cl] [C8MIM][Cl] [C10MIM][Cl] [C4MPYR][Cl] [C4MPRRO][Cl] [C4MPIP][Cl] [P4444][Cl] [P4444][Br] [N2222][Cl] [N4444][Cl] [N1111][Br] [N2222][Br] [N3333][Br] [N4444][Br] [N2222][BF4] [N2222][C1CO2] [N4444][BF4] [N4444][C3CO2] [N4444][C4CO2] [N4444][C5CO2] [N4444][C6CO2] [N4444][C7CO2] [N4444][C9CO2] [C4MIM][PF6] [C6MIM][PF6] [C2MIM][BF4] [C3MIM][BF4] [C4MIM][BF4] [C6MIM][BF4] [C8MIM][BF4] [C8MIM][SAC] [C2MIM][(C1)2PO4] [C2MIM][(C4)2PO4] [C2MIM][C0CO2] [DBU][C0CO2] [N1112OH][C0CO2] [N1112OH][C1CO2] [N1112OH][C2CO2] [N1112OH][C3CO2] [N1112OH][C5CO2] [N1112OH][C5CO2]

1-hexyl-3-methylimidazolium dicyanamide 1-butyl-3-methylimidazolium trifluoroacetate 1-ethyl-3-methylimidazolium thiocyanate 1-butyl-3-methylimidazolium thiocyanate imidazolium chloride 1-methylimidazolium chloride 1-hydroxyethyl-3-methylimidazolium chloride 1-allyl-3-methylimidazolium chloride 1-benzyl-3-methylimidazolium chloride 1-ethyl-3-methylimidazolium chloride 1-butyl-3-methylimidazolium chloride 1-hexyl-3-methylimidazolium chloride 1-ocyl-3-methylimidazolium chloride 1-decyl-3-methylimidazolium chloride 1-butyl-1-methylpyrilidinium chloride 1-butyl-1-methylpyrrolidinium chloride 1-butyl-1-methylpiperidinium chloride tetrabutylphosphonium chloride tetrabutylphosphonium bromide tetraethyllammonium chloride tetrabutylammonium chloride tetramethylammonium bromide tetraethylammonium bromide tetrapropylammonium bromide tetrabutylammonium bromide tetraethylammonium tetrafluoroborate tetraethylammonium acetate tetrabutylammonium tetrafluoroborate tetrabutylammonium butanoate tetrabutylammonium pentanoate tetrabutylammonium hexanoate tetrabutylammonium heptanoate tetrabutylammonium octanoate tetrabutylammonium decanoate 1-butyl-3-methylimidazolium hexafluorophosphate 1-hexyl-3-methylimidazolium hexafluorophosphate 1-ethyl-3-methylimidazolium tetrafluoroborate 1-propyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium tetrafluoroborate 1-hexyl-3-methylimidazolium tetrafluoroborate 1-ocyl-3-methylimidazolium tetrafluoroborate 1-octyl-3-methylimidazolium saccharinate 1-ethyl-3-methylimidazolium dimethyl phosphate 1-ethyl-3-methylimidazolium dibutyl phosphate 1-ethyl-3-methylimidazolium formate 1,8-diazabicyclo(5.4.0)undec-7-ene formate cholinium formate cholinium acetate cholinium propanoate cholinium butanoate cholinium pentanoate cholinium hexanoate

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[N1112OH][C6CO2] [N1112OH][C7CO2] [N1112OH][C9CO2] [N1112OH][C11CO2] [N1112OH][C1CO3] [N1112OH][C2CO3] [N1112OH][C7O2] [N1112OH][C2O4] [N1112OH][C6O7] [N1112OH][DTP] [N1112OH][(C4)2PO4] [N1112OH][TMPP] [N1112OH][BEP] [GUAN][C0CO2] [GUAN][C1CO2] [GUAN][C3CO2] [GUAN][C5CO2] [GUAN][C7CO2] [GUAN][C9CO2] [BE][C0CO2] [BE][C1CO2] [BE][C2CO2] [BE][C3CO2] [BE] [C2MIM][Br] [C3MIM][Br] [C4MIM][Br] [C6MIM][Br] [C8MIM][Br] [BTMA][Cl] [BTBA][Cl] [N1112OH][IA] [N1112OH][C3C] [N1112OH][GEN] [N1112OH][D-GAL] [N1112OH][ABI] [N1112OH][PYR] [N1112OH][L-ASC] [N1112OH][QUI] [N1112OH][VAN] [N1112OH] [GAL] [N1112OH][CAF] [N1112OH][SYR] [N1112OH][L-ALA] [N1112OH][ALA] [N1112OH][LYS] [N1112OH][GLY] [N1112OH][SER] [N1112OH][L-HIS] [N1112OH][L-ARG] [N1112OH][L-PRO] [N1112OH][L-VAL]

cholinium heptanoate cholinium octanoate cholinium decanoate cholinium dodecanate cholinium glycolate cholinium lactate cholinium benzoate cholinium oxalate cholinium citrate cholinium O,O-diethyl dithiophosphate cholinium dibutyl phosphate cholinium bis(2,4,4-trimethylpentyl) phosphinate cholinium bis(2-ethylhexyl) phosphate guanidinium formate guanidinium acetate guanidinium butanoate guanidinium hexanoate guanidinium octanoate guanidinium decanoate betaine formate betaine acetate betaine propanoate betaine butanoate betaine 1-ethyl-3-methylimidazolium bromide 1-propyl-3-methylimidazolium bromide 1-butyl-3-methylimidazolium bromide 1-hexyl-3-methylimidazolium bromide 1-ocyl-3-methylimidazolium bromide benzyltrimethylammonium chloride benzyltributylammonium chloride cholinium indole-3-acetate cholinium coumarin-3-carboxylate cholinium gentisate cholinium D-galacturonate cholinium abietate cholinium pyruvate cholinium L-ascorbate cholinium quinate cholinium vanillate cholinium gallate cholinium caffeate cholinium syringate cholinium L-alaninate cholinium alaninate cholinium lysinate, cholinium glycinate cholinium serinate cholinium L-histidine cholinium L-arginine cholinium L-proline cholinium L-valine

1. Catalysis and electrochemistry

Nomenclature

[N1112OH][CnCO2] [N1112OH][Cl] [N1112OH][BIT] [N1112OH][DHCit] [N1112OH][DHP] [N1112OH][MES] [N1112OH][TES] [N1112OH][CHES] [N1112OH][Tricine] [N1112OH][CHES] [N1112OH][HEPES] [N1112OH][BES] [N1112OH][MOPSO] [N1112OH][TAPSO] [N4444][MOPSO] [P4444][MOPSO] [N4444][BES] [P4444][BES] [N4444][TAPSO] [P4444][TAPSO] [P4444][EPPSO] [P4444][BICINE] [Pi(444)1][TOS] [C4MIM][TOS] [DMEA][C2CO2] [DMEA][C3CO2] [DMEA][C4CO2] [DMEA][C5CO2] [DEEA][C2CO2] [DEEA][C3CO2] [DEEA][C4CO2] [DEEA][C5CO2] [AMM110] NPTAB DNP THF TRICINE HEPES TES MOPSO BES MES CHES TAPSO

55

cholinium carboxylate cholinium chloride cholinium bitartrate cholinium dihydrogencitrate cholinium dihydrogenphospate cholinium 2-(N-morpholino)ethanesulfonate cholinium N-tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid cholinium 2-(cyclohexylamino)ethanesulfonate cholinium N-[tris(hydroxymethyl) methyl]glycinate cholinium 2-(cyclohexylamino)ethanesulfonate cholinium 2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonate cholinium 2-[bis(2-hydroxyethyl)amino]ethane sulfonicacid cholinium 2-hydroxy-3-morpholinopropanesulfonicacid cholinium N-[tris(hydroxymethyl)methyl]-3-amino-2-hydroxy propanesulfonic acid tetrabutylammonium 2-hydroxy-3-morpholinopropanesulfonicacid tetrabutylphosphonium 2-hydroxy-3morpholinopropanesulfonicacid tetrabutylammonium 2-[bis(2-hydroxyethyl)amino] ethanesulfonicacid tetrabutylphosphonium 2-[bis(2-hydroxyethyl)amino]ethane sulfonicacid tetrabutylammonium N-[tris(hydroxymethyl)methyl]-3-amino-2hydroxy propanesulfonic acid tetrabutylphosphonium N-[tris(hydroxymethyl)methyl]-3-amino-2hydroxy propanesulfonic acid tetrabutylammonium 4-(2-hydroxyethyl)-1piperazinepropanesulfonic acid tetrabutylammonium N,N-bis(2-hydroxyethyl)-glycine tri(isobutyl)methylphosphonium tosylate 1-butyl-3-methylimidazolium tosylate, N,N-dimethylethanolamine propanoate N,N-dimethylethanolamine butanoate N,N-dimethylethanolamine pentanoate N,N-dimethylethanolamine hexanoate, diethyl ethylene diamine propanoate diethyl ethylene diamine butanoate diethyl ethylene diamine pentanoate, diethyl ethylene diamine hexanoate Ammoeng 110 3-p-nonylphenoxy-2-hydroxypropyl trimethylammonium bromide N-(2,4-dinitrophenyl) tetrahydrofuran N-tris(hydroxymethyl)methylglycine 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid N-tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid 2-hydroxy-3-morpholinopropanesulfonic acid 2-[bis(2-hydroxyethyl)amino]ethanesulfonic acid 2-(N-morpholino)ethanesulfonate 2-(cyclohexylamino)ethanesulfonate N-[tris(hydroxymethyl)methyl]-3-amino-2-hydroxypropanesulfonic acid

1. Catalysis and electrochemistry

56

2. Recapitulation on the separation and purification of biomolecules

[NTf2]2 [PF6]2

bis(trifluoromethylsulfonyl)imide hexafluorophosphate

Proteins Cyt-C BSA IgY IgG

cytochrome C bovine serum albumin immunoglobulins immunoglobulins G

Enzymes TIL TtL HspL CrL CaL B α-CT HRP SOD

thermomyces lanuginosus lipase thermophilic lipase halophilic lipase commercial lipase candida antarctica lipase B α-chymotrypsin horseradish peroxidase superoxide dismutases

Salts K3C6H5O7 KCl K3PO4 K2HPO4 K2HPO4-KHPO4 K2CO3 KOH CH3COONa Na2CO3 NaCl NaOH NaNO3 Na2SO4 NaHSO4 NaH2PO4 Na2H2PO4 NaHCO3 C6H5Na3O7 (NH4)2SO4 Al2(SO4)3 CaCl2 MgCl2 C6H8O7 C12H22O11

potassium citrate potassium chloride tripotassium phosphate dipotassium phosphate potassium buffer potassium carbonate potassium hydroxide sodium acetate sodium carbonate sodium chloride sodium hydroxide sodium nitrate sodium sulfate sodium hydrogen sulfate monosodium phosphate sodium phosphate dibasic sodium bicarbonate sodium citrate tribasic dehydrate ammonium sulfate aluminum sulfate calcium chloride magnesium chloride polypropylene glycol sucrose

Acid CH3COOH C6H8O7

acetic acid citric acid

1. Catalysis and electrochemistry

References

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[89] E.V. Capela, A.I. Valentea, J.C.F. Nunesa, F.F. Magalhaesa, O. Rodrı´guez, A. Sotob, et al., Insights on the laccase extraction and activity in ionic-liquid-based aqueous biphasic systems, Sep. Purif. Technol. 248 (2020) 117052. [90] D.J. Huang, D.C. Huang, The research for the extraction of yeast’s nucleic acid with [BMIM] BF4-H2O-KH2PO4 ionic liquid aqueous two-phase system, Adv. Mater. Res. 455 456 (2012) 477 482. [91] G.E. Garcia, A.K. Ressmann, P. Gaertner, R. Zirbs, R.L. Mach, R. Krska, et al., Direct extraction of genomic DNA from maize with aqueous ionic liquid buffer systems for applications in genetically modified organisms analysis, Anal. Bioanal. Chem. 406 (2014) 7773 7784. [92] A.K. Ressmann, E.G. Garcia, D. Khlan, P. Gaertner, R.L. Mach, R. Krska, et al., Fast and efficient extraction of DNA from meat and meat derived products using aqueous ionic liquid buffer systems, New J. Chem. 39 (6) (2015) 4994 5002. [93] P. Xua, Y. Wanga, J. Chena, X. Weia, W. Xua, R. Nia, et al., A novel aqueous biphasic system formed by deep eutectic solvent and ionic liquid for DNA partitioning, Talanta 189 (2018) 467 479. [94] M.R. Almeida, D.C.V. Belchior, P.J. Carvalho, M.G. Freire, Liquid liquid equilibrium and extraction performance of aqueous biphasic systems composed of water, cholinium carboxylate ionic liquids and K2CO3, J. Chem. Eng. Data 64 (11) (2019) 4946 4955. [95] R.S. Sousa, M.M. Pereira, M.G. Freire, J.A.P. Coutinho, Evaluation of the effect of ionic liquids as adjuvants in polymer-based aqueous biphasic systems using biomolecules as molecular probes, Sep. Purif. Technol. 196 (2018) 244 253. [96] D.R. Canchi, A.E. Garcı´a, Cosolvent effects on protein stability annual reviews, Annu. Rev. Phys. Chem. 64 (2013) 273 293. [97] K. Nakashima, T. Maruyama, N. Kamiya, M. Goto, Comb-shaped poly (ethylene glycol)-modified subtilisin Carlsberg is soluble and highly active in ionic liquids, Chem. Commun. 34 (34) (2005) 4297 4299. [98] C. Pezzella, L. Guarino, A. Piscitelli, How to enjoy laccases, Cell. Mol. Life Sci. 72 (2015) 923 940. [99] A. Kunamneni, F.J. Plou, A. Ballesteros, M. Alcalde, Laccases and their applications: a patent review, Recent Pat. Biotechnol. 2 (1) (2008) 10 24. [100] A. Antecka, M. Blatkiewicz, T. Boruta, A. Gorak, S. Ledakowicz, Comparison of downstream processing methods in purification of highly active laccase, Bioprocess Biosyst. Eng. 42 (2019) 1635 1645.

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C H A P T E R

3 Current trends and applications of ionic liquids in electrochemical devices Ayaz Mohd1, Shaista Bano2, Jamal Akhter Siddique3 and Aftab Aslam Parwaz Khan4 1

Applied Biotechnology Department, University of Technology and Applied Sciences, Sur, Sultanate of Oman, 2Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India, 3Marie Curie fellow (List-B), SASPRO-2, Slovak Academy of Sciences, Bratislava, Slovakia, 4 Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia

3.1 Introduction In addition to polluting the environment, solvent deposition releases toxic pollutants into the atmosphere and groundwater [1 6]. The utilization of solvents in pharmaceutical manufacturing is estimated to account for around 60% of overall energy consumption in the industry and 50% of post-treatment greenhouse gas emissions, according to some estimates [7,8]. As a result, solvent assortment ought to be systematic to optimize synthesis settings while adhering to green chemistry principles and minimizing environmental impact. Many approaches and tools have been advanced to assist in identifying and indicating solvents that are suitable for synthesis purposes. A few recommendations on solvent selection may be found in the literature [1 8]. The top three solvents in the ideal column are the most preferred; ionic liquids (ILs) encompass a huge class of solvents in and of themselves. Not all ILs, on the other hand, are considered ecologically friendly solvents.

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For the sake of this notion, molten salts with melting temperatures of fewer than 100 C are classified as ILs. Most of these salts are considered organic with a broad diversity of designability as well as application possibilities. Their nonvolatility, in addition to high thermal stability, besides high ionic conductivity, among others, distinguishes them from the other two classes of solvents (and electrolytes), which are classified as the first and second classes, respectively. Furthermore, Lewis’ acidity/basicity of cations/anions (i.e., Coulombic interactions) and the contact directivity between cations and anions, as well as van der Waals interaction qualities between ions, among others, were found to have a striking effect on these features of cations and anions. It is important to remember that not all ILs exhibit the characteristics listed above, which allows for the development of novel task-specific ILs when examining their characteristics [9].

3.1.1 History of ionic liquids in electrochemical devices Welton [10], Holbrey and Seddon [11], and Seddon [12] are just a few of the authors who have written about the field of interstitial lymphomas. Furthermore, despite the fact that many parties agree with the assertion that ILs have been recognized and widely used in specific areas for a long time, there is still a great deal of confusion about what they are and how they work. Although their wide-ranging usages as solvents in chemical synthesis and catalysis have been known for some time, it has only later come to the consciousness of the scientific research community and academic institutions, which has seen a substantial rise in their popularity as a consequence. The precise date of discovery of the very first IL and the identity of the person who made the discovery are both up for debate. When Gabriel and Weiner [13] discovered ethanol ammonium nitrate (m.p. 52 C 55 C) in 1888, they were the earliest to report it. Room-temperature IL (RTIL) [EtNH3][NO3] (melting point 12 C) was discovered for the first time in 1914 [14]. According to the current definitions of ILs, this is perhaps the first material to be identified in the literature as meeting those criteria. In this regard, as far as a term or the concept of ILs, it would be very reasonable to acknowledge and note that Walden, of course, was completely unaware of it at the time of his writing. Therefore it should come as no surprise that little thought was given to the prospective of this particular class of materials at the time of their invention. Also described were the physical characteristics of ethyl ammonium nitrate, [C2H5NH3] NO3, a compound with a melting point of 12 C that is formed when ethylamine reacts with concentrated nitric acid. Walden’s paper was published in the journal Science. Nonetheless, it was not

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until the discovery of combinations of aluminum (III) chloride and N-alkyl pyridinium [15] or 1,3-dialkylimidazolium chloride [16] that interest in binary ILs was piqued. Hussey et al. [16 18] conducted a significant study on organic chloride-aluminum chloride ambient temperature ILs in the 1970s and 1980s, and Hussey [19] wrote the first comprehensive review of ILs. Chum et al. and Oster berg et al. [15,20] has described about the Electrochemical scrutiny of organometallic iron complexes and hexamethylbenzene in a room. Temperature molten salt has described about the Electrochemical scrutiny of organometallic iron complexes and hexamethyl benzene in a room temperature molten salt. It is possible to consider AlCl3-based ILs to be the earliest cohort of ILs. Only in the late 1990s, the properties of ILs were discovered, further rising to make them one of the most promising solvents of their kind available at that time. It is imperative to note that the alkyl substituents, as well as the imidazolium/pyridinium and the halide/halogen aluminate ratios, can be used to alter the physical properties of imidazolium halogen aluminate salts [21], including viscosity, melting point, and acidity, among others. Water sensitivity and acidity/ basicity were two significant impediments to the usage of certain constituents in specific applications. Wilkes and Zaworotko demonstrated the second cohort of ILs in 1992 using “neutral” weakly synchronizing anions, for instance, hexafluorophosphate (PF62) in addition to tetrafluoroborate (BF42), enabling a considerably broader variety of utilizations [22]. It has been discovered that the initial report of publications regarding the synthesis and application of air- and water-stable ILs, among others 1-n-butyl-3-methlyimidazolium tetrafluoroborate ([BMIm] [BF4]) as well as 1-butyl-3-methlyimidazolium hexafluorophosphate ([BMIm][PF6]), the number of applications for these compounds has escalated swiftly [23]. This class of ILs, in contrast to chloroaluminate ILs, can be produced and stored in a noninert environment. While these ILs are generally insensitive to moisture, prolonged exposure to moisture may induce various changes in not only physical but also chemical characteristics, particularly when exposed to high temperatures. As a result, it has been possible to develop ILs based on more hydrophobic anions [24 26]. Tri-fluoro-methane sulfonate (CF3SO32) and bis(tri-fluoro-methane-sulfonyl) imide [(CF3SO2)2N2], besides tris(tri-fluoro-methane-sulfonyl) methide [(CF3SO2)3C2], are examples of such compounds. Not only have these ILs sparked the attention of academics for the reason of their relatively low reactivity with water, but they have also aroused the interest of researchers because of their broad electrochemical windows. In general, ILs could potentially and effectively be dried under vacuum at temperatures ranging from 100 C to 150 C to a moisture content of less than 1 part per million [27].

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3.2 Ionic liquids in energy storage devices and conversion materials IL consumption is unquestionably one of the most significant areas of application study for IL consumption, and this is especially true for their energy consumption. A growing number of people are seeking clean and sustainable energy sources, particularly in the form of energystoring and converting materials and technologies. Both lithium batteries and fuel cells are prominent examples, with commercial applications in hybrid and electric vehicles. In fact, their fixed uses in homes, buildings, and even huge off-grid systems are already in the works, according to the company. Extensive and continuing research has aided in the establishment of novel constituents for usage in these devices, which is now underway. The usage of carbonate-based electrolytes in lithium-ion (Li-ion) batteries, for example, led to the creation of secondary batteries with the highest energy density yet to be achieved. While this is true, the usage of combustible organic carbonates in large-scale applications like electric cars and power grids remains to pose a threat to public safety. A conventional carbonate-based electrolyte cannot be utilized in “beyond Li-ion batteries,” like lithium-sulfur and lithium-oxygen batteries, as a consequence of side reactions, dissolution of electroactive components, as well as solvent evaporation. It is not possible to utilize fuel cells at temperatures over 100 C because of the fast water evaporation and a subsequent decline in terms of conductivity of the proton that is conducting membranes, which are customarily made of Nafion. These concerns have prompted researchers to investigate the development of novel electrolyte materials based on ILs. While this is happening, some ILs have been used as precursors for carbon compounds due to their nonvolatility and excellent thermal stability. By means of this novel method, researchers were able to identify carbon compounds that were both highly functional and specific to specific energy applications [9]. It is well recognized that ILs are none other than room-temperature molten salts. They mainly comprise organic cations and inorganic anions. When exposed to temperatures below 100 C, they show virtually infinite structural diversity. They are, moreover, identified as room-temperature molten salts, and they are mixtures of organic and inorganic ions that exhibit almost infinite structural diversity at temperatures below 100 C. On account of a distinctive combination of physical and chemical characteristics, they are exceptionally well suited for a broad variety of energy-related utilizations, including energy storage and conversion, among other things. They can also be used in a variety of different applications, such as medical devices. The great thermal and electrochemical stability, besides the low volatility of ILs, have led to their use in the creation of electrolytes for a

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range of Li-ion, as well as Na-ion, Li-O2 (air), in addition to Li-sulfur (Li-S), batteries and supercapacitors. Furthermore, they are materials that are utilized as precursors for the manufacture and modification of electrode materials, just to mention a few of their many applications. This chapter was primarily to offer a thorough overview of diverse applications of ILs that would benefit the science and technology industries. Additional research findings from recent years were discussed, as well as the possibility of novel applications of ILs from a theoretical perspective [28].

3.3 Ionic liquid in energy sustainability and CO2 sequestration In recent years, the capacity to maintain a sustainable energy supply has emerged as one of the most urgent problems confronting our current generation. As public awareness of fossil fuels’ contribution to greenhouse gas-induced global warming and the viability of fossil fuel sources in the long term has intensified, the advancement as far as technologies of alternative-energy production and storage are concerned has become more indispensable. The issue of a growing number of people who are concerned about pollution from passenger cars and energy security as well as air quality and energy security is propelling the growth of alternative energy sources for transportation, particularly in the case of passenger cars and other compact vehicles. Solar and wind energy are intermittent sources of energy in nature; thus, as global solar and wind-energy aptitude grows, there is a growing necessity for farreaching energy storage scale and all-encompassing variable-load applications, including fuel production. A significant amount of effort is being put out to mitigate the effects of fossil fuel usage via the deployment of carbon capture technology, which is now underway in some countries [29]. When it comes to energy-related applications, ILs provide a one-of-akind mix of characteristics that make them exceptional candidates for a wide assortment of uses, together with solar energy. Low-volatility cation anion combinations with excellent electrochemical as well as thermal stability, on top of high ionic conductivity, are enabling further development of electrolytes for assorted applications, notably battery electrolytes, supercapacitor, actuator, dye-sensitive solar cells (DSSCs), thermoelectric electrochemical cells, and thermoelectric electrochemical batteries. By using specific members of the protic IL family, it has been made feasible to split water to generate hydrogen and cocatalyze a unique water oxidation process with very high energy efficiency that is unmatched anywhere else in the world. Because of their high proton conductivity when used as fuel cell electrolytes, fuel cells may operate

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at temperatures above 100 C when these protic IL families are employed as fuel cell electrolytes. The relatively low vapor pressure and various functionality of these liquids, in addition to various electrochemical applications, make them particularly well suited for use as CO2 absorbents in postcombustion CO2 capture systems. Furthermore, due to the fact that they have distinct phase properties of their own, the development of phase-change thermal energy storage materials has been made possible as well. Furthermore, by changing their phase properties, they may be tailored to specific applications by modifying their melting points.

3.4 Ionic liquids as a novel electrolyte medium for advanced electrochemical devices A large variety of organic ions are used to construct the most common types of ILs, which exhibit almost infinite structural diversity due to the ease with which a wide range of their constituents can be synthesized. For instance, the ILs formed by the reaction of an alternated imidazolium cation with an anion, namely, tetrafluoroborate, BF4, or hexafluorophosphate, PF6, serve as typical examples. A wide range of salts can be used to create an IL with the characteristics required for an assortment of applications that utilize ILs as new “green” reaction media in the future. To be exact, Li-ion batteries [30], fuel cells [31], capacitors [32], and solar cells [33] are all being touted as the next generation of electrolytes for electrochemical devices. To select ILs for their intended use, the primary characteristics to consider are particularly thermal stability and ionic conductivity. There is contention about the various categories of ILs in terms of current and anticipated utilization in complex electrochemical devices. Specifically, aprotic and protic ILs, as well as their thermal and transport characteristics, are addressed in depth. The second point of focus is the importance of a novel class of intermolecularly linked ILs with cation and anion connected intramolecularly in electrochemical technology. In view of their growing significance, IL-based polymers are ultimately addressed, along with an evaluation of the anticipated evolution of ILs research and development [34].

3.5 Ionic liquids’ electrochemical sensing properties Electrochemical sensors (ESs) are rapidly acquiring a niche in specific sectors when they are extensively utilized in environmental monitoring, medical diagnostics, and bioinspired technologies, among others. Their

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distinguishing characteristics enable them to be used extensively in the aforementioned areas and equipment. They are none other than realtime and offline operation, in addition to high reliability and cheap cost, as well as significant advancements coming from breakthroughs in material science and engineering, that are the driving forces behind this technology [35,36]. ESs undoubtedly have a range of uses, particularly in environmental monitoring, medical diagnostics, and bioinspired systems. As a matter of fact, in an effort to augment the sensitivity and selectivity of ESs, it is such a frequent practice to integrate particular enzymes or antibodies within the electrode structure itself. Due to the fact that enzymes are sensitive to ecological influences, they frequently result in electrode deterioration when exposed to them. Particularly sensitive to environmental factors such as humidity, temperature, and pressure [37], enzymes are a good example. As a result, nonenzymatic ESs are more commonly used [38,39]. However, while nonenzymatic electrodes are frequently stable [40,41], their sensitivity and selectivity are generally suboptimal. To improve these characteristics of nonenzymatic electrodes, additional research is required. ILs are liquid salt when they are at (or slightly beyond) room temperature. The diversity of ILs favorable properties (high intrinsic conductivity, broad electrochemical windows, low volatility, among others, in addition to high thermal solidity, and excellent solvating competence) permits them to be utilized as nonvolatile electrolytes for electrochemical sensing and voltametric sensing at solid/liquid, liquid/liquid, and carbon paste electrodes. In addition, applications encompass gas sensing, ion-selective electrodes, and the detection of biological substances, explosives, and chemical warfare agents, among others [42]. Palladium (Pd) nanoparticles, ILs, or a combination which is integrated with graphene sheets to create ESs that are capable of detecting a wide range of biological species. To name a few, ascorbic acid (AA), uric acid (UA), and dopamine are some of the model analytes used in this study (DA). Many ILs, including EMI SCN, EMI DCA, BMP DCA, BMI PF6, EMI NTF2, and BMP NTF2, are being studied to determine the impacts of ILs’ component ions on their sensing capabilities. According to the findings, the graphene/IL electrode has a higher detection sensitivity than the graphene/Pd or graphene/Pd/IL electrodes, which are both made of platinum. It is noteworthy that the IL anions are discovered to be essential for the functioning of the sensor. It has been demonstrated that graphene may produce an aligned cation/anion orientation in the adsorbed IL film using angle-resolved X-ray photoelectron spectroscopy, with the anions subjugating the topmost surface, consequently dominating the interaction with analytes [43]. Owing to their low volatility and safety, ILs are considered promising potential electrolytes for utilization in the production of batteries,

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FIGURE 3.1 Schematic diagram of pyrrolidinium-based IL electrolytes [44].

particularly next-generation sodium-ion batteries. The researchers of this paper described two classes of advanced pyrrolidinium-based IL electrolytes made by mixing sodium bis(fluoro-sulfonyl) imide (NaFSI) or sodium bis(tri-fluoro-methane sulfonyl) imide (NaTFSI) salts with Nmethyl-N-propyl-pyrrolidinium bis(fluoro-sulfonyl)imide (Pyr13FSI), Nbutyl-N-methyl pyrrolidinium bis(fluoro-sulfonyl) imide (Pyr14FSI), and N-butyl-N-methyl pyrrolidinium bis(tri-fluoro-methane sulfonyl) imide (Pyr13FSI) (Fig. 3.1). There are eight different electrolytes of single anion electrolytes, and binary anion mixes are studied in this paper. The thermal characteristics, besides density, viscosity, in addition to conductivity, and electrochemical stability window, on top of the cycling performance in room-temperature sodium cells, are all discussed. When it comes to cell performance, the blends containing Pyr14FSI outperform the others, allowing the layered P2-Na0.6Ni0.22Al0.11Mn0.66O2 cathode to provide approximately 140 mAh/g for more than 200 cycles [44].

3.6 Applications of room-temperature ionic liquids 3.6.1 Electrochemical applications of room-temperature ionic liquids Nowadays, workplace safety takes precedence over performance, and research and development operations have included material productions that are both reliable and secure for diverse utilization. As a result, new material properties are anticipated to transpire in the near

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future, with the goal of speeding up the evolution to sustainable technologies and achieving the aspiration of the EU 2030 Agenda for Sustainable Development [45] as well as addressing the aspiring agenda of the EU 2050 Green Deal [46]. Research into RTILs, or generally ILs, is an instance of a novel kind of research, which provides resolutions and answers regarding real-world applications [47 49]. ILs salts (typically organic) have meager melting points (liquids below 100 C) and a very low (negligible) vapor pressure [48]. Due to their unique qualities as liquids, they have lately caught the interest of the scientific research community, as they are perfect substitutes for volatile organic solvents, which are a significant source of waste in the chemical synthesis industry [50]. As a result of the unique physicochemical features of ILs, their use across a broad variety of fields, including chemistry, materials science, and chemical engineering, has risen rapidly in recent years. This chapter includes in-depth discussions of the basic physicochemical features of ILs at room temperature, as well as their applications. RTIL has physicochemical characteristics that are significantly different from those of typical molecular liquids, as seen in Table 3.1. As a matter of fact, it is crucial to bear in mind, however, that not every IL displays the features stated in Table 3.1 [51]. ILs are almost all the time consisted of organic ions; as a consequence, they have practically infinite structural variations owing to the simplicity with which they may be built up of a number of different components. The tune-ability of cation anion combinations and the possibility of modifying the cation and/or anion component enable the creation of ILs with desired characteristics. As a consequence, there has been a significant growth in the usage of multifunctional ILs. In this paper, the researchers discussed further solvent systems that have been suggested as replacements for traditional organic electrolytes. ILs are discussed in detail, as well as the characteristics that make them TABLE 3.1

The typical room-temperature ionic liquids’ properties [51].

General properties

Features

Melting points are low

• Liquid at room temperature • Wide application-specific temperature range

Volatility-free

• Thermal stability • Flame retardancy • High ion density

Composed by ions

• High ion conductivity • Designable/tunable

Organic ions

• Infinite combinations possible

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suitable for use in electrochemistry (e.g., decrease of ohmic losses), such as diffusive molecular motion and ionic conductivity. For those purposes, the researchers have restricted ourselves to a review of the most recent, most representative advancements and innovations in the utilization of ILs as electrolytes, as determined by the cyclic voltammetry technique. As a result, there are numerous characteristics of RTILs, ranging from their understanding to their application in electrochemical processes.

3.6.2 Room-temperature ionic liquid as a nonfaradaic biosensing component RTILs are employed as either transducers or signal amplifiers in electrochemical sensing, depending on the application. RTILs’ ionic conductivity is critical in applications where they are employed as transducers since it allows them to conduct electricity. This is attributable to the fact that the sensing mode is controlled by the electrochemical interface, which is a phenomenon significantly influenced by the solvents’ ionic behaviors. Furthermore, considering that RTILs have charge-storage capabilities, they are good fits for use in the creation of capacitive immunoassays, which are becoming increasingly popular. In light of these considerations, the scientific research society has used a number of RTILs for electrochemical sensing purposes in a variety of contexts, including environmental monitoring. To be specific, RTILs that include cations including alkyl imidazolium, alkyl pyridinium, or alkyl ammonium chains and anions containing halide, alkyl halide, alkyl borane, or triflate compounds are extensively utilized in the pharmaceutical industry. Since its establishment, the formulation of tailored RTILs has been more popular, owing to the decrease in the complexity of the synthesis techniques for these complex solvents. The anions and cations used to make them may be combined and mixed to satisfy the specific needs of the application in which they will be utilized. As a result of the discovery of this method of simplifying the synthesis methods for these difficult solvents, the fabrication of customized RTILs has become more prevalent. It is possible to build these custom-made solvents by mixing and matching different anions and cations to fulfill the specific applications’ requirements. The results of a recent study published by that group provide more information on the electrochemical characteristics of RTILs and their applications [52]. Prior to this, inventors have developed a number of sophisticated constituents with a variety of features, namely size variability, physicochemical tune-ability, and, most pertinently, constituents with a moderate-to-high specific capacitance. Numerous nanoparticles, such as ZnO, TiO2, besides MoS2, MoSe2, graphene, on top of graphene oxide (GO), reduced graphene

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oxide (rGO), and carbon nanotubes (multiwalled/single-walled), are used comprehensively to develop capacitive immunoassays for nonfaradaic impedimetric detection of biomedically relevant biomolecules [53]. In nonfaradaic EIS applications, RTILs are regarded as a promising prospect alternative owing to their fascinating design, exciting chemical and electrochemical features, and distinctive ability to separate desired species from massive inventories.

3.6.3 Room-temperature ionic liquids in electrochemical gas sensoring As a result of their prominent roles in conservational monitoring, industrial production, and human security, gas sensors have attracted the interest of a broad number of parties. Recently, it was shown that carbon-gold nanocomposites might be beneficial for utilization in an electrochemical gas sensor based on reduced graphene oxide to detect gases with extraordinarily high sensitivity. Carbon-gold nanocomposites (CGNs) had been synthesized for the first time by using hydrothermal carbonization and deposition of gold nanoparticles. The electrochemical deposition of reduced graphene oxide (RGO) on a screen-printed gold electrode was followed by the modification of carbon graphene nanosheets (CGNs). To accomplish a lengthy lifespan and stability, a thin-film RTIL electrolyte with minimal evaporation and a wide potential window was used as the electrolyte. As a result, a highly sensitive RTIL-based electrochemical gas sensor was developed and implemented. The effects of RGO and CGNs adjustment on amplification were explored utilizing, namely, cyclic voltammetry, chronoamperometry, as well as transient double potential amperometry (DPA), among other techniques. Moreover, the findings indicate that the application of RGO and CGNs in synergy results in a significant improvement in sensor performance. It was calibrated to detect oxygen concentrations ranging from 0.42% to 21% with high sensitivity and linearity in the range of 0.42% 21%. In addition, the sensor’s reproducibility was investigated using chronoamperometry and transient DPA, both of which had excellent repeatability. According to the findings, a path has been opened for the development of susceptible electrochemical gas sensors that can detect and measure gas exposure in real-time [54].

3.7 Ammonium, pyrrolidinium, phosphonium, and sulfoniumbased ionic liquids and electrochemical properties This section discussed electrochemical advancements made possible by the utilization of ILs other than imidazolium-based ILs. Compared to

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pyridinium- and imidazolium-based compounds, quaternary ammonium salts are undoubtedly a more cost-effective class of ILs. As an added advantage, they are also well suited for use in a broad range of different utilization. It is owing to their exceptional thermal and chemical stability that they are considered a cost-effective class of ILs available. Using N-trimethyl-N-hexyl ammonium bis(trifluoromethyl sulfonyl) amide, [N1116] [NTf2], Sultana et al. [55] investigated the electrochemical behavior of an acetylacetonate (acac) platinum (II) complex, [Pt(acac)2], serving as a function of temperature using N-trimethyl-Nhexyl ammonium bis(trifluoro A) dependence between the cathodic peak associated with the reduction of platinum (II) to platinum (0) and the subsequent electrodeposition of this metal on the electrode and Pt (acac)2 diffusion in the [N1116] [NTf2] IL. Bhujbal et al. [56] used the ammonium-based IL, tri-n-butylmethylammonium chloride, [N1444]Cl to investigate the electrochemical properties of the uranyl ion, UO221. An irreversible reduction of U(VI) to U(IV) was detected, which resulted in the electrodeposition of uranium oxide (UO2) on the glassy carbon working electrode. The electrochemical window for [N1444]Cl is 2.8 V, with a cation reduction potential of 21.9 V and an anion oxidation potential of 10.9 V. The utilization of [N1444]Cl as the electrolyte for the UO221/UO2 reduction pair resulted in a rapid (40 min) reduction despite the presence of a significant quantity of uranium in the uranium dioxide. Arkhipova et al. [57] investigated the redox characteristics of mesoporous graphene nanoflakes using tetraethylammonium bis(trifluoromethyl sulfonyl) imide [N2222][NTf2], which exhibited a capacitance of more than 100 F/g. Simultaneously, Pajak et al. [58] showed that some protic ammonium-based ILs containing the nitrate anion (ethyl ammonium nitrate and propylammonium nitrate) might serve as proton donors in the electro-sorption of H2 in palladium. The finding was significant since the majority of “first-generation” ILs have been abandoned throughout the years. ILs based on pyrrolidinium are considered to be some of the supreme advantageous electrolytes in the establishment of innovative and viable portable energy devices. As a result, a substantial rise in the number of electrochemical investigations in such ILs has been noticed lately. Bouvet and Krautscheid [59] demonstrated in 2016 that the redox potentials of a Fe(II)/Fe(III) system based on pyrrolidinium-based ILs with L-proline and ferrocene as building blocks are independent of the IL anion and alkyl chain length. Following that, a series of pyrrolidinium-based ILs containing the [NTf2]2 anion were utilized to electrochemically synthesize 2 3 nm Pt nanoparticles [60]. Cyclic voltammetry was used to characterize the electrochemical reduction of [Pt(acac)2] in the aprotic pyrrolidinium-based

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ILs, 1-R-1-methylpyrrolidinium bis(tri-fluoro-methyl sulfonyl)amide (R 5 butyl, hexyl, or decyl). The reduction of [Pt(acac)2] to metallic platinum happened via a two-electron transfer process lacking the formation of any intermediate species. Manjum et al. [61] investigated the electrochemical reactivity of samarium species in 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl)amide, [bmpy][NTf2]. The Sm system exhibited electrochemical activity, with a cathodic peak resultant from the reduction of Sm(III)/ Sm(II) and an anodic peak related to the oxidation of Sm(II). At 100 C, the cathodic peak current density was greater than at  25 C, indicating a reduction in the IL viscosity at 100 C. In addition, the removal of the anodic recent peak at 100 C had been noticed and attributed to a chemical reaction of Sm(II) in the IL. The equilibrium between Sm, Sm(II), and Sm(III) also resulted in the production of Sm nanoparticles in the IL at 100 C. In a hybrid electrolyte including a redox additive, an IL with the same cation but a [DCA] 2 (DCA 5 dicyanamide) anion was employed to improve the specific capacitance of N-doped reduced graphene aerogel capacitors [62]. In addition, a method for the electrochemical reduction of NbF5 and NbCl5 in the IL 1-butyl-1-methylpyrrolidinium tri-fluoro-methane sulfonate [bmpy][OTf] was recently described [63]. Lahiri et al. [64] synthesized germanium-tin alloys using two distinct ILs: 1-butyl-1-methylpyrrolidinium trifluoromethyl sulfonate ([Py1,4]TfO) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl)amide ([Py1,4]Tf2N). The researchers conducted voltametric tests on both ILs and found that [Py1,4]TfO was the paramount IL to serve as an electrolyte for preparing Ge1-xSnx. Cyclic voltametric studies conducted with the precursors (SnCl2 and GeCl4) revealed a peak linked with the production of germanium-tin alloys. Gligor et al. [65] synthesized and characterized new films based on poly-3,4-ethylenedioxythiophene for nitrite detection on two substrates (namely glassy carbon and gold) and four electrolytes (namely water, acetonitrile, 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl) imide, and 1-ethyl-3-methylimida). The researchers discovered that electrodes produced in ILs had increased electrocatalytic activity and sensitivity. Phosphonium cation-based ILs are a widely accessible class of ILs exhibiting better characteristics in certain applications than nitrogen cation-based ILs. Extraction and synthetic solvents are among the most recent possibilities and utilization for these compounds. They are as well mostly beneficial for use in electrochemical procedures such as corrosion prevention and electrolytes. Girard et al. [66] investigated the electrochemical properties of trimethyl(isobutyl)phosphonium bis-fluoro-sulfonyl)imide, [P111i4][NTf2], by varying the Li[NTf2] concentration.

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Voltammograms show that a quasi-reversible Li deposition dissolution process is occurring. According to the Nernst equation, a slight change in the onset potential toward fewer negative values was detected as the Li-ion concentration increased. As anticipated based on the transport property trends, the current oxidation densities for the reversible peak dropped substantially as lithium salt concentration increased. The researchers discovered that Li[NTf2] solutions in [P111i4][NTf2] IL showed acceptable transport characteristics, therefore, may be suitable electrolytes for lithium batteries. Khrizanforov et al. [67] used dodecyl(tri-tert-butyl) phosphonium tetrafluoroborate, [(t-Bu)3PH][BF4], in a novel carbon paste electrode, investigating the redox characteristics of an iron complex ({μ2-[FeII(η5C5H4-P(PhOO)(η5-C5H4 P(PhOOH))]3FeIII} THF). The researchers concluded that the produced electrode exhibited excellent conductivity, a wide electrochemical window of opportunity (5.6 V), long-term stability, and repeatability. The cyclic voltametric measurement of the iron complex performed with the novel carbon paste electrode IL showed the presence of two distinct iron redox centers in oxidation states (II) and (III). In addition, the cathodic peak observed by cyclic voltammetry corroborated Ota et al. [68], who researched electrodeposition of Nd(III) in triethyl pentyl phosphonium bis(trifluoro-methane sulfonyl)imide, [P2225][NTf2]. Sulfonium-based ILs were employed by Venker et al. [69], examining the impact of sulfur atom materials on the IL’s redox behavior. They utilized ferro-cenyl-sulfonium ILs to perform their study. According to their results, electron-withdrawing materials induced a positive shift in the redox potential of Fe(II)/Fe(III) ferrocene when employed in conjunction with Fe(III) (III). These RTILs, according to the results, may be helpful as redox mediators, redox electrolytes, or additives in DSSCs, supercapacitors, protection of overcharging the batteries, respectively. Rangasamy [70] investigated the electrochemical redox characteristics of diethyl methyl sulfonium bis(trifluoromethyl sulfonyl)imide, [S222] [NTf2], along with other lithium salts using the technique of electrochemical characterization. When exposed to a wide range of potentials, the system [S222][NTf2]-LiNTf2 appears to be stable and to exhibit reversible redox activity, both of which are desirable characteristics for use as an electrolyte in practical battery applications, as demonstrated in this study. When the [S222] [NTf2]-LiNTf2 IL electrolyte was heated to 60 C, cyclic voltammetry was used to characterize it. Researchers have discovered redox maxima in both the forward and reverse scan directions in both cycles, they claim. A process of lithium deposition is used in one case, while a method of lithium stripping is used in the other. It is possible that the IL electrolyte has excellent reversible redox activity because the




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current densities of the two peaks are nearly identical. This provides additional evidence for the possibility that this IL electrolyte can perform the function of a “workable” electrolyte for use in a lithium battery. In a “perfect” electrolyte, one of the most desirable characteristics is a broad electrochemical stability window, which is vital for the reason that the chemistry of the battery’s two electrode-electrolyte interfaces is dependent on the electrolyte’s properties. Generally, the anodic limit of ILs is dictated by the anion’s oxidative stability. The low Lewis acidity of the liquids leads to weak interactions with the organic cations that are similarly low in Lewis acidity, resulting in comparatively excellent anodic stability for the liquids. The difficulties that such cations and anions have discharging on the electrodes, particularly at lower potentials, result in an electrochemical stability window with a wide range of possibilities. Klein et al. [71] measured the differential capacitances of three different families of ILs and found them to be significantly different (ammonium, imidazolium, and pyrrolidinium). For example, butyltrimethylammonium bis(trifluoromethyl sulfonyl)imide, [N1114][NTf2], ethyl-methylimidazolium bis(tri-fluoro-methyl-sulfonyl)imide, [emim] [NTf2], and methyl-propyl pyrrolidinium bis(tri-fluoro-methyl-sulfonyl) imide, [Pyr13][NTf2]. Because of differences in water content, trace contaminants, and current cutoff, the authors [71] hypothesized that the quaternary ammonium-based IL had a distinct electrochemical potential window. One of the most important conclusions to be drawn from this study is that there are several variables that contribute to the increase in capacitance. Initially, it was anticipated that [N1114] would crowd the interface, whereas [Pyr13] and [emim] would combine to form an interface structure with a screened surface that was too large. According to these findings, bulk measures of molecular interactions, for instance, viscosity, have little effect on the IL’s behaviors at the electrode-electrolyte interface’s nanoscale lengths. Velez [72] used di-cationic pyrrolidinium and piperidinium ILs with [NTf2]- anion as lithium battery electrolytes in his research. Using an electrode made from carbon paste, researchers investigated the electrochemical response of dopamine utilizing an array of imidazolium-, pyridinium-, pyrrolidinium-, and piperidinium-based ILs. In terms of results, it was discovered that both sizes and types of cation and anion have an impact on the response [73]. The utilization of any of the previously stated IL classes would make it possible to actualize a wide variety of electrochemical applications that were previously just a concept. Their usages as electrolytes or electrolyte additives are intensifying the scope of these applications even further. In the future, it would be beneficial to investigate other IL families, particularly those based on guanidinium.

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3.8 Current and future prospects 3.8.1 Ionic liquids as electrolytes ILs come with a distinctive combination of physical and chemical properties, making them excellent candidates as electrolytes. These properties include, among others: 1. Extensive variety of liquids with superior electrochemical stability 2. Outstanding electrical conductivity and compatibility with electrode materials 3. Nonflammable and nonvolatile substances Supercapacitors, in addition to Li-ion batteries and dye-sensitized solar cells, as well as sensor materials, besides metal plating techniques, have all demonstrated remarkable performance.

3.8.2 Ionic liquids as lubricants and hydraulic fluids It is considered whether ILs, previously used as stand-alone neat lubricants [74], are now being reconsidered as oil additives [75]. As a result of their unique properties (high miscibility in oils, as well as antiwear and anticorrosion capabilities), phosphonium ILs are considered promising lubricant additives [75,76]. In addition, their organic phosphorus content, which is essential in tribo-chemistry, makes them attractive as lubricant additives. Phosphonium salts, on the other hand, do not react with the vast majority of organic compounds and are wellmatched with the vast majority of commonly used lubricant additives [77]. When used as lubricant additives, ILs form protective tribofilms across the surfaces that come into contact with one another [75]. The tribochemical interaction of ILs and/or their breakdown products with the contact surfaces and/or wear debris at lubricating interfaces results in the formation of the protective tribofilm [75]. In addition, because of the positive charge generated on rubbing surfaces as a result of tribostress, anions have a strong interaction with oppositely charged surfaces [78]. When surfaces lubricated with an IL are squeezed, lubricating films remain in place at higher pressures than when surfaces lubricated with an equivalent molecular weight molecular lubricant remain in place at lower pressures [78,79].

3.8.3 Ionic liquids as chemical production processes Environmentalists are concerned about rising energy consumption and the depletion of fossil fuels, which has prompted a shift toward cleaner energy sources in the contemporary world [80 84]. Today,

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lignocellulose is an economically viable, environmentally friendly, and long-lasting platform for delivering chemicals and biofuels in the most efficient manner possible. A significant, albeit unavoidable, byproduct of agriculture is lignocellulosic biomass (LCB). It is also none other than the world’s most significant renewable resources and a substantial byproduct of agriculture [85]. Polymers such as cellulose (30% 40%), hemicellulose (20% 30%), and lignin (15% 40%) are the primary constituents, with extractives such as waxes, resins, and inorganics rounding out the composition [86,87]. Extensive research has been conducted on the potential use of cellulose and hemicellulose in the production of critical chemicals and fuels, and a few commercially viable methods have been developed [86,88,89]. In LCB, the only remaining component is lignin, an amorphous complex aromatic molecule with high aromatic content. Among those who work in the ethanol-based biorefinery and paper pulp industries, it is considered a significant recalcitrant. Every year, approximately 50 million tons of lignin are produced around the world, with 1 kg of lignin being produced for every liter of bioethanol produced [90]. Despite this, little attention is paid to the control of the lignin that has been liberated. This situation, on the other hand, is expected to transform in the near future [91 93]. Lignin is an aromatic biowaste that can be easily burned, making it a low-cost energy source for the environment. In the proper hands, this aromatic polymer with complex building blocks can be converted into a wide range of products, thereby increasing the economic significance of the lignocellulose deconstruction process for the production of commercial chemicals [94]. Jusqua` re´cemment, the chemical compound lignin, was used in physicochemical techniques to synthesize macromolecules such as polymers, adsorbents, composite materials, and adhesives [95,96]. While lignin valorization is currently under investigation, the use of chemical catalysts or microbial strains and enzymes to produce a variety of low-titerhigh-value compounds such as vanillin, muconic acid, and other fine chemicals, as well as other fine chemicals, is quickly gaining momentum [97]. Lignin valorization comprises three stages: fractionation, depolymerization, and upgrading. Each stage is described below. There is something unique about each one of these three sectors that adds to the entire value chain from lignin to chemicals. When LCB is pretreated, lignin separation and depolymerization are relatively simple to achieve. A simple matter of fine-tuning or customizing lignin derivatives for the purpose of synthesizing target compounds is all that is required for the upgrading process [98,99]. The lignin valorization approach, which generates a diverse range of lignin-derived commodities, has the potential to ensure the long-term economic viability of a lignocellulosic biorefinery [98,100] while minimizing environmental impact.

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Chemical processes constitute a significant portion of the whole application spectrum for ILs. Because of their unique chemical and physical characteristics, they make it feasible to develop entirely new reactions and process designs in the future. For example, their remarkable solubility and catalytic characteristics, as well as their unique behavior when coupled with microwaves or simply because they are nonvolatile, may be utilized in various industrial processes in an ecologically acceptable, emission-free way without the loss of solvent [101].

3.8.4 Ionic liquids as hydrogen storage Hydrogen will continue to increase in significance as a storage medium for sustainable energy for transportable and stationary applications [102,103]. Unfortunately, the problem emerges when it comes to storing hydrogen at a substantial gravimetric and volumetric density owing to hydrogen’s physical properties. Specifically, its form is not liquid at room temperature; its critical temperature is 32K. Although compressed gas or cryogenic liquid are the most frequently used hydrogen storage methods [104,105], safety concerns and hydrogen’s low density have motivated researchers to explore novel storage methods. Metal hydrides have a volumetric hydrogen density that is more than double that of liquid hydrogen [106 108]. In addition, complex hydrides (borohydrides [109,110], ammonia borane [111,112], and alanates [113,114]) have a high hydrogen density and may desorb and absorb hydrogen under certain circumstances. ILs have the ability to store hydrogen devoid of the need for pressurization, and they can do so in a stable and safe manner at room temperature. In addition, the energy densities currently achievable in engines are approximately 20% 25% greater than the storage capacity of conventional fuels, putting them at the forefront of hydrogen storage technology. In an ideal world, this figure could be increased to approximately 35%, or even higher in some cases. Hydrogen is selectively liberated from ILs and utilized to generate electricity in fuel cells, which are used to power the electric vehicles. Afterward, the depleted IL can be replenished with hydrogen and recycled continuously for an extended period of time [115,116].

3.9 Conclusions These past few years, ILs have persisted in being extensively exploited both as media and as catalysts in a broad range of chemical processes. Even though they show considerable potential in this field,

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several investigations have shown that ILs may react with reactants and hence cannot be referred to be inert solvents in this context. As a consequence, while designing reactions in ILs, synthetic chemists should take care to avoid creating undesirable products. In addition, ILs have demonstrated outstanding electrolyte characteristics, not only due to the absence of organic solvents but also as a result of their electrochemical stability. Due to electrochemical stabilty ILs may be utilized throughout a wide electrochemical prospective range devoid of becoming oxidized or reduced. Furthermore, their utilization had made it possible to utilize the straightforward approach of cyclic voltammetry to analyze the redox behavior of both organic and inorganic compounds. In addition, ILs may be utilized as additives or in combination with inorganic salts, which has shown sound effects. As far as this discussion was concerned, it was discovered that the IL’s capacity to dissolve or crystallize materials may be used for energy or sensing applications, thereby opening up a slew of new options for further exploration. It is anticipated that the electrochemical characteristics of ILs will remain relevant to be used for a multitude of conditions on a spectrum of products. A considerable number of instances were offered involving the creation of innovative materials or technologies. They display physicochemical features such as polarity, basicity, besides acidity, solution temperature, and even the existence of a reaction step as evidenced by a color change in the presence of water. Besides ions, conductive liquids are capable of transferring electrons, microchips, and a variety of other materials and objects. Liquid batteries may be manufactured by coating three layers of electrodes and electrolytes with a coating agent. To a considerable extent, the electrochemical sector of ILs is gaining impetus in the chemical, biological, and energy science disciplines, justifying and warranting more exploration in a number of industries. Although ILs have considerable benefits over traditional solvents, more tests should be conducted owing to the large variety of ILs and factors to optimize. In addition, due to the complexity of the majority of chemical processes and separation systems, a single technique is incapable of achieving the necessary purity for ILs electrochemical investigations. Conjugation of multiple purification methods may be required to increase purification efficiency. The employment of RTILs in conjunction with impedimetric detection significantly improves the performance of biosensing platforms. A brief overview of the main characteristics that distinguish RTILs from other materials for electrochemical transduction has been given in this study. Following that, information on ESs’ applications is provided, together with corroborating evidence from the literature, and the technique by which RTILs improve the performance of impedimetric biosensing platforms is explained in depth. The researchers described the current situation of RTIL-based biosensing technologies via the use of

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case studies. Final considerations are given to the difficulties and opportunities associated with comprehending and critically evaluating the scope of this subject. RTILs have been shown to enhance the sensitivity and specificity of biosensing platforms, which has been shown in the current paper. Overall, the use of RTIL-based systems has the potential to significantly enhance the field of biomedical engineering, particularly in the areas of biomarker identification, illness diagnostics, and disease management, among others. When it comes to power fuel cell processes, protic ILs serve as electrolytes, taking advantage of the electrochemical activity of the liquids while also benefiting from their high thermal stability. Carbohydrate compounds are typically formed from polymers, both natural and manmade. The discovery that some ILs may act as precursors for carbon compounds, on the other hand, broadened the range of possible applications for them. Because of their liquid nature, they can be easily coated and carbonized uniformly over any surface without causing any distortion. It was previously thought that this type of processing was not possible with conventional polymer precursors. Moreover, materials and mechanisms for energy storage and conversion will continue to gain prominence in the coming years, as will their applications. To discover essential applications for ILs, strong research needs must first be established while also taking into consideration the unique characteristics of ILs.

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C H A P T E R

4 Green chemistry of ionic liquids in surface electrochemistry Abbul Bashar Khan Department of Chemistry, Jamia Millia Islamia, New Delhi, India

4.1 Introduction In comparison to the preceding century, presently a large number of materials have been made available for various applications which will increase in the future. Still, the scientific community tries to sort out environmentally friendly and greener materials. Ionic liquids (ILs) alias room temperature ionic liquids (RTILs) have been one the examples of such materials that are mainly organic salts with very low melting temperature liquids, that is, below 100
C and extremely low vapor pressure (almost negligible) [1]. Although disputed, Gabriel and Weiner [2] reported first IL was termed as ethanolammonium nitrate with m.p. 52
C55
C in 1988 while the foremost fitting RTIL, was ethylammonium nitrate (C2NH3NO3), which was started in 1914 with m.p. as 12
C [3]. Later, the AlCl3-based ILs were regarded as the first organized edition of ILs with the observations of Gale and Osteryoung [4] for the mixtures of AlCl3 and 1-butylpyridinium chloride (C4PyCl) in basic conditions on Aluminum electrode in 1:1 proportion. Although the potentiometric readings were untrustworthy owing to the reaction of products with Aluminum [4]. That was later confirmed by their new findings, implying the cation reduction of dissociated 1-butylpyridinium due to Aluminum in the release [5], which led to the interconnection of the electrochemical arena with basic chloroaluminate liquifies. In the early 1990s hexafluorophosphate (PF62) and tetrafluoroborate (BF42) were

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conveyed as second-generation ILs with lightly bound anions by Wilkes and Zaworotko [6]. In addition, ILs based on more hydrophobic anions such as tris(trifluoromethanesulfonyl)methide [(CF3SO2)3C2], tri-fluoromethanesulfonate (CF3SO32), and bis(trifluoromethanesulfonyl)imide [(CF3SO2)2N2] have been developed that have wide consideration due of their low reactivity with water as well as their large electrochemical windows [711]. Out of thousands of articles regarding ILs, there are various research groups published a large number of articles that explore the contribution of ILs in electrochemistry and specifically on the surface electrochemistry [11,12]. In this chapter, I want to discuss the various applications of ILs in surface electrochemistry. The electric charge transfer between the electronic and ionic conductors through the electrified interface is the main concern of the electrochemistry that is integrally linked with the voltages and currents. Up to the late 1970s, there were very restricted studies possible by the current and voltage measurements but later on, in the last 50 years several advanced techniques to be established related to surface science to explore more insights regarding the atomic as well as molecular scale for interfaces and surfaces [1220]. To understand the surface chemistry, we should know some characteristics of the electrochemical reactions, such as electrochemical current and potential, electrochemical interfaces and models of electrochemical electron transfer, and electrochemistry at the molecular scale that includes the surface structure, ions, and water bonding as well as experimental facets related to current and voltage [21].

4.1.1 Important characteristics of electrochemical reactions 4.1.1.1 Electrochemical current and potential A potential difference and current flow between two half-cells at their respective electrode surface, are the fundamental measurements for an electrochemical reaction in the electrochemical studies, during which the electrons and ions flow take place between the reactions of half-cells. During electrochemical measurements there are two types of currents observed, one is the faradic current that corresponds to the reduction or oxidation of chemical species in the cell and convoys the charge transfer across the interface while the other flows in the absence of electrochemical reactions known as capacitive current that is observed during a change in potential of the electrode or the change in ions distribution near the interface. The work done essentially for transfer of charge among the opposite electrodes in the solution of an electrolyte is termed in electrochemistry

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as electrochemical potential as similar to the solids in a vacuum it may express that includes two steps. In the former step, an element of charge is carried near the boundary in an electrolyte corresponds to the Volta potential while the second step corresponds to the interfacial potential in which the charge transport across the interface takes place [21]. 4.1.1.2 Electrochemical interfaces On immersing a metal in an aqueous solution of electrolyte which has several ions and various developments take place on the interface which disturbs the distribution of the electron density at the surface, such as the presence of adsorption of an ion on the surface that tempts charging on the surface. Consequently, an electrochemical double-layer is formed that consists of a layer of charges on the surface as well as counter ions that are available at the interface [21]. 4.1.1.3 Models of electrochemical electron transfer The transfer of a single electron through an electrified interface is usually either an outer or inner sphere process that is concerned with the chemical bond formation or breaking. In the case of outer-sphere reactions, only the reorganization of the outer solvent sphere is to be observed and that is only very few acknowledged to date [21]. For calculating the electrochemical rates, the Butler Volmer concept is very useful but on the molecular basis, its linear free energy relationship assumption is not to be defensible. In addition, during an electron transfer process for an improved version of the outer solvation sphere changes, Marcus-Hush proposed a formalism that predicts the transfer rate of electrons based on reacting medium and reactant’s molecular structure. The Gibbs free energy related to the electron transfer steps reaction changes, with the change in Gibbs free energy of activation because of the difference of the molecular coordinates that involve alignment of surrounding dipoles, solvation shell, bond distance and bond angles, the rate of this process by the change in electrode potential [21].

4.1.2 Electrochemistry at the molecular scale 4.1.2.1 Surface structure The surface has a certain threshold to accommodate an excess charge and it inclines to reconstruct for accommodating the charge. The importance of surface charge changes in terms of the work function in such a way that the surface sustains, with a larger work function due to the positive charge and a smaller work function by the negative charge forces. On changing the potential externally, the surface charge

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magnitude is affected which influences the work function and finally alters the electrode electrochemical potential during the electrochemical systems [21]. 4.1.2.2 Bonding of ions As we know the electrochemical reactivity regarding metal electrodes in various aspects like etching, corrosion, electrocatalysis, and deposition is affected due to the presence of specifically adsorbed anions and the extent of specific adsorption surges as F , Cl , Br , l while the bond strength surges adversary as I , Br , Cl , F for the electrochemical system [2224]. As we have seen the extent of specific adsorption does not follow the same trend as the bond strength to the surface and suggests the concept of solvation of such types of ions in specific as well as nonspecific adsorption [21]. 4.1.2.3 Bonding of water There must be a solvation water monolayer formed among the available ion and surface of the electrode in case of non-specific absorption and water molecule bonds through oxygen or hydrogen atom to the surface [25]. Though they form weak interactions with approximately 0.5 eV as bond energies [26,27] and there is a remarkable redistribution of charge in both O- and H-bonding on the surface of metal [21]. 4.1.2.4 Experimental aspects of current/voltage properties The information regarding a particular cell reaction is obtained by measuring electrode currents as a function of the applied voltage for that generally two-electrode setup is used for current/voltage measurements through which a minute current is passed. But for bulky samples trouble to observe in conducting accurate measurements and cannot neglect the resistance between two electrodes on a high current flow.

4.1.3 Ionic liquids properties pertinent to surface electrochemistry Generally, the ILs properties pertinent to electrochemistry depend on the properties such as conductivity, viscosity, and electrochemical potential windows. As we know the electrolyte conductivity that measures the accessible charge carriers and their mobility. The ILs that are so-called semi-organic salts have sensibly good ionic conductivity as compared to the best organic solvent electrolytes and for a particular anion with various cations the conductivity follows the order as imidazolium . pyrrolidinium . ammonium. It is observed due to a decline in planarity of the cationic core in such a way that the flatness of the imidazolium ring appears to

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deliberate the higher conductivity as compared to the ammonium salts that exhibit the tetrahedral arrangement of alkyl groups while the pyrrolidinium based ILs assuming an intermediate geometry and conductivity [9]. The IL conductivity varies with the temperature near room temperature linearly as per the Arrhenius equation while as approaches its glass transition temperature the conductivity shows the significant deviation from the linear behavior that is described most significantly by the empirical VogelTammannFulcher (VTF) equation [28,29]. The viscosity of ILs is very much associated with its conductivity and as per Walden’s rule, the molar conductivity and viscosity for a given IL do not change irrespective of temperature [30]. In general, there is an inverse relationship between both the properties, on tumbling ILs viscosity a higher conductivity is to be obtained. Owing to the large internal friction of fluids ILs are viscous as compared to the most the molecular solvents and behave like a special solvent, despite that on adding of co-solvents such as acetonitrile, alcohol, dichloromethane, benzene, toluene, water, etc., a theatrical decrease is observed in viscosity of ILs deprived of modification in the cations or anions composition of the system [3138]. Furthermore, the collected data for the viscosity by various researchers reports concludes that the size of cations is relative to anions effects, and highly asymmetric alkyl substitutions in cations may lower, the viscosity of ILs [39]. The selected solvent’s oxidative and reductive stabilities are the basic factors for the electrochemical potential window in electrochemical studies while for ILs the resistance of the cation and anion to reduction and oxidation, respectively are fundamental factors for the potential window, that is, generally more than 2.0 V. But the anodic or cathodic potential limits and their corresponding electrochemical potential window are affected due to the existence of the impurities in the ILs, like the residual halide and water left during the synthesis of ILs. Therefore because of the substantial concentrations of the halide ions the anodic potential limit may be reduced appreciably while an IL contaminated by significant amounts of water can decrease the potential limits of both the anodic as well as cathodic. The electrochemical window for (C4mimBF4) IL, that reduced from 4.10 to 1.95 V on adding 3% of water by weight [40], produce electroactive species reacting with water for example, [PF6] anion form HF on reacting with water [39,41]. In the light of the important properties of the electrochemical reactions and pertinent properties of ILs, we are going to discuss the important applications of ILs in surface electrochemistry specifically by using cyclic voltammetry (CV). It measures the response of current in forward as well as backward voltage sweep in such a manner that positive and negative current peaks correspond respectively, for the oxidation and reduction of adsorbed species. The cyclic voltammogram shows both

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symmetrical and non-symmetrical forward as well as reverse peaks and reports the absorbance of both reactants and products on the electrode surface [21].

4.2 Role of ionic liquids in surface electrochemistry There are the following roles of ILs in surface electrochemistry:

4.2.1 Carbon ionic liquid electrode An active ion transfer across RTIL aqueous solution interface takes place at the very simple electrode, that is, the carbon paste electrode (CPE) [42,43] with RTIL as a binder [44]. This phenomenon is driven by electro generation of redox-active cation [4547] and monitored by differential pulse voltammetry and CV. In further progress, Norouz Maleki et al. [48] proposed the building of a novel CILE from n-octylpyridinum hexafluorophosphate (C8PyPF6) that acts as a binder. CILE has a set of various decent features that accounted for former distinctions of carbon nanotubes and CPEs as a CILE with surprisingly high electrochemical performance [48]. Under the following headings, we are going to discuss the application of CILE to explore the applications of ILs in surface electrochemistry. 4.2.1.1 Direct electrochemistry of hemoglobin Afsaneh Safavi et al. [49] reported the direct electrochemistry of hemoglobin (Hb) by direct immobilization of Hb on the CILE that exhibits good electrochemical and electrocatalytic activities specifically towards the reduction of hydrogen peroxide, nitride, and oxygen. The Hb modified CILE was quite stable for at least a month. The authors used 1-octyl pyridinium chloride, (C8PyCl) and modified CILE (C8PyCl/CILE) in the absence and presence of Hb. The CV of CILE, C8PyCl with CILE, and C8PyCl with CILE in presence of Hb are reported in Fig. 4.1, in similar conditions for plain CILE the curve (a) and C8PyCl with CILE the curve (b) and there is no peak shown in the employed range of potential. A well-defined pair of C8PyCl with CILE in presence of Hb, obtained as mentioned in the curve (c) with a potential value of Epc and Epa was 0.29 and 0.15 V, respectively. Perceptibly, due to the electroactive center of Hb, the redox reactions at the Hb/C8PyCl/CILE modified electrode take place, as a result, that it is mentioned in presence of C8PyCl and Hb it favors efficiently immobilized on the surface of the electrode while in absence of Hb could not be directly immobilized on the electrode surface. Hence, outcomes express

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FIGURE 4.1 CV plots of (a) CILE, (b) C8PyCl/CILE, and (c) C8PyCl/CILE in the presence of Hb (acetate buffer solution with pH 5.0 and scan rates of 0.1 V/s) [50].

that immobilization of Hb is very much enhanced due to the incorporation of C8PyCl. At various pH values, the CV of C8PyCl with CILE in presence of Hb was also studied and demonstrates that the value of peak potentials is visibly reliant in the range of 3.58.5 pH. The values of apparent formal 0 potential (E0 (in mV)) at different pH values, of Hb, immobilized on CILE were found at different pH values from the 38 varying from 0 125 to 370 reflects, the dependence of E0 value not only on the pH of the solution but also there is a shift to the negative potentials with the rise in pH of the solution, showing the presence of proton the Hb electron transfer process [49]. In addition, the authors also investigated the electrocatalytic effect of electrodes modified by the assembly of Hb with C8PyCl and CILE on the H2O2 reduction, as shown in Fig. 4.2 that on adding H2O2 to a solution that was under observation there is a peak at approximately 0.1 V to be detected. With more addition of H2O2 not only the increase in the reduction peak currents but also the shift in peak potentials towards higher negative values to be observed. The creation of biosensors for the advanced generation is founded on creating the CILE with direct immobilization as well as electron transfer of Hb deprived of the use of subsidiary films and redox intermediaries [49].

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4. Green chemistry of ionic liquids in surface electrochemistry

FIGURE 4.2 CV plots of C8PyCl/CILE in the presence of Hb (acetate buffer with pH 5.0) with added H2O2: (a) 0.0, (b) 5.0 3 104 M, (c) 1.0 3 103 M, (d) 1.5 3 103 M, (e) 2.0 3 103 M, and (f) 4.0 3 103 M [49].

Wei Sun et al. [50] report the creation of CILE by using the imidazolium-based RTIL, that is, 1-butyl-3-methylimidazolium hexafluorophosphate (C4mimPF6) by way of binder that was developed for the determination of H2O2 by immobilization of Hb in the Nafion/ nano-CaCO3 composite film that lies on the surface of CILE as a new electrochemical biosensor. The author also discussed a well-distinct pair for peaks of quasi-reversible redox by the Hb modified electrode by using the CV. The UVvis spectra explained the Soret band position by variation of not more than1 nm, hence the secondary structure of Hb is in its natural state even in a film of Nafion/nano-CaCO3. In addition, the results of FT-IR spectroscopy again confirmed that no change in the native structure of Hb in the film of Nafion/nano-CaCO3 is similar to the spectrum of free Hb. Hence, herein the authors concluded that during electron transfer reaction at electrode Hb involves the one proton and one electron as well as modified the CILE with Hb on a film of Nafion/nano-CaCO3. This favors the decent H2O2 catalytic reduction electrochemically which could be additionally implemented for the construction of an H2O2-based biosensor [50]. Wei Sun et al. [51] reported the construction of a new CILE that expressed the improved behavior electrochemically than the obsolete paraffin CPE by using N-butylpyridinium hexafluorophosphate (C4PPF6) which act as a binder. In addition, also discussed the immobilization of Hb on the surface of CILE with a film containing Nafion film and nano-CaCO3 regularly and CILE treated as a basal electrode.

1. Catalysis and electrochemistry

4.2 Role of ionic liquids in surface electrochemistry

97

As shown in Fig. 4.3 there are various CV plots of different compositions from (A) to (E), and specifically at 7.0 pH obtained voltammetric peaks with cathodic and anodic peak potentials values of 0.444 and 0.285 V, respectively [51]. As shown in Fig. 4.4A, the CV of Nafion/nano-CaCO3/Hb film a pair of crudely symmetric anodic as well as cathodic peaks and peak-topeak separation similarly rise in scan rate. The consequences showed a reduction of electroactive Hb Fe(III) to Fe(II) on the forward scan as well as on the reverse scan. In addition, peak current and scan rate establish a decent linear relationship as shown in Fig. 4.4B which was distinguishing for quasi-reversible surface controlled thin-layer electrochemical behavior [51]. As shown in Fig. 4.5A and B, there is no marked effect is observed in the native structure of Hb, as the spectra of Hb in the Nafion/ nano-CaCO3 film (1652.02 and 1532.05/cm) and natured Hb (1655.34 and 1535.80/cm) is almost same. In the case of denatured Hb, the intensities of the I and II amide bands will be meaningly reduced or may disappear, hence it is observed that the native structure of Hb is reserved on the CILE surface-immobilized by Nafion/ nano-CaCO3 film [51]. Furthermore, UVvis absorbance spectra of Hb as Fig. 4.6, in the various pH buffer solutions that show the Hb Soret band in aqueous

FIGURE 4.3 CV plots of (a) Nafion/nano-CaCO3/Hb/CILE, (b) bare CILE, (c) Nafion/ CILE, (d) Nafion/nano-CaCO3/CILE, and (e) Nafion/Hb/CILE in pH 7.0 BR buffer at a scan rate of 0.1 V/s [51].

1. Catalysis and electrochemistry

98

4. Green chemistry of ionic liquids in surface electrochemistry

FIGURE 4.4 (A) CV plots of Nafion/nano-CaCO3/Hb film in pH 7.0 B-R buffer solution with different scan rates (ai: 0.1, 0.15, 0.18, 0.2, 0.25, 0.3, 0.35, 0.4, and 0.5 V/s). (B) Plots of cathodic (a) and anodic (b) peak current with the scan rate (V) [51].

solution in the curve (a) at 405 nm and the spectra for the Hb with Nafion/nano-CaCO3 at the various pHs between 5.0 and 10.0, at nearly the similar position (curves c-f) as the native Hb solution. Hence it is observed that average pH ranges the native conformation of Hb retained in Nafion/nano-CaCO3 solutions [51].

1. Catalysis and electrochemistry

4.2 Role of ionic liquids in surface electrochemistry

FIGURE 4.5

99

FT-IR spectra of the films of Hb (A) and Nafion/nano-CaCO3/Hb (B) [51].

FIGURE 4.6 UVvis absorption spectra of Hb in water solution (a) and Nafion/nanoCaCO3/Hb in pH (b) 2.0, (c) 5.0, (d) 6.0, (e) 7.0, and (f) 10.0 buffer solutions, respectively [51].

Sun Wei et al. [52] report that the Hb is entrapped in the hydrogel film of sodium alginate (SA) on the CILE surface to exhibit the direct electrochemistry and offered a biocompatible interface with the increased electron transfer rate because of IL presence in the CILE. The bioactivity of Hb on

1. Catalysis and electrochemistry

100

4. Green chemistry of ionic liquids in surface electrochemistry

FIGURE 4.7 CV plots of SA/Hb/CILE in a pH 7.0 BR buffer solution containing 0, 20, 50, and 100 μmol/L H2O2 (curves ad) with the scan rate as 100 mV/s [52].

FIGURE 4.8 CV plots of SA/Hb/CILE in a pH 5.0 BR buffer solution containing 0, 20, 40, 50, 100, 200, 240 mmol/L NaNO2 (curves ag) with the scan rate as 100 mV/s [52].

the CILE surface of the modified electrode contains SA and Hb confirmed 0 by the E0 value of around 0.344 V at 7.0 pH of quasi-reversible cyclic voltammetric peaks that were accredited for Fe(III)/Fe(II) redox couple (Figs. 4.7 and 4.8) [52].

1. Catalysis and electrochemistry

4.2 Role of ionic liquids in surface electrochemistry

101

FIGURE 4.9 CV plots of DA (1.25 3 103 M), AA (1.25 3 103 M), and UA (1.25 3

103 M) at (A) CILE and at (B) CPE in phosphate-buffered solutions, pH 6.8. Scan rate was 50 mV/s [53].

4.2.1.2 Determination of various substances Afsaneh Safavi et al. [53] reported the study of the sensitive, simultaneous determination of dopamine (DA), ascorbic acid (AA), and uric acid (UA) as shown in Fig. 4.9. It is indicated from here that at the CPE the slow rate of electrons transfers to be observed for these biomolecules, their peaks for oxidation potentials are almost close to each other and report the quasi reversible or irreversible electrochemical reactions. From here it is observed that the electron transfer rates of DA, AA, and UA are to show strong improvement [53].

4.2.2 Quartz crystal microbalance Quartz crystals have been used as an analytical device after it was discovered that there is a linear relationship between frequency response against the deposited mass, as reported by the Saurbrey in 1959. He demonstrated that the linear relation between deposited mass and frequency response only holds if an ideal layer of foreign mass is strappingly combined with the resonator and hence such a device is called, “Quartz-Crystal Microbalance” (QCM) [54,55]. For particular organic vapors authors effectively developed and tested the use of the ILs as sensing materials for QCM, reported by the frequency shifts during its plasticization due to the induced sorption of the organic vapors. To measure the chemical processes which result in a substantial change of viscosities in the conforming reaction media

1. Catalysis and electrochemistry

102

4. Green chemistry of ionic liquids in surface electrochemistry

reported by the QCM device [5658]. Moreover, the authors observed that in ILs the mass transport rates of analytes are much faster as compared to solid inorganic or organic coatings [59]. QCM devices based on ILs are good contenders for the expansion of new detection systems for gas chromatography and industrial volatile organic vapors because of the short response time and excellent reversibility. The responses of QCM sensors coated with ILs with anions Tf2N and BF4 to selected organic vapors are reported in Tables 4.1 and 4.2, respectively [55]. Ilchat Goubaidoulline et al. [60] report, a concept for vapor sensing with the quartz crystal microbalance where the vapor phase is absorbed into small droplets of an ionic liquid. The liquid is contained in the pores of a nanoporous alumina layer, created on the front electrode of the quartz crystal by anodization. Ionic liquids are attractive for vapor sensing because being liquids they equilibrate very fast, while at the same time having negligible vapor pressure. Containing the ionic liquids inside cylindrical cavities of a solid matrix removes all problems related to the liquid’s softness as well as the possibility of dewetting and flow. The absence of viscoelastic effects is evidenced by the fact that the bandwidth of the resonance remains unchanged during the uptake of solvent vapor. The Henry constants for a number of solvents have been measured. In Fig. 4.10 and Table 4.3 the results discussed by the author for the determination of Henry’s constants in various organic solvents in IL [60]. Authors investigate the RTIL, 1-butyl-3-methylimidazolium hexafluorophosphate (C4mimPF6) as a selective deposition for QCM, that produces versatile mediums of tunable physicochemical properties to provide high solute mobility in comparison to polymer matrices because of short response and desorption times. The viability of RTIL-based QCM as vapor sensors with specific attention to the physicochemical interactions between the RTIL and ethyl acetate, that is, a model solute has been investigated. It has been found that ILs, deliver not only high solute mobility but also an excellent baseline recovery of the sensor after exposure to sample vapors, having physicochemical properties that can be personalized to a significant extent to a specific application. However, to explore basic mass transport phenomena for simple sample vapors it is believed that quartz resonators are a complementary and cheap method that happens on dissolving solutes in RTILs [61]. To recognize the nitro aromatic compounds like ethyl nitrobenzene (ENB) and dinitro toluene which are correspondents of redox-active explosives a single miniaturized platform was established that cartels electrochemical and piezoelectric transduction mechanisms as an integrated sensor. The 1-butyl-3-methylimidazolium tetrafluoroborate (C4mimBF4) was used for this as not only an electrolyte but also as the

1. Catalysis and electrochemistry

TABLE 4.1

Values of Δfmod for QCM sensors with various ILs (ILs with Tf2N anion) [55]. Ionic liquid, Δfmoda EtMeImTf2N (ethyl)

PrMeImTf2N (propyl)

BuMeImTf2N (butyl)

HepMelmTf2N (heptyl)

UnMelmTf2N (undecyl)

PrMe2ImTf2N

5740

5701

6345

5392

6171

6215

Toluene

2324

1680

3525

2065

3228

1532

Methanol

1377

588

2341

112

567

429

Ethanol

977

670

1565

53

459

250

2 Propanol

1801

1422

3338

637

iS.I’l

553

1 Butanol

1511

796

3991

1525

5378

653

Acetone

2650

3372

4320

3212

3501

3461

Acctonitrilc

2886

2899

4939

2041

2925

3009

Dichl oroimethane

1318

1124

1195

987

1121

1073

Chloroform

1557

1021

1291

541

1342

1291

Ethyl acetate

3654

3654

4289

4151

2781

4471

Tetrahydrofuran

4150

4155

5054

4358

5164

4450

a

Δfmod: frequency shift induced by coating ionic liquids on quartz crystals.

104

4. Green chemistry of ionic liquids in surface electrochemistry

TABLE 4.2 Values of Δfmod for QCM sensors with various ILs (ILs with BF4 anion) [55]. Ionic liquid, Δfmod EtMeImBf4N (ethyl)

PrMeImBf4N (propyl)

BuMeImBf4N (butyl)

5504

5559

5697

Toluene

578

765

1627

Methanol

2598

431

1241

Ethanol

818

447

769

2 Propanol

1453

1406

1905

1 Butanol

819

854

1645

Acetone

2469

3319

4090

Acetonitrile

2341

3996

4672

Dichloromethane

435

918

1933

Chloroform

747

1243

2372

Ethyl acetate

1730

2988

4702

Tetrahydrofuran

2381

3500

4777

FIGURE 4.10 Weight fraction of solvent inside the pores as a function of the solvent activity. The straight lines are linear fits [60].

sorption solvent and studied by various voltametric techniques including CV. The volatile ENB vapor’s recognition with the assembled chip of electrochemical quartz crystal microbalance (EQCM) was confirmed

1. Catalysis and electrochemistry

4.2 Role of ionic liquids in surface electrochemistry

TABLE 4.3

105

Henry’s constants of ionic liquids and their detection limits [61].

Solvent

Henry’s constant (Pa)

Detection limit (mg/m3)

Acetonitrile

9300

1232

Cyclohexane

760,000

1875

Isooctane

400,000

7634

Methanol

14,000

1429

THF

7400

321

Toluene

46,000

411

using amperometric methods as well as QCM, which may lead to enormously sensitive, precise, and quick recognition gas sensor devices [62].

4.2.3 Chemical warfare agent These are extremely lethal and considered the most sinful chemicals that are synthesized by the humans and due to their harmful effects the critical need for time to detect these chemicals very rapidly and sensitively [63] Chemical warfare agents (CWAs) intimidations are rising worry for many nations, despite various peace programs between the nations, like biological, chemical, nuclear, and radiological still very much [64]. It used to be done significantly by destructive organizations or even states due to its very easy synthesis technique that can produce dangerous deadliness. In past regrettably, CWA’s were used against the human race in various countries as the significant amount used in World War I and results in approximately 1.3 million deaths [65]. In addition, a few other chemicals like Sulfur Mustard (SM) caused around 4% of all causalities during the IraqIran dispute from 198088 [65]. As per the report on the deadly attack of SM on humans, the probability of its attack in the future from enemies is very high, so the very essential need of the hour us that to develop potential and proper way to detect it. Out of the enormous ways which have been in practice for CWA, detecting electrochemically is highly unsurpassed due to less cost, appreciably sensitive, transferability, and working ease [6670]. Singh et al. [71] reported the electrocatalysis of SM for RTIL, methyltrioctylammonium bis (trifluoromethylsulfonyl) imide (C1TOamTf2N) on Pt electrode, and the obtained data during electrocatalysis show that the oxidation of SM is an irreversible phenomenon which involves the participation of two-electron during SM oxidation.

1. Catalysis and electrochemistry

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4. Green chemistry of ionic liquids in surface electrochemistry

The contact of the anionic part of RTIL is responsible contrary to CWA activity, that is, electrocatalytic activity of RTIL that supports the polarization and leads to dissociation of the CCl bond. In this way, the application of RTIL is treated as a likely medium for detection owing to not only their negligible vapor pressure but also their electrocatalytic nature, which helps this methodology to be field deployable which is instantly required to encounter the terrorism movement by using CWA. Therefore, such kind of study establishes a much better way for the whole CWA with RTILs by using greener electrochemical sensing because of RTILs properties specifically nonflammability, wide liquid range, huge electrochemical potential windows, good thermal stability, as well as negligible vapor pressure that helps to treat it as ecofriendly and finally make it capable for the applications for a better environment. In addition, the financial issue related to the RTIL is also a matter of concern, hence nowadays chemists trying to develop better economical RTIL that makes it cheaper compared to the present scenario. Hence in the coming days, such kinds of studies will be protracted to prepare a common detection system for CWA [71].

4.2.4 Electrochemical oxidation Villagran et al. [72] used linear sweep and square wave voltammetry, for the electroanalytical quantification study of chloride in tetrafluoroborate, hexafluorophosphate, and bis(trifluoromethanesulfonylimide) of 1-butyl-3-methylimidazolium, respectively (C4mimBF4), (C4mimPF6), and (C4mimNTf2). The cathodic stripping voltammetry is of the utmost sensitivity at a silver disk electrode. The trace chloride in water-miscible/immiscible ILs has been examined with linear sweep and square wave voltammetry by using the electroanalytical method. Besides, at a silver disk electrode, the cathodic stripping voltammetry was found to be ideal for sub-ppm levels of quantification of chloride by exploring the electroanalysis [72]. The study on five RTILs that includes four 1-alkyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide [CnmimN(Tf)2], with different alkyl group (i.e., n 5 2, 4, 8, 10) and n-hexyltriethylammonium bis(trifluoromethylsulfonyl)imide [N6222N(Tf)2] as solvents reported by Evans et al. [73] to discuss the electrochemical oxidation of N,N,N0 ,N0 -tetramethyl-para-phenylenediamine (TC1PD) and N,N,N0 ,N0 tetrabutyl-paraphenylenediamine (TC4PD) under vacuum conditions by using 20 mL of samples. The study revealed the consequence of the accessible electrochemical window by the dissolved atmospheric gases. Such effect was explored and resolute, that explored to be not as substantial as got formerly for ILs comprising substitute anions [73].

1. Catalysis and electrochemistry

References

107

Buzzeo et al. [74] report the CV investigation at a gold microdisk electrode for two electrochemical reductions of oxygen in two different RTILs, 1-ethyl-3-methylimidazolium and hexyltriethylammonium with bis((trifluoromethyl)sulfonyl) imide, respectively [C2mimN(Tf)2] and [N6222N(Tf)2]. Because of the morphological variation in the cationic components of discussed RTILs, the viscosity of [N6222N(Tf)2] is more as compared to [C2mimN(Tf)2] and gives rise to meaningful diffusion coefficients of oxygen and superoxide that approximately contrary by extra than a value of 30 [74].

4.3 Conclusions Herein we can conclude the various applications of the ILs in the surface electrochemistry by specifically discussing the CVs results from the various articles. The role of ILs in the modification of CILE in the surface electrochemistry by direct electronation of Hb specifically by the role of C8PyCl in modified CILE absence and presence of Hb at various pH values. Furthermore, the importance of ILs to modify electrode (Hb/C8PyCl/CILE) for the electrocatalytic effect on the H2O2 reduction. In the same context also discussed the role of C4mimPF6 as a binder from the constituent of CILE and used for H2O2 determination with the help of Hb immobilization in the Nafion/nano-CaCO3 composite film on the CILE surface as a new electrochemical biosensor. In addition, effectively developed and tested the use of the ILs as sensing materials for QCM and also the role of ILs in CWA.

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[52] W. Sun, D. Wang, R. Gao, K. Jiao, Direct electrochemistry and electrocatalysis of hemoglobin in sodium alginate film on a BMIMPF6 modified carbon paste electrode, Electrochem. Commun. 9 (2007) 11591164. [53] A. Safavi, N. Maleki, O. Moradlou, F. Tajabadi, Simultaneous determination of dopamine, ascorbic acid, and uric acid using carbon ionic liquid electrode, Anal. Biochem. 359 (2006) 224229. [54] A. Janshoff, H.-J. Galla, C. Steinem, Piezoelectric mass-sensing devices as biosensors—an alternative to optical biosensors? Angew. Chem. Int. Ed. 39 (2000) 40044032. [55] C. Liang, C.-Y. Yuan, R.J. Warmack, C.E. Barnes, S. Dai, Ionic liquids: a new class of sensing materials for detection of organic vapors based on the use of a quartz crystal microbalance, Anal. Chem. 74 (2002) 21722176. [56] L. Deng, L. Bao, W. Wei, L. Nie, S. Yao, Rapid bacteria detection based on gelatin liquefaction with a piezoelectric viscosensor, Inst. Sci. Technol. 25 (1997) 69. [57] A. Bund, G. Schwitzgebel, Viscoelastic properties of low-viscosity liquids studied with thickness-shear mode resonators, Anal. Chem. 70 (1998) 2584. [58] R. Lucklum, C. Behling, P. Hauptman, Role of mass accumulation and viscoelastic film properties for the response of acoustic-wave-based chemical sensors, Anal. Chem. 71 (1999) 2488. [59] J.W. Grate, Acoustic wave microsensor arrays for vapor sensing, Chem. Rev. 100 (2000) 2627. [60] I. Goubaidoulline, G. Vidrich, D. Johannsmann, Organic vapor sensing with ionic liquids entrapped in alumina nanopores on quartz crystal resonators, Anal. Chem. 77 (2005) 615619. [61] T. Scha¨fer, F.D. Francesco, R. Fuoco, Ionic liquids as selective depositions on quartz crystal microbalances for artificial olfactory systems—a feasibility study, Microchem. J. 85 (2007) 5256. [62] Y. Lei, H. Yue, J. Xiaoxia, J.M. Andrew, Z. Xiangqun, Ionic liquid thin layer EQCM explosives sensor, Sens. Actuators B 140 (2009) 363370. [63] C.-Y. Chen, K.-H. Li, Y.-H. Chu, Reaction-based detection of chemical warfare agent mimics with affinity ionic liquids, Anal. Chem. 90 (2018) 83208325. [64] R.A. Falkenrath, R.D. Newman, B.A. Thayer, Americas Achilles Heel: Nuclear, Biological, and Chemical Terrorism and Covert Attack, the MIT Press, Cambridge, MA, 1998. [65] S.M. Somani, Chemical Warfare Agents, Academic Press, London, 1992. [66] K.A. Joshi, J. Tang, R. Haddon, J. Wang, W. Chen, A. Mulchandani, A disposable biosensor for organophosphorus nerve agents based on carbon nanotubes modified thick film strip electrode, Electroanalysis 17 (1) (2005) 5458. [67] S. Cinti, G. ValdesRamırez, W. Gao, J. Li, G. Palleschi, J. Wang, Microengineassisted electrochemical measurements at printable sensor strips, Chem. Commun. 51 (41) (2015) 86688671. [68] M. Pumera, A. Merkoci, S. Alegret, New materials for electrochemical sensing VII. Microfluidic chip platforms, TrAC. Trend Anal. Chem. 25 (3) (2006) 219235. [69] V.V. Singh, Recent advances in electrochemical sensors for detecting weapons of mass destruction. A review, Electroanalysis 28 (5) (2016) 920935. [70] A. Rabti, N. Raouafi, A. Merkoci, Bio (Sensing) devices based on ferrocenefunctionalized graphene and carbon nanotubes, Carbon 108 (2016) 481514. [71] V.V. Singh, P.K. Sharma, B. Sikarwar, K. Ganesan, M. Boopathi, B. Singh, Electrocatalysis of chemical warfare agent sulfur mustard in room temperature ionic liquid, Electroanalysis 29 (2017) 702707. [72] C. Villagrana, C.E. Banks, C. Hardacre, R.G. Compton, Electroanalytical determination of trace chloride in room-temperature ionic liquids, Anal. Chem. 76 (2004) 19982003.

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C H A P T E R

5 An evolution in electrochemical and chemical synthesis applications in prospects of ionic liquids Vijaykumar S. Bhamare and Raviraj M. Kulkarni Department of Chemistry and Centre for Nanoscience and Nanotechnology, KLS Gogte Institute of Technology, Udyambag, Belagavi, Karnataka, India

5.1 Introduction The term “Ionic Liquid” (IL) was initially used by S. Gabriel and J. Weiner in the year 1888 and investigated ethanol ammonium nitrate having the melting point 55
C [1]. Thereafter Walden synthesized ethyl ammonium nitrate having the melting point 12
C. Ethyl ammonium nitrate is the protic IL which is extensively investigated. On the other hand, other protic IL ethanol ammonium nitrate was neglected by researchers. Ethyl ammonium nitrate showed water like properties and three-dimensional structure [2,3]. The field of ILs has been developing rapidly due to its unique physicochemical properties and its utility in different fields especially electrochemical and chemical synthesis. The protic ILs can be synthesized by Bronsted acid and Bronsted base. The role of protic ILs and nonprotic ILs in chromatography was also investigated [4]. Room-temperature ionic liquids (RILs) are organic salts made of cationic and anionic components. The different kinds of RILs can be made by interchanging organic cations and inorganic anions. RILs melt at very low temperature.

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In order to overcome limitations of conventional organic solvents employed in organic synthesis, RILs were widely utilized because of tunability, inherent conductivity, wider electrochemical windows, low volatile nature, high thermal stability, etc. They are designed by combining a large size asymmetric organic cation with a small size inorganic anion and possess low melting points [5]. RILs were seen as a forest in desert due to their unique features [6]. Many investigations reported that electrochemical windows of RILs are varying in the range of 3.07.0 V. Organic solvent acetonitrile has electrochemical window 5.0 V. Tetrabutylammoniumtris(pentafluoroethyl)trifluorophosphate (TPeFETFP) has widest electrochemical window (7.0 V). There were small additional peaks observed in cyclic voltammogram for RILs because of chloride and bromide (Br) impurities. Hence, halogen free technology was demanded to prepare RILs. Merck KGaK synthesized halogen free RILs such as triflates [711]. It was found that nonhygroscopic conducting salt tris(perfluoroalkyl)trifluorophosphate-anion shows wider electrochemical window than other RILs. RILs are found to be less volatile than all conventional solvents. Therefore evaporative losses using RILs are very less at higher temperature [1217]. The viscous nature of RILs depends on different anionic components. RILs show very remarkable feature of inherent conductivity. This inherent conductivity property of RILs is influenced by movement of cations and anions components. This property of RILs depends on viscous nature, size of cations as well as anions, and interionic force of attraction. Due to these significant features, RILs are widely utilized in electrochemical synthesis. RILs are extensively employed for catalysis, synthesis, pharmaceuticals, gas sensors, biosensors, etc. [1821]. They are promising materials for organic synthesis due to their lower toxic nature, inherent conductivity. Moreover, they are thermally stable and exhibit lower vapor pressure [6,22]. The polar nature of RILs makes them excellent green solvents for electrochemical synthesis. They can dissolve many organic compounds, molecules, and organometallic compounds [23]. Electrochemical organic synthesis is very environment friendly, efficient, cheaper, effective field. On the one hand, conventional organic synthesis involves toxic volatile organic compounds [2428]. There are many reagents and solvents are utilized in conventional organic synthesis. On the other hand, electrons and electrodes are used as reagents and catalyst, respectively, in electrochemical organic synthesis [2527]. Electrochemical organic synthesis and conventional organic synthesis are having merits and demerits in different fields. RILs play a very prominent role in organic synthesis electrochemically. They act as a solvent and an electrolyte. This makes them promising candidate because it evades the toxic volatile organic solvents [29]. They exhibit wider electrochemical windows [2931].

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The N-heterocyclic carbenes (NHCylC) are electrochemically generated from the reduction of imidazolium- and thiazolium-based RILs. The NHCylC can be used as nucleophiles or bases in many electrochemical organic syntheses. They catalyze many electrochemical organic synthesis reactions. The chemical reduction of carbon dioxide gas is not very easy and convenient because of drastic conditions like very high pressure and temperature. It also involves the utilization of sophisticated catalysts. Electrochemical carbon dioxide reduction using various RILs is very fascinating field. This is carried out at low pressure and temperature. It involves radical anion intermediates which are stabilized by RILs [32]. In these reactions, RILs are referred as “ruler of solvents” because of their salient features. In these reactions, two modes are involved. The first mode involves direct electrochemical reduction of carbon dioxide and then coupling with alcohols and amines to obtain desirable products. The second mode of electrochemical reduction involves reduction of halogenated arenes to corresponding reactive radical species and then coupling with carbon dioxide gas to obtain carboxylic acids [33,34]. These two approaches utilized carbon dioxide gas and converted into useful carboxylic compounds. As a result, it reduces the excess of carbon dioxide greenhouse gas present in our environment using RILs. This greenhouse gas is more soluble in RILs. It is less soluble in conventional organic solvents [35]. Literature survey shows that there are many papers published related to synthesis and catalysis using RILs. It means that the research in the field of ILs in electrochemical and chemical synthesis has grown by leaps and bound in last decade. Fig. 5.1 demonstrates the number of research publications per year from 2010 to 2021 showing “ionic liquids in electrochemical and chemical synthesis” in the content as per Science Direct record data base (date of search: September 1, 2021). This chapter includes electrochemical and chemical synthesis applications in prospects of ionic liquids. This chapter has covered green chemistry of RILs in organic synthesis. This chapter encompasses many sections to focus on different organic reactions which were performed using RILs.

5.2 Electrochemical oxidation reactions using roomtemperature ionic liquids The oxidation at the anode surface and self-coupling with different molecules and polymerization using RILs found very common. Esmail and Amani utilized cyclic voltammetry method for the electrochemical oxidation of mesalazine using barbituric acid derivatives using RIL such as 1-butyl-3-methylimidazoliumtetrafluoroborate (BMITFB) as green solvent. Electrochemical study and synthesis are very useful green chemistry methods using RIL [36].

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Number of research publications

7000 6000

5575

5000

4601 3787

4000 3000 2000 1000 0

1008 1154

1361

1726 1866

2086

2450

2899

3321

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2010

2011

2012

2013

2014

2015

2016

2017

2018

2019

2020

2021

Years FIGURE 5.1 The number of research publications per year from 2010 to 2021 showing “ionic liquids in electrochemical and chemical synthesis” in the content as per Science Direct record (date of search: September 1, 2021).

Guoying Zhao et al. carried out the electrochemical oxidation of C6H5CH2OH by passing electric current in two-phase system such as RIL/ supercritical carbon dioxide gas using an undivided cell. The temperature of 318.2 K and the pressure of 10.3 MPa were maintained for this electrochemical oxidation reaction. There were two RILs such as BMITFB and BMIhexafluorophosphate (HFP) utilized as solvents and electrolytes for this electro-oxidation reaction. This investigation reported that C6H5CH2OH underwent easy electrochemical oxidation and formed C6H5CHO. The experimental data showed that BMITFB was good medium for electrochemical oxidation reaction. This research group investigated the influence of different parameters like variation in concentration of C6H5CH2OH, pressure, temperature on electrochemical oxidation reaction using BMITFB. During this study, it was found that the Faradaic efficiency and selectivity of C6H5CHO enhanced with the pressure of carbon dioxide gas (lower than about 9.3 MPa) and thereafter Faradaic efficiency declined with the increase in pressure of carbon dioxide gas. The selectivity of C6H5CHO can be tuned by changing the pressure of carbon dioxide gas. The solubility difference of reactants and products plays a crucial role in this electrochemical oxidation. RIL like BMITFB can be easily separated from C6H5CHO and reused in electro-oxidation reactions. This study suggested that the utilization of low volatile RILs, supercritical carbon dioxide gas, and clean electrochemical technology offers efficient and cleaner methods for electrochemical oxidation of C6H5CH2OH [37].

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Jalal Ghilane et al. carried out electrochemical oxidation of primary amine in RIL such as 1-ethyl-3-methylimidazolium (EMI)bis(trifluoromethylsulfonyl)imide (BTFMSI). The electrochemical oxidation of 4-nitrobenzylamine and 2-aminoethylferrocenylmethylether was performed in this investigation. The experimental results showed the surface concentration of attached group to be 13 3 1010 mol/cm2 in this media. This study highlighted that the RIL declines the surface concentration of grafted layer. There was a formation of less dense grafted layer on the electrode surface using EMIBTFMSI than traditional solvent acetonitrile [38]. Florin A. Hanc-Scherer et al. in 2013 studied in detail the electrochemical carbon monoxide oxidation on three Pt (hkl) electrodes using two different RILs such as EMITFB and EMIBTFMSI. Pt (hkl) electrodes were flame-annealed before carrying out the electrochemical oxidation reaction. Pt (110) surface selectively activated carbon monoxide oxidation in RILs. These findings could be applied to fabricate the Pt nanosized electrocatalysts for the electrochemical oxidation of carbon monoxide at higher temperature [39].

5.2.1 Oxidative self-coupling reaction Mellah et al. in 2005 investigated the oxidative self-coupling of aromatic compounds (anisole, mesitylene, naphthalene, and anthracene) using different RILs to give products of anodic dimerization. This study highlighted that the electrosynthesis of anodic self-coupling products in RILs could be possible in an undivided cell. This investigation concluded that the RILs could be utilized as easy and suitable media for electrochemical anodic oxidation in one-compartment mode. These findings could be utilized in different anodic reactions in which formation of equimolar proton takes place in target process. Mesitylene formed dimesityl product with 82% yield and small amount of m-terphenyl (9%) in RIL BMIHFP on the surface of platinum disk. Methoxybenzene formed 4,40 -dimethoxybiphenyl dimerized product (76%) at similar reaction conditions. The oxidation of 1,2-dimethoxybenzene with nucleophiles formed unsymmetrical dimerized and trimerized products. On the other hand, trimerized hexamethoxytriphenylene product was formed in the presence of nonnucleophiles. Hexamethoxytriphenylene on oxidation forms polymeric tubular systems having different morphologies which depend on the RIL utilized in this synthesis. It was also highlighted that aromatic compounds such as anthracene and naphthalene formed dimeric products [40].

5.2.2 Shono oxidation of carbamates Steven Bornemann and Scott Handy in 2011 reported Shono-type carbamate electrochemical oxidation reactions. This study highlighted the

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influence of viscous nature of solvent on the Shono oxidation of carbamates using RILs. It was reported that high viscous nature of RILs is serious issue for electrochemical oxidation reactions. This research group investigated the Shono oxidation of arylamines and carbamates in RILs which are used as recyclable solvents. It was found that cosolvent can be used to obtain better results and easy recovery of RILs. Methyl alcohol (5 equiv.) utilized as methoxylating agent in RIL such as tributyldecylammoniumtosyl (TBDATSL) for Shono oxidation of carbamates (Scheme 5.1). This reaction yielded more amount of product when 33% methanol was used. This may be due to the decrease in viscosity of solvent used. Volatile organic compounds can be used in this reaction to enhance the conversion efficiency but they will decrease the electrochemical window of RIL such as TBDATSL. The higher viscous nature of RIL declines mixing speed of reacting materials toward the surface of electrode. Consequently, there are not any sufficient reacting materials available around the surface of electrode which can react with electric current. Due to this, over oxidation of most easily oxidized materials occurs at the surface of electrode. Volatile organic compounds can be utilized as cosolvents to prevent or minimize over oxidation of materials. Volatile organic compounds are not eco-friendly and affect the significance of RILs. The use of RILs with low viscosities and wider electrochemical window is suggested for Shono oxidation of carbamates [41].

5.2.3 Oxidation of alcohols Barhdadi et al. in 2007 described the electrochemistry of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl mediated oxidation to form oxidized aldehydes and ketones from alcohols using RIL. This research group reported that diffusion currents were slowed in RIL as compared to conventional solvent acetonitrile. The reason behind this suppression of diffusion currents may be due to higher viscosity of RIL. The viscosity of RIL was declined in the presence of base and alcohol substrate. This enhances the rate of diffusion currents. This investigation showed that (2,2,6,6-tetramethylpiperidin-1-yl) O

OC2H5

N

O

e- 0.24 A

H3CO

OC2H5

O

N H3CO

+

OC2H5

N

OCH3

CH3OH, Bu3DecylN-OTs Pyrrolidine-1-carboxylic acid ethyl ester

SCHEME 5.1

2-Methoxy-pyrrolidine-1-carboxylic acid ethyl ester

2,5-Dimethoxy-pyrrolidine-1-carboxylic acid ethyl ester

Shono oxidation of carbamate in TBDATSL RIL.

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5.2 Electrochemical oxidation reactions using room-temperature ionic liquids O

OH

R

Alkyl substituted benzoyl alcohol

(2, 2, 6, 6-tetramethylpiperidin-1-yl)oxyl

H

Pt-Pt electrochemical cell, Electric charge 2.0 F/mol

R

Alkyl substituted benzaldehyde

SCHEME 5.2 Synthesis of aldehydes from alcohols electrochemically.

oxyl underwent reversible redox reaction in RIL (Scheme 5.2). This produces oxoammonium species that are very active. These species play important role for the oxidation of alcohols to aldehyde or ketone. This study also reported around 100% Faradaic and 100% chemical efficiencies when the oxidized product is nonenolizable aldehyde. This showed that the electrochemical synthesis method is very effective when oxidation of alcohol produces nonenolizable aldehyde. It was also highlighted by this research group that the catalyst gets deactivated slowly over prolonged electrolysis which involves the enolizable products. It was found that primary alcohols oxidize to aldehydes with good yield when 2 F/mol electric charge was passed. This investigation carried out oxidations of C6H5CH2OH and halogenated C6H5CH2OH. The oxidation of these alcohols to aldehydes yielded good amount of products irrespective of the alcohol substrate. The electrochemical oxidation of secondary alcohols produced 50%60% ketone products [42]. Ajith Herath and James Becker reported the electrochemical oxidation of C6H5CH2OH to C6H5CHO in RIL BMIHFP at the surface of electrode [43]. This study reported 90% yield of C6H5CHO product formed in this electrochemical synthesis. The electron-transfer reactions are found to be faster in methyl cyanide than RIL BMIHFP. This may be due to difference in viscous nature of solvents. The standard heterogeneous electrontransfer reactions in BMIHFP were influenced by the anodic material, where platinum shows higher rates as compared to glassy carbon.

5.2.4 Bromination reaction Gary Allen et al. in 2004 studied the reactivity of electrogenerated Br2 with C6H10 on the surface of Pt microelectrode using RIL and conventional aprotic solvent. In this study, BMIBTFMSI utilized as RIL. Acetonitrile was utilized as conventional aprotic solvent. The linear sweep and cyclic voltammetry were used for this bromination reaction. This study reported the change in voltammetric response in two different solvents. This indicated that the present reaction progressed through

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separate pathways. The bromination of organic substrate in RIL BMIBTFMSI produced trans-1,2-dibromocyclohexane. The bromination of organic substrate in conventional solvent acetonitrile produced trans-1(N-acetylamino)-2-bromocyclohexane. This direct oxidation of Br in two different solvents yielded different products due to their different nucleophilicities. This investigation suggested potential application of bromination reaction in organic electrosynthesis on a huge scale due to the electrogenerated bromine from Br [44].

5.3 Electrochemical reduction reactions using roomtemperature ionic liquid Literature survey revealed that many research papers have been published on electrochemical reduction reactions and following conversion into significant organic compounds using RILs as compared to electrochemical oxidation reactions. Jia-Xing Lu and coworkers in 2003 described the significant features of RILs and their role in electrochemical reduction reactions to synthesize useful organic compounds [45]. RILs employed initially in the electrochemical activation to synthesize cyclic carbonates without using any catalyst and cosolvent. This investigation indicated that RILs can play a significant role of effective reaction media in electrochemical synthesis reactions. Therefore this research group carried out reduction of benzoylformic acid electrochemically using RIL such as EMI-Br. This electrochemical reduction was carried out at temperature 80
C and formed mandelic acid with 91% yield on the surface of glassy carbon electrode. This investigation reported that RIL EMI-Br as promising material due to its lower reduction potential as compared to 1-alkylpyridinium salts. This salt can be used for electrochemical reduction of carbonyl compounds to obtain higher yields with good selectivity. It can be utilized as universal solvent in electrolysis due to its significant electrochemical character. It was also found that carbonyl compounds underwent direct electrochemical reduction and formed hydroxyl compounds. Many research groups also reported in their investigations that alkyl/aryl halides, α-haloketones, carbonyl compounds, carbon dioxide underwent irreversible electrochemical reduction and forms more reactive radical intermediate. Depending on the reactions conditions, these compounds underwent self-coupling reaction to form useful organic compounds. Irene Reche and coworkers in 2014 reported that cations of RILs stabilises the anionic radicals which are formed in electrochemical reduction reactions [46]. Different types of electrochemical reduction reactions reported by many researchers in their investigations are described here systematically.

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5.3 Electrochemical reduction reactions using room-temperature ionic liquid

5.3.1 Electroreductive coupling of organic halides Rachid Barhdadi et al. in 2003 presented that direct or Nickel-mediated electrosynthesis through reductive coupling achieved formation of carboncarbon bond in RILs. This study reported that electroreductive homocoupling of aromatic organic halides on the surface of nickel cathodic electrode and magnesium or aluminum sacrificial anodic electrode using 1-octyl-3-methyl imidazolium (OMI)TFB which formed dimer with better yields [47]. This synthesis is similar to Cu catalyzed Ullmann reactions. On the other hand, direct electroreductive homocoupling of alkyl halides on Nickel grid cathode using OMITFB RIL as solventelectrolyte media gave moderate yield of product (Scheme 5.3). The same research group also investigated electroreductive coupling of aryl halides with activated olefins using OMITFB. This electroreductive coupling of aryl halide gave moderate yields (Scheme 5.4). Mellah et al. in 2003 studied the electrosynthesis such as electrodimerisation of aromatic halogen compounds which was catalyzed by reduction of Ni(II) complexes. In this investigation, different RILs made of cation BMI and anionic parts such as TFB, heptafluorobutyrate (HFB), methylsulfonate, and BTFMSI were utilized. It was reported that BMIBTFMSI gave better yields of products. The voltammograms of 0.1 mmol [NiCl2(bipy)] in 2.0 mL of BMIBTFMSI for 1.0 mmol of C6H5CH2Br on glassy carbon electrode by passing 2.1 F/mol electric current gave excellent yields of products [48]. Dongfang Niu and coworkers demonstrated electrochemical reductive dimerization of aromatic Br such as C6H5CH2Br using different electrodes such as copper, nickel, silver, and titanium in RIL such as BMITFB. OMI-TFB, [NiBr2(bipy)], Nickel grid cathode 2 RX + 2 e-

R-R

Alkyl halide

+

2 X-

Alkane

SCHEME 5.3 Direct electroreductive homocoupling on Nickel grid cathode using OMITFB RIL as solventelectrolyte media to form alkane from haloalkanes.

R X

+

Aryl halide

Z

R

Z

OMI-TFB, Catalyst NiBr2, e- , Nickel grid cathode

Activated olefin

SCHEME 5.4 Electroreductive coupling of aryl halides using OMITFB RIL.

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This was found to be easy and environment friendly approach to dimerize aromatic Brs. The electrochemical reductive dimerization of C6H5CH2Br was investigated by cyclic voltammogram on the surface of silver electrode with the scan rate 100 mV/s using BMITFB in the presence of inert N2 gas. Cyclic voltammogram was also used to study the electrochemical reductive dimerization of C6H5CH2Br at copper, nickel, and titanium. It was reported that silver electrode showed excellent electrocatalytic effect for the electrochemical reductive dimerization of C6H5CH2Br. RIL BMITFB was recovered and reused fivefold without affecting the yields of products considerably [49]. Laura Duran Pachon and coworkers presented Pd nanoparticles generated in situ catalytic Ullmann-type reactions. This study described Pd catalyzed the electroreductive homocoupling of haloarenes. This study highlighted that electrons play a vital role for closing the catalytic cycle. This helps for electroreductive homocoupling to produce better yields of products. This reaction proceeds with passage of electric current and H2O as reagent. RIL OMITFB showed better inherent conductivity in this investigation. This solvent was recovered and reused at least fivefold without affecting the yields of desirable products. The projected mechanism involved formation of radical anion [50] (Scheme 5.5).

5.3.2 Pinacol coupling reaction Corinne Lagrost and coworkers reported Pinacol coupling of aldehydes or ketones for the synthesis of 1,2-diols which are promising materials for the organic synthesis. This investigation carried out the influences of RILs such as TEBABTFMSI, BMIBTFMSI, and TMBABTFMSI on the Pinacol coupling of ketones or aldehydes. This study reported that RILs showed strong effects on reaction kinetics of Pinacol coupling of ketone [51]. Fabien Andre et al. in 2010 demonstrated irreversible dimerization of acetophenone radical anionic species. During this investigation, the influences of different parameters such as ionic solvation, viscosity, and polarity on dimerization of acetophenone were observed. This

X

R

OMI-TFB, Catalyst Paladium, e- , Platinum cathode

2

+

R

Alkyl substututed aryl halide

SCHEME 5.5

R

Pd nanoparticles generated in situ catalytic Ullmann-type reactions.

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2 X-

5.3 Electrochemical reduction reactions using room-temperature ionic liquid

123

investigation involved six imidazolium organic salts having same anionic part. The experimental data showed that the nature of RIL cations with respect to ion radical influences the dimerization of acetophenone radical anionic species [52] (Scheme 5.6). Hannah Kronenwetter et al. highlighted that pinacol coupling is very useful in CC bond formation reactions in organic synthesis. This research group demonstrated this reaction using BMITFB and water reaction media. The experimental results showed that the coupling reaction is eco-friendly and easy in a mixture of 80% BMITFBwater. This coupling reaction reported excellent yields of products with moderate diastereoselectivity. There was not involvement of any metallic byproducts formed in this reaction. RILs were recovered and reused at least fivefold without affecting the yields of products considerably. This is a simple and easy synthetic method for formation of 1,2-diols [53] (Scheme 5.7).

R

2

R

2 e-

R'

2 R'

80% BMI-TFB-H2O mixture

O

O[BMI]+

R

[BMI]+

OR'

+

2 H+

R'

dl-isomer

OR

[BMI]+

[BMI]+

R

OR'

+

2 H+

R'

meso-isomer

O[BMI]+

R

SCHEME 5.6 Pinacol coupling to synthesis pinacol products in BMITFB and water mixture.

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R

3 H

e-

R

R H

O

80% BMI-TFB-H2O mixture, Platinum cathode

OH

HO

H

+ HO

H

HO

H

R

dl-isomer

R

meso-isomer

SCHEME 5.7 Synthesis of pinacol products from aldehydes using BMITFB and H2O mixture media.

5.3.3 Electrochemical reduction of carbon dioxide gas Mahinder Ramdin and coworkers reported in their review paper that emission of major greenhouse gas such as carbon dioxide has been increasing day by day across the world. It is responsible for air pollution and global warming phenomenon. Therefore the elimination of excess carbon dioxide gas or its conversion into harmless chemicals is highly demanded [54]. There are many methods applied for the elimination of excess carbon dioxide. These methods were found to be very expensive and less effective. RILs were proposed to reduce the excess carbon dioxide gas present in our surrounding. Giulia Fiorani and coworkers reported the latest developments in the field of carbon dioxide catalysis and organomediated conversions. Many researchers utilized chemicals, photochemical, and electrochemical methods to reduce excess greenhouse gas carbon dioxide by converting it into helpful products such as alcohols, fuels, or hydrocarbons. It was reported that electrochemical method to reduce CO2 gas was found very inexpensive and environment friendly. This is easy and convenient method. It is found to be highly solvent and substrate specific [55]. The electrochemical reduction of carbon dioxide gas can be carried out by two ways. The first approach involved the conversion of excess carbon dioxide gas into alcohols, carboxylic acids, hydrocarbons, and fuels [56]. Benjamin Martindale and Richard Compton reported that the direct electrochemical reduction of carbon dioxide gas forms useful methanoic acid on the surface of platinum microelectrode. 1-butylmethylpyrrolidinium (BMPyr)BTFMSI and EMIBTFMSI were RILs utilized in the presence of source of proton. The platinum electrode was preanodized to get desirable product [57]. The second approach involved the coupling of excess carbon dioxide gas with organic compounds which were electrochemically reduced. This approach formed very useful products which can be used in the medical field. Weinberg and coworkers synthesized electrochemically 60% α-amino acid product from benzaniline derivative by carboxylation [58].

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125

Yeonji Oh and Xile Hu in their review paper described that organic molecules could be utilized as mediators and catalysts to reduce carbon dioxide gas. In this review paper, it was concluded that reactions catalyzed by organic molecules such as pyridinium derivatives and RILs are more product selective and energy efficient in comparison to metalcatalyzed counter parts [59]. Lynnette Blanchard reported that supercritical carbon dioxide gas can be utilized to separate nonvolatile organic molecules from RILs. The separation of product was possible due to carbon dioxide which dissolved in liquid [60]. Joan Brennecke and Burcu Gurkan reported that RILs having amine groups showed better results to separate CO2 and also involved postcombustion of CO2 capture. RILs were utilized without water in this investigation. This method is energy efficient for the elimination of CO2 gas from flue gas [61]. Electrochemical CO2 reduction using RILs has many benefits such as lower volatile nature and higher solubility of carbon dioxide gas in comparison to different gases in RILs. Many researchers reported conversion of CO2 gas into different useful products such as carbamates, dimethyl carbonate, cyclic carbonates, and carboxylates by electrochemical reduction and coreaction [6267]. Niu and coworkers in 2009 reported that bromo precursors can be used to prepare aromatic carboxylic acids with the help of silver electrode [68]. Organic carbonates have been utilized as solvents in different manufacturing processes in industries [69,70]. Organic carbonates are biodegradable and less harmful materials. However, organic carbonates are prepared by using highly toxic reagents such as propylene oxide or phosgene [70]. In view of this, electrochemical methods are used for the fixation CO2 gas to alcohols or carbonates [7173]. Hongzhou Yang and coworkers synthesized epoxides by electrocatalytic cycloaddition of CO2 gas using mild reaction conditions. There was not any addition of harmful organic solvents, supporting electrolytes, and catalysts in this electrocatalytic cycloaddition. The excellent yields of desirable products can be obtained by optimizing suitable reaction conditions [74] (Scheme 5.8). Dandan Yuan and coworkers reported that it is difficult to prepare dimethyl carbonate from CH3OH and CO2 directly. Therefore electrochemical technique is used to prepare dimethyl carbonate easily. Electrocatalytic activation and conversion of CO2 into dimethyl carbonate on the surface of Pt electrodes was performed at optimized conditions to get maximum yields of dimethyl carbonate. Potassium methoxide was used as cocatalyst. RIL BMIBr was utilized as an electrolyte [75]. Fangfei Liu and coworkers synthesized dimethyl carbonate by electrochemical reduction of CO2 gas at indium electrode using mild

1. Catalysis and electrochemistry

126

5. An evolution in electrochemical and chemical synthesis applications O

BMI-TFB

O

+ R

O

CO2 (1 atm pressure)

O

-2.4 V vs Silver/Silver chloride R

SCHEME 5.8 Synthesis of epoxides by electrocatalytic cycloaddition of CO2 gas using mild reaction conditions.

reaction conditions. BMITFB was utilized as RIL for obtaining 76% yields of dimethyl carbonate. There was not any addition of harmful organic solvents, supporting electrolytes, and catalysts in this electrochemical reduction of CO2 gas. Cyclic voltammetry was utilized to investigate the electrochemical behavior of electrocatalytic reduction CO2 at the surface of electrode. It was observed that the yields of dimethyl carbonate strongly influenced by quantity of charge, working electrode potential and temperature of reaction [76]. La-Xia Wu and coworkers utilized NHCylC and carbon dioxide to synthesize dialkyl carbonates by transferring carbon dioxide gas to alcohols. This synthesis was performed at mild reaction conditions to get maximum yields of dialkyl carbonates with better selectivity. This has provided excellent pathway for the fixation of carbon dioxide [77] (Scheme 5.9). There are another important class of compounds called as carbamates which are very useful in cosmetics, preservatives, and pharmaceutical fields [78,79]. These compounds are synthesized from carbamic acid. Another research groups reported that phosgenation technique can used to synthesize carbamates via rearrangement reactions. Carbamates can also be synthesized by reduction of CO2 electrochemically and coupling with amines [80,81]. This reaction is considered as green pathway for the synthesis of carbamates because there is no involvement of any harmful substances. On the other hand, chemical methods to synthesize carbamates involve harmful substances. Feroci and coworkers demonstrated electrochemically formation of CN bond using raw materials such as amines and carbon dioxide in RIL BMITFB to synthesis carbamates [63] (Scheme 5.10).

5.3.4 Electrocarboxylation reaction Yusuke Hiejima reported the electrochemical carboxylation using RILs compressed with CO2 gas. It was observed that the current efficiency and diffusion coefficient of the substrate such as α-chloroethylbenzene increases with increase in pressure as well as temperature and vice versa. There was significant improvement in mass transfer due to compression

1. Catalysis and electrochemistry

127

5.3 Electrochemical reduction reactions using room-temperature ionic liquid

H3C

N

N+ Bu X-

eH3C

N

OCH3

R

Bu

ROH

H

O

CO2 (1 atm pressure)

N

O

CH3I

X = BF4, Cl

SCHEME 5.9 Synthesis of dialkyl carbonates.

BMI-TFB

R NH R'

+ CO2

OC2H5

R N

(1) + e-

R'

O

(2) + C2H5 I Amine

Carbamate

SCHEME 5.10 Synthesis of carbamates using BMITFB.

of CO2 gas. It was reported that the sacrificial Mg electrode controlled auxiliary oxidation. This has increased current efficiency for electrocarboxylation of α-chloroethylbenzene [82]. Hiroyuki Tateno and coworkers studied and determined electrochemical properties of RIL/supercritical CO2 gas systems. This research group has described the electrocarboxylation of different halogenated organic compounds using these systems. It was observed that the solubility of carbon dioxide gas in RIL enhanced due to supercritical carbon dioxide gas. This study concluded that the RIL/supercritical CO2 gas systems are very efficient and convenient for electrocarboxylation of halogenated organic compounds. RIL DEMMOEABTFMSI was utilized in this investigation. This investigation highlighted that different carboxylic acids and their derivatives can be synthesized easily by this method. Hence, it is very advantageous in the reduction of carbon dioxide as major greenhouse gas present in our atmosphere [83]. Qiuju Feng and coworkers studied electrocarboxylation of acetophenone with carbon dioxide gas using RIL. The reactions conditions to get maximum yields of products were mild. There was not any addition of harmful organic solvents, supporting electrolytes, and catalysts in this electrocarboxylation. Cyclic voltammograms were recorded for the electrocarboxylation of acetophenone using BMITFB. This study reported 62% yields of hydroxycarboxylic acid methyl ester. It was also found that different parameters such as electrode material, current density, concentration of substrate reagents, temperature of reaction, and quantity of electric charge used alter the yields of desirable products. RIL BMITFB was recovered and reused without affecting the yields of products considerably under optimized reaction conditions [84]. It was concluded that

1. Catalysis and electrochemistry

128

5. An evolution in electrochemical and chemical synthesis applications

electrocarboxylation will play a very vital role for fixation of carbon dioxide electrochemically. Therefore RILs can be considered as green solvents. Huan Wang et al. studied electrocarboxylation of activated olefins using carbon dioxide gas saturated BMITFB solution. This electrocarboxylation was performed at pressure 1 atm and temperature 50
C in an undivided cell. There was not any addition of harmful organic solvents, supporting electrolytes, and catalysts in this electrocarboxylation. This study reported around 35%55% moderate yields of monocarboxylic acids. RIL BMITFB was recovered and reused at least fivefold without affecting the yields of products considerably [85] (Scheme 5.11). This electroreduction was found to be diffusion controlled for electrocarboxylation of ethyl cinnamate in BMITFB on glassy carbon electrode.

5.3.5 Synthesis of aryl zinc compounds Sven Ernst et al. prepared an aryl zinc compound on the surface of zinc microelectrodes by reduction of aromatic compound 1-bromo-4nitrobenzene (13.5 mM) electrochemically using RIL such as BMPyrBTFMSI. It formed a radical anion on electrochemical reduction. This radical anion reacted with zinc electrode surface and formed aryl zinc compound in vacuum when kept for the period of 3 hours to eliminate other gases and H2O. This proposed mechanism was verified by passing carbon dioxide gas through the electrochemical cell to entrap aryl zinc species like Zn(C6H4NO2)2. Zn21 ion and deprotonated 4-nitrobenzoic acid were formed in this reaction. ZnðC6 H4 NO2 Þ2 1 2CO2 -Zn21 1 2O2 NC6 H4 COO2

(5.1)

The Zn21 ions formed in the electrochemical cell will get reduced to Zn metal on the surface of electrode due to considerable negative potentials of 1-bromo-4-nitrobenzene. Cyclic voltammograms were recorded for this reduction using RIL on the surface of zinc microelectrode [86].

5.3.6 Electrochemical reductive coupling to form 1,6-diketone Renuka Manchanayakage et al. demonstrated reductive coupling of 2-cyclohexen-1-one electrochemically to form 1,6-diketone. This reductive + eCOOC2H5 C6H5

BMI-TFB, CO2 (1 atm pressure), 50 oC, undivided cell

+

C6H5

COOH

Carboxylic acid

SCHEME 5.11

COOC2H5

COOC2H5 C6H5

Electrocarboxylation of activated olefins.

1. Catalysis and electrochemistry

Reduced product

5.3 Electrochemical reduction reactions using room-temperature ionic liquid

129

O O

O

eBMI-TFB-H2O mixture

2-cyclohexen-1-one 1, 6-diketone product

SCHEME 5.12 Preparation of 1,6-diketones using BMITFB at room temperature with water.

coupling was performed using RIL BMITFB with water. This reaction yielded 90% of (1,10 -bicyclohexyl)-3,30 -dione product (Scheme 5.12). This desirable product was formed through regioselective coupling at β-carbon. During this synthesis, inherent conductivity of BMITFB was enhanced by mixing H2O. This was considered as eco-friendly and convenient method since there was no utilization of volatile organic compounds, supporting electrolytes, and any other reducing reagents. RIL BMITFB was recovered and reused at least sevenfold without affecting the yields of desired products. BMITFB and water are miscible into each other. This investigation was performed using sacrificial Sn anodic electrode and Pt cathodic electrode in an undivided cell [87].

5.3.7 Electrochemical reduction of benzoyl chloride Qiuju Feng et al. studied synthesis of benzil by electrochemical reduction of benzoyl chloride using RIL BMITFB at room temperature. Cyclic voltammograms were recorded using BMITFB for this work. There was not any utilization of harmful solvents, supporting electrolytes, and catalysts. This electrolysis was performed in an undivided cell made of silver cathode and magnesium anode. This investigation reported benzil product with 51% yield. This research group studied the effects of different parameters such as working potential, electrode material, temperature, and the nature of RIL on the yields of benzil product. Experimental data showed that all these parameters influenced the yields of benzil product. This electrocatalytic dimerization of benzoyl chloride to synthesize benzil was found to be very convenient, effective, and eco-friendly [88]. Benjamin Martindale and coworkers reported mechanistic investigation of H1 reduction on the surface of palladium. It was observed that palladium is in a partly passivated state in BTFMSI-based RIL. There was formation of Pd/H as supported by theoretical model [89].

1. Catalysis and electrochemistry

130

5. An evolution in electrochemical and chemical synthesis applications

Debbie Silvester and coworkers reported the formation of nitrosobenzene. Cyclic voltammograms were recorded for reduction of nitrobenzene as well as 4-nitrophenol electrochemically on the surface of Au microelectrode. 1-Butyl-2,3-dimethylimidazoliumBTFMSI was utilized as RIL in this synthesis [90]. Ying-Zi Liu et al. presented synthesis of 2-anisidine electrochemically using RIL BMITFB at mild reaction conditions. 2-Nitroanisole was utilized as starting material for this synthesis. This investigation reported 51.3% yield of 2-anisidine from 2-nitroanisole on the surface of coppergraphite couple electrodes. The electric current of 6.0 F/mol was passed through electrochemical cell at the temperature of 50
C. Cyclic voltammograms were recorded. This investigation did not utilize any kind of harmful organic solvents, catalysts, or supporting electrolytes to obtain 2-anisidine [91] (Scheme 5.13).

5.3.8 Organocatalysis using electrogenerated bases Sotgiu and coworkers reported synthesis of β-lactams using RILs. The starting materials used are bromoamides which underwent electrochemically induced cyclisation without using conventional solvent molecules. RILs utilized for the synthesis of β-lactams were imidazolium-based salts. This work involved C4 carbanion formation. Thereafter, C4 carbanion underwent electrochemically induced cyclisation to form β-lactams via C3C4 bond formation. This study reported higher yields of β-lactams. RILs because of their unique features played a significant role in this electrochemical synthesis. NHCylC were synthesized using dialkylimidazolium cation. This research group totally prevented the utilization of volatile organic solvents and supporting electrolytes [92]. It was also observed that diesters at C4 carbon provided excellent amount of products.

NO2 NH2 OCH3

6 e+

OCH3

6 H3O+

+ 8 H2O

BMI-TFB, copper-graphite couple electrodes, electric current of 6.0 F / mol, 50 oC

2-anisidine

2-nitroanisole

SCHEME 5.13 Synthesis of 2-anisidine electrochemically on the surface of coppergraphite couple electrodes in BMITFB.

1. Catalysis and electrochemistry

5.4 Electrochemical polymerization reactions using room-temperature ionic liquids

131

5.4 Electrochemical polymerization reactions using roomtemperature ionic liquids Many researchers have utilized RILs for electropolymerization reactions. Zane et al. in 2007 prepared thin films of poly(o-phenylenediamine) electrochemically at the surface of Pt electrodes using RIL BMPyrnonafluorobutanesulfonylTFMSI. The characterization of these polymeric films was performed by electrochemical analysis. The prepared polymeric films using RILs-based electrolyte were found to better than polymeric films using conventional sulfuric acid aqueous solution. These prepared polymeric films have strong attraction toward the surface of Pt electrode [93]. Du YanFang et al. in 2007 used EMI-Br and MITFB as solvent and electrolyte to form o-phenylenediamine at the surface of glassy carbon electrode. The electropolymerization of o-phenylenediamine was carried out with the help of cyclic voltammetry. Experimental results showed that there was easy oxidation of monomer in RIL (oxidation potential 0.725 V) as compared to aqueous acidic solution (oxidation potential 0.455 V) [94]. Carstens et al. in 2008 studied electropolymerization of C6H6 in RIL. HMITPeFETFP was used as RIL for this synthesis. During this synthesis, a polymer reference electrode was employed effectively. This investigation includes cyclic voltammetry and IR spectroscopy measurements. The band gap of poly(p-phenylene) was found to be 2.9 6 0.2 eV [95]. Mohammad Al Zoubi and Frank Endres in 2011 synthesized poly(pphenylene) nanowires using RIL by electrochemical synthesis. This study performed the template assisted electropolymerization of C6H6 at room temperature. The characterisations of synthesized nanowires were performed in this work. During this investigation, track-etched polycarbonate membranes employed as templates. The diameters of nanowires were found to be 90 nm. The aspect ratio of the synthesized nanowires was found to be higher than 100. These synthesized materials can be used in batteries or catalysis [96]. Di Wei et al. in 2006 developed polyaniline nanotubules by electrochemical synthesis using BMIHFP. This RIL contained 1 M CF3COOH. The scanning electron microscopy shows that the diameter of polyaniline nanotubules as 120 nm. This study revealed that these tubular structures are in the conducting form. This study suggested the application of polyaniline nanotubules for organic photovoltaic and light emitting diodes [97]. Keke Liu et al. in 2008 has developed a thin film poly(3,4-ethylenedioxythiophene)by electropolymerization method. In this work, BMITFB was utilized as RIL. Experimental study reported specific capacitance of 130 F/g of synthesized thin film. About 1 mol/L sulfuric

1. Catalysis and electrochemistry

132

5. An evolution in electrochemical and chemical synthesis applications

acid solution was used to find out the electrochemical performance of thin film. The increased cycling lifetime was reported as 70,000 cycles. The higher stability of polymer electrode makes it useful to fabricate supercapacitor with enhanced performance [98]. Shahzada Ahmad et al. accumulated layers on Pt decorated flat Si surfaces using electropolymerization. The hydrophobic RILs were utilized as a medium for synthesizing poly(3-hexylthiophene) film. This study reported a conductivity of 105 S/cm at 298 K. The higher cycling life and a holecurrent of 7 mV/cm at 3.0 V were found. This investigation indicated the transformation of continuous film to fibrils and then to higher porous structure. The higher porous structure of poly(3-hexylthiophene) film indicated three-dimensional hierarchical growth [99]. Yuehong Pang et al. in 2006 used potentiodynamic and galvanostat methods for the synthesis of homopolymer and copolymer with the help of BMIHFP as RIL. These synthesized polymers were characterized by different sophisticated techniques. This investigation reported color changes from deep red to deep blue for homopolymer. This study reported significant change of color from bright red to greenish blue for copolymer. This investigation concluded that copolymerization is very significant approach to obtain required electrochromic properties [100]. Yuehong Pang et al. synthesized electrochromic poly(3-methylthiophene) by electrochemical polymerization method. The derivatives of poly(3-methylthiophene) were also synthesized in this work. The synthesized polymers were characterized by different experiment techniques. BMIHFP was utilized as RIL. This study noted different colors such as bright red for poly(3-methylthiophene), orange red for poly(3-hexylthiophene), and orange yellow for poly(3-octylthiophene). There was a reversible change observed after the oxidation of these undoped polymers. These synthesized polymers possess higher chromatic contrast, longer switching stability, and higher electrochromic efficiency [101]. Yuehong Pang and coworkers in 2007 prepared poly(3-octylthiophene) by electrochemical synthesis via galvanostat method. The prepared polymer poly(3-octylthiophene) was found in orange yellow color. This undoped polymer showed a reversible change after oxidation. BMIHFP was employed as RIL for electrochemical synthesis of poly(3-octylthiophene). This polymeric thin film was utilized to fabricate electrochromic device. The electrochromic properties of thin filmbased device were studied [102]. Eric Naudin and coworkers synthesized electroactive polymer 3-(4-fluorophenyl) thiophene. This synthesis was performed by electrochemical synthesis at the surface of Pt electrode using RILs EDMIBTFMSI and DEMIBTFMSI. This study reported that the reduction of 3-(4-fluorophenyl) thiophene to the fully n-doped state. There was removal of anions and inclusion of RIL cations found in this investigation [103].

1. Catalysis and electrochemistry

5.5 Electrochemical partial fluorination using room-temperature ionic liquids

133

Damlin and coworkers in 2004 synthesized poly(3,4-ethylenedioxythiophene) polymer films by electrochemical synthesis. The characterisations of polymeric thin films were carried out using cyclic voltammetry. BMITFB and BMIHFP were utilized as RILs to prepare poly(3,4-ethylenedioxythiophene) films. There was different pattern observed at a higher degree of doping using RILs as compared to organic media [104]. Petter Danielsson and coworkers synthesized poly(3,4-ethylenedioxythiophene) using two RILs by electrochemical synthesis. BMIdiethylene glycol monomethyl ether sulfate and BMIoctyl sulfate were RILs utilized to synthesize polymer electrochemically. Poly(3,4-ethylenedioxythiophene) has shown an anionic potentiometric response in potassium chloride aqueous solution. This investigation reported that water content of RIL affects the charge transport properties of poly(3,4-ethylenedioxythiophene) [105]. Graeme Snook and Adam Best studied the codeposition of poly(pyrrole) and poly(3,4-ethylenedioxythiophene) in BMPyrBTFMSI. This study showed that the codeposition of poly(pyrrole) and poly(3,4-ethylenedioxythiophene) created one layer which was found an excellent polymeric layer. The characterization of polymeric layers was performed using different sophisticated techniques. This investigation reported that the codeposited film of two conducting polymers shows faster ionic transport, enhanced morphology, high electrical conductivity, and stronger adherence [106]. Feng Yang et al. reported that RILs are advantageous because of physicochemical properties such as easy recycling, low volatility, higher thermal stability, wider electrochemical windows, better solubility, and good inherent conductivity. Due to these salient features, RILs are extensively utilized in polymerization as solvents or electrolytes. It means that RILs play very crucial role in electrochemical synthesis and applications. These investigators studied the role of RIL as initiators in polymerization. This present investigation studied the kinetics of electroinduced polymerization of vinyl ethers using RIL as initiator. BMITFB was employed as initiator in polymerization. Real-time Fourier transform near-IR spectroscopy was used in this study. The experimental data indicated that BMITFB was found to be capable and promising initiator in electroinduced polymerization of unsaturated ethers [107].

5.5 Electrochemical partial fluorination using roomtemperature ionic liquids The incorporation of fluorine atom(s) in organic compounds gives special properties to these organofluorine compounds. These fluorinated organic compounds are very beneficial in different fields such as agrochemical, pharmaceutical, and material science [108113]. The

1. Catalysis and electrochemistry

134

5. An evolution in electrochemical and chemical synthesis applications

preparations of fluorinated organic compounds (chlorinated, brominated, and iodinated organic compounds) are very difficult. The fluorinated reagents are needed to prepare fluorinated organic compounds. The fluorinated reagents show high toxicity and corrosiveness. The fluorination is carried out at drastic conditions. The selective fluorination or partial fluorination of organic molecules by electrochemical methods was found safe, efficient, and environment friendly [114,115]. Hence, organic solvents containing poly(salts) were utilized for selective electrochemical fluorination of organic compounds. Poly(HF) salts were used as source of fluorine atom or electrolyte in this synthesis [116118]. The anodic fluorination of different organic compounds contain heteroatom was carried out by many researchers [119127]. Et3N nHF and Et4NF nHF utilized for selective electrochemical fluorination were known as adducts. Thereafter, they are categorized as ionic liquids. The product selectivity is serious problem due to formation of unwanted products in electrochemical methods. In addition, the product selectivity is influenced by applied potential/charge density. The product selectivity is also influenced by galvanostatic or potentiostatic electrochemical methods. It was also reported that methyl cyanide used as organic solvent is responsible for anodic passivation. This declines the efficiency of anodic fluorination [117,118]. Therefore anodic fluorination was performed using suitable HF salts in place of any organic solvents to prevent the anodic passivation during electrolysis [121124]. Many researchers investigated the influence of additives, solvents, reaction conditions, and substituents on the anodic fluorination of organic compounds systematically and thoroughly [128131]. Fuchigami and research group contributed a lot in the electrochemical fluorination of organic compounds. Literature survey also revealed that many review papers are published on electrochemical perfluorination or partial fluorination of organic compounds [121,132135]. Electrochemical perfluorination of aliphatic carboxylic acid chlorides has attracted many researchers across the world. This process is usually employed to synthesize fluorinated organic compounds on large scale by industries [136139].





5.5.1 Anodic fluorination of dithioacetals Bin Yin and coworkers carried out anodic fluorination of dithioacetals. Et3N nHF and Et4NF nHF were utilized for this selective electrochemical fluorination (Scheme 5.14). Experimental results showed that the product selectivity was influenced by supporting electrolytes and the substituents. It was observed that a dithioacetal forms fluorodesulfurization product due to weaker electron-withdrawing amide group. On the other hand, a dithioacetal forms α-fluorinated product due to





1. Catalysis and electrochemistry

135

5.5 Electrochemical partial fluorination using room-temperature ionic liquids

SC6H5

C6H5S

R

-2 n eF-/Methyl cyanide

F C6H5S

SC6H5 R

SCHEME 5.14 Selective electrochemical fluorination of dithioacetal derivatives.

stronger electron-withdrawing nitrile group irrespective of the poly(HF) salts utilized. This investigation revealed that all the fluorinated organic compounds displayed irreversible multiple oxidation peaks [140].

5.5.2 Electrochemical fluorination utilizing mediators Kohta Takahashi et al. reported that electrochemical partial fluorination of organic compound is eco-friendly. This anodic oxidation of organic compound introduces fluorinated atoms during the synthesis. It was frequently performed using ionic liquid such as poly(hydrogen fluoride) salts. These salts are less viscous in nature at room temperature. Hence, poly(hydrogen fluoride) salts are found suitable solvent for partial fluorination of organic compounds electrochemically. Mediators utilized in electrochemical partial fluorination of organic compounds suppressed anodic passivation (Scheme 5.15). As a result, nonconducting polymer film was produced at anode. Consequently, anode surface is no longer electroactive. Additionally, mediator enhances efficiency of reaction by increasing electron-transfer rate as compared to direct electrochemical fluorination. Many mediators are synthesized with recycle ability to develop green fluorination method. Mediator helps in the electrolysis at very low potential with small amount of charge passed as compared to direct electrochemical fluorination. Mediator helps for obtaining monofluorinated product selectively. On the other hand, direct electrochemical fluorination forms difluorinated product. The halogen mediator using poly(hydrogen fluoride) salts can be reused repetitively by extracting the product with C6H14. This system was found to be very efficient for electrochemical fluorination of ester, carbonate, and sugar derivatives. The cheaper halides such as KI and KBr are soluble in poly(hydrogen fluoride) salt and worked as halogen mediator [123]. Takahiro Sawamura et al. in 2010 reported the fluorination of organosulfur compound electrochemically in (C2H5)3N/HF under constant current. This electrochemical fluorination gave good amount of product. The polystyrene-supported C6H5I can be recycled because of its easy extraction. The study reported that mediator activity of C6H5I was not affected even after repetitive use. There was a loss of smaller quantity

1. Catalysis and electrochemistry

136

5. An evolution in electrochemical and chemical synthesis applications

S

S

F

F

(C2H5)3 N. 5 HF

4.0 F/mol R

SCHEME 5.15

R

R

R

Electrochemical fluorination using mediator.

(C2H5)3 N. 5 HF R

R EWG

SCHEME 5.16

F

3.0 F/mol EWG

α-Fluorination reaction using (C2H5)3N5HF.

of mediator C6H5I during the product separation from the IL in homogeneous mediatory system. Therefore heterogeneous mediatory system was suggested in order to recycle and reuse mediator effectively [120]. Takahiro Sawamura and coworkers continued the work and performed indirect anodic fluorination with the help of mediator iodoarene in IL HF salts. This was highly selective. The movement of mediator iodoarene having IL moiety decreases in IL having higher viscosity. This resulted in indirect electrochemical fluorination efficiently. This gives it benefit over conventional mediatory systems [119]. Kohta Takahashi et al. prepared (C6H5)3 N mediators using 4,4-dibromotriphenylamine. In this investigation, the electrocatalytic reactions of dithioacetal compounds were performed using (C6H5)3N in an undivided cell. The required deprotected and difluorinated products were obtained by electrocatalytic reaction. This study reported that mediators are more soluble in (C2H5)3N5HF because of incorporation of ionic tag [122] (Scheme 5.16).

5.5.3 Fluorination of methyl adamantane-1-carboxylate electrochemically Miki and Shoji Hara carried out fluorination reaction of methyl adamantane-1-carboxylate electrochemically using C5H5N5HF. In this reaction, the fluorinated organic compound was formed. The synthesized electrochemical fluorinated product separated from C5H5N5HF using hexaneCH2Cl2. Thereafter, the C5H5N5HF utilized repetitively. The concentration of HF presents in the complex and the yield of product declined. The yield of product was increased by supplying HF to increase its concentration. Consequently, only HF and electric current needed to perform anodic fluorination. This method is found to

1. Catalysis and electrochemistry

137

5.6 Other electrochemical reactions using room-temperature ionic liquids

profitable for bulk synthesis of fluorinated organic compound electrochemically [141].

5.6 Other electrochemical reactions using room-temperature ionic liquids 5.6.1 Electrogenerated N-heterocyclic carbenes The electrogenerated NHCylC are nucleophilic catalytic reactive species. Due to this, many researchers used it widely in the field of organic synthesis. NHCylC species are found neutral. They have a bivalent carbon atom with a sextet of electrons. Wanzlick in 1960s studied first time the stability and reactivity of electrogenerated NHCylC. His study highlighted the significance of electrogenerated NHCylC acting as ligands. Thereafter, it remained as forgotten material for around 20 years. Arduengo et al. in 1991 discovered first time the storable, isolable, and stable NHCylC. Brian Gorodetsky et al. produced nucleophilic carbene such as 1,3-bis (2,4,6-trimethylphenyl)imidazol-2-ylidene. This was formed by reducing imidazolium chloride [142] (Scheme 5.17). Marta Feroci et al. in 2012 referred electrochemically modified imidazolium-based RILs as dynamic materials and suggested as very beneficial “green” solvents [143]. Marta Feroci et al. in 2013 reported that RILs and electrogenerated NHCylC can be used as green solvents and organocatalysts [144]. To understand the versatile nature of RILs and NHCylC medium, its efficacies in various organic reactions have tested and presented here systematically. In all these investigations described here, it was found that the mixing of particular quantity of substrate into RIL/NHCylC system gave moderate to higher yields of products. 5.6.1.1 Synthesis of β-lactams Sotgiu and coworkers in 2008 reported that β-lactams can be synthesized using electrogenerated bases [92]. Marta Feroci et al. in 2008 synthesized β-lactams using BMItetrafluorophosphate (TFP) RIL and electrogenerated NHCylC [145]. It was observed that electrogenerated NHCylC are useful for deprotonating the H1 from the C4 carbon. As a + eN

N+

R

N R

R'

BF4-

N

R'

- 1/2 H2

H

SCHEME 5.17 Synthesis of carbene by electrochemical reduction of imidazolium RIL.

1. Catalysis and electrochemistry

138

5. An evolution in electrochemical and chemical synthesis applications

result, intramolecular cyclisation forms product β-lactams with better yields. During this investigation, different materials such as copper, titanium, aluminum, platinum, and carbon were used as cathode. Different RILs such as BMImethyl sulfate (Ms), BMITFP, and BMIHFP were utilized in this investigation to get maximum yields of β-lactams. This research group reported that platinum cathode electrode using BMITFP gave better yields as compared to other electrodes and RILs. 5.6.1.2 Henry reaction It is also known as nitroaldol condensation reaction. This reaction involves the formation of carboncarbon chemical bond between nitroalkanes and unsaturated carbonyl compounds. In this nitroaldol condensation reaction, imidazolium-based NHCylC play the role of catalyst. Marta Feroci et al. in 2009 reported that imidazolium-based NHCylC are stable nucleophiles or bases which plays important role to form β-nitroalcohols from nitroalkanes [146] (Scheme 5.18). This reaction took place at mild reaction conditions and gives higher yields of products. The yield of β-nitroalcohols depends on the nature of RILs, reaction time, and concentration of NHCylC. This study reported better yields of products at optimized conditions. RIL acts as green solvents and precursors. They permit to prevent the utilization of harmful volatile organic compounds as conventional solvents. It also prevents mixing of any catalysts or electrolytes. 5.6.1.3 Benzoin condensation Benzoin is α-hydroxy ketone. NHCylC are found to be very useful in organic synthesis due to their catalytic behavior. They are very reactive species. NHCylC are generally synthesized from deprotonation of azolium salts. The electrogenerated NHCylC are nucleophilic catalysts which are very useful in many organic syntheses. Monica Orsini et al. in 2009 studied the benzoin condensation using NHCylC as catalyst in imidazolium-based RILs [147] and volatile organic compounds [148] (Scheme 5.19). Four different RILs such as BMIHFP,

N

N

H3C

R'-CHO Aldehyde

+

C2H5

HO

NO2

NO2

CH3NO2

+ R'

R'

Nitromethane

N+

N

H3C

C2H5

Nitro alcohol

Nitro aldol

BF4-

SCHEME 5.18 Electrogenerated β-nitroalcohols from nitroalkanes.

NHCylC

catalyzed

1. Catalysis and electrochemistry

Henry

reaction

to

form

139

5.6 Other electrochemical reactions using room-temperature ionic liquids

OH N

CHO

N

H3C

Bu

2 O N+

N

H3C

Benzaldehyde

H

Bu

BF4-

Platinum cathode and anode electrode, temperature 65 oC

SCHEME 5.19 Benzoin condensation using NHCylC as catalyst in RIL.

BMITFP, BMIMs, and 1-benzyl-3-methyl imidazolium (BzMI)TFB were utilized for benzoin condensation. The excellent yields of products were achieved using BMITFB and BzMITFB. It was also noticed that byproduct was formed due to overreaction of benzoin product at the temperature 100
C or longer period of time (20 hours). In this study, 2.3 g BMITFB mixed in reaction mixture to produce 0.89 g of benzoin. It was found that benzoin condensation produced 84% of yield in the presence of NHCylC (20%) using BMITFB. This synthesis took 2 hours for completion. This study reported that the utilization of harmful volatile organic compounds and bases can be prevented. 5.6.1.4 Stetter reaction Triethyl amine or potassium carbonate bases were utilized as a deprotonating agent in Stetter reaction and catalyzed by thiazolium salt. Due to this, an electrogenerated NHCylC was produced in situ. The formation of carboncarbon bond in umpolung mechanism was mediated by electrogenerated NHCylC. It was also found that the yields of 1,4-dicarbonyl compounds influenced by different parameters [149] (Scheme 5.20). NHCylC was formed by bulk electrolysis and galvanostatically at the temperature of 65
C in volatile organic compounds (dimethyl sulfoxide, methyl cyanide, and dimethyl formamide) and 3-butyl-1methylimidazolium (BMI0 )TFB RIL. After electrolysis, the aldehyde and more amount of α,β-unsaturated compound mixed into the reaction mixture with continuous stirring for the period of 2 hours at the temperature 65
C and for the period of 12 hours at room temperature. In this Stetter reaction, carbene adduct was formed from the starting material aldehyde. Thereafter, carbene adduct underwent addition reaction to produce 1,4-diketone and benzoin product. It was also highlighted that yields of benzoin products were in larger quantities as compared to 1,4-diketones in BMI0 TFB RIL.

1. Catalysis and electrochemistry

140

5. An evolution in electrochemical and chemical synthesis applications OH H3C

C6H5 O

N

O

S

OH

O

C6H5 C6H5

C6H5

H

+

BMI'-TFB

SCHEME 5.20

O

1,4-diketone

Benzaldehyde

BMI0 TFB.

+

C6H5 O

Benzoin product

Electrochemical synthesis of 1,4-diketone and benzoin product by using

5.6.1.5 Staudinger reaction This involves [2 1 2] cycloaddition of imine to ketene which forms β-lactams. The synthesis of β-lactams was carried out using electrogenerated imidazolium-based NHCylC as a base and organocatalyst from CH3COCl and a deactivated imine in RIL. BMI0 TFB was electrolysed under galvanostatic conditions with the help of platinum cathode and anode at the temperature 60
C [150]. In this reaction mixture, CH3COCl and deactivated imine were mixed with continuous stirring for the period of 2 hours. The yields of product were around 60% with optimized conditions. The selectivity of trans-product was found to be 90%. The influence of different parameters such as substituents, the presence of external base, and temperature on the formation of products was investigated by Marta Feroci [151]. The plausible mechanism for Staudinger cycloaddition reaction in RIL was projected. β-Lactams were found to be in good yields. Feroci et al. in 2010 reported that the imidazole-2-ylidene mixed with RILs was found beneficial in organic synthesis. This has avoided the utilization of volatile organic compounds. It was produced easily and efficiently. This is a strong base, catalyst, and good nucleophile. This can be utilized in different reactions such as the Henry reaction, alkylation and acylation reactions, the benzoin condensation, in the preparation of β-lactams, and also the preparation of imidazole-2-thiones. It was reported that there was a formation of carbene, imidazole-2-ylidene, and molecular hydrogen at the surface of cathode [152]. 5.6.1.6 Preparation of γ-butyrolactones Isabella Chiarotto et al. in 2011 performed synthesis of γ-butyrolactone from umpolung of α,β-unsaturated aldehydes using electrically generated NHCylC. It was highlighted that NHCylC played a role of precatalyst and solvent [153]. In this investigation, BMI0 TFB RIL was electrolysed. During this experiment, α,β-unsaturated aldehyde and ketone were mixed in the reaction mixture. Four different RILs were utilized in this synthesis. It was found that BMI0 TFB and

1. Catalysis and electrochemistry

141

5.6 Other electrochemical reactions using room-temperature ionic liquids

BzMITFB RILs gave excellent yields of products at reaction conditions such as temperature 80
C and time duration of 2 hours. Different molecular solvents such as dimethyl sulfoxide, methyl cyanide, and dimethyl formamide were employed in this synthesis of γ-butyrolactones. These molecular solvents gave lower yields for the cross-annulated γ-butyrolactones products. It was also highlighted that cis- and transisomeric products were formed. Simple column chromatography technique was used to purify all the synthesized products. RILs can be reused and reactions were completed within short time (2 hours) in NHCylC catalyzed reactions. On the other hand, electrogenerated NHCylC catalyzed reactions in molecular solvents were completed in longer time (16 hours) [154156]. 5.6.1.7 Esterification reaction Marta Feroci and coinvestigators obtained first time esters using unsaturated aldehydes in RILs through umpolung reaction and electrically generated NHCylC acting as catalyst. This investigation reported moderate to excellent yields of esterified products in RILs. These investigators performed and analzsed the influence of charge density, nature of RIL, mole ratio of aldehydes to alcohols, etc. on the product formation thoroughly. This investigation highlighted that different parameters influenced the formation of ester. This investigation reported 50%91% yields of esterified products [157] (Scheme 5.21). The electrically generated NHCylC was simply prepared under galvanostatic reduction of BMI0 TFB RIL. Thereafter, NHCylC acted as catalyst for internal oxidationreduction reaction. In this investigation, amidation of α,β-unsaturated aldehydes was achieved without the imine formation [158]. The experimental results were compared between organic solvents using classical procedures and RILs using electrochemical procedures. 5.6.1.8 Transesterification Isabella Chiarotto and coworkers reported that transesterification in BMI0 TFB RIL with an electrogenerated NHCylC acting as catalyst. This transesterification reaction formed nicotinate esters with good yields. BMI0 TFB was recovered and reused without affecting the yields of the products formed. It was also found that nicametate and benzyl O

O HO C6H5

H

+

NHCylC BMI'-TFB

SCHEME 5.21 Esterification of α,β-unsaturated aldehydes.

1. Catalysis and electrochemistry

O

142

5. An evolution in electrochemical and chemical synthesis applications

nicotinate products with good yields can be prepared by using ester such as methyl nicotinate and alcohols such as 2-(diethylamino)ethanol or C6H5CH2OH. This research group projected a plausible mechanism for this electrogenerated NHCylC-based transesterification reaction. The mechanism showed that NHCylC help for transferring the proton effectively. It was highlighted that benzylic alcohols and vinyl alcohols can be undergo transesterifcation using NHCylC as catalyst in RILs [159]. 5.6.1.9 Oxidative esterification of aromatic aldehydes Gianpiero Forte et al. prepared aromatic esters from aldehydes and organic Brs using BMI0 TFB RIL. NHCylC obtained from BMI0 TFB acts as catalyst as well as base in the reaction. It prevents the need of external bases. RIL was recovered and reused up to nine times. The synthesized product was extracted using C2H5OC2H5 solvent. This research group highlighted that it was the first successful attempt of oxidative esterification mediated by NHCylC in BMI0 TFB [160]. 5.6.1.10 Preparation of N-acyloxazolidin-2-ones Feeney et al. in 2009 reported that oxazolidin-2-ones can be N-acylated with BMI0 TFB RIL using NHCylC as a base. This mixture electrolysed using galvanostatic control (I 5 16 mA/cm2) under nitrogen atmosphere with the help of platinumplatinum electrode system. The saturated or unsaturated anhydrides (0.5 mmol) were mixed into this electrolysed solution. The electric current was switched off when 1.0 F/mol of chiral oxazolidin-2-one was consumed. This reaction mixture was placed for continuous stirring for the period of at least 3 hours at room temperature condition. Thereafter, solvent C2H5OC2H5 was utilized three times to extract the cathodic solution. Subsequently, C2H5OC2H5 was separated from the residue at low pressure applied. Then, N-acyl derivative was obtained with the help of flash chromatography technique in which a mixture of hexaneethyl acetate (ratio 8:2) was utilized. This study reported the better yields of N-acyloxazolidin-2-ones. It was observed that there was an involvement of base in deprotonation [161]. 5.6.1.11 N-Functionalisation of benzoxazolones Chiarotto et al. in 2009 reported derivatives of benzoxazol-2(3H)-ones with better yields. These investigators showed better results using RIL such as BMI0 TFB preventing the utilization of volatile organic solvents. In addition, RIL recovered and thereafter utilized five times efficiently. The reduction of RIL at the surface of cathode electrode produced electrogenerated carbene. This shows the involvement of base in deprotonation. The mixing of anhydrides in reaction mixture formed the desirable products with better yields. The study highlighted that reusing the RIL efficiently makes it very beneficial for electrochemical and organic synthesis [162].

1. Catalysis and electrochemistry

5.7 Conclusions

143

5.6.2 Functionalisation of nitroaromatic compounds Hugo Cruz et al. reported that RILs can be utilized as solvent to prepare substituted nitroaromatic compounds. This approach of utilizing RILs instead of organic protic solvents was found to be attractive “green route” in this synthesis. In this investigation, four RILs were utilized. Here, RILs exhibit different solvation effects which enhance the regioselectivity. BMIacetate, BMIHFP, BMITFB, and BMIBTFMSI were utilized as RILs. In this investigation, classical hydride, ketones, amines, cyanides, and methoxide were utilized as nucleophiles for studying the feasibility of this electrosynthesis reaction. The different nitroarenes were chosen for this study. The regioselectivity and electrocatalysis effects were studied by utilizing four different RILs. The experimental results are found to be better using RILs. This electrosynthesis approach to synthesize substituted nitroaromatic compounds using RILs was found cost-effective, eco-friendly, and efficient [163].

5.6.3 Epoxidation reaction using room-temperature ionic liquids Wong et al. had electrosynthesized H2O2 from O2 gas using BMI0 TFB and further utilized in epoxidation of alkenes. It was reported that BMI0 —TFBwater or sodium hydroxide mixtures are potential electrolytes to electrogenerate H2O2 in situ effectively. This entire process is clean system since it involves H2O, O2, and electricity. A large number of substituted cyclohexenes were epoxidised with around 80% of yields [164].

5.7 Conclusions RILs are found very advantageous because of prominent features such as wider electrochemical windows, better conductivity, lower volatility, higher thermal stability, and tunability. Many RILs, which are made of cationic components such as tetraalkyl ammonium, imidazolium, pyrrolidinium piperidinium, and anionic components such as HFB, TFB, and BTFMSI, were utilized for the electrochemical organic synthesis successfully. Most of these salts are nontoxic, inexpensive, easily available, convenient, recyclable, reusable, energy efficient, and selective. BMI-based salts with different anionic counter parts are extensively utilized for electrochemical synthesis as solvent and an electrolyte. The NHCylC could be regenerated electrochemically in situ by using imidazolium-based salts. The NHCylC are electrogenerated nucleophilic bases and catalyzed many reactions using RILs at optimized conditions

1. Catalysis and electrochemistry

144

5. An evolution in electrochemical and chemical synthesis applications

to obtain maximum yields of desirable useful compounds. In this chapter, many electrochemical organic syntheses using RILs are discussed systematically in detail. Firstly, electrochemical oxidation reactions such as oxidative selfcoupling, Shono oxidation, and bromination reaction in RILs were reviewed thoroughly. These reactions produce radical cation intermediate which is stabilised by RILs. Oxidative self-coupling reactions showed better yields to synthesize dimerized products from anisole, mesitylene, naphthalene, and anthracene. Shono oxidation of carbamates using TBDATSL and 33% methyl alcohol as the methoxylating agent provided better yields of desirable products. Volatile organic compounds can be utilized as cosolvents to prevent or minimize the over oxidation of materials. They are not ecofriendly and affect the significance of RILs. The use of RILs with low viscosities and wider electrochemical window for Shono oxidation of carbamates provided better yields of products. It was found that primary alcohols (C6H5CH2OH, halogenated C6H5CH2OH) oxidize to aldehydes with good yields when 2 F/mol electric charge was passed irrespective of the alcohol substrate. The electrochemical oxidation of secondary alcohols produced 50%60% ketone products. The electrochemical oxidation of C6H5CH2OH to C6H5CHO using BMIHFP yielded 90% C6H5CHO at the surface of electrode. The direct oxidation of Br using RILs and conventional organic solvents yielded different products due to their different nucleophilicities. It suggested potential application of bromination reaction in organic electrosynthesis on a huge scale due to the electrogenerated bromine from Br. The electrochemical reduction produces radical anion intermediates that are stabilised by RILs. This stabilization shows significant effect on the yields of products formed. The reduction of CO2 electrochemically using RILs is a promising field for researchers to explore. RILs considered as “ruler of solvents” to reduce CO2 gas electrochemically. This is due to unique physicochemical properties of RILs. There is direct reduction of CO2 gas electrochemically and then coupling with alcohols for obtaining carbonates at optimized conditions with better yields. Similarly, coupling with amines gave carbamates using same approach. The second approach of electrochemical reduction involves reduction of halogenated arenes to corresponding reactive radical species and then coupling with carbon dioxide gas to obtain carboxylic acids at optimized conditions for better yields. Many researchers reported conversion of carbon dioxide gas into different useful products such as carbamates, dimethyl carbonate, cyclic carbonates, and carboxylates by electrochemical reduction and coreaction. These approaches found very inexpensive and environment friendly. This is an easy and convenient method. It is found to be highly solvent and substrate specific. In these

1. Catalysis and electrochemistry

Abbreviations

145

two approaches, carbon dioxide is utilized and converted into useful carboxylic compounds. This reduces the excess of carbon dioxide present in our atmosphere RILs. Many researchers have utilized RILs for electropolymerization reactions. Polymeric thin films can be prepared on the surface of Pt electrodes. The prepared polymeric films using RILs-based electrolyte were found to better than polymeric films using conventional sulfuric acid aqueous solution. These prepared polymeric films have strong attraction toward the surface of Pt electrode. The selective fluorination of organic molecules electrochemically was found safe, efficient, and environment friendly. Hence, organic solvents containing poly(HF) salts were utilized successfully. The perfluorination of aliphatic carboxylic acid chlorides electrochemically found useful for synthesizing fluorinated products on large scale. RILs were recovered and reused without affecting the yields of desirable products considerably in electrochemical organic synthesis. These were extracted by using organic solvents. RILs are referred as “green solvents” by many investigators. This is due to nontoxicity, reusability, and recyclability of RILs. However, many RILs are costly from industrial point of view. It was also observed that harmful volatile organic solvents were needed to separate the product in reactions where RILs used as solvents. This affected green aspects of RILs. This shows that RILs are not 100% green solvents. Many researchers reported that the use of RILs has completely avoided the involvement of harmful volatile organic solvents, catalysts, any other coelectrolyte, and use of additional reducing agent in electrochemical organic synthesis. Therefore RILs can be refereed as green solvents up to some extent than harmful volatile organic solvents. However, there is a lot of scope available in the field of electrochemical organic synthesis to utilize various RILs for selectivity, recyclability, reusability, cost-effectiveness, and better yields of products. These future investigations will certainly utilize only RILs in the field of electrochemical organic synthesis to open up a way toward efficient green chemistry.

Abbreviations BMI BMI0 BMPyr Br BTFMSI BzMI DEMI DEMMOEA

1-butyl-3-methylimidazolium 3-butyl-1-methylimidazolium 1-butyl-methylpyrrolidinium bromide bis(trifluoromethylsulfonyl)imide 1-benzyl-3-methylimidazolium 1,3-diethyl-5-methylimidazolium N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium

1. Catalysis and electrochemistry

146 EDMI EMI HFP HMI MI Ms NFBS OMI TBA TBDA TEBA TFB TFMSI TFP TMBA TPeFETFP TSL

5. An evolution in electrochemical and chemical synthesis applications

1-ethyl-2,3-dimethylimidazolium 1-ethyl-3-methylimidazolium hexafluorophosphate 1-hexyl-3-methylimidazolium N-methylimidazolium methyl sulfate nonafluorobutanesulfonyl 1-octyl-3-methyl imidazolium tetrabutylammonium tributyldecylammonium triethylbutylammonium tetrafluoroborate (trifluoromethanesulfonyl)imide tetrafluorophosphate trimethylbutylammonium tris(pentafluoroethyl)trifluorophosphate tosyl

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C H A P T E R

6 Recent changes in the synthesis of ionic liquids based on inorganic nanocomposites and their applications Raju Kumar Sharma1,2, Jamal Akhter Siddique3, Chien-Yen Chen2 and Jyoti Prakash Maity2,4 1

Department of Chemistry and Biochemistry, National Chung Cheng University, Min-Hsiung, Chiayi County, Taiwan, 2Department of Earth and Environmental Sciences, National Chung Cheng University, Min-Hsiung, Chiayi County, Taiwan, 3Marie Curie fellow (List-B), SASPRO-2, Slovak Academy of Sciences, Bratislava, Slovakia, 4Department of Chemistry, School of Applied Sciences, KIIT Deemed to be University, Bhubaneswar, Odisha, India

6.1 Introduction 6.1.1 Inorganic nanocomposite materials—an overview Composite material is fetching an important role in the current field of research, which is generally formed by two different kinds of materials with dissimilar physico-chemical properties [1]. In another word, the composite material is produced using different heterogeneous/phases materials to improve the surface properties [2], where one of the materials among them must have less than 100 nm in pore size. The nanocomposite material synthesis has been reported last more than five decades [3], which signified highly efficient materials than conventional materials. Inorganic nanocomposite materials comprise very specific properties

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such as rigidity and high thermal stability and are greatly used for easy incorporation of other materials (e.g., organic materials, polymers, biomaterials, minerals, metals, etc.) to enhance the toughness, mechanical strength, and thermal or, electrical conductivity. Organic materials have a great interaction with inorganic nanocomposite materials due to their ductility, flexibility, and processability, which introduce the building blocks of inorganic materials at the nanoscale with a high surface-tovolume ratio. Additionally, inorganic nanocomposite materials were introduced to increase molecular strength, optical activity, surface activity, etc. [4,5]. Generally, the small molecules (APTES), biomolecules (LOX, HA, etc.), photosensitizers (Ce6, Porphyrin, etc.), drugs (DOX, GMP, etc.), polymers (PEG, PLGA, etc.), and immune active substance (OVA, anti-PD-1, CpG, etc.) were used as organic compounds to synthesize the nanocomposite materials, where, inorganic ions (Cu21, Cr31, Ti31, Ru21, Pd21, Cd21, Fe21, Mn21, Hf41, etc.) and inorganic nanoparticles (Au NPs, Ag NPs, Fe3O4, MnO2, MoSe2, Graphene, SiO2, CaCO3, UCNP, etc.) are used to synthesize the nanocomposite materials [6
8]. Moreover, the combination of these two or, more types of materials (Organic and Inorganic) was responsible to prepare the organic-inorganic nanocomposite particles in material science. There are various types of hybrid inorganic nanocomposite materials, was shown as follows; 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Organic-inorganic based nanocomposite materials Mesoporous silica-inorganic based nanocomposite materials Polymer-inorganic based nanocomposite materials Bioinspired-inorganic based nanocomposite materials Metals-inorganic-based nanocomposite materials MWCNT-inorganic based nanocomposite materials Carbon-inorganic based nanocomposite materials Modified organic-inorganic based nanocomposite materials Modified polymer-inorganic based nanocomposite materials Modified metal ions-inorganic based nanocomposite materials.

6.1.2 Development of inorganic nanocomposite materials synthesis The inorganic nanocomposite materials were being synthesized in the 15th century in the research field. The incorporation of gold and silver nanoparticles with glass as inorganic nanocomposite was first time prepared and modified in the 15th century by A. Libavius (1595) and 16th century by A. Cassius (Mid), A. Neri (1614), J. R. Glauber (1648) and O. Tachenius (1668). Furthermore, the different types of color nanocomposite materials were synthesized in the early 18th century as violet, the brown and black colors of gold nanomaterials in

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paint. In the 18th century, Richter reported that the change in color of nanoparticles mainly depended upon the observation angle, which was ensured using refraction of light in their optical study. In 1905, Maxwell Garnett modified the gold-glass nanocomposites through the heating whilst formation of ruby glass. Moreover, the types of elements such as Cu, Hg, P, As, Sb, Bi, S, Se, Te, Br, Rh, Os, Pt, Pd, I, etc. were also used in the synthesis of nanocomposite materials (dichroic fibers) [9]. In the meantime, a carbon black nanoparticle was synthesized to strengthen the rubber through filler in 1904, which was greatly useful in the rubber industry to enhance the softness, dispersion, and strength of polymer chains in rubber [10]. In the 20th arena of research, nanocomposite materials have shown excellent and promising results due to the advancement of their mechanical, optical, and thermal properties in synthesized materials [11]. The key challenges of nanocomposite materials were considered to scale-up progressions and their interaction between nanostrengthening and matrix [12]. In recent years, the wound healing application and antimicrobial activity in food packaging indicated the great importance of inorganic nanocomposite materials in the chemical and food industries [13
15]. Moreover, the synthesized nanocomposite materials are used in different advanced applications such as electrical devices, water treatment, polymer/molecular semiconductors, oil recovery, energy storage, sensors, medical sciences, etc. [16
25].

6.1.3 Role of ionic liquid in the synthesis of inorganic nanocomposite Ionic liquids (ILs) refer to the combination material of two or, more ions (cations and anions) to form the salt, where the melting point of salt was limited to standard ambient temperature (,100 C) or, room temperature. In most of ILs materials, the ILs included organic cations and inorganic anions with different suitable polarities [26]. Generally, the ILs are greatly thermodynamic favorable or, less kinetic favorable due to bulky size and their conformational flexibility behavior of ions, which is subjected to lessen the enthalpy and enhance the entropy of salt [27]. The increment of entropy is supported by the melting point of salt, which is projected to enhance the accuracy of ILs. The ILs were acknowledged exclusively for thermal and electrical properties with their low toxicity [28,29]. Recently, the ILs were demonstrated in industrial and analytical chemistry as a solvent, where solvent like ILs was greatly emphasized in microextraction techniques, electrophoresis, and chromatographic process [30,31]. Moreover, the ILs were categorized in three different ways, that is, simple salts (one cation and one

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anion ILs), binary ILs (more than one cation and anion ILs), and polymer ILs (two or, more cations and anions ILs). A general formula can also be predicted for different kinds of ILs such as 1. Simple salts ILs: [A]1[X]2 2. Binary salts ILs: [A]a[B]b[X] and [A][X]x[Y]y 3. Polymer salts ILs: [A]a[B]b[X]x[Y]y. . ., [A]a[B]b[C]c[X]. . . and [A]a[X]x[Y]y[Z]z. . . (Where, A and B: Cation ILs, X and Y: Anion ILs, a: mole fraction of [A], b: mole fraction of [B]: x 5 X{[X]} and y 5 X{[Y]}) Considering simple salts, there have been specified many examples, where cations was proposed as imidazolium, piperidinium, morpholinium, phosphonium, N-alkyl-pyridinium, tetraalkyl-ammonium, cholinium, choline, etc., and anions was projected as Chloride, bromide, dicyanamide, triflate, bis(trifluoromethylsulfonyl)imide, methyl sulfate, tetrafluoroborate, ethylsulfate, hexafluorophosphate, tosylate, carboxylate, alkylsulfate, adipate, etc. In case of binary ILs, there had been reported many examples, which was given as [o-Xyl-(bimS)2][Cl]2, [pXyl-(bimS)2][Cl]2, [m-Xyl-(bimS)2][Cl]2, [p-Xyl-(bimS) (bimS)][Cl]2, etc. [32]. Finally, polymer ILs are very useful materials in current research fields, where some of the examples are followed as [CnC1im][NTf2] (n 5 4
16), Terpy-PIL [{C27H22BrN5}n], poly(3-methyl-1-vinylimidazolium iodide), poly(3-n-propyl-L-vinylimidazolium iodide), poly(3-nhexyl-L-vinylimidazolium iodide), poly(3-n-heptyl-L-vinylimidazolium iodide), poly(3-n-dodecyl-1-vinylimidazolium iodide), etc. [33]. The ILs were also unified with nanocomposite or, modified nanocomposite materials to produce the ILs metal-organic frameworks (ILs-MOFs), which are highly efficient to use in purification, chemical reactions, separation, catalysis, gas storage, sensors (Nanosensor or, biosensors), extraction (microextraction), etc. [34
37]. The ILs-MOFsbased synthesized nanocomposite materials were a promising group of materials to challenge the uniform pore size, adjustable pore structure, easy synthesis, and structural diversity. Additionally, this material is accomplished with high thermal stability during the regulation of physical and chemical properties [38]. The most interesting ILs nanocomposites were categorized into four types, that is, polyionic liquids (PILs), task-specific liquids, room temperature liquids, and supported IL membranes, which were generally used to help in the formation of MOFs via intermolecular interactions between different ions [39]. The interactions of ILs with MOFs have concluded a better outcome in their physicochemical properties in recent studies, where the mesoporous oxide networks with ILs were shown high ionic conductivity, effectual chromophore solvation, and slight decrement of dynamics in monolith [40].

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6.1.4 Application-based importance of ionic liquids in inorganic nanocomposite ILs-based inorganic nanocomposite materials are currently shown a great ranking in advanced nanotechnology. These nanocomposite materials were used to prepare the advanced technological instruments, which are popular to develop nanolevel devices effectively. Mainly, it covers credit their surface activity in most of the industrial applications, were used in different fields of science and technology such as chemical industries, lithium/sodium-ion battery, drug delivery systems, bone substitutions, biosensors, pollutants removal (heavy metals: As, Pb, Ni, Cu, Cr, Co, PBV (patent blue V) dye), CO2 adsorption, ammonia adsorption, fuel cell, etc. [41
50]. Moreover, ILs were also used as a bridging agent in the synthesis of mesoporous materials such as SBA-CIL-CSPPL (Santa Barbara Amorphous-carboxyl-functionalized IL-chitosanporcine pancreas lipase), which are generally using in medical science for the provision of enzyme immobilization [51]. This type of mesoporous material improves the activity, stability, and recyclability at optimum pH and temperature. Most importantly, ILs could be combined with minerals such as Montmorillonite, kaolinite, etc., which are highly stable and efficient as compared to materials. Considering the excellent properties of these composite materials, they could be versatile nanosupport materials to improve the chemical or, biological reactions, and their separation processes.

6.2 Synthesis of inorganic nanocomposite materials using ionic liquid 6.2.1 Sol-gel method The sol-gel method is a very significant and innovative method to synthesize the nanomaterials/nanocomposite/ILs nanocomposite materials (Fig. 6.1). Generally, the Sol-gel process is a combined reaction of hydrolysis and condensation process using different types of precursors, where the hydrolysis of chemicals is easy to control in this method at room temperature [52,53]. The sol-gel method is also called the inorganic polymerization reaction process, which precedes the reaction system in aqueous and nonaqueous mediums. However, Dai [54] developed the sol-gel process first time to synthesize the inorganic materials using IL as a template/medium with an organic solvent. Additionally, 1-n-ethyl-3-methylimidazolium bis(tri-fluoromethylsulfonyl) imide was used with silica (precursor) to form the gel. Interestingly, some of the ILs behave as a hydrophobic and more polarizability character,

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FIGURE 6.1 Classification of methods of nanomaterials/nanocomposite/ILs nanocomposite materials synthesis.

which stimulates the well-ordered and homogeneous distribution of synthesized inorganic nanocomposite. Furthermore, it is also constituted a high surface area with less pore size distribution on the surface of materials. This synthesis process explored the great combination of ILs with functional inorganic nanomaterials to form a long chain of the hydrophobic tail and well-ordered lyotropic molecules [55]. Recently, Suo [56] conveyed a very important application, where ILs were supporting liquid salt to modify the cellulose and coated it

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with magnetic nanoparticles to enhance the activity and the stability in enzyme immobilization. In addition, this method-based synthesized material also increases the recovery rate of enzyme immobilization easily and efficiently, where synthesized nanoparticles played an important role to support in immobilization process. Moreover, Zhang [57] designed an advanced electrode using ILs-based nitrogen-doped porous carbon materials, which was further used as a fabrication to exhibit the high capacitance of the supercapacitor. However, presently, this type of advanced material is generally supported in environmental and medical applications via different processes.

6.2.2 Hydrothermal method A very unique and important branch of inorganic materials synthesis was established as a hydrothermal method in the material science era (Fig. 6.1). This method is generally used for the crystallization of constituents at high temperatures and pressure in different vessels. Byrappa [58] reported an effective approach first time to synthesize the zeolite nanocrystals using the hydrothermal method. This method was signposted various significant advantages over other types of methods. Remarkably, it is an effectual process in the synthesis of unstable nanoparticles, which lost their homogeneity of particles on the surface at higher temperatures and pressures [59]. There was particularized in the synthesis of ILs-based nanocomposite materials, which is majorly improved thermal stability and surface area of synthesized particles [60
62]. On the other hand, the significant disadvantage of the hydrothermal process was proposed as the higher cost of the instrument. This process also needed more space to adjust their instrument. Therefore, it results in a very expensive and time-consuming process, where this method is a more efficient and useful process as compared to other methods. Recently, a new method was approached as one-pot hydrothermal synthesis to reduce the cost and space in the new research arena, which process is exclusively established to prepare the optimized and stable materials with control of their composition, particle size, and their shape [63]. However, the efficiency of chemical reactions was amplified using autoclave steel with a high surface area of nanocrystals [64].

6.2.3 Microemulsion method The microemulsion method is one of the thermodynamically stable and specific micelle formation processes, which exploits the small size of particles via a reverse process (Fig. 6.1). The micelles are

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generally formed through two immiscible liquid components such as water and oil, where surfactants and co-surfactants (e.g.,
OH,
NH2, etc.) are also responsible for the formation and stabilization of micelles [65,66]. It was found that the microemulsion method can control the physicochemical properties of materials such as surface area, particle size, homogeneity, morphology, and geometry [67]. These types of different unique properties were exhibited due to the polar nature of ILs, which can easily provide the source to combine with nanomaterials/ nanocomposite materials [68]. Additionally, these materials can furnish a virtuous platform to use in several industrial applications. Conversely, the major drawback of the microemulsion method is signified in three categories (1) limited solubility at high temperatures, (2) temperature instability via pH and microemulsion, and (3) required a higher amount of surfactants. However, an advanced application was reported as poly (methyl methacrylate)
TiO2 nanocomposite supported through [bmim] [BF4] IL in this method, which was substantially used as a photocatalyst. The synthesized nanocomposite materials exhibited more efficiency to degrade the concentration of methylene blue pollutants from wastewater [67,69]. Recently, a new photoinduced IL-based nanocomposite material poly(methyl methacrylate) (PMMA)/TiO2-IL was introduced, which was able to enhance the hydrophilicity of nanomaterials [70]. Furthermore, a new eco-friendly cellulose/Silver nanocomposite material was synthesized using the castor oil on cellulose surface, where the material surface was modified through IL 1-ethyl-3-methylimidazolium acetate for easy and homogeneous separation of nanoparticles. The synthesized material was further applicated to diminish the 4-Nitrophenol through catalytic activity [71].

6.2.4 Precipitation and co-precipitation method A very crucial, easy, and promising technique was developed to change the state of matter from liquid to solid. The precipitation method illustrated the most common process in science and engineering arenas to synthesize the materials using a significant precipitating agent (Fig. 6.1). Primarily, the influence of the precipitation method depends upon the change in pH and precipitating agent, where, the interfering ions are also responsible to disturb the precipitation progression [72]. Recently, this method was significantly projected to prepare the advanced electrochemical sensor, increase the removal efficiency, easily metal co-detection, photo-degradation, adsorption, separation, etc. [73
78]. Some precipitation method-based synthesized particles are generally decomposed easily at very high temperatures due to less interaction between their bonding, where the co-precipitation process

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helps to reduce the temperature of decomposition in the reaction procedure and provides the strong bonding between ions of different particles [79]. Interestingly, in a few cases of nanocomposite material synthesis, the capping agent is not required and the SEM images can also be depicted easily [80]. Furthermore, this method is less timeconsuming, low cost, recyclable and large amount of final product and it increases the efficient bonding in the liquid medium of reaction [81].

6.2.5 Rays mediated method The rays mediated process was signposted as a direct method of particle synthesis through the irradiation process of light (Fig. 6.1). This process is a highly flexible, appropriate, and selective method for nanoparticle synthesis, where light is passed through metal ions to produce metal nanoparticles. The ultra-violet radiation, visible light, and gamma rays have played an important role to generate the nanoparticles over this advanced method. In this method, the metal ions are mixed with different solvents to make a colloidal solution, where the UV or, gamma rays are passed through the prepared colloidal solution. In the meantime, some of the colloidal solutions are required stabilizers and different rays to synthesize the nanoparticles [82]. This method was mainly categorized in three ways: (1) Photochemical method, (2) Photocatalytic deposition, and (3) Sonochemical method [83,84]. 6.2.5.1 Photochemical method The metal nanoparticles are produced using a direct light source or, a light source with a sensitizer in the photochemical method. This method is a more appropriate, less expensive, and clean process and it is easily controlled due to the unavailability of reducing agents in material synthesis [85]. The continuous penetration of light over metal ions was signified the main key benefits, which were mainly responsible to produce the homogeneous nanoparticles. Additionally, the control of exposure time and temperature is a great challenge in the current arena of research, which was significantly controlled in this method. However, this method will be illustrated more fruitful results in batch schemes. 6.2.5.2 Photocatalytic deposition The photocatalytic deposition process is an easy and less timeconsuming process, where the metals are generally adsorbed on the surface of supporter materials (e.g., TiO2, ZnS@CdS, etc.) using the light source (e.g., UV-source) to produce the more efficient materials. The produced materials are significantly used in water, soil, and air treatment [86]. Furthermore, it is also used in H2 production, disinfection of

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bacteria, and immobilization procedure. The main drawback of this method is less recovery rate and poor thermal stability. 6.2.5.3 Sonochemical method The sonochemical method is one of the rays mediated processes, which is established as a unique process of material synthesis using ultrasonic waves. This method was used to synthesize the nanomaterials and mesoporous materials. Generally, the bond between molecules are get fragmented whilst passing the ultrasonic waves (20 kHz
10 MHz) in the reaction procedure of synthesis. The method of synthesis is easy and it can be used at the industrial level as a green methodology [87]. Moreover, the morphology of materials was also controlled easily.

6.2.6 Electrochemical method The electrochemical method was a renowned and efficient process in electrochemistry and materials chemistry (Fig. 6.1). This method is mainly depending upon the electrode reaction between two or, more different ions and produces the particles. It is generally used to measure the chemical reaction activity of various analyte concentrations/charge, potential, or, current, which is helpful to synthesize the nanoparticles. It was expected that thickness and shape can be controlled significantly and synthesized materials could be highly uniform. Furthermore, the nanoparticles or, films could be also deposited on the substrates and determine the complex shape of the particles. The IL was expected to be greatly electrochemically stable, nonvolatile, and, highly conduction solvents to modify and synthesize the nanocomposite materials. Besides, a unique method was introduced as the plasma electrochemical method, which can combine with ILs and generate metal nanoparticles with different pore sizes. This method is very helpful to prepare the semiconductor easily using ILs [88,89].

6.3 How organic-inorganic is different from inorganic nanocomposites? The inorganic nanocomposite materials are generally considered as chemical inertness, high thermal stability, a significant amalgamation of dielectric properties, and a low melting point of electrolytes [90]. In addition, it is predicted to have low vapor pressure and solvation capacity in a new types of materials. Inorganic nanomaterials comprise less thermal stability and their applications in the magnetic, electrical, mechanical, and optical study were signified as less compatible as compared to IL-based organic-inorganic nanocomposite materials (nano/

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meso). On the other hand, the combination of nanoparticles with ILs and other inorganic or, polymer organic materials devised the nanosized (1
100 nm) of inorganic materials with organic components. Most commonly, organic/inorganic nanocomposite materials are available in nature (e.g. plants, minerals, etc.) [91], which were modified using the IL salts artificially in the lab to intensify the reactivity, thermal stability, ductility and reduce the melting or, the boiling point. In these nanocomposite materials, generally organic components polysaccharides molecules, drugs, small particles, immune active constituents, photosensitizers, nucleic acids, proteins, carbohydrates, lipids, etc., were involved as an eco-friendly components, whereas inorganic components comprise metal ions and inorganic nanoparticles [6]. This type of specific combination of materials with ILs provides an effective particle, which will be useful at a large scale in different industrial applications. Remarkably, to some extent, the incorporation of both organic and inorganic components could also produce synergistic effects, which would be greatly beneficial in medical applications (Fig. 6.1).

6.4 Recent advancements and advantages of inorganic nanocomposites with ionic liquids 6.4.1 Storage of heat energy ILs are one of an evolving group of green compounds synthesis, which has a great capacity to enhance the thermal stability at very high temperatures and generates a huge amount of energy storage. Due to the high compatibility of this chemical compound, the nanoparticles are incorporated with an IL to improve the collection of heat energy in solar panels [92]. Additionally, the ILs incorporated with silica nanoparticles and used in sorption desalination devices exceed the performance of water sorption capacity. 6.4.1.1 Advantages 1. Increase electrode reaction kinetics 2. The high amount of energy storage capacity 3. Less volatile composite materials 4. Reduce toxicity 5. Low cost.

6.4.2 Electrolytic support The electrodes are generally highly sensitive and volatile, which is handled carefully in chemical reactions. There are many electrolytic

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support chemicals, which was used to increase the electrode reaction performance such as acetonitrile, acetone, DMF, DMSO, water, etc. However, some of the interferences were still less governable for green industrialization. For example, recently lithium-ion battery production caused a lot of problems due to slow electrode reactions, less mass transfer, volatility, and low production rate, which was overcome through the sodium-ion battery. The non-volatile, inflammable, and temperature control chemical as ILs was supported by the sodium battery to improve the production rate and their performance in industries [93]. 6.4.2.1 Advantages 1. Highly sensitive 2. Recyclability 3. High current density 4. Easily hormonal or, drug detection 5. Simultaneous detection.

6.4.3 Solvents improvement The solvents play an important role in material synthesis and their polymerization with the incorporation of ILs. It is very helpful in the catalysis process. In some cases, ILs were governed more efficiently in reaction medium than other polar or, coordinate solvents [94], where the morphologies were shown more significant as compared to other types of materials. Most importantly, ILs are also decreased the number of steps in the reaction procedure. In the case of electrospinning, the amount of solvent is required very less (or, single) to complete this process. 6.4.3.1 Advantages 1. High thermal stability 2. Highly soluble 3. Better dispersion 4. Less hazardous 5. Industrial use 6. Low vapor pressure 7. Polydispersity enhancement.

6.4.4 Analytics and purity Analytics is one type of important character, which was generally executed in the crystallization of biomolecules. It is used to support

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mass spectrophotometer and chromatography (e.g., Gas and Liquid chromatography). This analysis illustrated to decontaminate of the preliminary material, where the ILs supported to purify of the samples under the inert atmosphere. Most of the recent applications are mainly dependent on the analytics and the purity of samples for better accomplishment in further research. 6.4.4.1 Advantages 1. Remarkable solvation capacity 2. Easy in hydrophobic ILs 3. Purification and recyclable 4. High accuracy.

6.4.5 Additives The additives are the most important supplementary requirements such as dispersing agents, plasticizers, suitable stabilizers, and solubilizing agents, which are mainly mediated to control the viscosity, surface interactions, and thermal diffusivity in IL-based materials. Appropriate addition of additives in IL salts with materials established a good function in the dispersion and lubrication process. Both processes are attributed to the best way to process and produce promising results in industries at a very low cost in current research. 6.4.5.1 Advantages 1. High chemical stability 2. Good thermal conductivity 3. High ionic mobility 4. Increase lubricating properties 5. Strongly surface interactions.

6.5 Current applications and their future perspective The concept of nanomaterials and their applications in various fields are innovative in research fields last more than 50 years, whereas the early nanotechnological industries and R&D were enduring approximately more than 20 years in the research arena [95]. On the other hand, ILs-based synthesized nanomaterials and nanocomposite materials was representing almost more than 15 years. The development of these advanced materials has stepped up in many sectors of science and engineering such as biomedical, environmental science, nuclear science, food science, energy storage and transfer, catalysis, lubricants, sensors,

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electrochemistry, etc. The description of significant applications in recent research are described below:

6.5.1 Biomedical The research and development in medical science have provided a great advantage in a new era of life (Fig. 6.2). Most pharmaceutical compounds are sparingly soluble in water or, other solvents, where the ILs were executed as highly soluble in water and ILs modified materials, even with very less soluble solvents. It has been stimulated in numerous forms of studies and widely explored as beneficial features for industries and hospitals, which are implemented in the treatment of different parts of the body in living organisms. However, recently polymer-based composite materials with ILs have achieved a new era of research to

FIGURE 6.2 Recent applications of ionic liquids nanocomposite materials in biomedical and environmental sciences.

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employ in the medical research field. This research was integrated to reduce the drug size and amount of drug (solid/liquid) in clinical studies, which was attributed to increasing the surface activity and reducing the cost of medicines. Some important study was ascribed to use in the treatment of living organisms such as anticancer drug sensor, antiviral drug detection, ultrasensitive determination, tumor microwave thermal ablation, enhance catalytic activity, chemotherapy, microwave thermal therapy, photocatalytic degradation, antibacterial activity, antibiofilm, wound healing, adsorption study, biocides, lubricant additives, bone regeneration, drug delivery, gene delivery, etc. [96
106]. However, these types of advanced research are required to implement the healthcare system to boost life in the world. The most important applications of IL-base materials were exhibited in some recent review papers [107
109].

6.5.2 Environmental science In the current era of research, most researchers are mainly focusing to synthesize the eco-friendly (composite) material to reduce toxicity and increase efficiency. In most of the studies, this type of material had been instituted as less expensive and significantly useful in different applications of environmental science (Fig. 6.2). 6.5.2.1 Water treatment Heavy metals and organic pollutants (e.g., dyes) are very crucial problems for human health due to the continuous increment of the world population. There are many methods exhibited to treat (remove the heavy metals and organic pollutants) the wastewater such as ion exchange method, distillation, reverse osmosis, filtration, photocatalysis, adsorption, etc. [110,111]. Recently, most researchers are fascinated by the green materials synthesis to remove the contaminants from polluted water and enhance the surface area to volume ratio, increasing the percentage of removal rate. Remarkably, green synthesis was comprising the low cost of raw materials and a simple procedure to control the pore size and their morphology. 6.5.2.2 Soil treatment All living organisms are mainly dependent on their food source to survive their life. Soil pollution is one of the major problems for living organisms, which can be easily arisen from the food source. Most of the soils are predominantly contaminated due to chemical and textile industries. Some heavy metals are also very dangerous in soil, which can be seriously caused by carcinogenic effects due to quick accumulation in various parts of the body [112,113]. The detection of

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heavy metals in soil is a very serious task due to the very less concentration of heavy metals in water. There were many techniques used to decontaminate heavy metals from soil such as liquid-liquid extraction, solid-phase extraction, ion-exchange method, sorption method, cloud point extraction, etc. Still, the research is going on to decontaminate the heavy metals from the soil, which can save the life from different hazardous diseases. 6.5.2.3 Air pollution treatment The different types of natural and manmade gases are available in the atmosphere. Some of the natural gases (e.g., CO2) and manmade gases (e.g., CFCs, HFCs, PFCs, etc.) are very harmful to human health. Moreover, vehicles and chemical industries produce toxic gases, which are mainly responsible to pollute the air. ILs-based materials were reported very efficient to capture the CO2 and CO gases from the atmosphere [114,115] and investigated the innovative way to separate the toxic gases from the atmosphere in the nanotechnology era.

6.5.3 Nuclear science Nuclear research revealed an innovative, safe, and great efficacy in industrial processes of science and technology (Fig. 6.2). Recently, it was used in quick responding hydrogen gas sensors, where response time, sensitivity, and limit of detection were heightened on their microchannel electrodes than on other electrodes [116]. The R&D is working to improve the production of energy as a renewable source. This research could fulfill the requirement of energy and also save the loss of energy through renewable processes.

6.5.4 Food science The study of food science in the research arena is most essential and sensitive than other types of relative applications (Fig. 6.2). In food science research, the mutual investigation of physicochemical, thermodynamic, and toxic properties presented a very important part of research to control the quality of products. Currently, the food industries are predominantly directed on the green chemical constituents; where thermal stability and melting temperature of preferred cations and anions play an important role to produce the high-quality product for a healthy life [117,118]. Moreover, the packaging of food products was having a great deal because the improper packaging and storage of food created fungal infections. However, food industries have been using the green

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chemical-based assembled materials to preserve and package food items for long terms storage capacity.

6.5.5 Energy storage and transfer Due to the extreme use of fossil fuels in daily life, the shortage of energy has shown a common problem. The energy storage and transfer in solar power and wind were maintained using oil-based energy production sources (Fig. 6.2), where the consumption of energy is higher than energy production and less eco-friendly process. Now a day, sustainable energy is being more predominantly in green research field development, where the ILs fabricated nanomaterials have turned the way of production and production rate. It has been conceived green and renewable resources in energy production for easily store and transfer as per necessity. The rechargeable lithium-ion battery, metal-air batteries (e.g., Zn-O2, Li-O2, etc.), sodium-ion battery, and supercapacitors were reported the great invention in energy storage and transfer through ILs modified nanomaterials [119,120].

6.5.6 Catalysis In the finding of promising and predictable reaction procedures, high aggregation and less stability of particles generated a significant problem in the value of surface to volume ratio. On the other hand, some reaction processes are time-consuming and less production rate in industries. The influence of catalysis was shown a great impact to improve the reaction rate and production rate with very less timeconsuming (Fig. 6.2). The green materials executed a pioneering role to enhance the catalytic properties of ILs-modified materials. The catalytic activity will be taking part in different types of organic/inorganic solvents for easy hydrogenation (e.g., ketone, arene, benzene, etc.) and functionalization in chemical reaction [121
123].

6.5.7 Lubricants There have been possessing two types of lubrication, that is, solid and liquid lubrication (Fig. 6.2). The solid lubricants comprise splendid tribological behavior in material science and engineering, which provides efficient lubrication in different fields of nanotechnological applications. However, some applications in materials science are having less efficient lubrication properties due to the strong delocalization of electrons such as graphene, benzene, etc. [124]. In the case of liquid, lubrication exhibits better efficiency due to reasonable durability, high fluidity,

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and easy cooling [125]. Recently, ILs nanocomposite materials are involving to functionalize with liquid lubricants to increase the loading capacity and preparation of the anti-wear film.

6.5.8 Sensors Sensors are emerging as the most innovative research in science and engineering to improve the performance of electronic devices (Fig. 6.2). Due to technological advancement of sensors, the detection of the unique position of metal ions or, chemical/biological compounds has been revealed very easily and flexible. In addition, the detection of hydroquinone in food and pharmaceutical compounds is carried out the best way through ILs nanocomposite materials based on prepared voltammetric sensors in current research life. Moreover, it is capable to identify two or, more chemical/pharmaceutical compounds simultaneously in the same reaction process [126,127].

6.5.9 Electrochemistry In the last few decades, electrochemistry has been preferred promising alternative process, which is ascribed highly selective, less timeconsuming, and less expensive in the chemical science and engineering sector (Fig. 6.2). The efficiency of ions is generally depending upon the electrode (redox) potential and their conductivity. Numerous scientists are working in this field to deploy a significant way to employ structure-function associations, the key role of pH values, interaction between covalent and non-covalent, etc., which control the activity, conductivity, and sensitivity of the interfaces.

6.6 Reaction mechanism of ionic liquids-based synthesized nanocomposite materials The reaction mechanism of ILs-based synthesized nanocomposite materials is recommended as a very crucial way to understand the formation of particles in nanoscience. The predictable general reaction mechanism of materials synthesis was shown in Fig. 6.3, where these methods were expected highly significant and popular to synthesize the materials recently. Generally, there are two types of methods to synthesize IL-based materials: (1) the Controlled method, and (2) the Uncontrolled method. It is very easy to understand the formation of ILs, where the emerging process of organic cation and organic/inorganic anion is used to form the ILs.

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FIGURE 6.3 Reaction mechanism of ionic liquids-based nanocomposite particles.

In the case of the uncontrolled synthesis method, the ILs are assorted with different types of precursors as their required products in presence of non-activating or, activating agents (e.g., ligands, polymers, or surfactants). The incorporation of precursor with ILs produces the nanoparticles

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to use in further applications or, in combination with other metal nanoparticles to grow ILs modified nanocomposite materials. Due to the uncontrollable temperature, concentration, and time in the synthesis procedure, the produced nanomaterials are highly aggregated and agglomerated, which reduces the surface area and increases the pore size of synthesized particles. On the other hand, in recent days, the researchers are mainly focusing on morphology, pore size, and surface area, which could be an efficient way to enhance the surface activity and their use in advanced applications at a very low cost. In the case of the controlled synthesis method, the ILs are associated with various types of precursors in the presence/absence of activating agent by controlling the temperature, pressure, and physical characteristic of particles. The produced particles are further incorporated with other metal nanoparticles to produce the ILs-modified nanocomposite materials with less/lowest agglomeration and aggregation of synthesized materials.

6.7 Conclusions The remarkable expansions have been made in inorganic synthesis applying, or in the existence of ILs, which has led to the development of sundry compounds with fascinating characteristics. This chapter significantly discussed the applications of ILs and PILs to nanocomposite construction, their impact during the processes, and in final composites properties, highlighting their interfacial effects, particularly when colloidal processes were involved. ILs interact with the growing system through H-bond “co-π-π stacking” mechanism, forming a variety of morphologically different hybrid materials under mild reaction conditions. This creates an ordered solvation layer of ILs on the surface, which influences the interfacial interactions. Thus differences in size, geometry, polarity, and coulomb coupling forces between cations and anions, which influence ILs’ viscosity and transition temperatures, directly contribute to the final produced particle’s morphology, size, and compactness. Moreover, the presence of C-H unities in the imidazolium ring and functionalization with polar groups could further intensify the ILs multiple H-bonding. These ILs’ features significantly influence the nanocomposites/hybrids interfaces, also adding to them an ionic character. However, not many studies discuss the interface ionicity influence on nanocomposite properties. Only very recently, this subject has been raised and very promising results were observed. Besides the outstanding mechanical properties reinforcements, very desirable properties, such as selective gas/liquid permeability, shape-memory, and self-healing, were added to

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the final nanocomposites. Thus we hope to have convinced researchers, especially from the areas of colloidal and interface sciences, that further investigations in this subject are worthwhile and could allow the developing of prospective new materials for many areas, such as biomedicine, electronics, aerospace, smart fabrics, intelligent packaging, sensors, and actuators. The updated applications of ILs in the synthesis of inorganic nanomaterials were briefly outlined. Since, its introduction to organic, catalytic, and electrochemistry, the ILs have captured and held the imagination of chemists working in these areas, and tremendous progress have occurred. Despite the that the application of ILs in inorganic nanomaterials is still in its infancy, however, many signs imply that the ILs are becoming an important emerging field in nanomaterials in upcoming years. The author wishes the present brief chapter is not only of interest but also particularly timely, to the nanoscience community.

Abbreviations anti-PD-1 APTES [bmim][BF4] Ce6 CFCs CpG DMF DMSO DOX GMP HA HFCs LOX Li-O2 MWCNT OVA PEG PFCs PLGA UCNP Zn-O2

antiprogrammed cell death protein 1 (3-aminopropyl)triethoxysilane 1-butyl-3-methylimidazolium tetrafluoroborate chloride e6 chlorofluorocarbons 50 —C—phosphate—G—30 dimethylformamide dimethyl sulfoxide doxorubicin gemcitabine monophosphate hyaluronic acid hydrofluorocarbons lactate oxidase lithium-air multiwalled carbon nanotubes ovalbumin poly(ethylene glycol) perfluorocarbons poly lactic-co-glycolic acid upconversion nanoparticles zinc-air

Author contributions Raju Kumar Sharma prepared the manuscript; Jamal Akhter Siddique, Jyoti Prakash Maity, and Chien-Yen Chen revised and gave necessary inputs.

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Conflicts of interest There are no conflicts of interest.

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7 Ionic liquids as green and efficient corrosion-protective materials for metals and alloys Mohd Amil Usmani1, Imran Khan2,3, Abul Hasnat1 and A. Moheman1 1

Department of Chemistry, Gandhi Faiz-E-Aam College, Shahjahanpur, Uttar Pradesh, India, 2Department of Chemistry, College of Science, Sultan Qaboos University, Muscat, Oman, 3CICECO—Aveiro Institute of Materials, Chemistry Department, University of Aveiro, Aveiro, Portugal

7.1 Introduction 7.1.1 Effect of corrosion Metals or alloys undergo irreversible chemical or electrochemical interactions with the air, resulting in spontaneous degradation [1]. Metals’ physical degradation is referred to as erosion, galling, or wear in general, despite the fact that corrosion only has a negative influence on metals [2]. Corrosion is thought to be the cause of massive economic losses throughout the world owing to lost metallic resources. Corrosion is seen as a concern in both academic and industrial settings [3]. Corrosion is a problem in most parts of the globe, limiting economic progress. Corrosion is expected to cost billions of dollars worldwide, including in India, and has a significant impact on practically every country’s economy [4]. Corrosion costs may be decreased thanks to newly developed and cost-effective corrosion prevention techniques.

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7.1.2 Causes of corrosion Corrosion occurs when metals interact with their environment, either directly or indirectly, to generate more stable compounds. Metals (excluding gold, silver, and platinum) have a natural tendency to oxidize owing to their chemical reactivity, which strives to stabilize them via chemical interactions with the elements of their surroundings. Metals and their alloys interact to the surrounding in acid solutions, and the corrosion protectors, which create a protective layer over the metallic surface and separate the metals from the environment, erode, forming stable corrosion compounds and preventing corrosion [5].

7.1.3 Techniques of corrosion protection The flow diagram of various corrosion prevention approaches is shown in Fig. 7.1. Depending on the nature of the metal, several corrosion prevention tactics and procedures have been developed and tested in the environment. These operations include painting, alloying and dealloying, galvanization, coatings, and the application of metal corrosion inhibitors [6,7]. The synthesis and deployment of ecologically acceptable corrosion protectors are critical to recent advancements in

FIGURE 7.1 Corrosion protection techniques/methods for metals and alloys.

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corrosion protection technologies. These protectors seem to grow more potent as they become adsorbent on metal surfaces [8,9]. As a result, they provide a barrier that prevents the metal contact from interacting with sensitive environments. As a consequence, corrosive species such as O2, H2O, and anions have a tough time diffusing over the metal surface. Corrosion inhibitors adsorb on the surface of metals and alloys by a variety of chemical, physical, and physiochemical methods.

7.1.4 Ionic liquids as green corrosion protectors “Green chemistry” permits designing and processes only the products that reduce the practice and fabrication of toxic substances [10]. That is why ecological knowledge and harsh environmental rules do not allow the development and application of harmful corrosion protectors, which emphasize to improve the synthetic chemistry by using environmental acceptable reactants; otherwise it needs appropriate strategy for production using latest energy sources like microwave heating and ultrasound. In this regard, multicomponent reactions practice along with microwave irradiation and ultrasonic emerge as better choice of synthetic methods toward “green synthesis.” Extracts of different plants and phytochemicals are considered as the green corrosion protector due to their nontoxic nature [11]. Nevertheless, their extraction and distillation are very costly and time-consuming. Apart from that, large amount of organic solvent is required for synthesis which causes adverse effect on environment, ultimately on human being. Additionally, the extraction of material from plants occasionally needs high temperature and may decay the active components, due to which the relative protection efficiency may decreases. Likewise, application of drugs as corrosion protectors is not possible because of their complicated production method and high cost [12]. Therefore it is important to produce “green protectors” either by managing their synthesis in such a way that accomplished by applying economic and environment acceptable reactants or by reducing the synthetic steps, preferably in single step. In this regard, ILs have emerged as green solvent as they showed excellent properties like lower melting point, chemical stability, low toxicity, very high thermal, lower vapor pressure and high polarity, a smaller harmful effect on environment and living being [13]. Nowadays, ILs have been applied in various fields of research like energy conversion, catalysis, extraction and separation, and synthesis of nanomaterials [13,14]. Ionic liquids (IL) are known for their marvelous properties due to which they can be tailored, as per the requirement by suitable choice of anions and cations, which provides ample opportunity to develop ILs of specific properties [15]. These fabulous properties make ILs as “designer chemicals” that have prospective to

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act as catalysts, solvents and reagents for production of numerous naturally and technically useful chemicals [15,16]. The old organic corrosion protectors not only contaminate the environment but also badly affect the human health. ILs are known as green and solvents by showing high thermal and chemical stability, noninflammability, nontoxic, and negligible volatility. ILs have extremely low vapor pressure, which also categorized them in a special class of chemicals. The rapid use of ILs grasp the attention of researchers and scholars in almost all fields of chemistry and enable them as “green and sustainable chemicals” which shows their capability as a strong solvent for inorganic and organic compounds [17,18]. As a result, ILs gained particular attention as environmental acceptable substitutes for old corrosion protectors. ILs action against corrosion significantly be governed by the nature of cations and anions [11,12]. This chapter aimed to define the recent progress in the corrosion protective nature of ILs for various metals and alloys.

7.1.5 Applications of ionic liquids ILs have shown promise to be used in various fields, depending on their extraordinary behavior. Fig. 7.2 exemplifies a concise view of ILs applications [19,20].

7.1.6 Classification of ionic liquids Brief classification of ILs has been described [21]. Some well-known types of ILs are illustrated in Fig. 7.3.

7.2 Ionic liquids as corrosion protector for metals and alloy As stated, numerous interesting features of the ILs make them noble candidates to substitute the old corrosion protectors that have numerous effects on environment and human beings. Numerous research works have been carried out with ILs as corrosion protector.

7.2.1 Ionic liquids as corrosion protector for iron and alloy Iron and its alloys are recognized for their cheap cost and great mechanical strength, making them excellent building materials for a variety of industries [22,23]. However, these materials are readily corroded by the environment and experience corrosive deterioration via a variety of industrial practices like acid descaling, acid pickling, acid etching, and acid cleaning procedures, necessitating the development of

1. Catalysis and electrochemistry

FIGURE 7.2 Common applications of ionic liquids.

FIGURE 7.3 Classification of ionic liquids.

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new materials [23]. Organic compounds containing polar fictitious groups and heterocyclic rings reported to use as additive for the protection of metals and alloys from spontaneous chemical reactions [22,23]. Electrons from double and triple bonds, as well as heteroatoms from polar functional groups, may serve as adsorption sites on metallic surfaces, allowing these compounds to adsorb on metallic surfaces [21]. However, since these corrosion protectors are exceedingly hazardous and volatile, they are incompatible with environmental awareness and tight environmental standards. As a result, ILs may be employed as corrosion protectors for corrosion prevention as a green option. Because of their unique qualities, ILs are increasingly used to replace very volatile, hazardous corrosion inhibitors. Numerous synthetic ILs have been investigated and found to be efficient corrosion defenders for carbon steel and mild steel in various electrolytic solutions. Electrolytes based on hydrochloric acid (HCl) and sulfuric acid (H2SO4) are largely used in educational and industrial settings, although they are corrosive and aggressive. ILs are widely utilized as defenders to metallic corrosion for iron and iron-based alloys, like carbon steel and mild steel, because of their particular characteristics. It was also discovered that imidazoliumbased ILs were mostly employed for corrosion prevention. This is because polar hydrophilic group of imidazolium that adsorbs onto the metallic surface, while the nonpolar hydrocarbon component adapts to the solution end. In HCl, corrosion protection of ILs based on imidazolium, 1-methyl imidazolium, and 1,2-dimethyl imidazolium for carbon steel demonstrated that increasing the hydrocarbon chain size from decyl to dodecyl carbon chain resulted in a considerable improvement in corrosion protection performance [24]. Furthermore, it was discovered that ILs based on imidazolium with the same amount of hydrocarbon atoms in their chain but varied numbers of methyl groups had variable levels of protection. In HCl [25], [C16M2Im] [Br] containing two methyl groups demonstrated stronger protection efficacy for mild steel corrosion than [C16MIm] [Br] containing just one methyl group at varied doses. Furthermore, in the HCl system, corrosion protection of ILs based on imidazolium (HMIMI, BMIMI, and PMIMI) for mild steel illustrated that on lengthening alkyl chain from propyl to hexyl protection efficiency increases [26]. Similar to these, another imidazoliumbased ILs known as [(CH2)3COOHMIm][HSO4], [(CH2)2COOHMIm] [HSO4], and [(CH2)3COOHMIm][HSO4] demonstrated an increase in protection efficiency as the length of the alkyl chain increased for carbon steel in the HCl system [27]. It was concluded that these three ILs inhibit carbon steel corrosion by hindering the cathodic reduction and anodic oxidation reactions and thereby overcrowding the energetic sites responsible for the vigorous corrosion. Significant smoothness was found in the metallic surfaces, and the flatness in the surface orientations remained

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stable in accordance with their practical protection efficiency. The ILs 1,3-dioctadecylmidazolium bromide (ImDC18Br) and N-octadecylpyridiniumbromide (PyC18Br) were found to be effective corrosion inhibitors for mild steel in an aqueous solution of H2SO4. They adsorbed on the metallic surface through a chemisorption mechanism and obey the Langmuir adsorption isotherm [28]. In addition, the performance of the 1-ethyl-3-methylimidazolium dicyanamide (EMID) on mild steel corrosion in H2SO4 revealed that EMID shields metals against corrosion by adsorption on the surface and thereby increasing surface area along inhibitor. The EMID adhered to the metal surface according to the Langmuir adsorption isotherm [29]. Furthermore, both ILs, 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM]HSO4) and 1-butyl-3-methylimidazolium chlorides (BMIC), showed high protective efficacy against mild steel corrosion on HCl. It was also discovered that these compounds followed the Langmuir adsorption isotherm and behaved as mixed type defenders on mild steel surfaces [30]. Both ILs, 1-allyl-3-octylimidazolium bromide ([AOIM]Br) and 1-octyl-3-methylimidazolium bromide ([OMIM]Br), worked as effective corrosion protectors in H2SO4 [31]. The IL based on chitosan was reported to reduce the rate of metallic suspension and hydrogen evolution when combined with chloride, a corrosive medium, making them mixed type inhibitors [32]. In a study evaluating the corrosion properties of pure titanium (Ti), stainless steel, and carbon steel in an aluminum chloride 1-ethyl-3-methylimidazolium chloride IL, it was discovered that stainless steel had the best corrosion resistance in the extreme chloride environment [33]. Different metals, such as stainless steel, nickel, and copper, had similar results [34]. The IL 1,4-di[1-methylene-3-methyl imidazolium bromide]-benzene was able to resist mild steel corrosion in H2SO4 by adsorbing on the surface and serving as mixed type protectors by following the Langmuir adsorption isotherm. These talks conclude that, although various classes of ILs have been used as prospective mild steel protectors in a variety of violent media, imidazolebased ILs are the most common among them.

7.2.2 Ionic liquids as corrosion protector for Al Aluminum is one of the extensively applied metals owing to its numerous appealing characteristics, such as its lightweight and insignificant standard electrode potential. Like mild steel, it is also prone to corrosion, especially in aqueous solutions of alkali. They react with the aluminum oxide protective surface coating, which has a strong negative potential. Extracts of various plants has reported as corrosion inhibitors for aluminum. Good corrosion protectors, on the other hand, would be ecologically benign, easily accessible, acceptable, and substantially less costly, and

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could be made using simple processes. Because of its unmatched properties, ILs have been used as corrosion protectors. The aluminum protection efficiencies of the ILs 1-octyl-3-methylimidazoliumchlorides, BMIC, and 1hexyl-3-methylimidazolium chlorides increased as their concentration in HCl increased [35]. The corrosion resistance of 6061 Al-15 alloy was investigated utilizing ILs, 1,3-bis(2-oxo-2-phenylethyl)-1H-imidazol-3-ium bromide (OPEIB) in H2SO4 solution [36]. Poly(ionic liquid)s, specifically poly (1-vinyl-3-butylimidazolium) (PImC4), poly(1-vinyl-3-dodecylimidazolium) (PImC12), and poly(1-vinyl-3-octylimidazolium) (PImC8) hexafluorophosphate) hexafluorophosphate) hexafluor. When the quantity of ethylene glycol or water in the mixture rises, the stability of the aluminum in 1butyl-3methylimidazolium tetrafluoroborate IL and ethylene glycol mixture falls in polarization resistance and increases in capacitance [37]. The protection behavior of OPEIB on 6061 Al-15 vol. pct. SiC (p) composite in H2SO4 acid solution after Temkin adsorption revealed that the IL has extreme corrosion protection efficiencies and performs as a cathodic type protector [36]. By adsorbing on the metallic surface and inhibiting corrosion in HCl, tetradecylpyridinium bromide shields aluminum against corrosion [38].

7.2.3 Ionic liquids as corrosion protector for Cu and Zn Copper and copper-based alloys are mostly used in electronics, coinages, building construction, electricity, development of industrial apparatus and ornaments, because of their corrosion resistance characteristics, electrical, mechanical and good thermal properties [39]. Nevertheless, when they encounter aggressive anions like nitrate, sulfate, and chloride, they corrode resulting in loss of materials [40]. Likewise, zinc and its alloys are commonly used in in petroleum and chemical industries. Due to constant fluctuation in the pH, these industries results in the loss of mechanical properties of zinc and its alloys, which allows corrosion. Keeping in mind all the drawbacks of organic protectors, numerous researchers have applied ILs as green corrosion inhibitor. According to this approach, ILs, 1-octyl-3-methylimidazolium hydrogen sulfate, 1-butyl-3-methylimidazolium hydrogen sulfate, and 1-hexyl-3-methylimidazolium hydrogen sulfate were observed for the protection efficiency on copper corrosion in H2SO4. Results showed that ILs followed the Langmuir adsorption isotherm and ILs behaved as mixed type protectors. In continuation, the protection performance of (2-hydroxyethyl)-trimethyl-ammonium and butyltrimethyl-ammonium ILs on steel 100Cr6 and CuSn8P observed in water, result found in good agreement [41]. The protection performance of 1-ethyl-3-methylimidazolium

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phosphonate, diprotic di[bis-(2-hydroxyethyl)ammonium] adipate, and triprotic di[(2-hydroxyethyl)ammonium] succinate on corrosion of copper together with 1-decyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3methylimidazolium tetrafluoroborate on corrosion of zinc in HCl, revealed that both the ILs performed as good corrosion protectors. On increasing the concentrations of ILs the protection efficiencies remarkably increased [42].

7.3 Corrosion protection mechanism The working style of ILs is comparable to that of organic protectors, just by hindering the anodic and cathodic pores on the surface of metal. Concisely, ILs are smart enough to hinder the rate of cathodic reduction and anodic oxidation processes, although, anodic oxidation reactions are accelerated in the presence of sulfate and chloride anions. Nevertheless, existence of ILs hinders the oxidation on anode due to the adsorption of ILs cation (ILs1). As a consequence, the anodic site is preferred by ILs cations. The electrostatic force of attraction between inhibitor molecules and charged metallic surfaces normally causes IL adsorption. The cationic species, on the other hand, get electrons later and produce a neutral species with free unshared pairs of electrons, which are responsible for adsorption by relocating their unbonded and/or electron into the metal’s d-orbital through coordination bonding. Because metals have a large number of electrons, this kind of electron donation generates interelectronic hindrance, making it more difficult for the metal to transfer electrons back into the guardians’ antibonding molecular orbitals (back-donation). As a result, synergistic effects bolster the donation and retro-donation phenomena [43].

7.4 Conclusions and future perspectives ILs are the most effective, environmental acceptable, and highly soluble class of corrosion protectors. Among the numerous classes, ILs based on imidazolium are extensively applied as corrosion protectors for various metals and alloys. They are able to hinder the corrosion current density and hence improve charge transfer resistivity. Corrosion protection efficiency of ILs, in electrolytes such as HNO3 and H2SO4, is reported to improve as the concentration increases and drops with the temperature. ILs are mostly applied in aqueous phase for the protection of corrosion; therefore they are highly useful as corrosion protecting materials.

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8 Ionic liquids as valuable assets in extraction techniques Jamal Ahmad Khan1 and Shagufta Jabin2 1

Applied Sciences and Humanities section, University Polytechnic, Zakir Husain College of Engineering and Technology, Aligarh Muslim University, Aligarh, Uttar Pradesh, India, 2Department of Chemistry, Faculty of Engineering, Manav Rachna International Institute of Research and Studies, Faridabad, Haryana, India

8.1 Introduction The extraction techniques are the powerful tool to produce extract and purify a wide range of target materials from different samples. They can be either sorbent based or solvent based. Many integrated and sustainable extraction techniques have been developed and employed that make the attention to researchers. Common extraction techniques are ultrasonicationassisted, microwave-assisted, supercritical fluid extraction, percolation, maceration, digestion, decoction, infusion, and counter current extraction. These techniques have low extraction efficiencies, poor selectivity, and difficulties in isolation. Most of the solvent based extraction techniques utilize toxic organic solvents and also add costs to the final product because of their high complexity, high time, and energy demands. Shortcomings of these techniques are the main challenges for the modern scientists. Due to this reason, ionic liquids (ILs) have been getting attention in various fields of research and become an attractive subject in greener extraction techniques [1 3] because they act as significant solvents for a wide range of different polar as well as nonpolar compounds. Various interesting properties of ILs distinguish them from volatile organic compounds. They have been widely used in various kinds of extraction techniques, including single-drop microextraction, dispersive liquid liquid microextraction, liquid liquid extraction, hollow fiber liquid-phase microextraction, and solid-phase microextraction.

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IL-based extraction, a novel and important separation method, can be used as an alternative diluents and/or extractants for organic, inorganic, and polymeric materials to reduce the environmental, energy, and cost impacts. They can be used in the extraction technique from organic or aqueous systems, which is much safer than that based on conventional organic solvents. Functional groups, high conductivity, and prevention of a third phase formation make them efficient for both nonaqueous and aqueous processes, such as extraction, electro-deposition, and adsorption. The reported IL-based extraction, such as solid liquid extraction, liquid liquid extraction, and IL-modified materials, emerged as a potential strategy for the researchers in the science and technology. The IL-based extraction technique was developed in the mid-1960s [4]. Since then, the interest of the researchers on this technique is continuously growing. The unique reactivity and miscibility behavior and nontoxic to the environment or living organisms and environmentally friendly properties is the main driving force of IL-based technologies. Their solubility is entirely different from other chemical compounds [3]. The sustainability of ILs as solvents, cosurfactants, electrolytes, and adjuvants provides the way to meet the needs of the present generation without compromising the capability of future ones. The ILs have also been used in the creation of IL-supported materials for the purpose of separation. IL-based extraction processes are used to carry out the extraction of desired substances, such as biomolecules [5 9], pharmaceuticals and drugs [10 16], metal ions [17], and rare earths [18], from different samples of human urine, food, biomass, water, soil, and other raw matrices. Their extraction ability depends on the different structures and the source from which they are obtained. They easily undergo biphasic separation with aqueous medium and other less polar and nonpolar organic solvents. ILs with large aromatic groups show greater activity for the extraction of aromatic in aqueous biphasic systems [19]. This chapter aims to provide the information about the ILs and their applications as fast and green extraction techniques for the extraction of ionic as well as nonionic substances from different samples. In this context, we have been able to provide the information about IL-based extraction methods using ILs in place of classical organic solvents, such as benzene, methanol, and diethyl ether.

8.2 Ionic liquids ILs (also referred to as molten salts, room temperature ILs, designer solvent [20], ionic fluid, liquid organic salt, fused salt, or neoteric solvent) are salts in the molten state whose melting points are generally below 100 C [21]. They continue to be liquid in a temperature range

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that is broader than that of water and other liquids. They consist of bulky and asymmetric organic cation (such as imidazolium, pyridinium, pyrrolidinium, phosphonium, and ammonium) and wide range of organic (such as acetates, formates, citrates, lactates, sulfonate, salicylates, benzoates, saccharinates, and thiazolanes) [22] or inorganic anion (such as nitrates, tetrafluoroborates, hexafluorophosphates, chloroborates, chloroaluminates, and bisulfates) [23,24] (Fig. 8.1). According to the needs, ILs with specific properties can be designed using suitable cation and anion [25,26]. On this basis, polarity and hydrophilicity hydrophobicity of ILs can also be tuned. Due to this reason, these are also identified as designer solvents [20]. These are thermally stable up to 200 C. Their thermal and chemical stability depend on the choice of anion and alkyl chain length [27]. The first IL, [C2H5NH3][NO3], was synthesized by Paul Walden et al. [28,29] in year 1914 by neutralizing ethylamine with con nitric acid. The physicochemical properties of ILs including viscosity, thermal stability and solubility in water and other organic solvents depend upon cation and anion but are mainly influenced by their anion and are considered to be the foremost factor that affects the extraction rate of the target compounds [30]. The properties of ILs can be wide ranging by simply changing the combination of cations and anions. For instance, BF42 has a better extraction effect than HSO42 and Br2, which may be due to the H-bond interaction between BF42 and target molecules [31]. The low vapor pressure, volatility and combustibility, good chemical and thermal stability, high conductivity, good ionic conductivity, and wide liquids range provide advantageous properties to ILs. The low vapor pressure is a result of the strong Coulombic forces between cations and anions of ILs. These are able to dissolve many polar and nonpolar inorganic as well as organic compounds (both simple and complex molecules) in liquid form, which occur due to ion ion interaction, dipole dipole interaction, van der Waal’s interaction, and π π interaction. These are soluble in water as well as organic solvents. Their solubility depends chiefly on the anion but may also be modified by the length of the alkyl substituent in cations. Due to their nonflammability and low toxicity, they have the advantage of being easy to work with them. One more significant characteristic of ILs is to be regenerated after use.

Bulky cation with alkyl chains

Ionic Liquid

FIGURE 8.1 Formation of ionic liquids.

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8.3 Ionic liquids for the extraction of natural products from the plants Natural products are significant resource of leading compounds for drug discovery [32 34] due to their interesting biological activity and chemical structures. About 40% of drugs available for clinical use are either naturally occurring or derived from natural chemical structures [35,36]. To get the target natural products from different herbals [37], organic solvents have been employed in the traditional extraction techniques (e.g., solvent extraction, cloud point extraction, and supercritical fluid extraction). Due to the optimal choice and enhancement of cost, energy, and environmental impact of organic solvents [38], ILs are now being examined as an alternative solvents so as to solve the issue caused by the traditional methods for the extraction of natural products with organic chemical reagents. The extraction effect of the active ingredients from natural products is influenced by the structure of ILs [39]. For instance, the order of the influence of the three IL types on their extraction effects is solvent extraction: imidazole , benzoxazoles , pyridines [40]. The IL-based materials present advantages in capturing the target natural products by chemical selectivity so as to extract and improve the products in a short time. Their applications as a kind of novel solvent in the extraction techniques of natural products, such as phenolics [41,42], alkaloids [43], fats [44], essential oils [45], caretenoids [46], vitamins [47], amino acids [48], nucleic acids [49], proteins [50], enzymes [51], and antibodies [52], from plants continue to prove the effectiveness as green solvent [53,54]. They can decrease the environmental damage produced by the huge quantity of organic reagents used in the field of natural products chemistry [38]. Therefore to comfort the environmental burden and to turn waste into useful products, a new technology is needed. Currently ultrasonic-assisted IL approach, microwave-assisted IL approach, and reactive dissolution of biomass in ILs have been applied for the extraction of natural products from plants.

8.3.1 Ultrasonic-assisted ionic liquid approach for the extraction of natural products Ultrasonic-assisted IL extraction approach is a simple, green, highly efficient and environment-friendly that has been developed for the effective and selective extraction of target natural products, such as alkaloids [55], biflavonoids [40,56,57], and polysaccharides [58]. This method has many advantages, such as simple operability, time-saving, high extraction efficiency, and safety.

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The ultrasonic-assisted IL extraction method also reveals the tremendous potential applications of ILs. By using this technique, the performance of ILs and the effects caused by changes in the anion and the alkyl chain length of the cation on the extraction efficiency in ultrasonicassisted extraction process can also be evaluated.

8.3.2 Microwave-assisted ionic liquid approach for the extraction of natural products Microwave-assisted IL extraction is one of the nonconventional methods for extracting the target compounds from medicinal plants using ILs as solvent [59]. In this extraction technique, the microwave energy is used for heating the solvents in contact with a sample, which increases the kinetics of extraction. ILs have unique excellent characteristics in extracting and microwave-absorbing ability [60]. Therefore they have been successfully applied as alternative solvents in the microwave-assisted extraction of bioactive substances for several years [61 63]. Success in applying microwave-assisted IL extraction technique to obtain the optimal of target constituents has been widely reported [64 69].

8.3.3 Reactive dissolution of biomass in ionic liquids for the extraction of natural products Biomass, also known as lignocellulosic biomass, is a widely abundant renewable organic raw material (such as agricultural crop residues, forestry residues, algae, wood and wood wastes, municipal waste, wet waste, and other waste materials) that is abundant in natural products and chemical energy [70]. Its utilization is increasing day by day as it provides unique resources for the sustainable production of bio-products. The biomass mainly constitutes carbohydrates namely cellulose, hemicellulose, and lignin with complex and rigid structure due to several covalent and noncovalent interactions between them. ILs are employed to achieve the green processing and cost-effective solvent and reaction media for the extraction of valuable natural ingredients from crude biomass. They have unique ability to selectively dissolve a wide range of both woody and herbaceous biomass components as well as whole biomass [71 74]. This is due to their broad liquid temperature range, disrupting the hydrogen bonds between cellulose chains [75] and multiple solvation interactions. Therefore the reactive dissolution of biomass using IL-strategy attracts great attention to obtain natural products (e.g., cellulose, hemicellulose, and lignin) with purity and

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efficiency. The lignocellulosic biomass can be deconstructed by ILs that makes these fractions susceptible for easier transformation to large number of commodities including energy, chemicals, and material within the concept of bio-refinery [76]. Only limited attention has been paid to the isolation of active components from biomass. The recent literature reveals the application of ILs for extracting bioactive natural ingredient with purity and good extraction yield.

8.4 Ionic liquids in extraction of pharmaceuticals from biological and environmental samples Pharmaceuticals are compounds with high biological activity. The development of these compounds seems very interesting because of their systematic delivery to the environment. The biological and environmental samples often identified in both human and veterinary pharmaceuticals [77], such as antibiotics and antipyretics [78], in many countries at low concentration levels. The determination and extraction of pharmaceuticals is particularly challenging because of their diverse structures and rich matrices. These are accumulative and toxic to aquatic and terrestrial ecosystems leading to the drug resistance [77]. So it is very significant to develop quick and simple methods without the need to introduce any additional steps to increase the safety of analyst [79]. The extraction and recycling of pharmaceuticals from solid pharmaceutical wastes is used an integrated approach comprising solid liquid extraction. Chromatography or electrophoresis among the many available techniques attached to different types of detectors, including fluorescence, ultraviolet, or mass spectrometry, is most often used for the extraction of pharmaceuticals in different and biological environmental samples. With reduced organic solvents and additional chemical and physical factors (e.g., sonication and temperature), these techniques brought significant progress but are time-consuming process with other difficulties [80]. Due to too many difficulties and inadequate selectivity in their quantification with traditional techniques, IL-modified methods have been applied for the sample preparation and extraction of pharmaceuticals in biological and environmental samples [81,82] that is based on the gas chromatography, liquid-based chromatography and electro migration techniques (e.g., capillary electrophoresis). For example, ibuprofen can be recovered from solid pharmaceutical wastes using IL-based solid liquid extraction techniques [83]. According to the data presented in various research and review papers, it is clear that researchers pay attention in improving these methods by introducing modifications using chemical and physical factors [84 89].

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8.5 Ionic liquids for the extraction of contaminants from wastewater Water contaminated with pollutants, such as heavy metals, toxic and hazardous organic compounds (such as nitroarenes, hydrocarbons, pesticides, and dyes), and radioactive metals, is of concern to all living systems. These are considered important industrial activities, mining as well as coal combustion [90 92]. There is a need to focus on wastewater extraction to prevent these pollutants from entering the environment and damaging ecosystems. For this purpose, various extraction techniques have been used. Since the most widely used traditional solvents are harmful and volatile for the extraction of toxic contaminants from wastewater; therefore ILs are actively used as an alternative solvents to meet the technology requirements. The IL-based extraction mechanism is considerably differed from that in conventional organic solvents. ILs are efficient and cost-effective solvents for the extraction of even small amounts of emerging contaminants from water [93]. They have several advantages as extraction media for the separation of water contaminants [94] with variety of applications including sequestration of transition metals, the separation of organic compounds, desalinating aqueous media, and capturing carbon dioxide [95 99]. Properly engineered ILs are enable to achieve the desired directional solubility. Selection of nonhazardous ILs, technological applications, disposal, and IL regeneration process are the main challenges in IL-based technologies for wastewater treatment. Therefore the wastewater treatment and its reuse are needed to develop efficient technologies regarding the water scarcity motivation.

8.5.1 Extraction of toxic metal ions Metals mainly heavy metals are the most dangerous water contaminants [100,101]. Their toxicities are serious problems across the world and are challenging to eliminate the long-term effects [102,103]. Several techniques including microfiltration [104], chemical precipitation [105], coagulation and flocculation [106], electrochemical removal [107], liquid liquid extraction [108,109], osmosis [110], crystallization and distillation [111], photocatalysis [112], membrane filtration [113], ion exchange [114], and adsorption have been applied for the extraction of metals from different sources of water. These techniques have high requirement of reagents, unpredictable removal of metal ions, and other several disadvantages. Therefore IL-based extractants that are economically significant, practical, and versatile and can be used for the removal of toxic and useful metals, such as cadmium, mercury, lead, gold, and platinum, from wastewater. The extracted valuable metals,

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such as platinum, gold, and rare earth elements, from industrial wastes can be recycled that are important for their strategic, economic, and national security value. IL-based extraction, such as adsorption technique supported with ILs, can increase the efficiency and selectivity of metal ions extraction. The adsorption isotherms equations of Langmuir and Freundlich and relevant kinetic studies can be applied for the calculation of adsorption efficiency of IL-based extracting agents. Kakaei et al. in 2020 proposed clay modified with triazolium and triazole ILs, an eco-friendly adsorbents, for the removal of various heavy metal ions, such as Pb21, Co21, and Zn21 ions. These adsorbents are eco-friendly and can be characterized by different techniques, such as X-ray diffraction, scanning electron microscopy (SEM), thermogravimetric analysis, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, along with SEM-map analysis [115].

8.5.2 Extraction of organic pollutants Organic pollutants present in wastewater, especially persistent organic pollutants from various kinds of industries, including coke plants, oil refineries, plastics, chemical, leather, paint, and pharmaceutical industries [116], are of the most important environmental problems in the world. Common organic pollutants in wastewater include fertilizers, hydrocarbons, pesticides, phenols, biphenyls, plasticizers, detergents, oils, greases, pharmaceuticals, carbohydrates, and proteins [117,118]. There are environmental restrictions and regulations of these pollutants; therefore these pollutants must be removed before final discharge of wastewater. Several methods, such as adsorption over polymeric and inorganic adsorbents, membrane-solvent extraction and liquid liquid extraction with different solvents [119], can be used for wastewater extraction. Due to some drawbacks in organic solvents, there is a need in the development of rapid, efficient, and significant technologies for the extraction of organic pollutants from wastewater. In this regard, IL-based adsorbents have been examined as solvent extraction [120,121], solvent membrane technologies [122 124], IL/water partitioning [125], and IL-modified materials [126,127] to many analytical and industrial separation processes for the recovery of organic pollutants. IL-based extractions have the great potential for rapid, effective, and selective extraction of wide range of organic pollutants, such as phenolic compounds, such as amines, herbicide, bisphenol A, toluene, phthalates, pesticides, toluene, pharmaceuticals, and dyes [128 130], from aqueous effluents. They have been applied as solvent membrane technologies, solvent extraction, and IL-modified materials to analytical separation processes. Their applications in the wastewater treatment in the removal of organic pollutants are still need to explore.

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8.6 Ionic liquids for the extraction of soil contaminants and soil organic matter The whole society has been concerned with the problem of soil pollution that is caused by both natural and man-made activities. Main sources of soil pollution are industrial and domestic sewage sludge and various agricultural chemicals. Sewage sludge obtained from different sources is enriched with micronutrients, pharmaceuticals, heavy metals, organic contaminants, pathogenic microorganisms, and many other contaminants. The amendment of soil with sewage sludge changes the behavior of soil. The presence of sludge normally increases soil salinity, which causes physiological damage to plants [131]. The high concentrations of heavy metals and other harmful chemicals in sludge-amended soil can cause its incremental abundance in soils and crops (Fig. 8.2). On the other hand, soil organic matters have numerous physical, chemical, and biological benefits. ILs have been investigated for their ability to dissolve soil contaminants and soil organic matter. Therefore ILs can be applied as extractant to extract soil contaminants [132,133] and soil organic matter [134].

8.6.1 Extraction of soil contaminants Soil contamination exceeds naturally occurring levels and causes human health risks and is largely caused by human activities. For example, sewage sludge directly applied in soil for nutrients supplement are enriched in contaminants, such as organic pollutants and heavy metals [135], which cause environmental problems. These substances can disturb the availability of nutrients, trace metals, and useful chemicals in the soils and plants [136 140], posing risks to humans. Therefore before use attention should be paid to sludge amended soil. 8.6.1.1 Extraction of soil organic pollutants Organic pollutants in soil are produced by natural or anthropogenic activities (such as direct amendment of sewage sludge in soil, coal burning,

FIGURE 8.2 General steps in extraction techniques.

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industrial activities, waste incineration, motor vehicle emissions, and waste dumping). Polybrominated biphenyls, polychlorinated biphenyls, polychlorinated dibenzofurans, polycyclic aromatic hydrocarbons, carbamate insecticides, organophosphorus, herbicides, and organic fuels are commonly found organic pollutants in soils. However, soil organic pollutants are typically found at low range in sewage sludge amended soil, but their environmental behavior and risk is challenging [141 143]. The extraction of organic pollutants in soil samples is the major challenge for better managing soils’ quality [144]. To minimize these problems, ILs have been used as green extraction solvents and absorbents in various techniques [145] in sample preparation and analytical chemistry [146], such as liquid liquid extraction [147], liquid phase microextraction [148], solid phase extraction [149], and solid phase microextraction [150], for the extractions of organic pollutants in soil samples. 8.6.1.2 Extraction of soil heavy metal ions The sludge mainly contains high amount of mobile heavy metals (viz. Pb, Cd, Hg, Cr, As, etc.), which generally cause its incremental abundance in soil and crop contamination posing risks to humans. The presence of traces of heavy metals contamination in soils is a serious worldwide problem, as it is challenging to eliminate their long-term effects [102,103]. Soil pollution due to heavy metals is considered to be more threatening than other types of soil pollution as it causes grave ill effects due to high degree of toxicity [151]. Therefore the extraction of metal ions is an area of attraction in the field of pollution control [152]. Heavy metals’ extraction by conventional processes for sewage sludge includes ion exchange, electrolysis, chemical precipitation, membrane separation, solvent extraction, coagulation, and adsorption. Out of these technologies, liquid liquid extraction is one of the most significant and simple techniques to extract metal ions from the sewage sludge. The use of ILs as extracting agents for minimizing amount of heavy metals from the sludge amended soil could be an important alternative to conventional extractants [153 155].

8.6.2 Extractions of soil organic matter Soil organic matters are the most dynamic of the soil components. These are extremely important source of nutrients, such as nitrogen, sulfur, and phosphorous, for plants [156]. These are mixture of components of different forms and types of biotic origin [157] with variable charge, which can strongly affect the overall properties of soil, such as electro-chemical properties, soil pH, and adsorption ability. However, the alkaline extraction and modern analytical techniques, including infrared spectroscopy, 13C nuclear magnetic resonance spectroscopy, and pyrolysis gas chromatography/mass

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spectroscopy are still in practice to extract soil organic matter [156], but ILbased extractions [158] have been proved as significantly advanced techniques due to their improved characteristics.

8.7 Extraction of rare earth metals Rare earth metals or rare earths are significant in our daily life. Their properties allow them to play crucial roles in many modern green and other advanced technologies [159] and in low carbon emission applications [160 162]. With the increasing uses of both light and heavy rare earth metals, their global demand is growing. They are subsequently extracted using various methods with conventional solvents, but the solvent extraction [163,164] is preferred over other methods because it offers high selectivity, continuous operation, and the wide range of extraction. The extraction of rare earth metals from earth crust or waste materials is still challenging for future research. Due to the poor selectivity and adverse effects on the environment and health, ILs have been proposed as significant replacements of traditional solvent for solvent extraction of rare earth metals [165,166]. The IL-based extraction is different from the traditional solvent-based extraction and has engrossed much attention because of attraction of ILs for both neutral hydrophobic and charged species representing an alternative to conventional processes for rare earth metals’ recovery. A lot of studies were carried out on the selective and sustainable extraction of rare earth metals using ILs as diluents [167 169]. Hidayaha and Abidin [170] explored bis(2ethylhexyl)phosphoric acid, 2-thenoyltrifluoroacetone, and Aliquat 336 (A336) diluted in imidazolium-based ILs to extract a wide range of rare earth metals. IL-base extraction technique is an effective approach for improving the extraction efficiencies of rare earth metals [171].

8.8 Ionic liquids for the extraction of food contaminants Food materials contain contaminants along with the beneficial ingredients. Harmful chemicals in foods are diversified, such as heavy metal ions, pesticides, mycotoxins, drugs, and many illegal additives. In recent years varieties of extractants have been applied for the extraction of food contaminants, which focused on the emerging extractants for food safety evaluation. The extraction techniques of food samples are used for functions of sample clean-up, separation, enrichment, and derivatization. Liquid-phase extraction and solid-phase extraction are most traditional extraction technologies. Most of the analytical techniques for the extraction of food samples are energy and time-consuming and normally require relatively large

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amounts of organic solvents. Based on the liquid-phase and solid-phase extractions, for food sample extraction, the innovative IL-based [172] approaches have been explored to determine toxic metals and organic contaminants in the variety of liquid or gel food samples, including oils, ketchups, eggs, juices, honey, wines, milk, and ice cream.

8.9 Applications of ionic liquids The separation and extraction of biologically active compounds and pollutants from the different samples is a great challenge for scientists. In this framework, ILs are proven to be promising media to solve and minimize such type of challenges. Due to the possibilities of designing physicochemical properties and to modify the chemical structure into the selected chemical compounds by selecting appropriate cation and anion, ILs have variety of applications. Their unique abilities make them usable as targeted agents in many areas, such as refinery industry, agricultural industry, biotechnology, science, and medicine [173 176]. They can be used as green solvents in extraction as well as microextraction processes for the separation techniques (such as thin layer, liquid, and gas chromatographies), synthesis, catalysis, spectroscopy, electrochemistry or capillary electrophoresis replacing toxic and flammable classic organic solvents [23,177 180]. The IL-based extraction is a novel and promising separation method. The unique property of ILs to dissolve many organic or inorganic compounds, make them a better choice for volatile organic compounds in solvent extraction. The extraction systems with some ILs, for instance, imidazolium IL systems have been found as higher extractability and selectivity than those with traditional solvents. For monometallic and bimetallic nanoparticles’ synthesis, ILs have been explored as reaction media, hydrogen sources, templating agents, catalysts, and stabilizers [181]. The stability of nanoparticles in ILs is influenced by IL viscosity, hydrogen-bonding capability, and the relative ratio of polar and nonpolar domains [181]. The water-insoluble ILs have crucial role in the liquid liquid extraction technique [182], such as single drop micro-extraction, liquid-phase microextraction, and dispersive liquid liquid micro-extraction. As it can be observed, ILs have been found more effective as masking agents than triethylamine, dimethyloctylamine, or ammonia for the separation of bases.

8.10 Conclusion and future prospective To minimize the use of organic solvents, there is a need of special efforts on ILs. These are reducing cost-effective, safe, reusable, and friendly with environment. Compared to ordinary molecular liquids,

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the ILs that show lower vapor pressure, higher stability, lower combustibility, strong electrostatic force among ions, and higher conductivity have been proved to advantageous for industrial applications and analytical chemists. The properly selected ILs are able to derive the high amount of extraction yields than traditional solvents. The aim of this chapter is to focus on the extensive application of ILs for the extraction of different classes of natural as well as synthetic organic compounds (e.g., vitamins, lipids, proteins, amino acids, nucleic acids, flavonoids, alkaloids, terpenes, terpenoids, antioxidants, and phenolic compounds), drugs/pharmaceuticals, metal ions and rare earths from biological (e.g., urine and blood), and environmental samples (e.g., water, soil, and biomass). In the future, the present chapter facilitates researchers to achieve new, measurable benefits.

Acknowledgments The authors would like to acknowledge with thanks for the consideration of this work. Jamal Ahmad Khan gratefully acknowledges Applied Sciences and Humanities, University of Polytechnic, Z.H. College of Engineering and Technology, Aligarh Muslim University, Aligarh, and Shagufta Jabin is thankful to Faculty of Engineering, Manav Rachna International Institute of Research and Studies, Faridabad, Haryana, India.

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[158] A.F. Patti, M. Clarke, J.L. Scott, Ionic liquid extractions of soil organic matter, in: 19th World Congress of Soil Science, Soil Solutions for a Changing World, Brisbane, Australia, 1 6 August 2010. [159] V. Balaram, Rare earth elements: a review of applications, occurrence, exploration, analysis, recycling, and environmental impact, Geosci. Front. 10 (2019) 1285 1303. Available from: https://doi.org/10.1016/j.gsf.2018.12.005. [160] Q. Gao, I. Opahle, O. Gutfleisch, H. Zhang, Designing rare-earth free permanent magnets in heusler alloys via interstitial doping, Acta Mater. 186 (2020) 355 362. Available from: https://doi.org/10.1016/j.actamat.2019.12.049. [161] J.M.D. Coey, Perspective and prospects for rare earth permanent magnets, Engineering 6 (2020) 119 131. Available from: https://doi.org/10.1016/j.eng.2018.11.034. [162] K. Habib, S.T. Hansdo´ttir, H. Habib, Critical metals for electromobility: global demand scenarios for passenger vehicles, 2015 2050, Resour. Conserv. Recycl. 154 (2020) 104603. Available from: https://doi.org/10.1016/j.resconrec.2019.104603. [163] H. Wei, Y. Li, Z. Zhang, W. Liao, Synergistic solvent extraction of heavy rare earths from chloride media using mixture of HEHHAP and Cyanex272, Hydrometallurgy 191 (2020) 105240. Available from: https://doi.org/10.1016/j.hydromet.2019.105240. [164] C. Tunsu, Y. Menard, D.Ø. Eriksen, C. Ekberg, M. Petranikova, Recovery of critical materials from mine tailings: a comparative study of the solvent extraction of rare earths using acidic, solvating and mixed extractant systems, J. Clean. Prod. 218 (2019) 425 437. Available from: https://doi.org/10.1016/j.jclepro.2019.01.312. [165] T.T. Tran, M.S. Lee, Separation of Mo(VI), V(V), Ni(II), Al(III) from synthetic hydrochloric acidic leaching solution of spent catalysts by solvent extraction with ionic liquid, Sep. Purif. Technol. 247 (2020) 117005. Available from: https://doi.org/ 10.1016/j.seppur.2020.117005. [166] M. Matsumiya, Y. Song, Y. Tsuchida, H. Ota, K. Tsunashima, Recovery of platinum by solvent extraction and direct electrodeposition using ionic liquid, Sep. Purif. Technol. 214 (2019) 162 167. Available from: https://doi.org/10.1016/j.seppur.2018.06.018. [167] N.N. Hidayah, S.Z. Abidin, The evolution of mineral processing in extraction of rare earth elements using liquid-liquid extraction: a review, Min. Eng. 121 (2018) 146 157. Available from: https://doi.org/10.1016/j.mineng.2018.03.018. [168] M. Iqbal, K. Waheed, S.B. Rahat, T. Mehmood, M.S. Lee, An overview of molecular extractants in room temperature ionic liquids and task specific ionic liquids for the partitioning of actinides/lanthanides, J. Radioanal. Nucl. Chem. 325 (2020) 1 31. Available from: https://doi.org/10.1007/s10967-020-07199-1. [169] F. Kubota, M. Goto, Application of ionic liquids for rare-earth recovery from waste electric materials, in: F. Veglio`, I.B.T.-W.E., E.E.R. Birloaga (Eds.), Woodhead publ. Ser. Electron, Opt. Mater., Woodhead Publishing, 2018, pp. 333 356. Available from: https://doi.org/10.1016/B978-0-08-102057-9.00012-3. [170] N.N. Hidayah, S.Z. Abidin, Extraction of light, medium and heavy rare-earth elements using synergist extractants developed from ionic liquid and conventional extractants, C. R. Chim. 22 (2019) 728 744. Available from: https://doi.org/ 10.1016/j.crci.2019.10.006. [171] H. Okamura, N. Hirayama, Recent progress in ionic liquid extraction for the separation of rare Earth elements, Anal. Sci. 37 (1) (2021) 119 130. Available from: https://doi.org/10.2116/analsci.20SAR11. [172] A. Martı´n-Calero, V. Pino, A.M. Afonso, Ionic liquids as a tool for determination of metals and organic compounds in food analysis, TrAC Trends Anal. Chem. 30 (10) (2011) 1598 1619. [173] A. Soto, Ionic liquids for extraction processes in refinery-related applications, in: H. Rodrı´guez (Ed.), Ionic Liquids for Better Separation Processes, Springer, Berlin/ Heidelberg, Germany, 2016, pp. 39 65.

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C H A P T E R

9 An involvement of ionic liquids and other small molecules as promising corrosion inhibitors in recent advancement of technologies in chemical industries Shweta Pal1, Mansi Chaudhary1, Pallavi Jain2, Prashant Singh1, Anita Kumari Yadav3, Shailendra Kumar Singh4 and Indra Bahadur5 1

Department of Chemistry, Atma Ram Sanatan Dharma College, University of Delhi, New Delhi, India, 2Faculty of Engineering and Technology, Department of Chemistry, SRM Institute of Science and Technology, NCR Campus, Ghaziabad, Uttar Pradesh, India, 3Department of Chemistry, Rajdhani College, University of Delhi, New Delhi, India, 4Department of Chemistry, Hans Raj College, University of Delhi, New Delhi, India, 5 Department of Chemistry, Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Natural and Agricultural Sciences, North-West University, Mmabatho, South Africa

Researchers are trying to learn, prevent and control the corrosion of different metallic objects. The scientists and researchers across the world are working on it with the more focus on iron (metal). Different metals are extracted from their ores, using selective protocols and techniques, involves the usage of lots of energy and incurs heavy cost. These corrosive metals can be combined with different other elements to give

Advanced Applications of Ionic Liquids DOI: https://doi.org/10.1016/B978-0-323-99921-2.00004-5

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9. An involvement of ionic liquids and other small molecules as promising corrosion inhibitors

Fe

Fe

2+

H2O, Cl

Fe(II)m(OH, Cl)n

-e

β-FeOOH

-

-e

Fe(II,III), oxide amorphous

α-FeOOH

+e

Fe3O4

+ e

γ-FeOOH

FIGURE 9.1 Electrochemical phenomenon of corrosion.

characteristic and promising compounds/materials having low energy levels, leading to inhibition of corrosion process. Corrosion in simple words can be explained as the deterioration of a metal at its surface when they are exposed to a specific environment, like oxygen, moisture, acidic, etc. [1
7]. Corrosion is an electrochemical phenomenon, which is explained in Fig. 9.1. Electrons are transferred between an aqueous electrolytic solution and a metal surface during corrosion. It results due to the tendency of the metal to react with water, oxygen, and other molecules electrochemically in the aqueous environment [8
11].

9.1 Consequences of corrosion Corrosion has shown an huge impact on health, technology, economy, safety, and culture of our society as outlined in Fig. 9.2.

9.2 Economic effects Corrosion is a constant, expensive/costly, and big problem to the world. To determine the economic losses due to corrosion in a particular country, studies have been performed by the researchers. Findings showed that the total hit on the US economy due to metallic corrosion is an estimate of a few hundred million dollars per annum. Based on reported literature, corrosion influences nearly every sector like government, transportation, utilities, foundation and infrastructure, and industry: manufacturing and assembling. Most of the instruments used in general medicine are metallic

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9.2 Economic effects

Architecture

Equipment

Defense Sector

Corrosion

Transports

Ships

FIGURE 9.2 Impact of corrosion in different areas of the society.

in nature. Plates, pacemakers, pins, hip joints, and other implants are metal prosthetic gadgets, and the utilization of these gadgets has been increasing in the body recently. Many composites and alloys have been developed for implantation but corrosion is still a cause of worry. There are immense disappointments through broken associations for pacemakers, aggravation due to corrosion in the tissue around implants, and break of weightbearing prosthetic gadgets have been worrisome. An illustration is the utilization of metallic hip joints, which can ease a portion of the issues of ligament hips. In the last few years, the situation is improved where hip joints were used only for over 60 years and now can be used for younger persons due to their longer lasting effects on hip joints [12
16]. The more considerable critical issue is the degeneration of infrastructure, which can bring about serious damages or even death toll. Well-being is being undermined by corrosion adding to disappointments of damages to, airplanes, cars, gas pipelines, and so forth, the entire complex of metal structures and gadgets that make up the advanced world. The monetary losses of corrosion influence innovation. A lot of the advancement of innovation is kept down by corrosion issues since new materials are needed to withstand, associated pressing factors, and overcome profoundly destructive conditions. Tackling corrion is a tough problem to hadle and not being addressed properly as per the demand. One can expect materials to withstand exceptionally focused arrangements of destructive salts at high temperatures and pressing factors. It has been observed that the

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corrosion occurs in the oil pipeline bored in ocean and it might be due to various microbes, sulphides and cliamte. This corrosion creates lots of hurdles/problems like in extraction of oils. In a significant number of examples, corrosion is a restricting component forestalling the improvement of financially or even innovatively functional frameworks [17
22].

9.3 Methods to control corrosion 9.3.1 Material selection Each metal and compound has interesting and distinctive corrosion behavior that can go from the high obstruction of nobel metals like platinum/gold to sodium/magnesium. However, there are a few contending or withstanding materials that can meet the corrosion necessities and requirements. A specific disposal alloy for the corrosion issue is replaced with a more notable alloy that is resistant to corrosion [3,23
29].

9.3.2 Coating Coatings used for protection from corrosion can be isolated into two general categories: metallic and non-metallic (can be organic and inorganic). With one or the other kind of covering, the goal is the equivalent, that is, to separate the fundamental metal from the media of corrosion [30
35]. 9.3.2.1 Metallic coating The idea of applying a more praiseworthy metal covering the metal that is active or prone towards corrosion takes advantage of the more prominent corrosion resistance of the respectable metal. The examples for this application, that is corrosion inhibition, is tin-plated steel. On the other hand, a more dynamic metal can be applied, and for this situation, the covering is consumed specially in preference to the substrate. An example to this framework is galvanized steel, where the conciliatory zinc coating corrodes specially and ensures the steel. 9.3.2.2 Organic coating The essential capacity of organic coatings in protection from corrosion is to separate the metal from the climate of corrosion. In addition to create a barrier layer for corrosion, the organic coating may bring corrosion inhibition.

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227

9.3.2.3 Inorganic coatings It incorporates porcelain lacquers, chemical compound setting silicate concrete linings, glass coatings, and other corrosion-safe ceramics. Like organic coatings, inorganic coatings for applications of corrosion may fill in as hindrance coatings. Some coatings of ceramics like carbides and silicides are utilized for safe-wear applications.

9.4 Inhibitors Restraint is a preventive measure against the destructive assault on metallic materials. It comprises the utilization of organic compounds that provide little fixations or alterations to an influential climate and could diminish corrosion of the uncovered or open metal. Inhibitors decrease the impact of either or both of the incomplete responses (anodic oxidation as well as cathodic decrease) responsible for corrosion. Consequently, they are named anodic, cathodic, and blended (mixed) inhibitors. Inhibitors can be utilized in electrolytes at various pH values from acidic to neutral and then to alkaline medium. As a result of multiple circumstances created by changing different factors like medium inhibitor in the arrangement of metal/corrosion medium/inhibitor, different hindrance mechanisms are conceivable. Interface hindrance assumes a solid association between the inhibitor and the corroding surface of the metal. Herein, the inhibitor adsorbs as layer on the surface of metal. This layer can impact the essential corrosion responses in an unexpected way; by a mathematical obstructing impact of the surface of electrode because of the adsorption of a steady inhibitor at high degree of inclusion of the metal surface or by an impending impact on active surface sites due to the adsorption of a steady inhibitor. For this situation, the adsorption is trailed by the electrochemical or substance responses of the inhibitor at the interface [9,12,36]. Researchers are working toward finding green corrosion inhibitors as depicted in Fig. 9.3.

9.5 Anodization Aluminum alloy is genrally applied for the treatment of surface and is known as anodization in a chemical substance bath. Electrochemical conditions in the chemical bath are thoroughly changed so that the uniform pores of a few nanometers width show up in the metal oxide’s film. These pores permit the development of thick oxide than passivating conditions would permit. Toward the finishing of the treatment, the pores are permitted to close, framing a harder-than-expected and, hence, more defensive

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9. An involvement of ionic liquids and other small molecules as promising corrosion inhibitors

Organic inhibitor

Amino acids Drugs Surfactants Plant extracts Biopolymer Ionic liquids, etc.

Eco-friendly Corrosion inhibitors

Inorganic inhibitor

Rare earth materials etc.

FIGURE 9.3 Types of green corrosion inhibitors.

surface layer. If this protecting layer is damaged, then a typical passivation take place and may harm the zone.

9.6 Cathodic protection Cathodic protection succesfully inhibit the corrosion. Subsequently, the corrosion or metal disintegration is avoided. Cathodic protection can be accomplished by two approached, which vary depending on the wellspring of the proactive current. A current framework utilizes a force source to constrain current from inactive anodes to the structure to be ensured. A conciliatory anode framework utilizes active metal anodes (zinc or magnesium), are associated with the structure to give the cathodic-insurance current.

9.7 Structure of electrical double layer A basic model of the twofold layer is proposed by Helmholtz, and herein, the charges at the metal/arrangement interface were viewed as two plates establishing an equal plate capacitor. The model was further changed by Stern and anticipated the two unique areas of charge barrier. In liquid aqueous arrangement, the adsorption of water particles and different molecules go as dielectric. The solution side of the twofold layer is believed to be comprised of multiple layers. Nearest to the electrode, the internal layer contains dissolvable atoms and, at the same times, other species that are supposed to be adsorbed. This layer is additionally called the compact Helmholtz layer. The center of the electric places of explicitly adsorbed particles is the Inward Helmholtz plane. The solvated ions can move toward the metal just to a distance, that

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is the center of the focus of these closest solvated ions and is known as Outer Helmholtz plane. The association of the solvated particles with the charged metal is through long-range electrostatic forces so their communication is basically independent of the ions. The overabundance or huge amount of charge at the mercury/electrolyte arrangement interface with the interface potential has been widely concentrated by the Lippmann electrometer, in which mercury is in a fine narrow contact with the arrangement under examination. The interfacial pressure increases as the difference between the interface and the reference electrode increases. It arrives at a greatest when a further increase in possible outcomes declines in interfacial tension. The inclination of the electrocapillary bend at a given potential gives the charge density, and it is positive on the climbing state and negative on the dropping state. At the electrocapillary, the inclination is zero and there is no overabundance charge at the interface. Electrocapillary has the potential of zero charge compared to the most extreme one.

9.8 Influence of temperature on the action of Inhibitors When the temperature rises, corrosion increases due to increase in the generation of hydrogen. The action of inhibitors is diminished at higher temperatures as these adsorb less significantly at a higher temperature. This significant rise in the response rate inferable from the surrounding region of metal presented to corrosive media. For these inhibitors, the Eeff is more significant/important with respect to the inhibitor than acid. Increase in temperature of metal coated with inhibitor weakens the corrosion inhibition ability of the inhibitors (tetraphenylphosphonium iodide etc). The activity of inhibitors is portrayed by a lower estimation of the temperature coefficient than for the corrosive cycle. However, corrosion process in their non-appearance, are of significant interest to avoid the corrosion at higher temperatures. It means dibenzyl sulfoxide and dibenzylsulfide in hydrochloric acid (nitrogenous bases) are immovably coated on the metal surface. They are bound to the surface of metal by forces of adsorption during chemisorption. The decrease in the estimation of Eeff is presumably represented by the expansion in surface area of the metal covered by molecules of inhibitor as temperature rises. The involvement or role of the inhibitor leads to sort of passivation of the metal, and therefore the response between metal and acid can happen just by dispersion of acidic anions through the fine pores of the defensive framed layer. The energy of the corrosion obtains the personality of a dissemination cycle, which justifies the low-temperature coefficient of the response. The term “materials” refers to those substances utilized in the development of machines, measure gear, and other manufactured items [37
46].

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9. An involvement of ionic liquids and other small molecules as promising corrosion inhibitors

These materials incorporate metals, polymers, and production of ceramics. The conditions are liquids or gases albeit under extraordinary conditions, as for corrosion certain solid
solid reactions may be incorporated. Rusting occurs at a small point in the beginging and the particle size of metal oxide is very small and led to the corrosion later. Some of the exampled of rusing are as: • Rusting of cast iron and steel in water and air, as what happens with modern water tanks, and uncovered steel structures. • In automotive cooling systems, corrosion of cast iron, aluminum, and copper. • In chemical processes, corrosion of nickel-base, copper-base, ironbase, and many other alloys. • Automobile corrosion of vehicle exhausts frameworks by direct response of the metal with high-temperature gases having moisture, and retention of the oxides of sulfur and nitrogen to create aqueous acidic conditions. • Corrosion of turbine edges in gas turbines by hot burning gases. • By liquid metals, corrosion of nickel-base and iron-base alloys. • Enhanced disintegration of underlying concrete and stone by association with dense dampness and acidic pollutants noticeable all around like the oxides of sulfur and nitrogen. • Stress-corrosion cracking of brass and gold by mercury. An assemblage of metal could frame expected distinction on its surface due to nonconsistency of properties on its surface as in Fig. 9.4. The metal itself is a channel and henceforth permits the movement of electrons. The point when the surface of metal exposed with the electrolyte, the electrochemical responses occurs on the metal and it caused rusting [40,47
52]. These days, carbon steel is broadly utilized as a construction material in numerous ventures because of its incredible mechanical properties

FIGURE 9.4 An understanding of the corrosion process.

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9.8 Influence of temperature on the action of Inhibitors

231

and ease of use. Unavoidably, the corrosion issues are of incredible interest for the businesses since they can cause tremendous financial misfortunes just as safety dangers. One strategy for shielding it from corrosion is to utilize organic inhibitors or heterocyclic compounds containing P, S, O, or N. Broadly utilized harmful corrosion inhibitors and the natural guidelines encompassing their utilization and removal of incite incredible interest in replacing unkind inhibitors with compelling non-dangerous options. Ionic liquids (ILs), being made altogether out of ions, have been of interest to electrochemists. ILs have an enormous potential as an ecologically generous response mode for reasonable synthetic combinations. Since the mid-1990s, quickly expanding research endeavors have indicated that ILs can be a substitute for regular and conceivably dangerous solvents in a wide scope of applications. There is an additional ability that the use of ILs can widen the scope of control of corrosion. ILs appear to be an ideal contender to take the place of conventional toxic, harmful corrosion inhibitors. Apporaches being used to avoid corrosion are expensive/costly and can be immplemented at a small scale, therefore, the approached could be questioned to apply on large scale at a later stage. The issue of corrosion is not simple and could not be tackled in less span of time, There is a need to work to find promising approaches at a large scale to tackle this issue. The molecular structure of the inhibitor play an important role and taken based on the adsorption ability, and therefore limit effectiveness. This issue calls for the consideration of numerous examiners. Studies in the region of electrochemistry have presumed that the request for restraint productivity inclination by compounds containing heteroatoms is with the end goal of O , N , S , P reactivity order. On account of long-time use with leading cost structures, it is beyond expectations to refresh the data consistently. Investigations have showed that consumption costs are significantly huge and a figure of around billions of US dollars has been spent in past on the repairing or replacement. In any event, 35% of the above amount might have been saved by taking suitable control measures for the corrosion menace. Till this point of time, the most generally utilized inhibitors to battle or prevent corrosion in these frameworks have been founded on chromates and nitrites. These are currently perceived as being hurtful/harmful to humankind and the climate, and their utilization is being debilitated and new options looked for. The improvement of “green” inhibitors is in direct reaction to a few distinct guidelines that have been set up in numerous locales/place of the world. From all these, the significance of this investigation is appropriately featured, and accordingly, research around it can’t be sabotaged. ILs have huge impressive consideration because of their attractive/fascinating abilities and applications. The utilization of ILs as a

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9. An involvement of ionic liquids and other small molecules as promising corrosion inhibitors

Task specific ILs Chiral ILs

Bio ILs

Ionic Liquids (ILs) Polarizable ILs

Amphiphile ILs

Switchable Polarity Solvents

FIGURE 9.5 Types of common ionic liquids.

responses medium may offer a helpful framework to restrain the corrosion. ILs have positive solvating practices for a wide scope of polar and non-polar solvents (Fig. 9.5). Therefore, they are named green solvents because of their amazing abilities. The properties of ILs can be tweaked or tunes for explicit applications by changing their ionic segments. Some common ions present in ILs are bis((trifluoromethyl)sulfonyl)imide (NTf2), hexafluorophosphate (PF6), tetrafluoroborate (BF4), carboxylates, pyridine, and imidazolium (Fig. 9.6). These ILs have distinctive side chains, every blend bringing about ILs with various thermodynamic, synergist, and transport properties. Based on the interest or applications to be explored, the ILs can be designed easily.

9.9 Corrosion inhibition—an inevitable arena of research The corrosion restraint is a surface process, which includes the adsorption of specific compounds called inhibitors on the metal surface. The adsorption relies mostly upon the molecular electronic structure. The restraint effectiveness of inhibitors relies upon the method of collaboration with the molecular structure and the metal surface. The investigation of ILs as corrosion inhibitors might be an optional field of exploration because of their handiness in different enterprises. Mostly heterocyclic compounds and other molecules having bonding abilities can be adsorbed on surface of metal [5
7,30,53
60].

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233

FIGURE 9.6 Common cations and anions to get various ILs.

9.10 Importance of ionic liquids (ILs) ILs have shown applications in different areas of science like catalysis, organic synthesis, protein stabilization, preparation of nanomaterials, etc. Presently, the point of the part is to clarify the potential of ILs in the inhibition of corrosion. A large portion of the basic substances utilized as inhibitors or fused in anti-corrosive pretreatments of metals is profoundly harmful and their utilization produces genuine natural risks (Fig. 9.7). Thus, an extreme examination/investigation is needed to be done to supplant these by more naturally agreeable details with low toxicity, phenomenal biodegradability, little bioaccumulation, and adequate safety limit. Hence, an endeavor needs to be made to synthesize green inhibitors (ILs) and to assess their presentation remembering the previously mentioned necessities [12,36,37,39,42,45,46,61
63].

9.11 Corrosion is a costly problem to the world Metal surfaces require the application of coatings possessing high passivating properties. The system should have a strong tendency toward surface adsorption and the ability to form a relatively strong and stable coating on the metal surface. For a proper selection of a maintenance coating system, the risk conditions have to be defined as in Fig. 9.8.

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FIGURE 9.7 Classification of corrosion inhibitors.

FIGURE 9.8 Various conditions of corrosions on metal.

9.12 Ionic liquids as promising coating agents and inhibitors With the coming of the 21st century, corrosion science is having its spot on the world platform as a developing science with the accentuation on climate kind disposition. One of the significant strategies for limiting corrosion is the utilization of corrosion inhibitors. Corrosion inhibitors are widely utilized in different applications and many industrial plants activities are reliant on their fruitful application. There exists an extraordinary need of knowledge into corrosion inhibition to empower all who are confronted with the assignment of choice and utilization of corrosion inhibitors to have an appropriate comprehension of the component of activity of inhibitors as in Table 9.1. ILs are being considered green substitutes for organic solvents like DMSO, (CH3)2CO, and others. The standing of these solvents as “environmentally friendly” synthetics depends fundamentally on their insignificant fume pressure. ILs give perhaps the best answer for battling corrosion in a way that would be most savvy and climate cordial just as appropriate for people and other living creatures/living organisms [7]. Diminishing or dispensing with poisonous materials and utilizing all

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9.12 Ionic liquids as promising coating agents and inhibitors

TABLE 9.1

ILs as corrosion inhibitors.

S. no.

ILs as corrosion inhibitors

Type of metal

Method

1

1,4-di [1-methylene-3-methyl imidazolium bromide] and 2-MeHImn 4-OHC, 2-MeHImn Br

Mild steel

Electrochemical

Carbon steel

Electrochemical impedance

[TBA][L-Met] 2

N, N0 -(cyclohexane-1,4-diylbis (oxy))bis(2-oxoethane-2,1-diyl) bis(N,N-dimethylalkan-1aminium) methylsulfate 1,4-Di(vinylimidazolium)butane bisbromide (DVIMBr) [BMIM][BF4], [BMIM][Otf], and [Cholinium][Acetate] [BMMB]1[Br]2 1

Electrochemical noise analysis

2

[BMEB] [BF4] 3

1-Butyl-3-methylimidazoliumbased ILs with [BF4] and [PF6] as anion

Stainless steel

Electrochemical impedance spectroscopy (EIS)

[BsMIM][HSO4] and [BsMIM] [BF4] 4

1.3-bis[2-(4-methoxyphenyl)-2oxoethyl]-1H-benzimidazol-3ium bromide

Aluminum alloy composite

Electrochemical impedance spectroscopy and Potentio dynamic polarization methods

5

1-Allyl-3-butylimidazalium bromide

X65 steel

Electrochemical

6

1-Butyl-3-methyl imidazolium tetrafuoroborate (bmimBF4)

Steel/steel, steel/ copper and steel/ aluminum

Electrochemical

the more earth inviting the innovative corrosion inhibitors acknowledge quantifiable cost investment funds. Various ILs including imidazolium as cation are explored for their anti-corrosive ability for the mild steel and various metals. Based on theoretical knowledge and computational calculations, the anti-corrosive nature depends on the number of electronegative atoms and imine bonds present in the molecules, termed inhibitors. This conclusion is made due to interaction of the hetero-atoms, groups showed interaction with the surface of the metal. ILs have shown a promising

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9. An involvement of ionic liquids and other small molecules as promising corrosion inhibitors

ability to inhibit the corrosion even in acidic medium. Researchers have reported the impact of the alkyl chain on the ionic liquids on the inhibition efficiency. Some of the ILs with different anions were studied for their corrosion inhibition ability for copper in an acidic medium. ILs with chloride as anion was found to be better than with bromide. They followed the Langmuir adsorption and the inhibition effects occur because of the creation of an insoluble layer over the metal due to the adsorption of ILs on the copper. Imidazoilum ring-based ILs with different chains [1-hexyl-3-(4phenoxybutyl)-1H-imidazol-3-ium bromide (Ms35) and 3-(4-ethoxy-4-oxobutyl)-1-hexyl-1H-imidazol-3-ium bromide] were synthesized to explore their inhibition activity on steel and were found to be promising. They followed the Langmuir adsorption isotherm. Further, the negative change in free energy indicates their adsorption on the steel. The quantum calculation also corroborated the results obtained. ILs are employed in the aqueous form or solution for the treatment of surface. They have the ability to make a layer on the surface to protect them from the corrosive medium. Imidazolium ring-based ILs have been reported to be more promising when benzyl is replace by alkyl chain, as it can enhance the adsorption. On increasing the chain length in the cation can increase the adsorption of inhibitor on the surface of metal and so can enhance the corrosion inhibition. As mentioned earlier, studying the effect of the anion keeping the cation same to inhibit corrosion ability is important. The anion, bis(trifluoromethanesulfonyl)amide, has been explored to find the ability of the IL to inhibit corrosion. A synergistic effect of the IL, imidazolinium cinnamate, was studied at pH of 2 to inhibit the corrosion. At this pH, the IL gives imidazolinium chloride and sodium cinnamate for the protection of steel. Cetrimonium nalidixate also showed promising corrosion inhibition ability. 1-Octyl-3-methylimidazolium was taken as cation, and the anions (chloride and hydrogen sulfate) were chosen; found to show promising anodic inhibitive effect on mild steel at a low concentration. Brønsted acidic ILs (BAILs) have unique properties and ability in corrosion inhibition in different medium. 1-Butyl-3-methylimidazolium chloride with different anions (chloride and hydrogen sulfate) has shown good corrosion inhibition ability in mild steel. On increasing the alkyl chain of ILs, the corrosion inhibition ability on mild steel increases. The inhibition efficiency for hydrogen sulfate as anion is found better than chloride as an anion in the IL and showed Langmuir isotherm absorption. Further, on taking dibencilimidazolio acetate and dibencilimidazolio dodecanoate as anion, a dense film on the surface of metal is observed and saves the metal from corrosion in corrosive medium of sulfuric acid and hydrochloric acid. Different imidazolium ring-based ILs on varying the alkyl chain (ethyl, allyl, hexyl, and octyl) are taken to study their anticorrosive ability. They were found too good to inhibit the corrosion by forming a layer on metal surface. Corrosion inhibition for copper using ILs with different anions is

2. Separation technology

9.13 Other corrosion inhibitors

237

studied under strong acidic conditions. Both the ILs based on imidazolium contain active sites to interact with the copper and form a coating on the surface and save the metal from corrosion. Nanomaterials are reported as good corrosion inhibitors and the impact can be enhanced on adding the ionic liquids. The inclusion of ILs increases the chemical properties of nanomaterials like surface area, stability, and conductivity. They have also shown amazing anticorrosive ability in different mediums [7,8,64
76].

9.13 Other corrosion inhibitors Sunset Yellow (SS), Amaranth (AM), Allura Red (AR), Tartrazine (TZ), and Fast Green (FG) were reported to be green corrosion inhibitors for steel in 0.5 M HCl using gravimetric, potentiodynamic polarization (PP) techniques [77]. Amino acids were mixed to polysaccharide esters and were assessed as corrosion inhibitor for mild steel in 0.5 M HCl by weight method [78], Starch based polymers are used as promising inhibitors for metal [79]. The authors investigated the antibacterial activities of as-orchestrated silver nanoparticles (Ag-NPs) against Escherichia coli, Staphylococcus aureus, and Streptococcus faecalis. Likewise, the counterconsumption activities affirmed that the Ag-NPs demonstrated as great inhibitors [80]. Two novel corrosion inhibitors based in bispyrano[2,3-c] pyrazoles, BP-1 and BP-2 and assessed their consumption hindrance property on mild steel (MS) in corrosive arrangement through weight reduction and electrochemical consumption methods [81]. Cobalt and tin sulfide NPs are utilized to integrate chitosan
cobalt and chitosan
SnS2 nanocomposites and utilized for corrosion assurance of mild steel (MS) in 1 M HCl at room temperature [82]. Two chitosan subsidiaries were blended unexpectedly as green corrosion inhibitors for the carbon dioxide consumption of P110 steel [83]. Two chitosan oligosaccharide subordinates (PHC and BHC) were integrated to use as consumption inhibitors. The outcomes showed that hindrance proficiency expanded with expanding centralization of inhibitor. Estimations demonstrated that the inhibitors had been effectively adsorbed to the outside of the P110 steel [84]. The current examination focuses on the examination of minimal effort nontoxic sugar biopolymer chitosan as a consumption inhibitor alone and in blend with KI for gentle steel in 1.0 M sulfamic corrosive medium [85]. Cerium acetylacetonate (CeA) was stacked into a betacyclodextrin (beta-CD) nano-compartment and desorption was surveyed [86]. The corrosion hindrance qualities of the subsidiaries of biopolymer hydroxypropyl methylcellulose, hydroxypropyl methylcellulose phthalate, and hydroxypropyl methylcellulose acetic acid derivative succinate film were explored [87].

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9. An involvement of ionic liquids and other small molecules as promising corrosion inhibitors

2-isobutylthiazole and 1-(1,3-thiazol-2-yl)ethanone are explored to prevent the corrosion of X65 steel in H2SO4 by different techniques [88]. Two thiourea functionalized glucosamine subordinates were studied as ecoaccommodating consumption inhibitors for mild steel (MS) in a 1.0 M HCl arrangement. Electrochemical tests and surface examinations showed GA-1 and GA-2 have high hindrance execution [39]. A Schiff’s base named N,N0 -bis(4-hydroxybenzaldehyde)-2,2-dimethylpropandiimine (pHBDP) was chosen as green inhibitor for the consumption of low carbon steel in a 1.0 M HCl solution. Results demonstrated that the restraint happens through the adsorption of the inhibitor atoms on the metal surface. Polarization information showed that this compound work as a blended inhibitors and the adsorption isotherm fundamentally complies with the Langmuir adsorption isotherm to indicate the prevention of corrosion [89]. Novel amiable superior consumption inhibitors depending on biopolymer were performed in situ utilizing diverse cellulosic materials and niacin. The anti-corrosive ability of cellulose composites for copper in 3.5% NaCl arrangements was assessed. The information separated from EIS was fitted through an identical circuit to show the corrosion hindrance/prevention [90]. Two polymers, chitosan and carboxymethyl cellulose (CMC) were taken as inhibitors against the steel pipeline having CO2 immensed in 3.5% NaCl arrangement. Decrease in the rate of corrosion or the inhibition of corrosion could be achieved by using inhibitors. Potentiodynamic polarization (PP) results showed the inhibition of the corrosion using the inhibitors. The adsorption of every inhibitor on the steel surface complies with Langmuir’s isotherm [91]. Extracellular polymeric substances (EPSs) as corrosion inhibitors were tried with carbon steel in 3.64% NaCl immersed with CO2 at 25 C, which is the run of the mill oilfield climate. EPS restrains consumption on both anode and cathode locales of metal surfaces [92]. Chitosan (CHI), dextran (Dex), CMC, sodium alginate (ALG), gelatin (PEC), hydroxylethyl cellulose, and Gum Arabic (GA) enhance the corrosion [93] Tamarindus indica separate-Zn21 (GON-Ti.E-Zn) through an easy green course. It affirmed the incredible hindrance impact of GO and observable corrosion restraint effect of T. indica alongside the zinc cations on the gentle steel consumption moderation [94]. ILs with various Brønsted corrosive destinations were combined and their hindering properties for the corrosion of carbon steel in 0.5 M HCl arrangement have been assessed. The desgined cation and the alkyl chain of Brønsted acidic ILs (BAILs) can be inhibitors of significant potential due to their adsorption on the steel surface to smother both anodic and cathodic cycles and obeying Langmuir adsorption isotherm [95]. The corrosion restraint of Luffa cylindrica leaf extract was examined as corrosion inhibitor [96]. Eucalyptus leaves extract found to inhibit or avoid inhibition of the mild steel (MS) corrosion in the HCl arrangement was inspected by consolidated exploratory and computational

2. Separation technology

9.14 Conclusion

239

examinations [97]. Copper NPs doped carbon quantum dots (Cu/CQDs) nanohybrid was utilized as an inhibitor to decrease the minimum inhibitory concentration (MIC) [98]. Extracts of persimmon husk were explored as corrosion inhibitors by utilizing weight reduction and potentiodynamic polarization methods [99]. Three synthesized chitosan schiff’s bases (CSBs) were studied as corrosion inhibitors. The adsorption of CSBs on the gentle steel surface complies with the Langmuir adsorption isotherm [100]. The aqueous extract of Esfand seed was studied as corrosion inhibitors using experimental techniques and quantum calculations [101]. Plant extracts, drugs, and ILs can be used as promising inhibitors [7]. Researchers utilized extract of henna leaves, aloe vera, tobacco, and others as promisng inhibitors [102]. The impact of two thiazole compounds [4-methyl-5-vinylthiazole and 5-(2hydroxyethyl)-4-methylthiazole] on Cu corrosion in NaCl arrangement was investigated [103]. Sulfonated chitosan (SC) was presented as a promising green corrosion inhibitor. The weight reduction tests demonstrated that SC gives good corrosion inhibition proficiency [104]. Three sunflower-based compounds were used as corrosion inhibitors through potentiodynamic polarization methods [54]. Restraint efficiencies of three amino acids [tryptophan (B), tyrosine (C), and serine (A)] have been concentrated as green consumption inhibitors on corrosion of carbon steel through DFT. Quantum calculations showed that the order of inhibition activity of amino acids is tryptophan (B) . tyrosine (C) . serine (A) [105]. Chitosanthiosemicarbazide (CS-TS) and chitosan-thiocarbohydrazide (CS-TCH) were assessed as new corrosion inhibitors for mild steel consumption in 1 M HCl. They were found to obey Langmuir adsorption isotherm and showed both physical and chemical adsorption [106].

9.14 Conclusion Traditional types of insulation create the perfect environment for moisture and the resulting corrosion and spread more rapidly to create a groove for the rust. Neoteric solvents are an alternative to the traditional organic solvents used in different areas of science. ILs have shown a promising role in different disciplines of the sciences [107
112]. They are considered as non-volatile and less toxic corrosion inhibitor. Further, their polarity can be tuned by changing anions or cations as per the interest. They have been explored in protein stabilization, protein extraction, catalysis, conducting material, corrosion inhibitor and others. ILs can be used in corrosion protection of the metals. ILs assisted metal is playing an important role in supporting innovative technological progress to manage the corrosion of the material. ILs assisted coatings provide a medium for corrosion resistance and insulation for pipelines, tanks, and others.

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9. An involvement of ionic liquids and other small molecules as promising corrosion inhibitors

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1028. [92] L.C. Go, et al., Evaluation of extracellular polymeric substances extracted from waste activated sludge as a renewable corrosion inhibitor, Peer J. 7 (2019) e7193. [93] S.A. Umoren, et al., Exploration of natural polymers for use as green corrosion inhibitors for AZ31 magnesium alloy in saline environment, Carbohydr. Polym. 230 (2020) 115466. [94] S. Akbarzadeh, et al., A green assisted route for the fabrication of a high-efficiency self-healing anti-corrosion coating through graphene oxide nanoplatform reduction by Tamarindus indica extract, J. Hazard. Mater. 390 (2020) 122147. [95] S. Cao, et al., Green Bronsted acid ionic liquids as novel corrosion inhibitors for carbon steel in acidic medium, Sci. Rep. 7 (1) (2017) 8773. [96] O.O. Ogunleye, et al., Green corrosion inhibition and adsorption characteristics of Luffa cylindrica leaf extract on mild steel in hydrochloric acid environment, Heliyon 6 (1) (2020) e03205. [97] A. Dehghani, G. Bahlakeh, B. Ramezanzadeh, Green Eucalyptus leaf extract: a potent source of bio-active corrosion inhibitors for mild steel, Bioelectrochemistry 130 (2019) 107339. [98] S.T. Kalajahi, et al., Green mitigation of microbial corrosion by copper nanoparticles doped carbon quantum dots nanohybrid, Environ. Sci. Pollut. Res. Int. 27 (32) (2020) 40537
40551. [99] J. Zhang, et al., Investigation of Diospyros Kaki L.f husk extracts as corrosion inhibitors and bactericide in oil field, Chem. Cent. J. 7 (1) (2013) 109. [100] J. Haque, et al., Microwave-induced synthesis of chitosan Schiff bases and their application as novel and green corrosion inhibitors: experimental and theoretical approach, ACS Omega 3 (5) (2018) 5654
5668. [101] M.T. Majd, et al., Production of an environmentally stable anti-corrosion film based on Esfand seed extract molecules-metal cations: integrated experimental and computer modeling approaches, J. Hazard. Mater. 382 (2019) 121029.

2. Separation technology

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[102] P. Poornima Vijayan, M. Al-Maadeed, Self-repairing composites for corrosion protection: a review on recent strategies and evaluation methods, Materials (Basel) 12 (17) (2019). [103] S. Mo, H.Q. Luo, N.B. Li, Study on the influences of two thiazole flavor ingredients on Cu corrosion caused by chloride ion, J. Colloid Interface Sci. 505 (2017) 929
939. [104] A. Farhadian, et al., Sulfonated chitosan as green and high cloud point kinetic methane hydrate and corrosion inhibitor: experimental and theoretical studies, Carbohydr. Polym. 236 (2020) 116035. [105] M. Dehdab, M. Shahraki, S.M. Habibi-Khorassani, Theoretical study of inhibition efficiencies of some amino acids on corrosion of carbon steel in acidic media: green corrosion inhibitors, Amino Acids 48 (1) (2015) 291
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1757. [107] I. Bahadur, M. Mabaso, G. Redhi, P. Singh, S. Kumar, et al., Separation of aromatic solvents from oil refinery reformates by a newly designed ionic liquid using gas chromatography with flame ionization detection, J. Sep. Sci. 38 (2015) 951
957. [108] M. Kumari, J.K. Maurya, U.K. Singh, A.B. Khan, M. Ali, et al., Spectroscopic and docking studies on the interaction between pyrrolidinium based ionic liquid and bovine serum albumin, Spectrochim. Acta A Mol. Biomol. Spectrosc. 124 (2014) 349
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220.

2. Separation technology

C H A P T E R

10 Role of ionic liquids in bioactive compounds extractions and applications Alam Nawaz1,*, Moghal Zubair Khalid Baig2,*, Mohmmad Umiad3, Fahmeena Asmat3, Young-A Son2 and Moonyong Lee1 1

School of Chemical Engineering, Yeungnam University, Gyeongsan, Republic of Korea, 2Department of Advanced Organic Materials Engineering, Chungnam National University, Daejeon, Republic of Korea, 3 Aligarh Muslim University, Aligarh, Uttar Pradesh, India

10.1 Introduction The bioactive compounds extraction from biomass utilizing conventional solvents and techniques is strenuous and time-consuming. With the development of sustainable and integrated technologies, the extraction, purification, and processing of bioactive ingredients were made easy and fast, not only from biomass matrices but also from synthetic routes, and fermented processes. Earlier, conventional solvents were used in the extraction of bioactive ingredients from medicinal plants or herbs. With advancement in extraction and separation techniques, the purification process has been simplified, but a challenging task of high purity and high yield remained. Extraction phenomenon is extraction of mixture of target compounds from biomass and raw matrices, whereas separation technique * These authors Alam Nawaz and Moghal Zubair Khalid Baig contributed equally. All authors have given approval to the final version of the manuscript.

Advanced Applications of Ionic Liquids DOI: https://doi.org/10.1016/B978-0-323-99921-2.00005-7

247

© 2023 Elsevier Inc. All rights reserved.

248

10. Role of ionic liquids in bioactive compounds extractions and applications

is isolation of each bioactive ingredient from the mixture. When these two methodologies were combined into a single operating procedure, time and manpower consumption were reduced, but some other limitations like low extraction efficiency, poor selectivity, toxic volatile organic solvents, and complexity are to be addressed. Considering these drawbacks, many researchers have focused on greener solvents that are compatible with integrated and sustainable technologies. Thus they switched to ionic liquids (ILs), a more greener, nonvolatile, chemically, thermally, electrochemically stable with nonflammable nature. ILs were used alongside ecofriendly solvents for the extraction of medicinal and pharmaceutical ingredients, through which extraction efficiency was improved by multiple folds. ILs are liquefied salts comprised organic or inorganic anions and organic cations at temperatures less than 100
C [1]. The most aprotic ILs displayed negligible volatility, excellent thermal, electrochemical, negligible vapor pressure, low nucleophilicity, chemical stability, and nonflammability. Besides, they are recognized for their extraordinary dissolution capacity for diverse compounds from natural to synthetic, biological compounds like proteins, amino acids, nucleic acids, lipids, vitamins, saponins, essential oils, carbohydrates, and pharmaceutical ingredients [2,3]. From the beginning of the 21st century, ILs have revolutionized the extraction and separation techniques from being possible and potential solvents to selective and appropriate solvents [47]. Besides their solvation ability for wide range of target compounds, they also showed excellent hydrotropic and surfactant characteristics [8,9] that improved multiphasic aqueous solutions and distinctive solvation performance. In addition, ILs are acknowledged for their mutliion tunable solvents as they possess multiple ion-combinations characteristic, which overcome the volatility limitation of conventional organic solvents. Thus nonvolatile ILs can easily replace the volatile organic solvent that helps in making greener environment and reduces the valuation cost of processes. The extraction efficiency and level of purity of target compounds are important parameters that are to be considered during the extraction and separation process development. Biomass from the natural extracts are one of the sustainable renewable resources for biochemicals, biofuels, power, heat, and biomaterials with modern applications [10]. ILs have capability of dissolving cellulose and lignin, from where fabric or pulp extraction and separation can be processed [1113]. This chapter mainly focuses on the ILs utilization as biocompatible solvents for extraction, separation, and purification of bioactive ingredients that are synthesized, or produced through fermentation process or obtained from natural plant extracts. Several bioactive compounds are discussed here, that is, proteins, aminoacids, enzymes, fats, lipids, essential oils, phenolic acids, alkaloids, flavonoids, carotenoids, vitamins, nucleic acids, etc. A list of ILs (acronym and full name of cations and anions) that were used in bioactive compounds extraction is mentioned in Table 10.1.

2. Separation technology

TABLE 10.1

Ionic liquids cation and/or anion name and their acronym [14].

Cation name

Acronym 1

Anion name

Acronym

Alkyl-1,8-diazabicyclo[5.4.0]undec-7-enium

[CnDBU]

Acesulfamate

[Ace]2

Alkyl(tributyl)phosphonium

[Pn444]1

Acrylate

[Acr]2

Alkyltropine

[Cntro]1

Alaninate

[Ala]2

Ammoeng 100

[N114(2OmOH) (2OnOH)]1

Alkylphosphonate

[CnPO3]2

Ammoeng 102

[N218O(2OmOH) (2OnOH)]1

Alkyl sulfonate

[CnSO3]2

Ammoeng 110

[N221(O)nOH]1

Alkyl sulfate

[CnSO4]2

Chirally functionalized methylimidazolium

[CwHxNyOz]1

Aminoate

[AA]2

Decyltris(3-hydroxypropyl)phosphonium

[P10(3OH)(3OH)(3OH)]1

Asparatinate

[Asp]2

Ethyl l-phenylalaninium

[C2(l-Phe)]1

Benzoate

[Bz]2

Hexaalkylguanidinium

[CnCnCnCnCnCnguan]1

Bicarbonate

[Bic]2

N,N,N,N-tetramethyl-3-(triethoxy)silylpropyl-guanidinium

[(C2H5O) 3SiC3C1C1C1C1guan]1

Bis(2,4,4-trimethylpentyl) phosphinate

[TMPP]2

N,N,N-trialkylammonium

[N0nnn]1

Bis(2-ethylhexyl) phosphate

[BEP]2

N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium (cholinium)

[N111(2OH)]1

Bis(trifluoromethylsulfonyl)imide

[NTf2]2

N,N-dialkylammonium

[N00nn]1

Bitartrate

[Bit]2

N,N-dialkyl-N-(2-hydroxyethyl)ammonium

[N0nn(2OH)]1

Bromide

Br2 (Continued)

TABLE 10.1 (Continued) Cation name

Acronym 1

Anion name

Acronym

N,N-didecyl-N-methyl-d-glucaminium

[C10C10C1gluc]

Calkanoate

[Calc]2

N,N-dimethyl(2-methoxyethyl)ammonium

[N11(2(O)1)0]1

Carboxylate

[CnCO2]2

N,N-dimethyl(cyanoethyl)ammonium

[N011(2CN)]1

Chloride

Cl2

N,N-dimethyl-N-(2- hydroxyethoxyethyl)ammonium

[N11(2(O)2OH)0]1

Cinnamate

[Cin]2

N-alkyl-N,N-dimethyl-N-(2-hydroxyethyl)ammonium

[N11n(2OH)]1

Citrate

[Cit]32

N-benzyl-N,N-dimethyl-N-(2-hydroxyethyl)ammonium

[N11(2OH)(C7H7)]1

Cysteinate

[Cys]2

N-butyl-N-methylmorpholinium

[C4C1mor]1

Dialkylphosphate

[(Cn)2PO4]2

N-ethyl-N-[3-(triethoxy) silypropyl] morpholinium

[(C2H5O)3SiC3C2mor]1

Dicyanamide

[N(CN)2]2

N-methyl-N,N,N-trioctylammonium

[N1888]1

Dihydrogencitrate

[DHCit]2

Tetraalkylammonium

[Nnnnn]1

Dihydrogenophosphate

[H2PO4]2

Tetraalkylguanidinium

[CnCnCnCnguan]1

Dimethylcarbamate

[N(C1)2CO2]2

Tetraalkylphosphonium

[Pnnnn]1

Glutarate

[Glut]2

Tetrakis(hydroxymethyl)phosphonium

[P(1OH)(1OH)(1OH) (1OH)]1

Glycinate

[Gly]2

Trihexyltetradecylphosphonium

[P66614]1

Glycolate

[Glyc]2

Triisobutyl(methyl)phosphonium

[Pi(444)1]1

Good’s buffers

[GB]2

1-(2-Cyanoalkyl)-3-methylimidazolium

[(NC)CnC1im]1

Hexafluorophosphate

[PF6]2

1-(4-Sulfonylbutyl)-3-methylimidazolium

[(HSO3)C4C1im]1

Hydrogenosulfate

[HSO4]2

1,3-Dihexyloxymethylimidazolium

[(C6H3OCH2)2im]1

Hydroxide

[OH]2

1-Alkyl-1-methylpiperidinium

[CnC1pip]1

Iodide

I2

1-Alkyl-1-methylpyrrolidinium

[CnC1pyrr]1

Itaconate

[Ita]2

1-Alkyl-2,3-dimethylimidazolium

[CnC1C1im]1

Lactate

[Lac]2

1-Alkyl-3-methylimidazolium

[CnC1im]1

Levulinate

[Lev]2

1-Alkyl-3-methylpyridinium

[CnC1pyr]1

Lysinate

[Lys]2

1-Alkylimidazolium

[Cnim]1

Malonate

[Mal]22

1-Alkylpyridinium

[Cnpyr]1

Methacrylate

[MAcr]2

1-Allyl-3-alkylimidazolium

[aCnim]1

N-[tris(hydroxymethyl)methyl]-3amino-2-hydroxypropanesulfonate

[TAPSO]2

1-benzyl-3-methylimidazolium

[C7H7C1im]1

Nitrate

[NO3]2

1-Butyl-3-trimethylsilylimidazolium

[C4(C1C1C1Si)im]1

N-trifluoromethanesulfonyl leucinate

[Tf-Leu]2

1-Carboxyethyl-3-methylimidazolium

[(HOOC)C2C1im]1

N-tris(hydroxymethyl) methylglycinate

[Tricine]2

1-Hexyloxymethyl-3-methylimidazolium

[(C6H13OCH2)C1im]1

N-tris(hydroxymethyl)methyl-2aminoethanesulfonate

[TES]2

1-Hydroxyalkyl-3-methylimidazolium

[(OH)CnC1im]1

O,O-diethyl dithiophosphate

[DTP]2 (Continued)

TABLE 10.1 (Continued) Cation name

Acronym 1

Anion name

Acronym

1-Methyl-3-(triethoxy)silypropyl imidazolium

[(C2H5O)3SiC3C1im]

Oxalate

[Oxa]22

1-Propylamine-3-methylimidazolium

[(NH2)C3C1im]1

Perchlorate

[ClO4]2

1-Vinyl-3-(2-methoxy-2-oxylethyl)imidazolium

[VC1O(O)C2im]1

Phenilalaninate

[Phe]2

2-(Alkyloxy)-N,N,N-trimethyl-2-oxoethanaminium (betain)

[N111[2 O(O)n]]1 ([bet]1)

Phenylacetate

[PhAc]2

2-(Hydroxyethyl)-N,N-dimethyl-3-(triethoxy) silypropyl ammonium

[N11[3Si(2 O)(2 O)(2 O)] (2OH)]1

Prolinate

[Pro]2

3-(2-(Butylamino)-2-oxoethyl)-1-ethylimidazolium

[(CH2CONHC4H9) C2im]1

Saccharinate

[Sac]2

3-(Dimethylamino)-1-propylammonium

[N011(3 N)]1

Salicylate

[Sal]2

3-Alkyl-1-vinyl-limidazolium

[VCnim]1

Serinate

[Ser]2

[C12mim]

Sorbate

[Sor]2

1-Butyl-3-methylimidazolium tetrafluoroborate (BMIM [BF4])

Succinate

[Suc]2

1,5-Diazabicyclo[4.3.0]non-5-enium acetate ([DBNH] [OAc])

Sulfate

[SO4]22

7-Methyl-1,5,7-triazabicyclo[4.4.0]dec-5-enium acetate ([mTBDH][OAc]) 1,5-diazabicyclo[4.3.0]non-5-enium acetate

Tetrachloroaluminate

[AlCl4]2

1-Ethyl-3-methylimidazolium acetate N-hexyl-4,40 bipyridinium bromide ([C6byp]Br)a

a

Reference from [15].

Tetrafluoroborate

[BF4]2

Thiocyanate

[SCN]2

Tricyanomethanide

[C(CN)3]2

Trifluoroacetate

[CF3CO2]2

Trifluoromethanesulfonate

[CF3SO3]2

Tris(pentafluoroethyl) trifluorophosphate

[FAP]2

Valinate

[Val]2

2-(2-Methoxyethoxy)ethylsulfate

[C1(OC2)2SO4]2

2-(N-Morpholino)ethanesulfonate

[MES]2

2-[Bis(2-hydroxyethyl)amino] ethanesulfonate

[BES]2

2-Hydroxy-3morpholinopropanesulfonate

[MOPSO]2

4-(2-Hydroxyethyl)-1piperazineethanesulfonate

[HEPES]2

4-Chlorophenoxyacetate

[CPA]2

Bis(trifluoromethylsulfonyl)imide

([(CF3SO2)2 N])#

254

10. Role of ionic liquids in bioactive compounds extractions and applications

FIGURE 10.1

Properties, potentials, current applications, and research dealing of ionic

liquids.

In addition, Fig. 10.1 illustrates several IL-based processes such as liquidliquid extractions that are processed out either with IL-based aqueous biphasic systems (ABS) or hydrophobic ILs; solidliquid extractions that include ultrasound-assisted, microwave-assisted, or ultrasonic-microwave-assisted; solidphase extractions that include IL-modified materials; backward extractions including induced-precipitation techniques like three-phase partitioning and crystallization are also addressed. Last, the challenges still to be confronted in the ILs methods/techniques, applications, and extraction processes are addressed in this chapter.

10.2 Bioactive compound extraction from biomass In recent times, demand for nutraceuticals, cosmetics, and pharmaceutical ingredients has been increased because of more health consciousness and naturalism. However, traditional extraction processes have many flaws such as nonselectivity, low efficiency, high energetic input, lengthy process, usage of volatile, and toxic organic solvents, which negatively impact the human and the environment. With a constant increase in the need for natural compounds in pharmaceutical products, cosmetics, and nutraceuticals, much attention for safe solvents from renewable sources, for example, ILs [16] and more sustainable extraction techniques, was considered. ILs are one of the most studied alternate solvents that is suitable for these purposes [5,1719]. ILs are used in a combination of liquids (i.e., water-rich extract from biomass), aqueous or nonaqueous solvents that are applied in IL-based extraction techniques like:

2. Separation technology

10.2 Bioactive compound extraction from biomass

255

1. Liquidliquid extraction (LLE), a. Liquidliquid extraction with hydrophobic ILs, b. IL-based aqueous biphasic systems (IL-ABS) Furthermore, they are used as a mobile phase on solid interfaces, thus giving rise to solidliquid extraction. Moreover, they also associate with electromagnetic fields, leading to development of microwave and/or ultrasonic-assisted extraction techniques. 2. Solidliquid extraction (SLE), a. Simple/basic IL-based solidliquid extractions (IL-SLE) b. IL-based ultrasound-assisted extraction (IL-UAE), c. IL-based microwave-assisted extraction (IL-MAE), d. IL-based ultrasonic/microwave-assisted extraction (IL-UMAE), e. IL-based ultrahigh pressure extraction (IL-UPE), f. IL-based negative-pressure cavitation extraction (IL-NPCE), g. IL-based microwave-assisted homogeneous liquidliquid microextraction (IL-MA-HLLME), and ILs can be restricted to solid matrices giving rise to solidphase extractions: 3. Solidphase extractions (SPE).

10.2.1 Ionic liquid-based liquidliquid extractions Extracts from the biological sources are rich in hydrophilic content, thus initially hydrophobic water-immiscible ILs are used to extract lipophilic compounds (IL-LLE). In the next phase, water-miscible ILs along with salting-out agents are used to create IL-ABS. An impression of IL-based extraction processes and ILs used during the bioactive compounds extraction is illustrated in Table 10.2. 10.2.1.1 Liquidliquid extraction with hydrophobic ionic liquids Absalan et al. successfully used hydrophobics ILs (hexafluorophosphate-based) in the extraction of 3-indole-butyric acid (IBA), plant growth regulator from its aqueous extracts [36]. The extraction efficiency was recorded higher with a preconcentration factor of 100 when [C4C1im][PF6] was used. When back-extraction of IBA was done, IL was reutilized for five more times [36]. Concurrently, another group utilized [C4C1im][PF6], [C4C1mim][NTf2], [C8C1im][BF4] imidazolium-based ILs in the plant growth regulators extraction from plant fluid of fresh Kappaphycus alvarezii seaweed. They found that [C4C1im][PF6] was effective in total trans-zeatin extraction by 65%, total indole-3-acetic acid by 18% from the plant fluid. They did not observe any extraction when [C4C1mim][NTf2] was used [38]. These study proved that [C4C1im][PF6] is one of the best IL finding. In another effective finding of IL-LLE

2. Separation technology

TABLE 10.2 Extraction of bioactive (or organic) compounds from biomass using IL-based LLE. Bioactive compound

Natural source

Technology

Used ILs (solvents)

Medium

Reference

Aloe anthraquinones

Aloe powder

IL-ABS

[C2C1im]Br, [C4C1im]Br, [C6C1im]Br, [C4C1im] [BF4], [C2C1im][BF4], [C4C1im][N(CN)2]; salt 5 NaH2PO4, (NH4)2SO4, Na2SO4, and MgSO4

Salt 1 water

[20]

Caffeic acid, vanillic acid, gallic acid, and vanillin

Lignin depolymerization

IL-ABS

[C12C1im]Cl, [C14C1im]Cl

PEG8000 1 NaPA8000

[21]

Caffeine



IL-ABS

[C4C1im][CF3SO3], [C4C1im][BF4], [C4C1im] [N (CN)2]; amino acids 5 l-Pro, l-Lys

Sugars 1 water, amino acids 1 water

[22,23]

Caffeine and nicotine

Mixture of alkaloids

IL-LLE

[P66614][NTf2], [P66614]Br, [P66614]Cl, [P66614][C1SO3], [P66614][N(CN)2], [P66614][TMPP]

K3PO4 1 water

[24]

Caffeine and nicotine

Synthetic urine

IL-ABS

[C2C1im]Cl, [C4C1im]Cl, [C6C1im]Cl, [C4C1C1im]Cl, [C7C1im]Cl, [C8C1im]Cl, [aC1im]Cl, [C10C1im]Cl, [C12C1im]Cl, [OHC2C1im]Cl, [C7H7C1im]Cl, [C2C1im][CF3SO3], [C2C1im][CF3SO3]

K3PO4 1 water

[25]

Capsaicin

Capsicum frutescens

IL-ABS

[N111(2OH)]Cl, [N111(2OH)][Bit], [N111(2OH)][DHCit]

Acetonitrile 1 water

[26]

Codeine and papaverine

Pericarpium papaveris

IL-ABS

[C4C1im]Cl

K2HPO4 1 water

[27]

Ellagic acid

Acacia catechu and Terminalia chebula

IL-LLE

[N1100][N(C1)2CO2]

Pure

[28]

Eugenol and propyl gallate



IL-ABS

[C2C1im]Cl, [C4C1im]Cl, [C6C1im]Cl, [C8C1im]Cl, [C4C1pip]Cl, [C4C1pyrr]Cl, [CnC1im]Cl (n 5 28)

C6H5K3O7/C6H8O7 1 water, PEG 1 K2HPO4/KH2PO4

[29]

Flavonoids and pectin

Ponkan peels

IL-ABS

[N111(2OH)][Ala], [N111(2OH)][Ser], [N111(2OH)][Cys], [N111(2OH)][Pro], [N111(2OH)][Asp], [N111(2OH)][Val], [N111(2OH)][Leu], [N111(2OH)][Phe]

K3PO4 1 water

[30]

Gallic acid



IL-ABS

[C7C1im]Cl, [C8C1im]Cl, [C2C1im][CF3SO3], [C4C1im][CF3SO3], [C4C1im][N(CN)2], [C4C1im] [C1SO4], [C4C1im][C8SO4], [C4C1im][C2SO4], [C4C1im]Br; salt 5 K3PO4, Na2SO4, and K2HPO4/ KH2PO4

Salt 1 water

[31]

Gallic, vanillic, and syringic acids



IL-ABS

[C4C1im][CF3SO3], [C4C1im][SCN], [C4C1im] [C1SO3], [C4C1im][C2SO4], [C4C1im][C1SO4], [C4C1im][Tos], [C4C1im]Br, [C4C1im][N(CN)2], [C4C1im][(C1)2PO4], [C4C1im]Cl, [C4C1im][C1CO2]

Na2CO3 1 water

[32]

Gallic, vanillic, and syringic acids

Mixture of phenolic acids

IL-ABS

5 wt.% [C4C1im][Tos], 5 wt.% [C4C1im][SCN], 5 wt.% [C4C1im][N(CN)2], 5 wt.% [C4C1im][C1CO2], 5 wt.% [C4C1im]Cl, 5 wt.% [C4C1pyrr]Cl, 5 wt.% [C4C1pip]Cl

PEG300 1 Na2SO4 1 water

[33]

Glaucine

Glaucium flavum Cr.

IL-ABS

[C4C1im][Ace], [C4C1im][Ace], [C4C1im][Ace], [C4C1im][Ace]

Na2CO3 1 water, MgSO4 1 water, NaH2PO4 1 water

[34]

Glycine

Pharmaceutical sample

IL-LLE

[C4C1im][PF6] 1 dicyclohexano-18-crown-6

Water

[35] (Continued)

TABLE 10.2 (Continued) Bioactive compound

Natural source

Technology

Used ILs (solvents)

Medium

Reference

Indole-3-butyric acid

Pea plants

IL-LLE

[C4C1im][PF6], [C6C1im][PF6], [C8C1im][PF6], [C6C1im][BF4], and [C8C1im][BF4]

Water

[36]

L-tryptophan, caffeine, and β-carotene



IL-ABS

[C4C1im][CF3SO3], [C4C1im][BF4]

Sugars 1 water

[22]

Nicotine, caffeine, theophylline, and theobromine

Mixture of alkaloids

IL-ABS

[C4C1im]Cl, [C6C1im]Cl, [C7C1im]Cl, [C8C1im]Cl, [C10C1im]Cl, [C4C1im]Cl, [C6C1im]Cl, [C8C1im]Cl, [C10C1im]Cl

C6H5K3O7/C6H8O7 1 water, C6H5K3O7 1 water

[23]

Para red and Sudan dyes

Chilli powder

IL-LLE

[C4C1im][PF6] and [C8C1im][PF6]

Water

[37]

Trans-zeatin, indole-3-acetic acid

Kappaphycus alvarezii sap

IL-LLE

[C4C1im][PF6], [C8C1im][BF4], and [C4C1C1im] [NTf2]

Water

[38]

Tyrosol

Olive mill wastewater

IL-LLE

[P4441][NTf2], [N4441][NTf2], and [N1888][NTf2]

Water

[39]

Vanillin



IL-ABS

[C2C1im]Cl, [C4C1im]Cl, [C6C1im]Cl, [C4C1C1im]Cl, [C7C1im]Cl, [aC1im]Cl, [C10C1im]Cl, [C12C1im]Cl, [OHC2C1im]Cl, [C7H7C1im]Cl, [C4C1im][CF3SO3], [C4C1im][N(CN)2], [C4C1im][C1SO4], [C4C1im] [C1SO3], [C4C1im][C1CO2], [C4C1im]Br

K3PO4 1 water

[40]

10.2 Bioactive compound extraction from biomass

259

approaches using [C8C1im][PF6] and [C4C1im][PF6] for the extraction of Sudan and red dyes from the red chili powder, [C8C1im][PF6] was found to be very effective [37]. Smirnova et al. has used dicyclohexano18-crown-6 and [C4C1im][PF6] for the extraction of glycine from pharmaceutical wastes [35]. Larriba and coworkers have effectively used ([N4441][NTf2]), ([P4441] [NTf2]), and ([N1888][NTf2]) in tyrosol extraction, a natural antioxidant from wastewaters of olive mills. They have successfully achieved tyrosol extraction in 94% by using [N4441][NTf2] and [P4441][NTf2], which is much higher than conventional extractions of ethyl acetate. They have improved the extraction efficiency by salting-out technique and also recycled the ILs by performing back-extractions of tyrosol using 0.1M NaOH aqueous solutions [39]. From the studies and also from literature, it is observed that the investigations carried out with hydrophobic water-immiscible ILs for LLE motives are significantly lesser than water-miscible ILs. This is because of limited variability and tuning of ILs. Most of the studies were carried with fluorinated-based anions such as [PF6]2 and [BF4]2 that have poor water stability. Because of poor solubility, the number of extraction times has increased, resulting in high cost and thus having an additional burden on economic viability [41]. 10.2.1.2 Ionic liquid-based aqueous biphasic systems It is emphasized that hydrophobic ILs in IL-based LLE methodologies have many limitations. Most favorable combinations for the extraction of the targeted bioactive compounds from biomass extracts are an aqueous mixture of water-miscible imidazolium-based ILs combined with anions like Br2, Cl2, [BF4]2, and [C1CO2]2 [5,1719]. After the extraction, IL-ABS approaches can be explored for fraction extraction before reaching the total selectivity [42]. The two liquid aqueous-rich phases were created by the salting-out of ILs with inorganic salts in aqueous media. For example, in two steps, Li et al. isolated opium alkaloids (papaverine and codeine) from Pericarpium papaveris. First, by ILSLE method using [C4C1im]Cl, then created IL-ABS method by adding K2HPO4 to an aqueous solutions of [C4C1im]Cl. Their investigation showed similar amount of extraction efficiency when compared to regular LLE method, but it had an advantage as it reduced number of extraction times and circumvention of volatile organic solvents. Some studies reported that the extraction of caffeine, especially drifts to the IL-rich phase, and higher extraction efficiencies were obtained in IL-salt ABS methodologies due to the intense salting-out phenomenon [22,23,43]. Passos et al. [44] examined the role of alkyl side-chain length of cations in ILs ([CnC1im]Cl, n 5 410) on the partition coefficients of the alkaloids (i.e., caffeine, nicotine, theobromine, theophylline). Alkyl

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260

10. Role of ionic liquids in bioactive compounds extractions and applications

chain length up to n 5 6 is favorable in high extraction efficiencies, whereas with an increase in chain length over six (n . 6) decreases the extraction efficiency because of self-aggregation caused by lengthy alkyl chains. Few studies were investigated based on different molecular weights. Almeida et al. [33] examined different ABS compositions of varied molecular weights of polyethylene glycol and Na2SO4 with ILs in 510 wt.%. Only 5 wt.% of IL in PEGs helped in extraction of all phenolic acids with 80%99% extraction efficiencies. Thus it confirms the capability of ILs even in low amounts in the extraction of phenolic acids by tuning the polarity of PEG-rich phases. In these studies, two major ILs were used ([CnC1im]Cl, n 5 12, 14) among major surfactants. In a recent report, anthraquinone derivatives, namely chrysophanol and aloe-emodin, were extracted through aqueous solutions of ILs, and later with IL-based ABS created by a combination of imidazolium ILs and sodium sulfate. Chrysophanol and aloe-emodin were obtained in 90.46% and 92.34%, respectively. Subsequently, the authors have subjected it to back-extraction and recovered the ILs by forming a new ABS system by altering the alkaline salts. After isolating glaucine from Glaucium flavum by aqueous biphasic methodology using [C4C1im]acetate and varied salts, Keremedchieva et al. recovered ILs [34]. In most cases, IL-ABS were mainly carried out with imidazoliumbased ILs for the segregation of bioactive products, combined with acetate, bromide, chloride, tetrafluoroborate anions, and dicyanimide. Few reports have been published that showed cholinium as a replacement for imidazolinium moieties, as the former showed higher biocompatible and biodegradable advantages over the latter. Recently, a new combination of ILs is available, which is comprised glycine-, cholinium-, and glycine-betaine-based cations accompanied by anions from biological buffers [45], carboxylic acids [46], and amino acids [47]. This new combination has a good scope for bioactive compounds extraction from biomass. Moreover, the combination strategies of IL aqueous solutions with IL-ABS are minimal. This combination brings the two processes, bioactive compounds extraction from biomass and the purification of target compounds into a single step is still challenging. Thus developing an integrated and sustainable strategy in recycling ILs will be more economical and reduces environmental corruption.

10.2.2 Ionic liquid-based solidliquid extractions Many studies were reported regarding the usage of SLE methods in natural products extraction like terpenoids, flavonoids, alkaloids,

2. Separation technology

10.2 Bioactive compound extraction from biomass

261

phenolic acids, lignans, saponins, etc. In the IL-based SLE methods, ILs are sometimes used in pure form as a solvent, and sometimes, it is combined with aqueous solution and or ethanol/methanol mixtures [4857]. In advanced SLE methodology, when solvents are integrated with optimal temperature and heat, targeted compounds’ extraction efficiency can be enhanced. Thus SLE techniques are incorporated with ultrasonic and microwave energies to result in IL-UAE and IL-MAE on improving the extraction efficiency, decreasing the use of solvent amount and extraction time. In this section, we have categorized three different IL-based SLE depending on extraction conditions like temperature, pH, extraction time, solidliquid ratio, and IL selection. 1. Simple/basic IL-SLE 2. IL-UAE 3. IL-MAE The bioactive compounds obtained from distinct biological sources through ILs are illustrated in Table 10.3. 10.2.2.1 Simple/basic ionic liquid-based solidliquid extractions There are many reports that used this technique in bioactive compounds extraction from biomass. Some of them are noted below: Glaucine extract from Glausium flavum [4850]: Herein, the authors investigated a series of ILs [CnC1im]1 as cation with different alkyl side-chain lengths linked to diverse anions including ([Ace]2, Br2, Cl2, [Sac]2) for glaucine extraction. They have optimized the concentration of IL in aqueous solution, the ratio of biomass-solvent, and the extraction time at 80
C for 1 hour using potassium acesulfamate aqueous solutions. The extraction yield of glaucine was obtained in 85% that was enhanced with the usage of an aqueous solution of [CnC1im][Ace] (n 5 4, 6, and 8) and are relatively more when compared to conventional methanol solvent. In the extraction process, imidazolium cation [CnC1im]1 played a substantial role as it formed an aromatic π-cloud that allowed strong and dynamic interactions with aromatic solutes. Galantamine, ungiminorine, N-desmethylgalantamine, narwedine from Leucojum aestivum aestivum [51]: Galantamine, ungiminorine, N-desmethylgalantamine, and narwedine were extracted from Leucojum aestivum by using aqueous mixtures of a sequence of ammonium-, pyrrolidinium-, and imidazolium-based ILs. The investigation revealed at optimum conditions that [C4C1im]Cl was the best IL among investigated ILs. Some reports extracted galantamine (used in treating neurological disorders like neurological, Alzheimer, and poliomyelitis) by SLE and then performed quantification using HPLC.

2. Separation technology

TABLE 10.3

Extraction of bioactive compounds from biomass using IL-based LLE, SLE, MAE, and UAE.

Bioactive compound

Natural source

Technology

Used ILs (solvents)

Medium

References

Tocopherol homologs

Mixed tocopherol

IL-LLE

[BMIM]Cl

Water

[58]

Ferulic acid, caffeic acid



IL-LLE

[BMIM][PF6]

Water

[59]

3-Indole butyric acid

Pea plants

IL-LLE

[BMIM][PF6]

Water

[36]

Para Red and Sudan dyes

Chilli powder, chilli oil, and food additive

IL-LLE

[OMIM][PF6]

Water

[37]

Aesculetin and aesculin

Fraxinus rhynchophylla

IL-UAE

[C7H7C1im]Br, [C7H7C1im]Cl, [C10C1im]Br, [C12C1im]Br, [C2C1im][BF4], [C2C1im]Br, [C4C1im][BF4], [C4C1im][ClO4], [C4C1im][HSO4], [C4C1im][Tos], [C4C1im]Br, [C4C1im]Cl, [C4C1im]I, [C8C1im]Br, [C6C1im]Br, [(HSO3)C4C1im][HSO4], and [(OH)C2C1im]Cl

Water

[60]

Artemisinin

Artemisia annua

IL-SLE

[N11(2(O)1)0][C2CO2], [N11(2OH)0][C7CO2]

Pure IL

[61,62]

Baicalin, wogonoside, baicalein, and wogonin

Scutellaria baicalensis Georgi

IL-MAE

[C4C1im]Br, [C4C1im]Cl, [C4C1im][BF4], [C4C1im][Oac], [C4C1im][CF3SO3], [C2C1im]Br, [C6C1im]Br, [C8C1C1im]Br, [C10C1im]Br, and [C12C1C1im]Br

Water

[63]

Caffeine

Paullinia cupana (guarana´)

IL-SLE

[C2C1im]Cl, [C2C1im][C1CO2], [C4C1im]Cl, [C4C1im][Tos], [C4C1pyrr]Cl, [(OH)C2C1im]Cl

Water

[64]

Caffeoylquinic acids

Flos Lonicerae Japonicae

IL-UAE

[C4C1im]Br

Water

[65]

Carnosic acid and rosmarinic acid

Rosmarinus officinalis

IL-MAE

[C10C1im]Br, [C2C1im]Br, [C4C1im][BF4], [C4C1im][NO3], [C4C1im]Br, [C4C1im]Br, [C4C1im]Cl, [C6C1im]Br, and [C8C1im] Br

Water

[66]

Catharanthine, vinblastine, and vindoline

Catharanthus roseu

IL-UAE

[aC1im]Br, [C2C1im]Br, [C4C1im][BF4], [C4C1im][ClO4], [C4C1im][HSO4], [C4C1im][NO3], [C4C1im][Tos], [C4C1im]Br, [C4C1im]Cl, [C4C1im]I, [C6C1im]Br, and [C8C1im]Br

Water

[67]

Cryptotanshinone, tanshinone I, and tanshinone II A

Salvia miltiorrhiza

IL-UAE

[C10C1im]Br, [C12C1im]Br, [C14C1im]Br, [C16C1im]Br, [C8C1im] Br, [C2C1im]Cl, [C4C1im]Cl, [C6C1im]Cl, and [C8C1im]Cl

Water

[68,69]

(1)-Catechin, ellagic acid, and pyrocatechol

Acacia catechu and Terminalia chebula

IL-SLE

[N1100][N(C1)2CO2]

Pure IL

[28]

Fangchinoline and tetrandrine

Stephaniae tetrandrae

IL-UAE

[C4C1im][BF4]

Water

[70]

Forskolin

Coleus forskohlii

IL-UAE

[C4C1im]Cl, [C4C1im]Br, [C4C1im][BF4], [C4pyr][BF4], [N000(2OH)][C0CO2], [C1C1C1C1guan][Lac]

Water

[71]

Galantamine, narwedine, Ndesmethylgalantamine, and ungiminorine

L. aestivum

IL-SLE

[C4C1im]Cl, [C6C1im]Cl, [C8C1im]Cl, [C10C1im]Cl, [C4C1im]Br, [C4C1im][Sac], [C4C1im][Ace], [C4C1C1im]Cl, [C4C1im][C1CO2], [C4C1im][CF3CO2], [C4C1im][SCN], [C4C1im][N(CN)2], [C4C1im][C(CN)3], [C7H7C1im]Cl, [C4C1pyrr]Cl, [N11(2OH)(C7H7)] Cl, and [N221(O)nOH]Cl

Water

[51]

Gallic acid

Suaeda glauca Bge.

IL-UAE

[C2C1im]Cl, [C4C1im]Cl, [C6C1im]Cl, and [C8C1im]Cl

Water

[72]

Glaucine

Glaucium flavum (papaveraceae)

IL-SLE

[C4C1im][Ace], [C10C1im][Ace], [C6C1im][Ace], [C8C1im][Ace], [C4C1im][Sac], [C4C1im]Br, and [C4C1im]Cl

Water

[48,49]

Hydroxycamptothecin and camptothecin

Camptotheca acuminata

IL-UAE

[aC1im]Br, [C7H7C1im]Br, [C2C1im]Br, [C3C1im]Br, [C4C1im] [BF4], [C4C1im][ClO4], [C4C1im][HSO4], [C4C1im][NO3], [C4C1im]Br, [C4C1im]Cl, [C6C1im]Br, [C8C1im]Br, and [C6H11C1im]Br

Water

[73]

Isoliensinine, liensinine, and neferine

Nelumbo nucifera

IL-MAE

[C2C1im][BF4], [C4C1im][BF4], [C4C1im][PF6], [C4C1im]Br, [C4C1im]Cl, [C6C1im][BF4], and [C8C1im][BF4]

Water

[53] (Continued)

TABLE 10.3

(Continued)

Bioactive compound

Natural source

Technology

Used ILs (solvents)

Medium

References

Iristectorin A, iristectorin B, and tectoridin

Iris tectorum

IL-UAE

[C4C1im][BF4], [C6C1im]Br, and [C8C1im]Br

Water

[74]

Myricetin, quercetin, kaempferol

Bauhinia championii

IL-MAE

[C2C1im]Br, [C4C1im][BF4], [C4C1im][H2PO4], [C4C1im][HSO4], [C4C1im][PF6], [C4C1im]2[SO4], [C4C1im]Br, [C4C1im]Cl, [C6C1im]Br, and [(HOOC)C1C1im]Cl

Water

[75]

Nuciferine, N-nornuciferine, O-nornuciferine

Nelumbo nucifera

IL-MAE

[C2C1im]Br, [C4C1im][BF4], [C4C1im][PF6], [C4C1im]Br, [C4C1im]Cl, [C6C1im]Br, and [C8C1im]Br

Water

[76]

Piperine

Piper nigrum (white and black pepper)

IL-SLE

[C10C1im]Cl, [C12C1im]Cl, [C12C1im]Br, [C12C1im][CF3SO3], [C12C1im][C1CO2], [C12C1im][N(CN)2], [C14C1im]Cl, and [N111 [2O(O)12]Cl

Water

[77]

Piperine

Piper nigrum (white and black pepper)

IL-UAE

[C4C1im][BF4], [C4C1im][H2PO4], [C4C1im][PF6], [C4C1im]Br, [(HSO3)C4C1im]Br, and [C6C1im][BF4]

Water

[54]

Polycyclic aromatic hydrocarbons

Petroleum Source Rock

IL-MAE

[C4C1im]Br, [C4C2C1im]Cl, and [C1C1C1im]2[SO4]

Water

[78]

Quercetin, ellagic acid, gallic acid, pyrocatechol, trans-resveratrol

Psidium guajava (guava) and Smilax china

IL-MAE

[C2C1im][BF4], [C2C1im]Br, [C4C1im][BF4], [C4C1im][C1SO4], [C4C1im][H2PO4], [C4C1im][N(CN)2], [C4C1im]Br, [C4C1im]Cl, [C4py]Cl, and [C6C1im]Br

Water

[79]

Rutin

Saururus chinensis and Flos sophorae

IL-MAE

[C4C1im][BF4], [C4C1im][Tos], [C4C1im]Br, and [C4C1im]Cl

Water

[80]

Rutin, hyperoside, and hesperidin

Sorbus tianschanica leaves

IL-MAE

[C4C1im]Cl, [C4C1im]Br, [C4C1im][BF4], [C4C1im][NO3], [C4C1im][HSO4], [C4C1im][ClO4], [C2C1im][BF4], [C6C1im][BF4], and [C8C1im][BF4]

Water

[53]

Saponins and polyphenols

Ilex paraguariensis (mate) and Camellia sinensis (tea)

IL-SLE

[aC1im]Cl, [C2C1im][Lac], [C2C1im][CF3SO3], [C2C1im][C2SO4], [C2C1im][C1CO2], [C2C1im][N(CN)2], [C2C1im]Cl, [C4C1im]Cl, [C6C1im]Cl, [C7H7C1im]Cl, [C8C1im]Cl, [N111(2OH)][NTf2], [N111 (2OH)]Cl, and [(OH)C2C1im]Cl

Water

[81]

Saponins and polyphenols

Ginkgo biloba

IL-SLE

[C4C1im]Cl

Pure IL

[82]

Shikimic acid

Illicium verum (star anise)

IL-MAE

[C2C1im][BF4], [C2C1im][CF3SO3], [C2C1im][C1CO2], [C2C1im] [NTf2], [C2C1im][PF6], and [C2C1im]Cl

Pure IL

[83]

Shikimic acid

Illicium verum (star anise)

IL-SLE

[C2im][HSO4], [C2C1im][HSO4], [(HSO3)C4C1im][H2PO4], [(HSO3)C4C1im][HSO4], [(HSO3)C4C1im][NTf2], [(HSO3) C4C1im]Br, and [(HSO3)C4C1im]Cl

Ethanol

[84]

Senkyunolide H, senkyunolide I, Z-ligustilide

Ligusticum chuanxiong

IL-MAE

[N11(2(O)2OH)0][C2CO2] and [N011(2CN)][C2CO2]

Pure IL

[85]

Shikimic acid

Chinese conifer needles

IL-UAE

[C4C1im]Cl, [C4C1im]BF4, [C4C1im][NO3], [C2C1im]Br, [C3C1im] Br, [C4C1im]Br, [C5C1im]Br, [C6C1im]Br, [C8C1im]Br, [C10C1im] Br, [C7H7C1im]Br,

Water

[86]

Shikonin β,β0 -dimethylacrylshikonin

Arnebia euchroma

IL-UAE

[C2C1im][BF4], [C4C1im][BF4], [C6C1im][BF4], [C6C1im][PF6], [C8C1im][BF4], and [C8C1im][PF6]

Pure IL

[87]

Biflavones

Selaginella doederleinii Hieron trachyphylla (Warb.)

IL-ABS

(Hmim)(PF6)

Pure IL

[88]

Scopolamine

Dendrobium nobile L.

UMAE

[Bmim]BF4

[89]

Flavanoids

Ginkgo biloba L.

IL-ABS

[C8 mim]BF4, [C4mim] BF4

[90]

Total flavanoids

Punica granatum L.

MAE

[Bmim]Cl

[91]

Rutin

Sophora japonica

IL-ABS

[Bmim]BF4

Ethanol

[92] (Continued)

TABLE 10.3

(Continued)

Bioactive compound

Natural source

Technology

Used ILs (solvents)

Medium

References

Gingerols

Zingiber officinalis

UAE

[C4mim]BF4

Pure IL

[93]

Isoflavones

Belamcanda chinensis

UAE

[Emim][BF4]

Sinomenine

Sinomenium acutum

UAE

([C2OHmim]FeCl4)

Water

[95]

Anisodamine, atropine, scopolamine, aposcopolamine scopoline

Physochlainae infundibularis

SLE

[C3tr]PF6

Water

[96]

Cynaropicrin

Cynara cardunculus

LLE

1-Alkyl-3-methylimidazolium chloride

Water

[97]

Morroniside, sweroside, loganin, cornuside

Cornus officinalis

Maceration

[Doomim]HSO4

Water

[98]

Schisandrin, schisantherin, deoxyschisandrin, γ-schisandrin

Schisandra chinensis

IL-ABS

Bmim-BF4

Water

[99]

Gallic acid, quercetin, and kaempferol

Orostachys japonicus

LLE



Water

[100]

[94]

10.2 Bioactive compound extraction from biomass

267

Caffeine extract from Paullinia cupana (guarana´ seeds) [64]: Similarly, Claudio et al. extracted caffeine in higher extraction yields of 9.4% from Paullinia cupana by IL-SLE method when compared to 4.3 wt.% by conventional Soxhlet extraction using dichloromethane. Piperine extract from Piper nigrum [77]: Since piperine is an aromatic alkaloid compound with hydrophobic nature, whose dispersion/solubility in aqueous media is increased by adding surface-active ILs. Bica et al. have used the surface-active ILs [C10C1im]1, [C12C1im]1, [C14C1im]1 incorporated with Br2, Cl2, [C1CO2]2, [CF3SO3]2, [N(CN)2]2 anions, in addition to biodegradable long-chain, betain-obtained IL, [N111[2O(O)12]]Cl). They had observed that extraction efficiency was as low as ,0.2 wt.% when the ILs concentrations were less than the critical micellar concentration (CMC). In contrast, when the ILs concentration was increased above the CMC, the extraction efficiency yields also increased to 4.0 wt.%. This enhancement in extraction yield is attributed to the self-aggregation of ILs in an aqueous medium and not due to the hydrotropic phenomenon. Most bioactive compounds have good solubility in water, but ABS ILs with low alkyl chains are not suitable candidates for hydrophobic biocompounds extraction. To increase the hydrophobic compounds extraction efficiency, Jin et al. put forward a new class of aqueous ILs with amphiphilic long-chain carboxylates (LCC-ILs) [52] for solubility and extraction of hydrophobic bioactive ingredients. The LCC-ILs showed strong hydrogen bonding and held weak polarity simultaneously, thus improving dissolution capacity for several hydrophobic compounds, that is, rutin, ginkgolides, perillyl alcohol, and tocopherol. This methodology promised higher extraction yields by 212 folds in comparison to conventional organic solvents extraction. 10.2.2.2 Microwave-assisted extractions Simple SLE technique discussed earlier requires a large volume of solvent and longer extraction time but MAE technology can provide faster extraction and higher yields of bioactive compounds from the biomass. It is reported in a study [79], the extraction of trans-resveratrol using aqueous IL solution from Rhizma polygoni by MAE technique is an excellent example. They inspected the role of ILs ([C4C1im]1[Br]2 or [Cl]2 or [BF4]2) at optimum operational conditions using an orthogonal design methodology. They found that [C4C1im]1[Br]2 was the best solvent IL to extract trans-resveratrol in 93% yield at optimum conditions. Further studies showed the use of only imidazolium-based ILs and no other solvent combination were used in this IL-MAE technique. More effective and environment friendly method was reported by Refs. [101,102], where they investigated the extraction of many alkaloids, namely liensinine, isoliensinine, nuciferine, neferine, o-nornuciferine,

2. Separation technology

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10. Role of ionic liquids in bioactive compounds extractions and applications

and n-nornuciferine from Nelumbo nucifera, namely liensinine, isoliensinine, nuciferine, neferine, o-nornuciferine, and n-nornuciferine. They illustrated that IL-MAE improves extraction efficiency up to 50% and by cutting the extraction time from 2 hours to 90 seconds. They also showed the chemical structure [C4C1im]1[BF4]2 was the most efficient IL in neferine, isoliensinine, and liensinine extraction at optimum conditions. On the other hand, [C4C1im]1[Br]2 was the most efficient IL in o-nornuciferine, n-nornuciferine, and nuciferine extraction. They also reported that an increase in IL alkyl chain length up to hexyl using [CnC1im]1[Br]2 ILs increases the extraction efficiency of alkaloids. A further increase in octyl significantly drops the extraction efficiency of alkaloids. Extraction of flavonoids was controlled by the ILs anions and they were achieved in higher extraction efficiencies when ILs have stronger hydrogen bond acceptor ability like [C4C1im]Br, [C4C1im][H2PO4], and [(HOOC)C1C1im]Cl [75,79,80]. ILs-MAE was used in hyperoside, rutin, and hesperidin extraction from Sorbus tianschanica leaves [53]. It was also applied in the extraction of wogonoside, baicalin, wogonin, and baicalein from Scutellaria baicalensis Georgi [63], and in the extraction of 10 flavonoid glycosides extraction from Chrysanthemum morifolium Ramat [76] where IL-MAE showed more superior performance when compared to conventional solvent and techniques. By this technique, baicalin was extracted with 22.28% yield in 90 seconds in comparison to water-based MAE with 9.77% yield in 90 seconds and IL-SLE with 16.94% yield in 30 minutes. The results from the above studies indicate that surface-active ILs are not only in favor of extraction of high hydrophobic alkaloids but also in favor of extraction of additional hydrophobic target bioactive compounds. Based on reports, most IL-MAE was processed using imidazolium-ILs with two exceptions: (1) by utilizing protic ILs, (2) in assessing pyridinium cation role. Furthermore, it also showed an increase in extraction efficiency with short alkyl chain lengths and cations. Most authors focused their achievement by improving extraction efficiency despite less information on the mechanism. This increase in extraction efficiency is attributed to hydrogenbonding and pipi interactions among the ILs and target compounds. A decrease in solidliquid proportions increased extraction yields. 10.2.2.3 Ultrasonic-assisted extractions Microwave-assisted extraction technology is swift and efficient in bioactive compounds extraction but has few drawbacks like difficulty to scale up, rapid heating by irradiation, and intensive energy in mass transfer [5]. UAE is a sustainable alternate method to MAE as it permits mechanically enhancing the mass transfer. It is a better choice in extracting thermosensitive biomolecules and thus overcomes one of the main

2. Separation technology

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269

drawbacks of MAE. The IL-UAE was found to be advantageous as it helps in obtaining higher extraction efficiency in shorter extraction times [54,73] at low temperatures [67]. It guarantees sustainability in process development and successful elimination of environmentally toxic organic solvents [55,67]. IL-UAE was employed not only in the extraction of alkaloids but also in the extraction of flavonoids (iristectorin A, tectoridin, and iristectorin B) from Iris tectorum [74]. Authors have obtained best results by using aqueous solutions of 0.5 M [C8C1im]Br when compared to a mixture of methanol and sodium chloride aqueous solutions. Shikmic acid was successfully extracted from conifer needles, gallic acid from Suaeda glauca leaves, caffeoylquinic acid extraction from Flos Lonicerae Japonicae using IL-based UAE. They observed with increase in temperature and lowering of solidliquid ratio, the extraction efficiency can be increased [65]. Li et al. in 2017 used magnetic ILs ([C2OHmim)FeCl4) in the extraction of sinomerine from Sinomerine acutum. They compared ethanolwater and pure water mixtures and obtained increased yields by 2.4 and 2.8 times, respectively [95]. Dong et al. in 2018 used five alkaloids from Physochlainae infundibularis. They optimized the extraction at 0.05 mol/L [C3tr]1[PF6]2 aqueous solution to achieve 95.1% extraction yield in 55 minutes [96]. Overall performance of IL-based UAE is effectively implemented to the variety of bioactive compounds extraction from various plant species, but lack in scalability process. It has an advantage over IL-MAE technique as the extraction processes can be conducted at lower temperatures. It is also seen that in alkaloids extraction, IL anion played a vital function, whereas alkyl chain length of cation played a significant role in flavonoids extraction. In the absence of complete studies and less information, it is inappropriate to conclude the above rule, thus there is a huge scope in this field of research. 10.2.2.4 More complex/rigid solid-liquid extractions 10.2.2.4.1 Ultrasonic/microwave-assisted extractions

Based on the benefits of IL-UAE and IL-MAE methodologies, a combination of both methodologies into single approach was proposed by Lu et al., known as ultrasonic/microwave-assisted extractions (UMAE). Author applied this unique combination technique for extraction of anthraquinones from Rheum spp. (rhubarb) and gallotannins from Galla chinensis [56,103]. Authors have investigated the extraction efficiency by replacing organic solvents with ILs in UMAE, namely [C4C1im]1[Br]2, [C4C1im]1[BF4]2, and [C4C1im]1[Cl]2. They found [C4C1im]1[Br]2 to be the best solvent for higher extractions. IL-UMAE considerably reduces the extraction time from 6 hours to 1 minute compared to IL-based UAE.

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A comparative evaluation among UMAE and other methods/techniques was carried out considering different conditions like extraction under reflux and heat condition. These methods help in achieving higher extraction yields with 24% improvement and shorter extraction times of within 2 minutes [103]. 10.2.2.4.2 Ultrahigh pressure extraction

Liu et al. have extended the IL-UMAE with ultrahigh-pressure extraction for tanshinones extraction from Salvia miltiorrhiza [104]. This approach IL-UPE in ethanol along with 0.5 M of [C8C1im][PF6] provided excellent extraction yields with lower operating time, energy, and solvent. This study assisted in enhancing the disruption of plants tissue cells. 10.2.2.4.3 Negative-pressure cavitation extraction

All the methodologies/techniques that are explored earlier such as IL-based SLE, MAE, UAE, UMAE, and UPE have drawbacks like difficulty extracting labile, thermosensitive, unstable compounds, and susceptibility to oxidation when coming into contact with air. A novel extraction method was proposed to overcome the mentioned drawbacks, a novel extraction approach was proposed, named NPCE. Using [C8C1im]Br, Duan et al. have extracted flavonoids like genistein, genistin, and apigenin from the roots of Cajanus cajan (pigeon pea) [105]. The outcome from the above study suggest that this methodology: 1. can be scaled-up of the process on an industrial scale and 2. can be processed at low temperature under inert atmosphere. 10.2.2.4.4 Microwave homogeneous liquidliquid microextraction

This proposed microextraction (IL-MA-HLLME) [106] technique is applied in the extraction of four anthraquinones, viz., aloe-emodin, chrysophanol, emodin, and physcion, from Rheum palmatum L roots. This methodology showed that it: 1. circumvents the usage of volatile organic solvents, 2. extraction can be obtained in a short time, and 3. requires only a low amount of solvent in comparison to IL-based reflux and UAE extraction.

10.2.3 Solidphase extractions The SPE persuades target compound adsorption from the extraction of liquid. However, the extraction performance is maintained because of

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ILs liquid nature like solid support immobilization, chemical properties, and multiple interactions. Many studies based on SPE are applied on different compound as: 1. To extract ferulic, protocatechuic and caffeic acid from Salicornia herbacea L in greater yields of about 79.09%, 94.69%, and 87.32%, respectively [107], 2. To extract alkaloid oxymatrine from Sophora flavescens Ait in optimized conditions, that is, IL alkyl chain length, contact time, solidliquid ratio, temperature, anionic nature, and recovered oxymatrine yields over four cycles and in the range of 89.7%93.4%. Authors also revealed that silica material column (SiO2[Im]1[PF6]2) was efficient for adsorption, desorption, and reusability [108]. 3. Imidazolium-based polymers extract alkaloids more efficiently, like oxymatrine, matrine from S. flavescens Ait extracts and theophylline, caffeine from green tea extracts, than conventional polymers. This efficiency is attributed to strong and specific interactions with the target biocompounds [109,110]. SLE can overcome the limitations caused by silica materials because silica involves in higher cost, longer synthesis protocol, limited pH stability, and fewer functional group present per contact area (Table 10.4). Most bioactive compounds are aromatic in nature and possess many hydroxyl groups that assist in achieving higher extraction efficiency. Much exploration is carried out with imidazolium/pyridinium rings and much more studies are to be conducted using other cations. Similarly, many studies were done with anions (i.e., Cl2, Br2, [PF6]2, and [BF4]2) and many more anions are to be explored.

10.2.4 Backward (or back)-extractions Most of the bioactive compounds extraction were conducted on lab and pilot scale using IL-based extraction techniques. Earlier studies are focused on extraction yield, number of bioactive compounds extracted, extraction time, purity, etc., rather than process sustainability. This technique pays attention in recovering and recycling the solvents. Few authors have worked on extraction, purification, and recovery of bioactive compounds and the ILs used [28,32,64].

10.3 Applications of ionic liquids 10.3.1 Green solvents—a gentle suspension of biomass ILs have the capability to dissolve biomass gently when compared to other solvents.

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TABLE 10.4 Extraction of bioactive compounds from biomass using IL-Modified material in solid-liquid extractions. Bioactive compound

Used IL-based material

Natural source

Technology

Caffeine and theophylline

Green tea

IL-SPE

[C1im]-modified polymer

[110]

Liquiritin and glycyrrhizic acid

Licorice

IL-SPE

[C2C1im]Cl-based silica and poly (butylpyridine chloride) divinylbenzene

[111,112]

Matrine, oxymatrine, sophocarpin, and sophoridine

Sophora flavescens Ait

IL-SPE

Poly(3-aminopropyl imidazole bromide hydrobromide) 4(chloromethyl) styrene

[109]

Oxymatrine

S. flavescens Ait

IL-SPE

Imidazolium [BF4]-, [PF6]- and [NTf2]based silica

[108]

Phenolic acids

S. herbacea

IL-SPE

Poly([aC2im]Br) ethylene

[113]

Protocatechuic aldehyde, sodium danshensu, rosmarinic acid, lithospermic acid, and salvianolic acid B

S. miltiorrhiza Bunge

IL-SPE

SiO2 Im1 PF62

[114]

Tanshinones

S. miltiorrhiza Bunge

IL-SPE

Imidazolium chloride-based silica; [C1im]Cl-based silica; and poly ([R1im]Cl) imprinted on 4-(chloromethyl) styrene

[115117]

Three phenolic acids

Saliconia herbacea L.

IL-SPE

Imidazolium chloridebased silica

[107]





References

ILs have unique properties with strongly coordinating hydrogen bonds that empower them to entirely dissolve biomass or particular biomass components selectively, which is not possible for conventional solvents by keeping the biopolymers fully intact. Besides this, ILs have additional ecofriendly advantages as:

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1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

273

no boiling point, no vapor pressure, high electrochemical stability, nonflammable, high mechanical and chemical stabilities, unusual solvation properties (biomass), excellent thermal stability up to 300
C, low compressibility, attractive tribological properties (lubrication), electrical conductivity from 50 mS/cm down to μS/cm (20
C), nanostructural organization, low solubility for apolar gases, antimicrobial properties, no biofouling, no cavitation at high tension, highly tunable properties by cation and anion variation and combination.

Thus ILs are excellent tools in expanding the source of raw ingredients, developing new processes, refining technologies, and restoring technical and natural fibers. In addition, ILs benefit from their dissolving power by strongly coordinating 1-ethyl-3-methylimidazolium acetate (EMIM-OAc) or 1-butyl-3methylimidazolium acetate (BMIM-OAc) in biomass application. In addition, these types of ILs were successfully implemented in various fiber welding techniques.

10.3.2 High-purity, inflammable electrolytes for battery and supercapacitor applications For electrolyte applications, to be an ideal candidate in terms of the performance and safety, ILs display exclusive blend of physicochemical properties such as: 1. 2. 3. 4. 5.

high conductivity, broad liquid range, high electrochemical stability, neither volatile nor flammable, and compatibility with electrode materials.

Applications like dye-sensitized solar cells, lithium ion batteries, sensor materials, super-capacitors, metal plating processes have been shown the most promissing. Some of the latest electrolytes as with their molecular formula are illustrated in Fig. 10.2: 1. 1-ethyl-3-methylimidazolium-tetrafluoroborate (EMIM-BF4) and 2. 1-ethyl-3-methylimidazolium-bis(fluorosulfonyl)imide (EMIM-FSI).

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10. Role of ionic liquids in bioactive compounds extractions and applications

FIGURE 10.2 Molecular formula of most used electrolytes.

Lately, for EMIM-FSI the CBILS-process has been improved, augmented, and sophisticated in kilogram scale. The EMIM-FSI has following advantages over other IL electrolytes: 1. low viscosity (21 mPas @ 20
C), 2. low freezing point till 248
C, and 3. high electric conductivity at 17.5 mS/cm.

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275

The EMIM-FSI showed remarkable result in lab-scale range (a halide content less than 5 ppm and water 15 ppm); the process can be upgraded to ton scale. 1. 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI), 2. 1-Butyl-3-methylimidazolium-tetrafluoroborate (BMIM-BF4), 3. 1-Butyl -3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM-TFSI), 4. 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPyrr-TFSI), and 5. 1-Butyl-1-methylpyrrolidinium bis(fluoromethylsulfonyl)imide (BMPyrr-FSI).

10.3.3 Antistatic agents 10.3.3.1 Liquid antistatic agents ILs successfully reduced ohmic resistance when applied on polymers and improved antistatic properties. Unwanted effects occur when surface charge accumulates like impurities or dust attraction (hygienic concern), electric shocks, and flammable liquids ignition (arcing). ILs are designated and prepared by considering solubility of the IL in the matrix and electrical discharge enhancement capability. The characteristics of ILs as antistatic agents in polymers are: 1. 2. 3. 4. 5. 6.

ohmic resistance reduction in polymers, nonflammability, high electric conductivity, miscibility and compatible with various kinds of polymers, minimal vapor pressure, high thermal stability.

10.3.4 Intrinsically safe high-temperature cooling The medium of standard water cooling is replaced by IL-B2001 (ILTEC Technology, Mettop GmbH). It generates an eruption-free environment that allows safe and thorough metallurgical devices cooling. IL-B2001 thus can be safe for people, plants, production capacity, and profit as it has benefits as: 1. low operative temperature of 210
C, 2. noncorrosive because of chlorine-free composition and manufacture process, 3. operation temperature between 50
C and 200
C,

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4. below decomposition temperature of approximately 450
C an almost zero vapor pressure, and 5. no explosion or harsh reaction when interacted with liquid metal. Table 10.5 presents the physical properties of IL-B2001, and Fig. 10.3 demonstrates the risk associated with standard cooling [118].

TABLE 10.5

Physical properties of the IL-B2001 cooling medium.

Property Operation temperature Short term stability Decomposition temperature

Symbol   

Value

Unit

Range

50200




C



250




C



450




C



C



Crystallization temperature



,15




Specific heat capacity

Cp

1.381.70

J/gK

50
C200
C

Density

ρ

1.251.14

kg/dm3

50
C200
C

Dynamic viscosity

η

205

mPa s

50
C200
C

Electrical conductivity

K

30130

mS/cm



FIGURE 10.3 Critical consequences depletion and 2D risk matrix by replacing water.

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277

10.3.5 Further more advanced applications ILs allow construction and modeling of machines in a particular way with its hydraulic and lubricating properties. These potentials are realistic with its material compatibility and easy formation of suitable additives. ILs strengths in the hydraulic and lubricants sector are extraordinary with viscosity index (VI) values of B400 whose outcome is very low variation in viscosity with low compressibility, increased temperature, high shear stability, insignificant cavitation under high tensile stresses (B1000 bar) with incredible cooling, lubricating, greasing, and nonflammable properties. 10.3.5.1 Air conditioning A liquid that is nonvolatile desiccant, noncorrosive, and nonevaporation prevents contamination and loss. It is better to decrease or eliminate toxic air constituents. A proionic company developed a process in segregating polar constituents, like water or CO2 from air, and saved energy of about 20%30% as compared to the existing systems using the liquid desiccant under operating air conditioning systems. The liquid desiccants are simpler to be manipulated when compared to regular solid desiccants. This allows air conditioning installions more costeffective and compact, attractive for consumers, and automotive trade. 10.3.5.2 Hydrogen storage Hydrogen is stored safely and stably without pressurization at standard temperature. The existing attainable energy in engines is B20% 25% of conventional fuels storage capacity. Thus they are leading in the field of hydrogen storage technology. Later, there is a potential to enhance this rate to B35%. Moreover, the release of hydrogen from ILs is selectively utilized to produce electricity power in fuel cells. In addition, through hydrogen the exhausted ILs can be recharged and recycled for multiple charging and discharging cycles. 10.3.5.3 Chemical production processes Chemical production processes are usually a wide range of ILs application, as they deliver process designs totally and new reactions due to their evident physical and chemical properties that produce new conceivable directions. Apart from their nonvolatile, catalytic properties, extraordinary solubility their unusual performance when merged with ultrasound, microwaves, many manufacture procedures can be functioned in an emission-free and ecofriendly ways.

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10.4 Conclusions and future prospects ILs are low melting point liquids that are used in many separation and extraction techniques. The usage of ILs is gradually increasing for their extraordinary chemical and physical properties. On daily basis, they are replacing conventional solvents as they are ecofriendly and environmental safe, and also their usage as solvents helped in extravagant increase in extraction efficiencies and extraction yields. In this chapter, many bioactive compounds extraction, purification, and yields were highlighted. The integration of ILs in different technologies like SLE, ILL, UAE, MAE, and UMAE mainly enhanced yields and reduced time by multiple folds. Back-extraction techniques helped in recovery of biocompounds and recycling of ILs. Their applications in hydrogen production in fuel cells are also potentially discussed. In the future, there is a rich scope of ILs usage in fuel cells, energy storage, in extraction of phytocompounds and pharmaceutical products with higher efficiencies and yields. Economic and sustainable extraction processes are crucial toward commercialization in nutraceuticals, cosmetics, and pharmaceutical industries.

Acknowledgments This work was supported by the 2020 Yeungnam University Research Grant and, the Priority Research Centers Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Education (2014R1A6A1031189). This work was also supported by Technology Innovation Program (2004439, Development of safety protective workwears and high visibility fluorescent dye materials on modacrylic and its blended fabrics) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

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C H A P T E R

11 Developments in gas sensing applications before and after ionic liquids Vijaykumar S. Bhamare and Raviraj M. Kulkarni Department of Chemistry and Centre for Nanoscience and Nanotechnology, KLS Gogte Institute of Technology, Udyambag, Belagavi, Karnataka, India

11.1 Introduction Solution chemistry plays a vital role in the field of chemistry as there are many significant chemical species found in the solution phase. The solution is a mixture which is prepared by dissolving a suitable amount of solute in the given amount of solvent. A solvent plays a very crucial role in solution chemistry and attracts researchers toward it because of its toxic effect to living beings. As a result, academic and research communities across the globe was in a need of new class of solvents called “ionic liquids” (ILs) as a substitute for conventional solvents. ILs were found to be like a forest in desert due to their unique features [1]. The term “ionic liquid” was first used by Gabriel and Weiner in 1888 and studied the ILs ethanol ammonium nitrate (EAN) having melting point 52
C55
C [2]. Thereafter, Walden revealed a room temperature ionic liquid (RIL), namely, EAN having melting point 12
C. EAN is the protic IL which is widely studied whereas another protic IL EAN could not receive more attention of research community. EAN is more remarkable than other ILs due to its water-like properties [3]. EAN can form a three-dimensional hydrogen-bonded network due to equal number of hydrogen donor and acceptor sites [4]. This field of ILs is developing at a very faster speed due to its significant properties and various applications.

Advanced Applications of Ionic Liquids DOI: https://doi.org/10.1016/B978-0-323-99921-2.00006-9

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The ILs are classified as protic and nonprotic ILs. The protic ILs can be prepared using Bronsted acid and Bronsted base. Poole in 2004 described significant role of protic ILs and nonprotic ILs in chromatography [5]. ILs or liquid salts were made of cations and anions. They have low melting points. The different kinds of ILs can be made by interchanging organic cations and organic or inorganic anions as shown in Fig. 11.1. The literature study revealed that RILs have attracted research community because of its use as substitute electrolytes for gas sensors [6,7]. R

R

N+

R'

N

R'''

P+

R'

+

N R''

R N-alkylpyridinium cation

N-N-dialkylimidazolium cation

tetraalkylphosphonium

R R

R'

R'

N+

+

N

R'''

N+

R'

R'' R''

dialkylpyrolidinium cation

F

F

F

B-

-

C2F5

F

F F

F

F

P

C2F5 F

F

hexafluorophosphate anion

O S

F

F

F

C2F5

-

CF3 F

F P-

tris(pentafluoroethyl)trifluorophosphate anion

tetrafluoroborate anion

F

tetraalkylammonium

dialkylpiperidinium

O-

N

S O

CF3 S

O

O

O

O

trifluoromethanesulphonate anion

bis(trifluoromethylsulphonyl)imide anion

FIGURE 11.1 The chemical structures along with IUPAC names of the cations and anions of generally used RILs. RILs, Room temperature ionic liquids.

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289

Conventional solvents or non-ILs have higher vapor pressure whereas ILs have very low vapor pressure which is almost negligible [8]. The Bhopal gas tragedy in India was the most horrible industrial disaster that the world has seen in the year 1984 due to leakage of poisonous gas methyl isocyanate [9]. It was reported that 520,000 persons were exposed to this poisonous gas. About 8000 persons were very severely affected and lost their lives within the first week immediately. More than hundred thousand persons have been suffered from permanent injuries and 38,473 persons affected from temporary partial injurious to their body due to this tragedy [9]. This gas leakage tragedy served as an alarm clock. It was reported that this tragedy took place due to casualness of the Union Carbide Corporation toward the leakage of poisonous gas. It has shown unwillingness to help affected persons economically and denied its responsibility in this huge industrial disaster. This major industrial disaster could have been avoided or controlled up to some extent using technology of gas sensing and detection. Many lives could have been saved by detection and proper measurement of toxic gas. The significance of proper detection, measurement, and monitoring of different gases has been appreciated and accepted by everyone from health and safety point of view for the last few decades. Precise methods of gas detection and accurate measurement of different gases present in our environment are highly demanded and required [1012]. Consequently, digital gas sensors with the latest technology are utilized in industries across the world for the protection of human beings, animals, and our environment from harmful poisonous gases. It is reported that O2, CO2, and CO are major gas sensors accounted for more than 50% of the gas detectors market size. Food processing and storage industries are extensively using CO2 gas sensors. It is also reported that manufacturing and automobile sectors are generally using O2 and CO gas sensors [6]. The life span of gas sensors is considered very significant factor in industrial sector. It is reported that liquid-state electrolytes used in gas sensors are found to be efficient due to its more durability than solid-state electrolytes. The conventional aqueous or organic solvents (sulfuric acid and water mixture, acetonitrile) are not durable and efficient at drastic conditions of low humidity and high temperature. The efficiency of gas sensor is also affected by evaporation of solvent used in conventional electrolyte. Membrane used in gas sensors could not prevent evaporation of solvent entirely. The demand of gas sensors in market depends upon the special features such as sensor lifetime, response time, selectivity, and sensitivity. The lifetime of gas sensor depends on membranes, solutions, and electrode materials used in manufacturing of gas sensor. Response times are significant while selecting gas sensors because the sensor needs response quickly to variation in concentration of gases. The gas sensor is selective toward a particular gas and detects it by filtering out [13].

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Total number of scientific publications

30000 26036 25654 25000

23312 20057

20000 13910

15000 10000

9314

10936

14826

16426

21444

17436

11720

5000 0

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021

Years 2010

2011

2012

2013

2014

2015

2016

2017

2018

2019

2020

2021

FIGURE 11.2 The total number of scientific publications per year from 2010 to 2021 showing “gas sensors” in the content as per Science Direct record (Date of search: August 10, 2021).

Literature survey reveals that there are many research papers published on IL gas sensors since the 1950s. Today, there is a huge demand of IL gas sensors in the market across the world. Fig. 11.2 demonstrates rapid increase in the total number of scientific publications every year in last decade showing the content “gas sensors” as per Science Direct record database (Date of search: August 10, 2021). The prime objective of present work is to highlight a comprehensive overview of latest progress in the area of gas sensors before and after ILs.

11.2 Layout of the chapter This chapter contains electrochemical gas sensors, optical gas sensors, piezoelectric gas sensors, semiconducting metal-oxide gas sensors, carbon-IL composite gas sensors, and gated IL gas sensors. Most of these gas sensors are further divided into different subsections.

11.3 Electrochemical gas sensors Based on the operating principles, electrochemical gas sensors are divided into two types, potentiometric gas sensors (Fig. 11.3) and

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11.3 Electrochemical gas sensors

FIGURE 11.3

291

A schematic diagram of potentiometric gas sensor electrode.

amperometric gas sensors (Fig. 11.4). Both of these gas sensors are based on solid electrolyte. These gas sensors show higher ionic conductivity due to movement of ions through point defect sites present in the lattice structure. The electronic conduction due to movement of electrons or holes is almost negligible in these gas sensors. It is also reported that the ionic transport number of solid-based electrolyte is greater than 0.99. This is calculated by dividing the value of ionic conductivity with total conductivity value [14]. In potentiometric gas sensor, there is no flow of ionic current through electrolyte. This gas sensor involves opencircuit conditions. It means that there is no passing of electric current through the external electric circuit of this sensor. Weppner in 1987 [15] had categorized the equilibrium potentiometric gas sensors into main three types. The first type is direct measurement of mobiles species which is conventional potentiometric gas sensor. This type of sensor involves measurement of the potential difference between reference and sensing electrodes due to mobile species present in the electrolyte [16]. It is mainly utilized to analyze O2. The second type is indirect measurement of mobile components which involves establishment of equilibrium between the gas analyte and component which is different from the primarily mobile species. Potassium carbonate and silver sulfate are the examples of solid electrolytes which are utilized for the detection of carbon dioxide and sulfur trioxide gases, respectively [17,18]. This second type of gas sensor is not found suitable at high-temperature conditions. The reason behind this is the lack of thermal stability of solid electrolytes. Third type

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Diffusion Barrier Porous Membrane

Sensing Electrode Reference Electrode Room Temperature Ionic Liquids

Counter Electrode

FIGURE 11.4 A schematic diagram of amperometric gas sensor electrode.

of potentiometric gas sensor is analysis of other species by employing auxiliary solid phase. Third type of gas sensors was manufactured to avoid the restrictions of particular electrolytes in former two types gas sensors. It involves an auxiliary phase that consists of similar ionic species as those from gas. These ionic species are attached to solid electrolyte surface. There is formation of equilibrium between target species and mobile ions. The measured potential difference across two electrodes depends on the concentration of target gases. As a result, third type of equilibrium potentiometric gas sensor is utilized for detection of different gases such as carbon dioxide [19], nitrogen oxides, and sulfur oxides gases [20,21] at higher temperature. Pasierb and Rekas reported in detail the working principles involved in equilibrium potentiometric gas sensors [22]. Equilibrium potentiometric gas sensors have limitations to detect reducing gases such as carbon monoxide and hydrocarbons beyond 600
C. Moreover, suitable electrolyte and auxiliary electrodes required for equilibrium potentiometric gas sensors are not easily available. Therefore, nonequilibrium approach is preferred in such cases. In nonequilibrium potentiometric gas sensors, gases do not exist in thermodynamic equilibrium at the interface between electrode and solid electrolyte. There are many electrochemical reactions that take place at the surface of electrode material. As a result, a mixed potential is produced at the surface of electrode [23]. It is reported that mixed potential gas sensors are found to be useful for the detection of harmful gases such as carbon monoxide [24,25], nitrogen oxides [26,27], and hydrocarbons [28] and released through automobile exhaust. Many reactions involved in different potentiometric gas sensors using different aqueous electrolytes are presented in Table 11.1. Leland C. Clark is also known as “Father of biosensors” due to his contribution toward electrochemical gas sensors. These gas sensors are based on Clark oxygen electrode. He is very popular American

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11.3 Electrochemical gas sensors

TABLE 11.1 Many reactions involved in different potentiometric gas sensors using different aqueous electrolytes. Reactions taking place at the electrode surface

Target gas

Aqueous electrolyte

Carbon dioxide

Sodium bicarbonate/ sodium chloride

CO2 1 2H2O ——- HCO32 1 H3O1

Nitrogen dioxide

Sodium nitrite/potassium nitrate

2NO2 1 3H2O ——- NO32 1 NO22 1 2H3O1

Sulfur dioxide gas

Sodium hydrogen sulfite

SO2 1 2H2O ——- HSO32 1 H3O1

Ammonia

Potassium chloride

NH3 1 H2O ——- NH41 1 OH2

biochemist who invented Clark electrode device that is utilized for the detection and measurement of oxygen in blood, water, and other liquids. Amperometric gas sensors are developed from “Clark oxygen electrode” design [29]. He has developed this first time for clinical purpose and tested on dogs. Thereafter, this device was used as oxygen generator for the purpose of cardiac surgery [30]. Clark oxygen electrode consists of cellophane membrane, a platinum metal as cathode electrode, a potassium chloride-calomel anode, and power supply. Cathode and anode electrodes were kept inside a glass tube. The function of cellophane membrane in Clark oxygen electrode was to avoid contact between internal and external environment. It prevented the effect of RBCs (red blood cells) in the detection of oxygen gas. In this work, oxygen gas was diffused into internal solution through the cellophane membrane. Oxygen gas was reduced at the surface of platinum electrode when it was connected to power supply of 0.6 V. O2 1 2H1 1 2e2 -H2 O2

(11.1)

The recorded current by galvanometer for the reduction of oxygen gas was found to be directly proportional to oxygen gas concentration. In Nernst’s laboratory, Heinrich Danneel reported first time the linear dependence relation involving O2 gas and current [31]. Many investigations were performed by Leland C. Clark to optimize suitable conditions for the detection of dissolved oxygen gas. He discovered membranes that were very efficient and reduced the response times for the detection of oxygen gas [29]. The cellophane membrane in Clark oxygen electrode showed response time approximately 20 seconds. On the other hand, response time for the detection of dissolved oxygen without any type membrane was found approximately 10 seconds [29].

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Thereafter, many investigations were performed on blood to check oxygen gas in vitro and in vivo. Galvanometer was used to record oxygen gas responses. Clark oxygen electrode design was found to be very accurate to detect and measure the oxygen gas with optimized experimental conditions. Clark electrochemical cell design was also applied for the development of glucose sensors [32]. This great invention of Prof. Leland C. Clark motivated and actuated many researchers to develop gas detectors using the principles of Clark oxygen electrode. Subsequently, Modern Clark type gas sensors were developed based on classical design of Clark oxygen electrode. Initially, the gas enters through particular membrane and dissolves into internal solution of the electrochemical cell. Thereafter, redox reaction takes place. The basic design of Leland Clark oxygen electrode was used by various manufacturers (Casella, Alphasense, Geotech, Honeywell, Drager, etc.) today across the world to manufacture a variety of gas sensors such as oxygen, carbon dioxide, carbon monoxide, phosphine, hydrocarbons, ammonia, hydrogen sulfide, hydrogen cyanide, sulfur dioxide, chlorine dioxide, hydrogen halides, halogens, nitrogen monoxide or nitric oxide, nitrogen dioxide, ozone, organic vapors, hydrogen peroxide, hydrogen gases. Amperometric gas sensors are found to be more efficient to detect and measure even higher concentration of gases. While potentiometric gas sensors are not found efficient to detect and measure higher concentration of gases, modern amperometric gas sensors with suitable designs and appropriate materials are found to be efficient for the detection of numerous gases such as oxygen, nitrogen oxides, sulfur dioxide, hydrogen, carbon monoxide, hydrogen sulfide, and organic vapors with aldehyde or alcohol functional groups [33]. Many electrochemical reactions involved in different amperometric gas sensors using different aqueous electrolytes are presented in Table 11.2.

TABLE 11.2 Many electrochemical reactions involved in different amperometric gas sensors using different aqueous electrolytes. Target gas

Aqueous electrolyte

Reactions taking place at the electrode surface

Oxygen

Potassium hydroxide

O2 1 2H2O 1 4e2 ——- 4OH2

Oxygen

Sulfuric acid/water

O2 1 4H1 1 4e2 ——- 2H2O

Nitrogen dioxide

Sulfuric acid/water

NO2 1 2H1 1 2e2 ——- NO 1 H2O

Carbon monoxide

Sulfuric acid/water

CO 1 H2O ——- CO2 1 2H1 1 2e2

Hydrogen

Potassium hydroxide

H2 1 2OH2 ——- 2H2O 1 2e2

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295

In spite of huge number of applications of Leland Clark type gas sensors, they have some demerits. The response time, sensitivity, and life span of Clark type gas sensors are adversely affected due to the use of membrane and conventional solvents. It is reported that the diffusion coefficient of oxygen gas is higher in air, lower in water [34], and lowest in polymer membrane [35,36]. This is due to very low gas permeability to oxygen gas by polymer membrane as compared to air and water [37]. It is understood that response time is influenced by material of membrane and its thickness in Clark type gas sensors. The response time in original work of Clark type gas sensor was found to be increased from 20 to 30 seconds when the thickness of cellophane membrane was increased by two times. E. I. Rogers reported that conventional solvents [38] that are utilized to detect and measure target gases have limitations due to evaporation of these solvents at higher temperature. This resulted in failure of gas sensors [39]. Gas sensors are failed due to boiling of solutions at higher temperature and freezing of solution at lower temperature operating conditions. It was highlighted by P. T. Moseley that electrochemical gas sensors have life span of 2 years at optimum conditions such as temperature 25
C and relative humidity 60% [40]. It was reported by O’Mahony [41] and Koschel [42] that sensors are needed to work at drastic temperature conditions from 260
C to 60
C for the detection and measurement of hydrogen sulfide gas in oil field. It was also found that cross sensitivity of other nontarget gases affects the accurate detection and measurement of target gas. It is reported that while using oxygen gas sensor, 20% of carbon dioxide increased the signal by 6% and causes inaccurate measurement of target gas. Oxygen gas sensor was found damaged when more than 25% of carbon dioxide gas is involved. The reason behind this damage of oxygen gas sensor may be due to carbon dioxide gas adsorbed by the electrolyte. It was also highlighted by J. Wang that strong oxidizing gases such as O3, Cl2, Br2, and ClO2 gains electrons which are transferred during the oxygen reduction process. This adversely affected an efficiency of oxygen gas sensors for accurate detection of target gas [43]. To overcome limitations of conventional solvents used in gas sensors, RILs are used and found to be very advantageous due to their prominent features such as wider electrochemical windows, maximum water uptake (Table 11.3), higher thermal stability (Table 11.4), density and viscosity (Table 11.5), inherent conductivity (Table 11.6), lower volatility, and tenability. RILs are made up of a large size asymmetric organic cation which are associated with a smaller size inorganic anion and present in liquid state at or below room temperature condition [44]. Due to these significant features, RILs are employed for the detection and measurement of target gases. Rogers et al. [38] reported the reaction mechanisms of different gases such as oxygen, carbon dioxide, hydrogen, ammonia, hydrogen sulfide, sulfur dioxide, and nitrogen dioxide in RILs.

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TABLE 11.3 Maximum water uptake by tris(perfluoroalkyl)trifluorophosphate room temperature ionic liquids (RILs). RILs

Maximum water uptake by RILs in ppm

1-Butyl-3-methylimidazolium hexafluorophosphate

22,600

1-Butyl-1-methylpyrrolidinium bis (trifluoromethylsulfonyl)imide

14,800

1-Butyl-1-methylpyrrolidinium tris(perfluoroalkyl) trifluorophosphate

3500

1-Hexyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide

10,670

1-Hexyl-3-methylimidazolium tris(perfluoroalkyl) trifluorophosphate

2030

TABLE 11.4 temperature.

Room temperature ionic liquids (RILs) with decomposition Decomposition temperature (
C)

RILs S-Ethyl-tetramethylthiouronium tris(perfluoroalkyl) trifluorophosphate

250

O-Ethyl-tetramethyluronium tris(perfluoroalkyl) trifluorophosphate

220

N,N,N0 ,N0 ,Nv-Pentamethyl-Nv-i-propylguanidinium tris (perfluoroalkyl)trifluorophosphate

240

1-Butyl-1-methylpyrrolidinium tris(perfluoroalkyl) trifluorophosphate

250

1-Hexyl-3-methylimidazolium tris(perfluoroalkyl) trifluorophosphate

290

1-Pentyl-3-methylimidazolium tris(perfluoroalkyl) trifluorophosphate

300

1-Ethyl-3-methylimidazolium tris(perfluoroalkyl) trifluorophosphate

300

David Williams in 2020 reviewed different types of electrochemical gas sensors for environmental gas analysis. The electrochemical gas sensors can be used to detect different harmful gases present in environment in ppb range (109). The influence of different parameters such as humidity and temperature on the performance of different electrochemical gas sensors was described [45].

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TABLE 11.5 Density and viscosity of room temperature ionic liquids (RILs) containing cations with different anions. RILs

Density of RILs (20
C) (g/cm3)

Viscosity of RILs (20
C) (mm2/s)

1-Hexyl-3-methylimidazolium [BF4]2

1.150

195

1-Hexyl-3-methylimidazolium [(CF3SO2)2N]2

1.377

44

1-Hexyl-3-methylimidazolium [(C2F5)3PF3]2

1.560

74

1-Hexyl-3-methylimidazolium [(C3F7)3PF3]2

1.620

227

1-Pentyl-3-methylimidazolium [(C4F9)3PF3]2

1.693

594

1-Hexyl-3-methylimidazolium [PF6]2

1.297

548

1-Hexyl-3-methylimidazolium chloride

1.050

7453

TABLE 11.6 (A) Conductivity of ionic liquid (RILs) with tris (perfluoroalkyl) trifluorophosphate anion and (B) conductivity of 1-ethyl-3-methylimidazolium RILs with different anions. Conductivity of RILs (20
C) (mS/cm)

RILs

(a) Conductivity of RILs with tris (perfluoroalkyl)trifluorophosphate anion 1-Ethyl-3-methylimidazolium tris(perfluoroalkyl) trifluorophosphate (60
C)

14.25

1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide

7.63

1-Ethyl-3-methylimidazolium tris(perfluoroalkyl) trifluorophosphate

4.40

1-Pentyl-3-methylimidazolium tris(perfluoroalkyl) trifluorophosphate

1.66

1-Hexyl-3-methylimidazolium tris(perfluoroalkyl) trifluorophosphate

1.32

(b) Conductivity of 1-ethyl-3-methylimidazolium RILs with different anions 1-Ethyl-3-methylimidazolium trifluoroacetate

8.53

1-Ethyl-3-methylimidazolium triflate

7.74

1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide

7.63

1-Ethyl-3-methylimidazolium tris(perfluoroalkyl) trifluorophosphate

4.40

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11.3.1 Electrochemical oxygen gas sensors The detection and measurement of oxygen gas is very much necessary in mining, steel manufacturing, and food processing industries. In addition, oxygen gas sensors or detectors are widely employed in automobile industries as solid-state gas sensors [46]. The oxygen gas sensors play a very vital role to monitor and control air to fuel ratio which is supplied to automobile engine connected to three-way catalyst. This has resulted in decline of emission of harmful air pollutants from vehicles into our environment. The oxygen gas sensor has very significant role in monitoring effectiveness of catalyst and decreases harmful emission of gases up to some extent. Catalyst helps for oxidation of hydrocarbons and reduction of nitrogen oxides [47]. Due to easy availability of oxygen gas cylinders, oxygen gas sensor has been investigated by many researchers across the world. Wang et al. have studied and played an imperative role for the progress of amperometric oxygen sensor. This oxygen gas sensor was based on Leland C. Clark basic design [48]. Platinum gauze is fitted in this Clark design as working electrode. RILs are utilized as electrolytes in this oxygen gas sensor. The present investigation reported that different analytical parameters of gas sensors are highly influenced by cations of RILs having bis(trifluoromethylsulfonyl)imide anion. The developed oxygen gas sensor is found to be more stable and efficient. This study highlighted that the modified electrochemical oxygen gas sensor with suitable RILs can be also used for the measurement and detection of CH4 and NO gases. This investigation reported response time as 120 seconds and limit of detection as 0.05% for oxygen gas sensor. There was no obvious fouling of electrode surface observed for the period of 90 days in this electrochemical cell. In this chapter, three RILs having different cations with the same anion were used. The calculated rate of diffusion was found to be very slow and recorded response time was longest when tributylmethylammonium bis(trifluoromethylsulfonyl)imide IL was used. This may be due to more viscosity of this RIL due to mass-transfer considerations as compared to other two electrolytes. This oxygen gas sensor containing tributylmethylammonium bis(trifluoromethylsulfonyl)imide IL showed excellent selectivity as compared to other two ILs. Baltes et al. in 2013 presented a prominent electrochemical membrane sensor using RILs for the detection and measurement of oxygen gas in ppm concentrations. In this study, nonvolatile with thermal stability RILs were used to detect oxygen gas in extremely low concentrations (ppm) at drastic temperature condition beyond 100
C [49]. This work reported cheap and simple oxygen gas sensors with different composition of electrolyte. This present work highlighted oxygen gas sensors with high sensitivity, good stability, and a quick response time. The

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best reproducibility of oxygen gas sensor was reported at room temperature. These developed oxygen sensors were constantly employed for at least 7 days. This has indicated long-term stability of oxygen gas sensors. This chapter concluded that RIL oxygen gas sensors can be used for gas sensing applications up to higher temperature 200
C. It has also pointed out that oxygen gas sensor with certain RILs suffers from hysteresis at very high temperature. Tonilio et al. in 2012 studied RILs and reported compounds showing quinone moieties which were able to promote oxygen reduction at very low potential. This innovative and interesting work highlighted that amperometric oxygen gas sensors without membrane can be developed using promising features of RILs like higher inherent conductivity, very low volatility, and higher thermally stable nature. The developed amperometric oxygen gas sensors were found to be very efficient. The efficiency of oxygen gas sensor can be enhanced with the addition of low melting salts having quinine moiety. This has permitted the reduction of oxygen gas taking place through an electrocatalytic pathway at very low potentials as compared to standard potential. This work concluded that oxygen gas detection up to 10 ppm (v/v) could be achieved at higher temperature (100
C) [50]. Xiong et al. in 2013 utilized chronoamperometry for the detection and measurement of gas concentrations and diffusion coefficient using 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide IL as electrolyte [51]. It was found that the solubility of oxygen gas in 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide IL does not depend on temperature condition. Copper metal can be employed as nonexpensive cathode electrode to check oxygen gas. Xiong and coworkers in other study discussed in detail about a simple and cheaper method to fabricate low-priced disposable electrodes made up of different types of materials and having different geometries for gas sensing applications [52]. There is always a demand of cheaper, easily manufactured, and disposable electrodes in the market across the world. The method which is based on lamination of electrodes was used to manufacture inexpensive, easily disposable, and effective electrodes. In this investigation, Au macrodisc and Pt microband homemade electrodes were fabricated. This paper highlighted that Au macrodisc electrode was found to be competent for carrying out quantitative electroanalysis in 0.1 M aqueous potassium chloride solution. Both the electrodes were analyzed in aqueous electrolyte and ILs. Experimental results show that both the electrodes work properly in aqueous solution and ILs. Both these electrodes were utilized for the detection and measurement of oxygen gas. In this investigation, Au macrodisc electrodes employed to record the peak currents of O2 gas at each volume percentage analyzed. Pt microband electrodes have shown

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steady-state currents for different volumes of O2 gas. Experimental data show less than 4% difference in the measurement of concentrations of oxygen using these two electrodes. Henry’s law constants for these two electrodes were found to be in good agreement. This paper concluded that these two electrodes are capable of quantitative measurement. Xiong et al. in 2011 studied Au nanoparticle (NP) composites using RIL as electrolyte [53]. This composite was fabricated to create a very thin layer of RIL on electrode in amperometric oxygen gas sensors without using membrane. In this work, it was observed that a very thin layer of RIL avoids the oxidation of Au NPs. As a result, Au NPs remain stable, protected, and active. These Au nanocomposites reported fastest response time, large electric current, and low reduction potentials. Many RILs having different viscous nature and surface tensions were used for the measurement of analyte oxygen gas. These Au NP sensors without membrane were found to be very efficient and effective due to quick response time. In this investigation, chronoamperometry and cyclic voltammetry methods were used to measure response of Au nanocatalysts. Au nanocatalysts oxygen sensors are found to be very versatile. Lee et al. in 2013 used several RILs for gas sensing purposes using screen-printed electrodes [54]. The present work employed four different electrodes such as platinum, silver, gold, and carbon. Experimental results indicate the suitability of platinum, silver, gold, and carbon electrodes for the detection and measurement of oxygen gas. In this study, cyclic voltammetric wave shapes were observed on platinum screenprinted electrode surfaces. These observed wave shapes were found to be different as compared to traditional solid platinum macroelectrode wave shapes. This may be due to chemical reaction between superoxide and compounds. It was concluded that this kind of sensing platform can be used for “single-use” oxygen sensors [55]. Gebicki et al. in 2011 investigated O2 gas sensor using suitable solid electrodes and different RILs [56]. Response time was determined. The product of permeability coefficient and solubility of oxygen in ionic liquids product was calculated in RILs for oxygen gas sensing application. Experimental results show that minimum exposure time to target gas is different for different RILs and influenced by viscous nature of liquids and volumetric flow rate. Hu et al. in 2012 developed cheaper and simple-structured oxygen gas sensors using nanoporous Au electrode arrays on cellulose as membrane [57]. This paper-based oxygen gas sensor showed better efficiency as compared to earlier RIL-based O2 gas sensors. This fabricated paperbased Au electrode arrays are found to be cheaper, highly flexible, and good conductor. RIL 1-butyl-3-methylimidazolium hexafluorophosphate was used to develop this paper-based oxygen sensor with a limit of detection 0.0075% and quick response time less than 10 seconds. This

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study proposed paper-based solid-state Au electrode oxygen gas sensor using RIL is very cheap and eco-friendly for the detection and measurement of target oxygen gas. Gunawan et al. in 2014 developed an amperometric microarray oxygen gas sensor [58]. It was suggested that miniaturized liquid systems can be preferred to sort out the difficulties such as evaporative solvent losses and slow mass transport in more viscous ILs in lab-on-a-chip devices for gas sensing applications. Yu et al. in 2019 developed metal oxide with reduced graphene oxide (RGO) gas sensor using IL such as 1-butyl-3-methylimidazolium hexafluorophosphate for oxygen gas sensing applications. This gas sensor shows sensitivity of 0.1087 µA/[%O2]. This study concluded that the interaction between 1-butyl-3-methylimidazolium hexafluorophosphate and NiCo2O4/RGO improves the performance of this gas sensor for the detection and measurement of oxygen gas [59]. Stine et al. in 2020 demonstrated bioprocessing online device platform for monitoring of dissolved oxygen gas. This gas sensor shows a sensitivity of 37.5 nA/dissolved oxygen. The limit of detection was found to be 8.26 dissolved oxygen percentage. This wireless bioprocessing online device platform can be used for diverse bioprocess monitoring applications [60].

11.3.2 Electrochemical ammonia gas sensors Among the various hazardous gases, ammonia gas is manufactured on large scale and widely used in different fields across the world [6167]. Ammonia is manufactured on large scale by HaberBosch process. The optimized conditions to get maximum yield of ammonia gas are high pressure (20 MPa) and high temperature (700K). The catalyst used in this process is iron oxide with small amounts of potassium oxide and alumina. The chemical reaction involved in this process is


N2 g 1 3H2 g -2NH3 g (11.2) This is exothermic and reversible reaction [68]. Ammonia gas is widely used in our day-to-day life such as in ice cream factories, dairy products plants, food processing plants, alcoholic production, and petrochemicals. In addition, ammonia is main source to produce hydrogen gas. Ammonia is also used in automobile engines to convert toxic nitric oxide to harmless nitrogen gas as mentioned in the equation [69]. 4NO 1 4NH3 1 O2 -4N2 1 6H2 O

(11.3)

It was reported that 2.18.1 Tg mass of ammonia gas is released annually in our environment through various human activities.

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Generally, ammonia gas found in our environment is very low in amount (15 ppb) [70]. Human beings can easily detect ammonia gas when it is present in higher concentration in environment. Ammonia gas has very strong smell. However, it is not possible for us to detect ammonia gas present at low concentration in our environment. Exposure to ammonia gas in lower concentration also adversely affects health of human beings and animals. Consequently, researchers developed a variety of ammonia gas sensors to detect and measure ammonia [71]. Murugappan et al. in 2011 investigated oxidation of NH3 gas using 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide as electrolyte on different screen-printed electrodes such as carbon, gold, and platinum using voltammetry technique [72]. This investigation concluded that screenprinted electrodes using microliters volumes of nonvolatile RILs are inexpensive gas sensors than voltammetric or amperometric gas sensors. Experimental data show well-defined oxidation peaks on gold and platinum electrode surfaces. The calibration graphs on gold and platinum screenprinted electrode surfaces are found to be linear in nature. Platinum screenprinted electrode surfaces are more capable for sensing ammonia gas than gold screen-printed electrode surfaces. Carter et al. in 2012 developed novel amperometric gas sensors on the basis of screen-printing and lamination strategies for the detection and measurement of ammonia gas [73]. The performance of printed ammonia gas sensors using RILs was studied and compared with traditional amperometric gas sensors using conventional electrolytes. The performance of printed amperometric gas sensors was studied in 0%95% relative humidity air. The performance was found better on exposure to higher percentage relative humidity air. A very small quantity of RIL was sufficient for printed amperometric gas sensors. The precision for printed amperometric ammonia gas sensors is 6 18.1% at 100 ppm ammonia and 6 5.2% at 1000 ppm ammonia. Experimental data indicated better performance of printed amperometric gas sensors using RILs in ammonia gas sensing. However, there is further scope to improve the performance of these gas sensors with optimized conditions. Carter et al. in 2014 reported the latest studies of RILs in printed amperometric gas sensors [74]. This study described in detail the effect of hydrophobic nature of RILs used as electrolyte in printed amperometric gas sensors. This study highlighted the fabrication of these gas sensors by laminating with many polymeric layers. Rational selection of RILs for the fabrication of printed amperometric gas sensors is outlined in this investigation. Ammonia gas undergoes oxidation and produces ammonium ion at the surface of platinum electrode as shown in the following equation. 4NH3 -3NH4 1 1 1/2N2 1 3e2

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There was equilibrium established between ammonium ion and free ammonia in diffusion layer of gas sensor. Thereafter, ammonium ion formed at the platinum electrode undergoes further oxidation. This paper also highlighted that the extent of electrode wetting is influenced by nature of RILs. The experimental data revealed that printed amperometric gas sensors using hydrophilic RILs are more capable and efficient than hydrophobic RILs as electrolytes for the detection and measurement of ammonia gas. Liu et al. in 2016 reported room temperature ammonia sensor on the basis of platinum and platinumiridium porous ceramic electrodes. Polarization and chronoamperometry methods were applied to measure the different parameters of gas sensor. The electrochemical activity of ammonia gas sensor was found to be enhanced by water vapors. The reaction path of ammonia gas on platinum surface is described. The sensitivity was found to be increased from 1.14 to 12.06 µA/ppm cm2 [75]. Praveen Sekhar and Jesse Kysar in 2017 demonstrated a room temperature electrochemical ammonia gas sensor on a paper substrate. Platinum electrodes and IL were used to fabricate this electrochemical ammonia gas sensor. The limit of detection was found to be 1 ppm. Experimental results show that the rise time was 8 seconds and fall time was 7 seconds. This gas sensor shows very quick response toward ammonia gas [76].

11.3.3 Electrochemical nitrogen oxide gas sensors Automobile engines released huge amount of harmful air pollutants such as nitrogen monoxide, nitrous oxide, and nitrogen dioxide in our environment across the world. These nitrogen oxides are responsible for respiration problems and other health-related issues in living organisms. Therefore there was a huge demand of electrochemical gas sensors for the detection and measurement of these nitrogen oxide gases released in the atmosphere. Basil Dimitriades in 1972 reported that nitric oxides gases are responsible for photochemical smog formation [77]. The role of nitric oxides in photochemical smog formation was studied. In this investigation, automobile exhaust with different levels of air pollutants nitric oxides was irradiated in smog chamber. Nitric oxides levels varied from 0.08 to 1.4 ppm. Photooxidation of air-pollutant nitric oxides and formation of oxidants were utilized as smog manifestations. During this work, smog levels found in smog chamber and atmospheric levels of reactants were analyzed. It has been observed that European limit value for average nitrogen oxides concentration has increased every year. The European community set the level of nitrogen oxide gases as 20 ppb (v/v) per year. Environmental Protection Agency (EPA) of the United States set the

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level of nitrogen oxide gases as 53 ppb (v/v) [78,79]. The present concentration levels of nitrogen oxide gases were found to be increased than the limit value across the number of places in the Netherlands. Toniolo et al. reported that smog formation can be monitored and controlled systematically by decreasing the harmful nitrogen oxide gases present in atmosphere [80]. In this study, amperometric nitrogen oxide gas sensor using RILs without membrane was proposed for the detection and measurement of these gases. Experimental data indicated that this gas sensor is capable and efficient to monitor nitrogen oxide gases present in environment. This gas sensor shows detection limit 0.55 ppb (v/v) which is quite lower than the permitted level of 53 ppb (v/v) for nitrogen oxide gases. It was also highlighted that there was not any considerable disturbance caused by other components present in environment. During this study the present gas sensor was used for detecting nitrogen oxides at low pressure present in environment. Kubersky et al. in 2015 at the University of West Bohemia developed electrochemical nitrogen oxide gas sensors using graphite paste and solid-polymeric electrolyte on two substrates [81]. One substrate was flexible whereas other was rigid in nature. This gas sensor was found to be cheaper, eco-friendly, and suitable for mass production. This gas sensor was developed without utilizing metal-based printing pastes. The experimental data indicated that this fully printed gas sensor shows sensitivity (590 nA/ppm) and resolution (0.2 ppm). It also shows fast response (70 seconds), recovery times (60 seconds), and linear response (010 ppm range). This work highlighted that fabricated gas sensor combined with potentiostat electric circuit used in this study displays good sensitivity and clear resolution. In addition, it shows good response time and hysteresis. It was concluded that the presented gas sensor in this study can be utilized for the detection and measurement of nitrogen oxide gases in workplaces. Kubersky et al. in 2015 also investigated the fluctuation phenomena in amperometric nitrogen oxide gas sensors using same setup. Experimental results concluded that diffusion noise at greater concentration of nitrogen oxide causes the current fluctuations in the gas sensor [82]. Kubersky et al. also reported intrinsic parameters of amperometric gas sensor with solid-state polymer as electrolyte. The present gas sensor shows good sensitivity and response time for nitrogen oxide gases. The work concluded that the amperometric gas sensor using organic IL can be utilized for battery-powered application. Electrical and mechanical parameters were investigated for this gas sensor [83,84]. Nadherna et al. in 2011 used ionic-liquid polymeric electrolyte in amperometric nitrogen oxide gas sensor. The electric current produced due to reduction of nitrogen dioxide was used to detect target gas. The experimental result shows ionic conductivity value 1.6 3 1024 S/cm at

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temperature 20
C. It also shows higher electrochemical stability and thermal stability. This gas sensor response is linear for the concentration of nitrogen dioxide varies from 0.3 to 1.1 ppm. It is highlighted that present gas sensor is reproducible and more stable [85]. Nadherna et al. in 2012 continued the study on electrochemical nitrogen oxides gas sensors. The planar amperometric gas sensor using binary solid-polymeric electrolyte containing RILs was developed. The indicator electrodes of different geometric surface areas were used in this study. This research group reported that the present gas sensor provides longer response stability and lower sensitivity to the test air humidity [86]. Li et al. in 2020 reported physically transient electrochemical nitric oxide gas sensor for the detection of nitric oxide. This fabricated gas sensor showed limit of detection 3.97 nM. This gas sensor shows quick response to nitric oxide gas. It can be used for a broad sensing range from 0.01 to 100 µM. This fabricated gas sensor was flexible, biodegradable, and wirelessly operated for continuously monitoring nitric oxide gas in vitro and in vivo [87].

11.3.4 Electrochemical volatile organic compounds gas sensors Volatile organic compounds are having very high vapor pressure. These organic compounds undergo evaporation or sublimation. On evaporation, some of these compounds are converted from liquid state to gaseous state. On sublimation, some of these compounds are converted from solid state to gaseous state directly. Pesticides, paints, printers, cleaning appliances are the sources of volatile organic compounds in atmosphere. Volatile organic compounds have adverse effects on human beings. Therefore many electrochemical sensors are fabricated to detect and measure volatile organic compounds present in atmosphere. Zevenbergen et al. in 2011 developed electrochemical gas sensor using Au electrode and IL 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate for the detection of ethylene analyte [88]. The limit of detection was found to be 760 ppb. The sensitivity was measured as 59 pA/ppm. In other investigation, Zevenbergen et al. developed electrochemical gas sensor using Au electrode and IL 1butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. The limit of detection (25 ppm), sensitivity (12 pA/ppm), and quick response time (3 seconds) were reported [89]. Dossi et al. in 2012 developed electrochemical gas sensor using glassy carbon disk electrode modified with IL Co(II) phthalocyanine and 1-butyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide for the detection

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and measurement of 1-butanethiol and phenol analytes. The limit of detection was determined as 0.5 µM. The sensitivity was 54.42 nA/µM. The response time was very quick (less than 6 seconds) [90]. Toniolo et al. in 2013 fabricated electrochemical gas sensor using glassy carbon disk electrode and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide or 1-butyl-3-methylimidazolium acetate ILs for the detection of 1-butanethiol and phenol. The limit of detection was found to be 0.29 µM. The sensitivity was found to be 1.09 mA/µM. The response time was less than 6 seconds [91]. Gebicki et al. in 2013 developed electrochemical gas sensor using Au layer and different ILs such as 1-butyl,3-methylimidazolium dicyanamide, 1-octyl,3-methylimidazolium tetrafluoroborate, and 1-butyl,3-methylimidazolium bis(trifluoromethanesulfonyl)imide. This gas sensor was utilized for the detection and measurement of three analytes such as benzaldehyde, methylbenzoate, and acetophenone. The limit of detection is calculated as 1 ppm. Sensitivity was found to be 200 nA/ppm [92]. Murugappan et al. in 2014 fabricated electrochemical gas sensor using platinum microelectrode and N-butyl-N-methylpyrrolidinium, bis (trifluoromethylsulfonyl)imide for the detection of methylamine analyte. The limits of detection were found to be 33 and 34 ppm for ILs such as N-butyl-N-methylpyrrolidinium and bis(trifluoromethylsulfonyl)imide, respectively. The sensitivity was found to be 1.24 and 2.13 pA/ppm for N-butyl-N-methylpyrrolidinium and bis(trifluoromethylsulfonyl)imide respectively [93]. The experimental data show that the higher solubility of methylamine analyte in RILs (230400 mM) as compared to 212 mM for O2 [94,95], 310 mM for H2 [96], and 200500 mM for H2S [97].

11.3.5 Electrochemical carbon dioxide gas sensor Lee et al. in 2017 developed microcarbon dioxide gas sensor with the help of solid electrolyte Li3PO4 thick film. This gas sensor shows lower consumption (59 mW) and higher selectivity toward carbon dioxide gas. The recorded EMF was found to be 50.5 mV/decade for air-based CO2 gas. Surface micromachining technique was used in the development of this gas sensor [98]. Schwandt et al. in 2018 demonstrated solid-state electrochemical gas sensor for the detection and measurement of carbon dioxide gas. This developed gas sensor has a modular design. It has ceramic and salttype ion conductors. The experimental data show response time of seconds to minutes toward carbon dioxide gas. It was also reported that this gas sensor offers thermodynamically expected cell voltage. This gas sensor is found to be working and drift-free for many weeks [99].

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11.3.6 Electrochemical methane and oxygen dual gas sensor Wang et al. developed electrochemical dual gas sensor using N-butyl-N-methylpyrrolidinium, bis(trifluoromethylsulfonyl)imide for the detection and measurement of analytes CH4, and O2 gases at the same time. The limit of detection for CH4 gas is 3000 ppm. The unique, localized, and coupled chemical reactions help for the detection of two gases at the same time. In this gas sensor, carbon dioxide and water are produced due to oxidation of methane on the surface of platinum electrode. There is formation of carbon monoxide due to partial oxidation of CH4 gas [100,101].

11.3.7 Electrochemical hydrogen sulfide carbon nanotubemodified electrode gas sensor Nauber and Tschuncky in 2014 used carbon nanotube (CNT) modified electrode using IL 1-ethyl-3-methylimidazolium acetate for the detection and measurement of hydrogen sulfide gas from 0 to 200 ppm. The response of this sensor to hydrogen sulfide gas was very quick (6 seconds). The recovery was also very short (6 seconds). This electrochemical gas sensor works at drastic condition such as very low humidity. This gas sensing is possible due to low volatility of IL. As a result, it can be used to detect and measure hydrogen sulfide gas in drastic conditions. For example, gas sensing used in oil fields is found in desert [102].

11.4 Optical gas sensors Optical gas sensor involves detection of target gas with the help of our naked eye or quantified spectroscopic methods. Chemical dyes are usually used in this gas sensor. There are many benefits of optical gas sensors. This is due to its salient features such as quick response time, simple construction, cost-effectiveness as compared to traditional instruments [103]. Optical gas sensors shows excellent chemical selectivity, higher extinction coefficient, better sensitivity due to variety of dyes and their derivatives [104,105]. Optical gas sensors can be used to detect many analytes [106]. Huang et al. in 2011 developed an artificial olfaction system on the basis of colorimetric sensor array. This technique was utilized for fast evaluation of fish freshness [107]. Salinas et al. in 2012 developed optoelectronic nose to check chicken meat aging. This technique was

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based on 16 pigments that were synthesized by inclusion of various dyes into inorganic materials [108].

11.4.1 Optical oxygen gas sensors Yuki Fujiwara and Yukata Amao in 2003 studied pyrene derivative chemisorption layers onto alumina plate. This technique was utilized for developing O2 sensing device with high sensitivity. This device has longer lifetime and higher quantum yield [109]. Pyrene-based dyes are not found stable in aqueous matrices due to evaporation losses. Similarly, these pyrene-based dyes are also not found stable in solid matrices due to the sublimation. Ozlem Oter and Sahin in 2015 studied pyrene dye which possesses many salient features such as better sensitivity and higher quantum yield [110]. It has also longer fluorescence lifetime. Due to its greater lipophilic character and better sensitivity, pyrene dye was chosen over ruthenium metal dyes. It faces difficulties of degradation under light. This dye was immobilized on an ethyl cellulose matrix. It was utilized for the detection and measurement of O2 gas. The response time of optical gas oxygen sensor was varying from 6 to 20 seconds. Electrospun polymer fibers used in optical gas sensors are having better sensitivity and higher surface area. These sensors were used for the detection of O2 gas [111]. Biring et al. in 2019 worked on the fabrication of optical gas sensors which are cost-effective and simple in construction for detection and measurement of more number of gases present in industrial areas. This research group demonstrated optical gas sensor for the detection of O2 and NH3 gases. This gas sensor consists of fluorescent dyes that are coated individually on both sides of a glass substrate. The maximum sensitivity toward O2 was found to be 60 at 100% O2 environment. This study reported maximum sensitivity toward NH3 as 20 in a 1000 ppm NH3 environment. These results indicated that the present optical dual gas sensor can be utilized for the detection of O2 and NH3 gases simultaneously [112].

11.4.2 Optical carbon dioxide gas sensors The amount of carbon dioxide gas is increasing day by day in our environment. The increase in carbon dioxide gas is due to excess use of fossil fuels by human beings. This causes global warming. Hence, there is urgent need for controlling carbon dioxide gas released in our environment. Optical carbon dioxide gas sensors work on the basis of absorbance or fluorescence changes of pH indicators. It has many significant features. Oter et al. in 2006 described the carbon dioxide gas sensor

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which was based on change in fluorescence signal intensity of ILs. Optical carbon dioxide gas sensor was fabricated using water-miscible ILs. The response of this gas sensor to gaseous and dissolved carbon dioxide was calculated [113]. Beat Muller and Peter Hauser in 1996 fabricated optical carbon dioxide gas sensor for detecting and measuring low concentrations of dissolved carbon dioxide gas [114]. Aydogdu et al. in 2010 fabricated optical chemical sensing devices using electrospinning method. This method is promising, novel, and simple method. It was reported that fabricated electrospun films exhibits highly porous nature and enhanced surface area. As a result, it offers higher gas permeability. In this investigation, poly(methyl methacrylate) and ethyl cellulose were utilized to fabricate optical gas sensor. The ion pair of 8-hydroxypyrene-1,3,6-trisulphonic acid is selected as the fluorescent indicator. This indicator has superior photostability and long life. Scanning electron microscopy technique was used for the characterization of nanofibrous. The experimental analysis reported that electrospun nanofibrous membranes show higher sensitivity toward carbon dioxide gas as compared to thin filmbased gas sensors. This present electrospun nanofibrous optical carbon dioxide gas sensor shows very response toward analyte gas. The signal changes were found entirely reversible. This gas sensor was found to be stable over the period of seven months [115]. Chen et al. in 2011 reported that there is need of simple, inexpensive, highly sensitive, and selective optical gas sensor for the detection of carbon dioxide gas. Therefore novel optical carbon dioxide sensor was developed on the basis electrochemiluminescence which produces species at the surface of electrode [116]. These species undergo electrontransfer reactions [117]. This fabricated gas sensor shows very quick response toward carbon dioxide gas. It operates at room temperature and wide range of concentrations of carbon dioxide. It was found free of interferences from various other gaseous components. Ramos et al. in 2019 prepared two dyes and were used for the detection of carbon dioxide gas using an inner filter process. The indicators are noncovalently trapped inside the polymers. This gas sensor exhibits better photostability. Therefore it can be used for longer duration measurements. This investigation reported response time from 42 to 60 seconds. The limit of detection was 0.04% and 0.57% carbon dioxide. The recovery time was reported from 103 to 120 seconds. The life span of sensing membrane is more than 570 days. It is stored in darkness [118].

11.4.3 Optical ammonia gas sensors Ammonia has many applications in different fields. Ammonia gas is main irritant to human beings. Chen et al. in 2012 fabricated optical

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ammonia gas sensor using ammonia as coreactant for electrochemiluminescence material in RILs. This optical sensor exhibits higher sensitivity toward ammonia gas. The limit of detection of this sensor was found to be 10 ppt. This fabricated ammonia gas sensor has exceptional selectivity against some interfering gaseous components. This gas sensor has very wider linear response ranging from 10 ppt to 10ppm [119]. Yung et al. in 2011 described chemosensory RIL. The RILs contains three nonfunctional conventional cationic components which are associated with the photoacidic trianion 8-hydroxypyrene-1,3,6-trisulfonate. The synthesis and characterization of RIL were discussed in this investigation. The limits of detection range for different analytes such as ammonia and alkyl amines were found to be 2200 ppm. The response time toward analyte gases was reported 5.5 minutes. Experimental results show better sensitivity toward ethyl amine and propylamine analyte gases [120]. Smith et al. in 2014 reported photonic crystal responsive material for gas sensing applications. The mobile phase used for organogel networks was RIL ethylguanidine perchlorate. There was a volume phase transition that shifts the light wavelength diffracted from a photonic crystal responsive material. This study concluded that 2D polymerized photonic crystal responsive materials can be used to detect ammonia gas visually [121]. Jia et al. in 2021 developed ammonia optical gas sensor on the basis of silicon microring resonator which is covered by graphene material. The experimental results indicated that the geometry of this gas sensor is found to be more sensitive toward the detection and measurement of ammonia gas [122].

11.4.4 Optical volatile organic compound gas sensors Galpothdeniya et al. in 2014 reported that several electronic man-made devices such as electronic tongues and noses have received attention of researchers across the world [123]. These devices are intelligent chemical array sensor systems. This study reported that optoelectronic noses and tongues are quick, cheaper, and highly sensitive. These devices are developed using an array of various dyes. The change in color of dyes takes place due to interaction between molecules. These devices are highly chemical selective and also highly sensitive in performance. This motivated research community to fabricate colorimetric sensor arrays to detect and measure analyte gases. This was also reported that optoelectronic devices were developed by fixing water repellent dyes on the surface of hydrophobic membranes. The optoelectronic devices were also developed by embedding different chemical-sensitive dyes into nanoporous structure of silica microspheres. Huang et al. in 2011 utilized a colorimetric sensor array for quick determination of fish freshness [107]. Salinas et al. in 2012 reported that

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a colorimetric sensor array can be employed for monitoring chicken meat freshness [108]. These optoelectronic gas sensor devices are used by quantifying the information present on a sensor array. This was carried out by scanning the array optically [123]. The color of each spot on the colorimetric sensor array is quantified. These optoelectronic sensors were fabricated to detect and quantize chemicals in aqueous and vapor phase. Twelve types of chemosensory RILs were prepared by combining trihexyl(tetradecyl)phosphonium cationic component with anionic component pH indicator dyes. The hydrophobic nature is due to trihexyl(tetradecyl) phosphonium cationic component present in RIL. As a result, they are less soluble in aqueous solutions. This study reported the use of four matrices to develop colorimetric sensor array which were utilized to detect acidic and basic vapors. Aqueous solutions pH were analyzed using these sensor arrays. These colorimetric sensor arrays were found applicable for the detection of chemical analytes from cigarettes. There are two different approaches utilized in constructing the discriminant models. This study reported the utilization of cotton thread for fabrication of sensor array which shows flexibility, low volume and found lighter in weight. This was used to determine pH and for the detection of different analyte gases. These can be used for day-to-day life applications such as diapers, bandages, and sweatbands. This study also discussed about the advanced colorimetric sensor arrays to detect and measure large number of gases in various applications such as qualitative analysis of water, military applications, and food safety.

11.5 Piezoelectric gas sensors Sauerbrey in 1959 developed the foundations for using piezoelectric crystals to fabricate piezoelectric gas sensors. It can be used to detect and measure the analyte gas. There was change in frequency which was correlated with change in mass deposited on surface of the sensor. Piezoelectric gas sensors are categorized into types on the basis of quartzcrystal microbalances and surface acoustic waves. Both piezoelectric gas sensors were used to detect and measure the target gas on the basis of change in mass accumulated on the sensor surface [124]. Additionally, piezoresistive (viscosity)-based gas sensors are also included in this part.

11.5.1 Quartz-crystal microbalance gas sensors Quartz-crystal microbalance gas sensors contain a resonating polymer-coated disk with metal electrodes. Lead wire was used for

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connecting each side of disk. The odorant mass and reduction in frequency are indirectly proportional to each other. Quartz-crystal microbalance gas sensors were found to be sensitive up to 1-ng mass change. This gas sensor becomes noisier when dimension decreases. Pandey et al. in 2012 coated tetramethylammonium fluoride tetrahydrate on the surface of electrode for gas sensing application. This gas sensor was used to detect and measure CO2 gas present in our environment. Reaction with hydrogen sulfide gas was reported in this study [125]. The metallic NPs of sulfide were formed due to reaction between fatty acids and hydrogen sulfide gas. Many investigations reported deposition of various fatty acids on LangmuirBlodgett film matrix. Thereafter, this film matrix was used to deposit on surfaces of Quartzcrystal microbalance gas sensors for the detection and measurement of hydrogen sulfide gas. Nanogravimetric technique was used to study the chemical kinetics between LangmuirBlodgett film and hydrogen sulfide gas. Hydrogen sulfide gas responded quickly when interacting with LangmuirBlodgett film. The experimental data show that fatty acid salts LangmuirBlodgett films can play a very role in the detection and measurement of hydrogen sulfide gas. This may be due to quick and irreversible reaction between hydrogen sulfide gas molecules and LangmuirBlodgett films [125]. Microorganisms are also responsible for the production of hydrogen gas in environment. To detect this gas, Ag electrode was suggested for the fabrication of piezoelectric crystal sensor. Hydrogen gas has affinity toward Ag electrode. As a result, hydrogen gas gets accumulated on the surface of silver. Consequently, there is decline in resonant frequency of quartz-crystal microbalance gas sensors to a large extent. Rehman et al. in 2011 reported that variation of cationic or anionic components RILs plays a very crucial role to increase or decrease the incorporation and interaction with analyte gases [126]. The interaction between many target gases with RILs on quartz-crystal microbalance gas sensors was observed and described in this study. RIL-based virtual sensor array was developed by Spellar et al. in 2015 [127]. In this investigation a very thin layer of RILs was coated on the surface of quartz-crystal microbalance-D transducer. The experimental data show that there were increases and decreases in frequency shifts. Schutze et al. in 2004 used different organic solvents such as C6H6, CH3OH, and C2H5OC2H5 as model substances which get accumulated on the quartz-crystal microbalance electrodes. In this study, frequency shift on sensor exposure to different organic vapors was analyzed [128]. Hou et al. in 2011 studied immobilized RILs on conducting polymers to fabricate RIL/polymer composite films. The experimental results indicated that there is enhancement of sensitivity, stability, and selectivity of these gas sensors [129].

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Tseng and Chu in 2014 presented tailored ILs on quartz-crystal microbalance chips. These chips were tested for online and chemoselective measurements of organic azide gases. These developed chips were inexpensive. The selectivity of these fabricated gas sensors was found to be different for all azide species [130].

11.5.2 Surface acoustic wavebased gas sensors Surface acoustic wave gas sensor is the second type of piezoelectric gas sensor. This type of gas sensor measures a traveling Rayleigh wave. This sensor is working at higher value of frequencies as compared to quartz-crystal microbalance gas sensors as discussed in the previous section. Therefore surface acoustic wave gas sensor is sensitive to mass changes down to 1 pg [125]. Murakawa et al. in 2013 fabricated a wireless surface acoustic wave gas sensor using RILs for the detection and measurement of hydrogen sulfide gas. The selectivity toward particular was found to be good. This work highlighted that there was linear correlation between exposure times of the target gas (hydrogen sulfide) to gas sensor and frequency shift. The sensitivity of this gas sensor improved when the accumulated film was exposed to hydrogen sulfide gas at higher temperature [131].

11.5.3 Piezoresistive-based gas sensors Piezoelectric gas sensors are found to be more sensitive in nature toward target gases. The introduction of RILs to the surfaces of piezoelectric-based gas sensors makes it more beneficial. Ohsawa et al. in 2011 presented a viscosity-based gas sensor. The viscosity of RIL plays a very crucial role in this gas sensor [132]. This sensor contains piezoresistive cantilever dipped in an IL droplet. This acts as viscosity-sensitive detector. There is decline in viscosity of IL when gas was absorbed. Therefore concentration of target gas was calculated with the help of viscosity of IL. The dimension of developed chip was 1.5 mm 3 1.5 mm 3 0.3 mm. In this study, acetone gas sensor was developed and found more capable for the detection and measurement of target gas [133].

11.6 Other forms of gas sensors 11.6.1 Semiconductor metal-oxide gas sensors Semiconductor metal oxide gas sensors are very beneficial in many aspects such as less expensive, light in weight, minimum power consumption, and smaller in size. These gas sensors are fabricated by

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simple process. Undoped or doped SnO2 is frequently used for the detection and measurement of hydrogen sulfide gas. 1-Butyl-3methylimidazolium chloride was used as IL in SnO 2 gas sensor for the detection and measurement of volatile organic compounds. The experimental results show that SnO 2 gas sensor was found to be capable for the gas sensing applications [134]. Zinc oxide semiconducting metal oxide was studied using IL benzyltrimethylammonium hydroxide for the detection of analyte ethanol. The limit of detection was found to be 10 ppm. The response time was less than 5 seconds [135]. The semiconducting metal oxide WO 3 was employed using RIL 1-butyl-3-methylimidazolium chloride for the detection of volatile organic compounds. The limit of detection for target gas was found to be 10 ppm and response time less than 25 seconds [136]. Li et al. in 2013 utilized In 2O 3 semiconducting metal oxide for the detection of volatile organic compounds using IL. The limit of detection for target gas was found to be 5 ppm. The response time was observed less than 16 seconds [137]. Taubert et al. in 2012 used CuO using IL to detect ethanol. The limit of detection was found to be 5 ppm. The response time was observed less than 10 seconds [138]. Jiao et al. in 2012 developed semiconducting metal oxide gas sensor using Co3 O 4 and RIL to detect ethanol analyte. The limit of detection was found to be 10 ppm. The response time was recorded as 6 seconds [139]. Kim and Lee in 2014 reviewed semiconducting metal oxide gas sensors that were developed using nickel oxide, copper oxide, chromium oxide, cobalt tetraoxide, and trimanganese tetraoxide. All these materials are p-type metal oxide semiconductors. The ionized adsorption of O2 on p-type semiconducting materials forms hole-accumulation layers. It was reported that p-type metal oxide semiconductors show better selectivity. These semiconductors can be used to fabricate highly sensitive and selective gas sensors [140]. Arunkumar et al. in 2017 fabricated gas sensor by doping gold NPs on ZnO crystal lattice. This study reported nanostar architecture of zinc oxide which shows better sensitivity, improved sensing, and quick response time (8 seconds). The recovery time was 15 seconds. This gas sensor exhibits better selectivity for carbon monoxide than other interfering gases such as CH3OH, C2H5OH, CH3COCH3, and H2 [141]. Ma and coworkers presented strategy to fabricate platinum sensitized mesoporous WO3 semiconductor gas sensor for the detection of carbon monoxide gas. This WO3/Pt gas sensor has bigger pores (13 nm) and higher surface area (128 m2/g). The calculated pore volume for this gas sensor was found to be 0.32 cm3/g. The experimental data show that WO3/Pt gas sensor exhibits better sensitivity and selectivity toward carbon monoxide gas [142].

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Deng in 2019 reported that semiconductor metal oxide sensors work on their own principle. The sensitivity, selectivity, stability of gas sensors depend on metal oxideresistive gas sensing mechanism [143].

11.6.2 Carbon ionic liquid composite gas sensors Research community is attracted toward the carbon materials due to its special features. Niu et al. in 2013 synthesized nitrogen and silica co-doped graphene nanosheets. These nanosheets were synthesized through hightemperature annealing. These synthesized graphene sheets were used for the detection and measurement of nitrogen dioxide gas. The limit of detection for nitrogen dioxide was found to be 1 ppm. The recovery time was recorded as 635 seconds. The response time of gas sensor was 68 seconds [144]. Tung et al. in 2012 synthesized composites of poly-IL-mediated Fe3O4 and Ag NP-decorated RGO. These synthesized materials were utilized for gas sensing applications. Ag NP-decorated RGO using poly-IL displays quick response toward methyl alcohol. On the other hand, Fe3O4 at RGO poly IL showed high sensitivity toward ethyl alcohol. The limit of detections for these two gas sensors was found to be 1 ppm [145,146].

11.6.3 Gated ionic liquid gas sensors RIL 1-ethyl-3-methylimidazolium tetrafluoroborate was used in combination with CNTs to develop gas sensor [147]. This gas sensor absorbs carbon dioxide/ammonia gases. The polymer such as polyethyleneimine absorbs carbon dioxide gas. This study used IL-gate for gas sensing application. It was highlighted that a strong gate field can be obtained by the IL-gate with applying a low voltage in this gate sensor. It was reported that a gate voltage can be applied at the interface between CNTs and the RILs effectively with the help of liquid gate. Ammonia and carbon dioxide gases are highly soluble in 1-ethyl-3-methylimidazolium tetrafluoroborate [148]. Yavari and Koratkar in 2012 fabricated IL-gated gas sensors using graphene by replacing CNTs. The response of this gas sensor was different for ammonia and carbon dioxide gases. This shows the good selectivity of this gas sensor. The graphene sensor offers ppm level of detection of target gases [149]. Ortiz and Pinto used ion gel gating for poly(3,4-ethylenedioxthiophene) nanoribbons. This gas sensor works as an electrochemical and electrostatic transistor. This study reported the ON/OFF ratio of this gas sensor as 730 with a hole mobility of 5.5 cm2/V s in the accumulation mode. This study suggested that the present device can be used in gas sensors and biosensors [150].

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Ersoz et al. in 2020 synthesized miniaturized CO2 gas sensor on the basis of an electrolyte-gate transistor. The mechanism of this gas sensor depends on electrochemical reactions and surface chemistry. This investigation reported excellent sensitivity toward CO2. The sensitivity coefficient for CO2 gas was found to be 175 ppm21. This mechanism of sensing involves simultaneous electrochemical reactions of water, oxygen, and carbon dioxide gases [151]. Ersoz and coworkers in 2021 demonstrated electrolyte-gated transistor for the detection and measurement of carbon dioxide gas. Electrochemical Impedance Spectroscopy was used to improve sensitivity of electrolyte-gate transistor toward carbon dioxide gas in lowfrequency region. The equivalent circuit model shows the superposition of metal oxide and IL physicochemical characteristics [152].

11.7 Conclusions The Bhopal gas tragedy in India was the most horrible industrial disaster that the world has seen in the year 1984 due to leakage of poisonous gas methyl isocyanate. A large number of people were very severely affected and lost their lives. This gas leakage tragedy served as an alarm clock. This major industrial disaster could have been avoided or controlled up to some extent using technology of gas sensing mechanism. These many lives could have been saved by detection and proper measurement of toxic gas. Precise methods of gas detection and accurate measurement of different gases present in our environment are highly demanded and required from health and safety point of view. These gases include oxygen, carbon dioxide, carbon monoxide, nitrogen oxides, ammonia, sulfur dioxide, hydrogen, phosphine, hydrogen cyanide, hydrogen sulfide, chlorine dioxide, hydrogen chloride, hydrogen bromide, hydrogen iodide, halogens, ozone, hydrogen peroxide, ethylene, and volatile organic compounds such as ethanol, 1-butanethiol, phenol, methyl ammine, methylbenzoate, acetophenone, benzaldehyde, and methylbenzoate. Consequently, digital gas sensors with the latest technology are utilized in industries across the world by detecting, measuring, and controlling these gases accurately for the protection of human beings, animals, and our environment from harmful poisonous gases. It is also found that conventional aqueous or organic solvents (sulfuric acid and water mixture, acetonitrile) are not durable and efficient at drastic conditions of low humidity and high temperature. The efficiency of gas sensor is also affected by evaporation of solvent used in conventional electrolyte. Membrane used in gas sensors could not prevent evaporation of solvent entirely. The demand of gas sensors in market

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depends upon the special features such as sensor lifetime, response time, selectivity, and sensitivity. The lifetime of gas sensor depends on membranes, solutions, and electrode materials used in manufacturing of gas sensor. Response times are significant while selecting gas sensors because the sensor needs response quickly to variation in concentration of gases. The gas sensor is selective toward a particular gas and detects it by filtering out. Sensitivity of a gas sensor shows the dependence of the measured current on variation in gas concentration. To overcome limitations of conventional solvents used in gas sensors, RILs are used and found to be very advantageous due to their prominent features such as wider electrochemical windows, inherent conductivity, lower volatility, higher thermal stability, and tenability. Due to these significant features, RILs are employed for the detection and measurement of target gases. Different types of widely used electrochemical gas sensors for the detection of gases such as oxygen, carbon dioxide, ammonia, nitrogen oxides, volatile organic compounds, methane, and hydrogen sulfide are discussed. These electrochemical gas sensors are very useful for environmental gas analysis. These gas sensors can be used to detect different harmful gases present in environment up to ppb range. The influence of different parameters such as humidity and temperature on the performance of different electrochemical gas sensors was described. The performance of these gas sensors is influenced by drastic conditions such as humidity and temperature. The electrochemical dual gas sensor using N-butyl-N-methylpyrrolidinium, bis(trifluoromethylsulfonyl) imide is found capable for the detection and measurement of analytes CH4 and O2 gases simultaneously. Another electrochemical CNTmodified electrode gas sensor applied for the detection and measurement of hydrogen sulfide gas shows very quick response at very low humidity. As a result, it can be used to detect and measure hydrogen sulfide gas in drastic conditions. For example, gas sensing used in oil fields is found in desert. Recently, physically transient electrochemical nitric oxide gas sensor is developed for gas sensing applications. These gas sensors are biodegradable, wirelessly operated, and flexible for continuous monitoring of nitric oxide gas in vitro and in vivo. Optical gas sensor involves detection of target gas with the help of our naked eye or quantified spectroscopic methods. Chemical dyes are usually used in this gas sensor. Optical gas sensors show many salient features such as quick response time, simple construction, costeffectiveness as compared to traditional instruments. Optical gas sensors show excellent chemical selectivity, higher extinction coefficient, better sensitivity due to variety of dyes and their derivatives. Optical gas sensors can be used to detect many analytes such as oxygen, ammonia, nitrogen oxides, carbon dioxide, and volatile organic compounds.

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Piezoelectric crystals are promising materials to fabricate piezoelectric gas sensors. It can be used to detect and measure the analyte gas. There was change in frequency which was correlated with change in mass deposited on surface of the sensor. Different types of piezoelectric gas sensors, including quartz-crystal microbalances and surface acoustic waves, are found to be very useful to detect and measure the target gas on the basis of change in mass accumulated on the sensor surface. Additionally, piezoresistive (viscosity) based gas sensors are also discussed. Afterwards, semiconductor metallic oxide gas sensors, carbon-IL composite gas sensors, gated IL gas sensors are also discussed with their gas sensing applications using RILs. These gas sensors show good selectivity, sensitivity, thermal stability, and life span. It is estimated that the overall gas sensors market will grow rapidly with the impact of COVID-19. Among the various gas sensors, there major gas sensors such as O2, CO2, and CO are accounted for more than 50% of the gas detectors market size across the world. Food processing and storage industries are extensively using CO2 gas sensors. It is also reported that manufacturing and automobile sectors are generally using O2 and CO gas sensors. Therefore there is a huge demand of different types of gas sensors with the latest technology in these sectors. However, there is need for further research to develop novel advanced gas sensors to detect and measure large range of gases present in our environment. There are many gas sensors which do not function properly at drastic conditions of temperature and humidity. The life span of gas sensors is also affected by different drastic operating conditions. The performance of gas some sensor toward particular target gas is adversely influenced by other interfering gases. Therefore there are challenges to develop different types of gas sensors with the latest technology to meet the demand of the market. Therefore studies should focus on enhancing selectivity, stability, response time, lifespan, sensitivity, and overall performance of various gas sensors in different fields. There is also need to develop cheaper, membrane free, wireless and biodegradable gas sensors using combinations of different RILs as electrolyte. In this regard, there is further scope for researcher community to fabricate multigas sensors to detect and measure large number of gases at the same time.

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C H A P T E R

12 Ionic liquids: a tool for CO2 capture and reduced emission Indrajit Das, K. Rama Swami and Ramesh L. Gardas Department of Chemistry, Indian Institute of Technology Madras, Chennai, India

12.1 Introduction The increase in the emission of greenhouse gases such as carbon dioxide (CO2), CH4, SO2, and NO2 into the atmosphere creates worldwide concern about global warming [1,2]. Among all, CO2 emission contributes a higher amount and it is rapidly increasing by every year due to an increase in industrialization and other sources worldwide [2,3]. As per international records, CO2 emission in 2008 was about 29.4 Gt worldwide, whereas it was 20.9 Gt in 1990, which indicated that 40% rapid growth occurred within 20 years of span [4,5]. The CO2 emission to the atmosphere will further increase and be expected to double by 2050 [6]. One of the significant contributors to CO2 emission was the power and energy sector about 41%. Other sectors such as the transport sector, industry sector, and buildings sectors contribute 23%, 20%, and 10%, respectively [7,8]. The significant contribution of power sectors is mainly due to coal, oil, and gas combustion to produce heat and electricity [8]. The global distribution of CO2 emissions is shown in Fig. 12.1. However, the continuous increase in CO2 emissions would lead to climate deterioration which causes social and economic problems. In this regard, CO2 capture and storage or CO2 capture and utilization has been envisioned by several researchers to mitigate the emission of CO2 in the atmosphere [9,10]. The separation of CO2 is a difficult task and leads to many obstacles to making it commercial. The carbon-capturing process from various sources is classified into three

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5%

4%

21% 20%

15% 7%

28%

China India USA Russian Federaon Japan Rest of the world Europian Union

FIGURE 12.1 Regional distribution of global CO2 emissions [8].

categories such as postcombustion, precombustion, and oxycombustion [11 14]. (i) Postcombustion In this process, CO2 will be separated from fuel gas after the combustion of fuel in the presence of air [11,15]. This process is being most widely used process in large-scale operations due to consistent CO2 content. Postcombustion is the most straightforward and uncomplicated process to implement as a retrofit route in thermal power plants and other sources. (ii) Oxyfuel CO2 combustion In this process, oxygen is used as the oxidant instead of air for the combustion of fuel. Later CO2 is separated from fuel gas [15]. This method is mainly applicable to small-scale industries process only. (iii) Precombustion In this process, the hydrocarbon-containing fuel is firstly converted into synthetic gas such as carbon monoxide (CO) and hydrogen (H2). Later CO is converted into CO2 using water then H2 is separated from CO2 [14,16]. Among all, postcombustion is an easier and more conventional method used in power plants to separate CO2 from fuel gases. The carbon capture from the postcombustion technique was further divided into absorption, adsorption, membrane, and cryogenics [17,18]. The details are shown below: (a) Absorption The absorption process is a conventional and widely used technology for the efficient capture of CO2 from various sources. Generally, absorption of CO2 occurs through chemical or physical interactions or both depending on the materials, where the chemical absorption occurs through specific solvents reacting

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with the CO2 resulting in the formation of covalent or coordinate covalent bonds [13,17,19]. In absorption, regeneration can be achieved simply by heating at higher temperatures, and captured CO2 is released. This mechanism process reveals that faster kinetics and high selectivity are obtained by synthesizing specific chemical complexes [19]. Amines or ammonia-based solutions are examples of chemical absorption. Physical absorption of CO2 with solvents or materials occurs based on Henry’s law. Henry’s law stated that “the solubility of any gas is directly proportional to the partial pressure of the corresponding gas at a constant temperature in equilibrium condition” [13,17]. The absorption capacity of CO2 would be maximum at high partial pressures and lower temperatures, dominated by the interaction between CO2 and solvent through van der Waals interactions. While the regeneration can be accomplished just by increasing the temperature or lowering the pressure of the absorbing system [18,19]. (b) Adsorption Adsorption is the process where CO2 capture occurs only at the surface of the material [13,17]. These interactions can be either chemical or physical, depending upon material like absorption. Generally, solid materials containing large surface area materials such as silica gel, zeolites, metal oxides, activated carbons, and ion-exchange resins are considered to be the best adsorbents for the CO2 separation process [20]. Presently, the adsorption process is not yet employed for largescale usage due to slow kinetics, and the CO2 selectivity in existing sorbents is low. (c) Membrane separation Membrane separation is one of the best technologies being developed to capture CO2 from fuel and for sweetening gases [13,14]. This process takes place through the interaction of the membrane material by either physical or chemical interactions. The absorption and desorption of gases can be achieved simultaneously using membrane technology. Also, the rate of gases passing through the membranes can be easily controlled. As the flue gas-containing mixer of gases passed through the membrane, the liquid in the membrane selects and captures the CO2 [21]. Membrane separation for CO2 capture was again classified into different processes such as polymeric membranes, composite membranes, and porous inorganic membranes [22]. Among all, the porous membranes are favored for absorption and stripping of CO2 from fuel gas.

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(d) Cryogenic separation Another process employed for the separation of CO2 is cryogenic separation. In this process, CO2 separation is occurred due to cooling and condensation [13,23]. This technique has an advantage such as the direct separation and production of liquid CO2. However, cryogenic technology needs high amounts of energy to reach cooling for the separation of CO2. This process is more suitable for when the concentration of CO2 is low in gas streams. This technique is more appropriate for oxyfuel combustion and precombustion methods than the postcombustion process [23,24]. Among all, absorption of CO2 is the more appropriate and is widely used in commercial CO2 capture from postcombustion in power plants. In the absorption process, the flue-gas present in the long columns or towers contacts the solvent like aqueous amines and captures CO2 from the flue gas in the industrial process [14,25]. The rate of CO2 absorption with solvent is the main deciding factor in the chemical absorption method. Various solvents have been developed by researchers from different countries for the efficient absorption of CO2 from fuel gases. Among them, aqueous soluble amines are used as the absorbents for the separation of CO2 due to their excellent properties [26].

12.2 Aqueous amines used in postcombustion Aqueous soluble amines are called “conventional absorbers” because they are being used as a solvent for the recovery of CO2 from the postcombustion process in industries since the late 1930s [26 28]. Conventional aqueous amines are used as solvent-based chemical absorbers [27]. Amines are so effective for CO2 capture due to their high reactivity, high absorbing capacity, high thermal stability, and CO2 selectivity from other gases. Generally, primary amines and their derivatives have been employed to capture CO2 from natural gas and fuel gas [28 30]. Some potential amines used for CO2 separation are monoethanolamine (MEA), methyldiethanolamine, diethanolamine, etc. The postcombustion capture with amines involves the separation of CO2 is happened by passing a flue gas stream into an aqueous amine solution chamber. The primary amines react with CO2 through chemical absorption and form bicarbmate or carbamates [27]. Whereas secondary and tertiary amines, do not contain a hydrogen atom in nitrogen atom, and they react with CO2 and form bicarbonate through

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the hydrolysis process [29,30]. All the chemical reactions between amines and CO2 are reversible and the captured CO2 can be released at high temperatures, then amines can be used for further recycling process [27 30]. In the recent past, piperazine-based solvents are being employed for CO2 capture to enhance the rate of absorption of secondary and tertiary amines and reduce the regeneration process costs [31,32]. These types of amines are prominent for CO2 capture in the means of high reactivity with CO2, relatively high thermal stability, high capture capacity, and CO2 selectivity [33]. Though all kinds of amines have superior advantages for the capture CO2 from the postcombustion process. However, these amines have several inherent disadvantages for CO2 capture studies, such as high vapor pressure, corrosive nature, reaction rate, and high energy needed for regeneration [34,35]. Amines, upon reacting with CO2 forms several degradation products such as amides, nitrosamines, and nitramines, and these are highly unstable and cause several problems [35]. The regeneration process requires high energy consumption to break the chemical bonds formed between CO2 and amine. For instance, the energy consumption upon using 30 wt.% MEA solvents nearly requires 2 4 GJ of energy for regeneration of CO2 [36]. All these factors by amines allow researchers to explore new green solvents to perform better than amines properties.

12.3 Ionic liquids as solvents for CO2 capture In the recent past, researchers from various fields seeking great potential alternatives for conventional volatile organic compounds for various applications, and ionic liquids (ILs) are matching all the characteristics [37]. ILs are molten salts which are having melting points below 100 C. This is due to ILs comprised of large organic cations and organic/inorganic anions. ILs show fascinating, unique properties such as negligible vapor pressure, high thermal and chemical stability, wide electrochemical window, tunable nature, and excellent solvent properties for over the range of nonpolar and polar compounds [38 40]. Different combinations of cation and anion can easily tune the physicochemical properties of ILs and these are named designer solvents [40,41]. All these characteristics of ILs have been adopted as solvents for CO2 capture for postcombustion, compensating existing aqueous amines’ limitations. The capture of CO2 from postcombustion using ILs has been classified into three different categories, as illustrated in Fig. 12.2.

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Physical absorpon Absorpon Chemical absorpon

By impregnaon Ionic liquids for CO2 capture

Adsorpon By Graing Supported ionic liquid membranes Poly ionic liquid membranes Membranes Composite membranes

Mixed membranes

FIGURE 12.2 The techniques ionic liquids used for separation of CO2 from the post combustion process.

12.3.1 Ionic liquids in the absorption process for CO2 capture The ILs have been proposed as a potential candidate for replacement for aqueous amines for CO2 capture from postcombustion processes [42,43]. Since ILs have negligible vapor pressure allows easier regeneration and recycling. The high chemical and thermal stability of ILs results in no degradation upon reaction and is less corrosive [14]. Different combinations of cations and anions can tune the physicochemical properties such as viscosity and heat capacity to enable and enhance the absorbance of CO2 [44]. Blanchard et al. have explored pioneer studies on CO2 using various imidazolium-based ILs with CO2 solubility of 0.6 mole/mole at 8 MPa and 298.15K and found that bistriflimide anion contains ILs have a higher affinity for CO2 capture than other ILs [45,46]. Amine-containing imidazole functionalized IL was used for the first time by Bates et al. to explore CO2 capture behavior and noticed that ILs can absorb about 0.5 moles of CO2 per mole of IL at 0.1 MPa pressure [47]. However, it was found that using ILs for CO2 capture results in high viscosity of captured samples. This behavior makes ILs have a slower rate for and less mass transfer of CO2 to ILs. Generally, the solvents that have a lower viscosity are more favorable due to the easy mass transfer rate. Given this, Krupiczka et al. modified the ILs using a proper combination of cation and anions to reduce the

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viscosity and high absorption capacity [48]. It was reported that the effect of anion on viscosity is follows the order of [bmim][NTf2] , [bmim] [CF3SO3] , [bmim][BF4] , [bmim][PF6] in imidazolium-based ILs. The viscosity of ILs can also be reduced by adding an appropriate amount of cosolvents such as water and MEA. The addition of a small number of water results in the drastic reduction of the viscosity of ILs and also enhances CO2 separation [49]. Later, researchers explored quaternary phosphorous IL-containing amino acid as anions and these ILs showed CO2 capture of 1 mole/mole of IL [50]. Song et al. studied CO2 solubility in different aqueous amino acid ILs and found CO2 absorption of 1 mol CO2/mol [51]. Generally, the CO2 absorption or solubility is due to the dipole and quadrupole interaction of gas and solvent or ILs. 12.3.1.1 Ionic liquids as physical absorbents The CO2 absorption using ILs mainly depends on different parameters such as pressure, temperature, and cation/anion choice. Literature reports showed that conventional ILs CO2 are captured mostly by physical absorption [46,52,53]. For example, when lowpressure CO2 approximately 1 bar is applied to the ILs results in low CO2 absorption in the liquid phase. Whereas an increase in the pressure increases, that is, about 100 bar led to an increase in CO2 solubility [53]. These results revealed that ILs are acting as physical absorbers. Generally, the solubility or absorption capacity of CO2 in ILs increases with increasing applied pressure and decreases with increasing temperature as per Henry law. The physical absorption mechanism that occurs between CO2 with IL is essentially due to large Van der Waals forces and quadrupole moment, which allows CO2 to occupy the “free space” [53,54]. Zhang et al. investigated the CO2 solubility in anion containing different alkyl chain lengths and observed that 70% increase in the CO2 solubility with tris(pentafluoroethyl)trifluorophosphate anionic IL compared to hexafluoro phosphate IL [55]. Babarao et al. reported that at low concentrations of ILs, CO2 occupied small voids inside the ILs, whereas at higher concentrations, all the small voids formed larger voids, allowed to accommodate more CO2, and led to higher solubility [56]. Generally, the solubility of CO2 in ILs will increase, with an increase in cation chain length, for example, CO2 solubility increased in the following order for the imidazolium-based ILs, that is, [Omim] . [Hmim] . [Bmim]. The densities of ILs decrease with an increase in alkyl chain length, leading to an increase in the free volume of ILs, and increasing CO2 solubility. Aki et al. systematically studied the effects of the cation on CO2 solubility and the increase of the alkyl chain on the cation resulted in a slight increase in CO2 absorption due to the increased volume available for CO2 interaction [57].

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12.3.1.2 Ionic liquids as chemical absorbents In addition to the physical absorption, ILs also possess chemical absorption through Lewis acid base interactions. Kroon et al. verified the solubility of CO2 in ILs using a theoretical approach using perturbed chain-statistical associating fluid theory (PC-SAFT) and observed that the dipolar interactions, quadrupole between CO2 and IL, as well as the Lewis acid base interactions between the IL and the CO2 results for chemical absorption [58]. These interactions are the more prominent and stronger ability for CO2 capture than physical absorption. Kazarian et al. confirmed Lewis acid base interactions of ILs with CO2 using attenuated total reflectance (ATR)-IR spectroscopy and noticed weak acid 2 base interactions CO2 solubility with fluoride anions [59]. Similarly, Yokozeki et al. studied CO2 solubility experiments with different room temperature ILs. The authors found that room temperature ionic liquids (RTILs) that showed a strong chemical bond with CO2 all anions containing carboxylates. These properties can be improved by the addition of carbonic or halide acids. Whereas, basic functions like amino and fluorine groups are also more prominent for chemical absorption [60]. All the reports from the literature found that imidazolium cation is the most stable and commonly used cation of choice [61]. Due to some limitations of existing IL systems such as physical absorption taking place and high absorption capacity occurring at only high pressures. In the recent past, researchers have developed taskspecific ionic liquids (TSILs) due to their advantages on the solubility of CO2 by covalently attached with a functional group to either or both anion or cation [62,63]. The CO2 absorption capacity of various ILs is shown in Table 12.1. These task-specific ILs are made by a chemical bond with CO2 in chemical absorption. In TSILs, the CO2 reacts with the one amine group on the IL and then reacts with another amine group resulting in the formation of an ammonium carbamate double salt that leads to one CO2 captured for every two ILs and results in a 1:1 ratio can be achieved when amines attached to ILs [47]. Similarly, superbase-based ILs have also been introduced and the basicity of superbases enhances the solubility of CO2. In the recent past, Zhu et al. explored 1,8-diazabicyclo[5.4.0]-undec-7ene based super base ILs for the separation of CO2 and found that the CO2 absorption capacity values were achieved at 1 mol CO2/mol ILs [64]. It is also found these TSILs act as both physical and chemical absorbents which means that TSIL at lower pressures CO2 absorption takes place through chemical interaction between IL and CO2. Whereas high-pressure absorption takes place through physical interactions, as discussed above. But TSIL shows high viscosity, which creates problems in the regeneration process and requires a lot of

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TABLE 12.1 The CO2 absorption behavior of various ionic liquids at 295.15 300.15K and 1 bar pressure [8,14]. Name of ionic liquid

CO2 solubility (mol/mol) IL

Temperature (K)

Pressure (bar)

[bmim][PF6]

0.03

298.15

1

[bmim][BF4]

0.02

298.15

1

[C8-mim][Tf2N]

0.15

300.15

1

[bmim] [DCA]

B0.12

298.15

1

[bmim] [NO3]

B0.1

298.15

1

[bmim] [TfO]

B0.15

298.15

1

[bmim] [Tf2N]

B0.15

298.15

1

[bmim] [methide]

B0.13

298.15

1

[bmim][NO3]

B0.09

298.15

1

[P66614][Ile]

0.99

295.15

1

[P66614][Sar]

0.9

295.15

1

[P66614][Gly]

1.2

295.15

1

[P66614][Pro]

0.56

295.15

1

[P66614][Met]

0.8

295.15

1

energy during desorption. To resolve the viscosity problem Bara et al. attempted to mix TSIL with conventional RTIL. He noticed that the mixer showed high CO2 solubility of 1:2 molar ratio as well as less viscous [65]. In addition, the employment of ILs as a substitute for conventional absorbents such as aqueous amines in the CO2 absorption shows significant advantages. Camper and coworkers studied a blend of RTIL solution along 16% (v/v) of MEA and found that 1:2 mole of CO2 to MEA was achieved even at low pressures [66]. In the recent past, Perumal et al. studied a blend of IL and MEA at different combinations and found that 30% less energy is required for desorption upon mixing with a proper combination of IL and MEA [67].

12.3.2 Ionic liquids in the adsorption process for CO2 capture Another method where ILs have been employed for CO2 capture is adsorption using different hybrid materials [68]. Adsorption of CO2 has particular merits as shown below. 1. The adsorbent is easier to regenerate and requires less energy.

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2. Mixing ILs with adsorbents can effectively reduce the amount of ILs for CO2 separation and cost. 3. Using adsorption technology, we can easily avoid the problem viscosity of ILs unlike the absorption process. 4. The high affinity of ILs towards CO2 can increase the efficiency of adsorbents for capturing CO2. Adsorption using ILs for capturing CO2 has been further classified into two types based on the adsorption mechanism. The first one is adsorption through the impregnation process. Uhera et al. studied the CO2 adsorption studies using RTILs attaching hybrid porous silica by impregnation method [69]. He reported that the CO2 capture of 50% impregnated ILs poly(methyl methacrylate) is about 1.2 mmol/g. Zhang et al. studied ILs impregnated mesoporous silica and found superior CO2 adsorption capacities [70]. Son et al. have revealed that the mesoporous silica supports having high pore size shows better CO2 adsorption capacities [71]. Sayari et al. reported that sorbent containing long alkyl chains of the surfactant in mesoporous silica support enhanced dispersion of the loaded amine leading to higher CO2 adsorption performance [72]. Similarly, Wan et al. investigated CO2 separation studies using the impregnation method for various mesoporous materials as CO2 adsorbents, and the highest adsorption capacity was found to be 1.84 mmol/g for MCM/ILs. The electrostatic interactions between mesoporous materials and ILs are played a crucial role in the adsorption of CO2 capture [73]. Cheng et al. employed molecular sieves for impregnation with ILs and found that hybridization had greater advantages than unsupported ILs [74]. The second method in adsorption is attaching ILs to adsorbents using the grafting method. Nkinahamira et al. observed employing mesoporous material MCM-41 with quaternary ammonium salt-based ILs using grafting showed that the loss of pores reduced the amount of adsorption [75]. The grafting process results in good retention for ILs and forms stable adsorption with ILs. Yuan et al. reported that amine-functionalized ILs to titanate nanotubes through grafting showed significant enhancement in adsorption performance up to 2.46 mmol/g [76]. Soll et al. developed mesoporous adsorption materials using self-complexation of ILs and noticed that the surface of the material and the copolymer matrix material are also involved in the adsorption process [77]. In the recent past, solid poly-ILs were explored for higher CO2 adsorption performance than monomer ILs since these undergo adsorption process through the free radical polymerization process [78].

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12.3.3 Ionic liquids in membranes process for CO2 capture Membrane technology comprised of ILs hybrid with membranes is another method that has been employed for CO2 separation, mainly from flue gases and biogas purification. In this method, the permeability and selectivity of gases are major deciding factors in membrane separation [79,80]. Therefore it is necessary to focus research on developing new membrane materials to improve the selectivity for CO2. Conventional ILs have high viscosity and slow mass rate concerns, allowing for the combination ILs with other solvents. As discussed above, ILs show selective absorption of CO2 from other gases due to their affinity with CO2 molecules, which gives directions for preparing CO2 separation using membranes [81]. There are several advantages to combining ILs with membranes. There is the selectivity of CO2 using membrane materials will be greatly improved upon the combination with ILs, the quantity of ILs used in the entire membrane process can be drastically reduced due membrane diffusion process and the slow diffusion of CO2 in ILs can be diminished upon using membrane separation [82 84]. The hybridization or combination of membranes with ILs has been further classified into three types and will be discussed below. 12.3.3.1 Supported ionic liquid membrane for CO2 separation The first category of ILs hybridizing with membranes is supported ionic membranes. The supporting ionic liquid membrane (SILM) comprises a thin solid support layer such as a polycarbonate membrane, polyacrylonitrile membrane, or anodized aluminum membrane, attached to ILs for the separation of CO2 from the postcombustion process [85 87]. The separation process of CO2 using SILM occurs as follows. Firstly, CO2 molecules pass through the membrane and get absorbed at the surface layer of the liquid. Then the captured CO2 molecules diffuse from one side of the membrane layer to the other side liquid layer due to the concentration gradient effect. Finally, the CO2 molecules will be desorbed on the side close to the support layer by passing the other side of supported membranes [80,85]. By using SILM, absorption and desorption can be achieved simultaneously and reduce the cost and energy required for the desorption process. The supporting IL membrane shows special properties of the ILs allowing them more appropriate for the preparation of membrane materials than liquid support membranes [88]. Since ILs are nonvolatile it enables SILM to have no problems due to solvent volatilization. The high thermal stability of ILs provides the possibility SILM would be able to conduct hightemperature experiments on CO2 separation from fuel gas [89]. In this process, ILs act as supported layers, and porous materials act as

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additives, allowing a selective layer with ILs [88,89]. In addition, selective doping of porous materials in ILs would improve the permeability of SILM. It was reported in the literature that the preparation of the support membrane to immobilize the ILs into the pores is a critical step for the separation of the CO2 process [90]. This can be achieved in two different ways. The first one is the addition of ILs onto the surface of the membrane by applying vacuum filtration [91]. The second one is the adding membrane with the surface covered by ILs using an autoclave. Ilyas et al. confirmed that membranes prepared using the autoclave method were more efficient and stable even at high pressures without leaching of ILs observed [92]. Lan et al. employed hollow fiber membranes for CO2 separation. He observed that the SILM containing hollow fibers was highly stable under high pressure [93]. Researchers have explored further SILM to enhance stability and selectivity and developed the ILs-nanomaterials into SILM for the separation of CO2. Chen et al. introduced 2D nanochannels membranes and the ILs were acting as fillers into the nanochannels. It was evident that these kinds of membranes showed long-term durability, high-temperature resistance, highpressure stability, and good selectivity [94,95]. Hwang et al. introduced applying additional electric fields to nanoconfined hybrid membranes has improved the separation of CO2. It was observed that an increase in the external forces leads to rearranging the structure of ILs results in the enhancement of the adsorption/desorption free energy, interaction energy, and free volume of IL for better CO2 separation [96]. 12.3.3.2 Polyionic liquids membranes for CO2 separation Supported ILs membranes have shown excellent behavior on CO2 capture. However, the mechanical properties are affected upon implementing large-scale operation, and also stability of SILM membrane under high pressure was less [97]. These problems can be resolved either making ILs binding into the nanochannels as discussed in the previous section. Another method is using polyionic liquids membranes for the separation of CO2. Poly-ILs show a greater affinity for CO2 and higher stability than ILs. Also, poly-ILs possess excellent mechanical and physical polymeric materials [98]. The preparation of poly-ILs membrane (PILM) is can be achieved using the casting process. These poly-ILs have highly capable to dissolve in a volatile solvent, and the thin membranes will be formed upon solvent evaporation [99]. Tang et al.’s investigations showed that the poly-ILs have higher capture capacities of CO2 and offer quick absorption/desorption rates compared ILs monomers or SILM, and these are considered potential membranes for separation of CO2 [97]. The properties of PIL can be adjusted by simply changing the type of anion or cation. Some literature reports showed that pure PIL membranes prepared less capable of CO2 capture.

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Vollas et al. studied pyridinium-based PILMs and a combination of PILs-ILs membranes. He found the absorption performance combined system was greatly improved than pure membrane [99]. Similarly, Tome et al. [100] investigated membranes within different combinations, ranging from pure ILs to poly-ILs along with mixtures, and observed that the addition of free ILs to the polymer ILs was the major factor in the increases in selectivity and permeability of the prepared PIL combinations for CO2 separation. 12.3.3.3 Ionic liquid composite membranes for CO2 separation In addition to polyionic liquid membranes, IL-containing composite membranes also showed good mechanical properties and separation for the gas separation process which will be prepared by using a polymer as the matrix and IL used as an additive [101]. These IL-based composite membranes are further divided such as crosslinked membranes composed of polymer and ILs, inorganic porous material, and mixed matrix membranes (MMMs). Cheng et al. [102] explored different blending membranes with various solvents such as MEA and ILs into the Pebax membranes. The study showed that the permeability and solubility of CO2 in a solvent and membranes increased significantly. Similarly, Lu et al. showed that polysulfone with ([Bmim] [TFSI]) IL-containing membranes possess higher CO2 capture capacity due to strong interaction between IL and CO2 [103]. In the recent past, MMMs such as inorganic membranes and polymeric membranes have been shown great attention due to their mutual advantages. The MMM consists of a dispersed particulate phase such as MEA or ILs and a continuous polymer matrix phase [104]. These membranes are formed through the interaction between the filler and the matrix. Because of this, Liu et al. reported that ILsbased MMMs showed a higher affinity for CO2 separation [105]. These are again classified into two types. The first is ILs or materials modified by ILs as the main component of MMM for improving the selectivity of CO2. The other is using ILs employed to fix the surface defects that arise in membrane materials. Lu et al. investigations showed that the proper selection of mixing of ILs and other polymers increases membrane efficiency for CO2 separation [103]. Similarly, Dai et al. found that blends of ILs-polymers show good separation ability with improved humidification of membranes [106]. The general inorganic porous materials employed for the preparation of MMMs mainly are zeolites, silica, graphene oxide (GO), molecular sieves, and metal-organic frameworks due to these substantial-high surface properties [107]. It is important to note that the bonding between the matrix and filler matrix is a deciding factor

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affecting membrane performance as per literature [108]. Huang et al. carried out studies on Pebax 1657 and ILs modified GO to prepare MMMs. He reported that bonding between Pebax matrix and ILs led to a more homogenous MMM system. Interface defects observed during the preparation of MMM would be improved by hydrogen bonding of ILs [109]. In addition, the preparation of ILs with MMM improves the compatibility between zeolite and matrix which led to higher enhancement of the selectivity and permeability of membranes [110].

12.4 Regeneration of CO2 from ionic liquids The main disadvantage of using ILs for capturing CO2 is the high viscosity, higher production cost, and high regeneration energy of ILs. To overcome high production costs generally, we separate ILs and CO2 from their mixture and the same as we can use for the next batch of CO2 capture. This regeneration process can be continued five to six times and it can be done by simple heating of IL 1 CO2 mixture. Another major problem increase viscosity drastically during CO2 capture. To solve this issue, researchers are studying CO2 capture by using ILs with different solvents (generally water, methanol, and MEA) as solvents are less expensive than ILs and decrease viscosity drastically. Moreover, in the binary study of ILs, ILs need less amount which can compensate higher production cost of CO2 capture [111,112]. Most regeneration techniques are based on either temperature, pressure, or vacuum swing adsorption (VSA), widely known as temperature swing adsorption (TSA), pressure swing adsorption (PSA), and VSA, respectively. Though the adsorption pressure is higher, and desorption pressure is lower for both PSA and VSA; the adsorption operates at atmospheric pressure in VSA, and for PSA desorption pressure is maintained at atmospheric pressure. In the TSA regeneration process, CO2 is regenerated by passing hot air or steam [113].

12.5 Designing ionic liquids for CO2 capture Theoretically, there are millions of cation and anion combinations possible and every year thousands of ILs have been published for investigating the properties and various applications. Among all identifying one ILs that suits the purpose of CO2 capture is a very tedious job and time taking. Designing ILs is important to accelerate the search for potentially suits ILs [114]. Generally, ILs are designed with a particular

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341

FIGURE 12.3 The designing of new ionic liquids for CO2 capture.

application, and the physicochemical properties are the basis for the designing process. Designing ILs for CO2 capturing is the most important topic nowadays. The pictorial representation of designing ILs for CO2 capture is shown in Fig. 12.3. 1. Viscosity is one of the most important factors for capturing CO2. Generally, low viscous ILs can be made but during the CO2 absorption, process viscosity increases drastically. To make the recycling process easier and regeneration energy less it is better to choose ILs which can maintain relatively low viscosity during CO2 absorption. 2. Basicity is another important factor for capturing CO2. More the basic ILs in nature interaction between ILs and weekly acidic CO2 will be more [59]. 3. Generally, the higher the free volume the ILs have, will capture more CO2 [115 118]. It was observed increasing alkyl chain length or fluorination of cations or anions increases the free volume of ILs and as a result, increases the CO2 solubility of ILs [57,119,120]. 4. The modern theoretical methods for screening ILs are faster and less expensive and usually more efficient. COnductor-like Screening MOdel for Real Solvents (COSMO-RS) is one of the most popular methods for designing ILs [121,122]. COSMO-RS is a simulationbased software. Using COSMO-RS, we can predict the properties of various solvents and their CO2 capturing ability at various

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conditions, specifically when experimental data does not exist. In this method. After putting input software will optimize molecular geometry and electrostatic charge distribution and predicts the properties of ILs. COSMO-RS model is most widely used for predicting trends rather than exact physical properties of ILs and can be used for a large number of ILs and ILs combinations quickly and accurately [123]. 5. Most advanced methods involve machine learning for the rapid and precise prediction of various properties of solvents [124]. In this method, the first available data is collected for independent cation and anion and functional groups, and ILs are designated as a combination of these building blocks. Then machine learning methods will calculate the overall properties of ILs for capturing CO2 [125]. Various databases are available in the open literature for researchers to construct and develop various predictive models, including artificial neural networks for predicting viscosities [126], melting points [125], and other properties of ILs. The accuracy of the predicted properties depends on an available number of data and computational power. 6. For specific tasks, screening ILs can be done using a combination of the theoretical and experimental methods where the experimental method finds data under specific parameters and the theoretical method will extrapolate the data and predict results under different conditions. In the case of CO2 capture desired condition, low viscosity and high conductivity combinations can be achieved using quantitative structure property relationship, and screening of ILs can be done based on viscosity and conductivity relationship [127].

12.6 Conclusions The present study reviews the application of ILs for the efficient separation of CO2 from various sources using absorption, adsorption, and membrane technology compared to existing conventional reagents. The unique and designing ability of ILs proved their potential for capture of CO2. The introduction of ILs to the above processes greatly increases the CO2 separation due to greater bonding affinity between IL and CO2. In the absorption process, it was observed that ILs and CO2 absorption capacities were more compared to conventional solvents. ILs acted both as physical and chemical absorbents depending on the structure and functional groups attached to the cation or anion. The addition of cosolvents to ILs or mixing of solvents showed enhancement in the CO2 absorption capacities. In the adsorption process, the addition of ILs to the solid materials by

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Abbreviations

343

impregnation or grafting method found improved CO2 adsorption than existing materials. The maximum capacities were achieved for the high pore size of the materials. In the membrane separation process, ILs have improved both selectivity and permeability of membranes and observed maximum capture capacities for mixed membranes. The addition of ILs helps to reduce the inference defects in MMMs with improved separation performance. Given all these observations, ILs can be employed as an excellent tool for CO2 capture and reduced emission into the atmosphere. Therefore, environmentally benign ILs can be further explored for research and large-scale industrial process to decrease the emission of CO2.

Acknowledgments The authors are grateful to the Department of Science & Technology (DST), India for financial support through two projects: EMR/2016/005810 and DST/INT/Portugal/P-01/2017. Indrajit Das would like to thank the Council of Scientific and Industrial Research (CSIR), India, for the doctoral fellowship: 09/084(0748)/2019-EMR-I.

Abbreviations [Bmim] [bmim][BF4] [bmim][CF3SO3] [bmim][DCA] [bmim][methide] [bmim] [NO3] [bmim][NTf2] [bmim][PF6] [bmim][TfO] [C8-mim][Tf2N] COSMO-RS GO [Hmim] ILs MCM MEA MMMs [Omim] [P66614][Gly] [P66614][Ile] [P66614][Met] [P66614][Pro] [P66614][Sar] Peba PILM PSA RTILs

1-butyl-3-methylimidazolium 1-butyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium trifluoromethanesulfonate 1-butyl-3-methylimidazolium dicyanamide 1-butyl-3-methylimidazolium methide 1-butyl-3-methylimidazolium nitrate 1-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide 1-butyl-3-methylimidazolium hexafluorophosphate 1-butyl-3-methylimidazolium trifluoromethanesulfonate 1-Octyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide COnductor-like Screening MOdel for Real Solvents graphene oxide 1-Hexyl-3-methylimidazolium Ionic liquids Mesoporous material Monoethanolamine mixed matrix membranes 1-Octyl-3-methylimidazolium trihexyl(tetradecyl)phosphonium glycinate trihexyl(tetradecyl)phosphonium isoleucinate trihexyl(tetradecyl)phosphonium methionate trihexyl(tetradecyl) phosphonium prolinate trihexyl(tetradecyl)phosphonium sarcosinate poly(ether-block-amide) poly-ILs membrane Pressure swing adsorption Room temperature ionic liquids

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SILM TSA TSIL VSA

supporting ionic liquid membrane Temperature swing adsorption Task-specific ionic liquids Vacuum swing adsorption

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C H A P T E R

13 Applications of ionic liquids in fuel cells and supercapacitors Sandeep R. Kurundawade1, Ramesh S. Malladi2, Prasanna S. Koujalagi3 and Raviraj M. Kulkarni3 1

Department of Chemistry, KLE Technological University, Hubballi, Karnataka, India, 2Department of Chemistry, BLDEA’s V. P. Dr. P. G. Halakatti College of Engineering and Technology, Vijaypur, Karnataka, India, 3Department of Chemistry and Centre for Nanoscience and Nanotechnology, KLS Gogte Institute of Technology, Udyambag, Belagavi, Karnataka, India

13.1 Introduction As the world is progressing at a rapid pace, the need for efficient energy storage and conversion systems is nothing less than an emergency. To supply the ever-increasing needs for the electrical energy and its by-products, we are contributing a lot more to environmental pollution. Therefore it is inevitable for us to move toward portable and efficient energy systems [1 3]. With all the new batteries, supercapacitors, and fuel cells (FCs) being invented, the hunger for the search for new and efficient materials has gone up. The conventional batteries and the automobile engines are now being slowly replaced with FCs. On the other hand, the supercapacitors are getting more efficient with new electrolytes being introduced, to supply the ever-mounting need for efficient storage devices. This is pushing researchers to pursue the investigation of better electrodes and the electrolytes [4]. Ionic liquids (ILs), therefore, have proven to be promising replacements for the conventional electrolytes we use in these devices. In this part of the chapter, let us look at the basics of ILs. The term “ionic liquid” was coined as early as in 1943. ILs are the salts that exist in liquid form at room temperature or a temperature close to

Advanced Applications of Ionic Liquids DOI: https://doi.org/10.1016/B978-0-323-99921-2.00016-1

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room temperature, typically made up of inorganic anions and organic cations [5]. In a context the ILs have also been called the salts that show melting point under the temperature in consideration, like 100 C. If any salt that can exist in molten form without decomposing or vaporizing, then it can result into an IL. For example, sodium chloride has the melting point at 801 C. When turned into a liquid, it consists of sodium cations (Na1) and chloride anions (Cl2). At the same time, if cooling of IL is considered, it gives an ionic solid that is either crystalline or glassy in nature. The normal liquids, like water, predominantly consist of electrically neutral molecules. On the other hand, ILs majorly consist of ions. These ILs are also known by other popular names such as ionic melts or ionic glasses. They are also known by other names such as liquid electrolytes, liquid salts, ionic fluids, and fused salts. Some examples of ILs are compounds based on the 1-ethyl-3methylimidazolium (EMIM) cation. They include EMIM:Cl, EMIMAc (acetate anion), EMIM dicyanamide, (C2H5)(CH3)C3H3N12 N(CN)22, which melts at 221 C, and 1-butyl-3,5-dimethylpyridinium bromide. The ILs are great electrolytes by their nature of being powerful solvents. Particularly, the salts with near-ambient temperature of melting find applications in electric batteries. They also have good sealant behavior by the virtue of their very small vapor pressure. ILs of lower temperature range are related to ionic solutions. These are the liquids that consist of neutral molecules and ions. Also, the mixtures of nonionic and ionic solid substances show considerably lower melting points than the pure compounds (Fig. 13.1).




13.2 The bonding in ionic liquids In ordinary liquids the ionic bonds that they have are mostly tougher than the Vander Waal’s forces among the molecules. Due to these strong interactions, salts possess higher lattice energies. Therefore they show higher melting point too. However, ionic liquids that contain organic cations possess lower lattice energies and hence they assume liquid state at room temperature.

13.3 Ionic liquids: evolution With experiments, as it was revealed that the ILs possess the properties fit enough to make them good electrolytes, the automotive industry fetched the maximum benefits with various ILs. Following the realization of importance of ILs, many researchers started finding out their applications in the automotive industry and other allied industries.

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13.4 Ionic liquids in fuel cells

FIGURE 13.1

355

Ionic liquids and their structures.

As a result, there were many materials proposed like imidazolium cation, piperidinium, pyrrolidinium, or ammonium cations and anions. Also, there is other inorganic range from the pretty small and chargelocalized halides (Cl2 or Br2) to bulkier, but with weak coordinating ions, with hexafluorophosphate and bis(trifluoromethylsulfonyl)imide. In the later stages, there were many ILs introduced which included protic, aprotic, composite, and polymer-based ILs with the conductivity ranging between 1.0 3 1024 and 1.8 3 1022 S/cm for both FC applications and supercapacitors. Following are the structures of some popular ILs (Fig. 13.2).

13.4 Ionic liquids in fuel cells ILs are drawing more consideration in current times because of their remarkable properties for several applications. The vital properties of these ILs include negligible vapor pressure, higher ionic conductivity,

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Cations

Anions F F

R1

N

+N

F

R2

+

O

Hexafluorophosphate

O

N S

F3C

O

CF3

O O

Bis(trifluoromethanesulfonyl)imide

F

Bis(fluorosulfonyl)imide

N B

N N

R

N Dicyanamide

R3 Tetraalkylammonium

N

N

Tetracyanoborate

O

O F3C

S

O

R

O

Tetraalkylphosphonium

O

Alkyl-sulfonate

O

+ R 2

P

R3

S O

O Trifluoromethanesulfonate

R1 R

F

O O

N

N+ R2

S

N

R1

O

N S

S

R2

N-alkyl-N-alkylpyrrolidinium

F F

Tetrafluoroborate

N

1-alkylpyridinium

F P

F

R

R1

F

F

1-alkyl-3-alkylimidazolium

+

B

F

O H3 C

C

R O

Acetate

O

P

O

H Alkyl-phosphonate

FIGURE 13.2 Some cations and anions in ionic liquids and their structures.

and, also, higher thermal stability across a huge range of temperatures and nonflammability [3]. Because of their acid base properties and tendency of forming hydrogen bonding, they prove to be good candidates for proton transport [6]. FCs are advanced versions of galvanic cells that we use to construct batteries. FC converts chemical energy directly into electrical energy. Today, we know that FCs are much more efficient than conventional batteries by the virtue of their power-to-weight ratio. Also, since the FC can function continuously with the continuous supply of fuel, it makes FCs much more efficient and convenient to use. On the other hand, they are eco-friendly too with nontoxic by-products.

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357

There are many FCs that are classified based on their electrolytes, namely, phosphoric acid, polymer electrolyte membrane, solid oxide, solid alkaline, and molten carbonate. Despite the efficient functioning of these FCs, the desire for achieving better efficiency has led to investigations in utilizing ILs as electrolytes for FCs. The results are encouraging with the many ILs like EMIM, 1-butyl-3-methylimidazolium trifluoromethanesulfonate, EMIM dicyanamide, and many more. The FCs are classified on the basis of their electrolyte and their operating temperature. The major types are molten carbonate FCs, proton exchange membrane FCs (PEMFCs), phosphoric acid FCs, alkaline FCs, and solid oxide FCs. PEMFCs use hydrogen or methanol as fuel and are used for transportation and other portable applications. It has been learnt that these FCs have advantages over conventional technologies due to the higher electrical efficiency, silent functioning, low or less toxic pollutant emissions, easy installation, and operation. Such FCs are composed of many fundamental elements such as diffusion layers, bipolar plates, electrodes, and the electrolyte. The main functioning portion of PEMFCs is called the membrane electrode assembly (MEA). It consists of the PEM placed between anode and cathode electrodes. PEMs are essential for carrying protons from the anode to the cathode, separating the gaseous reactants, ensuring electrical insulation of the electrons, and supporting the catalyst [7]. For the membranes to be applied in PEMFCs, it should have the following requirements [8]: • proton conductivity to be high in both dry and wet states • good dimensional stability and mechanical strength • thermal, electrochemical, and chemical stability under the operating conditions • low fuel and oxygen crossover • easy to fabricate for an efficient MEA • economically viable Also, to increase the energy efficiency of such FCs further, the researchers studied and experimented with various possible electrode materials, especially the electrolytes that are capable of conducting ions with ease and hence making the FC produce higher potential (Fig. 13.3). Over the course of time, ILs are found to be better replacements for the conventional materials. They have suitable properties like nonflammability, negligible volatility, and outstanding electrochemical stability. They also have good ionic conductivity and high thermal conductivity. Also, they show all these properties even under anhydrous conditions. These ILs entirely are made up of only ions and do not contain any solvent. Due to this reason, they can be combined for specific applications with all the desired properties [9]. With these ideas in mind, the applications of ILs are studied extensively for other electrochemical devices like dye-sensitized

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13. Applications of ionic liquids in fuel cells and supercapacitors

Electrical Current e-

OXIDANT

FUEL

FIGURE 13.3 Schematic diagram of a fuel cell.

solar cells and actuators. The major ones are supercapacitors and lithium batteries. However, the electrochemical devices that are more popular these days are FCs [10,11]. The ILs exhibit their conductivities at room temperature ranging from 1.0 3 1024 to 1.8 3 1022 S/cm. Today, there are many ILs available with various values of conductivities. ILs that are formed on dialkyl-substituted imidazolium cations base show conductivity in the range of 1.0 3 1022 S/cm. The other ILs from pyrrolidinium, tetra-alkyl ammonium, pyridinium, and piperidinium cations show inferior conductivities. The range is from 1.0 3 1024 to 5 3 1023 S/cm. It is worth noting that the reduction potential values observed for a cation and anion oxidation rely on the counterion. Hence, the anions such as F2 and Br2, which are halide ions, show stability up to 2 3 V. However, bis(trifluoromethanesulfonyl)imide anions are found to be oxidized at a higher anodic potential, favoring the stabilities of nearly 4.5 V [12]. Also, tetra-alkyl ammonium cation-based ILs have shown reduction at cathode at moderately negative potentials leading to enhancement in the stability close to 4.0 5.7 V [13]. Broadly, there exist two kinds of ILs, protic and aprotic. Aprotic ILs have a lower melting point due to their constraints of stacking large irregular cations with smaller anions. They show anion concentration and high mobility, which makes them suitable choice of electrolytes for lithium batteries, while protic types of ILs possess a mobile proton. This is positioned on the cation. The reactivity of such an active proton qualifies them as suitable electrolytes for FC applications [14]. Protic ILs are usually made by merging a Brønsted acid with a Brønsted base. These are capable of transferring protons from the acid to the base. As a result, it forms acceptor sites to accommodate proton donor. This can be utilized in building a hydrogen-bonded network

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359

[15]. In majority of the protic ILs, the proton movement takes place by a vehicular mechanism. Hence, protic ILs of the best fluidities are usually the ones with the highest conductivities [16]. A major advantage of employing protic ILs is that the FCs can function at temperatures above 100 C under anhydrous conditions. This is possible as the proton transportation is independent of the amount of water in it. The basics of synthesis of protic ILs lie in the acid base neutralization reaction. In the process a proton gets transferred from a Brønsted acid to a Brønsted base. This proton transfer is one of the most employed techniques in the synthesis of IL, provided the vapor pressure limitation is taken care of. The large drop in the partial pressures occurs. This is because of the decrease in free energy of the process of proton transfer which results in low vapor pressures. If the change in free energy for proton transfer is more, the proton usually becomes localized on the Bro¨nsted base and the chances of reformation of acid molecules become nearly nil at room temperatures. The properties possessed by protic ILs are due to the extent of their ionization. Here, we use Walden rule to assess the ionicity of ILs [17,18]. Various groups of researchers have assessed the functioning of various protic ILs in PEMFCs. A protic IL was prepared by Nakamoto and coworkers by combining various molar ratios of bis(trifluoromethanesulfonyl)imide and benzimidazole (BIm). The protic neutral salt resulting from this mixture possessed a thermal stability well above 350 C. It still remained hydrophobic and stable during electrochemical reactions at the equivalent molar ratio. A proton conductivity that it resulted into was 8.3 3 1023 S/cm at 140 C. In another investigation by Noda et al. [19], Brønsted acid base ILs were produced by adding various molar ratios of solid bis(trifluoromethanesulfonyl)imide and solid imidazole (Im). A neutral protic neutral salt, which was formed for a mixture with an equivalent molar ratio, proved to be stable over 300 C. The imidazole molecule works as a proton carrier and results in the improvement of the O2 reduction and H2 oxidation. FC testing was performed under nonhumidifying conditions through cyclic voltammograms. It showed that electric current gradually dropped at extended potential cycling. This observation was presumed to be because of the Imidazole adsorption on the surface of electrode. Another work was reported by Yoshizawa-Fujita et al. for the use of 3-(1butyl-1H-imidazol-3-ium-3-yl) propane-1-sulfonate added to three different acids (HTf2N, CH3SO3H, and CF3SO3H) which acted like proton transporting electrolytes. These experiments showed that the ionic conductivity of the mixtures of HTf2N improved with the amount of HTf2N till the concentration of 50 mol%. Past this proportion, the ionic conductivity did not increase any further (1.0 3 1023 S/cm at 100 C). For CH3SO3H and CF3SO3H the highest conductivity was recorded at the acid content of about 90 mol% for both compounds (1.0 3 1022 S/cm at 100 C).

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13. Applications of ionic liquids in fuel cells and supercapacitors

13.5 Ionic liquids in supercapacitors A supercapacitor is an electrochemical capacitor with a higher energy density compared to other normal capacitors and also has a lower voltage limit. They are also called as ultracapacitors. This fills the gap between rechargeable batteries and electrolytic capacitors. Supercapacitor has the capability to store 10 100 times more energy/volume or mass in comparison with electrolytic capacitors and a huge cycle life of more than 1,000,000 cycles. Also, they are capable of accepting and delivering charge quicker than the batteries. They can withstand several charging and discharging cycles more than the conventional rechargeable batteries [20]. The supercapacitors have seen a lot of progress since their invention. Supercapacitors are known for their applications involving rapid charge/discharge cycles. The applications of such nature are short-term energy storage or burst-mode power delivery or regenerative braking in trains, automobiles, cranes, buses, and elevators [21]. As the time progressed, the need for efficient supercapacitors was on the rise. Researchers working on the possible alternatives found out replacements for electrodes like activated carbons (ACs) [22,23], mesoporous carbon [24 30], carbon aerogels [31,32], carbide-derived carbon [33,34], and many more in the course of time with much better efficiency. The better efficiency is attributed to their higher specific surface area. However, at high charge/discharge rates in case of higher power uptake, it was observed that the energy density dropped drastically due to the slim electrochemical window of aqueous electrolytes. Therefore ILs came into the picture of being alternatives for the conventional aqueous electrolytes. Electrolyte in the supercapacitor plays vital role in its performance. The electrolyte being ionic conductive in nature in this case is a medium between the electrodes. This differentiates these supercapacitors from conventional electrolytic capacitors. ILs are extensively investigated for being used as an electrolyte for supercapacitors due to their huge ionic concentrations. The import properties that need to be considered for optimization are the electrical conductivity, electrochemical window, and the interfacial capacitances. ILs provide a hop of improvisation n to the former properties. However, they are limited by their high viscosity. In this part of the chapter, let us look at the various attempts of using different ILs for the supercapacitor applications. From materials perspective, supercapacitors are classified into electrostatic double-layer capacitors that use porous carbon electrodes (EDLC, electric double layer capacitor) [35,36] and pseudocapacitors that use metallic oxide materials [37]. In EDLCs the surface gets charge decreasingly by applying voltage over the adsorption of ions whereas

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361

Faradaic reactions of metal ions take place over the surface of pseudocapacitors. At the moment, most of the commercially available devices use EDLCs, because of their higher stability with respect to cycling. As pseudocapacitive mechanism is generally inactive in case of ILs employed as electrolytes, it makes sense to discuss about the EDLCs for the applications. The equations for energy density and the power of EDLCs are Em 5 P5

1 CV2 2 4V 2 R

In the abovementioned equations, C is the specific capacitance, V is the applied voltage, and R is the equivalent series resistance. To broaden the use of EDLCs, the important step is to upsurge the energy density. At this point, it is possible to enhance the operating voltage. It is also possible to improve the specific capacitance. In the meanwhile, one should not sacrifice the power density in this process, so R has to be kept as low as possible. In this regard, room-temperature ILs (RTILs) have evidenced to influence the relevant quantities, and hence they have the center of curiosity for EDLC community in the past.

13.6 Conclusion The continued attempts to improve the performance of PEMs display the importance of FCs in the upcoming days. For low-temperature devices, perfluorosulfonic acid membranes have been used mostly. However, for further improvements in the efficiency of this technology, a set of good performing membranes are required. There are many other ILs of protic and aprotic nature that are employed in the FCs that are found to show significant improvement in the gross performance of the cell. However, such membranes have limited life. Higher ionic conductivity is observed in basic membranes doped with acidic components. Further efforts are to be made for the hunt of new materials with better performance which will be able to meet the requirements as electrolytes for FCs. Their high proton conductivity under anhydrous conditions encourages the use of these materials at higher temperatures. Such assets of ILs broaden their applications in the electrochemical devices. ILs have also shown their potential to be electrolytes to run supercapacitors. Despite the intense research, it is yet a question whether they are

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13. Applications of ionic liquids in fuel cells and supercapacitors

good enough to be used as replacements for conventional organic electrolytes like acetonitrile or propylene carbonate. There are some effective ILs with good results, they also display enhanced stability at high temperatures, and with their much higher boiling point, they also ensure improved safety. They show a broad electrochemical window, a vital factor. This means that the energy density is proportionate to the square of the operating voltage. On the other hand, their use at extremely low temperatures seems impossible due to their high melting point. However, this limitation can be defeated by employing IL mixtures. It is also evident that their electrical conductivities and the individual diffusion coefficients are also inferior by at least one order of magnitude in comparison with the organic electrolytes. This makes it a very important factor for the power density of the electrochemical devices; as this may nullify the major advantage of supercapacitors over batteries. After 10 1 years of investigations, the understanding of these interfaces is much better today. The summary of all experiments showed that RTILs at planar surfaces have a layered structure. Ordered or disordered structures inside these layers have an impact on properties like differential capacitance. Investigations have also shown that nanoporous carbons have shown the best performance in the field. In a nutshell, this indicates that the potential of ILs to be employed in supercapacitors is by using them with organic solvents and making profit from their structurally versatile options to functionalize them. In this line, the recent development in the synthesis of biredox ILs is promising.

13.7 Future scope The world is progressing at a higher pace toward potable energy sources. With the need for the potable electrochemical devices raking up, the research has to be contributing to greater extent in finding more efficient and reliable materials to replace the current ones. Looking at the magnitude of need, the supply and options are hardly sufficient. In future, there is a lot of scope to enhance the efficiency of existing electrolytes and also to find the novel materials. Particularly, the polymers as we know will be the future of the world, there will be a lot of scopes to find polymer-based ILs for potable electrochemical storage and supply devices.

References [1] M.C. Buzzeo, R.G. Evans, R.G. Compton, Non-halo aluminate room-temperature ionic liquids in electrochemistry: a review, ChemPhysChem 5 (2004) 1106 1120. [2] M. Galinski, A. Lewandowski, I. Stepniak, Ionic liquids as electrolytes, Electrochim. Acta 51 (2006) 5567 5580.

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[3] M. Armand, F. Endres, D. Macfarlane, H. Ohno, B. Scrosati, Ionic-liquid materials for the electrochemical challenges of the future, Nat. Mater. 8 (2009) 621 629. [4] R. Mysyk, V. Ruiz, E. Raymundo-Pin˜ero, R. Santamaria, F. Be´guin, Capacitance evolution of electrochemical capacitors with tailored nanoporous electrodes in pure and dissolved ionic liquids, Fuel Cell 10 (2010) 834 839. [5] H. Ye, J. Huang, J.J. Xu, N.K.A.C. Kodiweera, J.R.P. Jayakody, S.G. Greenbaum, New Membranes based on ionic liquids for PEM fuel cells at elevated temperatures, J. Power Sources 178 (2008) 651 660. [6] K. Hooshyari, M. Javanbakht, M. Adibi, Novel composite membranes based on PBI and dicationic ionic liquids for high temperature polymer electrolyte membrane fuel cells, Electrochim. Acta 205 (2016) 142 152. [7] H. Zhang, P.K. Shen, Recent development of polymer electrolyte membranes for fuel cells, Chem. Rev. 112 (2012) 2780 2832. [8] B. Smitha, S. Sridhar, A.A. Khan, Solid polymer electrolyte membranes for fuel cell applications—a review, J. Membr. Sci. 259 (2005) 10 26. [9] A. Ortiz, A. Ruiz, D. Gorri, I. Ortiz, Room temperature ionic liquid with silver salt as efficient reaction media for propylene/propane separation: absorption equilibrium, Sep. Purif. Technol. 63 (2008) 311 318. [10] B. Qiu, B. Lin, F. Yan, Ionic liquid/poly (ionic liquid)-based electrolytes for energy devices, Polym. Int. 62 (2013) 335 337. [11] M.J. Park, I. Choi, J. Hong, O. Kim, Polymer electrolytes integrated with ionic liquids for future electrochemical devices, J. Appl. Polym. Sci. 129 (2013) 2363 2376. [12] R. Hagiwara, J.S. Lee, Ionic liquids for electrochemical devices, Electrochemistry 75 (2007) 23 34. [13] M. Galinski, A. Lewandowski, I. Stepniak, Ionic liquids as electrolytes, Electrochim. Acta 51 (2006) 5567 5996. [14] A. Fernicola, B. Scrosati, H. Ohno, Potentialities of ionic liquids as new electrolyte media in advanced electrochemical devices, Ionics 12 (2006) 95 102. [15] T.L. Greaves, C.J. Drummond, Protic ionic liquids: properties and applications, Chem. Rev. 108 (2008) 206 237. [16] U.A. Rana, M. Forsyth, D.R. Macfarlane, J.M. Pringle, Toward protic ionic liquid and organic ionic plastic crystal electrolytes for fuel cells, Electrochim. Acta 84 (2012) 213 222. [17] M. Yoshizawa, W. Xu, A. Angell, Ionic liquids by proton transfer: vapor pressure, conductivity, and the relevance of ΔpKa from aqueous solutions, J. Am. Chem. Soc. 125 (2003) 15411 15419. [18] J. Stoimenovski, E.I. Izgorodina, D.R. MacFarlane, Ionicity and proton transfer in protic ionic liquids, Phys. Chem. Chem. Phys. 12 (2010) 10341 10347. [19] A. Noda, M.A.B. Hasan Susan, K. Kudo, S. Mitsushima, K. Hayamizu, M. Watanabe, Brønsted acid base ionic liquids as proton-conducting nonaqueous electrolytes, J. Phys. Chem. B 107 (2003) 4024 4033. [20] F. Ha¨ggstro¨m, J. Delsing, IoT energy storage: A forecast, Energy Harvest. Syst. 53 (2018) 43 51. [21] Z. Tehrani, D. Thomas, T. Korochkina, C. Phillips, D. Lupo, S. Lehtima¨ki, et al., Large-area printed supercapacitor technology for low-cost domestic green energy storage, Energy (2016). Available from: https://doi.org/10.1016/j.energy.2016.11.019. [22] H.Y. Liu, K.P. Wang, H.S. Teng, A simplified preparation of mesoporous carbon and the examination of the carbon accessibility for electric double layer formation, Carbon 43 (2005) 559 566. [23] X.L. Li, C.L. Han, X.Y. Chen, C.W. Shi, Preparation and performance of straw based activated carbon for supercapacitor in non-aqueous electrolytes, Microporous Mesoporous Mater. 131 (2010) 303 309.

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[24] S. Alvarez, M.C. Blanco-Lpez, A.J. Miranda-Ordieres, A.B. Fuertes, T.A. Centeno, Electrochemical capacitor performance of mesoporous carbons obtained by templating technique, Carbon 43 (2005) 866 870. [25] B. Xu, F. Wu, R.J. Chen, G.P. Cao, S. Chen, Z.M. Zhou, et al., Highly mesoporous and high surface area carbon: a high capacitance electrode material for EDLCs with various electrolytes, Electrochem. Commun. 10 (2008) 795 797. [26] Y. Lv, F. Zhang, Y.Q. Dou, Y.P. Zhai, J.X. Wang, H.J. Liu, et al., A comprehensive study on KOH activation of ordered mesoporous carbons and their supercapacitor application, J. Mater. Chem. 22 (2012) 93 99. [27] A.B. Fuertes, G. Lota, T.A. Centeno, E. Frackowiak, Templated mesoporous carbons for supercapacitor application, Electrochim. Acta 50 (2005) 2799 2805. [28] J. Jin, S. Tanaka, Y. Egashira, N. Nishiyama, KOH activation of ordered mesoporous carbons prepared by a soft-templating method and their enhanced electrochemical properties, Carbon 48 (2010) 1985 1989. [29] Z.Q. Niu, J. Chen, H.H. Hng, J. Ma, X.D. Chen, A leavening strategy to prepare reduced graphene oxide foams, Adv. Mater. 24 (2012) 4144 4150. [30] Z.H. Feng, R.S. Xue, X.H. Shao, Highly mesoporous carbonaceous material of activated carbon beads for electric double layer capacitor, Electrochim. Acta 55 (2010) 7334 7340. [31] N. Liu, S. Zhang, R. Fu, M.S. Dresselhaus, G. Dresselhaus, Carbon aerogel spheres prepared via alcohol supercritical drying, Carbon 44 (2006) 2430 2436. [32] J. Li, X.Y. Wang, Y. Wang, Q.H. Huang, C.L. Dai, S. Gamboa, et al., Structure and electrochemical properties of carbon aerogels synthesized at ambient temperatures as supercapacitors, J. Non-Cryst. Solids 354 (2008) 19 24. [33] J. Leis, M. Arulepp, A. Kuura, M. Ltt, E. Lust, Electrical double-layer characteristics of novel carbide-derived carbon materials, Carbon 44 (2006) 2122 2129. [34] J. Chmiola, G. Yushin, R. Dash, Y. Gogotsi, Effect of pore size and surface area of carbide derived carbons on specific capacitance, J. Power Sources 158 (2006) 765 772. [35] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845 8454. [36] F. Be´guin, V. Presser, A. Balducci, E. Frackowiak, Carbons and electrolytes for advanced supercapacitors, Adv. Mater. 26 (2014) 2219 2251. [37] R. Paulo, J. Bueno, Nanoscale origins of super-capacitance phenomena, J. Power Sources 414 (2019) 420 434.

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C H A P T E R

14 Role of polymeric ionic liquids in rechargeable batteries Manjunath S. Hanagadakar1, Raviraj M. Kulkarni2 and Ramesh S. Malladi3 1

Department of Chemistry, S.J.P.N. Trust’s Hirasugar Institute of Technology, Nidasoshi, Karnataka, India, 2Department of Chemistry and Centre for Nanoscience and Nanotechnology, KLS Gogte Institute of Technology, Udyambag, Belagavi, Karnataka, India, 3Department of Chemistry, BLDEA’s V. P. Dr. P. G. Halakatti College of Engineering and Technology, Vijaypur, Karnataka, India

14.1 Introduction The depletion of fossil fuels has created an urgent demand for alternative energy supplies. Inexhaustible energy sources for example the sun and wind are considered possible options. As a result of the exhaustion of fossil fuels, there is a pressing need for alternate energy technologies. Renewable energy sources like solar and wind are being considered as possible alternatives, to store these energies batteries are playing an important role as a source of storage device. Research on pure energy sources like solar, wind, and hydro has begun, however, the most important drawback in creating smart use of this energy is the correct current storage thanks to the sporadic nature of those sources of energy. This requires the efficacious energy storage technologies of batteries and supercapacitors [1,2]. Ionic liquids (ILs) are salts dissolved in water in a convincing situation, the term is limited to salts with melting points below certain temperatures, such as 100 C (212 F). Lack of electrical charge molecules is distinguished from ions by their making up the majority of ordinary liquids

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14. Role of polymeric ionic liquids in rechargeable batteries

like water and gasoline, whereas ions make up the majority of ILs. Liquid electrolytes, ILs, molten salts, liquid salts, ionic fusions, and ionic glasses are some of the terms used to describe these materials [3,4]. IL is used in many aspects of energy storage and conversions, as well as clean and sustainable energy, are in high demand. Lithium batteries and fuel cells, for example, have already made a substantial impact in this area. Extensive research supports the development of innovative materials for these devices. Carbonate electrolytes are used in lithium-ion batteries (LIBs), making them the most energetic rechargeable battery. A comprehensive and active research program has been undertaken to develop new materials for use in these devices. For example, LIBs used carbonate-based electrodes, which resulted in higher energy densities in the development of secondary batteries. However, these volatile organic compounds (VOCs) can create safety concerns when using LIBs in high-energy applications for example in electric means of transport or power grid network. In addition, side reactions, the solubility of the electro-active components, and the solvent volatility make conventional electrolytes in the form of carbonates unsuitable for use in non-LIBs for example lithium-sulfur and lithiumoxygen batteries. Rapid water evaporation and the associated reduction in proton conductivity proton-conducting films, mainly Nafion-R, limit the use of fuel cells above 100 C. When studying chemistry, ILs opened up a new era of matter. ILs have also had a major impact on the chemical, pharmaceutical, biotechnology, and energy industries as well as gas processing, processing, and recycling. In the field of materials chemistry, ILs are a relatively recent study topic. Chemical processing, medicines, biotechnology, energy, gas management, and material processing have all benefited from the usage of ILs [5,6]. A battery’s LIBs and supercapacitors use ILs as alternate electrolytes because of these advantages [7,8].

14.2 Classification of ionic liquids based on their chemical structure ILs are divided into different classes based on their chemical structure. One of the most important classes of ILs, ILs contain non-proton and proton ions [9]. The characteristics of aprotic ionic liquids (AILs) and polymeric ionic liquids (PILs) are similar, but the most significant distinction between the two types of ILs is the presence of “accessible” (or free) protons on the PIL cation. PILs (protic and aprotic ILs) are two of the most significant types of ILs with applications (Fig. 14.1).

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14.2 Classification of ionic liquids based on their chemical structure

(A) aprotic ionic liquid (AIL) N

N

+

Imidazole

N

Br Alkyl halide

+

N

LiTFSI

Br -

O

+

N

O –

N

F3C S N S CF3 O O Aprotic ionic liquid

Imidazolium halide

(B) protic ionic liquid (PIL) N Brønsted base

O O H + F3C S N S CF3 O O

O +

H

Brønsted acid

O –

F3C S N S CF3 O O

N

Protic ionic liquid

Biocatalysts in ILs

Materials chemistry

Nuclear-based separations Catalyst for Chemical and biochemical transformations

Solvent extraction

Physico-chemical processes

Use of Ionic Liquids

Dye-sensitized solar cells Solvents for polymerization

ILs for the nuclear fuel cycle Oil shale processing

Media for nucleophilic substitution reactions

Chemical analysis

Mobile phase Electrodeposition of modifier in HPLC metals and semiconductors in ILs

Solvents for electrochemistry

Separation of petrochemical relevance

Synthesis of functional nanostructures

FIGURE 14.1

Ionic liquid classes and applications (A) Classes of ionic liquids AIL and PIL. (B) Applications of ionic liquids in various fields.

14.2.1 Protic ionic liquids as electrolytes for lithium-ion battery PILs are a kind of IL that is created by transferring protons from Bronsted acid for regeneration. Because of the “free” proton they have, they have several intriguing characteristics (e.g., hydrogen bond). Pure PILs or PIL mixes with acetonitrile or water have been proposed as super-capacitor electrolytes. Due to the “free” proton they have, they have several interesting characteristics (e.g., a hydrogen bond). Pure PILs or mixtures of PILs with acetonitrile or water have been proposed as electrolytes of supercapacitors. A study of proton conductivity of PIL-based polymers was also conducted in the context of its application in fuel cells [10]. Little attention has been made to the use of PIL as an electrolyte solvent in LIBs. As a result, PIL is much cheaper than aprotic

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IL, is easier to manufacture, and is environmentally friendly, so it has the potential to be used in LIBs [11]. Gel polymer electrolytes (GPEs) are the blends of organic solvents with inorganic salts, like ethylene carbonate/ propylene carbonate/ sodium iodide etc. [12,13]. To produce a pure bipolar moment, the polymer host must have excessive chemical resistance and strong functional groups without electrons [14]. Because most ILs are liquids at room temperature, they can be utilized as electrolytes without the need for a solvent medium. However, using organic liquid electrolytes has some drawbacks for devices, including leakage, corrosion, and a lack of supply. To solve these issues, the scientific community then proposed using ILs with polymer electrolytes (PEs) to maintain the mechanical qualities of ILs while still preserving their good electrochemical properties [15]. Because many PEs have low ambient temperature conductivities, this is the case. Therefore, they are ideal as solvents and electrolytes, and their internal ionic conductivity is important for electrochemical applications. ILs have unique properties that make them ideal for a wide variety of energy-related applications [16].

14.2.2 Aprotic ionic liquids as electrolytes for lithium-ion battery ILs are attractive for producing LIB electrolytes because of, their sturdy thermal stability, and low vapor pressure. For more than a decade, scientists have studied the use of IL in LIBs and used unusual forms of cations and anions to make AILs. The cations that have been researched the most include imidazolium, pyridinium, pyrrolidinium, and piperidinium. They are used in a mixture with lithium salt to generate electrolytes excellent for LIBs. Tetrafluoroborate (BF42), PF62, and per-fluoro-alkyl as di-(trifluoro methane sulfonyl imide) (TFSI) and di-(fluoro sulfonyl imide) (FSI) are the common anions shown in Fig. 14.2. These AILs are mixed with lithium salts to create LIB-compatible electrolytes. Lithium salts often contain anions that are used to reduce the weight of electrolyte solutions. It is often referred to as “solvent-free” because it does not require a solution to form these liquid electrolyte solutions [17]. The cation anion pair has a significant influence on the properties of ILs. For example, the addition of imidazolium cations enables the production of IL with higher conductivity than the pyrrolidinium and piperidinium cations. For example, the inclusion of imidazolium cations can produce IL with higher conductivity than pyrrolidinium and piperidinium cations [16,18]. Using these cations instead of imidazoles allows for the synthesis of ILs with a wide electrochemical stability window [17].

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FIGURE 14.2 Ion families in common ionic liquids used in lithium-ion batteries [16].

The FSI-based AILs have a lower viscosity than the TFSI-based AILs. With the properties of imidazole ions, it is clear that one of the main advantages of using an IL-based electrolyte is that the properties of the electrolyte can be modified by changing the composition of the cations and anions.

14.3 Introduction to Li batteries The power stored using a battery is converted from chemical energy into electrical energy by redox chemistry. An electrical source can be converted into chemical, power for the life of a battery. Rechargeable lithium batteries have become very popular in electrical energy storage

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14. Role of polymeric ionic liquids in rechargeable batteries

FIGURE 14.3 Compare battery types.

systems because of their high-powered capacity, operating current, prolonged existence life, and less self-discharge (Fig. 14.3) [19,20]. In effect, phosphorous oxide has a completely high potential for Li batteries, making it a very not unusual manner of improving their electricity density [21]. In recent years, Li batteries have exhibited a power density of 100 200 Wh/kg, making them improper for cars. Steel lithium has been used as an anode for a long term; however, when coupled with natural liquid electrolytes, lithium dendrite development is the most important trouble with Li batteries [22,23]. Furthermore, the usage of those flammable and volatile liquids is exposed for its safety at hazard. In addition, because of their electrochemical instability at better voltages, these natural liquid electrolytes cannot be used in batteries [24]. Therefore, an alternative electrolyte is required for the safe use of lithium metal in batteries. Owing to their mechanical, thermal, and electrochemical stability permanence in addition to their safety and flexibility, PEs have become established in Li batteries [25]. Ions can move about in the ion transport host matrix, which is a polymer matrix with free space. Organic salts are usually dissolved in a polymer concentration to form solid polymer electrolytes (SPEs). If a polar group is present in the polymer matrix, it affects whether it can easily interact with the cations and if the bond rotation is limited [26]. Due to their flexibility in large chains and their ability to remove many biological/unnatural chemicals, polymer poly(ethylene) oxide (PEO) electrolytes in various polymer matrices have been extensively studied [27,28].

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14.4 Basics of ionic liquids

371

14.4 Basics of ionic liquids Organic salts, ILs, molten salts, or organic salts are all examples of ILs. Bulky, distinct, weak organic cations and organic and inorganic anions are the most common IL mechanisms [29]. Water and organic solvents may have nonvolatility, high thermal stability, and high ionic conductivity are features of solvents (and electrolytes). In other words, when it comes to the features of ILs, it is important to remember that the abovementioned characteristics are not always present in all ILs, which opens up the possibility of creating new task-specific ILs. Fig. 14.4 demonstrates the general cations and anions of ILs that are used in batteries [30]. Because of low lattice energy and weak ionic bonds between cations and ionic salt anions, ILs are separated (NaCl, KCl, etc.). The result is high conductance, low vapor pressure, glass transition temperature and high melting temperature, excellent heat and electrochemical stability, low deposition, and easy recovery. Table 14.1 presents some of the properties of IL.

FIGURE 14.4

General cations and anions for ionic liquids in batteries.

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TABLE 14.1

Ionic liquids properties

Ionic liquids

Tg (C)

Tm (C)

PYP1,4-TFSI

287

26

[EMIM][BF4]

293

[EMIM][PF6]

211

Td (C)

450

σ (mS/cm)

η (cP)

2.69

60 

14 at 25 C

43 at 20 C

5.2 at 26 C

60

[EMIM][TFSI]

298

4

440

8.8 at 20 C

28 at 25 C

[BMIM][Cl]

—

41

254

—

1534 at 50 C

16.5 at 25 C

24.5 at 25 C

[EMIM][FSI]

12.9

585 at 25 C

[HMIM][PF6]

278

261

417

[BMIM][PF6]

276

10

390

1.8 at 25 C

312 at 25 C

[BMIM][TFSI]

2104

24

439

3.9 at 20 C

52 at 25 C

[OMIM][PF6]

282

240

376

[HMIM][BF4]

282.4

218

409

682 at 25 C 1.22 at 25 C

439

14.5 Organic and inorganic ionic liquids in electrical storage systems At normal temperatures, ILs are referred to as molten salts, a form of substance that contains both organic cations and inorganic/inorganic anions [31,32]. It is significant that the separation of polymer ions into ILs reduces the electrostatic force of the ions and separates them, lowering the melting temperature. In organic synthesis, chemical detection, life sciences, green chemistry, and storage systems that can generate electrical energy ILs are used as solvents [33,34]. ILs are utilized as electrolytes and solvents in different ways, since the advantages of ILs, like variable polar nature and ionic conductance, low volatile nature, exceptional thermal stability, and low flammability, have contributed to several advantages and uses in electrical energy storage [35,36].

14.6 Ionic liquid-based polymers electrolytes historical background Because of its interesting characteristics, synthetic polymer materials developed quickly in the industrialized world. Various research groups created physicochemical and theoretical techniques needed to explore polymeric materials at the same time.

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14.7 Polymeric ionic liquids for rechargeable lithium-ion batteries

373

Owing to their intriguing features, synthetic polymer materials have developed quickly in industrialised nations. Numerous researchers independently developed the physicochemical and computational techniques needed to study the polymeric materials. Wright et al. reported that poly (ethylene) oxide (PEO) compounds with sodium thiocyanate, potassium thiocyanate, and sodium iodide show ionic conductivity [37]. Instead of looking for high-performance electrolytes, the focus is shifting to more recent industries such as high-performance composites and fibers (such as Kevlar). The importance of Wright’s paper was explored by Armand et al. in 1978, who advocated using a salt-polymer combination as a solid electrolyte [38,39]. In 20 years in a new field of PE, inventions and widespread distribution of portable microelectronics and electronics have emerged, and there is a strong demand for extremely lightweight rechargeable batteries, high performance, and affordable. In the beginning, electrolytes become increasingly necessary as a result. Several methods for limiting the crystallinity of polymer materials were discovered. In the beginning, several methods for limiting the crystallinity of polymer materials were discovered [40]. The third decade, around the 1990s, saw the widespread acceptance of LIBs in addition to a fast increase in the production of low-cost portable devices. PEs made from amorphous PEO were referred to as “classics.” The new electrolyte is a high salt or angel salt PE, and a gel electrolyte containing solvent molecules is used in the polymer matrix [41]. Environmental contamination is a serious problem in today’s globe, owing to a variety of human activities, including electricity generation. As a result, a lot of attention is being paid to sharpening equipment with harmless materials that provide outstanding performance. PEs is also experiencing significant changes, such as the use of natural polymers and feasible solvent and plasticizer replacements [42]. The preparation and analysis of inorganic and inorganic polyethylene represented advancement in polyethylene research in the fourth decade. Owing to their mechanical, thermal, electrochemical, and chemical stability and their excellent conductance at room temperature, these materials promise excellent use in lithium rechargeable batteries [43]. Also, the addition of minerals to the chemical makeup and the creation of new classes of synthetic gels and IL-based PEs have created a whole new family of materials [44,45].

14.7 Polymeric ionic liquids for rechargeable lithium-ion batteries As IL molecules are added to a polymer chain, PILs or ionic polymerization liquids (polymer ILs) are produced. This is a new type of

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14. Role of polymeric ionic liquids in rechargeable batteries

functional polymer that blends the characteristics of room temperature ILs with the polymer structure. Produces a new type of functional polymer. As a result of Ohno’s and his co-workers’ insight [46,47]. PILs play an important role in various energy storage systems, as in batteries and supercapacitors, as well as conversions (fuel cells) and electromechanical applications (actuators and sensors) [48,49]. PIL, with a unique combination of polymer and anti-ion, has tuneable properties such as glass transition temperature and solubility that enable a multitude of new applications. The mechanical stability, increased process capability, robustness, spatial control capability, and other benefits of using PIL over IL are just a few of the benefits. Used as a SPE, PIL has a variety of architectures, as shown in Fig. 14.5. PILs with variety of architectures (Fig. 14.5) were extensively used as SPEs. Researchers made attempts to investigate the relationship between the structure-ionic conductivity of the PIL-based SPEs with respect to the type of cation and anion present in PILs [48,49].

14.7.1 Emerging of ionic liquid based polymer electrolyte There is serious research for energy solutions in vehicles with the rising need for clean, dependable, and internationally inexpensive electricity and energy. Many efforts to find novel materials and appropriate design methods have resulted as a result of this. PEs have made significant progress toward high safety and noticeable efficiency. ILs are suitable materials for integration with PE because of their high conductivity; chemical resilience, low toxicity, and favorable electrochemical characteristics [50]. These are molten salts with bulky asymmetric organic and mineral anions that can be used at a comfortable temperature. IL has been used as possible solvents that are inherently unsafe to PE. ILs have been dubbed “green solvents” since they are non-volatile, soluble, and evaporation resistant [51]. ILs have also been used to increase the ambient temperature conductivity of PEs. The IL, N-alkyl-N-methylpyrrolidinium per fluoro-sulfonylimide, was used by Passerini et al. to aid conductivity enhancement in a PE [52]. The development of a compelling amorphous phase was described using the zinc ionic conductivity of PE 1-ethyl-3-methyl-imidazolium, bis (trifluoro-methane-sulfonyl) imide (EMIMTFSI). PE EMIMTFSI ion conductor zinc ion di(trifluoromethanesulfonyl) imide (EMIMTFSI) ion conductor is effective, and the development of an amorphous phase is compelling proof of existence [53]. Because the properties of a liquid are determined by the careful selection of IL-imidazolium, pyridinium, alkylammonium, alkyl-phosphonium, pyrrolidinium, guanidinium, etc., IL is aprotic and aprotic depending on the type of cation. They can

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14.7 Polymeric ionic liquids for rechargeable lithium-ion batteries

FIGURE 14.5

375

Chemical structure of PIL used in SPE.

contain ILs that contain metal and anions like halides (Cl2, Br2, I2), polyatomic metals (PF62, BF42) and polyoxometalates, as well as organic anions like nitrate (NO32) and trifluoromethylsulfonylimide

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14. Role of polymeric ionic liquids in rechargeable batteries

Cations R1 N

R1

R1

+

H R1

N+

N+

1,3-dialkyl-imidazolium

N+

R1

R1

R1

R1

N+

N-alkyl-pyridinium

Tetraalkyl-ammonium

N-alkyl-pyrrolidinium

Anions F

F

O -

P

F

F

F

-

B

F3C

S O

F

F

F

F

F

PF-6

BF 4

-

O

O N-

S

CF3

F3C

O

O

S

O-

O

Bis((trifluoromethyl)sulfonyl)imide Trifluoromethanesulfonate

FIGURE 14.6 Examples of cations and anions used to make IL.

(TFSI2) and trifluoromethanesulfonate (Tf2). Fig. 14.6 depicts a few common ions involved in the production of ILs. They include halides (Cl2, Br2, I2), inorganic polyatoms (PF62, BF42), inorganic anions such as polyoxometallate, NO32, TFSI2, and Tf2. Some common ions are involved in IL production. These cationic chemicals and replacements have a significant impact on IL conductivity, hydrophobicity, melting temperature, viscosity, solubility, and other chemical and physical properties [34]. Conductivity is important in electrolytes because the balance between the interactions of ion pairs has a substantial influence in realworld applications. LIBs make up the vast majority of commercially available batteries. Because they generally employ electrolytes that are volatile and flammable, their use is severely limited Conductivity is important in electrolytes because the balance between the interactions of ion pairs has a substantial influence in real-world applications. LIBs make up the vast majority of commercially available batteries. Because they generally employ electrolytes that are volatile and flammable, their use is severely limited. In the early 1980s, lithium-ion polymer cells were presented as a possible answer to safety concerns [54]. Even though these efforts were partially effective, the primary issue of the system remained because liquid organic solvents are flammable. The non-toxic, eco-friendly nature of ILs provides value to the rechargeable battery business in general. Many reports in recent years have demonstrated that integrating ILs will result in a positive outcome shortly [55].

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14.7 Polymeric ionic liquids for rechargeable lithium-ion batteries

377

Due to their solubility in solvents or electrolytes, IL electrolytes are often used as suitable alternative battery energy storage systems (BESS) like LIBs. This chapter attempts to summarize the present state of knowledge on the electrochemical, cycle, and physical characteristics of IL-based electrolytes that are relevant to LIB. Solvents like IL can help enhance the performance of environmental and energy storage devices, particularly LIBs, by replacing more flammable organic carbon [56]. One of the most important studies is the active usage of IL. LIBs, for example, employ carbonate electrolytes to achieve the maximum energy density of secondary batteries. However, these VOCs are a safety concern when using LIBs in extensive applications such as electric motors and power grids. For example, LIBs use carbonate-based electrolytes, achieving the highest energy density of any secondary battery developed so far. However, when an LIB is used in more applications in electric motors and power grids, these VOCs pose safety concerns. These concerns have prompted the development of novel IL-based electrolyte compounds. Meanwhile, the nonvolatility and great thermal stability of ILs have allowed some of them to be used as carbon material precursors. For energy applications, this new approach reveals highly functional, task-specific carbon compounds. This article describes the energy consumption of IL. The carbon compounds produced from IL are shown. This shows that IL can be used as a new functional material for energy applications, especially as a new building block for catalysts and electrodes. Table 14.2 presents common cation and anion abbreviations found in IL. TABLE 14.2

General abbreviations for cations and anions in IL.

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14. Role of polymeric ionic liquids in rechargeable batteries

14.8 Li/Na-ion battery electrolyte LIBs are used as a light source because they have a high power density and high energy density. The generally common components in Li batteries are negative electrodes, highly volatile electrodes containing Li (Li-MO2, M: conversion metals), and organic electrolytes [57,58]. Li is inserted into a negative carbon electrode while charging an LIB and Li is evicted as of a positive LiMO2 electrode during the extraction process. Li salts are extensively employed as electrolytes in LIBs, ethylene carbonate (EC) and diethyl carbonate are aprotic molecular solvents used [59,60]. Organic molecular solvents are characterized by their flammability and volatility. The safety of LIBs must be enhanced, particularly in largescale energy storage systems used as electric cars and power grids. Alternative energy sources were also investigated, including sodiumion batteries at room temperature with operating electrolytes [61]. Thermally stable electrolytes are widely used to increase the thermal durability of these batteries [59,62]. Due to the thermal stability, low volatility, and flame retardancy of IL, many researchers plan to use IL as an electrolyte for batteries [63 65]. Chloroaluminate-based IL (AlCl4) was investigated in IL’s groundbreaking research on the battery used [66,67]. However, chloroaluminate anions are sensitive to water and corrosive, therefore, they have not been widely used in chloroaluminate-based ILs in recent years in lithium and sodium battery research. Therefore they have not been widely used in chloro-aluminate-based ILs in recent years in lithium and Na battery research. The electrolyte also has sufficient electrical stability. When the battery permanently damages the negative electrode and generates an oxidation voltage on the positive electrode, it affects the charging and discharging performance. When the battery is full, the negative and positive electrodes are lowered and discharged separately. Therefore, the electrolyte Li/Na ion batteries have great potential.

14.9 Polymer-electrolytes classification Electric polymer is used to develop electrochemical devices. PE is divided into the following groups based on materials (Fig. 14.7).

14.9.1 Electrolytes based on dry solid polymer Electrolytes based on the dry solid polymer are ion-conducting electrolyte that is made by integrating inorganic salt into a polar

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14.9 Polymer-electrolytes classification

Solid Polymer Electrolytes

379

High flexibility Thermal, mechanical and chemical stability Low ionic conductivity

Composite Polymer Electrolytes

High conductivity Thermal, mechanical and chemical stability

Polymer Electrolytes Plasticized Polymer Electrolytes

Gel Polymer Electrolytes

High conductivity Low mechanical and chemical stability

High flexibility Thermal and chemical stability High ionic conductivity

FIGURE 14.7 Polymer electrolytes classification.

polymer [68]. Coordination bonds are formed when the metal ions in the salt electrostatically interact with the polar polymer. The distance between the polymer’s functional groups, the molecular weight, the kind of branching, the metal’s charge, and other variables that might influence the metal-polymer relationship [69]. Ions begin to migrate from one coordination site to the next when the PE is exposed to an electric field. It happens because the metal ion and functional g have a weaker connection. Because the metal ion and the polymer chain’s functional group have a weaker bond.

14.9.2 Electrolytes based on plasticized polymer Plastic PEs are formed by the dissolution of low molecular weight substances such as EC, propylene carbonate, and polyethylene glycol [70]. This reduces Tg and crystallization while reducing intermolecular and intramolecular interactions between polymer chains and increasing the salt dissociation capacity [71]. This method improves the conductivity of poly-electrolytes but has limitations in terms of mechanical durability, solubility, and lithium electrode reactivity.

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14. Role of polymeric ionic liquids in rechargeable batteries

14.9.3 Electrolytes based on gel polymer GPEs have advantages over liquid electrolytes, have high conductivity and good electrode contact with the electrolytes, with the advantages of solid electrolytes, safety, mechanical and thermal properties for the production of PEs. It is becoming more and more popular among them. GPEs are safer than liquid electrolytes since they use a polymer to preserve liquid components while simultaneously providing mechanical support. GPE is a polymer that contains a lot of organic solvents and is shipped with the bulk polymer [72]. Due to the desirable properties of IL, IL-based GPE has recently been the focus of research, including excellent conductivity, thermal stability, and low vapor pressure. As a result, GPE can be used as an alternative to liquid electrolytes.

14.9.4 Electrolytes based on composite polymer Polymer-based composite electrolytes offer the advantages of nonmetals and polymers and can be used directly in solid metal batteries in the future, proton membrane membranes, and methanol fuel cells, among other applications. It is expected to have high safety, good flexibility, excellent thermal stability, and superior electrolyte performance. However, the lithium-ion guide (lithium-ion, proton, etc.) and weak electrolyte adaptation during the process of transport/discharge, especially when matching high voltage codes and metal lithium codes, are still waiting for better solutions. In addition, the multipurpose Kuwaiti functions are unique, such as healing, flexibility, and antiphizers, which are also desirable for natural disasters in different situations. This should be considered by ion, electrochemical/chemical movements, and polymer-based disasters in dealing with the above issues [70]. It can usually be divided into polymer/polymer mixture and polymer electricity. The components of production devices are affected by their performance. Each component and content/content/content is particularly important in high-performance disasters [71]. The purpose of this research is to progress in polymer-dependent rap, including the effect of materials and their components, on mechanisms for improving the relevant performance.

14.10 Ionic liquid-based gel polymer electrolytes application in lithium batteries IL-based electrolytes offer some favorable characteristics, and they are widely employed in supercapacitors, batteries, and fuel cells. Lithium batteries are essential because of their high energy density,

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14.11 Low melting point alkaline salts in lithium batteries

381

versatility, and safety. In recent years, polymer batteries have been used to power all electronic gadgets, including computers, cell phones, power banks, and portable players. One of the primary advantages of GPE, according to Gupta et al., is the creation of a strong interface between the electrodes and electrolytes, which improves the lithium battery cycle’s stability. The performance of PEO-based GPE battery has been studied in the literature [73]. The electrochemical performance of GPE-Li batteries based on PEO is shown in Table 14.3. The following researches demonstrate that utilizing GPE instead of liquid solvents in lithium batteries increases battery expandability and electrochemical stability substantially. IL-based GPE not only exhibits high ion conductivity, flexibility, and mechanical stability but also contributes to the capacity, recycling ability, and safety of lithium batteries.

14.11 Low melting point alkaline salts in lithium batteries The MP of conventional lithium salts is over 200 C such as LiClO4 is 236 C, LiBF4 is 296.5 C, and LiPF6 is 200 C. The high electrostatic interaction between lithium and anions is the underlying reason for this. The charge density of lithium ions is high due to the short ion radius, and there is a significant electrostatic attraction between lithium ions and anions. Thus fusion usually requires high temperatures. Some Li inorganic salts have low melting points and should be mentioned here. The M.P. temperature of LiClO3 is 128 C, but in the fourth system NaNO3, LiNO3, LiClO3, NaClO3 M.P. at 128 C [86]. LiAlCl3SCN melts at 110 C and may be kept at ambient temperature as a superheated liquid, whereas LiAlCl4 melts at 143 C. The LiAlCl3SCN anion is an asymmetric AlCl3SCN ligand compound with a symmetric structure, whereas the AlCl3 anion has a symmetric structure. Several factors affect solubility, including electrostatic interactions between Li and anions, anion size, anion symmetry, and anion morphology. All of these affect the melting temperature of Li salts [87]. The weak interaction between the fundamental Lewis anion and Li, according to the hard and soft acid and base (HSAB) theory, is feeble. On the other hand, even the Li salt of a large element anion with a low Lewis base has an M.P. significantly higher than room temperature. According to the HSAB theory, the weak interaction within the major Lewis and Lithium anions is weak. On the other hand, even salts of large element anions with low Lewis bases are M.P. substantially higher than ambient temperature [88,89]. Fujinami et al. studies have shown that the formation of liquid salts of Li was activated by the penetration of ether groups into the aluminate structure [90,91].

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382 TABLE 14.3

14. Role of polymeric ionic liquids in rechargeable batteries

The electrochemical performance of GPE-Li batteries based on PEO.

Polymer electrolytes

Li-battery

C-rate

Capacity (mAh/g)

References

PEO20LiTFSI[Pyr13TFSI]1.27

Li/LiFePO4 at RT

C/10

115 at 20 cycles

[74]

PEO20LiTFSI2[Pyr14TFSI]4

Li/NMC at 40 C

C/10

160 at 100th cycle

[75]

PEO20LiTFSI[Im12TFSI]

Li/LiFePO4 at 50 C

C/5

110 at 20th cycle

[76]

PEO20LiTFSI[Pip1.1O1TFSI]

Li/LiFePO4 at RT

C/20

120 at 35th cycle

[77]

PEO20LiTFSI[Pip1.101TFSI]

Li/Li4Ti5O12 at RT

C/20

150 at 40th cycle

[77]

PEO20LiTFSI2[Pyr14TFSI]4

Li/LiFePO4 at 40 C

C/10

140 at 450th cycle

[78]

PEO 1 20 wt.% LiTFSI 1 20 wt.%ThdpTFSI

Li/NMC622 at RT

C/10

148 at 150th cycle

[79]

PEO20LiTFSI2[Pyr14TFSI]2

Li/LiFePO4 at 40 C

C/5

160 at 180th cycle

[80]

PEO-LiTFSI-10wt%EMIMFSI

Li/NCA at RT

C/10

175 at 200th cycle

[81]

PEO 1 LiFSI 1 7.5 wt.% EMIMFSI

Li/LiFePO4 at RT

C/20

143 at 100th cycle

[82]

PEO 1 LiTFSI 1 12.5% EMIMTFSI

Li/LiMn2O4 at RT

C/10

120 at 10th cycle

[83]

PEO 1 20%LiTFSI 1 30% (1-butyl 3-methyl yridinium TFSI)

Li/LiFePO4 at 40 C

C/10

160 at 25th cycle

[73]

PEO 1 20%LiFSI 1 10% PYR13FSI

Li/GOLiFePO4 at rt

C/10

163 at 100th cycle

[84]

PEO 1 20 wt.% LiTFSI 1 20 wt.% BMIMTFSI

Li/LiMn2O4 at RT

140 μAh/cm2 at 25 cycle

[85]

The ether group acts as a coordination ligand for the Li cation, separating them from the anion’s nuclear atom. Researchers developed lithium ILs, which separate the lithium cations from the anions’ core atoms by combining them with borates that have electron-withdrawing moieties to minimize the anionic basicity. We created a lithium IL containing Licontaining borate with an electron-withdrawing group and the coordination of the ether ligand of Li. The ether group acts as a binding compound for Li, separating the Li cation from the central atom of the anion.

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14.12 Conclusion

383

FIGURE 14.8 Structures of Li ILs.

Watanabe et al. have developed a lithium IL containing Li and a borate with a single electron-removing group to reduce the presence of anions. Li also controls the ether and separates lithium via anionic atoms [92,93]. Other alkali metals—IL (Li, Na, and K) can be formed in the same way by using the same ether and larger anions. For example, at room temperature, alkali carboxylate is still a liquid [94], as shown in Fig. 14.8.

14.12 Conclusion The use of IL in combination with organic electrolytes is an interesting method in light of the development of better electrolytes. Some studies have already investigated the effects of various electrolytes on the performance and safety of LIBs, especially in terms of flammability. The IL solvent interactions were investigated in a few instances. Nonetheless, gaining a better knowledge of these interactions might help with the logical design of these electrolytes. This latest study seems to be important in this regard. In recent years, there has been increasing research showing that IL may help prevent unwanted reactions such as anodic decomposition of Al or improve the properties of additives such as redox shuttles. As a consequence, the use of

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14. Role of polymeric ionic liquids in rechargeable batteries

IL as an addition to traditional LIB electrolytes appears to be highly promising, and more studies in this area should be conducted as soon as possible. Finally, various methods have been proposed to improve the conductivity of PEO-based PEs below the melting temperature. GPEs based on ILs are considered to be the most promising. PEs with these GPEs have improved ionic conductivity, thermal stability, and electrochemical stability. Advantages of lithium batteries include improved electrode electrolyte interactions, mechanical stability, and safety. They can help to avoid the formation of undesirable dendrites and contribute to the safety of lithium batteries owing to their mechanical stability. It also has excellent electrochemical stability, making it suitable for use in high-voltage, high-energy batteries. These batteries offer excellent electrochemical and periodic stability, as well as flexibility and safety. As a result, these IL-based GPEs can be considered viable alternatives to liquid electrolytes from lithium batteries.

Abbreviations AILs BESS BF42 EC EMIMTFSI FSI GPEs HSAB KSCN ILs LIBs LiBF4 LiClO3 LiClO4 LiPF6 NaClO3 NaSCN PEs PEO PF62 PILs SPEs Tf2 TFSI VOCs

Aprotic ionic liquids Battery energy storage systems Tetrafluoroborate Ethylene carbonate 1-Ethyl-3-methyl-imidazolium,bis (trifluoro-methane-sulfonyl) imide Di-(fluoro sulfonyl imide) Gel polymer electrolytes Hard and soft acid and base Potassium thiocyanate Ionic liquids Lithium-ion batteries Lithium tetrafluoroborate Lithium chlorate Lithium perchlorate Lithium hexafluorophosphate Sodium chlorate Sodium thiocyanate Polymer electrolytes Poly(ethylene) oxide Hexafluorophosphate Protic ionic liquids Solid polymer electrolytes trifluoro-methane-sulfonate Di-(trifluoro methane sulfonyl imide) Volatile organic compounds

References [1] B.K. Kim, S. Sy, A. Yu, J. Zhang, Electrochemical super-capacitors for energy storage and conversion, Handbook of Clean Energy Systems, John Wiley & Sons, Ltd., 2015, pp. 1 25.

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C H A P T E R

15 Progress in optoelectronic applications of ionic liquids Sana Zafar1 and Mohd. Imran2 1

Department of Applied Sciences and Humanities, Jamia Millia Islamia (Central University), Delhi, New Delhi, India, 2Department of Physics, Jamia Millia Islamia (Central University), Delhi, New Delhi, India

15.1 Introduction In the last decades, the optoelectronic devices have been extensively studied and become an important research field for modern and microtechnology. The field of optoelectronic technology deals with the study and application of electronic devices interacting with light photons in either manner to detect, control or become a source of light. Since this technological field has become very vast, a wide range of devices fall under the category. They are the class of devices that convert light into electronic signals like solar cells or photovoltaic cells and photodetectors or emit light using an electrical signal like a light-emitting diode (LED), light-emitting chemical cells (LECs) or laser. In recent years, the rapid development and improvement in the efficiency of optoelectronic devices, especially solar cells, LEDs, and photodetectors, have been observed [1,2]. Owing to this, there is a great possibility to discover new materials that proposed significant advancements in the way that the device must be sustainable, stable, environmentally friendly, cost-effective, and more importantly energy efficient [3]. Over the centuries, the world population is reliant on the fossil fuels as an energy source. Fossil fuels though abundant in nature are nonrenewable source of energy and depleting with increase in demand. Apart from this, combustion of fossil fuels has adverse environmental effects such as climate change, greenhouse effect, global warming, and

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pollution. The exploration of clean and sustainable cheap and renewable source of energy thus becomes an attractive topic of research. Among the existing sustainable and renewable energy sources, solar energy is most abundant, clean, nonhazardous, infinite, and economically available. Since solar energy is a promising supplement for the future generation, several types of solar cells have been discovered in the past several years [4]. It is notable that only 10 minutes of solar irradiation onto the Earth’s surface is equal to the total yearly human energy consumption [5]. The photovoltaic device technology works on the principle of direct conversion solar energy into electrical energy and emerging to meet the demand for future generation. However, the solar cell technology requires improvement in materials required and an effective device to improve the power conversion efficiency (PCE) at low fabrication cost. Nowadays, scientists are much interested in designing, photovoltaic devices having high efficiency, low cost, ease in fabrication, sustainable, and safe. Therefore the dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) based on interlayer hybrid structure gained popularity considered to be as third-generation solar cells for future solar energy devices thus challenged the market of inorganichybrid solar cell technology [6,7]. DSSC is a photovoltaic electrochemical device having achieved significant popularity and studied extensively because of low manufacturing cost, simple device fabrication procedure, high-power conversion, and reasonable device efficiency than semiconductor-based solar cells. Besides the performance, DSSCs are flexible, available in different colors, light weight, showing acceptable performance even in low light and as they are less sensitive to incident angle and intensity of light [8]. This technology revolutionized since 1991 research achieving solar cell efficiency to 7% 8% [9]. Prof. Gra¨tzel et al. placed the milestone in photovoltaic conversion efficiency by incorporating monolayer of organic dye molecules as light absorber on the nanocrystalline wide band gap semiconductor titanium dioxide (TiO2). Since then, for the enhancement in cell efficiency, several strategies have been adopted by improving the main components of DSSCs such as optimizing the semiconductor, the electrodes, the electrolyte, and organic dye [10]. It has been found that the charge carrier transport properties are improved and cell efficiency has been enhanced to 11% just by introducing ruthenium-based organic dye on the surface of semiconductor alone [11,12]. So far, the highest efficiency of 14.30% was obtained by using complex liquid electrolyte cobalt (III/II) tris(1,10-phenanthroline) with co-sensitization of an organic dye in DSSC [13]. However, the high efficiency has been obtained by using volatile organic solvents as an electrolyte, which makes the development of sustainable and safe devices a significant issue. To resolve the stability issue, recently an effort has been made by incorporating solid state (SS) hole transport material by completely eliminating liquid electrolytes, though the efficiency

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drops to 11% [14]. Along with the long-term sustainability, the DSSC devices also face the issues like leakage and evaporation of liquid electrolytes at high temperatures [15]. Thus the photovoltaic technology including DSSCs and PSCs or lightgenerating devices like LED and LEC faces commercial feasibility issues. Researchers from across the world are in pursuit to improve device efficiency while resolving the stability and environment friendly problems also. Toward this way, ionic liquids (ILs) have gained huge popularity and gradually develop the keen interest among the global scientific community. ILs are ionic compounds made up of cations and anions in liquid form at ambient temperature, whose melting point is 100 C. So far, different types of ILs have been reported comprising organic cations viz. alkylammonium, alkyl pyridinium 1,3-dialkyl imidazolium, alkyl phosphonium, and BF42 ClO42, PF62, NO32, and AlCl42 as anions and many more [16 18]. Some of the factors behind the acceptance for ILs include low vapor pressure, nontoxic nature, high electrical conductivity, thermal stability, wide electrochemical window, and ease in structural modification that offers tunable physical and chemical properties. In particular, ILs are less volatile and inflammable and can be replaced by conventional volatile and flammable solvents and accepted as a green solvent in various applications [19 23]. The versatile set of physiochemical properties opens the wide range of applications as a “desired solvent” in new energy-related technology and improvising the existing one also. The optoelectronic conversion devices such as PSCs, LED, and DSSCs have been opting ILs to as a new material for their better performance and development thus gained extraordinary attention. Among them, the application of ILs has been emerged as a great substitute of organic solvents as an electrolyte in DSSCs and found to be more extensive. Due to the voluminous literature on application of IL in optoelectronic devices, this chapter reviews the research in growth and progress of ILs as an electrolyte in DSSCs done recently. The chapter mainly discusses the different types of Poly (ILs) (PILs) including copolymers, MWCNTs-based electrolytes used in DSSCs and tries to analyze the mechanism involved in ion transport for current generation with and without iodine redox species. To the best of our knowledge, the thorough analysis of PILs as a different form of electrolytes has not been discussed. Finally, it summarizes the challenges and future prospect of ILs for the practical applications in DSSCs.

15.2 Principle and structure of dye-sensitized solar cell The main components of a traditional DSSC consist of a rutheniumbased organic dye (black dye, N3, Z907, N719, etc.), a photoanode that

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GLASS Platine

Counter Electrode Electrolyte TiO2 particle with dye adsorbed onto the surface

TCO Photoelectrode Sunlight

FIGURE 15.1 Schematic diagram of dye-sensitized solar cell [10].

is made up of dye-sensitized particles on semiconductor TiO2 layer, a platinum (Pt)-coated cathode also known as counter electrode, and an electrolyte composed of triiodide/iodide (I2/I32) as a redox couple as shown in Fig. 15.1. In general, the IL-based DSSC combines a low viscosity IL (electrolyte) with a redox active salt that facilitates the rate of the redox couple reaction. The photon energy gets converted to electrical energy in a DSSC by following few basic steps and can be briefly explained as follows: The first step involves the absorption of photon energy by the dye-sensitized particles, and leads to the photoexcitation of dye molecules followed by the ultrafast injection of electrons to conduction band of the semiconductor TiO2 layer. In the next step, these injected electrons get collected at a transparent fluorine-doped tin oxide substrate via penetrating to the TiO2 layer. The electrons then flow through the external circuit to generate electrical energy and reintroduced at Pt counter electrode [24]. As in the electrolyte, the triiodide (I32) ions receive electrons at the counter electrode it gets reduced to iodide (I2) ion. In the last step, the (I2) ion regenerates the dye molecules (D1) through redox reaction. Thus, in this way the cycle continues repeatedly to generate current.

15.3 Role of ionic liquids as an electrolyte in dye-sensitized solar cells The electrolyte is a vital component and is crucial in the effective functioning of DSSC. The electrolyte provides the ionic conductivity between counter electrode and photoanode in DSSCs. It also facilitates the reduced redox couple reaction at the interface of photoanode and electrolyte, thus helps to regenerate oxidized dyes via the transfer of charged ions/redox species (I2 of I3/I2) [25]. Also, the electrolyte builds up a redox potential barrier differ from the Fermi level of TiO2 semiconductor layer, which is

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dependent on the value of open-circuit voltage (VOC) that determines charge flow in DSSCs [26]. Despite of achieving maximum efficiency by using volatile organic solvent as an electrolyte for traditional DSSCs, it hinders the commercialization of the solar cells. The organic liquid electrolytes cause severe technological problems that include lack of durability, longterm stability due to evaporation of solvents, and enclosed sealing of the cell at high temperature especially with long exposure of solar radiation [27,28]. The composition of electrolyte also affects the fast charge diffusion at TiO2 nanolayer and counter electrode thus maintain the energy conversion. Therefore various approaches such as the use of p-type inorganic semiconductors [29,30], organic hole-conducting materials [30,31], IL electrolytes [32 35], and polymer in the form of complex ionic salts or as a solid polymer or gel polymer have been extensively studied in the past years [36 39] and have found to be adopted to fabricate all SS or quasisolid state (QSS) electrolytes to achieve better cell efficiency in photocells. The use of p-type inorganic semiconductors and hole-conducting material offered poor efficiency, less stability due to carrier diffusion length of the dye-sensitized nanoporous TiO2 film, and imperfect filling [40]. Although, till date the highest achieved cell efficiency (13%) of DSSC has been obtained by Mathew et al. [35], using acetonitrile (ACN) as a liquid electrolyte with cobalt (II/III)-based redox species. However, the liquid electrolytes challenge the long-term performance and practical utility of solar cells [41]. Thereby, ILs have been considered to be the most appropriate choice as a liquid-based electrolyte owing to their extraordinary features described already. IL is being fluid at room temperature and faces leakage problem an unavoidable issue, that limits their practical usage in DSSCs. Contrary to them to meet the challenge of long-term sustainability and global environmental crisis, the gel polymer electrolytes (GPEs) and solid polymer electrolytes based ILs proved to have great potential to replace RTIL and organic solvents in DSSC [42]. Recently, there is a considerable growing interest that has been emerged in this new class of polymer electrolytes refer to as polyionic liquids: PILs [43,44]. PILs possess mechanical strength and provide good ionicity due to its own polar environment and thus efficiently transport the target ions without any mediator [45]. In the succeeding segments, we specifically focus the brief review on the recent research work progressed in PILs used as SS and QSS electrolytes in DSSCs.

15.3.1 Role of poly ionic liquids as solid or quasi-solid electrolyte in dye-sensitized solar cell For the applications of IL in polymer science, Lu et al. carried out extensive research to prepare high-conductive IL-incorporated polymer gels with

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different strategies [46]. PILs-based DSSC utilizes the concept of GPE, which involves PILs network dissolved in organic solvent or organic plasticizer in the presence of gelators [46]. The incorporation of ILs shows enhancement in ionic conductivity of GPEs thus results in efficient performance of DSSC [47]. A most commonly used host polymer in GPE science is polyethylene oxide (PEO). The popularity of PEO lies in its specific properties, which includes its good mechanical and chemical stability, acts as a good solvent. Although, the semicrystalline nature of PEO hinders the electron movement thus weaken the ionic conductivity and limits its usage in practical application [48]. Various studies suggest that imidazolium iodide IL has been preferred as an additive for IL polymers as an electrolyte. The incorporation of imidazolium-based ILs in PEO introduces the amorphous nature, promotes the charge transport through π π stacking, and thus enhances the ionic conductivity of polymer composite [47,49,50]. Kim et al. [51] successfully prepared the GPE by polymerizing PEO with imidazolium iodide salts as IL (PEOm-BImI). The photovoltaic parameters for different units of ethylene oxide (EO) at one sun illumination have been tabulated in Table 15.1. The long-chain effect of PEO has a significant effect on the photovoltaic performance of PEOmBIm-I-based liquid electrolyte. GPE was formed by adding 300 nanoparticles to liquid PEO13BIm-I electrolyte. GPE, PEO13-BImI based Q-DSSC obtained 5.25% of the cell efficiency, under AM 1.5 illumination (100 mW/cm2). The obtained open-circuit voltage, VOC 5 733 mV, and short-circuit current, JSC 5 10.71 mA/cm2 showed good photovoltaic performance and maintained conversion efficiency to 94% after 6 months of use. Syairah et al. [52] prepared GPEs incorporating imidazolium iodide ILs by varying alkyl chain into host polymer as PEO consists of two solvents, namely ethylene carbonate (EC) and propylene carbonate (PC). Different samples of GPE designated as S1, S2, and S3 were prepared by mixing PEO with ILs, namely 1-methyl-3-propylimidazolium iodide (MPII), 1-butyl-3-methylimidozium (BMII), or 1-hexyl-3-methylimidozium iodide (HMII), respectively. The study mainly focused on the effect of ILs with different alkyl chain on ionic conductivity and performance of DSSCs. The conductivity study reveals that the value of ionic conductivity increases on increasing the content of IL, which owes to the greater number of ions free to move thus enhancing the ionic conductivity of GPEs. The outcomes of the research performed also very well established a correlation between structure of IL and ionic conductivity. They out a decrease in conductivity with the longest alkyl chain of IL. Furthermore, the incorporation of low viscous IL S1 in the electrolyte increases the redox couple reaction and thus improves the ionic conductivity of GPE and performance of DSSC. The highest value of ionic conductivity (JSC) of 9.41 mS/cm and conversion efficiency of 9.35% were obtained for sample S1 with MPII, as compared to other samples consisting of BMI and HMII ILs (Table 15.1).

4. Electronic applications

TABLE 15.1 Literature reported photovoltaic performance data for Poly (ILs) as an electrolyte for solid state/quasi-solid state DSSCs. Poly(ionic liquids)

Gel electrolyte composition

VOC (V)

JSC (mA/cm2)

Fill factor (FF)

PCE, η (%)

Durability

References

PEO2BIm-I

PEOmBIm-I/MPN and hydrophilic silica nanoparticles

0.617

11.23

0.67

4.63

N.A

[51]

0.683

12.10

0.66

5.46

N.A

PEO13BIm-I 0.15 730 11.61 0.66 5.60

PEO13BIm-I/MPN liquid electrolyte 1 300 nanoparticles of aerosol

0.730

11.61

0.66

5.60

94% after 6 months

PEOMPII (S1)

PEO 1 MPII 1 NaI 1 EC 1 PC

0.559

24.55

68.1

9.35

NA

BMII(S2)

PEO 1 BMII 1 NaI 1 EC 1 PC

0.569

18.82

68

7.28

NA

HMII (S3)

PEO 1 HMII 1 NaI 1 EC 1 PC

0.564

20.80

67.1

7.87

NA

PVIM-W

Vinyl imidazolium iodide 1 PMMA 1 PEO 1 WO3 NPs

0.81 6 0.005

15.61 6 0.07

0.57 6 0.03

7.2 6 0.021

94.4% after 500 h

0.66 6 0.001

13.96 6 0.09

0.57 6 0.03

5.34 6 0.018

NA

PEO6BIm-I 0.15 12.10 0.66 5.46

PVIM

[52]

[53]

PAN

PAN 1 EC 1 PC 1 4.61 wt.% of LiI

0.65 6 0.03

21.0 6 1.1

5.4 6 0.1

NA

[54]

MPII

Poly(vinyl alcohol-co-ethylene) 1 dimethyl sulfoxide (DMSO), ruthenium dye (N719) 1 NaI 1 10 wt.% of MPII

0.67

9.97

65.39%

4.35%,

NA

[13]

ILP11

Poly(ethylene glycol) methyl ether methacrylate (POEM)of 10 wt.% 1 MEBIm-I 1 acrylonitrile 1 DMF

0.80 6 0.01

16.48 6 0.02

0.53 6 0.01

7.22

NA

[55]

ILPA431

Poly(ethylene glycol) methyl ether methacrylate (POEM) of 5 wt.% 1 MEBIm-I 1 acrylonitrile 1 DMF

0.82 6 0.02

15.99 6 0.01

0.56 6 0.01

7.57

NA

(Continued)

TABLE 15.1 (Continued) Poly(ionic liquids)

Gel electrolyte composition

VOC (V)

JSC (mA/cm2)

Fill factor (FF)

PCE, η (%)

Durability

ILPA422

Poly(ethylene glycol) methyl ether methacrylate (POEM) 1 MEBIm-I 1 acrylonitrile 1 DMF

0.81 6 0.01

15.19 6 0.01

0.54 6 0.01

6.85

NA

ILA11

MEBIm-I 1 acrylonitrile 1 DMF

0.82 6 0.01

13.58 6 0.02

0.58 6 0.01

6.66

NA

DMPII

DMPII 1 PEO 1 CAN 1 KI of 5 wt.%

0.71

14.11

0.59

5.87

NA

[56]

PEAII

Poly(1-ethyl-3-(acryloyloxy) hexylimidazolium iodide) 1 acetonitrile

838

9.75

65

5.29

85% after 1000 h

[57 59]

P-HI

1-Hexyl-3-methylimidazolium iodide (HMII) 1 1-allyl-3methylimidazoliumiodide (AMII) 1 P-HI of 20 wt.%

0. 643

15.10

0.72

6.95

NA

[60]

1-(2-Acryloyloxy-ethyl) 2 3methyl-imidazol-1-ium iodide (AMImI)

MWCNT/AMBImI

0.634

5.931

0.693

2.598

NA

[61]

MWNT-poly(AMImI)

0.630

3.294

0.733

1.681

Poly(AMImI)

0.630

2.367

0.778

1.156

POEI-II 1 ACN/MPN 1 LiI 1 I2

0.778

13.97

0.66

7.19

POEI II/MWCNTs

0.784

14.50

0.67

7.65

η maintained 1000 h

[BVIm] [TFSI] 1 I2 1 PMII 1 TBP

0.621

11.32

0.710

5.01

94% after 50 days

POEI-II

[BVIm] [TFSI]

References

[62]

[63]

399

15.3 Role of ionic liquids as an electrolyte in dye-sensitized solar cells

The recent work reported by M. Thomas et al. [53] incorporated the tungsten oxide nanoparticles (WO3 NPs) into PEO polymer matrix. The introduction of nanoparticles increases the dimensional integrity, ionic conductivity, interfacial stability, and thus power efficiency of PIL electrolytes. They partially grafted methyl methacrylate (MMA) and vinyl imidazolium iodide on PEO to prepare PIL and designated as poly(ethylene oxide vinylimidazolium methyl methacrylate iodide) (PVIM), which then incorporated into WO3 NPs via electrospinning to form a polymer nanocomposite membrane (PVIM-W) as an electrolyte for QSS-DSSC. Fig. 15.2 exhibits the current density-voltage (J-V) plots obtained for DSSCs, fabricated with liquid, PVIM, PEO, and PVIM-W. PCE (7.2%) of PVIM-W-based DSSCs was obtained to be more as compared to liquid electrolyte (6.85%) and attained a long-term stability (94.4%) and is attributed to the increase in the values (shown in Table 15.1) of JSC and VOC. The incorporation of NPs increases the ionic conductivity and provides the pathway for ion diffusion by providing free volume in the polymer network; MMA reduces the crystallinity of PEO and increases the diffusion of I2 ions; and the presence of nanofibers of certainly small diameter in the PVIM-W provides the easiness for the transportation charge carriers, therefore increasing JSC, VOC, and efficiency of PVIM-W-based DSSC. Apart from PEO-functionalized IL polymers, polyacrylonitrile (PAN) is another type of polymer that has been employed as a host matrix to prepare gel polymer [54,64,65]. The high dielectric constant of PAN

16 PVIM-W

Jsc /mA cm-2

14 12

LE 10 PVIM 8 PEO

6 4 2 0 0.0

0.2

0.4

0.6

0.8

1.0

V/V FIGURE 15.2

Current density-voltage (J-V) curves of prepared DSSCs with the liquid and PEO, PVIM, PVIM-W membrane as electrolyte [53].

4. Electronic applications

400

15. Progress in optoelectronic applications of ionic liquids

assists the doping salts to dissociate into free ions thus increasing the conductivity in polymer gel electrolytes. Also, the known compatibility of PAN with polar solvents provides easiness to form the polymer gel electrolyte. Furthermore, the PAN polymer also holds more free cations because of the polar characteristic of nitrile groups. Therefore, to form gel electrolytes, PAN with these specific properties believed to be a potential candidate as a host polymer [66,67]. Arof et al. [54] reported PAN-based gel electrolytes by selecting different ionic salts to observe the improved performance of DSSCs. The study involves the preparation and characterization of PAN-based seven GPE samples with varying contents of lithium iodide (LiI) (0 5.28 wt.%) keeping the amount of tetrapropylammonium iodide and BMII fixed. The effect of LiI on the ionic conductivity and PV performances of GPEs for DSSCs were investigated. The study reveals that adding LiI improves the performance parameters of DSSC as shown in Fig. 15.3. The enhancement in conductivity for DSSC with S6 in comparison to the DSSC with S1, without LiI. On adding 4.61 wt.% of LiI content in S1 improves JSC to (21.0 6 1.1) mA/cm2 from JSC (10.1 6 1.1) mA/cm2 and the corresponding decrease in VOC to (0.48 6 0.02) V from (0.65 6 0.03) V has been observed as shown in Fig. 15.3. The accumulated Li 1 cation on the TiO2 surface that leads to the shift of conduction band to positive potential thus enhancing the electrotransfer from excited state of dye to the conduction band of TiO2 semiconductor and thereby increasing the conductivity. Further increasing the LiI content beyond 4.61 wt.% reduces the number of Li 1 cations on the TiO2 layer and thus decreases the mobility and charge number density of electron in electrolyte for the DSSC with S7 electrolyte. Thus the slight addition of LiI salt in S7 electrolyte lowers the conductivity, JSC, to (17.0 6 1.7) mA/cm2 and enhancing the VOC by 0.04 V. Among all the prepared electrolytes, the DSSC with electrolyte containing 4.61 wt.% of LiI also shows the best photovoltaic performance. The PCE for S7 electrolyte obtained was (5.4 6 0.1)%, that is attributed to the highest value of JSC of the DSSC. From the application purpose, the DSSCs containing 4.61 wt.% LiI content have been utilized to design the prototype solar panels. The solar panels worked continuously for several weeks and delivered average PCE of (3.7 6 0.2)% with corresponding output power (17.1 6 0.9) mW. In the quest of improvisation in achieving photoconversion efficiency of SS-based polymer electrolyte copolymerization by introducing copolymers as a host in the electrolyte matrix gained much attraction [13]. The presence of polar groups in copolymer poly(vinyl alcohol-co-ethylene) abbreviated as PVA-co-PE promotes the ions transportation and thus increases the ionic conductivity. Since gel polymers drop the density of free charge ions in electrolyte as polymers do not diffuse into ions by itself, thus ILs have been incorporated as an additive in a gel

4. Electronic applications

Short circuit current density, Jsc / mA cm-2

24 21.0

20

19.3 17.0

16 14.9 12.3

12

11.2 10.1

8

Open circuit voltage, Voc / V

0.7 0.65 0.61

0.6

0.58 0.54 0.52

0.5

0.52 0.48

0.4

Fill factor, FF / %

70

60 58.1

56.9

56.0 51.6

51.2

50

53.4

53.0

40

Power conversion efficiency, PCE / %

6.0 5.4 5.1

5.0

4.6

4.0

4.0

3.9

3.8

4.1

3.0 0

1

2

3

4

5

6

Lil content / wt.% FIGURE 15.3 Variation of photovoltaic parameters JSC, VOC, FF, and PCE with LiI content of PAN-based DSSC [54].

4. Electronic applications

402

15. Progress in optoelectronic applications of ionic liquids

electrolyte. C.Y. Tan et al. [68] in their recent work prepared IL-incorporated GPE using different amounts of MPII and PVA-co-PE to synthesize DSSCs. The PV performance of DSSC improves with the increase of MPII in PVA-co-PE based GPE, attributed to the greater number of free ions that lead to improvise the ionic conductivity and thus cell efficiency. The maximum value of JSC reaches for IL7.5 (10.09 mA/cm2), although IL 10.0 depicts a slight decrease in current density due to experimental uncertainty. The maximum photoconversion efficiency of 4.35% has been achieved by GPE with 10 wt.% of MPII IL. The PV performance parameters have been displayed in Table 15.1. The DSSC fabricated with PVA-co-PE based GPE shows significant improvement in cell efficiency by employing MPII as an additive when compared with other DSSCs has been reported in literature [69,70]. Although the excessive addition of MPII IL (up to 15 wt.%) lowers down the efficiency of DSSC which may be due to the ion accumulation. Inferring from the data obtained, the 10 wt.% of MPII IL is in optimum range to achieve the maximum efficiency and better PV performance of PVA-co-PE based GPE DSSC. The mobility of ions, which has been influenced by the chemical structure of IL copolymers in gel electrolyte, is important attribute for achieving better photovoltaic performance of DSSCS. Recently, S. K. Tseng et al. [55] prepared a series of imidazolium IL copolymers through copolymerizationcontaining monomers (MEBIm-I), acrylonitrile, and polymer (ethylene glycol)methyl ether methacrylate (POEM). The as-synthesized copolymers, namely ILA11, ILP11, ILPA422, and ILPA431, were obtained through co-polymerizations of MEBIm-I, POEM, and acrylonitrile with varying molar concentration. They investigated the dependency of chemical structure and composition of IL copolymers on the ionic conductivities, diffusion coefficient, and photovoltaic performances. The ionic conductivities of IL copolymer solution-based electrolytes found to be low as compared to the pristine electrolyte, as gel electrolytes have higher viscosity. Among other copolymers under study, the IL copolymers ILP431and ILP11 achieved the highest conductivity. The higher amount of POEM and MEBIm-I in imidazolium-based IL polymer electrolyte accompanied by ethylene oxide, promoted the I2 ion charge mobility, and thus increases the ionic conductivities and diffusion coefficients in gel electrolytes. They have fabricated DSSCs, namely DSSC III-20, DSSC III-10, DSSC III-5, DSSC V-10, DSSC IV10, DSSC II-10, and DSSC I, by incorporating IL copolymer solution-based electrolyte. The PV performances were investigated to study the effect of varying content of ILPA431-based gel electrolyte by 5, 10, and 20 wt.% for different solar cell devices, figure is provided for reference (Fig. 15.4). The obtained values under AM 1.5 G illumination, 100 mW/cm2 of JSC, VOC, and PCE have been summarized in Table 15.1. It has been found that the current density-voltage (J-V) curves for DSSC III-5 show the almost similar

4. Electronic applications

403

15.3 Role of ionic liquids as an electrolyte in dye-sensitized solar cells 0

DSSC III-5 DSSC III-10 DSSC III-20

Current density (mA/cm2)

-2 -4 -6 -8 -10 -12 -14 -16 -18 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Voltage (V)

FIGURE 15.4 J-V characteristics for DSSCs incorporating 5, 10, and 20 wt.% ILPA431based liquid electrolytes [55].

behavior those obtained for DSSC III-10 (Fig. 15.4). Due to the higher content of ILPA431 (20 wt.%) contained in DSSC III-20 exhibits lower PV performance as compared to the lower content of DSSC III-5 and DSSC III-10. Poor performance displayed by DSCC III-20 was attributed mainly due to high viscosity of ILPA431 (20 wt.%) that leads to slow down the I2 ion diffusion, thus recombination resistance (Rrec) and diffusion resistance (Rd) increase, resulting in lowering the JSC value. The DSSC III-5 observed to have the highest value of VOC (0.82 6 0.02 V) and thus shows the enhancement in PCE to 7.57%. The comparable (J-V) curves for DSSCs prepared solution-based electrolyte IL copolymer have been shown in Fig. 15.5. As can be seen from the curve, the PV performances of devices prepared with IL copolymer were comparable to liquid electrolyte-based DSSC I. They observed the effect of different chemical composition of IL copolymers on the photovoltaic performances of DSSCs. In this array of study, the highest conductivity JSC (16.6 mA/cm2), catalytic activity ionic diffusion coefficient, and higher PEC (7.5%) for the DSSC III-10 with ILPA431 electrolyte in comparison to other solution-based electrolytes like ILP11-based DSSC II-10 have been observed. The enhancement in catalytic activity shown by ILPA431 solution-based electrolyte that has been associated with the lower viscosity due to suitable units of acrylonitrile, which promoted the chain mobility. Although in the IL polymers ILPA422 and ILA11 large amount of acrylonitrile suppresses the ionic conductivities and diffusion of ions and their PV performance were found to be inferior to the ILPA431-based DSSC III-10.

4. Electronic applications

404

15. Progress in optoelectronic applications of ionic liquids 0

DSSC I DSSC II-10 DSSC III-10 DSSC IV-10 DSSC V-10

-2

Current density (mA/cm2)

-4 -6 -8 -10 -12 -14 -16 -18 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Voltage (V)

FIGURE 15.5 J-V characteristics for DSSCs incorporating liquid electrolytes based on IL copolymers [55].

15.3.2 Iodine-free PIL as an electrolyte in dye-sensitized solar cells Polymer electrolytes consisting of iodide salts corrode metals, that is, counter electrode, and have volatile properties, which can decrease the durability of the cell. Recently to overcome the constraints caused few iodine-free electrolytes have been reported [56,71,72]. Chen et al. [56] developed iodine-free GPE by combining IL (DMPII, 1,2-dimethyl-3propylimidazolium iodide) as an intermediate for transferring charge along with PEO, potassium iodide (KI), and ACN for synthesizing DSSCs. Unlike the traditional charge transfer mechanism to create triiodide species, KI reacts with oxidized dye and promotes the charge carriers by producing more free mobile ions (I2) and also avoid crystallization of PEO. On experimenting with the preparation of electrolyte (solution of DMPII 1 PEO 1 ACN), they observed the formation of aggregates above the 7 wt.% of KI content, therefore kept the percentage of KI salt in optimum range (0 7 wt.%). They investigated the effect of different concentrations of KI contained in quasi-state iodine electrolyte on the ionic conductivities and PV performances. They obtained the enhancement in ionic conductivity to 16.32 mS/cm with concentration of KI to 5 wt.% from 6.97 mS/cm at a concentration of 1 wt.%, confirming the enhancement of ionic conductivity of electrolyte is depending on the transport mechanism of I2 ions. On further increasing the content of KI to 7 wt.%, the PEO-based electrolyte achieved the highest conductivity of 18.68 mS/cm although achieved at the slower rate as charge

4. Electronic applications

405

15.3 Role of ionic liquids as an electrolyte in dye-sensitized solar cells

Photocurrent density / mA cm-2

carriers may got saturated. As mentioned earlier, the enhanced concentration of KI in electrolyte facilitates more I2 ions thus regenerating oxidized dye that leads to increase in the short current (JSC) and decrease in VOC. With the increase in KI concentration from 1 to 7 wt.% in PEObased iodine-free electrolyte, they recorded increase and decrease in JSC and VOC from 10.21 to 15.00 mA/cm2 and from 0.73 to 0.68 V, respectively. The maximum PCE of 5.87% was achieved for an iodine-free PEO electrolyte with 5 wt.% content of KI for a DSSC (Table 15.1). A slight decrease in the cell efficiency to 5.72%, with the lowering of other parameters as well viz VOC and JSC has been observed [56]. A series of iodine devoid SS PIL electrolytes were synthesized by Wang et al. [57 59]. In the process, they substituted different alkyl chains using poly(1-alkyl-3-(acryloyloxy) hexylimidazolium iodide) (PAAII) to fabricate ss-DSSCs. Polymerization of monomers consists of ionic moieties form PAAII, an IL polymer. The attraction between the imidazolium cation and iodine anion is weak, because the latter is bound to the main chain of PAAII thus diffusion of iodide ion can only occur in electrolyte thereby acting as an intermediate for the charge transport from counter electrode to oxidizing dyes. The PV performance of DSSCs has found to be influenced by the alkyl chain attached to main chain of PAAII, the result has been displayed in Table 15.1. The current density-voltage curves obtained with illumination of 100 mW/cm2 for DSSCs have been illustrated in Fig. 15.6. Among them the DSSC with PEAII poly(1-ethyl3-(acryloyloxy)hexylimidazolium iodide) (Polymer B), that is, as a SS

10 8 6 Polymer B 4

Polymer A Polymer C

2

Polymer D

0 0

200

400

600

800

1000

Voltage / mV FIGURE 15.6 Current density-voltage curves of Poly (IL) PAAI electrolyte based ssDSCs using PAI as the electrolyte under AM 1.5 illumination (100 mW/cm2) [58].

4. Electronic applications

406

15. Progress in optoelectronic applications of ionic liquids

electrolyte achieved maximum conversion efficiency of 5.29% under 100 mW/cm2 (AM 1.5). DSSCs with PEAII electrolyte without sealing maintain good stability of almost 85% of their initial efficiency even after 1000 hours. Furthermore, they investigated the effect PAAII as a mono-ion transport electrolyte on the PV performance of DSSCs assembled with PEAII. It is to be mentioned here, mono-ion transport electrolyte is based on PIL without iodine. Wang et al. [58] carried out their study to vary the concentration of iodine in PEAII and recorded PV performance data. The result has been exhibited in Fig. 15.7; the obtained J-V curves clearly indicate that with increasing concentration of iodine in electrolyte, both JSC and VOC decrease. The increase of iodine in the electrolyte leads to the accumulation of I31 ions on TiO2 layer that promotes the recombination of injected electrons with triiodide, thus decreases the VOC across the cell. Also, the efficiency of PACB EOI based SS solar cell has found to be decreased with the increase of iodine concentration by Ikeda et al. [73]. Thereby they come to a conclusion that the DSSCs fabricated by them perform better with iodine-free electrolyte and for the functioning of SS DSSC, iodine need not to be required as an initial component of electrolyte. However, the PCE achieved for DSSCs employed with SS electrolyte due to lesser charge transfer and improper interfacial contact between TiO2 electrode and the electrolyte [74,75].

Photocurrent density / mA cm-2

10 8 6 4 2

e d

0 0

200

400

600

c b

a 800

Voltage / mV FIGURE 15.7

Current density-voltage characteristics of Poly (IL) PEAII electrolytebased DSSCs with various contents of iodine [58].

4. Electronic applications

15.3 Role of ionic liquids as an electrolyte in dye-sensitized solar cells

407

The acidic PIL P [((3-(4-vinylpyridine)propanesulfonic acid)iodide)-co(acrylonitrile)], named as (P-HI), was synthesized by Fang et al. [60]. The novel IL polymer has been incorporated with IL electrolyte to fabricate DSSCs. The aforesaid acidic polymer P-HI formed is devoid of iodine instead, containing of sulfonic acid group that gets bonded with ILs via strong electrostatic forces and forms the continuous and homogeneous framework, facilitating the redox couple reaction in electrolyte has strong electronegativity upon ionization that shows compatibility with hydrophilic ILs. The performance of acidic IL electrolyte/ionic PILs for DSSCs has been evaluated with varying content of P-HI and iodine content. The highest cell efficiency of about 6.95% has been obtained for DSSC based on the polymer composite electrolyte with 20 wt.% of P-HI illuminated under 100 mW/cm2 (AM 1.5) (Table 15.1). The PV performance has found to be improved on adding P-HI in electrolyte. It has been observed that JSC raises from 13.05 to 15.30 mW/cm2 although the electrolyte becomes more viscous. They also observed the influence of iodine on the performance of DSSCs. They found that the addition of iodine decreases the value of VOC primarily because of increase in dark current and a slight reduction in JSC when the concentration of iodine was up to 4.34% absorption of visible light by triiodide ion species in the electrolyte [72]. Thus they too concluded that the acidic P-HI-based DSSCs work best with iodine-free electrolyte. To improve the charge transportation in QSS electrolytes, incorporation of carbon nanotubes (CNTs) or multi-walled carbon nanotubes (MWCNTs) gained potential to fabricate highly efficient DSSC as they exhibit good thermal stability, mechanical strength, high electrical conductivity, and good option as an electrolyte for the redox coupling of triiodide/iodide [76,77]. By utilizing radical polymerization method, Chang et al. [61] synthesize an IL monomer poly(1-2-acryloyloxy-ethyl)-3-methyl-imidazole-1-ium iodide poly(AMImI). They grafted MWCNT-poly (AMImI) by utilizing physical adsorption denoted as (MWCNT/AMBIm) using poly(AMImI) to fabricate SS DSSC. The DSSC with poly(AMImI) yields 1.156% of PCE and MWCNT/AMBImI with 0.05 wt.%, attained the maximum cell efficiency to 2.598% (Table 15.1). The conversion efficiency rises from 1.16% to 3.55% as the amount of MWCNT/AMBImI varied from 0 to 0.05 wt.%. Notably, the presence of MWCNT/AMBImI and MWCNT-poly(AMImI) has promoted the diffusion of I32 in electrolyte leading to the increase in JSC. Chang et al. [49] reported to synthesize a novel gel electrolyte poly(oxyethylene)-imide imidazolium iodide (POEI-II) to fabricate QSS DSSC. The POEI-II GPE is one of its kind, having dual functional group that promotes the chelation of Li1 cations in the electrolyte, thus enhances the VOC and manifested the strong π π interactions with I2 ions thus improving the ionic conductivity of POEI-II. The POEI-II is highly soluble in both organic solvents and in aqueous solution because of the presence of hydrophilic site of POE and aromatic imides of POEI-II functional group. The POEI-II gel

4. Electronic applications

408

15. Progress in optoelectronic applications of ionic liquids

electrolyte based QSS-DSSC shows excellent cell efficiency of 7.19% with significant PV performance values as 778.2 mV and 13.97 mA/cm2 recorded for VOC and JSC, respectively. Furthermore, improvement in the cell efficiency by 7.65% and in values of VOC 5 784 mV and JSC 5 14.50 mA/cm2 has been observed by employing the POEI-II GPE into MWCNTs for the corresponding DSSC. The enhancement shown by MWCNTs/POEI-II gel electrolyte has been credited to the extended transfer of charge exhibited by MWCNT that promotes the charge transfer via redox mediator and thus assist the oxidized dye to regenerate. The efficiency of MWCNTs/POEI-II GPE-based DSSC was checked on the daily basis and showed unmatched durability even after 1000 hours, for storing at 50 C while the cell suffers a loss of 72% with organic solvent electrolyte. With the continuation of previous work, an iodide-free PIL functional group of selenocyanate (POEI-IS) was synthesized as a gel electrolyte by the same group for a QSS DSSC [62], obtained the efficiency of 8.18% and achieved 95% stability for 1000 hours. The synthesis route via in situ thermal polymerization reported by Song et al. [63] of IL monomer 1-butyl-3-vinylimidazolium bis(trifluoromethanesulphonyl) imide in short [BVIm][TFSI] and ILs mixture to prepare gel electrolytes [PBVIm][TFSI] have been reported without using volatile solvent for developing Q-DSSCs. The Q-DSSCs based on [PBVIm][TFSI] GPE obtained 5.01% of maximum efficiency under one sun illumination. It is to be noted that the DSSCs maintained 94% of their initial efficiency after 50 days soaked at 60 C. The long-term stability attained by [PBVIm][TFSI] GPE-based DSSC without volatile solvent overcomes the issues like leakage problem and instability of liquid electrolytes.

15.4 Challenges and future prospects ILs are molten salts containing anion and cation species that recently found to have applications in various optoelectronic energy conversion devices. In particular, DSSCs have been widely investigated for the future energy efficient device. The commercialized applications of DSSCs have found to be hindered due to the use of organic solvents as an electrolyte. In the past few years, ILs preferred over volatile organic solvents and emerged as a new class of electrolytes due to their low volatility, low vapor pressure, high ionic conductivity, thermal stability, and many more. In the earlier studies, liquid-state ILs have been explored, showing high efficiency of DSSCs. But the cell durability has still been an issue due to the fluidity and long-standing stability of liquid electrolytes. To overcome this, SS and QSS electrolytes have been introduced. Wherein, the polymer chain has been incorporated into the ILs to synthesize various PILs. The performance of PIL-based SS and

4. Electronic applications

References

409

QSS DSSCs showed noteworthy improvement in their photovoltaic performance, up to 9.53% efficiency. The DSSCs using PIL resolve the long-term sustainability issue, which has been missed in case of organic solvents based electrolytes. Despite the PIL-based DSSCs have successfully attained the sufficient cell efficiency but it is still low as compared to organic solvents. The proper functioning of DSSCs require both; long-term sustainability as well as high efficiency, thus the efficiency of solar cells needs to be improved. It has been studied that the incorporation of ILs into polymer matrix enhances the ion transport within an electrolyte. It has been found that imidazolium-based IL is the prevalent choice of most of the studies. To improve, the charge transport incorporation of MWCNTs into PILs has also been tried, although the efficiency is still low while maintaining cell durability [62]. Based on the review, most of the studies have been emphasizing on improving the cell efficiency without mentioning and testing the cell durability test, this needs to be considered. Furthermore, the iodine-free ILs are the good choice for the future PILs. The PV performance of solar cell shows better response with iodine devoid electrolytes. Thus the exploration of new ways and different combinations of ILs need to be made. It is to be mentioned that the usage of ILs as a source of ions to improve ion conductivity and thus enhancing the efficiency of energy conversion devices is an excellent choice.

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C H A P T E R

16 Role of ionic liquids and their future alternative toward protein chemistry Mansi Chaudhary1,2, Shweta Pal1,2, Kamlesh Kumari3, Indra Bahadur4, Geetu Gambhir5 and Prashant Singh2 1

Department of Chemistry, Atma Ram Sanatan Dharma College, University of Delhi, New Delhi, India, 2Amity Institute of Applied Sciences, Amity University, Noida, India, 3Department of Zoology, University of Delhi, New Delhi, India, 4Department of Chemistry, Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Natural and Agricultural Sciences, North-West University, Mmabatho, South Africa, 5 Department of Chemistry, Acharya Narendra Dev College, University of Delhi, New Delhi, India

16.1 Introduction Proteins are enormous, complex, and hereditarily heterogeneous restricting biomolecules in nature, and are connected by an amide covalent bond (
CO
NH
) [1
3]. The alteration of a protein is characterized in the form of an adjustment that includes the arrangement or loss of covalent bond, like deamidation. Definite fragility alludes to the protein and so the misfolding and conglomeration. Protein aggregation is viewed as more dangerous because it can happen during any phase of preparation and medication, etc. It can influence the safety and pharmacokinetics of protein
drug interaction because the activity of the protein is firmly connected to its forms. It can determine any slight change to a protein’s shape/structure, which may cause the protein to become

Advanced Applications of Ionic Liquids DOI: https://doi.org/10.1016/B978-0-323-99921-2.00013-6

417

© 2023 Elsevier Inc. All rights reserved.

418

16. Role of ionic liquids and their future alternative toward protein chemistry

dysfunctional or denatured [4
12]. Ionic liquids (ILs) are simple organic salts having melting points below the boiling point of water, that is 100 degree celcius. They are made of organic cations and the anions can be organic or inorganic and the types ILs are given in Fig. 16.1 [13
17]. The extraordinary properties of ILs are less volatile, heat stability, tunable consistency, and compatibility with water and organic solvents. Further, they possess ideal solvation properties, high thickness, low toxicity and are nonionizing [12,15,18
25]. Ions constituted the ILs can transform the property and the ability of ILs are per the interest or the applications. The cationic and anionic parts of ILs depend on the selectivity of specific properties like hydrophobicity and others. On account of the tunable properties of ILs, they are utilized as solvents for possible applications. The presence of common ILs demonstrated moderate polarization because of the delocalization of the sub-atomic charge. ILs are of two kinds; simple salt and paired ILs. The common cations in ILs are hexafluorophosphate, ethylsulfate, 1-alkyl-pyridinium, 1,1-dialkylpyridinium,1-ethyl-3methylimidazolium, 1,3-dialkylimidazolium, tetraalkylammonium,

1. Task specific ILs

10.

2. Bio ILs

Proc ILs

3. Amphiphile ILs

9. Chiral ILs

ILs

8.

4.

Deep Eutecc Solvents

Metal Salts ILs

7. Supported ILs

6. Polarizable ILs

5. Switchable Polarity Solvents

FIGURE 16.1 Types of ionic liquids.

5. Miscellaneous applications

419

16.1 Introduction

tetraalkylphosphonium, 1-alkylpyrrolinium, 1,3-dialkyltrizolium, hexaalkylgyaninium, and 1-alkylthiazolium; and the anions in ILs are chloride, bromide, iodide, isothiocyanate, [ethyl amino]
, [p-MeC6H4SO3]2, [CF3CO2]2, [OCN]2, [N(CN)2]2, [Al(Et)Cl3]2, [Al2Cl7]2, [Al3Cl10]2, [Al2(Et)2Cl5]2, [Al(CH2CF3)4]2, [MeCO2]2, [SbF6]2, [P(C2F5)3F3]2, [BF4]2, [(RO)2PO2]2, [ROSO3]2, [B(oxalate)2]2, [B(C6H44CF3)]2, [C(CN)3]2, [F(HF)n]2, [(CF3SO2)2N]2, [CF3SO3]2, [ROSO3]2, [(RO)2PO2]2, and [PF6]2 [2,3,13,21,26
30]. Some of the systems (ILs with other molecules/biomolecules), properties, methods, and applications of the ILs are mentioned in Fig. 16.2. There are three types of ILs: the physical and chemical properties like thickness, consistency, solvency, safety are considered in first generation. The second generation is significant for adjustable properties of ILs and the third generation is the new age including drug fixings. The plan of dynamic drug fixings as ILs is to give creative arrangements in the new remedy and conveyance choices [13,21,26
29,31
33].

Systems: IL – proteins IL – nanoparticles IL – water IL – carbohydrates IL – lipids

Applications:

IL - drugs

Drug delivery Drug synthesis Solvents Green chemicals

Applications Ap ns of ILs

Medical analytics

Properties: Structure Spectra Energy Dynamics Biological activity toxicity

Methods: Physicochemical methods (solubility, stability, etc.) Biological assays Theoretical approaches (molecular dynamics, DFT, semi-empirical)

FIGURE 16.2 Some of the systems, properties, methods, and applications of the ionic liquids.

5. Miscellaneous applications

420

16. Role of ionic liquids and their future alternative toward protein chemistry

16.2 Antibacterial and antitumor activities of ionic liquids ILs exhibit good activity against gram-positive and gram-negative bacteria, parasites, and green growth. ILs based on quinoline ring have a promising action against microorganisms. Cations like imidazolium, phosphonium, and ammonium ions of ILs act potentially against malignancy. The chain length of alkyl at the 3-position of the imidazole ring assumes a significant role in the antitumor and cytotoxicity of these ILs. ILs are applied in the biomedical and biotechnology area. Proteins interact with an IL to secure or get steadiness and found to chemically reactive. Several ILs are found to be efficacious in biotechnology because of solvency and being less toxic. Some of the ILs can be used as potential solvents that help to break the nonpolar substrates with unalterable protein movement. Henceforth, these ILs are considered as greener solvents in comparison with many other traditional organic solvents like hexane, methonol [34
37]. The potential applications and use of ILs are presently very much perceived. In this situation, ILs have step by step supplanted the picture of the stability of the protein in conventional traditional solvents. ILs are synthetically made of ions and are considered green alternate to hazzardous volatile organic compounds. Since 1990s, investigations are reported on the ILs interface with other materials including biosegments to recognize compounds. In any case, there is less promising significant strategy for predicting the properties of biomolecules with ILs. The impact of salts on the structure of water in the interfaces and the characterization of compounds that influences interfacial surface properties have been explored. The ions in ILs are as kosmotropes or chaotropes depending on their abilities to motivate the structuring of water. Some ions (kosmotropes) having high charge densities have harmful effects. According to definition, the Hofmeister arrangement greatly affects a wide scope of curiosities; are still unexplained in protein collapsing/ unfurling properties [28,38
42]. ILs are another class of biocompatible solvents in the biological field. They have versatile properties that make them a reason of primary curiosity to all scientists and bioengineers for the thermodynamics values. The physicochemical properties of ILs are exceptionally important to the structure and nature of the cations and anions. Various applications of ILs are reported and range from catalysis, synthesis of nanomaterials, and polymerization applications. The utilization and possible uses of ILs are quickly expanding for homogeneous biphasic catalysis and extractions. Mobility, structure, and strength of proteins in watery medium with ILs have been discussed [38,42
50]. ILs are considered organic solvents in synthesis and other applications. They became the solvent of interest because of their potential properties,

5. Miscellaneous applications

16.3 Protein instability and its influencing factors as well as analytical monitoring

421

recyclability and reusability with no or minimum loss of the activity. ILs have numerous properties like low vapor pressure, high ionic conductivity, wide liquid reach, high dependability, and capacity to break up a range of properties. Novel insightful uses of ILs are increasing expontially with time and with huge interest. In an assortment of investigation modes, ILs have been shown to incorporate electrochemically, partition, extraction, mass spectrometry, and detection [19,25,51
56]. ILs have likewise arisen as a remarkable interest in nonwatery enzymatic ability. The additional ability with ILs is their physicochemical properties and they can be made adjustable by chainging cations and/or anions. ILs can be mixed with water-miscible or immiscible compounds with various polarities, and viscosities. Researchers have just demonstrated biocatalyst action, enantioselectivity, warm steadiness, and reusability of ILs. Numerous enzymatic responses like the combination of Z-aspartame by thermolysis, alcoholysis, ammonolysis, and hydrolysis utilizing lipase, transesterification utilizing chymotrypsin, have been accounted for in ILs. Careful investigation of these enzymatic responses has uncovered that the physicochemical properties of the ILs assume a significant part in the change of the structure, steadiness, and action of the proteins. Numerous ILs (with anions BF42, PF62, and Tf2N2) showed less twisting properties of organic solvents; high enantioselectivity, and high reactant movement [19,25,51
60]. Henceforth, it is important to have a comprehension of the impacts of IL like safety, action, and primary/conformational structure of proteins, before exploring an IL as a dissolvable vehicle for enzymatic responses [48,49,59
62]. Researchers have studied the impact of ILs on the changes in the structure of proteins. The utilization of ILs as a responsible medium offers a solution for being the soluble and reusable solvent. ILs containing imidazolium cations can be utilized as a media in some ground-breaking synergist effects, to quicken the response rate with improved selectivity. ILs have been broadly lauded for being reasonable for a wide scope of natural responses. It has abilities like controlling, the higher rate and reactivity, efficient, faster/ rapid, and reusing. ILs offer wide reach contingent upon cations and anions physicochemical properties [44,45,48,49,59
66].

16.3 Protein instability and its influencing factors as well as analytical monitoring The most important troublesome issue in the improvement of proteins is to manage their physical and chemical abilities/stability. The well-known indeterminate factor is protein accumulation, which can be influenced by an assortment of components and chemical changes. A cautious assessment of these components affecting stability can help

5. Miscellaneous applications

422

16. Role of ionic liquids and their future alternative toward protein chemistry

to moderate certain safety issues. Besides, the choice of proper techniques and sufficient investigation for effective and precise scoring of protein shakiness can guarantee fruitful improvement in protein [40,44,45,48,49,59
64]. Under specific conditions, the tertiary and quaternary structure of a protein can be tunes along without deceasing the potency and manipulated immunogenicity (Fig. 16.3). Dang et al. reported the arrangement of lysozyme crystals inside the presence of 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim]BF4),1-butyl3-methylimidazolium chloride ([C4mim]Cl), 1-butyl-3-methylimidazolium bromide([C4mim]Br), and 1,3-dimethylimidazolium iodine([dmim]I) and the impacts of ILs on the structure and the difference in the lysozyme action was described by Fourier change infrared spectroscopy. It tells about the details or amount of the alpha-helix and beta-lysozyme sheet. They find an expansion in-band close to 2935 and 1656 cm21. A change in the amide III at 1230
1270/cm21 in the expansion of [dmim]I, showing a change from beta-sheet to irregular loop lysozyme. For [C4mim]BF4, [C4mim]Cl, and [C4mim]Br, the alpha and beta helix-sheet structures are dominating for lysozyme. The study indicated that the usage of the activity of the ILs brought a hopeful impact. Particularly, [dmim]I found to be most promising IL in stabilization [22]. Patel et al. [33] investigated the strategies utilized for the investigation of the IL interface, and is helpful for the information about the

FIGURE 16.3 Importance of aqueous ionic liquids liquids for better solubility of proteins.

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16.5 Effect of cations and anion of ionic liquids on the stability of proteins

423

primary and conformational elements of the protein. This can be valuable to expand information about the conduct of ILs on the stability and action of proteins. The impact of the physicochemical properties of ILs like hydrogen holding capacity and hydrophobicity on protein steadiness are examined [33,62].

16.4 Effect of alkyl chain length of cations of ionic liquids on the stability of proteins The length of the alkyl chain in cation of ILs significantly affects the activity and stability of the enzyme/proteins. It was also noted in few reported work that the long alkyl chain in the ILs has a destabilizing impact on the proteins. For instance, ILs having long-chain (decyl)based imidazolium cations; may be destabilizing the bovine serum albumin (BSA).

16.5 Effect of cations and anion of ionic liquids on the stability of proteins There is a need to study the impact of cations and anions present in ILs on their interaction with the proteins to understand its stabilization/ destabilization or the change in structures of proteins. The Hofmeister arrangement of ions affects a collaboration between ILs and the proteins. The coordination relies upon the kosmotropic and chaotropic properties of ILs. Hofmeister series detailed the ability of the ions for the salt formation on mixing with proteins. It has been shown that the Hofmeister arrangement has general utility in organic chemistry and other material sciences, colloids, polymer and surface science, etc. The impact of the ILs on protein has been inspected by various research groups. A new report portrayed mushroom tyrosinase within the sight of ILs with both cation and anion supporting the activity of the ILs in influences of chemical execution [33,62,67
69] (Fig. 16.4). Tadeos et al. [48] examined the impact of the anions of the Hofmeister arrangement on the protein strength. Changes in stability are acquainted with the transformation related to the region of the dissolvable side chain, explicitly for the used nonpolar solvation part [48].  1 Cation: K1 . Na1 . ½Me4 N1 . Li1 . ½Chol1 . ½Et4 N1 . bmpyr . ½bmim1 . ½Bu4 N1 Anion: SO4 22 . dhp22 . ac2 . F2 . Cl2 jjEtSO4 2 . BF4 2  Br2 . OTf2 . I2 . SCN2 . Tf2 N2

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FIGURE 16.4 Effect of different types of cations and anions in ionic liquids on folding and unfolding of proteins.

16.6 Effect of hydrophobicity of ionic liquids on the stability of proteins The hydrophobic part of ILs is to diminish the measure of the surface area of proteins. Hydrophobicity of ILs can be understood in terms of log P and it is the logarithm of the partition coefficient of IL in the water/octanol system.

16.7 Effect of viscosity of ionic liquids on the stability of proteins It is reported that the ILs have more consistency than ordinary organic solvents. Numerous reports are indicating the consistency of ILs; may affect the usability and action of the protein. Sankaranarayanan et al. [55] announced that the helix-beta adaptation progress of the beta-cow-like lactoglobulin (β-LG) at two different pH conditions (pH 4
7.5) in the presence of IL, 1-ethyl-3-methylimidazolium ethyl sulfate (EMES). The analysis was investigated by various instrumentation techniques like circular dichroism (CD) spectroscopy, fluorescence, isothermal titration calorimetry (ITC), and transmission electron microscopy (TEM). Impact of miniature thickness

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16.8 Protein folding in ionic liquids

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incited by EMES in auxiliary structure β-LG was arranged to utilize a quartz microbalance and was corrborated with the fluorescence results at a consistent state.

16.8 Protein folding in ionic liquids In the traditional perspective on two-state collapse, a protein exists in a strong agreement between (1) a condition of low energy and low entropy; and (2) high entropy. Tentatively, the level of foldedness is generally estimated by investigating spectroscopic measurements like ellipticity estimated from CD. In studying the disruption of thermodynamics parameteres, it is important to recognize energy-entropy understanding of a globular protein. At a point, when the polymer-dissolvable connection is ideal, then, there is no energy cost for the investigation of high entropy states swollen loop get together. Fiebig et al. revealed the consequences of thermodynamic depiction for denaturing myoglobin in two different dilute aqueous conditions of ILs by exploring spectroscopic techniques. Guanidine hydrochloride encourages denaturation due to the loss of ingestion limit. The study showed the arrangement of myoglobin in an aqueous solution of ethylmethylimidazolium acetic acid (EMIAc) and butylmethylimidazolium boron tetrafluoride (BMIBF4). Fundamentally, BMIBF4 stabilizes the structure of myoglobin efficiently, while EMIAc does not. This was affirmed by control contemplates that utilize the LiBF4 and NaAc arrangements. BMIBF4 can be utilized in protein stability and is hard to denature [70]. Sen et al. [45] recommended that the protein is covalently marked by a fluorescent-colored compound, Alexa Fluor 488. Expansion of RTILs causes a decrease in the size (hydrodynamic span) of Cyt C GdnHCl. The decrease in size demonstrates the growth of a comparatively closed-packed structure. Two sorts of protein conformational unwinding are observed. Kumari et al. [50] examined the connection of the IL, 1-butyl-1methyl-2-oxopyrrolidinium (BMOP) bromide, and bovine serum albumin (BSA) utilizing spectroscopic techniques, UV
Visible, Fourier transform infrared spectroscopy (FT-IR) and fluorescence, and docking. Relating thermodynamic boundaries; change in Gibbs free energy, change in entropy, and change in enthalpy indicated that the coupling cycle was unconstrained and entropy driven. Additionally, it demonstrates that hydrophobic forces play important role in the binding of BSA to BMOP. The analysis of fluorescence spectroscopy showed that the configuration of BSA within the sight of BMOP is changed. The change in the amide I band in FT-IR spectra recommended BSA sending

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16. Role of ionic liquids and their future alternative toward protein chemistry

the possible structure after incorporation of BMOP. The investigation of molecular modeling of BSA
BMOP shows that the BMOP interacts with BSA at interfaces between two areas IIA and IIIA, found simply over the passage of the IIA restricting pocket through different interactions. Bharmoria et al. [36] reported that dodecyl 3-methyl-1-octylimidazolium, [C8mim][C12OSO3] based IL, forming a vesicle biamphiphilic to activate huge changes in the structure of BSA in aqueous medium at pH 7.0. Such modifications have been researched in detail utilizing different physiochemical properties and spectroscopic strategies. Distinctive fixation systems of [C8mim][C12OSO3]
BSA interactions have been characterized by binding and adsorption isotherms utilizing ITC and tensiometry, CD, fluorimetry, and dynamic light dispersing (DLS) estimations. These have been demonstrated that [C8mim][C12OSO3] prompts a display of BSA in monomer framework at low concentration. It has been discovered that the [C8mim][C12OSO3]
BSA settled against conglomeration is significant for destabilization of the protein. Ferdjani et al. [71] considered the action and strength of a betaglucosidase and two α-galactosidases in various hydrophilic IL/water. Action of 1,3-dimethylimidazolium methyl sulphate [MMIM][MeSO4] and 1,2,3-trimethylimidazolium methyl sulphate [Tmim][MeSO4] in glycosidases has been investigated. A close connection between the thermostability of catalysts and their impact was studied in ILs. The outcome profoundly exhibits that thermostable glycosidases are more appropriate for biocatalytic responses in water-immiscible ILs.

16.9 Enzymes with ionic liquids Different approaches to protein extraction are reported by traditional unstable natural solvents. ILs have also explored media for biotransformation because of their extraordinary abilities (Fig. 16.5). Since 2003, the corresponding impact of ILs and watery arrangements of inorganic salts of high charge thickness prompted the improvement of a particular innovation-based watery biphasic framework. It has been perceived as a strategy for treatment that offers numerous points of interest like low energy utilization, recyclability, enhancement of stability, and a biocompatible climate. In light of the more prominent safety, movement and enantioselectivity, ILs can be explored in the extraction of biocatalysts. Lipases are catalysts fit for catalyzing a wide scope of responses like hydrolysis, esterification, alcoholysis, acidolysis, and aminolysis. Their biocatalytic potential has caused a great deal of interest in zones of food, cleansing, and paper businesses.

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16.10 Application of ionic liquids as biocatalysis

FIGURE 16.5

427

Effect of ionic liquids in folding/unfolding and the extraction of

proteins.

Wang et al. reported the strategies for cell interruption by ILs in the extraction of entire proteins in Chlorella pyrenoidosa. In this way, the hydrolysis of proteins (papain, trypsin, and others) was accomplished with different chemicals [71]. Proteins are insoluble in ILs or their solubility is very low. Therefore, not much data regarding this matter is available in the literature, and the immediate protein extraction with hydrophobic ILs has been viewed as unreasonable through direct extraction. There are numerous ways to deal with choosing the IL utilized in the extraction cycle for a particular reason. From the perspective of process engineering, the direct protein extraction with severely hydrophobic ILs ought to be looked for as it joins the upsides of extraction. Researchers have effectively extracted hemoglobin and cytochrome C by an imidazolium-based IL: 1butyl-3-trimethylsilylimidazolium hexafluorophosphate (BtmsimPF6).

16.10 Application of ionic liquids as biocatalysis Biocatalysis is a nonaqueous system is to grow the potential outcomes of proteins to substrates that are not soluble in water and the capacity to handle those with low dissolvability. In any case, the utilization of traditional natural solvents for enzymatic responses presents issues like toxicity and low biodegradability. To acquire a green biotechnological arrangement, the utilization of ILs as dissolvable or codissolvable in oxidative enzymatic responses for the debasement of an insoluble natural mixture is a promising choice. The utilization of immobilized proteins has ended up being more reasonable and

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important for catalysis. Immobilized catalysts can be readily isolated from the ILs due to consistent recyclability and reusability. The techniques for immobilization ordinarily impact the properties of the biocatalyst and it is essential to discover a system for improving the properties of immobilization of proteins. Moniruzzaman et al. [52] designed ILs-enzymes that can be utilized as active, steady, and reusable biocatalysts. Candida rugosa lipase was microencapsulated on surfactant-shaped IL, consolidated in the polymer, and explained through free extreme polymerization of IL (1-vinyl-3-ethylimidazolium bis (trifluoromethyl-sulfonyl) amide). The movement, steadiness, and reusability of these polymeric materials containing IL-lipase are analyzed by utilizing lipase-catalyzed hydrolysis of p-nitrorophenylbutyrate (p-PNB) as a model response. Moniruzzaman et al. [72] explained the outcomes of enzymatic action of horseradish peroxidase (HRP) microencapsulated into IL utilizing pyrogallol as substrate. The framework utilized in this examination comprises of anionic surfactant, AOT {sodium bis- (2-ethyl-1-hexyl) sulfosuccinate}/hydrophobic IL {1-octyl-3-methyl-imidazolium-bis-(trifluoromethyl-sulfonyl) amide}/water/1-hexanol. The ability of biomolecules in ILs is fascinating in the drug industry. The reliability of the local protein shows minor changes in the dissolvable medium influencing their properties and practical action, drastically. ILs are another class of solvents for a wide scope of modern applications. The physicochemical abilities of ILs are exceptionally sensitive to the structure and nature of cations and anions. They are thermally and synthetically steady, noncombustible, have a low softening point, a huge liquid reach, and great solvation conduct. ILs are utilized to solubilize proteins, DNA, biocatalysis, and biosensors while safeguarding the soundness of the chemical properties. Lu et al. [53] reported the lipase-catalyzed blend of palmitate starch combinations with ILs, 1-butyl-3-methyl-imidazolium acetate ([bmim] Ac) and 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF4]). The impact of the response boundaries in the measure of catalyst utilized, the response temperature, the amount of [bmim][BF4]/[bmim]Ac, the molar ratio of methyl palmitate/anhydrous glucose, and the response time was focussed or optimized. The starch ester arrangement was affirmed by the presence of the carbonyl sign in the FT-IR spectra, nuclear magnetic resonance, scanning electron microscopy, and powder X-ray diffraction. Weuster-botz et al. [58] revealed the biocatalysis with immiscible liquid for cell biocatalysts that overcome the low water solubility of the products and substrates, hindering impact of reagents on biocatalyst, or low steadiness of products and substrates in the aqueous phase. The noninvasive astonishing impacts on cell layers of hydrophobic ILs,

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particularly with bis-(trifluromethylsulfonyl)imide and hexaflurophosphate in IL/aqueous biphasic IL make these better than numerous natural solvents that were applied for bio-transformations. Pfruender et al. [66] reported that the water-miscible ILs have no bad impact on the layers of cells of Escherichia coli and Saccharomyces cerevisiae. Accordingly, they can be utilized as biocompatible solvents for microbial biochanges exemplified by an expansion in the yield of amalgamation of the chiral solvents. Temme et al. [73] contemplated the hydrolysis of rutin rhamnosidase catalyzed by flavonoid. The trihexyl hydrophobic IL, (tetradecyl)phosphonium-bis-(trifluormethylsulfonyl)imide was distinguished to join the good attributes of the 23 ILs explored regarding the protein similarity, solvency of the substrate, and the distribution coefficient of the chemical. On considering the cations like 1-ethyl-3-methyl imidazolium [emim], 1-butyl-3-methylimidazolium [bmim], 1-butyl-1methylpyrrolidinium [bmpl] and 1-octyl-3-methylimidazolium [omim] with the [Tf2N] anion is best with managed by naringinase.

16.11 Ionic liquids do not inactivate enzymes like polar organic solvents 1-Alkyl-3-methylimidazolium-based ILs are called polar in nature. They are miscible in polar solvents like methylene chloride (dichloromethane, chloroform) and immiscible in hexane, water. The capacity to utilize solvents with a more prominent extremity builds the solubility of the polar substrates like maltose, glucose, or ascorbic acid, showing sudden responses that show changes. Candida antarctica lipase B (CAL-B) catalyzed acylation of the ascorbic acid with oleic acid in an IL and the transformation was good (83%) than in natural solvents. Even though, BMIM-PF6, BMIM-BF4, and BMIM-NTf2 are in the same family, the dipole moment of BMIM-BF4 is the least.

16.12 Increased stability of enzymes in ionic liquids Literature focused on hydrolases (proteases and lipases); which are the most well-known proteins in biocatalysis. The resulting reports incorporate oxidoreductases like peroxidases and dehydrogenase. All ILs are not appropriate for biocatalysis. Proteins that are active in the ILs contain PF6, BF4, and NTf2 as anions; however, they are not active in ILs containing CF3SO3, Cl, NO3, and trifluoroacetate as anions. The negative charge in the BF4 anion stretches out due to four fluorine particles and the anion PF6 has six more fluorine (Fig. 16.6).

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16. Role of ionic liquids and their future alternative toward protein chemistry

FIGURE 16.6 Representation of protein in an aqueous solution of ionic liquids.

Ghosh et al. reported the impact of a sinusoidal strength on the decreasing energy of proteins. They indicated that the normal season of folding of the protein, increase in activation recurrence first and afterward diminishes [74]. Yamamoto et al. [59] considered the tunable property of the ILs and the impacts of new competitors like N-alkyl chlorides and chlorides of N-alkyl-N-methylpyrrolidinium on protein refolding. Denatured lysozyme was utilized as a model protein and refolded by 30-overlap weakening with refolding different cushions containing distinctive ILs comprise of an assortment of precise alkyl chains. The outcome gives various new powerful added substances dependent on ILs for protein refolding and information on the impact of the compound structures on protein refolding. Byrne et al. [61] showed that the denaturation temperatures of ribonuclease A and hen egg lysozyme are sensitive. A high solidness is a more essential arrangement, and the improvement cycle is all around represented by two-state models. Bae et al. [75] utilized a copper base with the point of improving the proficiency of refolding of laccase, which requires IL-copper as a cofactor. When 1-ethyl-3-methylimidazolium trichlorocuprate ([EMIM] [CuCl3]) was added to the refolding cushion instead of the urea, the laccase refolding yield is improved on more than 2.7 point in examination with the traditional refolding cradle containing urea. Das et al. [7] contemplated the impact of 1-pentyl-3-methyl-imidazolium bromide ([PMIM][Br]) in the unfolding of the protein, human serum albumin (HSA) using fluorescence spectroscopy. The underlying variances of protein display three trademark time constants. Within the sight of RTIL, the addition of GdnHCl causes huge changes in structure (CD

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16.13 Cytotoxicity of ionic liquids

431

range) and time constants of the conformational variance. In addition to 6 M GdnHCl, the dispersion coefficient decreases. The hydrodynamic span of denatured protein (in 6 M GdnHCl) is more noteworthy than that of local protein.

16.13 Cytotoxicity of ionic liquids On examination, quaternary ammonium salts with long alkyl chains may be used as disinfectants and antiseptics. In the early investigations, it was discovered that the ammonium salts containing long-chain alkyl were exceptionally harmful to microorganisms. It was noticed that the quaternary ammonium salts of long alkyl showed harmful effects in vitro, to a scope of test life forms that have good bactericidal properties and could be germ-killers. Further, the investigation of quaternary ammonium salts of the ILs, and the poisonousness or cytotoxicity of cetyl-pyridinium chloride was performed and the IL was accounted to be harmful to a wide scope of test living beings including both Grampositive and Gram-negative microscopic organisms. Understanding the poisonousness or toxicity of ILs to various living life forms is significant for the choice of ILs, which would permit one to comprehend the effect of the unexpected exposure of these solvents. At present, ILs have been utilized in various diverse mechanical cycles, including a technique for dissolving cellulose, in paints, and as drug intermediates. Moreover, the possible utilization of ILs was researched in various territories or areas. A portion of these potential applications incorporate the utilization of some phosphonium-based ILs dependent on ammonium malignancy treatment, uses as a cleanser, blood or tissue additives, phenol debasement, and the utilization as wood additive. In any case, the expanded utilization of ILs builds the chance of incidental exposure of these solvents into the climate. This implies that the comprehension of the poisonousness or toxicity of various classes of ILs to all taxa of life can’t be underestimated, as it gives a comprehension of how a coincidental arrival of dissolvable will influence the climate. The significance of understanding the harmfulness and biodegradability of ILs has been surely known by the examination. The number of distributions that have been managed either poisonousness/toxivity or biodegradability of ILs was huge. As a rule, the harmfulness and biodegradability studied have zeroed for popular ILs families, i.e. containing pyridinium, piperidinium, imidazolium, and quaternary ammonium cations. Cation of ILs played important role in the toxicity and poisonousness of the environment. Cvjetko et al. [76] investigated the cytotoxicity of imidazolium-based ILs in fish CCO ovarian cell line and HeLa cell line utilizing human tumor test

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16. Role of ionic liquids and their future alternative toward protein chemistry

MTT cell. The studies indicated that the IL, 1-n-butyl-3-methylimidazoliumbis-(trifluoro-methylsulfonyl)imide, [bmim][Tf2N] found to most toxic than other ILs, 1-n-butyl-3-methylimidazolium tetrafluoroborate of [bmim][BF4], 1,3-dimethylimidazolium hexafluorophosphate [MMIM][PF6] and 1-nbutyl-3-methylimidazolium hexafluorophosphate [bmim][PF6]. They relied upon the chain length of the alkyl chain and the nature of the anion. The examination for CCO fish and human cell lines HeLa cells proposes that the CCO is an acceptable organic framework for introductory testing of the poisonousness of ILs that could supplant live bioassays. Zhang et al. researched the impacts of three ILs, 1-butyl-3methylimidazolium hexafluorophosphate (BMIMPF6), N-ethylpyridinium trifluoroacetate (EtPyCF3COO), and N-ethylpyridinium tetrafluoroborate (EtPyBF4) on development and uranium absorption by Clostridium sp. ILs influenced the development of the microorganisms as proven by the observed diminishing optical thickness, the absolute gas creation, creation of natural acids, and glucose digestion. IC50 for 48 h, three ILs diminished for BMIMPF6 (8.26 mm) . EtPyBF4 (7.04 mM) . EtPyCF3COO (4.05 mm). Uranium absorption by bacterial cells was diminished by 75% in presence of 1% (v/v) BMIMPF6 and about 90%
1% (v/v) or EtPyCF3COO or EtPyBF4 contrasted with the control without ILs. The diminished absorption can be assign to EtPyBF4 and EtPyCF3COO based on TEM investigation. Energy dispersive X-ray spectroscopy investigation revealed a collection of uranium inside the cells. These outcomes suggested that the capability of ILs are harmful to microorganisms, is a significant issue to restrict their biotechnological applications [17,72,73].

16.14 What are neoteric solvents? Neoteric means new and is categorized into four major categories: biomass-derived solvent, supercritical CO2, ILs, and DESs as in Fig. 16.7 [75
80]. These have allowed researchers from chemistry and engineering to explore them in different areas. Neoteric solvents have emerged to achieve the target by minimizing the cost and time as well as decreasing the environmental pollution so that they can be scaled up and can be used in industries. Pharmaceutical companies have a demand for less toxic and less polluting solvents in the synthesis, drug solubility, protein stabilizations, and many more. Earlier, ILs were found to be a breakthrough in the field and followed the green chemistry in the areas of protein stabilizations, catalysis, separation, etc. Later, studies found ILs to be toxic; therefore, there is a need to find new species having permissible toxicity and more promising. So, DESs open a new possibility for chemistry in the area of corrosion, protein chemistry, catalysis, etc. Further, it is important to mention that DESs are easy to synthesize and eco-friendly [81
91].

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16.15 Role of deep eutectic solvents on protein chemistry

FIGURE 16.7

433

Types of neoteric solvents.

16.15 Role of deep eutectic solvents on protein chemistry DESs comprise combinations of a hydrogen donor and a hydrogen acceptor. The literature available explained that the ILs can be hydrogen acceptors and have the capacity to interface with various donors like urea, glycerol, and so forth. As of now, choline salt (choline chloride) is thoroughly investigated as one of the constituents of the DESs and it interacts with glycerol (donor); can be synthesized easily, available, biodegradable, and reasonable options in contrast to customary aprotic cation-anion matched ILs. DESs showed brilliant mobility with heat stability up to 200 C. DESs are used as green and economical solvents in contrast to existing ILs. It has been realized that some of DESs can be utilized as dissolvable in the enzymatic responses to get extremely green synthetic cycles. DESs, novel age solvents, have been portrayed as productive and time-saving reagents in different applications of proteins [92
110]. Transesterification exercises of immobilized cross-connected proteases were separately inspected utilizing novel DESs. In a 1:2 ratio for the combination of choline chloride (ChCl)/glycerol, crossconnected subtilisin showed a brilliant potential and high selectivity in the transesterification response of N-acetyl-L-phenylalanine ethyl ester with 1-propanol [92]. The dependability of hen’s lysozyme albumin in various ChCl-based pseudo-concentrated and pure DESs was investigated by fluorescence spectroscopy and CD spectroscopy. Experiments of the unfolding of proteins on exposure to heat were performed in a dilute solution of urea:ChCl and glycerol:ChCl (eutectic solvents), individually; indicates the gathering at specific temperatures of discrete, folded protein up to some extent showed a high amount of secondary structures of proteins and breaks the tertiary structure of a protein. A reverse of the unfolding of the proteins is not possible in urea-based DES. On the other side, a decrease in the concentration of the eutectic mixture promotes the reversible reaction, which means, folding
unfolding. Lysozyme movement was likewise influenced in GCCL-DES with an expanding recuperation of action upon dilution, and full rejuvenation after DES

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16. Role of ionic liquids and their future alternative toward protein chemistry

elimination through dialysis. These outcomes encourage that protein interaction with DES can be reversible [93]. Molecular dynamics simulations of the DESs in a liquid state have been performed. Urea:ChCl has been explored as it is a notable eutectic combination. The nature of the lipase as a biocatalyst was investigated. For a correlation, the enzyme construction was likewise mimicked in 8 M urea. The stability of the enzyme toward the heat in DESs, water, and 8 M urea was assessed. The enzyme showed excellent conformational stability in the urea:ChCl combination even on exposure to heat. The outcomes are in acceptable concurrence with recent observations [94]. Four types of green DESs, ChCl:urea, tetramethylammonium chloride (TMACl):urea, tetrapropylammonium bromide (TPMBr):urea, and ChCl:methylurea were prepared. An aqueous solution of ChCl:urea (DES) was investigated thoroughly for the extraction of cow-like serum albumin. Data demonstrated that these systems might offer additional opportunities in the extraction of proteins [95]. Four types of ChCl-based DESs were explored to isolate the bovine serum albumin and ChCl-glycerol was chosen as the appropriate extracting dissolvable. Experiments have been performed to examine the impacts of the extraction cycle, including the measure of DES, the centralization of salt, the mass of protein, the shaking time, the temperature, and pH information. Results propose that ChCl-based DES may give new opportunities in protein chemistry [96]. Dilute aqueous solution of reline and glyceline can be utilized as a phase for catalase reaction. UV
Visible, fluorescence, and circular dichroism (CD) spectroscopic strategies were utilized and explored to know the ability of catalase. These examinations demonstrated that the binding affinity of the substrate to the catalyst or enzyme gets enhanced, which in contrast to reline arrangement that presumably correlated with the secondary structure of the protein [97]. Six types of betaine-based DESs have been explored in the extraction of protein. Betaine-urea (Be-U) was chosen as the reasonable extractant. Single-factor tests were performed to decide the ideal states of the extraction through DES fixation, time, amount of protein, temperature, and pH. UV
Visible, FT-IR, and CD spectra affirmed that the conformation of protein was not changed during the extraction. These outcomes demonstrated that betaine-based DESs may be a better substitute for the separation of proteins [98]. Researchers have carried out a detailed study for understanding protein conformity in ChCl-based DES and in combinations with water. Lysozyme and serum albumin have been investigated by CD and other techniques [99]. The authors target the impact of the hydrogen bonding of cholinium chloride:carboxylic acid-based DESs for an understanding of lysozyme nanofibers. Mono-, di-, and tri-carboxylic acids (lactic, levulinic, malic, and

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16.16 Conclusion

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citrus extracts) were utilized to get various DES formations and were utilized on the fibrillation of lysozyme. The outcomes indicated that carboxylic acids assume a significant part of fibrillation proficiency [100]. DESs were explored as a new medium for the combination of alkyl glycosides catalyzed by thermally stable α-amylase. This enzyme was entirely deactivated when measured in a pure DES, although, with cosolvents, the results are better. ChCl:urea-based DES was further investigated for the alcoholysis response, utilizing methanol as a nucleophile [101]. Authors have created electro-spinning techniques to fuse the antimicrobial ILs/DESs, and choline geranate (CAGE). Coordination of CAGE was confirmed by spectroscopic techniques and the impact on the properties of materials of gadgets was surveyed via utilizing examining electron microscopy. CAGE-containing gadgets showed a focused subordinate inactivation of exogenously applied arrangements of both gram-positive and gram-negative microorganisms (Enterococcus sp and Pseudomonas aeruginosa, individually) [102]. The stability of BSA on exposure to heat was significantly improved in presence of ChCl-based DESs, in particular ChCl
urea (DES1) and ChCl
glycerol (DES2). The ability of DES1 is better than DES2. It was fascinating to take note that DES1 having urea as a hydrogen bond contributor prompts the outstanding warm solidness of the BSA structure. Fascinatingly, the mix of DES1 with PEG conveys promising outcomes, as they lift the warm solidness of BSA by around 16 C. In this way, the DES medium gives stability to BSA. The capability of DESs to lessen the unfavorable impacts of macromolecular crowded on protein stability has been examined [103]. High concentration of urea focused unfolding of proteins prompting denaturation. Interestingly, ChCl and urea in 1:2 proportion, structure a profound eutectic mixture at room temperature, shielding proteins from denaturation. Expansion of ChCl hinders the elements of urea, confining its aggregation around the protein backbone. Choline cations in the solvation of the protein show the most grounded subdiffusive conduct. This keeps the protein from denaturation and makes it primarily inflexible, which is upheld by the smaller radius of gyration [104]. The infiltration system of ChCl
glycerol (DES) through the layer as a likely dissolvable for a novel enhancer of protein penetration into the skin was examined using X-ray diffraction. It was found that DES entered through intercellular lipids [105].

16.16 Conclusion ILs are the solvents that surpass the inabilities of regular and traditional solvents. They are salts of ions and their properties can be tuned

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by changing the cation, anion, and alkyl chain. It means that by managing various anions or cations, ILs can be tuned. Specifically, the ILs are significantly more fascinating than the natural solvents, and hence can be utilized for some definite purposes [50,111
116]. For instance, utilization of nonsolvents, associated with the errand explicit ILs can be another territory of future exploration. The utilization of ILs, as of late revealed the immobilization of biocatalyst and protein extraction. Information move to the third-generation ILs, speaks of the sensible advance in the zone. Hence, ILs participates in improving the movement and steadiness of the chemical, which exhibits its potential as nonwatery media for enzymatic responses. Further, the toxicity of the ILs is also reported. Therefore, there is a need for its replacements and the research groups find DESs, an alternative to ILs, and are exploring the protein chemistry and are found to be more promising than the ILs.

References [1] L. Li, et al., Application of ionic liquid as additive in determination of three betaagonists by capillary electrophoresis with amperometric detection, Electrophoresis 34 (2012) 277
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8.

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C H A P T E R

17 Ionic liquids in metrological analysis and applications Umaima Gazal Chemistry Department, Raja Bahadur Venkata Rama Reddy Women’s College, Affiliated to Osmania University, Hyderabad, Telengana, India

17.1 Introduction Ionic liquids are the safe diluent in many elevated-temperature submissions because of their inflammable characteristics. They have been designated as “exclusive diluents” and also termed as “diluents prospect” as of their physicochemical characteristics which can be controlled and modified via altering their elements and then the configurations of pairs of the ions [1 3]. Ionic liquids have numerous benefits, comprising inflammability, biochemical solidity, non-volatility, tunable miscibility, high thermal, noble abstraction ability as well as extensive electrochemical space which are not made for unstable organic diluents [4 6]. Ionic fluids are not at all modest solutions. Its ions are usually irregular, by divisional columbic charges [7]. The atomic interfaces, as well as forces of these green solvents, are extra complex in comparison to collective standard alkali [8]. Ionic liquids have specific resemblances with water. Water subsists as a combination of open water molecules plus hydrogen-attached water bunches [9]. Similarly, ionic liquids seem to be a blend of hydrogen-attached ionic supramolecular shapes in addition to open ions [10,11]. The remarkable features of ionic liquids are mainly applicable to their ionic behavior. The arrangement of weak turning relations (containing dispersion interactions, Van der Waals inductive, cation p interactions, and hydrogen bonding interactions) and strong Columbic interactions may convince the foundation of nanoscale arrangements in green solvents and ionic liquids solute mixtures

Advanced Applications of Ionic Liquids DOI: https://doi.org/10.1016/B978-0-323-99921-2.00009-4

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© 2023 Elsevier Inc. All rights reserved.

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17. Ionic liquids in metrological analysis and applications

along with ionic liquids solvent [12]. An inspiring amount of the anion cation amalgamations prepared these composites appropriate for the subsequent areas of submission: fungicidal substances, liquid-liquid extraction, and production of liquid membranes, organic synthesis, capillary electrophoresis, liquid chromatography, separation techniques, biocatalytic reactions and bactericidal as well as mainly all green diluents [13,14]. Their lower air compression directed to a notion that ionic liquids might also be measured as green diluents, owing to short levels of distinctive contamination, though, it is nowadays well identified that the industrial, usage, plus removal of the diluent itself, should also be measured [15 19]. Amongst all the diverse members of ionic liquids, ammonium-centered ionic liquids are acknowledged to show a substantial part in biochemical plus chemical progressions. Through the globally aggregating call for ionic liquids, ammonium-centered ionic liquids are appealing cumulative consideration as green co-diluents in an extensive variability of study regions [20 23]. The attractive and explicit characteristics of ammonium-based ionic liquids might extant perfect benefits in dissimilar biomedical along with pharmacological submissions [24 26] also have stimulated their usage in fuel cell [27,28] nanotechnology [29,30] arrays as electrolytes [31 33] polymerization reactions [34 36] along with technical and commercial utilization [37 44].

17.2 Wide-ranging ionic liquids Owing to their inimitable electrochemical characteristics [45] and the probability of alteration of their biological and physicochemical factors, ionic liquids get effectively engaged in the plan regarding biomaterials as well as sensors. Ionic liquids can be reprocessed as a self-regulating receptor deposit or as a supportive constituent bringing added regulator above selectivity and sensitivity throughout in contact with vapor phase constituents. Thanks a lot to their elevated viscosity ionic liquids can be smeared as a folder, an intermediate among the bio-sensitive and transducer substantial inserted on the exterior of the biosensor [46]. Outstanding electrochemical characteristics, by the vast electrochemical space along with moderately high electrical resistivity, form ionic liquids a valued substantial for transforming as well as constructing electrochemical devices. Furthermore, their low instability, electrochemical plus thermal solidity means that they can be utilized below hugevoid situations in a secure as well as regulated manner. Due to the large standard of active viscidity factors, the ionic liquids can also be engaged such as layers in amperometric devices, that discrete the vapor atmosphere after an operational conductor. The metrological factors related to amperometric devices comprising dimensional range, selectivity, the

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17.3 Protic and aprotic ionic liquids

445

perimeter of recognition, sensitivity as well as quantitation or the reaction phase are influenced using physicochemical characteristics of the ionic liquids such as dispersion constant of analyte in ionic liquids, hygroscopic effects, dynamic viscosity, in addition to analyte solubility in ionic liquids, electrochemical stability range, electrical conductivity and water-solubility [47]. Attainments in the area of biosensors and sensors for nanotechnology, biology, electronics, biotechnology, and genetic engineering, allow the production and application of instruments from a microelectronic nose family (artificial nose, odor-detecting method, electric nose, e-nose) [48], whose progression centered on the application of bio sensible constituents ensued in bioelectronics noses [49]. The nature of electric nose gadgets, which has a straight application on the simple action factors in addition to utilization series, is a device medium. Exhaustive growth of microelectronic nose gadgets has been emphasized on (Table 17.1): i. Reduction, growing flexibility also cost deterioration, ii. Origination of the electric methods imitating the human brain and nose. iii. Manufacturing devices with high specificity, short response period, and sensitivity iv. Progression of the instrument schemes produces indicators identical to those of their biotic complements.

17.3 Protic and aprotic ionic liquids Ionic liquids initiate valuable uses in lubricants, fuels, solid-state photocells, hydraulic fluids, thermal fluids, sensors, ionogels, batteries, separation devices, and solar cells [51 55]. Thus two forms of ionic liquids expand cumulative importance in applied science and tools: protic as well as aprotic ionic liquids. The desired aprotic ionic liquids are those with tetraalkylphosphonium, pyridinium, pyrrolidium, or else tetraalkylammonium ions. Though, the utmost extensively active cation is 1-alkyl-3methylimidazolium. The imidazolium-centered alkali display exclusive characteristics that stalk after the microelectronic arrangement of the pungent cations. Therefore the imidazolium-centered ionic liquids are noble select for evaluating molecular interfaces for example hydrogen linking. The anions range starting after typically hydrophobic to intensely hydrophilic comprising Acetate, tris(pentafluoroethyl), hexafluorophosphate bis(trifluoromethylsulfonyl)imide, tetrafluoroborate, trifluoromethylsulfonate, thiocyanate, dicyanamide, diethylphosphate, trifluorophosphate and ethylsulfate. Protic ionic liquids are said to be an imperative subcategory of ionic liquids shaped through an arrangement of equimolar volumes of

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446 TABLE 17.1

17. Ionic liquids in metrological analysis and applications

Certain features of ionic liquids [50].

Biotic features

Physicochemical features

Can be reprocessed; regained and reusing of the catalytic agent are also possible with the ionic liquids.

Reactivity referring to gases, for example, CO, NO2, oxygen, C2H4, C2H6, etc., which construct them apposite diluents for aerobic oxidation, carbonylations, hydroformylation, and catalytic hydrogenations

Drug transporters in biotic media

Tribological properties, lubricating efficiency

Ecofriendly, the toxicity of several ionic liquids is still unidentified

Interionic interfaces: cross-connection details amongst anions and cations, polarization effectiveness because of several interfaces initiated using anions and cations and ionic liquids

Protection and maintenance of the easy tissues

Probability to modify a few properties through changing the cation anion amalgamation: Hydrophobicity, hydrophylity, electrochemical conductivity, density, polarity, electrochemical durability, viscosity, sorption properties, melting point, solubility in water

Capability to penetrate biotic tissues

Diphasic methods conceivable

Activities of several biotic constituents, containing interface with lipids, fungi, bacteria, proteins, etc.

Capability to liquefy both inorganic and organic complexes

Antimicrobial actions in addition to antistatic characteristics

Extraordinary thermal strength (equal to 450 C, breakdown: 300 500 C)

Pharmacological plus pharmacokinetic activity

Molten state beneath 100 C

Biocatalytic response factor

Incombustibility

a Bronsted base as well as Bronsted acid [56]. The main aspect that differentiates protic ionic liquids from further ionic liquids is proton transmission after the acid towards the base, primary to the existence of proton-acceptor/donor spots, which can also be recycled to form a hydrogen-link system. Protic ionic liquids have a variety of exclusive features in comparison to further ionic liquids, pure as water which is changed from “usual” molecular fluids. Therefore, protic ionic liquids are an impeccable method to examine the existence and stableness of hydrogen attachment. The amount of hydrogen bond contributors, the interface power, also the linkage foundation can be speckled as preferred.

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17.4 Physicochemical properties of ionic liquids defining metrological parameters Physicochemical characteristics of these green solvents hinge on both the kind such as the mass of cation and anion (extent of its irregularity in addition size of alkyl chain accountable for charge delocalization). A cation of ionic liquid is liable mostly on behalf of physical characteristics of the alkali, however, an anion defines ample biochemical characteristics along with the reactiveness of the complex. The features of ionic liquids which affect sensors are:

17.4.1 Miscibility and solubility These both are stimulating effects of ionic liquids and acquire a wide deviation in their miscibility as well as solubility properties. This also is influenced by the description of the anions and cations. Distinct forms of surroundings, for example, arrangement capability and polarity of its ions, furthermore hold an influence on the reactivity of ionic liquids [57]. Together covalent and ionic complexes can be diffused in ionic liquids also that feature shows an appropriate major character in its uses such as diluents for fabricated chemistry, isolation against mixtures including abstraction of constituents from fusions [58]. Thus this has untied novel zones of chemistry where they are recycled as catalytic agents and solvents [59]. Ionic liquids can be regulated through diverse anion/cation that can be useful in the suspension research of solids, liquids, also inorganic and organic gases.

17.4.2 Solvation with repulsion Repulsion is the utmost imperative characteristic in the application of diluents. Though diluent repulsion is well-marked as the probable activity of the diluent about the solute [59,60]. Solvation arises through the congregation of diluent particles or grains or else ions using solute-ions. Ionic liquids can also diffuse mutually nonpolar plus polar solutes including solvation research demonstrate interfaces for example solutesolvent interfaces, which comprise electron pair contributor acceptor interfaces, ionic, van der Waals forces, columbic, and numerous dipole interfaces as well as hydrogen bonding [60]. The diluent feature can also be improved through the apposite selection of anions and cations. Therefore, particular diluents can be recycled as a reinstatement otherwise wherever exclusive solvation is required. The diluents characteristics of ionic liquids are extra regulated equated to molecular biological

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diluents [61]. There are several diverse approaches to actuate diluent polarization. Dielectric persistent or comparative diffusivity processes the duality of a diluent or fluid. The fluid enclosing nonpolar particles, for instance, toluene, hold small dielectric coefficients, however polar fluid particles, such as acetone or methanol, have great dielectric coefficients. Hydrogen bonded fluids, too have great dielectric coefficients. Ionic liquids to carrying electrical energy.

17.4.3 Vapor pressure Vapor pressure is the main remarkable feature of these green diluents. They get trifling air compression [62,63]. This characteristic subsidizes green chemistry concerning the decrease in airborne contamination plus the exclusion of precarious impulsive materials. The shortage of vapor compression at thermal disintegration temperature is because of the stout columbic interfaces among the ions in the fluid [64]. Commonly biological diluents recycled in a biochemical progression are toxicants, have deprived solubility of stimulants, are not thermally constant then they taint the last yields.

17.4.4 Density and viscosity Usually, ionic liquids are extra viscid in comparison to molecular diluters. The viscidness of ionic liquids is commonly associated with the aspect of the viscosity in addition to the cation of ionic liquids rising with rising in the proportions of the cation (alkyl chain size) [65,66]. Ionic liquids with unstable linking anions, for example [PF6] and [BF4], have low viscidness then those with stable linking anions have greater viscidness. The viscidness of ionic liquids confides in hightemperature variations, as well as decreases with a rise in the temperature [59]. Advanced viscidness is not beneficial in mass solidification, however, can be beneficial in mineral manufacturing, where inert crystal development from dispersing diluents is a benefit [61]. The existence of slight extents of contaminations can also disturb the viscidness of ionic liquids. Normally, phosphonium ionic liquids are slightly compact in comparison to water whereas imidazolium ionic liquids are heavier in comparison to water. The compactness of ionic liquids reduces through an escalation in alkyl chain size on cations; which is caused by the Van der Waals interfaces of ions that are compact along concerning high temperature which inclines to lesser the effective stuffing of ions [67].

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17.6 Ionic liquids-centered devices

449

17.5 Electrochemical constancy and conductivity Ionic liquids are rousing due to their usage such as nonaqueous electrolytes in electrochemical operation whereas permeability is the main property. Mostly, the flexibility of control transporters in electrolytes commonly confide or is associated with their dispersion amount in electrolytes. The viscidness of ionic liquids is mostly advanced compared to further electrolyte which also tends to fall with rising temperature though, enclosing more charge transporters. Electrochemical solidity is noticeably inclined through contaminations confined in ionic liquids as well as hygroscopic features of the ionic liquids. Though, less quantity of water existent in the ionic liquid supplies near a fall of electrochemical constancy series. The ionic liquids hold an extensive variety of electrochemical constancy that is widespread series of transformations amongst the capacities of anode and cathode, for which here is oxidation and reduction of ionic liquids, correspondingly. Regular electrochemical stability series of ionic liquids are starting as of 4 5 V also it is an expressively greater than the aqueous resolutions of electrolytes, for which this series is equivalent to 1 3 V [47].

17.6 Ionic liquids-centered devices The distinctive characteristics of ionic liquids: dual process (electrolyte/solvent, inorganic/organic), diphasic nature (liquid/solid) succeed them to a varied collection of utilizations, containing vapor detectors transducers. The choice of an applicable ionic liquid, as well as the transducers used for an electric nose medium, is openly related to features of the ionic liquids. Owing to the weak analyte liquid / solid interfaces, it is necessary to use ionic liquids for the production of the aural transducers. Dual performance of ionic liquids can be consumed in planning tissue-free electrochemical devices. An apposite range of transducers with higher efficacy of gas exploration in specific operation is resolved through the synergistic grouping of characteristics of a particular transducer (reaction mechanism or transducing) with the physicochemical properties of the receptor substantial. These characteristics in addition to their suitable technique of device fabrication hold straight comportment on central metrological factors of the devices, for example, application series, constancy, sensitivity, etc. The ionic liquid-centered biochemical devices being the constituents of the electric nose exist in the group of gravimetric devices, specifically quartz crystal microbalance piezoelectric device plus surface acoustic wave device. The devices operating surface acoustic waves are constructed commencing a

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17. Ionic liquids in metrological analysis and applications

piezoelectric substrate, on which there are dual probes (transferring plus accepting ones) [68].

17.7 Configuration of ionic liquids in biosensors Outlining the improvement of electrolyte utilization in the planning of the organic devices displays that water has been the key constituent for an elongated period. Though, its lower thermal constancy, instability, and constricted series of process temperatures thus inadequate improvement of the biomaterials. The ionic liquids remained hosted in demand on the way to overthrowing this unavailability. The higher affinity among biomaterials along with ionic liquids [69] can affect in improved action of several biomolecules. The apposite configuration of the receptor level quieted the biomaterial, then the apposite range of the ionic liquids undergoes an enhancement of efficient features of the organic devices. The advancement of biosensors tools is intended to impend the sensitivity choice of the organic complements, wherever aromatic constituents can be identified at the nanomole level [70]. The idea, accomplished of contracting these prospects, centers on retaining organic mechanisms initiated after olfactory arrangements [71]. Operating ionic liquids such as adhesive representatives in the biodevices aids effective control of the bio complex constituents on the device’s outward. The device layout has to interpret for a central connection between mass transference as well as viscosity in the solution [46]. Electrochemical biodevices can be recycled for the quantitative plus qualitative study of a varied range of analytes. Confiding on its endpoint the receptor level of the biosensor can be constructed from diverse forms of bio complex components (receptors, enzymes, antibodies, nucleic acids, microbes, etc.). An important component of a bio device is a transformer fragment, in that a sign owing to the interface of the organic substantial through an analyte is altered within the electric sign. Ionic liquids as standard for organic constituents are effectively used in electrochemical biodevices, mostly cheers to a moderately noble bio affinity with several biomaterials adapted [72,73].

17.8 Aspects of ionic liquids as promoters in biodiesel fabrication Ionic liquids in reality are recognized to attain explicit desired organic features, for example, activation and enzyme counteraction. Because of their ecologically responsive plus tunable characteristics, ionic liquids are treated as a capable reinstatement for traditional

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17.9 Affinity attributed to ionic liquids in nanomaterials

451

diluents. The characteristics of ionic liquids are predominantly valuable for the fabrication of biodiesel as they can substitute traditional diluents in the purification along with reaction methods, through performing such as enzyme reinforcements, promoters, extorting diluents, or cosolvents. Instant profits comprise a decrease in the size of wastewater in contrast to predictable promoters and rarer stages for item parting plus ablution [74]. Numerous catalytic agent forms are recycled in the biodiesel construction procedure from animal fats along with vegetable oils for example enzymes [75 77], acid [78 80], and alkalines [79 81]. Otherwise, enzymes can also be recycled which are proficient in incapacitating the drawbacks of customary catalysis, for example, high vitality requirement in addition to apparatus erosion [82]. Though, various rebuffs are quite connected with the procedure, comprising high charges, and deactivation in the survival of intoxicants, also the prospective for condensed enzyme control. Various acid ionic liquids have been reconnoitered via scientists as a possible catalytic agent in biodiesel construction for instance Bronsted acid ionic liquids. These catalytic agents enclose the sulfonic acid chain in their cations and also have been established operative in the transesterification of numerous fats either esterifying free fatty acids. Further ionic liquids which have engendered substantial consideration as promoters in biodiesel construction progressions are multi and single sulfonic acid operationalized Bronsted acid ionic liquid [83 85]. In the accumulation of acid ionic liquids, elementary ionic liquids have also enforced biodiesel production for example choline and imidazolium-centered ionic liquids, however, not as commonly [86 88]. In maximum elementary ionic liquids, a substrate with a lesser free fatty acid complacent is necessary to elude lather creation.

17.9 Affinity attributed to ionic liquids in nanomaterials The grouping of ionic liquid with nanotechnologies provides additional striking and proficient potentials for advanced technically appropriate nanomaterial. The outstanding physicochemical characteristics of ionic liquids might support the creation of novel structural nanomaterial, as a result, it would perform as a stabilizing as well as strengthening mediator for nanomaterial. Applying ionic liquids in Nanocomposites would include an interface of constituents and can also benefit electron transference reactions. These novel constituents can be recycled as photonics, catalysts, electro sensors, and electronics. The profit of employing ionic liquids in nanomaterials is to enhance electrochemical as well as thermal constancy, decrease toxicity, control particle size, increase conductivity, and also rise reactivity in an environmentally friendly

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17. Ionic liquids in metrological analysis and applications

mode. Metallic nanoparticles fascinate substantial consideration centered on their new features [89,90]. Numerous studies have testified high product composite of metallic nanoparticles in ionic liquid [91 93]. As a point of fact, metallic nanoparticles created in ionic liquids are stated to be simple-discharged and also non-agglomerate as a consequence of ionic liquids counteraction [94,95]. Acceptable to formulate preferred nanoparticles in ionic liquid, the constancy of the nanoparticle diffusion must remain cautiously measured. Some aspects distressing the constancy of nanoparticles in ionic liquids could point to nanoparticle magnitude [96 99] and configuration [100 104] deviations in the course of production. The magnitude and magnitude circulation of nanoparticles formed in ionic liquids stay afflicted through physicochemical characteristics of ionic liquid that induced the nanoparticle’s equalization. Such as, reduced thickness and constricted circulations of manufactured zinc oxide and nickel nanoparticles continue to arrange in ionic liquids through lengthier cross chains, as the lengthy crosschain compressed the nanoparticle’s constancy in ionic liquids through varying the physicochemical characteristics of ionic liquids in addition to the interfaces concerning nanoparticles and ionic liquids [94,101]. Longer dimension or only wide-ranging format of produced nanoparticles can be initiated via accumulation of unbalanced primary nanoparticles, which can affect by lesser ionic liquid anions displaying tougher anion cation. Columbic inclination, [105] or a lesser amount of synchronizing anions of ionic liquids [104,106] and advanced temperature reduce the viscidness of the ionic liquids in addition to growing the diffuse rates of the spluttered nanoparticles [107,108]. Therefore advanced compliance with the constancy of nanoparticle diffusions in ionic liquids will pointedly support struggles focused on the production of nanoparticles with measured magnitude and fabrication in ionic liquids.

17.10 The implication of green diluents in space mechanics The practice of ionic liquids in space mechanics operations is considered as new fresh expansions, arising from the earlier period. Advancements in the area of spaceship formation, rocketry, and also satellite broadcasting machinery have modernized space investigation and then delivered the needed substructure for space-centered universal telecommunications, climate foreseeing, and Global positioning system triangulation. Corresponding science and engineering areas, for example, analytical technology, robotics, computer technology, materials science, and process intensification, have also proficient prompt developments as an artifact of this technical initiative, and several of the revolutions have established operations in non-space uses, for example,

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17.11 Space energy

flame-resilient fibers centered on polybenzimidazole [109] as well as gentle-respite manure [110]. Investigators have examined ionic liquids for an extensive variety of space-interrelated operations, comprising lubricants [111], hypergolic fluids [112], high-performance composite materials [113], and electrospray thrusters [114]. Ionic liquids hold numerous individualities that mark them predominantly suitable for space mechanics utilizations. One of the main benefits is their trivially lower vapor compressions [115], which empowers ionic liquids to be cast-off in lower compression and vacuum environs, for example, those practiced on the Moon, Mars, and also outside the universe, deprived of fall through vaporization. The inimitable features of ionic liquids sketched over, linked with their “drawing ability,” besides the latest obtainability of structural property projecting simulations process that it would be probable to plan ionic liquids with optimum characteristics for a variety of space-correlated operations.

17.11 Space energy Colloid thrusters are also identified as electrospray thrusters, which are electrical energy machines that operate the electrostatic stimulation of electric fluid propellant drops as the power of energy. Equated to reversibly impelled methods, electrospray thrusters deliver a definite lower full thrust [116], however, have the benefits of higher efficacy in addition to exact impulsion [117]. So, they are mainly suitable for the impulsion of minor satellite broadcasting, for instance, CubeSats, and also for the higher accuracy approach-controller of large spaceships [118] (Fig. 17.1). Ionic Liquid–Based Greases Space Emollients

CO2 Removal

Space Contracts

Life Assist Methods

Heat Transfer Fluids

Lunular Fluid Glass Contract

Applications of ILs in Space Technology Thermal Controller Complex Resources

Hypergolic Fluids Space Impulsions Electrospray Propulsive

Complex Cryotanks

FIGURE 17.1

Applications of ionic liquids in space technology [119].

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17. Ionic liquids in metrological analysis and applications

17.12 Compost properties Compost resources have improved significance [120], mostly owing to the capability to accomplish the preferred features for a particular user through the cumulation of the substantial configuration [121]. As, in the fields of space manufacturing and civic creation, there is a necessity for light constituents which are too inflexible and tough. Compost resources are recycled in planetary operations chiefly to decrease the mass and whole price of the cryogenic reservoirs [122]. The utmost imperative factors as well as benefits of compost resources, which permit them to be recycled in planetary operations, such as strength, bulk, toughness, as well as thermo-flexible constancy. National Aeronautics and space administration has constituted that using practicing the compost cryotanks, the mass of the skyrocket reservoirs can be compact by 30% as well as the price can be dropped by 25% [123]. For instance, ionic liquids have also been recycled in the formula of fluid absorbents [124], restrained on solids for practice as adsorbents [125], and also in skin furnishing [126]. The similar values can also be adapted to the elimination of carbon dioxide from log compartment midair in spaceships, lunular inhabitation entity, or else in the principal lifecycle care method in a spaceman’s spacesuit.

17.13 Life support techniques Humanoid space operations need several care schemes accomplished of supposing all necessary to endure a lifetime throughout the operation. On spaceships, this comprises oxygen for metabolic ingestion, adequate nitrogen to preserve the exact midair configuration, and filtered water for ingestion in addition to unpolluted water for diet research, sterility, and moisture controller. Appropriate to sustain nontoxic, breathable air, it is very essential to eliminate respired carbon dioxide, particulate material, microbes, and unstable organics from the compartment air. Various ionic liquids are recognized to have an exact higher solubility for carbon dioxide compared to further gases [127], which have directed to broad study interested in the usage of ionic liquids for carbon internment in earthly operations [128].

17.14 Hypergolic solutions Hypergolic solutions are recycled as great vitality stimulants in space missile devices and also in spaceship steering methods. They

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17.15 Space emollients

455

characteristically are made up of two modules, an oxidant and a fuel which will impulsively explode when got into connection. This methodology to impulsion eludes the necessity for an outer oxidization cause, which is the main benefit of rocket equipment, meanwhile, it considerably decreases the probabilities of collapse and drops the charge constraint compared to nonhypergolic space missile impulsion [129]. There are some required characteristics to reflect when choosing a hypergolic stimulant. The ultimate substantial is the exact instinct, or else the core per unit bulk of stimulant, which is a magnitude of the propellant efficacy. The adiabatic flash high temperature of the stimulant arrangement, which signifies the theoretic extreme heat achievable through the oxidation response, is an additional significant reflection. Explosion stay, which strengthens the interval between ignition and liquid interaction, is also the main limit though it has a solid association with the exact instinct [130]. Lastly, it is also imperative to study the security concerns in addition to the comfort of control of the hypergolic liquids. Various stimulants are extremely contaminated, therefore generating important security threats in treatment, transport, and storing. So, it is necessary to discover an appropriate stimulant with lesser harmfulness, better treatment features, and an improved ecological outline.

17.15 Space emollients Because of the noticeable complications of executing preservation and maintenance on a rocket, it is precarious that rocket appliances are planned and also fabricated to work easily and constantly. In takeoff, exciting pulsations might be practiced, also in the process, the oxygen-open situations of space joined with exciting high temperatures can boost metallic shells to repair together [131]. Consequently, devices must be sufficiently greased to avoid failure or loss to the apparatus [132]. Grease is a solid which is combined to decrease the abrasion stuck between two stirring shells to inhibit tear and wear as well as minimalize the expansion of heat. In earthly operations, usually used greases contain synthetic oils, fats, and mineral oils. The emollients presently in practice for space uses can be categorized as fluid oils, concrete fats, and fats [133]. Concrete fats comprise lamellar material, for example, tungsten disulfide and molybdenum disulfide, easy metals, for example, gold and lead, and polymers, for example, and poly tetra fluoro ethylene and polyimides [132]. Lubricants are semi-solid fluids that entail liquefied oil and also a thickener [133]. The best regular planetary emollients consist of ionic liquids, mineral oils, silicones, silicic hydrocarbons, synthetic hydrocarbons, and perfluoropolyethers [134].

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17.16 Lunular fluid-glass contract Additional possible planetary operation for ionic liquids comprises their usage in a lunular fluid-reflect contract. Such contracts practice a contemplative fluid as the mirror. Mercury is the best natural fluid recycled in these utilizations, though further fluid metals have also been recycled, for example, gallium-centered compounds. To yield the parabolic outward essential to take the farther light to a center in the contract, the fluid, and its ampule need to be revolved at a persistent swiftness. Even though this indication certainly yields a paraboloidal form in some ampules, the sum of fluid required can be minimalized by consuming an ampule with the requisite character. The main benefit of fluid-glass tools is the substantial decrease in equally cost and weight in contrast with predictable concrete crystal glasses. Additional benefits comprise their plainness, which creates the shipment as well as amassing at ease, and their outstanding ocular potential [119].

17.17 Conclusion The capability to plan ionic liquids with optimum chemical and physical characteristics for various operations has permitted them to be affianced in an extensive range of inquiries. The advancement in nanotechnology, electronics, biochemistry, and chemistry is essential for refining, mobility, potentialities, functionality, and reliability of biodevices centered on ionic liquids. The swift development in the area of sensors and rising nano-configurations-centered electrochemical devices will have a notable effect on the growth of novel bio-detecting boundaries for determining our forthcoming medical diagnostics and concern biotic expansions at a particular molecular level. Researches based on intermolecular contacts distressing the constancy of ionic liquidscentered nanoparticle diffusions and on the “interaction force structural functions” associations in nanoparticle ionic liquids complexes deliver essential awareness desired for the balanced plan of such constituents and their operations in numerous functioning environs. Ionic liquids have numerous promising features which mark them highly apposite for the exacting environments of planetary and spaceship. Ionic liquids have also been validated to denote harmless and low contaminated substitutes to customary hypergolic solutions for example hydrazine. It is predicted that the assimilation of ionic liquids into planetary tools will hasten advances in planetary examination though also serving in the construction of novel machinery for earthly uses.

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References

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C H A P T E R

18 Antibacterial properties of silver nanoparticles synthesized in ionic liquids Elham Avirdi1, Seyyed Emad Hooshmand2, Indra Bahadur1, Lebogang Maureen Katata-Seru1 and Rajender S. Varma3 1

Department of Chemistry, Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Natural and Agricultural Sciences, North-West University, Mmabatho, South Africa, 2Department of Medical Nanotechnology, Faculty of Advanced Technologies in Medicine, Iran University of Medical Sciences, Tehran, Iran, 3Regional Centre of Advanced Technologies and Materials, Czech Advanced Technology and Research Institute, Palacky´ University in Olomouc, Olomouc, Czech Republic

18.1 Introduction 18.1.1 Metal nanoparticles and ionic liquids The phrase “ionic liquids” (ILs) in literature universally is used to address salts that are in their liquid state [1]. Ordinary liquids, for example, water and oils, are almost made of neutral molecules, while ILs normally encompass a large number of ions, for instance electrically nonneutral molecules [2]. Different experimental data has demonstrated that the bonds between ions are tremendously stronger than Van der Waals forces between molecules, culminating in high melting points of ILs. In other words, this property results from high lattice energy in ILs.

Advanced Applications of Ionic Liquids DOI: https://doi.org/10.1016/B978-0-323-99921-2.00003-3

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This category of liquids has some outstanding properties eventually becoming a cornerstone of electrochemical studies. Their remarkable attributes include high conductivity, thermodynamic stability, nonvolatility, low evaporation pressure, and hydrophobicity, among others. Because of these notable properties, they appear in an assortment of industrial and medical applications [3 8]. In addition, on account of reducing the consumption of hazardous and pollution chemicals, they are becoming an integral part of the industrial green chemistry [9,10]. The process of producing metal nanoparticles, especially in the IL as a media needs to proceed with the definition of nanoparticles involved: that is, a tiny object with all three external dimensions in the nanoscale (1100 3 1029 m), or 0.1 10 Angstrom, mean a nanoparticle. Nowadays, thanks to advancements and evolvement of imaging techniques, and improvements in scanning instruments have led to an enormous growth of applications of metal nanoparticles [11,12]. There are a variety of methods aiming to produce metal nanoparticles, namely chemical reduction, thermal decomposition, electrochemical reduction, and microwave irradiation [13,14]. Fortunately, during the synthesis of these tiny particles their shape, size, size distribution, and other morphological properties can be manipulated. In doing so, before going through using ILs, there are two different possibilities: first is the deployment of strong reducing agents like sodium hydride which seldom could control properly both, the size and the shape of metal nanoparticles; second relates to mild agents namely sodium citrate and ascorbic acid which besides controlling the morphological properties of metal nanoparticles can function as coordinating capping ligands as well [15]. One could expect that by manipulating the reducing agent, temperature, pH, and the concentration of metal salt precursors one may observe the varying size of the ensued metal nanoparticles. Although the sizes in the dimension 5 3 1029 m, (5 nm) are accessible but simultaneously one has to cope with the so-called agglomeration mechanism. The agglomeration problem of metal NPs would be addressed in this chapter using ILs, instead of the other techniques.

18.1.2 Silver as antibacterial agent: historical background Throughout the history of science, humans have constantly tried to improve their quality of life particularly in terms of finding effective remedies for a vast number of illnesses. Initially, herbal remedies played a critical role, thereafter, the advent of chemical and synthetic drugs had a dramatic impact on global health. Historically, it was Louis Pasteur and Robert Koch who, through studies on animals, succeeded in underpinning the causal relationship between infectious diseases and

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microorganisms; in fact, their investigation of bacteria from the 1860s to the 1890s simultaneously revolutionized medicine [16]. On the one hand, several illnesses stem from bacterium which include a wide variety of species and exist in a plethora of places whether foodstuffs, drinking waters, farmland, or even hospitals and health centers. In addition to this, owing to the morbidity rate increasing due to pathogenic bacteria, the development and design of novel potential compounds and materials with high antibacterial activity have paved the way for a viable remedy [17]. Conversely, drug-resistant pathogenic bacteria are empowered with the ability to either circumvent or thwart the action of typically used therapeutic antibiotics [18]. Bacteria become resistant to various panacea over time, this fact can be considered an interesting lesson of Darwinian evolution. As an example, Escherichia coli could change the nature of the cell wall which rendered antibiotics inactive and becoming resistant to some commercially available antibiotics [19]. Accordingly, one could go through the antibacterial properties of the metal nanoparticles. In the nanomaterials arena, silver nanoparticles (Ag NPs), owing to their antimicrobial properties, have appeared as the cornerstone of deploying these engineered tiny particles in a wide range of consumer and medical applications [20,21]. Apart from E. coli, on account of the overuse of antibiotics, other bacteria, such as methicillin-resistant Staphylococcus aureus and vancomycinresistant Staphylococcus aureus, which are staphylococci, have become resistant. With the advancement of nanoscience and the creation of Ag NPs endowed with newer properties and morphology, silver could regain its lost position and once again garner attention because of its inherent antibacterial properties. By increasing the surface-to-volume ratio of Ag NPs, it is possible to change their physical and chemical properties and increase their efficiency in killing bacteria. It appears that silver metal nanoparticles will play a key and forward-looking role in the fight against microbes in the future. Ag NPs are made up of Ag atoms and able to form clusters with diameters from 1 to 100 nm, and on account of their characteristic antimicrobial activities, they have received widespread attention. It could be claimed that Ag nanoparticles in both, consumer and industrial applications are one of the most commonly used metal nanoparticles today. There are many examples indicating the benefits of using Ag as an antimicrobial agent encompassing agricultural industries, food packaging, medical devices, water petrification, water, and air filters, and odor-resistant fabrics, among others [8,22,23]. As mentioned above, despite the outstanding benefits of using Ag NPs still there are possible health and environmental risks in the widespread usage of Ag and its nanoparticles [24]. Usually, human exposure to silver can occur orally, dermally, or via inhalation which causes the argyria disease in patients who inhale it in large amounts

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and/or for a long period. Nevertheless, it should be noted that being exposed to tiny particles, nanosized, or in bulk form of silver, no such problems or diseases have yet been widely recognized. To determine the probable risks of Ag NPs, especially in the nanoecotoxicology field, because of their simple structure, bacteria have been considered enormously [25,26]. One of the important stages of interaction between nanoparticles and bacteria is the possibility of their internalization within the bacteria. Historically, Chamakura and coworkers reported that any fingerprint of the presence of the 10 50 nm Ag nanoparticles in Gram-negative E. coli has not been found [27]. Hence, they measured the antimicrobial activity of Ag nanoparticles after 2 hours of exposure at a particle concentration of order 40 µg/mL, and by conventional transmission electron microscopy (TEM). To be more precise, the structure of E. coli treated with Ag-NPs was investigated by fluorescent microscopy to determine wall/membrane integrity. High-resolution transmission microscopy (HR-TEM), equipped with X-ray energy dispersive spectroscopy (EDS) were deployed to obtain morphological images and elemental distribution. Moreover, electron energy loss spectroscopy (EELS) was employed to determine the distribution of the elemental composition, to allow further understanding of the mechanism of E. coli’s inactivation. Regarding the interaction of Ag NPs with E. coli, Dror-Ehre et al. used conventional TEM and surprisingly found Ag NPs inside and on the membrane of the aforementioned bacteria [28]. Their report is only based on two-dimensional imaging and they did not consider elemental analysis or electron tomography. Going ahead, Yuan and colleagues, qualitatively, revealed that a limited number of Ag NPs could be being internalized after 3 hours of exposure at 10 mg/mL particle concentration by Gram-negative bacteria like Nitrosomonas europaea [29]; nanoparticles with sizes between 4 and 10 nm were deployed. Thus, it is obvious that Ag NPs for the latter report are smaller by a factor of almost five. This comparison elucidates the importance and the role of the size of Ag NPs being aimed for internalization in the bacteria. The chapter is organized wherein after the initial introduction, an overview is provided on the synthesis of nanoparticles in ILs followed by their antimicrobial applications, including closing remarks and discussions.

18.2 Silver nanoparticles’ antimicrobial properties and activities If one wanted to provide a brief and adequate definition, the antibacterial activity of nanoparticles is assumed as an ability of these metal tiny particles that can destroy or reduce the progress of bacterial growth. Today’s antimicrobial agents can be established in terms of origin from either a chemical approach or natural extraction [30]. For the latter, one

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immediately recalls products like aminoglycosides and sulfonamides which have been well studied. From another viewpoint, the mediator molecules can be classified as bactericidal or bacteriostatic, which for the former refers to the ability to kill the bacteria while the latter term pertains to breaking down the bacterial growth. In regards to controlling the microorganisms, the chemical and physical agents are the most known, the physical ones being radiation, heat, and filtration, among others. Antimicrobial nanomaterials have a better performance against microorganisms owing to their high surface-to-volume ratio and higher contact surface [31]. Meanwhile, Ag NPs have shown unparalleled antimicrobial activity against a wide range of microorganisms and have recently garnered the widespread attention of many researchers [22,32]. In regards to describing the mechanism which governs the antimicrobial activities of metal nanoparticles, this may be divided into two main steps. The first step is devoted to analyzing the behavior of the system in the aforementioned chemical or physical modification and the second one pertains to the way that metal nanoparticles interact with bacteria aiming to demonstrate internalizing and destroying the microorganisms. For the first situation, one can envision the aggregation, reduction-oxidation photo reaction, the release of nanoparticles, adsorption or desorption of ions, molecular species, or polymers which influences the metal nanoparticles’ activity and also the shape, size, and other properties of these tiny particles. For the second step, the rate and consequences of the interaction of these ultrathin particles with microbes do matter as shown in Fig. 18.1. Nowadays, with advanced nanotechnology, one can create metallic Ag in the form of particles with a size of fewer than 100 nanometers, which contain 10,000 15,000 silver atoms: termed Ag NPs or nanosilver particles. Nanosilver production technology has revolutionized antibacterial

FIGURE 18.1 Parts (A) and (B) show the real and imaginary parts of permittivity’s of the metal candidates Ag, Au, Na, K, and Al, respectively [33].

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materials, and is the main impetus for the development of nanosilver products that have many advantages over conventional chemicals; these particles show special physicochemical properties and biological activities and are crucial antibacterial agents against an extensive variety of antibiotic-resistant bacteria. Nanomaterials, particularly metal nanomaterials, inactivate enzymes and DNA microorganisms by electron balance between electron donor groups such as thiol, carboxylate, amide, imidazole, indole, and hydroxyl, due to their high charge and surface-to-volume ratio. In general, the main properties of Ag NPs include relative nontoxicity, high stability, hydrophilicity, thermal resistance, and increase of resistance in microorganisms [34]. Also, traditional medicine comprising herbal medicines and biosynthesized nanoparticles with plants have been widely accepted in various fields of medical sciences. Therefore, the synthesis of Ag NPs using different plant extractives has been amply demonstrated; Varma’s research group has reported the greener production of Ag nanoparticles by plants and natural products in recent years [35 37] In addition, nanoparticles can alter reactive oxygen species (ROS) levels within the cell and affect the cell growth and differentiation. Various studies in vitro and in vivo suggest that they can produce ROS and therefore, may play a vital role in intracellular calcium concentrations, activating transcription factors, and altering the cytokines. ROS can damage cells through various methods such as DNA damage, interference with cellular signaling pathways, and changes in the transcription process of genes [38]. The main mechanism of the antibacterial property of Ag NPs is the release of silver ions. However, the mechanism of action of Ag ions from the point of view of molecular microbiology is not yet fully understood. Some of the main mechanisms entail the damage to cell membranes, production of ROS, cell attack of Ag ions (or even Ag nanoparticles due to membrane cavities) and further damage to adenosine triphosphate (ATP) products, and inhibition of DNA replication. Assorted studies have amply illustrated that Ag ions damage the cell membrane. These reports are mainly based on the observation of large cavities or holes in the bacterial membrane by TEM analysis. Silver ions may interact with sulfur-containing membrane proteins (e.g., respiratory chain protein thiols) to cause physical damage to the membrane [39]. Nam in his study deployed Ag NPs with varying weight percentages of 0.1% 3% to materials in contact with oral tissue, for a limited period, helping them to return to a healthy condition (tissue conditioner); its antibacterial effect showed mutation on S. aureus and Streptococcus bacteria [40]. Further, with increasing the concentration of nanoparticles, this effect gets intensified. The antibacterial effect of Ag NPs has been illustrated against E. coli, the theory is that in the face of Ag nanoparticles, microorganisms lose their ability to replicate, and cell proteins become inactive [41].

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FIGURE 18.2 Synthetic processes for Ag NPs: first preparation of IL-Met-cit, followed by formation of Ag NPs.

In this context, very recently, the Farjadian research group devised a novel class of Ag NPs capped with IL-based metronidazolium [42]. In this novel nanoparticle, metronidazole is a positive moiety of IL and citrate simultaneously plays a dual role as a negative portion of IL and a reducing agent (Fig. 18.2). The synthesized IL was precisely characterized via 1HNMR, 13CNMR, and fourier transform-infrared spectroscopy (FTIR). Finally, the amount of metronidazolium and citrate as capping agents as specified by thermal gravimetric analysis (TGA) and X-ray diffraction spectroscopy, that have been used for the characterization of the crystalline structure of Ag NPs. TGA diagrams, demonstrate that Ag NPs possess 2.65% organic layer coating while TEM images reveal Ag NPs prepared with sizes less than 50 nm. The prepared Ag NPs were generated and capped with IL-based metronidazolium citrate (IL-Met-cit) and the preparation of nanoparticles in two days was monitored by ultraviolet-visible spectroscopy (UV-Vis) spectroscopy. FT-IR spectroscopy illustrated the presence of capping agents surrounding Ag NPs. ILMet-cit may efficiently play dual roles as reducing and stabilizing Ag NPs. The prepared Ag NPs could be deployed as an antibacterial agent in the synthesis of a wound dressing with significant efficiency in both, aerobic and anaerobic bacteria. In this case, the antibacterial activity of disks from several concentrations of Ag NPs in calcium alginate medium has been investigated in two classes of bacteria encompassing S. aureus (as Gram-positive bacteria) as well as Pseudomonase aeruguinosa as (Gramnegative) bacteria.

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18. Antibacterial properties of silver nanoparticles synthesized in ionic liquids

FIGURE 18.3 Schematic preparation of Ag NPs colloids in 1-butyl-3-methyl imidazolium halide ILs. Source: Reproduced with permission from V. Patil, S. Mahajan, M. Kulkarni, K. Patil, C. Rode, A. Coronas, et al., Synthesis of silver nanoparticles colloids in imidazolium halide ionic liquids and their antibacterial activities for Gram-positive and Gram-negative bacteria, Chemosphere 243 (2020) 125302. Copyright (2019) Elsevier.

In 2020, Patil et al. reported four types of 1-butyl-3-methylimidazolium halide-based ILs which were prepared through metathesis and anion exchange processes [43]. The Ag NPs were synthesized as colloids in four different ILs in a pressurized reactor by reduction of silver nitrate with hydrogen gas, without using any stabilizing agents, additives, or solvents (Fig. 18.3). Antibacterial activities of Ag NPs colloids in ILs were investigated via the well-diffusion method against both Gram-positive and Gram-negative bacteria like Bacillus cereus and E. coli. It appears that the antibacterial activities of ILs and Ag NPs colloids in ILs affected not only by ILs anions but also by Ag NPs particle size. The 1-butyl-3methylimidazolium iodide ([C4MIM][I]) IL revealed higher antibacterial activities among the prepared ILs. In addition, the presence of Ag NPs in ([C4MIM][I]) resulted in improving its antibacterial activity against both B. cereus and E. coli bacteria. In a compelling report, Bains and coworkers introduced some ILbased benzimidazolium and silver hybrid nanocomposites, which were precisely characterized by several techniques [44] (Fig. 18.4). The morphology, elemental composition, crystallinity, and size were investigated by dispersive X-ray, powder X-ray diffraction, scanning electron microscopy (SEM), and dynamic light scattering. In addition, the authors evaluated the antibacterial efficacy of ILs and IL@ metal composites by screening their potential against Gram-negative and Grampositive pathogenic microorganisms through colony-forming unit assay. The outcome from preliminary antibacterial screening illustrated that the IL@Ag nanocomposites represented higher antibacterial activity in

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473

FIGURE 18.4 Schematic illustration of the mechanism of IL-1d@Ag nanocomposite. Source: Reproduced with permission from D. Bains, G. Singh, N. Kaur, N. Singh, Development of an ionic liquid@ metal-based nanocomposite-loaded hierarchical hydrophobic surface to the aluminum substrate for antibacterial properties, ACS Appl. Bio Mater. 3 (2020) 4962 4973. Copyright (2020) American Chemical Society.

comparison to the ILs without silver. The IL@Ag nanocomposites exhibited significant potent activity against both, the S. aureus and E. coli bacterial strains with minimum inhibitory concentration (MIC) 5 08 6 2 mg/mL and 12 6 2 mg/mL respectively. To implement this nanomaterial in biomedical applications, the synthesized IL nano-Ag was fabricated onto a micro-structured aluminum substrate with hierarchically arranged functionalities. Moreover, the possible mechanism of antibacterial activity for this IL@Ag nanocomposite was evaluated via generation of ROS and the morphology of treated pathogenic bacteria was investigated via atomic force microscopy (AFM) and SEM analysis. Finally, the hydrophobicity of the material coated onto the Al substrate was evaluated by the static water contact angle technic which depicted an enhancement in hydrophobic character.

18.3 Discussions and final remarks In this review, an adequate explanation has been provided for both, the Ag NPs synthesis in ILs and the application of such nanosized particles, especially their antimicrobial activity. Furthermore, the origin of ILs, Ag NPs, and their antibacterial activities have been reviewed with a discussion on the well-known ILs and their salient properties exemplified by [BMIm] [PF6], [BMIm][TfO], [Me2NH2][Me2NCO2] and so on. Thus, the synthesis and stabilization of Ag NPs, by some renowned ILs facilitated in examining their antimicrobial activities. Previous studies have reported that the size and shape of Ag NPs are critical factors for their antibacterial properties.

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These crucial morphological factors go back to the fact that reducing the particle size causes enhanced contact with bacteria and increases the cell death effectively. Reducing the particle size also increases the enhanced potential for the release of silver ions from the surface thus providing enhanced antibacterial properties. Besides the size of the nanoparticles, the shape of the particles is also effective and the spherical morphology can engage with the bacteria and ultimately kill bacteria. Silver nanoparticles are mostly spherical, and studies have shown that the best form of Ag NPs to prevent bacterial growth are perchlorinated angular nanoparticles, spherical nanoparticles, and rod-shaped nanoparticles, respectively, because they have a greater ability to engage, respectively with bacteria.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment Authors wish to acknowledge the SASOL, North-West University, Material Science Innovation and Modelling and the National Research Fund (NRF) for financial support.

References [1] J.P. Hallett, T. Welton, Room-temperature ionic liquids: solvents for synthesis and catalysis. 2, Chem. Rev. 111 (2011) 3508 3576. [2] J.G. Huddleston, H.D. Willauer, R.P. Swatloski, A.E. Visser, R.D. Rogers, Room temperature ionic liquids as novel media for ‘clean’ liquid liquid extraction, Chem. Commun. (1998) 1765 1766. [3] S.E. Hooshmand, R. Afshari, D.J. Ramo´n, R.S. Varma, Deep eutectic solvents: cuttingedge applications in cross-coupling reactions, Green Chem. 22 (2020) 3668 3692. [4] A. Kumar, K. Kumari, S. Singh, I. Bahadur, P. Singh, Noscapine anticancer drug designed with ionic liquids to enhance solubility: DFT and ADME approach, J. Mol. Liq. 325 (2021) 115159. Available from: https://doi.org/10.1016/j.molliq.2020.115159. [5] B. Shi, The strengths of van der Waals and electrostatic forces in 1-alkyl-3methylimidazolium ionic liquids obtained through Lifshitz theory and Coulomb formula, J. Mol. Liq. 320 (2020) 114412. [6] S.K. Singh, A.W. Savoy, Ionic liquids synthesis and applications: an overview, J. Mol. Liq. 297 (2020) 112038. [7] K.S. Egorova, E.G. Gordeev, V.P. Ananikov, Biological activity of ionic liquids and their application in pharmaceutics and medicine, Chem. Rev. 117 (2017) 7132 7189. [8] Z. Fallah, E.N. Zare, M.A. Khan, S. Iftekhar, M. Ghomi, E. Sharifi, et al., Ionic liquidbased antimicrobial materials for water treatment, air filtration, food packaging and anticorrosion coatings, Adv. Colloid Interface Sci. 294 (2021) 102454. Available from: https://doi.org/10.1016/j.cis.2021.102454.

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[9] P.C. Marr, A.C. Marr, Ionic liquid gel materials: applications in green and sustainable chemistry, Green Chem. 18 (2016) 105 128. [10] T. Welton, Ionic liquids in green chemistry, Green Chem. 13 (2011) 225. [11] R.K. Sharma, S. Yadav, S. Dutta, H.B. Kale, I.R. Warkad, R. Zboˇril, et al., Silver nanomaterials: synthesis and (electro/photo) catalytic applications, Chem. Soc. Rev. (2021). Available from: https://doi.org/10.1039/D0CS00912A. [12] C.N.R. Rao, S.R.C. Vivekchand, K. Biswas, A. Govindaraj, Synthesis of inorganic nanomaterials, Dalt. Trans. (2007) 3728 3749. [13] S. Navalon, A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Metal nanoparticles supported on two-dimensional graphenes as heterogeneous catalysts, Coord. Chem. Rev. 312 (2016) 99 148. [14] J.M. Walker, J.M. Zaleski, A simple route to diverse noble metal-decorated iron oxide nanoparticles for catalysis, Nanoscale 8 (2016) 1535 1544. [15] F. Davar, M.R. Loghman-Estarki, M. Salavati-Niasari, M. Mazaheri, Controllable synthesis of covellite nanoparticles via thermal decomposition method, J. Clust. Sci. 27 (2016) 593 602. [16] A.K. Tiwari, S. Gangopadhyay, C.-H. Chang, S. Pande, S.K. Saha, Study on metal nanoparticles synthesis and orientation of gemini surfactant molecules used as stabilizer, J. Colloid Interface Sci. 445 (2015) 76 83. [17] P.J. Johnsen, J.P. Townsend, T. Bøhn, G.S. Simonsen, A. Sundsfjord, K.M. Nielsen, Factors affecting the reversal of antimicrobial-drug resistance, Lancet Infect. Dis. 9 (2009) 357 364. [18] J. Hoque, M.M. Konai, S. Samaddar, S. Gonuguntala, G.B. Manjunath, C. Ghosh, et al., Selective and broad spectrum amphiphilic small molecules to combat bacterial resistance and eradicate biofilms, Chem. Commun. 51 (2015) 13670 13673. [19] A. Hozzari, P. Behzadi, P.K. Khiabani, M. Sholeh, N. Sabokroo, Clinical cases, drug resistance, and virulence genes profiling in uropathogenic Escherichia coli, J. Appl. Genet. (2020) 1 9. [20] C.M. Crisan, T. Mocan, M. Manolea, L.I. Lasca, F.-A. T˘ab˘aran, L. Mocan, Review on silver nanoparticles as a novel class of antibacterial solutions, Appl. Sci. 11 (2021) 1120. [21] C.G.A. Das, V.G. Kumar, T.S. Dhas, V. Karthick, K. Govindaraju, J.M. Joselin, et al., Antibacterial activity of silver nanoparticles (biosynthesis): a short review on recent advances, Biocatal. Agric. Biotechnol. 27 (2020) 101593. [22] J. Lalley, D.D. Dionysiou, R.S. Varma, S. Shankara, D.J. Yang, M.N. Nadagouda, Silver-based antibacterial surfaces for drinking water disinfection—an overview, Curr. Opin. Chem. Eng. 3 (2014) 25 29. [23] S.H. Lee, B.-H. Jun, Silver nanoparticles: synthesis and application for nanomedicine, Int. J. Mol. Sci. 20 (2019) 865. [24] J.-Y. Maillard, P. Hartemann, Silver as an antimicrobial: facts and gaps in knowledge, Crit. Rev. Microbiol. 39 (2013) 373 383. [25] M. Ajdary, F. Keyhanfar, M.A. Moosavi, R. Shabani, M. Mehdizadeh, R.S. Varma, Potential toxicity of nanoparticles on the reproductive system animal models: a review, J. Reprod. Immunol. (2021) 103384. [26] P.D. Howe, S. Dobson, Silver and Silver Compounds: Environmental Aspects, World Health Organization, 2002. [27] K. Chamakura, R. Perez-Ballestero, Z. Luo, S. Bashir, J. Liu, Comparison of bactericidal activities of silver nanoparticles with common chemical disinfectants, Colloids Surf. B Biointerfaces 84 (2011) 88 96. [28] A. Dror-Ehre, H. Mamane, T. Belenkova, G. Markovich, A. Adin, Silver nanoparticle E. coli colloidal interaction in water and effect on E. coli survival, J. Colloid Interface Sci. 339 (2009) 521 526.

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[29] Z. Yuan, J. Li, L. Cui, B. Xu, H. Zhang, C.-P. Yu, Interaction of silver nanoparticles with pure nitrifying bacteria, Chemosphere 90 (2013) 1404 1411. [30] F. Solo´rzano-Santos, M.G. Miranda-Novales, Essential oils from aromatic herbs as antimicrobial agents, Curr. Opin. Biotechnol. 23 (2012) 136 141. [31] O. Akhavan, E. Ghaderi, Toxicity of graphene and graphene oxide nanowalls against bacteria, ACS Nano 4 (2010) 5731 5736. [32] O. Akhavan, E. Ghaderi, Enhancement of antibacterial properties of Ag nanorods by electric field, Sci. Technol. Adv. Mater. (2009). [33] N. Chouhan, Silver Nanoparticles: Synthesis, Characterization and Applications, IntechOpen, 2018. [34] B. Le Ouay, F. Stellacci, Antibacterial activity of silver nanoparticles: a surface science insight, Nano Today 10 (2015) 339 354. [35] M. Khatami, S. Iravani, R.S. Varma, F. Mosazade, M. Darroudi, F. Borhani, Cockroach wings-promoted safe and greener synthesis of silver nanoparticles and their insecticidal activity, Bioprocess. Biosyst. Eng. 42 (2019) 2007 2014. [36] D. Hebbalalu, J. Lalley, M.N. Nadagouda, R.S. Varma, Greener techniques for the synthesis of silver nanoparticles using plant extracts, enzymes, bacteria, biodegradable polymers, and microwaves, ACS Sustain. Chem. Eng. 1 (2013) 703 712. Available from: https://doi.org/10.1021/sc4000362. [37] M.C. Moulton, L.K. Braydich-Stolle, M.N. Nadagouda, S. Kunzelman, S.M. Hussain, R.S. Varma, Synthesis, characterization and biocompatibility of “green” synthesized silver nanoparticles using tea polyphenols, Nanoscale 2 (2010) 763 770. [38] O. Choi, Z. Hu, Size dependent and reactive oxygen species related nanosilver toxicity to nitrifying bacteria, Environ. Sci. Technol. 42 (2008) 4583 4588. [39] S. Kim, D. Ryu, Silver nanoparticle-induced oxidative stress, genotoxicity and apoptosis in cultured cells and animal tissues, J. Appl. Toxicol. 33 (2013) 78 89. [40] K.-Y. Nam, In vitro antimicrobial effect of the tissue conditioner containing silver nanoparticles, J. Adv. Prosthodont. 3 (2011) 20 24. [41] J.R. Morones, J.L. Elechiguerra, A. Camacho, K. Holt, J.B. Kouri, J.T. Ramı´rez, et al., The bactericidal effect of silver nanoparticles, Nanotechnology 16 (2005) 2346. [42] F. Farjadian, A.R. Akbarizadeh, L. Tayebi, Synthesis of novel reducing agent for formation of metronidazole-capped silver nanoparticle and evaluating antibacterial efficiency in Gram-positive and Gram-negative bacteria, Heliyon 6 (2020) e04747. [43] V. Patil, S. Mahajan, M. Kulkarni, K. Patil, C. Rode, A. Coronas, et al., Synthesis of silver nanoparticles colloids in imidazolium halide ionic liquids and their antibacterial activities for Gram-positive and Gram-negative bacteria, Chemosphere 243 (2020) 125302. [44] D. Bains, G. Singh, N. Kaur, N. Singh, Development of an ionic liquid@ metal-based nanocomposite-loaded hierarchical hydrophobic surface to the aluminum substrate for antibacterial properties, ACS Appl. Bio Mater. 3 (2020) 4962 4973.

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C H A P T E R

19 Progressive function of ionic liquids in polymer chemistry Panmei Gaijon, Arun Kant, Sudipta Ghosh and M. Ramananda Singh Department of Chemistry, Kirori Mal College, University of Delhi, New Delhi, India

19.1 Introduction During the last few years, ionic liquids are attracting more and more attention from researchers as greener and designer mediums for a variety of chemical reactions. The ionic liquid has the potential to replace organic solvent at room temperature in the catalytic process, in industrial as well as laboratory-scale [1]. Such ionic liquids are also known as room-temperature ionic liquids and they exist as a liquid at room temperature due to the larger size of ions. In 1914, [EtNH3][NO3] is found as the first room-temperature ionic liquid which has a melting point of 12
C. It is made up of a cation and an anion. But the interest in the field of the ionic liquid has not emerged among the researchers until the invention of binary ionic liquids which are composed of two components like aluminum (III) chloride and N-alkylpyridinium or 1,3-dialkylimidazolium chloride [2]. Two main types of ionic liquids are (1) simple salts ionic liquid-like [EtNH3][NO3] which is made of a cation and an anion and (2) binary ionic liquids such as mixtures of aluminum (III) chloride and 1,3-dialkylimidazolium chlorides. In recent years ionic liquid has been employed as a green solvent and its applications in polymer science are mainly in a variety of polymerization process as polymerization media, atom transfer, conventional free radical polymerization, fragmentation

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transfer of reversible addition, and coordination polymerization [38]. Some of the Ionic liquids are also used as a component of matrixes of polymers like gel of polymer, novel electrolytes for electrochemical polymerization, and templates for porous polymer [9]. Applications of ionic liquid in the polymerization process were widely used as a solvent [10,11]. The ionic liquid in polymer science has been used in various types of polymerization processes mainly as polymerization media including conventional and atom transfer polymerization of free radical and coordination polymerization [6,8,12,13]. Since the Ionic liquid is a greener medium and its important properties like electrochemical stabilities and low flammability, it is considered an alternative safe medium compared to traditional liquid as electrolyte for electrochemical applications in the battery industry [14,15]. Some of the electrolyte systems containing poly-tetrafuran in which Li-ion is less bounded with backbones of polymers due to weaker interaction between Oxygen of the furan and Li-ion. Therefore it shows a high Liion transport number of about 0.53 as compared to traditional polyethylene oxide-based polymer electrolytes. Similarly, in the case of poly (diallyldimethylammonium) bis(trifluoromethanesulfonyl) imide, it also can enhance the transportation of Li-ion through the simplification of dissociation of Li salt [16,17]. The combination of a polymer membrane with ionic liquid leads to the formation of polymer inclusion of membrane, where an ionic liquid is entrapped in the membrane structure. Polymerizable ionic liquid monomers are synthesized by in situ polymerization method and it can be applied to structure the network of interpenetrating polymer ionic liquids [18,19]. The incorporation of ionic liquids which have different cations and anions into the matrix of poly(vinylidene fluoride) affects the polarity and electroactive phase in the composite and such composite is also used as a plasticizing agent [20]. A large variety of ionic liquid-polymer matrices can be prepared and a wide range of their applications are associated with them, moreover, some specific functionality such as thermochromism of ionic liquid-polymer matrix are found in them which are due to intrinsic properties of ionic liquid and different other properties can be due to various way of interaction between ionic liquid and polymer matrix [21]. Alternating copolymer and linear copolymer with the different structural units have the potential application as polymer electrolytes and optoelectronic devices [22,23]. The evaluations of the performance of solid-state polymer electrolytes have two key properties like mechanical strength and ionic conductivity [24]. This chapter focuses on the recent functions of ionic liquid in polymer science in terms of preparation of functional polymer, modification of polymer caused by the assortment of ionic liquid, ionic liquidpolymer matrix, composite, ionic liquid-polymer gels, etc.

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19.4 Some important advantages and characteristics of ionic liquid

481

19.2 Ionic liquid The ionic liquid is a salt at ambient temperature, which has a low melting point which is much lesser than the boiling point of water [25]. At room temperature, the state of ionic liquid is often achieved by choosing ion pairs where one is organic with a delocalized charge so that ions are poorly coordinated and the ionic liquid with a low melting point is formed. Ionic liquids are commonly made up of organic cation and inorganic anion. They are commonly known by various names such as greener solvent, designer, neoteric solvents, molten salts, and ionic fluids. They are also found to be used in numerous chemical reactions and the preparation of functional materials as a solvent [2634]. Ionic Liquids are currently considered to be a potential alternative as a green solvent as compared to non-conventional solvents which are toxic and hazardous in addition to their application in polymer chemistry as additives, plasticizers, polymer electrolytes, etc. [35]. They are also used as a task-specific and greener solvent that has extremely low vapor pressure and is capable of minimizing pollution associated with volatile organic solvents through recycling [25]. It also has various applications either as a catalyst or catalytic phase in the synthesis of nanoparticles and organometallic reactions etc. [36,37].

19.3 Structure of ionic liquid Different cations and anions are combined to form various molecular structures of Ionic Liquids. The cations are positively charged organic compounds which are bigger in size as shown in Fig. 19.1 and anions are negatively charged which are smaller in size as shown in Fig. 19.2. Due to the difference in the size of cation and anion, ionic liquid has a weak bond which is why this compound exists as a liquid at room temperature or below 100
C. Those ionic liquids which exist as liquids above 100
C are known as molten liquid whereas others that exist as a liquid at room temperature are known as room-temperature liquid.

19.4 Some important advantages and characteristics of ionic liquid Ionic liquids are liquids with low vapor pressure, less volatile, and highly polar. They exist as a liquid from a very low temperature up to the maximum of 200
C [38]. Some of the Ionic liquids are the liquids of high stability and resistivity to heat up to 300
C. They also have properties of high electrical conductivity.

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Sulfonium

Pyrollidium

Pyrazolium

Piperidinium

Ammonium

Morpholinium

Guanidinium

Phosphonium

Imidazolium

Pyridinium

Cholinium

FIGURE 19.1 Molecular structures of common cations used in ionic liquids.

19.5 Common methods of making ionic liquids Ionic liquids were synthesized in various ways. They are as followed.

19.5.1 Anion exchange The anion exchange method is commonly used for the synthesis of ionic liquid-containing tetrafluoroborate and hexafluorophosphate which are used in conventional studies. Despite this, aluminum trichloride salt is extensively used in the synthesis of Lewis acidbase ionic liquid through this method [39]. This method involves two steps which are as followed [40]. 1. In the first step, the cation is formed by either acidification of amine or quaternization of haloalkane with amine through a thermal process. 2. In the second step, a reaction is carried out between halide salts with Lewis acid to form the desired Lewis acid-based ionic liquid.

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19.5 Common methods of making ionic liquids

Cl

-

Br

Hexafluorophosphate

Triflate

Trifluoromethylacetate

-

-

Bromide

Chloride

FIGURE 19.2

-

BF4

NO3

Tetrafluoroborate

Nitrate

Alkylsulphate

Dicyanamide

Bis (trifluoromethylsulfonyl) imide

Acetate

Tetracyanoborate

Chemical structures of anions which are commonly used in ionic liquids.

19.5.2 Alkylation The alkylation method is used to get ionic liquid. Ionic liquid (phosphonium, pyridinium and imidazole, and ammonium-based alkyl cation, etc.) is synthesized using an alkylating agent like haloalkane that is the nucleophile. For example, Alkyl salts of pyrimidine are produced by halogens of alkanes with alkaline pyridine. The imidazole-based ionic liquid in which an alkyl imidazole is a basic unit that is synthesized by alkalizing imidazole [41].

19.5.3 Preparation of ionic liquid special performance Ionic liquids with special performance or functional ionic liquids are used as an important solvent in catalysis and chemical synthesis. In this synthesis, the materials used are lubricants and pharmaceutically active substances. A variety of ionic liquids are synthesized with a special performance. Haloalkane reacts with contributing agents like pyridinium, imidazolium, and phosphonium to form ionic fluids. Preparation of different ionic liquids is done using ionized agents through anionic exchange with alkaline salt and commercial acids [42]. Ionic Liquid with special performance can be prepared using functionalized alkyl sulfates anion. The ionic liquid with a special performance like immunosuppressive ionic liquid was developed with a focus on the remediation of metal ions from the polluted solution.

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19. Progressive function of ionic liquids in polymer chemistry

19.6 The function of ionic liquid in polymer Various polymerization processes employ Ionic liquid either as a medium or plasticizer for varieties of polymer materials and they are also used as components in a polymer gel. Polymer membrane and porous material absorbed the ionic liquid, which causes concomitant changes in mobility and conductivity of ions [43]. Ionic liquid-containing polymer, Polymer ionic liquid composites, and polymer ionic liquid gel are synthesized by using typical free radical polymerization, conventional radical polymerization, doping of polymer with ionic liquids, in situ polymerization, polymerization of polymerizable ionic liquid, etc. A schematic representation of the synthesis of polymer containing ionic liquid is shown in Fig. 19.3.

19.7 Polymer-doped ionic liquid Ionic liquid, poly(vinylidene fluoride)-hexafluoropropylene gels containing trapped ionic liquid such as tetrafluoroborate; BF42 and 1-ethyl-3methylimidazolium salts of triflate; CF3SO32, such ionic liquid-based polymer gels are flexible, potential as catalytic membrane, freestanding and have conductivity range at room temperature about 1.15.8 mS/cm [44,45]. Poly

Ionic liquid

Polymer

Wide range of Ionic liquid polymer

FIGURE 19.3 A diagram of synthesis of polymer containing Ionic liquids.

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19.8 Polymerization of vinyl monomer in ionic liquid

485

(vinylidene fluoride)-hexafluoropropylene ionic liquid gels were synthesized using ionic liquids of 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate and 1-(2-hydroxyethyl)-3-methyl imidazolium hexafluorophosphate. Such polymeric film at a temperature range of 20
C70
C exhibited ionic conductivity in a range between 1 and 0.01 mS/cm [46]. Polymers like poly(acrylonitrile), poly(vinylalcohol), and Ionic liquids like 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium hexafluorophosphate and1-butyl-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide mixed using the casting method to form thin-film polymer electrolyte that has the properties of elasticity and it shows the conductivity of 15 mS/cm at room temperature [47]. Ionic liquid gel synthesized through 5 wt.% of triblock copolymer such as poly(styrene-block-ethylene oxide-block-styrene) in 1-butyl-3methylimidazolium hexafluorophosphate shows conductivity in range of 1.21.6 mS/cm [48].

19.8 Polymerization of vinyl monomer in ionic liquid A simple method for the preparation of ionogels is used by mixing ionic liquids with polymers even though such a method does not allow much turning of the interaction of the polymer with ionic liquid. In the case of polymerization of a vinyl monomer, it can form ionic gels with cross-linking polymerization because of its high solubility in ionic liquid. These types of ionic liquid polymer-based gels have additional chemical and physical properties. A transparent, highly conductive, and mechanically strong polymer electrolyte was obtained by radical polymerization (in situ) of a polymer 2-hydroxyethyl methacrylate in 1butylpyridinium tetrafluoroborate (BPBF4) [49]. The polymer gels made up of ethylmethacrylate (MMA) in [1-butyl-3methylimidazolium][PF6] revealed brilliant thermal stability up to 260
C [50]. Various Polymer ionic gels can be synthesized at room temperature using ionic liquid, 1-ethyl-3-methyl imidazoliumbis(trifluoromethanesulfonyl) imide through polymerization of vinyl monomers like acrylonitrile, styrene, MMA, vinyl acetate, 2-hydroxyethyl methacrylate, methyl acrylate, and acrylamide. Some of the polymer ionic liquid gels are used as polymer electrolytes which shows the high conductivity of ions. Among these, the polymer ionic liquid gels prepared using methyl acrylate or MMA in 1ethyl-3-methyl imidazoliumbis(trifluoromethanesulfonyl) imide have the value of ionic conductivity of about 10 mS/cm and it is flexible at ambient temperature [51]. Some polymer electrolytes such as Ionic gels are also synthesized through polymerization of the monomer of ionic liquid. Ionic liquid gels of cross-link were formed by introducing the vinyl group on a cation or anion moiety of an ionic liquid covalently at a chosen

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(B) (A)

FIGURE 19.4 Molecular structures of (A) 11-(1-methyl-imidazolium-3-yl-) undecylacrylate tetrafluoroborate (Ionic Liquid-BF4) and (B) poly(1-vinyl-3-ethylimidazolium bromide).

temperature [5254]. Liquid, Ionic Liquid polymer prepared from a reactive monomeric ionic liquid is based on the imidazolium undecylacrylate as depicted in Fig. 19.4A. A waxy solid can also be formed when it is polymerized in dimethyl formamide or methylene chloride solution as a homopolymer and it has melting point that variation from 38
C to 42
C. A polymer of Bromide containing monomer of imidazolium undecylacrylate was synthesized in dimethyl formamide. Such polymer has a melting point of about 75
C and they are considered to be potential bactericide [55,56]. Ionic liquid 1-vinyl-3-ethylimidazolium bromide monomer and its homopolymer, as shown in Fig. 19.4B, were applicable as shale hydration inhibitors which were synthesized by typical free radical polymerization. It shows good thermal stability and they are applied as a sealing additive for controlling water ingress into shale and it has beneficial to high-temperature drilling [57].

19.9 Polymerizable ionic liquid Monomers of ionic liquid are also polymerized to give ionic gels and a variety of electrolytes of polymer are synthesized through the polymerization process in ionic liquid-based monomer [54,58]. Macromolecules are also obtained from polymerizing ionic liquid monomers, which have various applications as polymeric electrolytes, catalytic membranes, microwave absorbing materials, and ionic conducting materials [43,5861]. A variety of polymerizable ionic liquid-like polyanion type ionic liquid, polycation type ionic liquid, poly(zwitterion), and copolymer are used for the preparation of ionic gels but ionic conductivity of these polymerized ionic liquid are lowered because of having less number of mobile ions on covalently bonding and elevation of glass transition temperature [62,63].

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487

19.9 Polymerizable ionic liquid

Some functional groups containing ionic liquid are shown in Fig. 19.5. The polymerizable ionic liquid such as N-vinylimidazolium, acrylate, and vinylbenzyl are polymerized via UV photopolymerization or free radical polymerization [64,65]. Such polymer shows the change in the properties like decreasing in ionic mobility and ionic conductivity and increases in glass transition temperature [66]. Polymerized ionic liquid has also shown other properties like enhancing stability, flexibility, and durability of the polymeric membrane [67]. Liquid crystal hydrogel in nano-scale is prepared by self-assembly and photopolymerization of acrylate monomeric amphiphile, 1-(8-(acryloyloxy)octyl)-3-methylimidazolium chloride, [AcrC8mim1][Cl2] which are used to obtain a poly[AcrC8mim1][Cl2] hydrogel that is crosslinked and lamellar structure. This liquid crystalline hydrogel goes through reversible swelling in water up to 200 times its original forms [68]. The photosensitive material which can exist in the form of liquid crystalline alignment layers can also be synthesized through the coupling formed based on ionic self-assembly by the coupling of polymerized ionic liquids with azobenzene dye methyl orange which is photosensitive. A diagrammatic representation of photosensitive material is depicted in Fig. 19.6. It is employed as photosensitive material in various fields [69]. The unique physiochemical properties of the synthesized polymeric ionic liquid are that they have a large area of application as compared to the ionic liquid monomer. The polymeric ionic liquid

N-vinylbenzyl

Vinylbenzyl

Acrylate

FIGURE 19.5 Common function group used in polymerization of ionic liquid.

FIGURE 19.6 Structure of ionic complex of methylated poly (4-vinylpyridine) with methyl orange.

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19. Progressive function of ionic liquids in polymer chemistry

system generally displays constant viscosity scaling association in the semi-dilute un-entangled and semi-dilute entangled arrangement of the structure. Electrospinning can make polymerized ionic liquid as a fiber due to high ionic conductivity. Polymerized Ionic liquid such as poly(1-butyl-3-methylimidazolium tetrafluoroborate) exists as a fiber with ribbon structure because of being non-volatile and it also hinders the solidification of the fibers to a smaller size [70].

19.10 Adsorbed and covalently linked ionic liquids Polymeric Ionic liquid made up of polystyrene is commonly used as an efficient catalyst in the conversion of haloalkanes and sulfonyloxyalkanes to the fluorinated moiety via nucleophilic fluorination or other substitution. It is also used with tetrafluoroborate as a counter anion which has potential catalytic properties as compared to its counterpart. Such polymer can also be regenerated repeatedly without losing its activity as well as its composition [71,72]. The molecular structure of Ionic liquid, 11-(1-methyl-imidazolium-3-yl-) undecylacrylate and 2-acrylamido-2-methyl-1-propanesulfonicacid, AMPS is shown in Fig. 19.7. These components are polymerized to produce poly (ionic liquid AMPS) which has a high charge density and a melting point of 28
C. Poly(ionic liquid AMPS) is more interesting because it has high ionic conductivity due to which it can be used in the formulation of polymeric membrane for sensors, batteries and fuel cells, etc.

19.11 Microwave absorbing ionic liquid polymer The ionic liquid-containing polymer also acts as Microwave Absorbing material which can be employed in stealth aircraft technology, microwave darkroom protection, and microwave cookware; it can also reduce

FIGURE 19.7 Molecular structure of 11-(1-methyl-imidazolium-3-yl-) undecylacrylate AMPS (ionic liquid-2-acrylamido-2-methyl-1-propanesulfonicacid).

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19.12 Ionic liquid-polymer composite

electromagnetic interference from the computer [7378]. Since the poly (ionic liquid) like poly[p-vinylbenzyltrimethylammonium tetrafluoroborate] and poly[1-(p-vinylbenzyl)-3-butylimidazolium tetrafluoroborate] as shown in Fig. 19.8, act as an insulator and they exhibit no magnetic loss, they are also used as microwave absorbing material [79].

19.12 Ionic liquid-polymer composite Ionic liquid-polymer composite containing noble metal composite is commonly formed depending on the presence of noble metal as an oxidizing agent in an ionic liquid medium. Such polymers are also found to be conducting in nature. Many salts such as AuCl2 and AgNO3 which has a high solubility in ionic liquid-like [1-ethyl-3-methylimidazolium] [bis(trifluoromethylsulfonyl)imide] and such metal ions participate in polymerization of the monomer of terthiophene and pyrrole oxidatively. Poly(pyrrole)/silver composite and poly(pyrrole)/gold composite are found to have a uniform size in the range of 510 nm and 515 nm respectively [80,81]. Polymer ionic liquid composite (P(PEGDGEα-BDT)), as shown in Fig. 19.9 have a molar mass of 10,100 g/mol and

(A)

(B)

FIGURE 19.8 Structure of (A) poly[p-vinylbenzyltrimethylammonium tetrafluoroborate] and (B) poly[1-(p-vinylbenzyl)-3-butylimidazolium tetrafluoroborate].

FIGURE 19.9 Polymer ionic liquid composite; P(PEGDGE-α-BDT).

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19. Progressive function of ionic liquids in polymer chemistry

its dispersity of 1.6. it shows glass transition at low temperature, sharp melting point at 62
C, decomposition temperature over 270
C, low mechanical strength, and high ionic conductivity [82]. Composite electrolyte are obtained by casting method using ionic liquid N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)-imide and lithium bis(fluorosulfonyl)imide with poly(diallyldimethylammonium) bis (trifluoromethanesulfonyl)imide. It is used in various manufacturing units of batteries with high energy density [83].

19.13 Summary The progressive function of ionic liquid in polymer chemistry is described in the schematic diagram representing the various applications of ionic liquid and ionic liquid-containing polymers as shown in Fig. 19.10. A polymer containing ionic liquid has relevant properties which come into a novel class of materials that are synthesized by the Ionic liquid containing polymer (IL-Polymer composite IL-Polymer gels)

Application of Ionic liquid containing polymer

Properties and application of Ionic liquid

Use as green solvent with low melting temperature

Battery, device and electrical vehicle application

It is highly polar and More Stable

As a functional polymer

Non-volatile and have low vapor pressure

As a membrane and shale hydration inhibitor

It have biological properties and act as a medicine

In catalytic process

As catalyst for variety of reaction

Exhibit high ionic Conductivity

FIGURE 19.10 Use of ionic liquid-containing polymer and properties of ionic liquid.

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polymerization of ionic liquid monomers and they are found to be having auspicious properties of polymers and ionic liquids. Polymeric ionic liquids also have various applications in the analytical fields, issues related to the interaction of protein and Capillary electrophoresis and electro osmotic flow. Polymers containing ionic liquidbased electrolytes are cost-effective and environmentally compatible. Characterization of the best composition of ionic liquid-containing polymer was determined with an appropriate analytical techniques like X-ray diffraction, Raman Spectroscopy, Fourier transform infrared spectroscopy, Mass spectroscopy, Thermogravimetric analysis, and Nuclear magnetic resonance spectroscopy to find out the expected structure. Many studies were carried out to find out the thickness, porosity, thermal shrinkage behavior, and electrochemical performance like ionic conductivity, electrochemical study, activation energy, and ion transport mechanism. It has been observed that ionic liquid plays a very important role in polymer chemistry in the modification of the properties of the polymers and its applications in various fields of industries. It also encourages the researcher to work on various applications of ionic liquid in polymer chemistry due to many opportunities available to look into various spectrums of the progressive function of ionic liquid.

Conflict of interests The authors declared that there is no conflict of interest in the publication of this book chapter.

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20 Potential hazards of ionic liquids: a word of caution Saima Farooq and Zakira Naureen Department of Biological Sciences and Chemistry, College of Arts & Sciences, University of Nizwa, Nizwa, Oman

20.1 Introduction Almost two-thirds of the volatile organic compounds (VOCs), the common industrial solvents, are one of the major contributors to environmental pollution deleteriously affecting the ecosystem and biodiversity, contaminating the aqueous effluents, and causing pathogenesis in humans [1]. Their high levels of toxicity, volatility, and flammability are the characteristic features that make them a constant threat to humanity [2,3]. Recently, VOCs have been engineered into non-/or least hazardous alternatives to the harmful chemical solvents, as a “green-technology approach”; among those, ionic liquids (ILs) have garnered tremendous attention due to their fascinating properties Such as low volatility, high thermal and chemical stability, less flammability and excellent solvation compared to their conventional counterparts such as acetone, ethanol, etc. [4 7]. These characteristics have not only proven them as a revolutionary solvent in a wide range of applications but also privileged them in combating the horrendous issue of environmental pollution which was augmented with the use of conventional solvents. Generally, ILs, are composed of large, asymmetrical organic cations attached with a weakly-coordinating anion (Table 20.1) resulting in low melting point, below 100 C, composition [8]. Interestingly, the physical properties of ILs are linked to the structure of cation and anionic precursors, thus far, the selective choice of precursors can tailor their intrinsic properties [9,10]. Nevertheless, the possibility of diverse structural

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TABLE 20.1 Chemical composition of ionic liquids. Common cations

Common anions

Imidazolium

Tetrafluoroborate

Pyridinium

Bis (trifluoromethylsulfonyl) imide

Piperidinium

Hexafluorophosphate

Common inorganic anions

Morpholinium

Alkyl sulfate

Pyrrolidinium

Tosylate

Phosphonium

Acetate

(Continued)

TABLE 20.1 (Continued) Common cations

Common anions

Ammonium

Dicyanamide

Sulfonium

Alkyl sulfonate

Common inorganic anions

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501

variation in ILs makes them innovative solvents for task-specific applications. Considering the promising characteristics of ILs, their use as a reaction solvent in numerous applications such as bio-catalysis, organic synthesis, chemical catalysis, fuel cells, energy storage, etc., have been widely reported [11 18]. These findings have documented the biocompatibility of the solvent media due to their least volatility and toxicity. However, a substantial amount of research work has been done so far, highlighting the toxicological effects of ILs in both soil and aquatic environments on a variety of organisms including plants, microorganisms, and land and marine animals [19 25]. Overall, these findings have concluded the impact of their toxicity poses serious threats to the environment and demand preventive measures to combat the hazards of ILs. Henceforth, this chapter briefly reviews the toxicity of ILs to the environment.

20.2 Environmental concerns of ionic liquids ILs enter terrestrial and aquatic environments through wastewater discharges and landfill leaching. Because of their persistence and bioaccumulation properties, some of the ILs pose a tremendous threat to the survival of the susceptible species, as illustrated in Fig. 20.1. As a result, several studies have been conducted to estimate their longterm hazards and toxic effects on various organisms ranging from bacteria to humans. Initially, the ecotoxicity of ILs was focused on the aquatic environment and species [26]; however, the trends have

FIGURE 20.1

Environmental toxicity of ionic liquids.

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shifted to genotoxicity [27], cytotoxicity [28], enzymatic inhibition [29], phytotoxicity ([30]; Barbara [31]). In addition to in vivo assays and cellular studies, in silico models have been utilized to predict ILs toxicity such as small planktonic crustacean Daphnia magna [25], the bioluminescent marine bacterium Vibrio fischeri [32], various human and animal cell lines [33] and enzymes such as acetylcholinesterase (AChE) [34]. Studies have suggested that ILs mediate toxicity in living organisms not only because of their internal chemical structure but also due to their varying external influences on different biological systems under various environmental conditions. The data resorting ILs toxicity is mainly presented in the form of effective concentration (EC) or inhibitory concentration (IC) that causes a reduction in growth or reproductive activity. For Instance, EC50 means the EC that inhibits the growth or reproduction to 50%.

20.2.1 Cytotoxic action of ionic liquids Recent studies on mammalian cell lines such as murine fibroblasts and human keratinocytes and fibroblasts as well as cancer cell lines have revealed cytotoxic effects of ILs [35 37]. These studies have reported the ICs of ILs containing halide anions such as bromide [Br]2 or chloride [Cl]2 anions causing 50% inhibition of the biochemical or biological activity. In all types of cell lines used thus far, it has been noticed that ILs having cations attached with short alkyl chains have weaker biological effectiveness as compared to those containing long non-polar alkyl chains. This depicts that the length of the alkyl chain in the cations plays a significant role in enhancing biocidal activity. However, the type of anion has a small or no effect on the biocidal activity [35] which could be due to the scarcity of reports on examining the effects of anion on cytotoxicity. Although, some studies involving ILs have a vast variety of anions such as [Cl]2, [Br]2, tetrafluoroborate [BF4]2, and hexafluorophosphate [PF6]2 have shown a relatively stronger effect of anions on the cytotoxicity [38]. These ILs usually manifest EC50 values like that of the same cation, while bis (trifluoromethylsulfonyl)imide [NTf2]-based ILs are even more toxic than their halide analogs [38,39].

20.2.2 Enzymatic interaction of ionic liquids At the molecular level ILs exhibit inhibitory effects on enzymes. Studies involving enzymatic inhibition assays have provided useful information regarding the influence of ILs on humans. Though some ILs have potential inhibitory effects on enzymes others have

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shown a strong potential to replace the conventional organic solvents in reaction media using enzymes, for being less toxic to the living systems and environment as compared to organic solvents [40] (Table 20.2). Studies have revealed that the enzymatic inhibition by ILs might be attributed to the chemical structure that enables them to interact with the enzymes. For instance, the more hydrophobic ILs have a higher affinity toward binding the enzyme active-loop. This indeed is the driving force behind ILs-mediated inhibition of lactic dehydrogenase and lipase [42]. On the other hand, ILs containing anions, BF4, TfMs, perchlorate (ClO4), and dicyanamide [N(CN)2], interact with AChE mainly via hydrogen bonding. In general hydrophobicity of ILs anions has been linked to the (eco) toxicity in the tested system, however, in the case of cations, the

TABLE 20.2

Enzyme inhibition by ionic liquids (ILs).

Name of ionic liquids

Enzymes

Effect of ILs on enzymes

References

N-methylimidazolium derivatives with longer alkyl chain length

Lactic dehydrogenase (LDH) activity

Inhibitory effect on LDH with IC50 ranging between 0.57 and 0.05 mol/L, for bmim [Cl] and omim [Br], respectively. Anions influenced the LDH activity in the trend: TfMs . BF4 . Br . Cl

[41]

1-Decyl-3methylimidazolium bromide (C10mim [Br]) 1-Methyl-3-octylimidazoliumtetrafluoroborate (omim [Br]) 1-Hexyl-3methylimidazolium bromide (hmim [Br]) 1-Butyl-3methylimidazolium bromide (bmim [Br].)

Trypsin

Interaction and subsequent inhibition of enzyme activity. Indeed, the inhibitory abilities of tested ILs on the enzyme activity were closely related to their hydrophobicity: C10mim [Br] . omim [Br]  hmim [Br] . bmim [Br]

[42]

Dicyanamide, tricyanomethanide, tetracyanoborate, bistriflimide and tris (perfluoroalkyl) trifluorophosphate

Acetylcholinesterase

Dicyanamide was the least toxic while tris (perfluoroalkyl) trifluorophosphate was the most toxic anion

[34]

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20. Potential hazards of ionic liquids: a word of caution

1-Ethyl-3-methylimidazolium (EMIM) cation was observed to be more toxic than the respective alkali salts [43].

20.2.3 Phytotoxicity of ionic liquids The ILs are not only toxic to plants themselves but are hazardous for herbivores eating these plants. In addition to that, bioaccumulation of these ILs in plants used for food and fodder such as cereals rice, wheat, and Barley and vegetables radish and onion is a matter of great concern. Studies have revealed that the toxic effect of ILs on plants depends upon several factors such as the type and concentration of ILs, their side chains, anion moiety, and plant species. For instance, phosphonium derivatives, 80 mg of triphenylphosphonium iodide, and 100 mg of triphenylhexadecylphosphonium iodide per 1 kg of dry soil mass were highly toxic to spring barley Hordeum vulgare and common radish Raphanus sativus and inhibited their growth and germination [44]. Imidazolium-based ILs were also shown to exhibit phytotoxicity against H. vulgare and R. sativus but to a lesser extent. The common radish revealed a higher tolerance to tested imidazolium-based ILs comparatively to spring barley [45]. However, the length of the alkyl chain greatly influences the inhibition of spring barley development. In addition to the inhibition of development, the ILs have been reported to alter antioxidant enzyme activities and enhance guaiacol peroxidase and ascorbate peroxidase. The antioxidant system cannot effectively remove reactive oxygen species (ROS) in high ILs concentrations which leads to lipid peroxidation and damage to the photosynthetic system. On the contrary, the ionic salts with chiral menthol cation coupled with tetrafluoroborate anion [BF4] gave astonishing results on Spring Barley and common radish. Their phytotoxicity to radish plants decreased by increasing the side chain length till it became zero with the cation having 11 carbon atoms while the cation having 1 3 carbon atoms had the highest phytotoxicity [46]. In addition to cereals and vegetables, Imidazolium-based ILs have shown toxic effects on other superior land plants. For instance, Vicia faba seedlings are sensitive to imidazolium derivatives (C10mim [Cl], C10mim [Br], and C10mim [NO3]). The concentration of 50 mg/kg was observed to be enough in suppressing the shoot and root length, dry weight, and pigment contents. Besides that, an increase in ROS level resulted in cellular lipid peroxidation, DNA, and oxidative damage [47]. In another study changes in V. faba antioxidant system were observed in response to a higher concentration of him [Cl] and bmim [Cl] [48]. Protic ILs were, however, found to be less toxic even at higher concentrations against the Allium cepa, Lolium perenne, and R. sativus and soil microbiota [48]. EC50 values of protic ILs are higher than that of aprotic ILs. imidazolium-based ILs were found toxic to rice seedlings as

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depicted by diminished roots and stem growth and decreased weight, altered antioxidant system, and cellular structure [49]. While, omim [PF6] showed growth inhibition of Wheat seedlings (Triticum aestivum) at concentrations higher than 2 mg/L [50] evidenced by inhibition of root growth, gravity response, amyloplasts amount, and distribution pattern of root cap compared with controls.

20.2.4 Toxicity of ionic liquids to microorganisms Microorganisms are essential for the environment since they take part in carbon and nitrogen cycling. Perhaps one of the most studied organisms to ILs-mediated toxicity are microorganisms. Several papers report estimation of the ILs toxicity toward lactic acid-producing bacteria [51], V. fischeri, Escherichia coli, Pichia pastoris, Bacillus cereus, and others [52] as shown in (Table 20.3). Different bacterial species show varied susceptibility to the ILs. For instance, E. coli and Bacillus subtilis have shown a significant difference TABLE 20.3

Toxicity of ionic liquids against some microbial species.

Microbial species

Ionic liquids

Effects of ionic liquids

References

Vibrio fischeri, Escherichia coli, Staphylococcus aureus, Bacillus subtilis, Pseudomonas fluorescens, Saccharomyces cerevisiae

[BMIM][Br], [HMIM] [Br], [OMIM][Br], [BMPyr][Br], [HMPyr][Br], [OMPyr][Br]

The ILs with the longest alkyl chain length showed Significantly higher toxicity against B. subtilis

[53]

Lactobacillus rhamnosus NBRC 3863

[Bmim][PF6], [Hmim] [PF6], [Omim][PF6]

Imidazolium-based ILs inhibited bacterial growth by decreasing the survival rate and glucose assimilation

[54]

Vibrio fischeri

Alkyl (C3-C10) methyl-, ethylimidazolium with [BF4], [PF6], [Br], [Cl]

The ILs with the longest alkyl chain length showed higher toxicity.

[55]

Escherichia coli

[P6,6,6,14][NTf2], [N1,8,8,8][NTf2], [P6,6,6,14][Cl], [N1,8,8,8][Cl]

Rapid bioaccumulation of chlorides in the cell as depicted by FT-IR spectra

[56]

L. rhamnosus NBRC 3863, Lb. homohiochi NRIC 0119, Lb.

[BMIM][PF6], [HMIM][PF6], [OMIM][PF6]

The number of viable cells of bacteria in the presence of

[51]

(Continued)

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20. Potential hazards of ionic liquids: a word of caution

TABLE 20.3

(Continued)

Microbial species

Ionic liquids

homohiochi NRIC 1815

Effects of ionic liquids

References

imidazolium-based ILs (5% v/v) was lower than determined for the control group and decreased with the alkyl chain length

Vibrio fischeri

Guanidinium, phosphonium, [IM] with [Cl], [Br], [I], [CH3SO32], [TOS]

The increasing toxicity with the increase in the alkyl chain length, the alkyl chains with ether or ester groups decrease the toxicity, the phosphonium-ILs were more toxic, the EC50 (mg/L) 15 min, decreases in the order: [P4441][CH3SO4] (237.60) , [P4444][Br] (172.80) , [P4444][Tos] (169.60) , [P66614] [CH3SO3] (7.43) , [P66614][Cl] (7.10)

[57]

Saccharomyces cerevisiae

[BMIM]Cl

The reproduction rate decreased with the [BMIM]Cl concentration increasing. The EC50 and IC50 values were 0.53 and 0.39 g/L respectively

[58]

in sensitivity towards imidazolium, pyridinium, and the choline-aminoacid-derived ILs [59]. The study reported that ILs were more toxic to B. subtilis than E. coli. This study along with a couple of others demonstrated that gram-positive bacteria are generally more susceptible to ILs-mediated toxicity as compared to gram-negative bacteria [59,60]. This is because ILs reaction with the peptidoglycan layer and lipid component of the cell wall of gram-negative bacteria is different than that with cell wall components of Gram-positive bacteria.

20.3 Factors affecting the toxicity of ionic liquids The factors affecting the toxicity of ILs can broadly be grouped into two categories; chemical composition of ILs and environmental factors.

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507

The structural modification of ILs can be done via the replacement of anion, cation, and or alkyl side chains whereas dissolved organic matter (DOM) and salinity have been reported to be crucial for ILs’ activity.

20.3.1 Effect of the chemical composition of ionic liquids The degree of toxicity caused by ILs depends upon several factors such as the anion, the positive moiety cation, and the attached alkyl chain length [61]. Studies predicting the toxicity of ILs have mainly focused on the length of the alkyl chain and the type of cation, however, anion also plays a significant role in the toxicity of ILs having weak cations [55,62,63]. 20.3.1.1 Cations Cations have been given due consideration in studies determining the toxicity of ILs and many researchers reported that the structure of cations is the most important factor affecting the toxicity of ILs [55,62,64]. For instance, Stolte et al. [65] evaluated the toxicity of many cations such as dimethylpyridinium, pyridinium, imidazole, pyrrolidinium, morpholinium, and ammonium against Lemna minor, V. fischeri, Scenedesmus vacuolatus. The results indicated that the aromatic cation containing ILs such as dimethylpyridinium was most toxic as compared to quaternary ammonium salts. As the imidazolium and pyridiniumbased ILs are mostly not biodegradable [66] so they are highly toxic against enzymes, microbial cells, cell lines, animals, and plants [67]. On the other hand, ILs having alicyclic cations such as pyrrolidinium, or quaternary ammonium are less toxic as compared to aromatic cations such as imidazolium and pyridinium [68]. In addition to that, the presence of polar groups such as hydroxyl, ether, and nitrile groups in the alkyl side chains may result in lowering the toxicity of ILs towards enzymes such as AChE [69]. 20.3.1.2 Alkyl chain length Numerous publications have reported that the toxicity of ILs is strongly linked to the length of their side chain. There is compelling evidence that the longer the alkyl chain the higher the toxicity [70,71]. For instance, Matzke et al. [72] reported that the antagonistic effect of 1-alkyl-3-methylimidazole against marine organisms V. fischeri, S. vacuolatus, L. minor, and terrestrial plants such as T. aestivum and Lepidium sativum is highly dependent on the length of the alkyl chain and is irrespective of the type of anion. Other studies have reported similar effects of length of alkyl chain on the viability of marine and terrestrial organisms [73,74]. This possibly could be due to the fact the increase in alkyl

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chain length increases the hydrophobicity of the ILs which might, in turn, disrupt and dysfunctional the lipid bilayer, leading to cell lysis [65]. Moreover, an increase in the length of the alkyl chain of pyridinium salts, imidazolium, and quaternary ammonium salts increases their antimicrobial activity [73]. 20.3.1.3 Anions Although the cations and the alkyl chains are mainly responsible for toxicity manifested ILs the anion also contributes significantly to the overall toxicity of ILs having fewer toxic cations. Mostly lipophilic anions increase the toxicity of the ILs [75]. Alkylimidazolium and alkyl pyridinium salts containing anions such as [BF4], [PF6], and [C] are highly toxic against the marine bacterium V. fischeri [22,32], mammalian cell cultures [38,72], human cell HeLa18 [36], soil nematodes Caenorhabditis elegans [76] and freshwater snail Physa acuta [77], D. magna [25]. However, most of the studies proved that the anion type has a minimal effect on toxicity [61,65,68], however, the situation is different for the anions belonging to the polyfluorinated ion group; such anions hydrolyze to release highly toxic fluorides [78 80]. Monoatomic anions (Br2 and Cl2) however, cause less toxicity as compared to largesized anions [80]. For instance, Br2 and Cl2 showed less toxicity to Photobacterium phosphoreum as compared to [BF4]- and [PF6]- [80].

20.3.2 Effect of environmental factors The toxicity of ILs is strongly dependent upon the presence of DOM and the salinity of the medium. 20.3.2.1 Dissolved organic matter DOM plays important role in the bioavailability and related toxicity of ILs and eventually the fate of ILs in the environment. As evidenced by research the ILs make a complex with DOM in soil and pore water thus reducing its availability in the soil and consequently reducing the toxicity. For instance, the toxic effect of [C4MIM][BF4] and [C8MIM] [BF4] on T. aestivum and L. sativum was observed to decrease by 50% in response to a 5% increase in DOM [81]. Likewise, the toxicity of imidazolium to L. minor was slightly reduced by the natural [82]. In general, the DOM reduce the toxicity by binding them and consequently reduce their availability in free form that could cause toxicity. 20.3.2.2 Salinity Salinity can affect the bioavailability and toxicity of ILs in many ways such as competing ILs for ionic sorption sites in soil and tissues or

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509

enzymes in living cells, aggregate formation by the screening effect, salting in and salting-out strategy, and providing an ion-pairing environment for the cations macro- and macromolecular structures in the living cells. Studies have reported a decrease in toxicity of imidazolium ILs against algae-like Oocystis submarina, Chlorella vulgaris, Geitlerinema amphibium, and Cyclotella meneghiniana by an increase in salinity [83]. This could be due to the reason that increase in salinity reduces the permeability of IL cations through a cell wall.

20.4 Fate and transfer of ionic liquids to the environment Excellent chemical and thermal stability, high water solubility, and low volatility of ILs make them persistent in the soil/water media. It implies that a systematic protocol is mandatory for their treatment before discharge into the environment. Substantial efforts have been devoted to the degradation of ILs in soil, sediment, or aqueous solution, as illustrated in Fig. 20.2, to assess their persistence in the environment and to combat environmental pollution [84,85].

20.4.1 Adsorption of ionic liquids Realistically, the sorption of ILs is a dominant factor affecting the transport, bioavailability, and reactivity of hazardous compounds in the environment. Thus far, the environmental fate of ILs requires evaluation of their sorption onto porous substrates because ILs are typically found in the ground and/or surface water environment. Numerous reports are available on exploring suitable adsorbents materials such as activated carbon [86,87], clay [88,89], zeolites [90], or resins [91,92] for ILs adsorption from the soil, sediments, or aquifers. Generally, the adsorption of ionic compounds onto the solid substrate

FIGURE 20.2 Fate of ionic liquids in the environment.

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20. Potential hazards of ionic liquids: a word of caution

can take place via a variety of possible mechanisms such as electrostatic ionic interaction, ion-exchange mechanism, hydrophobic interaction, the polarization of π-electrons (electron-rich aromatic ring of adsorbate to the positively sites on adsorbent), and dispersion forces due to bulky adsorbent [43]. Commonly, the polar adsorbent surface is suitable for hydrophilic ILs adsorption due to adsorbent-adsorbate hydrogen bonding, while, non-polar adsorbent surfaces are highly efficient adsorbents for hydrophobic ILs [93]. ILs are potentially dangerous to both the terrestrial and aquatic environments which are considered their possible recipients; however, the distribution of ILs in receiving bodies strictly depends upon their mutual interaction. Scare information is available on the sorption behavior of ILs in soil, though, both the chemical structure of cation in ILs, mainly its electronic density and the alkyl side-chain length, and physicochemical properties of sorbent substrate, that is, organic matter (OM), cation exchange capacity, and clay mineral content, etc., have been reported as a major contributor in binding ILs to the soil surface. Generally, the cations with a longer alkyl side chain promote the adsorption of IL to the soil to a greater extent compared to the shorter alkyl chain carriers. Similarly, the high electron-receptor tendency of the cationic group facilitates the adsorption of ILs onto carbon-rich soil/ sediment adsorbent via electrostatic interaction. Stepnowski [94] evidenced irreversible adsorption of lyophilic imidazolium salts on soil and sediments via strong hydrophobic interaction with OM and nonhydrophobic interactions due to a highly polar ionic cation group. The double-layer interaction pathway has been observed for ILs sorption onto soil and minerals, one involving a short and/or hydroxylated alkyl side-chain and the other one constitutes the rest of the compound [95,96]. Soil, sediments, and aquifer substrates with acidic pH promote the sorption of anions via an anionic exchange mechanism due to the formation of positively charged surface sites on adsorbents. Furthermore, the deprotonation of anionic soil substrate takes place by increasing pH enhancing cationic exchange. Along the same lines, Mrozik et al. [97] investigated the sorption of phosphonium-, ammonium-, and pyrrolidinium- based ILs and confirmed enhanced mobility of weak and/or hydroxylated side-chain ILs in the soil/sediment substrate due to their weak sorption tendencies. Moreover, a low sorption tendency was observed for imidazolium and pyridinium chloride ILs on 11 different types of soil samples at acidic pH. In a preliminary investigation, Mrozik et al [98] reported the sorption of imidazolium-type ILs onto kaolinite clay minerals as multilayer-type adsorption via a combination of physical sorption and electrostatic interaction. During monolayer formation, the beginning of adsorption, both the Van der Waals interaction

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511

and ion exchange are responsible, whereas dispersive interactions are dominant at higher concentrations of ILs. Also, the authors in this work confirmed the spontaneous and exothermic nature of the sorption of ILs onto soil and sediment substrates based on thermodynamic investigations. Interestingly, some of the reports have observed a negative effect of alkyl chain length on the sorption of imidazolium-based ILs onto aquatic sediments [99]. These contrary findings imply that the sorption of ILs can be altered depending upon the nature of the environmental system.

20.4.2 Biodegradation of ionic liquids The widespread use of any chemical solvent depends upon environmental hazard assessment concerning persistence, bioaccumulation potential, and toxicity to biological systems. The persistence of a chemical in the environment is the key factor determining its fate and the resulting adverse effects. Chemicals that degrade rapidly either by themselves or by natural microorganisms present in the environment are least likely to cause any adverse effects however the recalcitrant chemicals not only persist longer but cause more damage to the environment. In addition, the persistent chemicals accumulate in the abiotic and biotic components of the ecosystem and therefore are highly likely to cause irreversible damage [21]. The biodegradability of ILs is a fundamental aspect that should be considered before applying ILs. For instance, biodegradability is desirable from an environmental perspective while undesirable from an industrial application point of view where repeated use of the ILs is required and fresh applications could considerably increase the overall cost of the system [67,70]. There are numerous reports on the biodegradability of ILs stating that imidazolium-based ILs are mostly not biodegradable, however, few studies have demonstrated the biodegradation of ILs by microorganisms [75,100 102]. Large-scale applications of ILs in industry and academia pose a threat of accidental release of these chemicals in the environment and consequent bioaccumulation, therefore, demanding evaluation of environmental impact including biodegradation by indigenous microorganisms present in soil and water [67,70,71]. Biodegradation of ILs is dependent upon many factors including the type of ILs, cation, alkyl chain length and the anions, inherent biodegradability, soil texture, DOM, salinity, microbial population, diversity, and interactions of ILs with the biotic and abiotic components of the environment [101]. Microbial biodegradation of ILs has been linked with alkyl side chain length. For instance, an increase in the alkyl side

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chain length especially around the C12 C14 corresponds to increased biodegradation owing to the availability of oxidizable carbons in the chain however the increase in the side chain length is highly toxic to microbial growth [101,103]. This means that the biodegradation of the ILs by microorganisms is dependent upon a compromise between the ILs side chain length mediated toxicity and microbial susceptibility. On the other hand, chains of C16 and C18 exhibit very poor water solubility thus having low bioavailability [71]. Hence design optimization of ILs by adding ether oxygen in alkyl chains can reduce toxicity while maintaining biodegradability [54].

20.4.3 Chemical degradation of ionic liquids The ILs are mostly found to be resistant to biodegradation presenting a wide range of contamination in soil, sediments, and water media. Consequently, several physical and chemical approaches have been devised for the removal of ILs from the environment including adsorption and degradation processes. The advanced oxidation process (AOPs) is one of the effective degradation processes which involves the in situ generation of strong oxidants to effectively degrade the pollutant [104]. The AOPs generally include ultrasonic, electrolytic, photocatalytic, and Fenton-based degradation processes [105]. The photocatalytic degradation employs a semiconductor catalyst that can be excited by light absorption generating electron-hole pairs which interact with the adsorbed species at the surface of the semiconductor to mineralize it [106]. The preliminary work in this context was performed by Stepnowski and Zaleska [96] who compared the oxidative degradation of imidazolium-based ILs using TiO2/H2O2 photocatalyst with and/without UV irradiation. They observed UV/H2O2 was highly effective for the degradation of ILs. Furthermore, Morawski et al investigated the comparative degradation of different kinds of ILs such as imidazolium, pyridinium, phosphonium, and ammonium using TiO2 catalyst under UV irradiation. The authors reported high sensitivity of both hydroxylated-ILs and long side-chain cation-based ILs under UV irradiation. Moreover, phosphonium-based ILs were found to be easily degradable compared to ammonium-based ILs due to their higher molar mass. Moreover, Itakura et al [107] evidenced high photodegradation of imidazolium ILs using PtTiO2/UV and TiO2/UV photocatalysts systems, but the anions degradation required additional hydrothermal treatment. Similarly, Li et al. [108] evidenced 93% degradation of 1,3-dialkylimidazolium-based ILs in a hydrogen peroxide/acetic acid substrate within 12 hours of UV light exposure. They proposed the mechanism involves the oxidation of three hydrogen atoms followed by the cleavage of alkyl groups. Surprisingly, the

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20.5 Conclusion and future perspectives

513

mechanism was not found to be affected by both the nature of the sidechain and or surrounding anions. Several groups investigated the degradation of ILs using reactive oxides generated by the Fenton reagent. The imidazolium-based IL was degraded up to 97% within 90 minutes irradiation using 1/100 mM of Fenton reagent [109 111]. Pyrrolidinium-based ILs were found to be the least stable under oxidative degradation compared to the imidazolium-based ILs due to the high molar mass of the cation. The rate of degradation strictly depends upon the alkyl side-chain length, type of anions, and cations, as suggested by [96]. These findings were consistent with DFT calculations done by Bedia et al. [104] who observed complete degradation and up to 80% mineralization of different pyridinium and imidazolium-based ILs under solar light irradiation in the presence of TiO2 photocatalyst. Based on theoretical results, the rate of degradation increased with the hydrophobicity of the alkyl sidechain and molar mass of the head group, while the type of anions showed a mere effect. Similar findings were evidenced by electrocatalytic [105,112,113] and ultrasonic [114 116] degradation involving the decomposition of ILs at anode/cathode electrodes in an aqueous solution or as a result of acoustic cavitation. The detailed spectroscopic investigations suggested the electrocatalytic degradation of ILs by undergoing an attack of hydroxyl radical and oxidative cleavage of the cationic ring via a series of reaction pathways. Interestingly, the probability of oxidative cleavage decreased with increasing hydrophobicity of ILs side-chain, which was not the case with previously discussed decomposition pathways [105,112]. Nonetheless, detailed investigations are still required to verify the decomposition of ILs chemically.

20.5 Conclusion and future perspectives ILs are a mixture of low melting point organic solvents with a pertinent characteristic of low volatility making them versatile solvents in a wide scale of experimental synthesis procedures. These characteristics prove them advantageous over conventional counterpart solvents in terms of atmospheric pollution. However, ILs are considered potential vehicles for pollution to both the terrestrial and aquatic environments. Most of the ILs are non-or least biodegradable and demonstrate a wide range of toxicities, being in common more toxic than the conventional solvents. Nevertheless, the toxicity of ILs can be minimized by tailoring their structure in terms of choice of anion, cation, and/or alkyl chain to develop better ‘green’ solvents which are easily biodegradable.

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Conflict of interest The authors declare no conflict of interest.

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[111] Ewa M. Siedlecka, P. Stepnowski, The effect of alkyl chain length on the degradation of alkylimidazolium- and pyridinium-type ionic liquids in a Fenton-like system, Environ. Sci. Pollut. Res. (2009). Available from: https://doi.org/10.1007/s11356008-0058-4. [112] S. Lee, H.S. Hwang, W. Cho, D. Jang, T. Eom, D.C. Martin, et al., Eco-degradable and flexible solid-state ionic conductors by clay-nanoconfined DMSO composites, Adv. Sustain. Syst. (2020). Available from: https://doi.org/10.1002/adsu.201900134. [113] E.M. Siedlecka, S. Stolte, M. Gołebiowski, A. Nienstedt, P. Stepnowski, J. Tho¨ming, Advanced oxidation process for the removal of ionic liquids from water: the influence of functionalized side chains on the electrochemical degradability of imidazolium cations, Sep. Purif. Technol. (2012). Available from: https://doi.org/10.1016/j. seppur.2012.09.012. [114] D.J. Flannigan, S.D. Hopkins, K.S. Suslick, Sonochemistry and sonoluminescence in ionic liquids, molten salts, and concentrated electrolyte solutions, J. Organomet. Chem. (2005). Available from: https://doi.org/10.1016/j.jorganchem.2005.04.024. [115] P.M. Kanthale, A. Brotchie, F. Grieser, M. Ashokkumar, Sonoluminescence quenching and cavitation bubble temperature measurements in an ionic liquid, Ultrason. Sonochem. (2013). Available from: https://doi.org/10.1016/j.ultsonch.2012.05.011. [116] J.D. Oxley, T. Prozorov, K.S. Suslick, Sonochemistry and sonoluminescence of roomtemperature ionic liquids, J. Am. Chem. Soc. (2003). Available from: https://doi. org/10.1021/ja029830y.

6. Future applications and studies

Index Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

A Absorption process, 328
329 ionic liquids in absorption process for CO2 capture, 332
335 Acetone, 447
448 gas sensor, 313 Acetonitrile (CAN), 394
395 Acetophenone, 306 Acetylcholinesterase (AChE), 501
502 Acidic PIL P [((3-(4-vinylpyridine) propanesulfonic acid)iodide)-co(acrylonitrile)], 407 Activated carbons (ACs), 329, 360 Adducts, 133
134 Adsorption process, 329 ionic liquids in adsorption process for CO2 capture, 335
336 Advanced oxidation process (AOPs), 512 Agglomeration problem, 466 Air conditioning systems, 277 Air pollutants, 303 Air pollution treatment, 170 Alcohols, oxidation of, 118
119, 119f Alicyclic cations, 507 Alkali metals, 383 Alkaloids, 202 Alkyl amines, 310 Alkyl chain length, 507
508 Alkyl chain length of cations of ionic liquids, effects of on stability of proteins, 423 1-alkyl-3-methylimidazole, 507
508 1-Alkyl-3-methylimidazolium-based IL, 429 Alkyl/aryl halides, 120 Alkylation method, 482 Allium cepa, 504
505 Alloys, 185 ionic liquids as corrosion protector for, 188
193 Allura Red (AR), 237 1-allyl-3-methylimidazolium chloride, 37 α-chymotrypsin (α-CT), 47

α-haloketones, 120 Aluminum (Al), 8 alloy, 227
228 electrode, 89 ionic liquids as corrosion protector for, 191
192 Amaranth (AM), 237 Amides, 331 Amine-containing imidazole functionalized IL, 332
333 Amines, 331 Amino acids, 26
39 IL-based ABS for extraction and purification of aminoacids, 27t of enzymes, 31t of nucleic acids, 34t of proteins, 29t 2-aminoethylferrocenylmethylether, 117 Ammonia, 294
295, 309
310 gas, 301 Ammonia optical gas sensor, 310 Ammonium, 73
77 Amperometric gas sensors, 290
294 Amperometric microarray oxygen gas sensor, 301 Amperometric nitrogen oxide gas sensor, 304 Analytics and purity, 166
167 advantages, 167 Anionic components, 143 Anions, 187
188, 418
419, 508 anion
cation amalgamations, 443
444 exchange, 482
483 effect of ionic liquids on stability of proteins, 423 Anodization treatment, 227
228 Anodized aluminum membrane, 337
338 Antimicrobial agents, 468
469 Antimicrobial properties and activities, silver nanoparticles, 468
473 Antistatic agents, 275

523

524

Index

Antistatic agents (Continued) liquid antistatic agents, 275 Aprotic ionic liquids, 358, 445
446 as electrolytes for lithium-ion battery, 368
369 Aqueous amines used in postcombustion, 330
331 Aqueous biphasic systems (ABS), 24, 248
254 Aqueous soluble amines, 330
331 Aromatic aldehydes, oxidative esterification of, 142 Aromatic amino acids, 36 Artificial olfaction system, 307
308 Aryl zinc compounds, synthesis of, 128 Ascorbic acid (AA), 69, 101 Aspergillus flavipes, 49 Attenuated total reflectance-IR spectroscopy (ATR-IR spectroscopy), 334 Automobile engines, 303, 353

B Bacillus cereus, 47
48, 472, 505 Bacillus sp., 45
46 Bacillus subtilis, 505
506 Backward-extractions, 271 Bacteria, 466
467 Battery applications, electrolytes for, 273
275 Battery energy storage systems (BESS), 377 Benzaldehyde, 306 Benzimidazole (BIm), 359 Benzoin condensation, 138
139 Benzoxazol-2-ones (3H-ones), 142 Benzoxazolones, N-functionalisation of, 142 Benzoyl chloride, electrochemical reduction of, 129
130 1-benzyl-3-methyl imidazolium (BzMI), 138
139 β-lactams, 140 synthesis of, 137
138 electrochemical reduction of imidazolium RIL, 137f Beta-cow-like lactoglobulin (β-LG), 424
425 Betaine (BE), 40
41 betaine-based DES, 434 Betaine-urea (Be-U), 434 Bhopal gas tragedy, 289 Biflavonoids, 202 Binary ILs, 158 Bioactive compounds, 248
254

extractions, 247
248 applications of ionic liquids, 271
277 from biomass, 254
271 ionic liquids cation and/or anion name and acronym, 249t Biocatalysis, application of ionic liquids as, 427
429 Biodegradation process, 511
512 Biodiesel fabrication, aspects of ionic liquids as promoters in, 450
451 Biomass, 248 bioactive compound extraction from, 254
271 backward-extractions, 271 ionic liquid-based liquid
liquid extractions, 255
260 ionic liquid-based solid
liquid extractions, 260
270 solid
phase extractions, 270
271 gentle suspension of, 271
273 in ILs for extraction of natural products, 203
204 Biomolecules IL-based ABS in separation and purification of, 26
51 amino acids, 26
39 enzymes, 45
49 nucleic acids, 50
51 proteins, 39
44 Bioprocessing online device platform, 301 Biosensors, 95 configuration of ionic liquids in, 450 4,4-bipyridinium ILs-based electrodeposition technique, 8 Bis (trifluoromethanesulfonyl) imide, 480 1,3-bis(2-oxo-2-phenylethyl)-1H-imidazol-3ium bromide (OPEIB), 191
192 Bis(trifluoromethanesulfonyl)imide, 358
359 Bis(trifluoromethylsulfonyl) imide (BTFMSI), 117, 306
307 Bonding in ionic liquids, 354 Bovine serum albumin (BSA), 11
12, 39, 423 Bromide, 418
419 Bromide (Br)2anions, 502 Bromination reaction, 119
120 Brønsted acidic ILs (BAILs), 234
237, 359 Brønsted corrosive ILs (BAILs), 238
239 Burkholderiacepacia ST8, 46 Butler Volmer concept, 91

Index

1-butyl,3-methylimidazolium bis (trifluoromethanesulfonyl)imide, 306 1-butyl,3-methylimidazolium dicyanamide, 306 1-butyl-1-methyl-2-oxopyrrolidinium (BMOP), 425
426 3-butyl-1-methylimidazolium (BMI), 139 3-(1-butyl-1H-imidazol-3-ium-3-yl) propane-1-sulfonate, 359 1-butyl-3-methyl-acidic imidazolium, 428 1-butyl-3-methylimidazolium acetate (BMIM-OAc), 273 1-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide, 305
306 1-butyl-3-methylimidazolium chlorides (BMIC), 39, 188
191, 313
314, 422 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6), 96, 301, 432 1-butyl-3methylimidazolium
tetrafluoroborate (BMI
TFB), 102
105, 115, 422, 428 1-butyl-3-methylimidozium (BMII), 396, 422, 472 Butylmethylimidazolium boron tetrafluoride (BMIBF4), 425 1-butylmethylpyrrolidinium (BMPyr), 124 1-Butylpyridinium tetrafluoroborate (BPBF4), 485

C Caenorhabditis elegans, 508 Cajanus cajan, 270 Candida antarctica lipase (CaL), 45 CaL B, 45, 429 Candida rugosa, 45, 428 Capacitive current, 90 Carbamates, 126 insecticides, 207
208 Shono oxidation of, 117
118 Carbon, 300, 302 Carbon capture technology, 67 Carbon compounds, 377 Carbon dioxide (CO2), 120, 124, 291
292, 294
295, 327
330 gas, 115, 124 electrochemical reduction of, 124
126 ionic liquid composite membranes for CO2 separation, 339
340 in CO2 sequestration, 67
68 from ionic liquids, regeneration of, 340

525

polyionic liquids membranes for CO2 separation, 338
339 supported ionic liquid membrane for CO2 separation, 337
338 Carbon grapheme nanosheets (CGNs), 73 Carbon ionic liquid composite gas sensors, 315 Carbon ionic liquid electrode, 94
101 determination of various substances, 101 direct electrochemistry of hemoglobin, 94
100 Carbon monoxide (CO), 292, 294, 328
330 Carbon nanotubes (CNTs), 72
73, 407
408 Carbon paste electrode (CPE), 94 Carbon steel, 188
191, 230
231 Carbon-capturing process, 327
330 Carbon-gold nanocomposites (CGNs), 73 Carbonate electrolytes, 366 Carbonyl compounds, 120 Carboxymethyl cellulose (CMC), 238
239 Carica payaya, 48 Catalysis, 171 Cathodic protection method, 228 Cations, 187
188, 418
420, 507 effect of ionic liquids on stability of proteins, 423 of ionic liquids on stability of proteins, alkyl chain length of, 423 Cerium acetylacetonate (CeA), 237 Chemical absorbents, ionic liquids as, 334
335 Chemical dyes, 307 Chemical production processes, 277 ionic liquids as, 78
80 Chemical synthesis, 115 Chemical warfare agent (SWAs), 105
106 Chemosensory RIL, 310
311 Chitosan, 238
239 Chitosan Schiff bases (CSBs), 238
239 Chitosan thiosemicarbazide (CS-TS), 238
239 Chitosan-thiocarbohydrazide (CS-TCH), 238
239 Chlorella pyrenoidosa, 427 Chlorella vulgaris, 508
509 Chloride, 418
419 Chloride anions (Cl2), 353
354, 502 Chlorine dioxide, 294 Chloroaluminate-based IL, 378 Choline chloride (ChCl) ChCl-based DES, 434 ChCl-methylurea, 434

526

Index

Choline chloride (ChCl) (Continued) ChCl-urea, 434
435 ChCl
glycerol, 435 eutectic solvents, 433
434 glycerol, 433 Choline geranate (CAGE), 435 Choline-based IL, 45
46 Cholinium alaninate, 41 Cholinium chloride, 434
435 Cholinium glycinate, 41 Cholinium lysinate, 41 Cholinium serinate, 41 Cholinium-based IL, 42, 48
49 Chromatography, 204 Chronoamperometry methods, 299, 303 Chrysanthemum morifolium, 268 Circular dichroism (CD), 40 spectroscopy, 424
425 Clark electrochemical cell design, 294 Clark oxygen electrode, 292
293 Clostridium sp., 432 Co-precipitation method, 162
163 Co-solvents, 93, 332
333 CO2 capture designing ionic liquids for, 340
342 ionic liquids as solvents for, 331
340 ionic liquids in absorption process for, 332
335 ionic liquids as chemical absorbents, 334
335 ionic liquids in adsorption process for, 335
336 ionic liquids in membranes process for, 337
340 as physical absorbents, 333 Coating method, 226
227 inorganic coatings, 227 metallic coating, 226 organic coating, 226 Cobalt (Co), 7 Colloid thrusters, 453 Commercial lipase (CrL), 45 Compact Helmholtz layer, 228
229 Complex/rigid solid-liquid extractions, 269
270 microwave homogeneous liquid
liquid microextraction, 270 negative-pressure cavitation extraction, 270 ultrahigh pressure extraction, 270 UMAE, 269
270 Composite material, 155
156

Composite membranes, 329 Composite polymer, electrolytes based on, 380 Compost resources, 454 Conductivity electrochemical conductivity, 449 internal ionic conductivity, 368 physical properties of ILs, 4
5 COnductor-like Screening MOdel for Real Solvents (COSMO-RS), 341
342 Conventional absorbers, 330
331 Conventional batteries, 353 Conventional ILs, 337 Conventional solvents, 288
289, 295 Conversion materials, ionic liquids in, 66
67 Coordination bonds, 378
379 Copper corrosion protector for Cu and Zn, ionic liquids as, 192
193 ionic liquids as corrosion protector for, 192
193 Corrosion, 223
224 consequences of, 224 as a costly problem to the world, 233 economic effects, 224
226 corrosion in different areas of society, 225f inhibition, 232 inhibitors, 234
239 for metals and alloy, ionic liquids as, 188
193 for Al, ionic liquids as, 191
192 for Cu and Zn, ionic liquids as, 192
193 for iron and alloy, ionic liquids as, 188
191 methods to control corrosion, 226
227 coating, 226
227 material selection, 226 protection mechanism, 193 protector effect of, 185 causes of, 185
186 ionic liquids as green, 187
188 techniques of, 186
187, 186f restraint, 232 Coulombic forces, 201 Critical micellar concentration (CMC), 267 Cryogenic separation process, 330 Cyclic voltammetry (CV), 74
75, 93
94, 115, 125
126

Index

Cyclic voltammogram, 121
122, 127
128, 130 [2-(cyclohexylamino) ethanesulfonate] (CHES), 42 Cyclotella meneghiniana, 508
509 Cytochrome c (Cyt-c), 40 Cytotoxicity of ionic liquids, 431
432

D Daphnia magna, 501
502 Deep eutectic solvents on protein chemistry, role of, 433
435 Degradation products, 331 Deoxyribonucleic acid (DNA), 50 Designer chemicals, 187
188 Dextran (Dex), 238
239 Di-(fluoro sulfonyl imide) (FSI), 368 FSI-based AILs, 369 1,3-dialkylimidazolium chloride, 64
65, 479
480 Dicyanamide [N(CN)2], 503 Diethyl ethylene diamine propanoate, 40 Diffusion resistance (Rd), 402
403 1,6-diketone, 128
129 Diluents prospect, 443
444 for fabricated chemistry, 447 Dilute aqueous solution, 434 1,2-dimethoxybenzene, 117 1,3-dimethylimidazolium iodine, 422 1,3-dioctadecylmidazolium bromide (ImDC18Br), 188
191 Dipole
dipole interaction, 201 Dispersive liquid
liquid microextraction technique, 199 Dissolved organic matter (DOM), 506
508 Dithioacetals, anodic fluorination of, 134
135 DNA microorganisms, 469
470 Dodecyl 3-methyl-1-octylimidazolium, 426 Dopamine (DA), 69, 101 Doped SnO2, 313
314 Doping of polymer, 484 Double potential amperometry (DPA), 73 Drugs, 187
188 solubility, 432 Dry solid polymer, electrolytes based on, 378
379 Dye-sensitive solar cells (DSSCs), 67
68, 391
393 iodine-free PIL as electrolyte in, 404
408 poly ionic liquids role as solid or quasisolid electrolyte in, 395
403

527

principle and structure of, 393
394 role of ionic liquids as electrolyte in, 394
408 Dynamic light dispersing (DLS), 426

E Effective concentration (EC), 501
502 Electric polymer, 378 Electrical conductivity, 444
445 Electrical double layer, structure of, 228
229 Electrical storage systems, organic and inorganic ionic liquids in, 372 Electro-spinning techniques, 435 Electrocarboxylation reaction, 126
128 carbamates using BMI
TFB, 127f dialkyl carbonates, 127f Electrocatalysis, 3
4 Electrocatalytic nature, 106 Electrochemical ammonia gas sensors, 301
303 Electrochemical analysis, 131 Electrochemical biodevices, 450 Electrochemical biosensors, 11
12 Electrochemical capacitors, 3
4 Electrochemical carbon dioxide gas sensor, 306 Electrochemical conductivity, 449 Electrochemical constancy, 449 Electrochemical devices ammonium, pyrrolidinium, phosphonium, and sulfonium-based ionic liquids and, 73
77 applications of room-temperature ionic liquids, 70
73 current and future prospects, 78
80 energy storage devices and conversion materials, ionic liquids in, 66
67 energy sustainability and CO2 sequestration, ionic liquid in, 67
68 history of, 64
65 ionic liquids electrochemical sensing properties, 68
70 as novel electrolyte medium for, ionic liquids, 68 Electrochemical double-layer, 91 Electrochemical dual gas sensor, 307 Electrochemical electron transfer model, 91 Electrochemical gas sensors, 290
307, 297t density and viscosity of RIL, 297t electrochemical ammonia gas sensors, 301
303

528

Index

Electrochemical gas sensors (Continued) electrochemical carbon dioxide gas sensor, 306 electrochemical hydrogen sulfide carbon nanotube-modified electrode gas sensor, 307 electrochemical methane and oxygen dual gas sensor, 307 electrochemical nitrogen oxide gas sensors, 303
305 electrochemical oxygen gas sensors, 298
301 electrochemical reactions involved in amperometric gas sensors, 294t electrochemical volatile organic compounds gas sensors, 305
306 reactions in different potentiometric gas sensors, 293t RIL with decomposition temperature, 296t room-temperature ionic liquids in, 73 schematic diagram of amperometric gas sensor electrode, 292f schematic diagram of potentiometric gas sensor electrode, 291f water uptake by tris(perfluoroalkyl) trifluorophosphate RIL, 296t Electrochemical impedance spectroscopy, 316 Electrochemical method, 164 Electrochemical organic synthesis, 114 Electrochemical oxidation, 106
107 using room temperature ionic liquids, 115
120 bromination reaction, 119
120 oxidation of alcohols, 118
119 oxidative self-coupling reaction, 117 Shono oxidation of carbamates, 117
118 Electrochemical partial fluorination using room temperature ionic liquids, 133
137 anodic fluorination of dithioacetals, 134
135 electrochemical fluorination utilizing mediators, 135
136 α-Fluorination reaction, 136f electrochemical fluorination using mediator, 136f fluorination of methyl adamantane-1carboxylate electrochemically, 136
137 Electrochemical polymerization method, 15
16

reactions using room temperature ionic liquids, 131
133 Electrochemical potential windows (EPWs), 13
14 Electrochemical quartz crystal microbalance (EQCM), 102
105 Electrochemical reduction reactions, 3, 90 important characteristics of, 90
91 electrochemical current and potential, 90
91 electrochemical interfaces, 91 models of electrochemical electron transfer, 91 using room-temperature ionic liquids, 120
130, 137
143 of benzoyl chloride, 129
130 of carbon dioxide gas, 124
126 electrocarboxylation reaction, 126
128 electrochemical reductive coupling to form 1,6-diketone, 128
129 electrogenerated N-heterocyclic carbenes, 137
142 electroreductive coupling of organic halides, 121
122 epoxidation reaction using roomtemperature ionic liquids, 143 functionalisation of nitroaromatic compounds, 143 organocatalysis using electrogenerated bases, 130 Pinacol coupling reaction, 122
123 synthesis of aryl zinc compounds, 128 Electrochemical sensors (ESs), 5
6, 68
69, 305 ClPrNTf2 IL-based electrochemical sensor, 6f Electrochemical solidity, 449 Electrochemical stability, 115, 444
445 series of ionic liquids, 449 Electrochemistry, 172 applications of ionic liquids in, 5
16 electrochemical biosensors, 11
12 electrochemical sensors, 5
6 electrodeposition, 7
8 electroredox, 9
10 at molecular scale, 91
92 bonding of ions, 92 bonding of water, 92 experimental aspects of current/ voltage properties, 92 surface structure, 91
92 Electrodeposition technique, 3
4, 7
8, 8f

Index

Electrodes, 165
166 Electrogenerated N-heterocyclic carbenes, 137
142 benzoin condensation, 138
139 esterification reaction, 141 Henry reaction, 138 N-functionalisation of benzoxazolones, 142 oxidative esterification of aromatic aldehydes, 142 preparation of N-acyloxazolidin-2-ones, 142 preparation of γ-butyrolactones, 140
141 Staudinger reaction, 140 Stetter reaction, 139 synthesis of β-lactams, 137
138 transesterification reaction, 141
142 α, β-unsaturated aldehydes, 141f Electrolyte(s), 360, 373, 378 for battery and supercapacitor applications, 273
275 in dye-sensitized solar cells, role of ionic liquids as, 394
408 iodine-free PIL as electrolyte in dyesensitized solar cells, 404
408 ionic liquids as, 78 for lithium-ion battery, aprotic ionic liquids as, 368
369 for lithium-ion battery, protic ionic liquids as, 367
368 medium for advanced electrochemical devices, 68 role of poly ionic liquids as solid or quasi-solid electrolyte in dyesensitized solar cell, 395
403 Electrolytic support, inorganic nanocomposite materials, 165
166 advantages, 166 Electronic conductor, 90 Electronic double-layer capacitor, 14 Electronic man-made devices, 310 Electronic noses, 310 Electronic tongues, 310 Electrophoresis, 204 Electropolymerization applications of ionic liquids in, 15
16 Electroredox, 9
10 biredox IL, 9f Electrospray thrusters, 453 Electrostatic double-layer capacitors, 360
361 Electrosynthesis, 3
4

529

Energy density, 14 Energy storage ionic liquids in energy storage devices, 66
67 and transfer, 171 Energy storage systems (ESSs), 12
13 Energy sustainability, ionic liquid in, 67
68 Energy-dispersive X-ray spectroscopy, 206 Energy-related applications, 67
68 Environmental contamination, 373 Environmental hazard assessment, 511 Environmental protection agency (EPA), 303
304 Environmental science, 169
170 air pollution treatment, 170 soil treatment, 169
170 water treatment, 169 Enzymes, 45
49 with ionic liquids, 426
427 in ionic liquids, increased stability of, 429
431 Equilibrium potentiometric gas sensors, 291
292 Escherichia coli, 47, 237, 429, 466
467, 505 Esterification reaction, 141 Ethanol ammonium nitrate (EAN), 287
288 Ethyl ammonium nitrate, 64
65, 113 Ethyl nitrobenzene (ENB), 102
105 Ethyl sulfate 1-ethyl-3-methylimidazolium (EMES), 424
425 1-ethyl-3-methyl-imidazolium, bis (trifluoro-methane-sulfonyl) imide (EMIMTFSI), 374
376 1-ethyl-3-methyl imidazolium salts, 484
485 1-ethyl-3-methylimidazolium acetate (EMIM-OAc), 273 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide, 302 1-ethyl-3-methylimidazolium chloride, 37 1-ethyl-3-methylimidazolium dicyanamide (EMID), 188
191 1-ethyl-3-methylimidazolium (EMI), 117, 354 Ethylene carbonate (EC), 378, 396 Ethylene oxide (EO), 395
396 Ethylmethylimidazolium acetic acid (EMIAc), 425 Evaporation, 305 Exclusive diluents, 443
444

530

Index

Extracellular polymeric substances (EPSs), 238
239 Extraction capacity, 35
36 Extraction techniques, 199 extraction of rare earth metals, 209 ionic liquids, 200
201 for extraction of contaminants from wastewater, 205
206 for extraction of food contaminants, 209
210 for extraction of natural products from plants, 202
204 extraction of pharmaceuticals from biological and environmental samples in, 204 extraction of soil contaminants, 207
208 for extraction of soil contaminants and soil organic matter, 207
209 extractions of soil organic matter, 208
209

F Fast Green (FG), 237 Fibers, 373 Filler matrix, 339
340 Flammable solvents, 393 Flavonoids, 268 Fluorescence, 424
425, 434 Fluorine atom, 133
134 Food industry, 49 ionic liquids for extraction of food contaminants\, 209
210 materials, 209
210 science, 170
171 Fossil fuels, 67, 78
79, 365, 391
392 Fourier transform infrared spectroscopy (FT-IR), 41, 96, 425
426 Free radical polymerization process, 336 “Free” proton, 367
368 Fuel cells (FCs), 353 ionic liquids in, 355
359 Fuel cells, 66, 366 Functional polymer, 480 Fused salts, 354

G Galantamine, 261 Galla chinensis, 269 Galvanometer, 294 γ-butyrolactones, preparation of, 140
141

Gas sensors, 73, 290, 294, 339 chemical structures with IUPAC names of cations and anions, 288f electrochemical gas sensors, 290
307 forms of gas sensors, 313
316 carbon ionic liquid composite gas sensors, 315 gated ionic liquid gas sensors, 315
316 semiconductor metal-oxide gas sensors, 313
315 optical gas sensors, 307
311 piezoelectric gas sensors, 311
313 Gated ionic liquid gas sensors, 315
316 Geitlerinema amphibium, 508
509 Gel polymer, electrolytes based on, 380 Gel polymer electrolytes (GPE), 367
368, 394
395 GPEs incorporating imidazolium iodide ILs, 396 Gelatin (PEC), 238
239 Gibbs free energy, 91 Glassy carbon electrodes (GCEs), 6 Glaucium flavum, 260
261 Gold, 300, 302 nanoparticle, 156
157 Gold (Au) electrode, 305 nanocatalysts oxygen sensors, 300 Good’s Buffers (GBs), 36 GB-IL, 43 Grafting process, 336 Gram-negative pathogenic microorganisms, 472
473 Graphene (GR), 11
12 Graphene oxide (GO), 72
73, 339
340 Grease, 455 Green chemistry, 187
188, 465
466 Green corrosion protectors ionic liquids as, 187
188 Green DES, 434 Green diluents in space mechanics, implication of, 452
453 Green ILs, 187
188 Green protectors, 187
188 Green solvents, 3, 231
232, 271
273. See also Solvents bioactive compounds, 272t for negligible vapor pressure, 24 Green synthesis, 187
188 Green-technology approach, 497 “Green” inhibitors, 231
232 Greenhouse gases, 327
330

Index

Guanidine hydrochloride, 425 Gum Arabic (GA), 238
239

H Haber
Bosch process, 301 Halides, 373
376 Halogens, 294 Halomonas sp., 45 Halophilic lipase (HspL), 45 Hard and soft acid and base theory (HSAB theory), 381 Hazardous, 504 Heat energy advantages, 165 storage of, 165 Heavy metals, 169, 208 Hemoglobin (Hb), 94
95 direct electrochemistry of, 94
100 CV plots, 95f, 98f FT-IR spectra, 99f UV
vis absorption spectra, 99f Hen’s lysozyme albumin, 433
434 Henry reaction, 138 Henry’s law, 328
329 Herbicides, 207
208 Hexafluorophosphate (HFP), 64
65, 116, 502 1-hexyl-3-methylimidozium iodide (HMII), 396 High-performance composites, 373 High-performance electrolytes, 373 High-temperature cooling intrinsically safe high-temperature cooling, 275 IL-B2001 cooling medium, 276t Hofmeister arrangement, 423
424 Hollow fiber liquid-phase microextraction technique, 199 Hordeum vulgare, 504 Horseradish Peroxidase enzyme (HRP enzyme), 48
49, 428 Humanoid space operations, 454 Hybrid electrolyte, 75 Hybrid inorganic nanocomposite materials, 155
156 Hybrid materials, 174
175 Hydraulic fluids, ionic liquids as, 78 Hydrocarbons, 292, 294 Hydrochloric acid (HCl), 188
191 Hydrogen (H2), 294
295, 312, 328
330 gases, 294 storage, 277

531

ionic liquids as, 80 Hydrogen cyanide, 294 Hydrogen halides, 294 Hydrogen peroxide, 294 Hydrogen sulfide, 294
295, 312 Hydrophobic forces, 425
426 Hydrophobic ionic liquids, 248
254 liquid
liquid extraction with, 255
259 Hydrophobic polymer, 24 Hydrophobicity of ionic liquids on stability of proteins, effect of, 424 Hydrothermal method, 161 1-hydroxyethyl-3-methylimidazolium chloride, 37 4-(2-hydroxyethyl)-1piperazineethanesulfonic acid (HEPES), 36 1-(2-hydroxyethyl)-3-methyl imidazolium hexafluorophosphate, 484
485 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate, 484
485 Hydroxyl ammonium-based ILs, 40 Hydroxylethyl cellulose, 238
239 Hypergolic solutions, 454
455

I Imidazolium, 143 cation, 334
335 imidazolium-based polymers, 271 imidazolium-centered alkali, 445
446 imidazolium-centered ionic liquids, 445
446 ring-based ILs, 234
237 Imidazolium chloride, 37 Immobilized catalysts, 427
428 Immobilized RIL, 312 Immunoglobulins G (IgG), 43 Impregnation process, 336 3-indole-butyric acid (IBA), 255
259 Inexhaustible energy sources, 365 Infiltration system of ChCl
glycerol, 435 Inhibitors, 227 corrosion inhibition, 232 corrosion inhibitors, 237
239 influence of temperature on action of, 229
232 common ionic liquids, 232f ionic liquids as promising coating agents and, 234
237 on metal surface, 232 Inhibitory concentration (IC), 501
502 Inorganic anions, 373

532 Inorganic coatings, 227 Inorganic ionic liquids in electrical storage systems, 372 Inorganic membranes, 339 Inorganic nanocomposite materials, 155
156 application-based importance of ionic liquids in, 159 current applications, 167
172 biomedical, 168
169 catalysis, 171 electrochemistry, 172 energy storage and transfer, 171 environmental science, 169
170 food science, 170
171 lubricants, 171
172 nuclear science, 170 sensors, 172 development of, 156
157 ionic liquid role in synthesis of, 157
158 with ionic liquids, recent advancements and advantages of, 165
167 additives, 167 analytics and purity, 166
167 electrolytic support, 165
166 solvents improvement, 166 storage of heat energy, 165 organic-inorganic, 164
165 reaction mechanism of ionic liquidsbased synthesized nanocomposite materials, 172
174 using ionic liquid, synthesis of, 159
164 electrochemical method, 164 hydrothermal method, 161 microemulsion method, 161
162 precipitation and co-precipitation method, 162
163 rays mediated method, 163
164 sol-gel method, 159
161 Inorganic polyatoms, 373 Inorganic polyethylene, 373 Inorganic polymerization reaction process, 159 Inorganic salts, 24, 42 Internal ionic conductivity, 368 Iodide, 418
419 Iodine-free PIL as electrolyte in dyesensitized solar cells, 404
408 Ion pairs (IP), 9 Ion-exchange resins, 329 Ionic conductor, 90 Ionic fluids, 354

Index

Ionic fusions, 365
366 Ionic glasses, 354, 365
366 Ionic liquid-based aqueous biphasic systems (IL-ABS), 44, 248
255 applications, 25
26 in separation and purification of biomolecules, 26
51 Ionic liquid-based microwave-assisted extraction (IL-MAE), 255 Ionic liquid-based microwave-assisted homogeneous liquid
liquid microextraction (IL-MA-HLLME), 255 Ionic liquid-based negative-pressure cavitation extraction (IL-NPCE), 255 Ionic liquid-based solid
liquid extractions (IL-SLE), 255, 260
270 complex/rigid solid-liquid extractions, 269
270 microwave-assisted extractions, 267
268 simple/basic, 261
267 biomass using IL-based LLE, SLE, MAE, and UAE, 262t ultrasonic-assisted extractions, 268
269 Ionic liquid-based ultrahigh pressure extraction (IL-UPE), 255 Ionic liquid-based ultrahigh/microwaveassisted extraction (IL-UMAE), 255 Ionic liquid-based ultrasound-assisted extraction (IL-UAE), 255 Ionic liquid-lipophilic compounds (IL-LLE), 255 Ionic liquid-metal-organic frameworks (ILsMOFs), 158 Ionic liquids (ILs), 3, 24, 63, 89, 102, 113, 157
158, 187
188, 199
201, 231, 247
248, 287
288, 303, 306, 331, 353
355, 365
366, 371, 393, 417
418, 443
444, 465
466, 497 adsorbed and covalently linked, 488 advantages and characteristics, 481 affinity attributed to ionic liquids in nanomaterials, 451
452 for Al, ionic liquids as corrosion protector, 191
192 ammonium, pyrrolidinium, phosphonium, and sulfonium-based ionic liquids and electrochemical properties, 73
77 anodization, 227
228 antibacterial and antitumor activities of, 420
421 applications of, 188, 210, 271
277

Index

air conditioning, 277 antistatic agents, 275 chemical production processes, 277 green solvents, 271
273 high-purity, inflammable electrolytes for battery and supercapacitor applications, 273
275 hydrogen storage, 277 ionic liquids as biocatalysis, 427
429 intrinsically safe high-temperature cooling, 275 room-temperature ionic liquids, 70
73 aqueous amines used in postcombustion, 330
331 aspects of ionic liquids as promoters in biodiesel fabrication, 450
451 bonding in ionic liquids, 354 cathodic protection, 228 cations and anions in ionic liquids and structures, 356f effect of chemical composition of, 507
508 alkyl chain length, 507
508 anions, 508 cations, 507 chemical composition of, 498t classification of, 188, 189f common methods of making, 482
483 alkylation, 483 anion exchange, 482 preparation of ionic liquid special performance, 483 composite membranes for CO2 separation, 339
340 compost properties, 454 configuration of ionic liquids in biosensors, 450 consequences of corrosion, 224 consumption, 66 corrosion as a costly problem to the world, 233 corrosion inhibition, 232 corrosion inhibitors, 237
239 for Cu and Zn, ionic liquids as corrosion protector, 192
193 current and future prospects, 78
80 ionic liquids as chemical production processes, 78
80 ionic liquids as electrolytes, 78 ionic liquids as hydrogen storage, 80 ionic liquids as lubricants and hydraulic fluids, 78

533 cytotoxicity of, 431
432 designing ionic liquids for CO2 capture, 340
342 economic effects, 224
226 electrochemical constancy and conductivity, 449 in electrochemical devices, 64
65 electrochemical phenomenon of corrosion, 224f electrochemical properties, 5 electrochemical sensing properties, 68
70 in electrochemistry, applications of, 5
16 in electropolymerization, applications of, 15
16 emerging of ionic liquid
based polymer electrolyte, 374
377 cations and anions in IL, 377t in energy storage devices and conversion materials, 66
67 in energy sustainability and CO2 sequestration, 67
68 to environment, fate and transfer of, 509
513, 509f adsorption of, 509
511 biodegradation of, 511
512 chemical degradation of, 512
513 environmental concerns of, 501
506, 501f cytotoxic action of, 502 enzymatic interaction of, 502
504 enzyme inhibition by, 503t to microorganisms, toxicity of, 505
506, 505t phytotoxicity of, 504
505 effect of environmental factors, 508
509 dissolved organic matter, 508 salinity, 508
509 enzymes with ionic liquids, 426
427 for extraction of contaminants from wastewater, 205
206 for extraction of food contaminants, 209
210 for extraction of natural products from plants, 202
204 for extraction of natural products, reactive dissolution of biomass in, 203
204 microwave-assisted ionic liquid approach, 203 ultrasonic-assisted ionic liquid approach, 202
203

534

Index

Ionic liquids (ILs) (Continued) in extraction of pharmaceuticals from biological and environmental samples, 204 for extraction of soil contaminants and soil organic matter, 207
209 factors affecting toxicity of, 506
509 formation of, 201f in fuel cells, 355
359 function of, 484 synthesis of polymer containing Ionic liquids, 484f as green corrosion protectors, 187
188 hypergolic solutions, 454
455 IL-base extraction technique, 209 IL-based electrolytes, 380
381 IL-based extraction processes, 200, 206 IL-based materials, 202 ILs-based inorganic nanocomposite materials, 159 implication of green diluents in space mechanics, 452
453 importance of, 233 inactivate enzymes like polar organic solvents, 429 increased stability of enzymes in, 429
431 influence of temperature on action of inhibitors, 229
232 inhibitors, 227 in inorganic nanocomposite, applicationbased importance of, 159 ionic liquid-based gel polymer electrolytes application in lithium batteries, 380
381 electrochemical performance of GPE-Li batteries based on PEO, 382t ionic liquid-based liquid
liquid extractions, 255
260 ionic liquid-based aqueous biphasic systems, 259
260 liquid
liquid extraction with hydrophobic ionic liquids, 255
259 ionic liquid-based polymers electrolytes, 372
373 ionic liquid-polymer composite, 489
490 ionic liquid-polymer matrix, 480 ionic liquids-based synthesized nanocomposite materials, 172
174 ionic liquids-centered devices, 449
450 for iron and alloy, ionic liquids as corrosion protector, 188
191

in Li-ion batteries, applications of, 12
14 life support techniques, 454 lunular fluid-glass contract, 456 for metals and alloy, ionic liquids as corrosion protector, 188
193 in metrological analysis and applications, 443
444 microwave absorbing, 488
489 molecules, 373
374 as novel electrolyte medium for electrochemical devices, 68 optoelectronic applications of principle and structure of dyesensitized solar cell, 393
394 role as electrolyte in dye-sensitized solar cells, 394
408 physical properties of, 4
5 conductivity, 4
5 electrochemical potential window, 5 viscosity, 5 physicochemical properties of ionic liquids defining metrological parameters, 447
448 density and viscosity, 448 miscibility and solubility, 447 solvation with repulsion, 447
448 vapor pressure, 448 in polymer chemistry, 481 polymer composite, 489
490 polymer-doped ionic liquids, 484
485 polymerizable ionic liquids, 486
488 common function group used in polymerization, 487f structure of ionic complex, 487f polymerization of vinyl monomer in, 485
486 molecular structures, 486f as promising coating agents and inhibitors, 234
237 ILs as corrosion inhibitors, 235t properties pertinent to surface electrochemistry, 92
94 protein folding in, 425
426 protic and aprotic ionic liquids, 445
446 recent advancements and advantages of inorganic nanocomposites with, 165
167 regeneration of CO2 from, 340 role in surface electrochemistry, 94
107 role in synthesis of inorganic nanocomposite, 157
158 as solvents for CO2 capture, 331
340

Index

in absorption process for CO2 capture, 332
335 in adsorption process for CO2 capture, 335
336 in membranes process for CO2 capture, 337
340 space emollients, 455 space energy, 453 stability of proteins effect of alkyl chain length of cations, 423 effect of cations and anion of, 423 effect of hydrophobicity of, 424 effect of viscosity of, 424
425 structure of, 481 chemical structures of anions, 483f molecular structures of common cations, 482f structure of electrical double layer, 228
229 in supercapacitors, 360
361 for supercapacitors, applications of, 14
15 supported ionic liquid membrane for CO2 separation, 337
338 synthesis of inorganic nanocomposite materials using, 159
164 Ionic liquids classification based on chemical structure, 366
369 aprotic ionic liquids as electrolytes for lithium-ion battery, 368
369 protic ionic liquids as electrolytes for lithium-ion battery, 367
368 Ionic melts, 354 Ionic mobility, 4
5 Ionic-liquid polymeric electrolyte, 304
305 Ion
ion interaction, 201 Ions, 370, 418
419 bonding of, 92 Iris tectorum, 269 2-isobutylthiazole, 238
239 Isothermal titration calorimetry (ITC), 424
425 Isothiocyanate, 418
419

K Kappaphycus alvarezii, 255
259 Kluyveromyces marxianus, 48
49

L L-phenylalanine, 36, 38 L-tryptophan, 26, 35
38

535

L-tyrosine, 36, 38 Lactobacillus brevis, 47 Langmuir
Blodgett films, 312 Laser, 391 Leland Clark type gas sensors, 295 Lemna minor, 507 Lepidium sativum, 507
508 Leucojum aestivum, 261 Leucojum aestivum aestivum, 261 Lewis acid
base interactions, 334 Lewis acid
base ionic liquid, 482
483 Lewis acidity, 76
77 Lewis’ acid, 64 Light-emitting chemical cells (LECs), 391 Light-emitting diode (LED), 391, 393 Lignin, 78
79 valorization approach, 78
79 Lignocellulose, 78
79 Lignocellulosic biomass (LCB), 78
79, 203
204 Lipases, 426 Lippmann electrometer, 228
229 Liquid antistatic agents, 275 Liquid electrolytes, 354, 365
366 Liquid salts, 69, 288, 354, 365
366 Liquid-phase extraction, 209
210 Liquid
liquid extraction (LLE), 23
24, 248
255 bioactive compounds from biomass using IL-based LLE, 256t with hydrophobic ionic liquids, 255
259 technique, 199 Lithium (Li) batteries, 3
4, 66, 366, 369
370 dendrite, 370 ionic liquid-based gel polymer electrolytes application in, 380
381 Li/Na-ion battery electrolyte, 378 low melting point alkaline salts in, 381
383 salts, 368 Lithium-ion batteries (LIB), 3
4, 66, 366 applications of ionic liquids in, 12
14 aprotic ionic liquids as electrolytes for, 368
369 protic ionic liquids as electrolytes for, 367
368 Lithium-oxygen batteries, 66 Lithium-sulfur batteries (Li-S batteries), 66
67 Lolium perenne, 504
505

536

Index

Long-chain carboxylates-ILs (LCC-ILs), 267 Low melting point alkaline salts in lithium batteries, 381
383 Lubricants, 171
172, 455 ionic liquids as lubricants fluids, 78 Lunular fluid-glass contract, 456 Lysozyme crystals, 422

M Macromolecules, 486 Magnesium chloride (MgCl2), 42 Mammalian cell, 502 Marcus
Hush theory, 9 Material selection method, 226 Mellow steel (Ms), 237 Membrane electrode assembly (MEA), 332
333, 357 Membrane separation process, 329 Membrane technology, 337 Membranes process for CO2 capture, ionic liquids in, 337
340 ionic liquid composite membranes for CO2 separation, 339
340 polyionic liquids membranes for CO2 separation, 338
339 supported ionic liquid membrane for CO2 separation, 337
338 Mercury, 456 Metal nanoparticles, 163 and ionic liquids, 465
466 Metal oxides, 13, 329 Metal surfaces, 233 Metallic coating method, 226 Metallic hip joints, 224
225 Metallic nanoparticles, 312, 451
452 Metals, 185
186, 205
206, 223
224 ionic liquids as corrosion protector for, 188
193 physical degradation, 185 Methanol, 447
448 Methyl adamantane-1-carboxylate electrochemically, fluorination of, 136
137 Methyl alcohol, 118 Methyl methacrylate (MMA), 399 Methyl sulfate (Ms), 137
138 Methyl sulfate 1,2,3-trimethylimidazolium [Tmim][MeSO4], 426 1-methyl-3-propylimidazolium iodide (MPII), 396 Methylbenzoate, 306 1-methylimidazolium chloride, 37

Methylsulfate-1,3-dimethylimidazolium [MMIM][MeSO4], 426 Metrological parameters, physicochemical properties of ionic liquids defining, 447
448 Metronidazole, 471 Microcarbon dioxide gas sensor, 306 Microemulsion method, 161
162 Microextraction techniques, 157
158 Microorganisms, 312, 505 Microwave homogeneous liquid
liquid microextraction, 270 Microwave-assisted extraction technology, 267
269 Microwave-assisted ILs approach, 202 for extraction of natural products, 203 Mixed matrix membranes (MMMs), 339 Modern Clark type gas sensors, 294 Molecular dynamics simulations of DES, 434 Molten salts, 365
366, 371
372 Monoethanolamine (MEA), 330
331 Multi-walled carbon nanotubes (MWCNTs), 8, 407
408

N

N,N,N0 ,N0 -tetrabutylparaphenylenediamine (TC4PD), 106 N,N,N0 ,N0 -tetramethyl-paraphenylenediamine (TC1PD), 106 N,N0 -bis(4-hydroxybenzaldehyde)-2,2dimethylpropandiimine (p-HBDP), 238
239 N,N-dimethylethanolamine butanoate, 40 N,N-dimethylethanolamine hexanoate, 40 N,N-dimethylethanolamine pentanoate, 40 N,N-dimethylethanolamine propanoate, 40 N-(2,4-Dinitrophenyl)-glycine (DNPglycine), 38 N-(2,4-Dinitrophenyl)-L-alanine (DNP-Lalanine), 38 N-(2,4-Dinitrophenyl)-L-leucine (DNP-Lleucine), 38 N-(2,4-Dinitrophenyl)-L-valine (DNP-Lvaline), 38 N-acyloxazolidin-2-ones, preparation of, 142 N-alkyl pyridinium, 64
65 N-butyl-N-methylpyrrolidinium, 306
307 N-butylpyridinium hexafluorophosphate, 96
97 N-desmethylgalantamine, 261

Index

1-n-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 159
160 N-ethylpyridinium tetrafluoroborate (EtPyBF4), 432 N-ethylpyridinium trifluoroacetate (EtPyCF3COO), 432 N-functionalisation of benzoxazolones, 142 N-heterocyclic carbenes (NHCylC), 115 [2-(N-morpholino) ethanesulfonate] (MES), 42 N-octadecylpyridiniumbromide (PyC18Br), 188
191 N-tris(hydroxymethyl)methyl glycine (TRICINE), 36 N-tris(hydroxymethyl)methyl-2aminoethanesulfonic acid (TES), 36 N-vinylimidazolium, 487 Nanocomposite materials, 156
157 Nanoecotoxicology field, 467
468 Nanomaterials, 156
157, 167
168 affinity attributed to ionic liquids in, 451
452 Nanoparticle diffusion, 451
452 Nanosilver production technology, 469
470 Nanotechnology, 469
470 Narwedine, 261 National Aeronautics and space administration, 454 Natural products microwave-assisted ILs approach for extraction of, 203 from plants, 202
204 reactive dissolution of biomass in ILs for extraction of, 203
204 ultrasonic-assisted ILs approach for extraction of, 202
203 Negative-pressure cavitation extraction, 270 Nelumbo nucifera, 267
268 Neoteric solvents, 432 Nitramines, 331 Nitrate, 374
376 Nitric oxide, 294 Nitroaldol condensation reaction, 138 2-Nitroanisole, 130 Nitroaromatic compounds, functionalisation of, 143 4-nitrobenzylamine, 117 Nitrogen dioxide, 294
295, 303 Nitrogen monoxide, 294, 303 Nitrogen oxides, 291
292, 294 Nitrosamines, 331

537

Nitrosomonas europaea, 467
468 Nitrous oxide, 303 Nonaqueous electrolytes, 449 Nonequilibrium potentiometric gas sensors, 292 Nonfaradaic biosensing component room-temperature ionic liquids as, 72
73 Novel amperometric gas sensors, 302 Novel optical carbon dioxide sensor, 309 Nuclear science, 170 Nucleic acids, 50
51 Nucleotides, 50

O 1-octyl,3-methylimidazolium tetrafluoroborate, 306 1-octyl-3-methyl imidazolium (OMI), 121 Oocystis submarina, 508
509 Optical ammonia gas sensors, 309
310 Optical carbon dioxide gas sensors, 308
309 Optical gas sensors, 307
311 optical ammonia gas sensors, 309
310 optical carbon dioxide gas sensors, 308
309 optical oxygen gas sensors, 308 optical volatile organic compound gas sensors, 310
311 Optical oxygen gas sensors, 308 Optical volatile organic compound gas sensors, 310
311 Optoelectronic conversion devices, 393 Optoelectronic devices, 391 Optoelectronic gas sensor devices, 311 Optoelectronic technology, 391 Organic anions, 374
376 Organic carbonates, 125 Organic coating method, 226 Organic fuels, 207
208 Organic halides, electroreductive coupling of, 121
122 aryl halides using OMI
TFB RIL, 121f nickel grid cathode, 121f Organic ionic liquids in electrical storage systems, 372 Organic materials, 155
156 Organic matter (OM), 510 Organic pollutants, 169, 207
208 extraction of, 206 Organic salts, 370
371 Organic solvents, 4
5, 114, 200, 312, 371 Organic vapors, 294

538

Index

Organic-inorganic nanocomposite, 164
165 Organocatalysis using electrogenerated bases, 130 2-anisidine, 130f Organophosphorus, 207
208 Organophosphorus hydrolase (OPH), 11 Outer Helmholtz plane, 228
229 Oxidative self-coupling reaction, 117 Oxidizing gases, 295 Oxycombustion, 327
330 Oxyfuel CO2 combustion, 328 Oxygen, 293
295 sensors, 298
299 Oxygen dual gas sensor, 307 Oxygen reduction process, 295 Ozone, 294

P p-nitrorophenylbutyrate (p-PNB), 428 3-p-nonylphenoxy-2-hydroxypropyl trimethylammonium bromide (NPTAB), 36 Palladium nanoparticles (Pd nanoparticles), 69 Paper-based oxygen gas sensor, 300
301 Paper-based solid-state Au electrode oxygen gas sensor, 300
301 Paullinia cupana, 267 Pebax matrix, 339
340 PEG dimethyl ether 250 (PEGDME250), 41 1-pentyl-3-methyl-imidazolium bromide ([PMIM][Br]), 430
431 Per-fluoro-alkyl as di-(trifluoromethane sulfonyl imide) (TFSI), 368 Perchlorate (ClO4), 503 Pericarpium papaveris, 259
260 Perovskite solar cells (PSCs), 391
393 Perturbed chain-statistical associating fluid theory (PC-SAFT), 334 Pharmaceuticals from biological and environmental samples, 204 companies, 432 ingredient, 247
248 Phosphine, 294 Phosphonium, 73
77 ionic liquids, 448 salts, 78 Phosphonium cation-based ILs, 75
76 Phosphorous oxide, 370 Photobacterium phosphoreum, 508 Photocatalytic deposition process, 163
164

Photochemical method, 163 Photovoltaic device technology, 391
392 Photovoltaic technology, 393 Physa acuta, 508 Physical absorbents, ionic liquids as, 333 Physochlainae infundibularis, 269 Pichia pastoris, 505 Piezoelectric gas sensors, 311
313 piezoresistive-based gas sensors, 313 quartz-crystal microbalance gas sensors, 311
313 surface acoustic wave
based gas sensors, 313 Piezoresistive-based gas sensors, 313 Pinacol coupling reaction, 122
123 BMI
TFB and water mixture, 123f situ catalytic Ullmann-type reactions, 122f Piperazine-based solvents, 331 Piperidinium, 143 Piperine, 267 Planar amperometric gas sensor, 305 Plasma electrochemical method, 164 Plasticized polymer, electrolytes based on, 379 Platinum (Pt), 8, 300, 302 electrodes, 303 Pt-coated cathode, 393
394 screen-printed electrode surfaces, 302 Polar organic solvents, ionic liquids do not inactivate enzymes like, 429 Polar solvents, 429 Polarization methods, 303 Pollutants, 205 Poly (diallyldimethylammonium) imide, 480 Poly (vinylidene fluoride), 480 Poly (vinylidene fluoride)hexafluoropropylene gels, 484
485 Poly ionic liquids (PILs), 158, 191
192, 373
374, 393
395 membranes for CO2 separation, 338
339 PILs-based DSSC, 395
396 role as solid or quasi-solid electrolyte in dye-sensitized solar cell, 395
403 literature reported photovoltaic performance data for Poly (ILs), 397t Poly-ILs membrane (PILM), 338
339 Poly(1-alkyl-3-(acryloyloxy) hexylimidazolium iodide) (PAAII), 404
406 Poly(3-hexylthiophene)film, 132

Index

Poly(3-methylthiophene), 132 Poly(3,4-ethylenedioxythiophene) polymer films, 16, 133 Poly(ethylene oxide
vinylimidazolium
methyl methacrylate iodide) (PVIM), 399 Poly(ethylene) oxide (PEO), 370 Poly(hydrogen fluoride) salts, 133
135 Poly(methyl methacrylate) (PMMA), 161
162 Polyacrylonitrile (PAN), 399
400 membrane, 337
338 PAN-based gel electrolytes, 400 Polyatomic metals, 374
376 Polybrominated biphenyls, 207
208 Polycarbonate membrane, 337
338 Polychlorinated biphenyls, 207
208 Polychlorinated dibenzofurans, 207
208 Polycyclic aromatic hydrocarbons, 207
208 Polyethylene glycol, 379 Polyethylene glycol-polysaccharide ABS (PEG-polysaccharide ABS), 35
36 Polyethylene oxide (PEO), 395
396 Polyethyleneimine, 315 Polymer, 315 polymer-based composite electrolytes, 380 Polymer (ethylene glycol)methyl ether methacrylate (POEM), 402
403 Polymer electrolytes (PEs), 368, 404
406 classification, 378
380 electrolytes based on composite polymer, 380 electrolytes based on dry solid polymer, 378
379 electrolytes based on gel polymer, 380 electrolytes based on plasticized polymer, 379 plastic, 379 Polymer-doped ionic liquids, 484
485 Polymer 1 inorganic salts, 24 Polymeric films, 131 Polymeric ionic liquid (PILs), 366 in rechargeable batteries basics of, 371, 372t classification of ionic liquids based on chemical structure, 366
369 ionic liquid-based gel polymer electrolytes application in lithium batteries, 380
381

539

ionic liquid-based polymers electrolytes historical background, 372
373 Li batteries, 369
370 Li/Na-ion battery electrolyte, 378 low melting point alkaline salts in lithium batteries, 381
383 organic and inorganic ionic liquids in electrical storage systems, 372 polymer-electrolytes classification, 378
380 polymeric ionic liquids for rechargeable lithium-ion batteries, 373
377 Polymeric membranes, 329, 339 Polymeric thin films, 133 Polymerizable ionic liquids, 486
488 Polymerization process, 479
480, 484 of vinyl monomer in ionic liquid, 485
486 Polymers, 78
79 polymer-salt systems, 44 Polyoxometallate, 374
376 Polypropylene glycol 446 (PPG400), 48 Polysaccharides, 202 Polystyrene, 488 Porous inorganic membranes, 329 Post-treatment greenhouse gas emissions, 63 Postcombustion method, 327
330 aqueous amines used in, 330
331 Potassium carbonate, 291
292 Potassium iodide (KI), 404
406 Potassium thiocyanate, 373 Potentiodynamic polarization techniques (PP techniques), 237 Potentiometric gas sensors, 290
291 Power conversion efficiency (PCE), 391
392 Precipitation method, 162
163 Precombustion method, 327
330 Pressure swing adsorption (PSA), 340 1-propyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide IL, 299 Propylene carbonate (PC), 379, 396 Protectors ionic liquids as green corrosion protectors, 187
188 Proteins, 39
44, 417
418, 429 aggregation, 417
418 chemistry

540

Index

Proteins (Continued) effect of alkyl chain length of cations of ionic liquids on stability of proteins, 423 antibacterial and antitumor activities of ionic liquids, 420
421 application of ionic liquids as biocatalysis, 427
429 effect of cations and anion of ionic liquids on stability of proteins, 423 cytotoxicity of ionic liquids, 431
432 effect of hydrophobicity of ionic liquids on stability of proteins, 424 enzymes with ionic liquids, 426
427 increased stability of enzymes in ionic liquids, 429
431 ionic liquids do not inactivate enzymes like polar organic solvents, 429 neoteric solvents, 432 protein folding in ionic liquids, 425
426 protein instability and influencing factors as well as analytical monitoring, 421
423 role of deep eutectic solvents on, 433
435 effect of viscosity of ionic liquids on stability of proteins, 424
425 folding in ionic liquids, 425
426 instability and influencing factors as well as analytical monitoring, 421
423 stability, 435 Protic ionic liquids (Protic IL), 287
288, 358
359, 445
446 as electrolytes for lithium-ion battery, 367
368 Proton exchange membrane FCs (PEMFCs), 357 Pseudocapacitive mechanism, 361 Pseudocapacitors, 360
361 Pseudomonase aeruguinosa, 471 Purification factor (PF), 45 Pyrene-based dyes, 308 Pyrrolidinium, 73
77, 143 pyrrolidinium-based IL electrolytes, 69
70

Q Quadrupole moment, 333 Quartz crystal microbalance (QCM), 101
105, 103t gas sensors, 311
313

Henry’s constants of ionic liquids, 105t Quasi-solid electrolyte in dye-sensitized solar cell, role of poly ionic liquids as, 395
403 Quasisolid
state (QSS), 394
395 Quaternary ammonium salts, 431

R Radical polymerization method, 407
408 Raphanus sativus, 504 Rare earth metals, extraction of, 209 Rays mediated method, 163
164 photocatalytic deposition, 163
164 photochemical method, 163 sonochemical method, 164 Reaction mechanism of ionic liquids-based synthesized nanocomposite materials, 172
174 Reactive oxygen species (ROS), 504 levels, 469
470 Rechargeable lithium batteries, 369 Rechargeable lithium-ion batteries, polymeric ionic liquids for, 373
377 emerging of ionic liquid
based polymer electrolyte, 374
377 Recombination resistance (Rrec), 402
403 Red blood cells (RBC), 293 Reduced graphene oxide (rGO), 72
73 Renewable energy sources, 365 Renewable resources, 248 Repulsion, solvation with, 447
448 Restraint, 227, 238
239 Rheum spp., 269 Rhizma polygoni, 267
268 Ribonucleic acid (RNA), 50 Ribulose-1,5-bisphosphate carboxylase/ oxygenase, 47 Room temperature ammonia sensor, 303 Room temperature electrochemical ammonia gas sensor, 303 Room temperature ionic liquids (RTIL), 64, 89, 113, 287
288, 334, 361 applications of, 70
73 typical room-temperature ionic liquids properties, 71t electrochemical applications of, 70
72 in electrochemical gas sensoring, 73 electrochemical oxidation reactions using, 115
120 electrochemical partial fluorination using, 133
137

Index

electrochemical polymerization reactions using, 131
133 electrochemical reactions using, 137
143 electrochemical reduction reactions using, 120
130 epoxidation reaction using, 143 as nonfaradaic biosensing component, 72
73 Room temperature liquids, 158 Room-temperature ionic liquids. See Ionic liquids Rubsico-1,5-bisphosphate carboxylase/ oxygenase, 47

S Saccharomyces cerevisiae, 429 Salicornia herbacea, 271 Salinity, 508
509 Salvia miltiorrhiza, 270 Scanning electron microscopy (SEM), 206, 309 Scenedesmus vacuolatus, 507 Screen-printed electrodes, 302 Semi-organic salts, 92
93 Semiconducting metal oxide gas sensors, 313
315 Sensors, 172 Separation and purification methods of biomolecules, 26
51 for compound, 23
24 Shikmic acid, 269 Shono oxidation of carbamates, 117
118, 118f Silica gel, 329 Silver, 300 as antibacterial agent, 466
468 Silver nanoparticles (Ag NPs), 156
157, 237, 466
467 synthesized in ionic liquids antimicrobial properties and activities, 468
473 historical background, 466
468 metal nanoparticles and ionic liquids, 465
466 real and imaginary parts of permittivity’s of metal candidates, 469f schematic illustration of mechanism of IL-1d@Ag nanocomposite, 473f schematic preparation of Ag NPs, 472f silver as antibacterial agent, 466
468 synthetic processes for Ag NPs, 471f

541

Silver sulfate, 291
292 Simple salts ionic liquid [EtNH3][NO3], 479
480 Single-drop microextraction technique, 199 Single-factor tests, 434 Single-phase system, 24 “Single-use” oxygen sensors, 300 Sinomerine acutum, 269 Size-exclusion high-performance liquid chromatography (SEHPLC), 41 Small-scale industries process, 328 Sodium alginate (SA), 99
100, 238
239 Sodium cations (Na1), 353
354 Sodium chloride (NaCl), 42 Sodium iodide, 373 Sodium sulfate (Na2SO4), 42 Sodium thiocyanate, 373 Soil contaminants, extraction of, 207
208 extraction of soil heavy metal ions, 208 extraction of soil organic pollutants, 207
208 ionic liquids for extraction of, 207
209 Soil heavy metal ions, extraction of, 208 Soil organic matter extractions of, 208
209 ionic liquids for extraction of, 207
209 Soil organic pollutants, extraction of, 207
208 Soil pollution, 169
170 Soil treatment, environmental science, 169
170 Sol-gel method, 159
161 Solar energy, 391
392 Solid electrolyte in dye-sensitized solar cell, role of poly ionic liquids as, 395
403 Solid polymer electrolytes (SPE), 370 Solid
liquid extraction (SLE), 248
255 Solid
phase extractions (SPE), 209
210, 248
255, 270
271 Solid-phase microextraction technique, 199 Solid
state (SS), 392
393 electrochemical gas sensor, 306 Solubility of polar substrates, 429 Solution chemistry, 287 Solvation with repulsion, 447
448 Solvents advantages, 166 based extraction techniques, 199 for CO2 capture, ionic liquids as, 331
340 ILs, 187
188 improvement, 166

542 Solvents (Continued) viscosity, 5 Sonochemical method, 164 Sophora flavescens, 271 Sorbus tianschanica, 268 Space emollients, 455 Space energy, 453 Space mechanics, implication of green diluents in, 452
453 Stability of proteins alkyl chain length of cations of ionic liquids on, 423 effect of alkyl chain length of cations of ionic liquids on, 423 anion effect of ionic liquids on, 423 effect of cations and anion of ionic liquids on, 423 cations effect of ionic liquids on, 423 effect of hydrophobicity of ionic liquids on, 424 increased stability of enzymes in ionic liquids, 429
431 effect of viscosity of Ionic liquids on, 424
425 Stainless steel, 188
191 Staphylococcus aureus, 237, 467 Staudinger reaction, 140 Steel lithium, 370 Stetter reaction, 139 Streptococcus bacteria, 470 Streptococcus faecalis, 237 Suaeda glauca, 269 Sublimation, 305 Sulfonated chitosan (SC), 238
239 Sulfonium-based ILs, 76 Sulfonium-based ionic liquids and electrochemical properties, 73
77 Sulfur dioxide, 294
295 Sulfur Mustard (SM), 105 Sulfur oxides gases, 291
292 Sulfuric acid (H2SO4), 188
191 Sunset Yellow (SS), 237 Supercapacitor (SC), 3
4, 353 applications of ionic liquids for, 14
15 electrolytes for supercapacitors applications, 273
275 ionic liquids in, 360
361 Superoxide dismutases (SOD), 48
49 Supporting ionic liquid membrane (SILM), 337
338 Surface acoustic wave
based gas sensors, 313

Index

Surface chemistry, 90 Surface electrochemistry. See also Electrochemistry characteristics of electrochemical reactions, 90
91 electrochemistry at molecular scale, 91
92 ionic liquids properties pertinent to, 92
94 ionic liquids role in, 94
107 carbon ionic liquid electrode, 94
101 chemical warfare agent, 105
106 electrochemical oxidation, 106
107 quartz crystal microbalance, 101
105 Surface micromachining technique, 306 Surface-active IL (SAIL), 15 Synthesis method of inorganic nanocomposite, 157
158 Synthesized IL, 43 Synthetic gas, 328
330

T Tamarindus indica, 238
239 Tartrazine (TZ), 237 Task specific ionic liquids (TSILs), 334
335 Task-specific liquids, 158 Temperature swing adsorption (TSA), 340 regeneration process, 340 Tetra-alkyl ammonium, 143 cation-based ILs, 358 Tetrabutylammonium
tris (pentafluoroethyl)trifluorophosphate (TP
FETFP), 114 (Tetradecyl)phosphonium-bis(trifluormethylsulfonyl)imide, 429 Tetraethylammonium tetrafluoroborate (TEA-BF4), 14
15 Tetrafluoroborate (BF42), 64
65, 368, 502 Tetrafluorophosphate (TFP), 137
138 Tetrahydrofuran (THF), 45
46 Tetramethylammonium chloride-urea (TMACl-urea), 434 Tetrapropylammonium bromide-urea (TPMBr-urea), 434 Thermogravimetric analysis, 206 Thermomyces lanuginosus lipase (TlL), 45 Thermophilic lipase (TtL), 45 Thermus thermophilus HB27, 45 1-(1,3-thiazol-2-yl)ethanone, 238
239 Titanium (Ti), 188
191 Titanium dioxide (TiO2), 392
393 Toxic metal ions, extraction of, 205
206

Index

Toxicity of ionic liquids to microorganisms, 505
506, 505t Transesterification, 433 reaction, 141
142 Transmission electron microscopy (TEM), 424
425 Tributyldecylammonium
tosyl (TBDA
TSL), 118 Tributylmethylammonium bis (trifluoromethylsulfonyl)imide IL, 298 Trichlorocuprate-1-ethyl-3methylimidazolium (EMIM), 430
431 Trifluoromethylsulfonylimide (TFSI2), 374
376 Trihexyl hydrophobic IL, 429 Triphenylhexadecylphosphonium iodide, 504 Triticum aestivum, 504
505 Tungsten oxide nanoparticles (WO3 NP), 399

U Ultracapacitor, 14, 360 Ultrahigh pressure extraction, 270 Ultrasonic-assisted extractions, 268
269 Ultrasonic-assisted ILs approach, 202 for extraction of natural products, 202
203 Ultrasonic/microwave-assisted extractions (UMAE), 269
270 Undoped SnO2, 313
314 Ungiminorine, 261 Uranium oxide (UO2), 74 Uric acid (UA), 69, 101 UV
Vis, 434

V Vacuum swing adsorption (VSA), 340 Van der Waals forces, 333 Van der Waals interaction, 510
511 Vapor pressure, 448

543

Vibrio fischeri, 501
502 Vicia faba, 504
505 Viscosity of ionic liquids on stability of proteins, effect of, 424
425 Vogel
Tammann
Fulcher equation (VTF equation), 92
93 Volatile organic compounds (VOC), 118, 305, 366, 497 Volatility, 497 Voltammetry technique, 302

W Wastewater ILs for extraction of contaminants from, 205
206 extraction of organic pollutants, 206 extraction of toxic metal ions, 205
206 Water, 332
333, 371 bonding of, 92 treatment of environmental science, 169 Wide-ranging ionic liquids, 444
445 features of ionic liquids, 446t Wireless bioprocessing online device platform, 301 WO3/Pt gas sensor, 314

X X-ray diffraction, 206 X-ray photoelectron spectroscopy, 206

Y Y-shape proteins (IgY), 43

Z Zeolites, 329 Zinc ionic conductivity, 374
376 ionic liquids as corrosion protector for, 192
193 Zinc oxide semiconducting metal oxide, 313
314