Nanostructured polymer membranes. Volume 1, Processing and characterization 9781118831779, 978-1-118-83173-1

Table of contents :
Content: Title page
Copyright page
Preface
Chapter 1: Processing and Characterizations: State-of-the-Art and New Challenges
1.1 Membrane: Technology and Chemistry
1.2 Characterization of Membranes
1.3 Ceramic and Inorganic Polymer Membranes: Preparation, Characterization and Applications
1.4 Supramolecular Membranes: Synthesis and Characterizations
1.5 Organic Membranes and Polymers to Remove Pollutants
1.6 Membranes for CO2 Separation
1.7 Polymer Nanomembranes
1.8 Liquid Membranes
1.9 Recent Progress in Separation Technology Based on Ionic Liquid Membranes
1.10 Membrane Distillation. 1.11 Alginate-based Films and Membranes: Preparation, Characterization and ApplicationsReferences
Chapter 2: Membrane Technology and Chemistry
2.1 Introduction
2.2 Membrane Technology: Fundamental Concepts
2.3 Separation Mechanisms
2.4 Chemical Nature of Membrane
2.5 Surface Treatment of Membranes
2.6 Conclusions
References
Chapter 3: Characterization of Membranes
3.1 Introduction
3.2 Physical Methods for Characterizing Pore Size of Membrane
3.3 Membrane Chemical Structure
3.4 Conclusions
References. Chapter 4: Ceramic and Inorganic Polymer Membranes: Preparation, Characterization and Applications4.1 Introduction
4.2 Recent Developments in Filler-doped Polymer Electrolytes
4.3 Methodology
4.4 Results and Discussion
4.5 Conclusions
Acknowledgment
References
Chapter 5: Supramolecular Membranes: Synthesis and Characterizations
5.1 Overview
5.2 Supramolecular Materials
5.3 Supramolecular Membranes
5.4 Membrane Fabrication Using Supramolecular Chemistry
5.5 Conclusions
References
Chapter 6: Organic Membranes and Polymers for the Removal of Pollutants. 6.1 Membranes: Fundamental Aspects6.2 Liquid-phase Polymer-based Retention (LPR)
6.3 Applications for Removal of Specific Pollutants
6.4 Future Perspectives
6.5 Conclusions
Acknowledgments
References
Chapter 7: Membranes for CO2 Separation
7.1 Introduction
7.2 Fundamentals of Membrane Gas Separation
7.3 Polymeric Membranes for CO2 Separation
7.4 Mixed Matrix Membranes
7.5 Supported Ionic Liquid Membranes (SILMs) for CO2 Separation
7.6 Conclusion
7.7 Overall Comparison and Future Outlook
Abbreviations
References
Chapter 8: Polymer Nanomembranes
8.1 Introduction
8.2 Materials. 8.3 Nanomembrane Fabrication8.4 Characterization
8.5 Applications
References
Chapter 9: Liquid Membranes
9.1 Introduction
9.2 Most Recent Developments
9.3 Liquid Membranes Based Separation Processes
9.4 Conclusion
References
Chapter 10: Recent Progress in Separation Technology Based on Ionic Liquid Membranes
10.1 Introduction
10.2 Ionic Liquid Properties
10.3 Bulk Ionic Liquid Membranes
10.4 Emulsified Ionic Liquid Membranes
10.5 Immobilized Ionic Liquid Membranes
10.6 Green Aspect of Ionic Liquids
10.7 Conclusions
Acknowledgments
References.

Citation preview

Nanostructured Polymer Membranes

Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106 Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Nanostructured Polymer Membranes Volume 1: Processing and Characterization

Edited by

Visakh P.M. and Olga Nazarenko

Copyright © 2017 by Scrivener Publishing LLC. All rights reserved. Co-published by John Wiley & Sons, Inc. Hoboken, New Jersey, and Scrivener Publishing LLC, Beverly, Massachusetts. Published simultaneously in Canada. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or other­­wise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at http://www.wiley.com/go/permission. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For more information about Wiley products, visit our web site at www.wiley.com. For more information about Scrivener products please visit www.scrivenerpublishing.com. Cover design by Russell Richardson Library of Congress Cataloging-in-Publication Data: ISBN 978-1-118-83173-1

Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

Contents Prefacexv 1 Processing and Characterizations: State-of-the-Art and New Challenges 1 Visakh. P. M.   1.1 Membrane: Technology and Chemistry 1   1.2 Characterization of Membranes 3   1.3 Ceramic and Inorganic Polymer Membranes: Preparation, Characterization and Applications 4   1.4 Supramolecular Membranes: Synthesis and Characterizations 5   1.5 Organic Membranes and Polymers to Remove Pollutants 7   1.6 Membranes for CO2 Separation 8   1.7 Polymer Nanomembranes 9   1.8 Liquid Membranes 11   1.9 Recent Progress in Separation Technology Based on Ionic Liquid Membranes 12 1.10 Membrane Distillation 13 1.11  Alginate-based Films and Membranes: Preparation, Characterization and Applications 14 References15 2 Membrane Technology and Chemistry Manuel Palencia, Alexander Córdoba and Myleidi Vera 2.1 Introduction 2.2 Membrane Technology: Fundamental Concepts 2.2.1 Basic Parameters 2.3  Separation Mechanisms 2.3.1 Pressure-driven Membrane Methods 2.3.2 Liquid Membranes 2.3.3 Other Methods

27 27 28 31 33 34 40 41 v

vi  Contents 2.4 Chemical Nature of Membrane 41 2.5  Surface Treatment of Membranes 42 2.5.1 Chemical Methods for Membrane Modification 42 2.5.1.1 Chemical Treatment 42 2.5.1.2 Grafting 43 2.5.1.3 Chemical Initiation Technique 44 2.5.1.4 Photochemical and Radiation Initiation Techniques44 2.5.1.5 Plasma Initiation Technique 45 2.5.1.6 Enzymatic Initiation Technique 45 2.5.2 Physical Methods for Membrane Modification 46 2.5.2.1 Coating 46 2.5.2.2 Blending 46 2.5.2.3 Composite 46 2.5.2.4 Combined Methods 47 2.5.3 Current Research about Membrane Modification 47 2.6 Conclusions 48 References48 3 Characterization of Membranes 55 Derya Y. Koseoglu-Imer, Ismail Koyuncu, Reyhan Sengur-Tasdemir, Serkan Guclu, Recep Kaya, Mehmet Emin Pasaoglu and Turker Turken 3.1 Introduction 56 3.2 Physical Methods for Characterizing Pore Size of Membrane 56 3.2.1 Microscopy 57 3.2.2 Bubble Pressure and Gas Transport 59 3.2.3 Porosimetry 61 3.2.4 Liquid-vapor Equilibrium 63 3.2.5 Liquid-solid Equilibrium (Thermoporometry) 64 3.2.6 Gas-liquid Equilibrium (Permporometry) 65 3.2.6.1 Capillary Condensation 65 3.3 Membrane Chemical Structure 67 3.3.1 Fourier Transform Infrared Spectroscopy (FTIR) 67 3.3.2 Raman Spectroscopy 68 3.3.3 Energy-dispersive X-ray Spectroscopy (EDS) 70 3.3.3.1 Basics of EDS 70 3.3.3.2 Applications of EDS in Membrane Characterization70 3.3.4 X-ray Photoelectron Spectroscopy (XPS) 71

Contents  vii 3.3.5 Electron Spectroscopy 73 3.3.5.1 Auger Electron Spectroscopy (AES) 73 3.3.5.2 Electron Energy Loss Spectroscopy (EELS) 74 3.3.6 Atomic Force Microscopy (AFM) 74 3.3.6.1 Basics of AFM 74 3.3.6.2 Applications of AFM in Membrane Characterization76 3.3.7 Secondary Ion Mass Spectrometry (SIMS) 78 3.3.8 Surface Hydrophilicity and Surface Energy 79 3.3.8.1 Determination of Hydropilic/ Hydrophobic Nature of Membranes 79 3.3.8.2 Contact Angle Measurement by Drop Profile Analysis 80 3.3.8.3 Surface Energy 83 3.4 Conclusions 85 References85 4 Ceramic and Inorganic Polymer Membranes: Preparation, Characterization and Applications 89 Chiam-Wen Liew and S. Ramesh 4.1 Introduction 90 4.1.1 Overview of Polymer Electrolytes 90 4.1.2 Methods to Enhance Ionic Conductivity of Polymer Electrolytes 90 4.1.3 Ionic Liquids 91 4.1.3.1 Advantages of Ionic Liquids 91 4.1.3.2 Applications of Ionic Liquids 92 4.1.4 Fillers 92 4.1.4.1 Types of Fillers 93 4.1.4.2 Advantages of Addition of Fillers 94 4.1.5 Applications of Nanocomposite Polymer Electrolytes (NCPEs) 95 4.2 Recent Developments in Filler-doped Polymer Electrolytes 95 4.2.1 Al2O395 4.2.2 TiO298 4.2.3 ZrO299 4.2.4 SiO2101 4.2.5 Supercapacitors 103 4.2.5.1 Types of Supercapacitors 103

viii  Contents 4.2.5.2 Advantages of Supercapacitors 104 4.2.5.3 Applications of Supercapacitors 104 4.3 Methodology 105 4.3.1 Materials 105 4.3.2 Sample Preparation 105 4.3.3 Sample Characterization 106 4.3.3.1 Ambient Temperature-ionic Conductivity and Temperaturedependent Ionic Conductivity Studies 106 4.3.3.2 Differential Scanning Calorimetry (DSC) 106 4.3.3.3  Linear Sweep Voltammetry (LSV) 107 4.3.4 Electrode Preparation 107 4.3.5 Electrical Double-layer Capacitors (EDLCs) Fabrication 108 4.3.6 Electrical Double-layer Capacitors (EDLCs) Characterization108 4.3.6.1 Cyclic Voltammetry (CV) 108 4.3.6.2 Galvanostatic Charge-discharge Analysis (GCD) 108 4.4 Results and Discussion 109 4.4.1 Ambient Temperature-ionic Conductivity Studies109 4.4.2 Temperature-dependent–ionic Conductivity Studies 112 4.4.3 Differential Scanning Calorimetry (DSC) 114 4.4.4 Linear Sweep Voltammetry (LSV) 117 4.4.5 Cyclic Voltammetry (CV) 119 4.4.6 Galvanostatic Charge-discharge Analysis (GCD) 124 4.5 Conclusions 127 Acknowledgment128 References128 5 Supramolecular Membranes: Synthesis and Characterizations Cher Hon Lau, Matthew Hill and Kristina Konstas 5.1 Overview 5.2 Supramolecular Materials 5.2.1 Porous Materials 5.2.1.1  Metal-organic Frameworks 5.2.1.2  Zeolitic Imidazole Frameworks (ZIFs)

137 138 138 139 140 146

Contents  ix 5.2.2 Porous Organic Materials (POMs) 148 5.2.2.1  Covalent Organic Frameworks (COFs) 149 5.2.3 Cages 153 5.2.4 PAFs 154 5.3 Supramolecular Membranes 157 5.3.1 Concepts of Supramolecular Chemistry in Polymeric Membranes 157 5.3.1.1 Poly(dialkylacetylenes) 158 5.3.1.2 Polymers with Intrinsic Microporosity (PIMs) 163 5.3.2 Supramolecular Concepts in Nanocomposite Membranes165 5.3.2.1 Metal Organic Frameworks (MOFs) in Polymer Membranes 166 5.3.2.2 Porous Aromatic Frameworks (PAFs) in Super-Glassy Polymers 168 5.4 Membrane Fabrication Using Supramolecular Chemistry 170 5.4.1 Molecular Recognition – CO2 Affinity 172 5.4.2 Host-guest Chemistry 175 5.4.3 Self-assembled Membranes 178 5.4.4 Self-assembled Polymers as Membranes 179 5.4.5 Self-assembled Molecules and Nanoparticles as Membranes 181 5.5 Conclusions 184 References186 6 Organic Membranes and Polymers for the Removal of Pollutants Bernabé L. Rivas, Julio Sánchez and Manuel Palencia 6.1 Membranes: Fundamental Aspects 6.1.1 Membrane Transport Theory 6.1.2  Pressure-driven Membrane Methods 6.1.3 Hybrid Methods Applied for Removal of Pollutants 6.1.3.1 Membrane Bioreactor (MBR) 6.1.3.2 Electro-ultrafiltration 6.1.3.3 Ultrafiltration Coupled to Ultrasound 6.1.3.4 Flotation Coupled with Microfiltration 6.1.3.5 Liquid-phase Polymer-based Retention (LPR)

203 204 205 208 209 209 210 210 210 211

x  Contents 6.1.3.6 Surfactant Liquid Membrane Coupled with Liquid-phase Polymer-based Retention 211 6.1.4 Fouling 211 6.2 Liquid-phase Polymer-based Retention (LPR) 212 6.2.1  Theory and Fundamental Aspects 213 6.3 Applications for Removal of Specific Pollutants 216 6.3.1  Removal of Inorganic Species by LPR 217 6.3.1.1  Heavy Metals 218 6.3.1.2 Inorganic Anions 222 6.4 Future Perspectives 228 6.5 Conclusions 228 Acknowledgments228 References228 7 Membranes for CO2 Separation 237 Abedalkhader Alkhouzaam, Majeda Khraisheh, Mert Atilhan, Shaheen A. Al-Muhtaseb and Syed Javaid Zaidi 7.1 Introduction 238 7.2 Fundamentals of Membrane Gas Separation 239 7.2.1 Membrane Gas Permeation Mechanisms 240 7.2.2 Robeson’s Upper Bound 242 7.3 Polymeric Membranes for CO2 Separation 245 7.3.1 Polyimides 246 7.3.2 Polysulfones 251 7.3.3 Polymer Blends 253 7.4 Mixed Matrix Membranes 258 7.5 Supported Ionic Liquid Membranes (SILMs) for CO2 Separation263 7.5.1 SILM Permeation Properties for CO2/N2 268 and CO2/CH4 Separation 7.5.2 SILMs Stability in Gas Separation 276 7.6 Conclusion 278 7.7 Overall Comparison and Future Outlook 279 Abbreviations282 References285

Contents  xi 8 Polymer Nanomembranes 293 Giuseppe Firpo and Ugo Valbusa 8.1 Introduction 293 8.2 Materials 294 8.2.1 Rubber Polymers 294 8.2.2 Glassy Polymers 295 8.2.3 Mixed Matrix and Nanocomposite 297 8.3  Nanomembrane Fabrication 298 8.3.1 Spin Coating 298 8.3.2 Layer-by-Layer Assembly 300 8.3.3 Chemical Vapor Deposition 301 8.3.4 Other Techniques 302 8.3.5 Surface Treatments 303 8.4 Characterization 304 8.4.1 Gas Permeability and Selectivity 304 8.4.2 Mechanical Properties 307 8.4.3  Long-term Stability 309 8.5 Applications 310 8.5.1 Gas Purification 310 8.5.2 Water Desalination 312 8.5.3 Biomedical Devices 313 8.5.4 Sensors 315 References316 9 Liquid Membranes Jiangnan Shen, Lijing Zhu, Lixin Xue and Congjie Gao 9.1 Introduction 9.2  Most Recent Developments 9.3  Liquid Membranes Based Separation Processes 9.3.1  Emulsionized Liquid Membranes 9.3.2  Immobilized Liquid Membranes 9.3.3  Applications of Immobilized Liquid Membranes 9.3.4  Molten Salt Membranes 9.3.5  Hollow Fiber Liquid Membranes 9.3.6  Bulk Hybrid Liquid Membranes 9.3.6.1 Instruction 9.3.6.2 Theoretical Aspects of Bulk Hybrid Liquid Membranes 9.3.6.3 Pertraction in a Multi-membrane Hybrid System

329 329 330 330 330 345 348 349 349 354 354 354 356

xii  Contents 9.3.6.4 Applications 358 9.3.6.5 Summary 359 9.3.7  Bulk Aqueous Hybrid Liquid Membranes 360 9.3.7.1 Introduction 360 9.3.7.2 Theoretical Aspects of Bulk Aqueous Hybrid Liquid Membranes 361 9.3.7.3 Applications 363 9.3.7.4 Summary 364 9.3.8  Liquid Membranes in Gas Separation 364 9.3.8.1 Introduction 364 9.3.8.2  Separation Mechanism 365 9.3.8.3  Materials for LM 367 9.3.9  Common Gas Separation Applications 375 9.3.9.1 Carbon Dioxide Separation from Various Gas Streams 375 9.3.9.2 Sulfur Dioxide Separation from Various Gas Streams 376 9.3.9.3 Hydrogen Separation from Various Gas Streams 377 9.3.9.4  Olefin Separation 377 9.3.9.5  Conclusion and Outlook 378 9.4 Conclusion 379 References379 10 Recent Progress in Separation Technology Based on Ionic Liquid Membranes M.J. Salar-García, V.M. Ortiz-Martínez, A. Pérez de los Ríos and F.J. Hernández-Fernández 10.1 Introduction 10.2 Ionic Liquid Properties 10.3 Bulk Ionic Liquid Membranes 10.4 Emulsified Ionic Liquid Membranes 10.5 Immobilized Ionic Liquid Membranes   10.5.1 Supported Ionic Liquid Membranes   10.5.2 Polymer Ionic Liquid Inclusion Membranes   10.5.3 Polymeric Ionic Liquid Membranes   10.5.4 Membranes Based on Gelation of Ionic Liquids   10.5.5 Non-dispersive Solvent Extraction (NDSX) and Pseudo-emulsion Hollow Fiber Strip Dispersion (PEHFSD) Based on Ionic Liquids

391 392 393 395 397 400 401 404 406 407 408

Contents  xiii 10.6 Green Aspect of Ionic Liquids 410 10.7 Conclusions 411 Acknowledgments411 References412 11 Membrane Distillation 419 Mohammadali Baghbanzadeh, Christopher Q. Lan, Dipak Rana and Takeshi Matsuura 11.1 Introduction 419 11.2 Applications of Membrane Distillation Technology 420 11.3 Different Kinds of Membrane Distillation Configurations422   11.3.1 Direct Contact Membrane Distillation (DCMD) 422   11.3.2 Air Gap Membrane Distillation (AGMD) 423   11.3.2.1  Memstill and Aquastill 423   11.3.3 Permeate Gap Membrane Distillation (PGMD) 425   11.3.4 Sweep Gas Membrane Distillation (SGMD) 425   11.3.5 Vacuum Membrane Distillation (VMD) 426   11.3.5.1 Vacuum Gap Membrane Distillation (VGMD) 427  11.3.5.2 Memsys 428   11.3.5.3 Differences between VMD and Pervaporation (PV) 429 11.4 Distillation Membranes 432   11.4.1 MD Modules 432   11.4.1.1  Plate and Frame 432  11.4.1.2 Hollow Fiber 432  11.4.1.3 Tubular 433  11.4.1.4 Spiral Wound 433   11.4.2 Applicable Membranes for MD 434  11.4.2.1 Nanocomposite Membranes 434   11.4.3 Membrane Characteristics in MD 435   11.4.3.1 Liquid Entry Pressure (Wetting Pressure) 435  11.4.3.2 Membrane Thickness 436   11.4.3.3  Porosity and Tortuosity 436   11.4.3.4 Mean Pore Size and Pore Size Distribution437  11.4.3.5 Thermal Conductivity 437  11.4.3.6 Membrane Fabrication 438

xiv  Contents 11.5 Transport Phenomena in MD 439   11.5.1 Mass Transfer in MD 439   11.5.2 Heat Transfer in MD 443   11.5.2.1  Thermal Efficiency and Heat Loss 445   11.5.3 Temperature and Concentration Polarization 447   11.5.4 Fouling 448   11.5.5 Operating Parameters 449  11.5.5.1 Feed Temperature 449  11.5.5.2 Permeate Temperature 449  11.5.5.3 Feed Concentration 449   11.5.5.4  Feed Flow Rate 449   11.5.5.5  Air Gap Thickness 449  11.5.5.6 Membrane Properties 450 11.6 Conclusion 450 References450 12 Alginate-based Films and Membranes: Preparation, Characterization and Applications 457 Jiwei Li and Jinmei He 12.1 Introduction 457 12.2 Recent Development 459   12.2.1 Cross-linking 460   12.2.2 Plasticizing 462   12.2.3 Blending 463   12.2.4 Compositing 465   12.2.5 Drying 467 12.3 Applications 468   12.3.1 Pharmaceutical and Medical Applications 468   12.3.2 Packaging Applications 470   12.3.3 Environmental Applications 472 12.4 Conclusion 473 References474 Index491

Preface Many recent research accomplishments in the area of polymer nanocomposite membrane materials are summarized in this book, Nanostructured Polymer Membranes: Processing and Characterizations. State-of-the-art on membrane technology and chemistry and new challenges being faced in the field are discussed. Among the topics reviewed are characterization of membranes; current techniques for the processing and characterization of ceramic and inorganic polymer membranes; supramolecular membranes; organic membranes and polymers for removal of pollutants; membranes for CO2 separation; polymer nanomembranes; liquid membranes; separation technology based on ionic liquid membranes; membrane distillation; and alginate-based membranes and films. This book is intended to serve as a “one-stop” reference resource for important research accomplishments in the area of nanostructured polymer membranes and their processing and characterizations. It will be a very valuable reference source for university and college faculties, professionals, post-doctoral research fellows, senior graduate students, and researchers from R&D laboratories working in the area of polymer nanobased membranes. The various chapters are contributed by prominent researchers from industry, academia and government/private research laboratories across the globe and comprise an up-to-date record on the major findings and observations in the field. Chapter 1 provides an overview of the techniques and processes detailed in later chapters, along with the state of art, new challenges and opportunities in the field. In chapter 2, principles and fundamentals of membrane separation are presented in addition to a description of different types of membrane processes (pressure-driven membrane methods and liquid membranes) and the chemical and physical methods for membrane modification. Chapter 3 provides fundamental ideas about all types of characterization techniques for polymer-based nanomembranes such as FTIR, Raman spectroscopy, X-ray spectroscopy, electron spectroscopy, atomic force microscopy mass spectrometry and surface hydrophilicity. The next chapter mainly concentrates on the preparation, characterization xv

xvi  Preface and applications of ceramic and inorganic polymer membranes. Chapter 5 gives an overview of supramolecular membranes, primarily focusing on polymeric membranes and mixed matrix membranes for gas separation applications. Other topics discussed in this chapter are membranes synthesized from self-assembly, hydrogen bonding, π-π stacking of block copolymer systems, small molecules, and nanoparticles; and a summary of recent research into gas membrane separations. Chapter 6 explains organic membranes and polymers to remove pollutants. It provides fundamental aspects of membranes as well as processes, including membranes as electro-ultrafiltration, ultrafiltration coupled to ultrasound, flotation coupled to microfiltration, liquid-phase polymerbased retention and liquid surfactant membrane. The next chapter is essential for tracking the progress in membrane development. It is a comprehensive review of recent studies in CO2 separation using different technologies, CO2 permeation properties, and breakthroughs and challenges in developing efficient CO2 separation membranes. Chapter 8 reports the state-of-the-art on fabrication methods of polymeric nanomembranes according to their specific needs and illustrates the most useful materials, employing mostly glassy and rubbery polymers. Often, to enhance membrane properties or to prevent undesired behavior, the fabrication is followed by different kinds of surface treatments. The authors discuss recent investigations on mechanical, thermal and gas transport properties of nanomembranes that frequently reveal a different behavior with respect to the polymeric membranes of greater thickness. The next chapter presents an introduction to liquid membrane separation techniques such as emulsion liquid membranes, immobilized liquid membranes, salts liquid membranes, hollow fiber contained liquid membranes, bulk hybrid liquid membranes and bulk aqueous hybrid liquid membranes. The author of this chapter also discusses the theory behind liquid membranes, along with their material design, preparation, performance and stability, and their applications in the separation and removal of metal cations from a range of diverse matrices, gas separation, etc. Chapter 10 provides an overview of the recent progress in separation technology based on ionic liquid membranes; moreover, it covers issues relevant to this technology such as methods of preparation, mechanisms of transport, stability and fields of application. Chapter 11 on membrane distillation provides comprehensive coverage of both the fundamentals and recent developments associated with the application, process design, and membrane fabrication in this field. The final chapter provides a comprehensive overview of general properties, recent developments, and applications of alginate-based films and membranes. Sodium alginate is water-soluble,

Preface  xvii nontoxic, biocompatible, biodegradable, reproducible, and can yield coherent films or membranes upon casting or solvent evaporation. In conclusion, the editors would like to express their sincere gratitude to all the contributors of this book, whose excellent support made the successful completion of this venture possible. We are grateful to them for the commitment and sincerity they showed towards their contributions. Without their enthusiasm and support, the compilation of a book would not have been possible. We would like to thank all the reviewers who have taken their valuable time to make critical comments on each chapter. We also thank the publisher, John Wiley and Sons Ltd. and Scrivener Publishing, for recognizing the demand for a book on the increasingly important area of Nanostructured Polymer Membranes Processing and Characterization and handling such a new project, which many other publishers have yet to address. Visakh. P. M. Olga Nazarenko September 2016

1 Processing and Characterizations: State-of-the-Art and New Challenges Visakh. P. M. Research Assistant, Department of Ecology and Basic Safety, Tomsk Polytechnic University, Tomsk, Russia

Abstract

A brief account of various topics concerning the processing and characterization of nanostructured polymer membranes is presented in this chapter. The different topics that are discussed include membrane technology and chemical characterization of membranes; ceramic and inorganic polymer membranes preparation, characterization and applications; supramolecular membranes synthesis and characterizations; organic membranes and polymers to remove pollutants; membranes for CO2 separation; polymer nanomembranes; liquid membranes; recent progress in separation technology based on ionic liquid membranes; membrane distillation; and preparation, characterization and applications of alginate-based ­membranes and films. Keywords:  Nanostructured polymer membranes, membrane processing, ­membrane characterizations, supramolecular membranes, organic membranes, liquid membranes, separation technology, ionic liquid membranes

1.1 Membrane: Technology and Chemistry Membranes are used in a broad range of applications such as protein fractionation, purification of drugs, separation of gaseous mixtures, sample simplification in analytical procedures, production of ultrapure water and

Corresponding author: [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 1, (1–26) © 2017 Scrivener Publishing LLC

1

2  Nanostructured Polymer Membranes: Volume 1 wastewater treatment, among others [1–5]. The membrane can be defined as a selective barrier that allows some species to permeate the barrier while retaining others. Membrane can be symmetric or asymmetric membrane according to their macroscopic configuration. Thus, asymmetric membranes consist of two layers; the top one is a very thin dense layer and is commonly called the skin layer or active layer and determines the permeation properties. In particular, separation methods directed by pressure can be categorized into four major membrane processes: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) [6, 7]. Liquid membrane processes are commonly identified as three main configuration types: bulk liquid membrane, surfactant liquid membrane (or emulsion liquid membrane) and supported liquid membrane. Membranes can be referred to as porous and dense, and this first classification permits defining the two main mass transport models through the membranes. In membrane processes, the retained or rejected species accumulate near the membrane surface and as a consequence concentration polarization is produced. According to the transport mechanisms, the separation methods by liquid membrane can be divided into six basic mechanisms of transport: simple transport, simple transport with chemical reaction in strip solution, facilitated transport, coupled counter-transport, coupled co-transport and active transport. The range of materials used for nanofiltration and reverse osmosis membranes is much smaller than that used for microfiltration and ultrafiltration, and is limited to polymers. Membrane material is required to be resistant to operation conditions and suitable for specific application. In many cases, additives are added to membrane phase during the fabrication to increase the permeability or reduce the fouling. Inorganic membranes have high selectivity and high permeability as well as thermal, chemical and mechanical stability but the cost of these are very high in comparison with polymer membranes. Organic and inorganic membranes can be modified for different applications by changes in the material chemical properties or by changes of pore size [8]. The above can be accomplished using methods such as chemical oxidation, incorporation of additives into the membrane matrix, plasma treatment, classical organic reactions, polymer grafting, interpenetrating polymer network, surfactant modification, self-assembly of the nanoparticles, among others [9]. Plasma surface treatment usually refers to a plasma reaction that either results in modification of the molecular structure of the surface, or atomic substitution. For example, simple inert gas [10], nitrogen, or oxygen plasmas have been used to increase the surface hydrophilicity of membranes [11], and ammonia plasmas have successfully yielded functionalized

Processing and Characterizations  3 polysulfone membranes [12]. There are several potential advantages for the use of enzymes in membrane modification. Currently, the pressure-driven membrane processes are widely used in water treatment, biotechnology, food industry, medicine, and other fields [13]. One of the main problems arising from the operation of the membrane units is membrane fouling, which seriously hampers the applications of membrane technologies [14]. New membrane modification methods have been proposed, including the modification of membrane surfaces via  microswelling for fouling control in drinking water [15], hydrogel surface modification of reverse osmosis membranes [16], modification of Nafion membrane using fluorocarbon surfactant for all vanadium redox flow ­batteries [17], modification of ultrafiltration membranes via interpenetrating polymer networks for removal of boron from aqueous solution [18], among others.

1.2 Characterization of Membranes Membrane morphology characterization is one of the indispensable components of the field of membrane research. Physical and chemical properties of membranes can be characterized with different laboratory techniques. Several microscopic techniques, both electronic as scanning and transmission electron microscopies, and atomic, as atomic force microscopy, have been used to analyze the pore structure and pore size distribution of the membrane. Microscopy methods, including scanning electron microscopy (SEM), transmission electron microscopy (TEM) or atomic force microscopy (AFM), are the most direct methods to characterize the membrane pore structure. SEM can be used in various pore size characterization studies to visually inspect pore sizes and shapes. The AFM has proven itself to be a useful and versatile tool in the field of surface characterization. Porometry measurements can also give information about the pore size distribution (PSD) of membrane surface area [19]. Gas adsorption is one of the most popular methods and is generally used for the surface characterization and structural properties of porous materials, allowing the determination of their surface area, pore volume, pore size distribution and adsorption energy distribution of polymer membranes. One of the most promising methods is permporometry, where a mixture of non-condensable gas and condensable vapor is fed to a porous membrane and the permeation rate of non-condensable gas is measured [20]. Fourier transform infrared (FTIR) spectroscopy is widely used in structural characterization of membrane surfaces. With

4  Nanostructured Polymer Membranes: Volume 1 recent advances in the technology, the instrument has become simplified and some of the problems are reduced [21]. Raman spectroscopy technique usually employs a laser source and the scattered light and analyzes in terms of wavelength, intensity and polarization. Raman scattering is capable of detecting elastic vibrations of an entire nanoparticle, therefore Raman scattering is good for detecting nanoparticles on the membrane surface [22]. Energy-dispersive X-ray spectroscopy (EDS) analysis can be helpful for both membrane characterization and foulant characterization. For example, Sile-Yüksel et al. [23] used EDS analysis to determine the location of silver nanoparticles in different polymer membrane matrix. Corneal et al. [24] coated tubular ceramic membranes with manganase oxide nanoparticles. They examined the coating layer using SEM-EDS. With the help of EDS analysis they observed that the manganase oxide nanoparticles were not just successfully placed on the surface but also penetrated the membrane matrix. Soffer et al. [25] used EDS analysis to show colloidal iron fouling on ultrafiltration membrane surface. Long-term fouling of a reverse osmosis membrane was examined by Melián-Martel et al. [26]. The measuring method must be adapted according to charge places whether it is on the surface or inside the pores.

1.3 Ceramic and Inorganic Polymer Membranes: Preparation, Characterization and Applications Liquid electrolytes are liquid state electrolyte used to conduct the electricity. However, these conventional liquid electrolytes possess several dis­advantages such as leakages of corrosive solvent and harmful gas, electrolytic degradation of electrolyte, formation of lithium dendrite growth, poor dimensional and mechanical stabilities, slow evaporation due to the gel state of polymer electrolyte, low safety performances, narrow potential window, poor interfacial stability and reduction in thermal, electrical and electrochemical stabilities as well [27]. Ionic liquids also offer some fascinating advantages, such as excellent chemical, thermal and electrochemical stabilities, high ionic conductivity due to high ion concentration, good oxidative stability and superior ion mobility as well as high cohesive energy density [28]. Krawiec et al. found that the particle size of filler is a vital parameter to govern the conductivity of the polymer electrolytes. They reported that the conductivity of nanosized Al2O3 added polymer electrolytes was higher about an order of magnitude that that of micrometer-sized Al2O3. High

Processing and Characterizations  5 surface area to volume ratio of nanoparticles has become a driving force in the development of nanotechnology in various research fields, especially in materials science. The small particle size of the fillers can improve the homogeneity in the sample and its electrochemical properties [29]. The higher conductivity of nanoscale filler compared to micro-sized filler is also attributed to the rapid formation of the space charge region between the grains [30]. Mica plays a role in reducing resin costs, enhancing processability and dissipating heat in exothermic thermosetting reaction. Other particulate fillers, such as graphite, carbon black, and aluminium flakes, are used to reduce mold shrinkage or to minimize the electrostatic charging. Electrochemical devices, especially batteries, show a wide range of electrical and electronic applications. These devices can not only be applied in portable electronic and personal communication devices, such as laptops, mobile phones, MP3 players, and PDAs, but also in hybrid electrical vehicles (EVs) and start–light–ignition (SLI), which serves as a traction power source for electricity [31]. The properties of the final alumina depend on the crystalline structure, morphology and microstructure of the polymorph. Therefore, many attempts have been studied with respect to their transformation mechanisms, changes in porosity, specific surface area, surface structure, chemical reactivity and the defect crystal structure of polymorph [32].

1.4 Supramolecular Membranes: Synthesis and Characterizations Supramolecular chemistry has typically been focused within the inorganic field, with our understanding of porous silicas [33] leading to breakthroughs in electrochemical energy storage. New approaches have been designed and investigated to improve the membranes performance; this involves the incorporation of porous composite materials. Metal-organic frameworks (MOFs) are a class of supramolecular coordination polymers that have emerged in the literature over two decades ago, when they could be identified by single-crystal X-ray crystallo­graphy [34–38]. The MOF structures are obtained by a self-assembling process starting from metal ions that assemble together with linker molecules. MOFs are successfully synthesized from solvothermal reactions with metal and organic building blocks which are dissolved in organic solvents and heated up to 130 °C. In addition to the conventional heating used for solvothermal reactions, MOFs can be synthesized using electrochemistry,

6  Nanostructured Polymer Membranes: Volume 1 mechanochemistry and ultrasonic methods. Because MOFs can reversibly absorb carbon dioxide gas, they are promising materials for the selective capture of carbon from the atmosphere and flue gas. The large quadrupole moment of carbon dioxide molecules causes them to interact with the framework, increasing the uptake of the gas over other inert adsorbents such as zeolites. Polycrystalline thin films are made from direct synthesis where a bare substrate is used with the appropriate mother growth solution for the given MOF, heat treated as required for solvothermal synthesis. The method involves the metal and organic linker crossing a porous membrane and crystallizing at the interface [39]. Zeolites are widely used in industry for water purification, adsorbents, catalysts and gas separations. They are naturally found but can also be synthesized to incorporate a range of small inorganic and organic species. Supramolecular chemistry describes chemical systems comprising a number of assembled molecular subunits or components arranged in spatial organizations using noncovalent bonding like hydrogen bonding, metal coordination, and hydrophobicity. The first part of this chapter will focus on supramolecular chemistry concepts in polymeric membranes, followed by a short discussion on how metal coordination and host-guest chemistry play important roles in mixed-matrix membranes. Membranes fabricated via supramolecular chemistry are rarely reported for gas separations, and are more common for liquid separation or purification and filtration membranes. Polytrimethylsilylpropyne (PTMSP) membranes operate as sizeselective membranes. Meanwhile, when PTMSP membranes are used to isolate hydrocarbons from mixtures containing condensable hydro­carbon vapors and permanent gases, these membranes operate in the reverseselective mode [40]. Despite its unique property of high hydrocarbon/gas selectivity and permeability, PTMSP has apparently found no industrial applications. This is due in part to PTMSP being highly soluble in liquid hydrocarbons [41, 42]. Additive incorporation into polymer matrices remains one of the most common ways in which supramolecular chemistry is observed in membranes. For example, Merkel et al. reported that the incorporation of nonporous fumed silica nanoparticles into a PTMSP polymer matrix enhanced gas permeability [43]. Schmidt et al. used a bottom-up approach to form supramolecular nanofibers inside a scaffold to prepare stable polymermicrofiber/supramolecular-nanofiber composites for filter applications [44]. Upon solvent evaporation, and filtration over commercial microfiltration syringes, three-dimensional supramolecular networks were formed within cellulose acetate membranes that are suitable for inexpensive and fast water separations.

Processing and Characterizations  7

1.5 Organic Membranes and Polymers to Remove Pollutants A membrane is a thin planar structure or interphase that separates two phases and permits mass transfer between the phases. Membranes can be classified into two main groups: (1) biological membranes and (2) artificial or synthetic membranes. The polymer membranes are the main type of membranes in the market because polymeric materials are easier to process and less expensive [45, 46]. The separation of various components of a mixture is related directly to their relative transport rate within the membrane, which is determined by their diffusivity and solubility in the membrane material. Ultrafiltration membranes are used in electrodialysis pretreatment, electrophoretic paint, cheese whey treatment, juice clarification recovery of textile sizing agents, separation of oil/water emulsion, water treatment, and reverse ­osmosis ­pretreatment. The permeate is the portion of the fluid that has passed through the membrane and the retentate, or concentrate, is the portion containing the constituents that have been rejected by the membrane [47–49]. For a membrane separation method to be denominated as a hybrid separation method, changes beyond the simple incorporation of a different configuration or the simple change in a sequence in a separation line should be incorporated. The main disadvantage of flotation lies in the fact that the removal efficiency may be reduced if some of the undesired substances are not sufficiently hydrophobic, thus remaining in the bulk dispersion or solution [50]. Consequently, in the flotation coupled with microfiltration, the solid particles are partially removed by flotation, while clean water is obtained from the membrane module. Fewer solid particles remaining in the dispersion are then deposited on the membrane’s surface, resulting in decreased membrane fouling [51, 52]. Fouling has a direct impact on operating costs because a large part of the energy consumption is required to overcome fouling resistance and for periodic cleaning operations [53]. Geckeler et al. carried out the first experimental advances and analytical applications related with this technique [54–56]. Later, many research groups worked on the evaluation and description of retention properties of different water-soluble polymers (WSPs) for environmental and analytical applications [57–65]. One of the most promising techniques used is the application of separation methods based on the membrane process [66, 67]. Membrane filtration easily allows this separation by means of the method known as the LPR technique [68–75]. Among these methods, the membranes are

8  Nanostructured Polymer Membranes: Volume 1 the most promising for the enrichment of several ions from solution and their separation, especially where very low arsenic is required. The value of retention in the system with regenerated cellulose membrane is different than that reported in the literature where poly(ethersulfone) membrane was used as a filter [76]. At the present time, new research is being directed at improving the properties of the membranes, which are being modified to be an active component during the separation process. Thus, modification of the membrane is not only directed at permeability or antifouling properties.

1.6 Membranes for CO2 Separation Numerous methods have been used for the separation of CO2. These methods include adsorption with porous solids (e.g., activated carbon and zeolites), amine absorption cryogenic separation and membranebased separations [77, 78]. Adsorption technology is also being used for CO2 ­separation using different types of adsorbents. The low energy ­consumption and environment friendly nature was the main reason and focus of a large number of research studies on membrane technology [79]. Membranes (which generally consist of a semipermeable, thin, polymeric film) allow selective and specific permeation of some molecules while retaining ­others [80, 81]. Permeability and selectivity are the two main criteria that must be achieved in a good membrane. Membrane systems give reductions of over 70% in size and about 66% in weight compared to conventional separation columns [82]. The highpurity CO2 separation may require numerous membranes with different characteristics, due to their limited ability to achieve high degrees of gas separation [83]. Gas separation membranes use the differences in partial pressure as their driving force for separation [84]. One component dissolves into the membrane, diffusing through the membrane before passing to the other side in the final stage [85]. Most of the research studies in membrane gas separation have been carried out on nonporous (dense) polymeric membranes. These membranes play an important role in gas separation. Different types of polymers have been used to develop dense polymeric membranes for CO2 separation from different gases, including polyimides, polysulfones, cellulose, and polycarbonates. Polyamides are one of the most extensively investigated polymeric materials for membrane gas separation since they possess very high CO2 permeability, mainly those incorporating the group 2,2-bis(3,4dicarboxyphenyl) hexafluoropropane (6FDA). Polyimides have become

Processing and Characterizations  9 widely used membrane materials for gas and vapor separation due to their excellent thermal, chemical and mechanical stability in addition to their high gas separation. Polysulfone (PSF) is considered to be one of the most widely studied polymeric membrane material for CO2 separation for several gas streams [86, 87]. Gas permeation properties of PSF blends have been extensively investigated because of their low cost, chemical stability and mechanical strength [88]. Mixed-matrix membranes (MMMs), or composite membranes, are well-known in polymeric membranes development for gas separation. These membranes incorporate an inorganic material in the form of microor nanoparticles, hence combining the ease of polymer processing with the efficient gas permeation of a molecular sieve [89, 90]. Incorporating an inorganic material into a polymeric membrane can serve as a molecular sieve that enhances the gas permeance through the membrane or as a barrier that reduces the gas permeability [91]. Guo and coworkers prepared polysulfone-based mixed-matrix membranes (MMMs) incorporated with amine-functionalized titanium-based metal organic framework. One of the growing processes in the development of membrane science is the supported ionic liquid membranes (SILMs) technology. Due to their special properties, e.g., high thermal and chemical stability and low vapor pressure, ILs have become an ideal alternative to conventional organic solvents in a wide range of chemical ­applications at lab scale, such as separation and purification and chemical catalysis [92–97].

1.7 Polymer Nanomembranes To cite some of the most recent examples of polymeric nanostructured nanomembranes, researchers have found that they may act as molecular sieves [98] or humidity sensors in optical microcavity [99]; others have found large applicability in advanced biomedical applications [100]. Watanabe et al. found that polystyrene (PS) is not a satisfactory material to build robust nanomembrane with big size because it is not tough enough [101], thus suggesting that PS is not a suitable material for nanomembranes. A semiconducting polythiophene derivative like poly(3-thiophene methyl acetate) (P3TMA) has been blended with poly(tetramethylenesuccinate) (PE44) [102] or with thermoplastic polyurethane (TPU) [103] in order to fabricate robust biodegradable nanomembranes for tissue engineering. Poly(diallyldimethyl-ammonium chloride) (PDMADMAC) and poly(styrenesulfonate) (PSS) [104] have been used to control the gas

10  Nanostructured Polymer Membranes: Volume 1 permeability of polymeric nanomembranes. Thanks to its biocompatibility and biodegradability, poly(lactic acid) (PLA) in the form of nanosheets, has been proposed as a physical barrier against burn wound infection [105], for sealing operations in surgery [106], and for cell adhesion [46, 107]. Polysaccharide nanomembrane with a thickness of 75 nm is suitable for repairing a visceral pleural defect without any loss of respiratory functions of the lung [108], and the same polymer is used together with a poly(vinylacetate) (PVAc) to sandwich tetracycline antibiotics against ­bacterial infection, to form an antibiotic-loaded nanosheet [109]. Mixed-matrix membranes (MMMs) consist of a mixture of rubbery or glassy polymer with inorganic materials like zeolites [110], carbon molecular sieves [111], or nanoparticles [112]. Yin et al. [113] summarized the scientific and technological advances in developing nanocomposite membranes for water treatment, including ultrathin films. Recent studies have revealed that the incorporation of small amounts of a dense monolayer of planar graphene oxide in polyelectrolyte nanomembranes significantly enhances their mechanical properties [114]. The surface of the substrate has to be ideally flat to fabricate membranes with uniform thickness, in particular where it is necessary to transfer it from a soft support as in [115]. Freestanding nanoscale membranes with outstanding mechanical characteristics, made with polyelectrolyte multilayers (PEM) with a central interlayer containing gold nanoparticle, have been built combining spin coating with layer-by-layer (LbL) assembly [116]; graphene oxide layers have also been incorporated into the same previous multilayers fabricated in LbL assembly via Langmuir-Blodgett (LB) deposition. Evans et al. [117] synthesized conducting polymer nanocomposite film in a vacuum chamber oven following vapor phase polymerization (VPP) technique, and Fabretto et al. [118] investigated the influence of the VPP parameters on the dynamics of the polymerization process for use in large-scale electrochromic devices. Angelova et al. [119] presented a modular scheme to efficiently fabricate carbon nanomembranes utilizing a three-step procedure: deposition of self-assembled monolayer of polyaromatic molecules on solid substrate followed by electron irradiation to induce two-dimensional crosslinking, and transfer on the final support. We should point out that the depth of surface plasma modification ranges from a few microns down to a few nanometers [120] depending on plasma energy. Recently, great attention has been devoted to the performance of polymeric nanomembranes with micro- or nanoporous structure. The LbL technique allows the fabrication of conjugated microporous nanomembranes with tunable selectivity and permeability. Zhou et al. targeted their attention on developing a thin composite polymer membrane

Processing and Characterizations  11 for CO2 separation with high stability with respect to aging and plasticization [121]. In addition, reaction conditions, surface area, and surface morphology are parameters that can affect the membranes performance [122, 123]. The properties of the surfaces of thin film influence antifouling behavior, in particular their hydrophilicity, roughness, and electric charge density. Nanostructured nanomembrane of polyaniline (PANI) has proven its suitability for pH sensing, both doped with HCl alternated with poly(vinyl sulfonic acid) in a LbL assembly, and synthetized in the presence of water-soluble polyvinyl alcohol (PVA) [124, 125].

1.8 Liquid Membranes Liquid membrane processes are those involving a selective liquid membrane phase in which simultaneous extraction/stripping occurs. Separation is achieved by permeation of solute through this liquid phase from a feed phase to a receiving phase. the liquid membrane serves a dual purpose of permitting selective transfer of one or more components through it from external phase to internal droplets and vice versa, and preventing mixing of external and internal phases. Ever since ELM was invented by Li [126] in 1968, the use of this method for the hydrometallurgical recovery of heavy metals has drawn the attention of many investigators. Frankenfeld and Li [127], Martin and Davies [128] and Kitagawa et al. [129] were among the earliest investigators to report the extraction of metal ions. The capture and separation of CO2 using facilitated transport membranes has shown particular promise as a potential substitute for the existing process due to its high diffusivity coefficient in comparison with polymer membranes and the selective permeation of CO2 by solubility selectivity [130]. To create a supported liquid membrane, a liquid material capable of reversibly bonding with the gas intended for separation, or a material that can perform such a role, is dissolved and the solution is loaded into the pores of a porous support [131]. Hydrophobic microporous hollow fiber membranes with high porosity can be formed by a simple and convenient method of nonsolvent-induced phase inversion separation (NIPS) process, and membrane structure and morphology can also be optimized by adjusting various preparation conditions if a suitable solvent can be found for the selected polymer. Therefore, the thermal stability of the membrane material decides the membrane performance and the economy of the operation under high temperatures. For such applications, fluorinated polymers are good candidates due to their high hydrophobicity and chemical stability [132–134].

12  Nanostructured Polymer Membranes: Volume 1 Liquid membrane technology has great potential for the removal of heavy metals from aqueous dilute solutions. Liquid membrane is a layer of an organic solvent separating two aqueous solutions. The liquid membranes can be classified into different types: bulk, emulsion, supported, and hollow fiber liquid membranes. Bulk liquid membranes consist of an aqueous feed and stripping phases, separated by a water-immiscible liquid membrane. They demonstrate stable transport properties. A strongly basic membrane can exchange anions or react with un-dissociated acid molecules [135], whereas a strongly acidic membrane can lower the dissociation of acids and enhance the sorption (extraction) of the weaker one [136]. Liquid membranes provide greater selectivity and permeability than the solid ion-exchange membranes. Data on electrodialysis of liquid membranes are rather low in comparison with membrane extraction. One of the first works in this field was conducted by Purin [137], who used electro­ dialysis through bulk liquid membranes to concentrate rhenium from industrial solutions. A liquid membrane is a layer of an organic solvent separating two aqueous solutions. Compounds promoting the transport of substances from one aqueous solution to another may be dissolved in the organic phase. Based on this point, liquid membrane is gradually applied in gas separations, which has been intensively studied in the separation process. Generally, liquid membranes with and without supports can be differentiated. The process of liquid membrane in gas separation is mainly explained by the dissolve-diffusion mass transfer mechanism [138].

1.9 Recent Progress in Separation Technology Based on Ionic Liquid Membranes Liquid membrane processes are based on membrane separation and ­liquid-liquid extraction (LLX) in a single step. The main principle of extraction processes lies in the use of a fixed or mobile reagent solution phase, which is often immiscible with water, placed between liquid or gaseous feeding and stripping phases. These are only some of the unique properties that make ionic liquids an alternative to organic solvents in many chemical separation and purification processes [139, 140]. The selection of adequate chemical structures and the high viscosity typical of ionic liquids allow the solubility of membranes to be reduced, avoiding losses of liquid and contributing to their stability. The application of liquid membranes has been widely investigated in separation processes, mainly for the separation of organic compound and metal ions. Heavy

Processing and Characterizations  13 metals, such as chromium (Cr), have also been extracted from wastewater by using a double ionic liquids-based ELM. 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [bmim+][NTf2–], was used to stabilize the membrane phase and tri-n-octylmethylammonium chloride was used as carrier. They achieved an extraction yield of chromium of approximately 97% and confirmed the capacity of [bmim+][NTf2–] to stabilize the membrane [141]. Emulsion liquid membrane is a promising technology for many separation processes, especially for heavy metals removal, wastewater treatment and separation of gases. However, the success of its application depends on several key factors, such as the adequate selection of emulsification method and emulsion formulation, factors that largely determine the stability of these membranes. The immobilization of ILs can increase efficiency, facilitate recycling and widen the range of applications of these compounds with unique chemical and physical properties. Ionic liquid can be immobilized as liquid phase in membrane materials for their application in separation processes. Another important application of these types of membranes is the separation of biobutanol, which has great potential as biofuel. This is produced during the fermentation of alcohols and ketones from biomass. Much progress has been made to improve the long-term stability of immobilized ionic liquid membranes. The most important application is the use of novel supporting materials [142] or the gelation of ionic liquids [143]. In the work which focused on gelled supported ionic liquid membranes, the membrane obtained had good mechanical stability and maintained similar gas transport properties to the original ionic liquid. This work demonstrated that gelation is a simple and promising method to control the mechanical properties of ionic liquids-based membranes without sacrificing separation performance.

1.10 Membrane Distillation Membrane distillation has potential for use in water desalination, feed solution degassing, treatment of industrial effluents, purification of pharmaceuticals, processing of foods, and removal of organic compounds, heavy metals from aqueous solutions [144–148] and radioactive wastes [149], and concentrating diluted nonvolatile acids such as sulfuric acid and phosphoric acid [150]. Direct contact membrane distillation is the simplest membrane distillation (MD) configuration. The membrane is in direct contact with the liquid phase and has the ability of producing a high flux. Hot feed is in direct contact with the hot side of the membrane surface and

14  Nanostructured Polymer Membranes: Volume 1 vapor molecules pass through the membrane toward the permeate side and condensation takes place inside the module. In this arrangement, pressure is maintained below the equilibrium vapor pressure to improve mass transfer. Vacuum membrane distillation (VMD) is beneficial for removing volatiles from an aqueous solution [151, 152]. Vacuum multi-effect membrane distillation (VMEMD) combines the advantages of multi-effect and vacuum concepts to make a multistage setup integrated into a compact plate and frame module. This configuration has been successfully commercialized [153]. Hollow fiber membranes have larger specific surface area than flat sheet ones, but they typically have low flux due to their poor flow dynamics and high degree of temperature polarization [154, 155]. The use of nanomaterials in developing the MD membranes is considered to be a new concept and only a few studies have been reported in the literature in that direction. New materials such as carbon nanotubes and fluorinated copolymers have been recently developed for membrane fabrication [156]. For membrane fabrication, different methods such as sintering, stretching, and phase inversion are mostly used. Sintering is used to prepare polytetrafluoroethylene (PTFE) membranes.

1.11  A  lginate-based Films and Membranes: Preparation, Characterization and Applications Since sodium alginate membranes are hydrophilic and soluble matrices, the crosslinking process with polyvalent cations has been used to improve their water barrier properties, mechanical resistance, cohesiveness and rigidity [157, 158]. It is well known that the formation of biopolymer films requires the addition of plasticizers to overcome their brittleness and improve their processing behavior [159–161]. For sodium alginate membranes, the addition of plasticizer enhances flexibility, decreases brittleness, as well as avoids shrinking during handling and storage [162–164]. Compared with pure alginate or alginic acid membranes, the mechanical properties of the blend films were significantly improved by introducing cellulose or regenerated cellulose. Bacterial cellulose (BC), a natural biopolymer synthesized in abundance by different strains of bacteria, displays high water content, high wet strength and chemical purity [165, 166]. Sodium alginate membrane is a polyelectrolyte with negative charges on it. Chitosan is the N-deacetylated derivative of chitin, a cationic polysaccharide composed of d-glucosamine and N-acetyl d-glucosamine residues with 1,4-linkages [167, 168]. The self-adhered (SA)/PVA-based IPN membranes extended drug release up to 24 h, while SA and PVA

Processing and Characterizations  15 membranes discharged the drug quickly. Polyethylene glycol (PEG) is a biocompatible polymer with excellent biocompatibility and nontoxicity. Immiscible SA/PEG blend films with enhanced thermal stability have been developed [169]. Several SA-based composite films have been developed by adding reinforcements (fillers) of cellulose origin to enhance their performance and applicability [170–172]. Recently, Thu et al. developed an alginate-based bilayer hydrocolloid slow-release wound dressing film composed of an upper layer impregnated with a model drug (­ibuprofen) and a drug-free lower layer that acts as a rate-controlling membrane [173]. Sodium alginate membrane-based films can be proposed as a material for drug delivery system because of their ability to enhance the efficacy and paracellular transport, as well as prolong the release time of drugs [174–177].

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20  Nanostructured Polymer Membranes: Volume 1 75. J. Sánchez, L. Toledo, B.L. Rivas, N. Rivera, E. Muñoz, Water-soluble cationic cellulose coupled to a ultrafiltration membrane for the removal of arsenic and chromium. J. Chil. Chem. Soc., 58(4), 1842–1846, 2013. 76. B.L. Rivas, M. Aguirre, E. Pereira, Cationic water-soluble polymers with the ability to remove arsenate through a ultrafiltration technique. J. Appl. Polym. Sci., 106, 89–94, 2007. 77. E. Drioli, M. Romano, Progress and new perspectives on integrated membrane operations for sustainable industrial growth. Ind. Eng. Chem. Res., 40(5), 1277–1300, 2001. 78. S. Wong, R. Bioletti, Carbon Dioxide Separation Technologies, Carbon & Energy Management Alberta Research Council, 2002. 79. R.W. Baker, Reviews – Future directions of membrane gas separation technology. Ind. Eng. Chem. Res., 41(6), 1393–1411, 2002. 80. P. Bernardo, E. Drioli, G. Golemme, Membrane gas separation: A review/ state of the art. Ind. Eng. Chem. Res., 48, 4638–4663, 2009. 81. S. Kentish, Investigating viable CO2 capture and separation options. Presented at Towards Zero Emissions, National Conference, Brisbane, Queensland/ Australia, 2003. 82. O. Falk-Pedersen, H. Dannström, Separation of CO2 from offshore gas turbine exhaust. Energy Convers. Manage., 38, 81–86, 1997. 83. J. Davison, P. Freund, A. Smith, Putting carbon back into the ground. IEA Greenhouse Gas R&D Programme Report: Cheltenham, Gloucestershire. pp. 1–28, 2001. 84. J.G. Wijmans, R.W. Baker, The solution-diffusion model: A review. J. Membr. Sci., 107(1–2), 1–21, 1995. 85. M.D. Jensen, et al., Carbon separation and capture. Plains CO2 Reduction (PCOR) Partnership Report: North Dakota, pp. 1–27, 2005. 86. C.E. Powell, G.G. Ciao, Polymeric CO2/N2 gas separation membranes for the capture of carbon dioxide from power plant flue gases. J. Membr. Sci., 279(1–2), 1–49, 2006. 87. H. Julian, I.G. Wenten, Polysulfone membranes for CO2/CH4 separation: State of the art. IOSR J. Eng., 2(3), 484–495, 2012. 88. M. Mulder, Basic Principles of Membrane Technology, 2nd ed., 564 pp. Dordrecht; Boston: Kluwer Academic, 1996. 89. H.B. Tanh Jeazet, C. Staudt, C. Janiak, Metal-organic frameworks in mixedmatrix membranes for gas separation. Dalton Trans., 41(46), 14003–14027, 2012. 90. D. Shekhawat, D.R. Luebke, H.W. Pennline, A Review of Carbon Dioxide Selective Membranes: A Topical Report, 2003. 91. C.A. Scholes, S.E. Kentish, G.W. Stevens, Carbon dioxide separation through polymeric membrane systems for flue gas applications. Recent Patents on Chemical Engineering, 1(2), 52, 2008. 92. F. Endres, D. MacFarlane, A. Abbott, Electrodeposition from Ionic Liquids, p. 1, Wiley-VCH: Weinheim, 2008.

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22  Nanostructured Polymer Membranes: Volume 1 106. Y. Okamura, K. Kabata, M. Kinoshita, D. Saitoh, S. Takeoka, Free-standing biodegradable poly(lactic acid) nanosheet for sealing operations in surgery. Adv. Mater., 21, 4388–4392, 2009. 107. T. Fujie, L. Ricotti, A. Desii, A. Menciassi, P. Dario, V. Mattoli, Evaluation of substrata effect on cell adhesion properties using freestanding poly(l-lactic acid) nanosheets. Langmuir, 27, 13173–13182, 2011. 108. T. Fujie, N. Matsutani, M. Kinoshita, Y. Okamura, A. Saito, S. Takeoka, Adhesive, flexible, and robust polysaccharide nanosheets integrated for tissue-defect repair. Adv. Funct. Mater., 19, 2560–2568, 2009. 109. T. Fujie, A. Saito, M. Kinoshita, H. Miyazaki, S. Ohtsubo, D. Saitoh, S. Takeoka, Dual therapeutic action of antibiotic-loaded nanosheets for the treatment of gastrointestinal tissue defects. Biomaterials, 31, 6269–6278, 2010. 110. D. Bastani, N. Esmaeili, M. Asadollahi, Polymeric mixed matrix membranes containing zeolites as a filler for gas separation applications: A review. J. Ind. Eng. Chem., 19(2), 375–393, 2013. 111. D.Q. Vu, W.J. Koros, S.J. Miller, Mixed matrix membranes using carbon molecular sieves: I. Preparation and experimental results. J. Membr. Sci., 211(2), 311–334, 2003. 112. J. Ahn, W.J. Chung, I. Pinnau, M.D. Guiver, Polysulfone/silica nanoparticle mixed-matrix membranes for gas separation. J. Membr. Sci., 314(1–2), 123– 133, 2008. 113. J. Yin, B. Deng, Polymer-matrix nanocomposite membranes for water treatment. J. Membr. Sci., 479, 256–275, 2015. 114. D.D. Kulkarni, I. Choi, S.S. Singamaneni, V.V. Tsukruk, Graphene oxide– polyelectrolyte nanomembrane. ACS Nano, 4(8), 4667–4676, 2010. 115. G. Firpo, E. Angeli, L. Repetto, U. Valbusa, Permeability thickness dependence of polydimethylsiloxane (PDMS) membranes. J. Membr. Sci., 481, 1–8, 2015. 116. C. Jiang, S. Markutsya, V.V. Tsukruk, Compliant, robust, and truly nanoscale free-standing multilayer films fabricated using spin-assisted layer-by-layer assembly. Adv. Mater., 16(2), 157, 2004. 117. D. Evans, M. Fabretto, M. Mueller, K. Zuber, R. Short, P. Murphy, Structuredirected growth of high conductivity PEDOT from liquid-like oxidant layers during vacuum vapor phase polymerization. J. Mater. Chem., 22, 14889–14895, 2012. 118. M. Fabretto, J.P. Autere, D. Hoglinger, S. Field, P. Murphy, Vacuum vapour phase polymerised poly(3,4-ethyelendioxythiophene) thin films for use in large-scale electrochromic devices. Thin Solid Films, 519, 2544–2549, 2011 119. P. Angelova, H. Vieker, N.E. Weber, D. Matei, O. Reimer, I. Meier, S. Kurasch, J. Biskupek, D. Lorbach, K. Wunderlich, L. Chen, A. Terfort, M. Klapper, K. Müllen, U. Kaiser, A. Gölzhäuser, A. Turchanin, A universal scheme to convert aromatic molecular monolayers into functional carbon nanomembranes. ACS Nano, 7(8), 6489–6497, 2013. 120. K.S. Houston, D. Weinkauf, F. Stewart, Gas transport characteristics of plasma treated poly(dimethylsiloxane) and polyphosphazene membrane materials. J. Membr. Sci., 205(1–2), 103–112, 2002.

Processing and Characterizations  23 121. J. Zhou, M.M. Tran, A.T. Haldeman, J. Jin, E.H. Wagener, S.M. Husson, Perfluorocyclobutyl polymer thin-film composite membranes for CO2 separations. J. Membr. Sci., 450, 478–486, 2014. 122. A.K. Ghosh, B.H. Jeong, X. Huang, E.M.V. Hoek, Impacts of reaction and curing conditions on polyamide composite reverse osmosis membrane properties. J. Membr. Sci., 311, 34–45, 2008. 123. Y. Jin, Z. Su, Effects of polymerization conditions on hydrophilic groups in aromatic polyamide thin films. J. Membr. Sci., 330, 175–179, 2009. 124. N.C.S. Vieira, E.G.R. Fernandes, A.D. Faceto, V. Zucolotto, F.E.G. Guimarães, Nanostructured polyaniline thin films as pH sensing membranes in FETbased devices. Sensor. Actuat. B, 160, 312- 317, 2011. 125. I. Mihai, F. Addiego, D. Ruch, V. Ball, Composite and free standing PANIPVA membranes as flexible and stable optical pH sensors. Sensor. Actuat. B, 192, 769–775, 2014. 126. N.N. Li, Separating hydrocarbons with liquid membranes, US Patent 3410794, assigned to Exxon Research Engineering Co, 1968. 127. R.A. Kumbasar, Selective extraction of nickel from ammoniacal solutions containing nickel and cobalt by emulsion liquid membrane using 5,7-dibromo-8-hydroxyquinoline (DBHQ) as extractant. Miner. Eng., 22, 530–536, 2009. 128. S. Praipruke, K. Kriausakul, S. antayanon, Recovery of palladium (ii) from acidic chloride solution by emulsion liquid membranes. Presented at 13th Asian Chemical Congress, Session: Organic Chemistry and Green Chemistry, 2009. 129. A.L. Ahmad, A. Kusumastuti, C.J.C. Derek, B.S. Ooi, Emulsion liquid membrane for heavy metal removal: An overiew on emulsion stabilization and destabilization. Chem. Eng. J., 171, 870–882, 2011. 130. F.F. Krull, C. Fritzmann, T. Melin, Liquid membranes for gas/vapor separations. J. Membr. Sci., 325, 509–519, 2008. 131. J.D. Way, R.D. Noble, Facilitated transport, in: Membrane Handbook, W.S. Ho, K.K. Sirka (Eds.), pp. 833–866, Chapman and Hall, New York, 1992. 132. M. Khayet, C.Y. Feng, K.C. Khulbe, T. Matsuura, Study on the effect of a nonsolvent additive on the morphology and performance of ultrafiltration hollow fiber membranes. Desalination, 148, 321–327, 2002. 133. M.L. Yeow, Y.T. Liu, K. Li, Morphological studies of poly(vinylidene fluoride) asymmetric membranes: Effect of the solvent, additive and the dope temperature. J. Appl. Polym. Sci., 92, 1782–1789, 2004. 134. J.H. Kim, B.J. Chang, S.B. Lee, S.Y. Kim, Incorporation effect of fluorinated side groups into polyimide membranes on their pervaporation properties. J. Membr. Sci., 169, 185–196, 2000. 135. R. Wódzki, J. Nowaczyk, Membrane transport of organics. II. Permeation of some carboxylic acids through strongly basic polymer membrane. J. Appl. Polym. Sci., 71, 2179–2190, 1999. 136. R. Wódzki, J. Nowaczyk, Membrane transport of organics. I. Sorption and permeation of carboxylic acids in perfluorosulfonic and perfluorocarboxylic polymer membranes. J. Appl. Polym. Sci., 63, 355–362, 1997.

24  Nanostructured Polymer Membranes: Volume 1 137. B.A. Purin, Electrochemical extraction as the method of the purification of metals using liquid membranes. Izv. Akad. Nauk Latv. SSR [in Russian], 5, 31–368, 1971. 138. P. Li, Z. Wang, Z. Qiao, Y. Liu, X. Cao, W. Li, J. Wang, S. Wang, Recent developments in membranes for efficient hydrogen purification. J. Membr. Sci., 495, 130–168, 2015. 139. L.J. Lozano, C. Godínez, A.P. de los Ríos, F.J. Hernández-Fernández, S. Sánchez-Segado, F.J. Alguacil, Recent advances in supported ionic liquid membrane technology. J. Membr. Sci., 376, 1–14, 2011. 140. J.F. Brennecke, E.J. Maginn, Ionic liquids: Innovative fluids for chemical processing. AIChE J., 47, 2384–2389, 2001. 141. R.K. Goyal, N.S. Jayakumar, M.A. Hashim, Chromium removal by emulsion liquid membrane using [BMIM]+[NTf2]− as stabilizer and TOMAC as extractant. Desalination, 278, 50–56, 2011. 142. A.J.B. Kemperman, H.H.M. Rolevink, D. Bargeman, T. van den Boomgaard, H. Strathmann, Stabilization of supported liquid membranes by interfacial polymerization top layers. J. Membr. Sci., 138, 43–55, 1998. 143. A.M. Neplenbroek, D. Bargeman, C.A. Smolders, Supported liquid membranes: Stabilization by gelation. J. Membr. Sci., 67, 149–165, 1992. 144. A. Alkhudhiri, N. Darwish, N. Hilal, Membrane distillation: A comprehensive review. Desalination, 287, 2–18, 2012. 145. J. Zuo, S. Bonyadi, T.S. Chung, Exploring the potential of commercial polyethylene membranes for desalination by membrane distillation. J. Membr. Sci., 497, 239–247, 2016. 146. S. Goh, J. Zhang, Y. Liu, A.G. Fane. Membrane distillation bioreactor (MDBR) – A lower green-house-gas (GHG) option for industrial wastewater reclamation. Chemosphere, 140, 129–142, 2015. 147. K.C. Wijekoon, F.I. Hai, J. Kang, W.E. Price, W. Guo, H.H. Ngo, et al., A novel membrane distillation-thermophilic bioreactor system: Biological stability and trace organic compound removal. Bioresour. Technol., 159, 334–341, 2014. 148. K. Gethard, S. Mitra, Carbon nanotube enhanced membrane distillation for online preconcentration of trace pharmaceuticals in polar solvents. Analyst, 136, 2643–2648, 2011. 149. G. Zakrzewska-Trznadel, M. Harasimowicz, A.G. Chmielewski, Concentration of radioactive components in liquid low-level radioactive waste by membrane distillation. J. Membr. Sci., 163, 257–264, 1999. 150. U.K. Kesieme, H. Aral, M. Duke, N. Milne, C.Y. Cheng, Recovery of sulphuric acid from waste and process solutions using solvent extraction. Hydrometallurgy, 138, 14–20, 2013. 151. S. Bandini, C. Gostoli, G.C. Sarti, Separation efficiency in vacuum membrane distillation. J. Membr. Sci., 73, 217–229, 1992. 152. G.C. Sarti, C. Gostoli, S. Bandini, Extraction of organic components from aqueous streams by vacuum membrane distillation. J. Membr. Sci., 80, 21–33, 1993.

Processing and Characterizations  25 153. K. Zhao, W. Heinzl, M. Wenzel, S. Büttner, F. Bollen, G. Lange, et al., Experimental study of the memsys vacuum-multi-effect-membranedistillation (V-MEMD) module. Desalination, 323, 150–160, 2013. 154. S. Bonyadi, T.S. Chung, Highly porous and macrovoid-free PVDF hollow fiber membranes for membrane distillation by a solvent-dope solution coextrusion approach. J. Membr. Sci., 331, 66–74, 2009. 155. M. Baghbanzadeh, D. Rana, C.Q. Lan, T. Matsuura, Effects of inorganic nano-additives on properties and performance of polymeric membranes in water treatment. Sep. Purif. Rev., 45, 141–167, 2016. 156. L.M. Camacho, L. Dumee, J. Zhang, J. Li, M. Duke, J. Gomez, et al., Advances in membrane distillation for water desalination and purification applications. Water, 5, 94–196, 2013. 157. A.C. Bierhalz, M.A. da Silva, M.E. Braga, H.J. Sousa, T.G. Kieckbusch, Effect of calcium and/or barium cross-linking on the physical and antimicrobial properties of natamycin-loaded alginate films. LWT-Food Sci. Technol. Int., 57, 494–501, 2014. 158. M.A. da Silva, A.C.K. Bierhalz, T.G. Kieckbusch, Alginate and pectin composite films crosslinked with Ca 2+ ions: Effect of the plasticizer concentration. Carbohydr. Polym., 77, 736–742, 2009. 159. M.G.A. Vieira, M.A. da Silva, L.O. dos Santos, M.M. Beppu, Natural-based plasticizers and biopolymer films: A review. Eur. Polym. J., 47, 254–263, 2011. 160. V. Jost, K. Kobsik, M. Schmid, K. Noller, Influence of plasticiser on the barrier, mechanical and grease resistance properties of alginate cast films. Carbohydr. Polym., 110, 309–319, 2014. 161. R.P.H. Brandelero, F. Yamashita, J. Zanela, E.M. Brandelero, J.G. Caetano, Mixture design applied to evaluating the effects of polyvinyl alcohol (PVOH) and alginate on the properties of starch‐based films. Starch‐Stärke, 67, 191–199, 2015. 162. R. Russo, M. Abbate, M. Malinconico, G. Santagata, Effect of polyglycerol and the crosslinking on the physical properties of a blend alginate-hydroxyethylcellulose. Carbohydr. Polym., 82, 1061–1067, 2010. 163. T. Pongjanyakul, A. Priprem, S. Puttipipatkhachorn, Investigation of novel alginate-magnesium aluminum silicate microcomposite films for modifiedrelease tablets. J. Control. Release, 107, 343–356, 2005. 164. Q. Tong, Q. Xiao, L.T. Lim, Effects of glycerol, sorbitol, xylitol and fructose plasticisers on mechanical and moisture barrier properties of pullulan– alginate–carboxymethylcellulose blend films. Int. J. Food Sci. Technol., 48, 870 878, 2013. 165. L. Zhou, D. Sun, L. Hu, Y. Li, J. Yang, Effect of addition of sodium alginate on bacterial cellulose production by Acetobacter xylinum. J. Ind. Microbiol. Biotechnol., 34, 483–489, 2007. 166. A. Svensson, E. Nicklasson, T. Harrah, B. Panilaitis, D. Kaplan, M. Brittberg, P. Gatenholm, Bacterial cellulose as a potential scaffold for tissue engineering of cartilage. Biomaterials, 26, 419–431, 2005.

26  Nanostructured Polymer Membranes: Volume 1 167. W. Liu, W. Liu, A. Ye, S. Peng, F. Wei, C. Liu, J. Han, Environmental stress stability of microencapsules based on liposomes decorated with chitosan and sodium alginate. Food Chem., 196, 396–404, 2016. 168. D. Algul, H. Sipahi, A. Aydin, F. Kelleci, S. Ozdatli, F.G. Yener, Biocompatibility of biomimetic multilayered alginate–chitosan/β-TCP scaffold for osteochondral tissue. Int. J. Biol. Macromol., 79, 363–369, 2015. 169. T.M. Swamy, B. Ramaraj, Siddaramaiah, Sodium alginate and poly(ethylene glycol) blends: Thermal and morphological behaviors. J. Macromol. Sci. Part A Pure Appl. Chem., 47, 877–881, 2010. 170. J.A. Sirviö, A. Kolehmainen, H. Liimatainen, J. Niinimäki, O.E. Hormi, Biocomposite cellulose-alginate films: Promising packaging materials. Food Chem., 151, 343–351, 2014. 171. H.M. Azeredo, K.W. Miranda, M.F. Rosa, D.M. Nascimento, M.R. de Moura, Edible films from alginate-acerola puree reinforced with cellulose whiskers. LWT-Food Sci. Technol. Int., 46, 294–297, 2012. 172. Q. Chen, U.P. de Larraya, N. Garmendia, M. Lasheras-Zubiate, L. CorderoArias, S. Virtanen, A.R. Boccaccini, Electrophoretic deposition of cellulose nanocrystals (CNs) and CNs/alginate nanocomposite coatings and free standing membranes. Colloids Surf., B, 118, 41–48, 2014. 173. H.E. Thu, M.H. Zulfakar, S.-F. Ng, Alginate based bilayer hydrocolloid films as potential slow-release modern wound dressing. Int. J. Pharm., 434, 375–383, 2012. 174. N. Khuathan, T. Pongjanyakul, Modification of quaternary polymethacrylate films using sodium alginate: Film characterization and drug permeability. Int. J. Pharm., 460, 63–72, 2014. 175. S.G. Reddy, A.S. Pandit, Controlled drug delivery studies of biological macromolecules: Sodium alginate and lignosulphonic acid films. J. Appl. Polym. Sci., 131, 2014. 176. W.W. Hu, S.L. Tsou, The effect of alginate on DNA delivery from layer-bylayer assembled films. Carbohydr. Polym., 101, 240–248, 2014. 177. T. Pongjanyakul, H. Suksri, Alginate-magnesium aluminum silicate films for buccal delivery of nicotine. Colloids Surf., B, 74, 103–113, 2009.

2 Membrane Technology and Chemistry Manuel Palencia1*, Alexander Córdoba1 and Myleidi Vera2 Department of Chemistry, Faculty of Natural and Exact Sciences, University of Valle, Cali, Colombia 2 Polymer Department, Faculty of Chemistry, University of Concepción, Concepción, Chile 1

Abstract

The science and technology of membranes is a current topic with many projections for industrial applications (drinking water and wastewater treatment, ­separation and purification of pharmaceutical substances, concentration of proteins and other solutes). In this chapter, the principles and fundamentals of membrane separation are shown, including basic parameters, membrane transport mechanisms (size exclusion model and solution-diffusion model), and fouling ­mechanisms (concentration polarization, cake filtration, adsorption of solutes and blocking models). In addition, different types of membrane processes (pressuredriven membrane methods and liquid membranes) and chemical and physical methods for membrane modification are described. In addition to describing membrane-based methods, this chapter can serve as an introduction to the fundamental aspects of membrane theory and an illustration of the state of the art of membrane theory. It is clear that fouling is the main problem for the application of this technology and that the efforts of researchers are being directed towards membrane modification. Keywords:  Membrane, transport mechanism, fouling, surface modification

2.1 Introduction Nowadays, membranes have an important place in the industrial field because membrane separation processes are simple and, consequently, *Corresponding author: [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 1, (27–54) © 2017 Scrivener Publishing LLC

27

28  Nanostructured Polymer Membranes: Volume 1 their industrial scaling is relatively easy; these processes do not require additives, and can be performed isothermally at low temperatures and at low energy consumption in comparison with other thermal separation processes. Membranes are used in a broad range of applications (e.g., protein fractionation, purification of drugs, separation of gaseous mixtures, sample simplification in analytical procedures, production of ultrapure water and wastewater treatment, among others) [1–5]. It is clear that the first term that is necessary to understand is the membrane concept. In relation to that, though different membrane definitions can be identified in the specialized bibliography, these include the same basic aspects related to the selective mass transfer through a material medium, which is the membrane. Thus, a membrane can be defined as a selective barrier that allows some species to permeate the barrier while retaining others [1] or as an interphase between two adjacent phases acting as a selective barrier, regulating the transport of substances between the two phases [2]. Also, according to the International Union of Pure and Applied Chemistry (IUPAC), a membrane is a structure with lateral dimensions much greater than its thickness through which mass transfer may occur under a variety of driving forces [3]. The difference of this last definition with the previous concepts is the consideration of geometrical aspects. Note that in all of the previous concepts the nature of membrane materials and separation substances are defined. Finally, it is important to note that the objective of this chapter is to provide a general introduction to membrane science and technology by describing topics fundamental to all membrane processes, such as transport mechanisms, separation mechanisms, chemical and physical properties, as well as surface treatments which are actually used to improve their properties.

2.2 Membrane Technology: Fundamental Concepts Previously, the concept of a membrane was defined and understood to be a membrane having a discrete structure that acts as a thin interface and moderates the permeation of chemical species in contact with it; however, terminology related to membrane technology must be clarified in order to ease the understanding of developed topics. Membranes can be classified into two main groups: (1) biological membranes (e.g., lipid bilayer systems as cell and nuclear membranes, physiological tissues as peritoneum, pericardium or the meninges that surround the brain) and (2) artificial or synthetic membranes which are fabricated

Membrane Technology and Chemistry  29 from organic and inorganic materials by different technological processes (e.g., membranes of reverse osmosis, microfiltration, ultrafiltration, pervaporation, dialysis, emulsion liquid membranes, membrane-based solvent extraction, membrane reactors, gas permeation, supported liquid membranes, electrodialysis, membrane bioreactor, among others) [1, 4,  6–8]. Here, synthetic membranes are the focus of attention and, consequently, biological membranes are not discussed. Synthetic membranes can be classified as organic membranes (e.g., polymer membranes or liquid membranes type water-oil-water, w/o/w), inorganic membranes (e.g., glassy, metallic and ceramic membranes) and mixed membranes (liquid membranes supported on porous ceramic fibers). In addition, according to their porosity, these can be classified as porous membranes (e.g., microfiltration and ultrafiltration membranes) or dense membranes (e.g., surfactant, supported and bulk liquid membranes) [1, 5, 6]. A more detailed classification can be found in the specialized bibliography related to the geometry or macroscopic structure (e.g., flat sheet membranes, cylindrical membranes, disperse membranes and ­ hollow fiber membranes), and transport mechanism (e.g., gaseous or liquid flow and diffusion for porous membrane, diffusion or coupled transport for liquid membranes, anionic or cationic membranes for ionic exchange m ­ embranes) [1–3]. Also, membranes can be symmetric or asymmetric membrane according to their macroscopic configuration. Thus, asymmetric membranes consist of two layers, the top one is a very thin dense layer, commonly called the skin layer or active layer, and determines the permeation properties; whereas, the bottom one is a porous layer, commonly called the support, and is a passive layer which provides mechanical strength to the membrane (an illustration of different membrane systems is shown in Figure 2.1; in (a), a liquid phase is supported in a porous material and mass transport occurs through liquid phase; in (b), numerous hollow fibers with tubular configuration act as membrane and, in (c), a micrograph shows an lamellar asymmetric membrane) [1]. Mass transport through membranes may occur as a consequence of a driving force as a result of a difference in chemical potential which is produced by a gradient across the membrane (e.g., concentration, pressure or an electrical field). In particular, separation methods directed by pressure can be categorized into four major membrane processes: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) [9, 10]. In these processes, the pressure applied across the membrane to drive the separation is the transmembrane pressure (TMP). The fraction of the feed stream that passes through the membrane is called filtrate or

30  Nanostructured Polymer Membranes: Volume 1 Porous support

A

C

Liquid membrane

(a)

Hollow fiber (b)

Active layer

(c)

Porous support

Figure 2.1  Illustration of different membrane systems: (a) Supported liquid membrane, (b) hollow fiber membrane and (c) asymmetric membrane.

permeate, and those that cannot pass through the membrane are called retentate. Commonly five types of ideal continuous flows used in membranebased separation methods are accepted: cross flow (or tangential flow) and dead-end flow, which are the more common flow types characterized as being parallel and perpendicular to membrane surface, respectively; and co-­current flow, completely-mixed flow and counter-current flow, which usually are less cited in the specialized literature about membranes [3, 9, 10]. On the other hand, liquid membrane processes are commonly identified as three main configuration types: bulk liquid membrane, surfactant liquid membrane (or emulsion liquid membrane) and supported liquid membrane. For this kind of membrane, the mass transport and separation process includes the incorporation of liquid-liquid extraction and membrane separation in one device which is continuously operated [11]. Other types of membrane-based separation methods have been developed, however, basic principles to describe the separation mechanisms of pressure-driven methods and liquid membranes (i.e., porous and dense membranes) can be extrapolated easily to other membrane systems.

Membrane Technology and Chemistry  31

2.2.1 Basic Parameters The first parameter to characterize the substance transport through the membrane is the volumetric flow rate of permeate per unit area of membrane; this is usually denoted by J,

1 dV (2.1) Am dt

J

where V is the total volume that has permeated through the membrane at time t, and Am is the area of the membrane. Also, in membrane terminology, the flux Ji can be defined as the number of moles, volume, or mass of a specified component i passing through the membrane per unit time and unit of membrane surface area. Commonly used units for Ji include [kmolm–2s–1], [m3/m–2s–1] or [kg/m–2s–1]. This parameter permits evaluation of the transport of various components of a mixture by comparison of their relative transport rate within the membrane [1–4]. Also, transmission of a species across the membrane is usually described by the apparent sieving coefficient or by observed rejection (apparent rejection coefficient). The denomination “apparent” is because the substance concentration on the membrane surface is different from the bulk substance concentration; this difference in concentration is produced by direction of substance flux from bulk to on membrane surface. Thus, the apparent sieving coefficient (S) is defined as the concentration of a species in the permeate (Cp) as a fraction of its concentration in the feed (C0):

S

Cp C0



(2.2)

whereas the apparent rejection coefficient (R) is defined as:

R 1

Cp C0



(2.3)

An important concept in transport across the membrane is the permeability coefficient (or simply permeability). IUPAC defines the permeability as a transport flux, Ji, per unit transmembrane driving force per unit membrane thickness. Commonly permeability units are [kmolmm–2s–1kPa–1], [m3mm–2s–1kPa–1] or [kgmm–2s–1kPa–1] (i.e., permeability can be expressed in terms of particle number, volume or mass).

32  Nanostructured Polymer Membranes: Volume 1 For dense membranes (e.g., liquid membranes), permeability (L) of molecules across the membrane can be expressed as:

L



KD x

(2.4)

where K is the partition coefficient, D is the diffusion coefficient, and Δx is the thickness of the membrane. Note that diffusion coefficient is a measure of the rate of entry of the substance to the membrane phase and is a function of molecular weight or size of a molecule, whereas K is a measure of the solubility of the substance in the membrane phase and depends on molecular structure and intermolecular interactions [11, 12]. On the other hand, the permeability of porous membrane is determined by the size of pores, membrane structure, and magnitude and character of driving forces. Darcy’s law states:

Ji

KCi

dP dx

(2.5)

where dP/dx is the pressure gradient existing in the porous medium, Ci is the concentration of component i in the medium and K is a coefficient associated with the nature of the medium. In a simpler form, when filtration occurs in the region controlled by pressure, Darcy’s law can be expressed to be:

J



LP

(2.6)

For porous membranes (e.g., ultrafiltration and microfiltration membranes), the simplest representation is one in which the pores are assumed to be parallel cylindrical pores and perpendicular to the membrane surface. In addition, the length of each one of the pores is equal to the membrane thickness and the pores have the same radius. Also, during filtration, fouling and concentration polarization are not present and the flow through the pores is laminar, incompressible, Newtonian and time independent. Thus, it is possible to define an ideal membrane called “Hagen-Poiseuille’s membrane” as a perfect porous membrane that has the characteristics previously defined [1–5]. In Figure 2.2 an illustration of Hagen-Poiseuille’s membrane is shown. Flux for a Hagen-Poiseuille’s membrane is given by:

J

rp2 P 8

x



(2.7)

Membrane Technology and Chemistry  33 Real membrane (SEM image)

Hagen-Poiseuille’s membrane 2rp

2r Am = r2

=

np rp2 Am

Figure 2.2  Illustration of a real membrane from SEM image (left) and Hagen-Poiseuille’s membrane (right); note that the shape of pores is not congruent with idealized cylindrical, perpendicular and parallel pores).

where rp is the channel radius (in this case, the mean pore radius is a experimental parameter difficult to determine; usually, the pore size is specified by the molecular-weight cutoff, MWCO, which is defined to be the molecular weight of the macromolecules at which the apparent retention coefficient is near 90%), η is the viscosity of the fluid permeating the membrane, Δx is the thickness of the membrane which is equal to the length of the channel and ε is the surface porosity of the membrane which is defined to be:

np r 2 Am



(2.8)

where Am is the membrane area and np is the number of pores. Note that ΔP must exactly correspond to the net driving force, and thus if the osmotic pressure, Δπ, is significant, then effective pressure should be ΔP–Δπ. Other more complex membrane models have been developed and described by several authors [2, 4].

2.3  Separation Mechanisms Membranes can be referred to as porous and dense, and this first classification permits defining the two main mass transport models through the membranes: solution-diffusion model (which is associated with extraction mechanism by dissolution in the membrane phase; examples of methods described by this mechanism are RO, NF and liquid membranes) and poreflow model (which is associated with size exclusion mechanism; examples of methods described by this mechanism are NF, UF and MF). Illustrations of diffusion-dissolution model and pore-flow model are shown in Figure 2.3.

34  Nanostructured Polymer Membranes: Volume 1

(a)

Porous membrane 1

Porous membrane 2

Membrane phase (b)

Figure 2.3  Membrane transport mechanism: size exclusion model (a) and solutiondiffusion model (b).

For the separation by porous membranes, all particles larger than the largest pore diameter are rejected by the membrane and particles smaller than the largest pore diameter, but larger than the smallest pore diameter, are partially rejected, and particles much smaller than the smallest pore diameter will pass through the membrane into the permeate stream. On the other hand, mass transport by non-porous membranes, or dense membranes, consists of a dense film through which solutes are dissolved in the membrane phase and transported by diffusion under the driving force. Though membrane transport models are physically understandable, mathematical expressions must be concluded for each case depending on the separation method. A description of mass transport mechanism for different membrane-based separation methods are presented below.

2.3.1 Pressure-driven Membrane Methods Pores of UF membranes are smaller than those of MF membranes (for UF, pore diameter is between 2–100 nm whereas for MF the pore diameter is between 100–1000 nm). Commonly, UF membranes are exclusively asymmetric in structure whereas MF membranes are symmetric in structure.

Membrane Technology and Chemistry  35 During separation by porous membranes different intrinsic phenomena affect the substance transport; these are the fouling and concentration polarization. Fouling is defined by IUPAC as a process resulting in loss of membrane performance as a result of the deposit of suspended or dissolved substances on its external surface or within its pores [3]. These phenomena produce a decline in flux over time of operation and consequently are the major drawback to the use of membrane separation methods. It is possible to differentiate two types of fouling: Irreversible fouling (or simply fouling) and concentration polarization (or reversible fouling). Fouling is produced by the solute adsorption on the surface or within pores, producing changes in membrane permeability, porosity, thickness, roughness and in the hydrophobicity/hydrophilicity; this type of fouling is determined by membrane–solute interactions and can be described by different models (e.g., blocking pore models and series resistance model) [13–18]. The simplest type of model relates the flux to the time and/or volume permeated. Most models are based on the assumption that the build-up of the fouling layer is a first order reaction, but since they are semi-empirical in nature, they do not help to explain or understand the phenomenon itself; mathematical expression for some these models are shown by Cheryan [2]. Though completely theoretical models can contribute to the understanding of the fouling phenomena, for the predicting of permeate flux decline in filtration processes, experimental data are required to estimate some parameters of the models. Therefore, semi-empirical models whose parameters have a physical meaning are a suitable solution to achieve an accurate prediction of permeate flux decline in filtration and explain the fouling mechanisms at the same time [13]. In the fouling description, four mechanisms of blocking pore (or Hermia’s models) can be identified for description of the total resistance as a function of the filtrated volume (see Figure 2.4) [13–16]: Cake filtration: it is assumed that the cake resistance is proportional to the thickness of the cake, Intermediate blocking: it is assumed that each location has an equal probability of being occupied and the blocked pores are impermeable, Standard blocking: which is based on the assumption that all particles are deposited inside the pores and, Complete blocking: it is assumed that blocked parts of the membrane are impermeable; consequently, the resistance is inversely proportional to the fraction of free pores.

36  Nanostructured Polymer Membranes: Volume 1

Cake filtration

Complete blocking

Standard blocking

Intermediate blocking

Figure 2.4  Illustration of blocking pore models.

Depending on the pore blocking mechanism, the permeate flow decline can be described by:

d 2t dV 2



dt k dVp

n

(2.9)

where t and Vp are the filtration time and permeate volume, respectively, and k and n are two experimental parameters where n is a dimensionless number that is related to the fouling mechanism [16]. Commonly, four pore blocking mechanisms are considered: Complete pore blocking (n = 2):

Jn



2

J 0 (kb Am ) (2.10) t

where Am is the membrane area, kb is a phenomenological coefficient “­fouling constant by complete pore blocking,” Jn=2 is the flux and J0 is the initial flux (or flux reference). Internal pore blocking (n = 1.5):

Jn

1. 5

4 J0 (2.11) [2 ks ( Am J 0 )1/2 t ]2

where ks is a phenomenological coefficient “fouling constant by internal pore blocking” and Jn=1.5 is the flux.

Membrane Technology and Chemistry  37 Intermediate pore blocking (n = 1):

Jn



J0 1 ki ( Am J 0 )t (2.12)

1

where ki is a phenomenological coefficient “fouling constant by intermediate pore blocking” and Jn=1 is the flux. Cake formation (n = 0):

Jn

0

J0 (2.13) [1 2kc ( Am J 0 )2 t ]1/2

where kc is a phenomenological coefficient “fouling constant by cake filtration” and Jn=0 is the flux. The easy determination of model parameters from operating experimental data, possibility to be solved analytically and their simplicity are some advantages of Hermia’s models, besides which, fouling mechanism is inferred from the shape of the filtration curve. There are many contributions in the literature that analyze Hermia’s models [13–16]. Analysis based on Hermia’s models assumes that fouling is controlled by only one mechanism which is not applicable for all situations, hence two or more mechanics could be occurring; or one can be a direct consequence of other. Another assumption that is not necessarily adequate is the impermeability of fouling formed. The forming of a porous and dense cake could be interpreted as an intermediate blocking mechanism and does not take into account the thickness of the cake formed or the change in the roughness produced. Another model widely used is the series resistance model, which is an approach derived from the “resistance-in-series” concept common in heat transfer. In this model, an increase in the membrane permeability is a consequence of membrane resistance changes produced by fouling. Thus, one intrinsic membrane resistance is defined and ­different contributions are assigned depending on different fouling factors ­present after or during the filtration [14, 15]:

1 Rtotal

i

1 (2.14) Ri

where Rtotal is the membrane resistance and Ri the different partial contributions of total resistance (i.e., resistance by gel, concentration polarization, intrinsic of the membrane, etc.).

38  Nanostructured Polymer Membranes: Volume 1 In membrane processes, the retained or rejected species accumulate near the membrane surface and as a consequence concentration polarization is produced. This fouling type is a natural consequence of preferential transport of some species through the membrane, is a reversible process in nature which increases membrane resistance (decrease of permeability) and can facilitate the production of irreversible fouling by promoting solute aggregation in solution and/or on the surface preadsorbed with solute by altering interactions between solvent, solute and membrane. Reversible fouling is determined by solute–solute interactions and can be described by different models [17, 18]. Film and gel layer models were developed to describe flux in pressuredriven membrane processes and assume the formation of film in the adjacent region of the membrane surface. According to the film model (see Figure 2.5), considering the steady state, a mass balance is possible in the boundary layer by taking into account convective and diffusive flows for the solute:

Js



J vC D

dC (2.15) dx

where, Js and Jv are the solute and dissolution flows, respectively, D is the solute diffusivity and C is the solute concentration. Assuming that (a) solute retention is not complete, Js = JvCp, where Cp is the permeate concentration, (b) the conditions of the boundary x P1 D

dC dx

c

Diffusion

Convection JvC0

Permeation JS Cm

C0 Cp Pz

y, p x

Figure 2.5  Solute concentration gradient and mass balance on the surface membrane according to film theory.

Membrane Technology and Chemistry  39 C = Cm) being δc the film thickness layer (x = 0  C = C0 and x = δc and (c) small transference rate, the mass transfer coefficient (Km) can be defined as:

D

Km



(2.16)

c

Thus, volumetric flux (Jv) is given by:

Jv



K m ln

Cm C p C0 C p

(2.17)

Considering gel layer model (see Figure 2.6), Cp = Cg, which supposes gel layer is dynamic (changes in operation conditions produce changes in characteristics of gel layer), the flow through membrane may be described in polarization conditions by:

P

J v ,lim



(Rm

Rg )

K m ln

Cg

Cp

C0 C p

(2.18)

where η is the solution viscosity, Rm is the intrinsic membrane resistance and Rg is an additional resistance produced by the gel layer [17, 18]. g

P1 D

dC dx

c

Diffusion

Convection JvC0

x

Permeation JS Cm

C0 Cp Pz

y, p x

Gel layer

Figure 2.6  Scheme of concentration gradient according to the gel model.

40  Nanostructured Polymer Membranes: Volume 1

2.3.2 Liquid Membranes According to the configuration definition, three groups of liquid membranes are usually considered (see Figure 2.1): bulk liquid membrane (which consists of a bulk aqueous feed and receiving phases separated by a bulk organic, water-immiscible liquid phase), supported liquid membrane (which is produced by liquid that is impregnated in the pores of a thin microporous solid support), and emulsion liquid membrane or surfactant liquid membrane (receiving phase is emulsified in an immiscible liquid membrane, and later, the emulsion is then dispersed in the feed solution in order to produce the mass transfer from the feed to the internal receiving phase) [11]. According to the transport mechanisms, the separation methods by liquid membrane can be divided into six basic mechanisms of transport: simple transport, simple transport with chemical reaction in strip solution, facilitated transport, coupled counter-transport, coupled co-transport and active transport (see Figure 2.7). In simple transport, solute passes through the membrane as a result of its solubility in the membrane phase and permeation stops when concentration equilibrium is reached. By this mechanism, the solute does not react chemically with membrane and is supposed to be in the same form in the feed (F), in the membrane phase (E), and receiving phase (R); whereas in the facilitated or carrier-mediated mechanisms, the transport can be described by subsequent partitioning, complexation, and diffusion. F

E

R

F

F

R

E

A

–

S

S

S

S

S

S S

SA

Simple transport

E

R

S

2+

SE2

S2+

2H+

2EH

2H+

R A–

E

SA

S

A–

Simple transport with chemical reaction in strip solution

Facilitated transport

S+

S2+

S2+ F

E SE

F

E

S+

ESA

A–

E

2H+

A–

Coupled counter-transport

Coupled co-transport

F

E

S+

S2+ red S+

SEA2

A–

2A–

E

R

2A

R + OX

S

2A– –

Active transport

Figure 2.7  Illustration of transport mechanisms in separation methods by liquid membranes.

S2+

Membrane Technology and Chemistry  41 Thus, solute partitioning (dissolving) in liquid membrane on a feed sideliquid membrane interface chemically reacts with a carrier, dissolved in the ­liquid membrane, to form complex. This complex reverse reacts on the liquid membrane-receiving side interface releasing the solute, partitioning to the receiving (strip) phase [3, 11].

2.3.3 Other Methods At the present time, there are several membrane-based separation methods (dialysis, electrodialysis, membrane bioreactor, etc.) and in each case the separation mechanisms can be different from the one previously exposed; however, these must be analyzed individually. This difference is more relevant in those called “hybrid methods,” which are membrane separation systems resulting from a combination of several techniques producing a change in the separation mechanism [19]. Examples of the more representative hybrid methods are membrane bioreactor [20–23], electro-­ultrafiltration [24–27], ultrafiltration coupled with ultrasound [28–30], flotation coupled with microfiltration [31–33], liquid-phase polymer-based retention or polymer-enhanced ultrafiltration [19, 34, 35] and surfactant liquid membrane coupled with liquid-phase polymer-based retention [36–38].

2.4 Chemical Nature of Membrane Similar to microfiltration membranes, ultrafiltration membranes are made from a wide range of polymers and inorganic substances. Polymer materials include polysulphone, polyvinylidene fluoride, polyamide, and cellulose acetate, whereas the inorganics are limited to ceramics, alumina, and zirconia. The range of materials used for nanofiltration and reverse osmosis membranes is much smaller than that used for microfiltration and ultrafiltration, and is limited to polymers. Cellulose acetate membranes are often used in reverse osmosis processes for the desalination of sea water because they have a relatively high permeability to water and a very low salt permeability. Polyamides are also commonly used. Although polyamides exhibit lower water permeability, they can be operated over a wider range of pH [1–3]. Membrane material requires resisting the operation conditions and being suitable for specific application. In many cases, additives are added to membrane phase during the fabrication to increase the permeability or reduce the fouling. Polymer membranes are severely limited in their applications in extremely high-temperature and corrosive environments. On the other hand, inorganic membranes have high selectivity and high

42  Nanostructured Polymer Membranes: Volume 1 permeability as well as thermal, chemical and mechanical stability, but the cost of these are very high in comparison with polymer membranes [2]. In the fabrication of liquid membrane, the chemical nature of membrane phase and carriers are important factors. Commonly, the more used liquid membrane is w/o/w and therefore the organic phase should be more immiscible with water. On the other hand, ideal carrier should have rapid kinetics of formation and decomposition of the complex on membrane interfaces, no side reactions, no irreversible or degradation reaction and low solubility in the aqueous feed and strip phases. Some typical carriers are 18-crown-6, benzo-18-crown-6, dibenzo-18-crown-6, and acyclic crown ether. Some ionic carriers, such as amines or carboxylic and phosphoric acids, are used for metals, organic acids, and amines (e.g., Aliquat 336, tri-n-octylamine chloride, naphthalene-1-carboxylic acid, undecanoic acid) [11].

2.5  Surface Treatment of Membranes Organic and inorganic membranes can be modified for different applications by changes in the material chemical properties or by changes of pore size [39]. Membrane modification offers a versatile means to improve several membrane properties (e.g., hydrophilicity, hydrophobicity, biocompatibility, antifouling, surface roughness, antistatic, antibacterial, conductivity, among others). Also, it permits adapting the membrane to novel functionalities (e.g., selective ion retention, photocatalysis capability, molecular imprinting and enzyme immobilization) [40–42]. The above can be accomplished using methods such as chemical oxidation, incorporation of additives into the membrane matrix, plasma treatment, classical organic reactions, polymer grafting, interpenetrating polymer network, surfactant modification, self-assembly of the nanoparticles, among others [43].

2.5.1 Chemical Methods for Membrane Modification The principal chemical methods of membrane modification are described below.

2.5.1.1 Chemical Treatment In chemical treatment, the membrane surface is modified through covalent bonding interaction by incorporating modifying agents that introduce various functional groups on the membrane surface. The membrane surface properties can be improved whereas the membrane bulk is not

Membrane Technology and Chemistry  43 significantly affected. Moreover, the covalent attachment of modifiers on membrane surface offers a long-term chemical stability, in contrast with physical methods [44]. Sulfonation, chloromethylation, aminomethylation, and lithiation reactions have been applied to polysulfone membranes [45]. The main disadvantage of modification by chemical treatment is that the modification agent may in part block the membrane pores. Even if the modified membranes are less prone to fouling, the total flux after modification is generally smaller than flux before modification. In some cases, chemical modification during membrane formation is preferred, since it seems to compromise the flux loss [46]. In addition, in order to reduce membrane fouling, various chemical reactions have been proposed for the introduction of charged groups, such as –SO3 or –CO2H, onto the ­membrane surface [47, 48]. For example, during the prolonged exposure of polyacrylonitrile membrane to 1 M NaOH, the surface nitrile groups turned into carboxylic groups [49]. The modified membranes obtained by this method were less prone to fouling, with bovine serum albumin showing a reduction in the average pore diameter of about 80%. The ultrafiltration membranes of ­polyethersulfone-polyacrylonitrile treated with aqueous NaOH solutions at room temperature for 24 h showed higher flux recovery ratio compared with the unmodified membranes after ultrafiltration of poly(ethylene g­ lycol) and dextran solutions. The increase in the fouling resistance is explained by the higher hydrophilicity of the modified membrane surface [50]. Modifications of blended chitosan-cellulose acetate membranes (C-CA) with heparin, or a quaternary ammonium, to change the hydrophilicity and the membrane charge, or by chemical reaction with an AgNO3 to introduce biocide properties, have been reported. The reaction of the heparin with the C-CA membrane was by the formation of polycation–polyanion complex, where –CH2SO3− and –NHSO3− groups on heparin interacted with \NH3+ groups in chitosan. The attachment of quaternary ammonium to the membrane was performed via both the –CH2OH and –NH2 positions on the chitosan polymer chains. On the other hand, in the membrane modification with AgNO3, silver ions were loaded onto the membrane through surface complexation with the amine groups in chitosan and through physical adsorption (C-CA–Ag membranes) [51].

2.5.1.2 Grafting Grafting is a method wherein monomers are covalently bonded onto the membrane. The techniques to initiate grafting are: (1) chemical, (2) photo­ chemical and/or via high-energy radiation, (3) the use of plasma, and

44  Nanostructured Polymer Membranes: Volume 1 (4) enzymatic method. The selection of specific grafting technique depends on the chemical structure of the membrane and the desired characteristics after surface modification [45]. Among the surface modification techniques developed to date, surface grafting has emerged as a simple, useful, and versatile approach to improve surface properties of polymers for many applications. Grafting has several advantages: (1) the ability to modify the polymer surface to have distinct properties by the choice of different monomers, (2) the controllable introduction of graft chains with a high density and exact localization on the surface, without affecting the bulk properties, and (3) long-term chemical stability, which is assured by covalent attachment of graft chains [52].

2.5.1.3 Chemical Initiation Technique In chemical grafting, free radicals are produced and transferred to the substrate to initiate polymerization and form graft copolymers. A few studies have shown that redox initiation-grafting could be successfully applied to polyethersulfone ultrafiltration membranes [53]. For these, peroxydisulfate and metabisulfite oxidizing agents have been used to initiate free radical polymerization grafting of methacrylic acid, polyethyleneglycol-­ methacrylate, and sulfopropylmethacrylate in aqueous solution at ambient temperature. In general, this strategy is a simple and economic ­modification technique, leading to membranes that are claimed to be less sensitive to fouling due to the presence of the hydrophilic grafted monomers, but it is a harsh treatment [45].

2.5.1.4 Photochemical and Radiation Initiation Techniques The irradiation of macromolecules can cause homolytic fission and thus forms free radicals on the membrane. UV irradiation and UV-assisted graft polymerization are techniques that can selectively alter membrane surface properties without affecting the bulk polymer. UV-assisted graft polymerization modifies the membrane surface by grafting polymer chains onto the surface and in the pores. UV irradiation can crosslink polymer chains and cleave polymer bonds, therewith forming functional groups such as hydroxyls, carbonyls, or carboxylic acids on the surface. The UV irradiation should be carefully used because it leads to severe degradation of the pore structure with loss of membrane function, which needs to be partially compensated by grafted polymer [54]. This technique was used to graft several hydrophilic monomers (N-vinyl-2-pyrrolidinone, N-vinylcaprolactam, and N-vinylformamide) onto 10 kDa PES ultrafiltration membranes. Membranes modified with

Membrane Technology and Chemistry  45 N-vinyl-2-pyrrolidinone (25% increase in hydrophilicity) exhibited the best combination of low fouling (50% decrease in BSA fouling) and high flux, although membrane permeability was significantly decreased because the grafted polymer chains blocked the membrane pores (over 25% reduction in flux due to modification) [55].

2.5.1.5 Plasma Initiation Technique Plasma surface treatment usually refers to a plasma reaction that either results in modification of the molecular structure of the surface, or atomic substitution. Plasma treatment is a useful tool in the modification of surface properties. Currently, more and more attention is being given to its applications in membrane separation science. The accelerated electrons from the plasma have sufficient energy to induce cleavage of the chemical bonds in the membrane structure and to form macromolecule radicals, which subsequently initiate graft copolymerization [56]. Plasma treatment can be done by either regular plasma treatment or plasma graft copolymerization (PGC) [57]. Low temperature plasma techniques, which are very surface selective, have been used to modify various types of membranes, specifically to reduce protein–surface attractive interaction. For example, simple inert gas [58], nitrogen, or oxygen plasmas have been used to increase the surface hydrophilicity of membranes [59], and ammonia plasmas have successfully yielded functionalized polysulfone membranes [45]. Plasma treatment can also be used as a source of radicals that act as active sites for graft polymerization. The active components generated in plasma can activate the upper molecular layers of the membrane surface to increase the hydrophilicity without affecting the bulk of the polymer [60]. A reported disadvantage of plasma modification is the time dependency of the induced changes [61].

2.5.1.6 Enzymatic Initiation Technique The principle involved in this membrane modification technique is the chemical/electrochemical grafting reaction by the use of some enzyme [56]. This method employs enzymes to convert the substrate (monomer, oligomer or polymer chains) into a reactive free radical(s), which undergoes subsequent non-enzymatic reaction with the membrane [62]. There are several potential advantages for the use of enzymes in membrane modification. With respect to health and safety, enzymes offer the potential of eliminating the need for (and hazards associated with) reactive reagents (and solvents). A potential environmental benefit for using enzymes is that their selectivity may be exploited to eliminate the need for wasteful

46  Nanostructured Polymer Membranes: Volume 1 protection and deprotection steps. Finally, enzyme specificity may offer the potential for precisely modifying macromolecular structure to better control polymer function. Some polymers that were successfully modified with enzymes are briefly mentioned here [63–65].

2.5.2 Physical Methods for Membrane Modification The physical modification of membrane surface implies a noncovalent interaction; however, a chemical reaction may be required during the modification. The main physical methods for membrane modification are described below.

2.5.2.1 Coating Surface coating or deposition is a simple but effective method for membrane surface modification, wherein the coating material forms a thin layer that noncovalently adheres to the substrate. Coating methods can be divided into five techniques: coating of a hydrophilic thin layer by physical adsorption possibly followed by curing with heat, coating with a monolayer using Langmuir–Blodgett or analogous techniques, deposition from a glow discharge plasma, and casting or extrusion of two polymer ­solutions by simultaneous spinning using, e.g., a triple orifice spinneret. In the last technique, using different solvents for each polymer solution ­facilitates adhesion between the upper coating layer and polymer ­membrane [45, 66–70].

2.5.2.2 Blending Blending is a process in which two (or more) polymers are physically mixed to obtain the required properties. The use of branched amphiphilic copolymers with polyethylene glycol (PEG) in an amphiphilic comb-copolymer with polystyrene as the hydrophobic part and PEG has been reported [71–73]. In this case, the hydrophilic PEG segments spontaneously segregated to the membrane surface during immersion precipitation, which increased hydrophilicity and reduced protein adsorption, whereas only a slight change in permeation properties was observed. Recently, amphiphilic copolymers such as phosphorylcholine copolymer were investigated [72].

2.5.2.3 Composite A composite is a material made from two or more materials with different physical or chemical properties which remain separate and distinct on a

Membrane Technology and Chemistry  47 macroscopic level within the finished structure [74, 75]. TiO2 nanoparticles were added to the polymer solution and then TiO2-entrapped polysulfone membranes were prepared by phase inversion. These membranes showed less flux decline compared with unmodified polysulfone membranes. Also, polyethersulfone-TiO2 composite membranes showed better flux behavior compared to polyethersulfone membranes [76]. The TiO2 nanoparticle acts mainly on hydrophobic substances, suggesting a possible use as a new antifouling component in composite membranes [77].

2.5.2.4 Combined Methods Recently, combined techniques were presented for modification of polyethersulfone membranes, in which the membrane was blended with a copolymer of acrylonitrile and acrylic acid, and subsequently grafted with bovine serum albumin [78]. Sulfonic acid groups were generated on the polyethersulfone membrane surface by chemical sulfonation, followed by dipping the membranes into the TiO2 solution. This modification reduced the loss of flux due to fouling from 80 to 65%. Another combined modification was carried out by blending polyethersulfone with polyimide and treatment with diethanolamine to introduce –OH groups on the membrane surface, followed by dipping in TiO2 colloidal solution, and irradiation with UV light [79]. Combined modification sequences led to lower fouling of the membrane. The results as such are interesting, although the complexity of the technique could prove to be a major difficulty.

2.5.3 Current Research about Membrane Modification Currently, the pressure-driven membrane processes are widely used in water treatment, biotechnology, food industry, medicine, and other fields [80]. However, one of the main problems arising from the operation of the membrane units is membrane fouling, which seriously hampers the applications of membrane technologies [81]. This is because it leads to higher operating pressures, more frequent chemical cleaning, shortened membrane life, and compromised product water quality [82]. In this context, most of the current research is directed towards solving the problem of fouling and various membrane modification methods for fouling reduction are currently being proposed. New membrane modification methods have been proposed, e.g., the modification of membrane surfaces via microswelling for fouling control in drinking water [83], hydrogel surface modification of reverse osmosis membranes [70], controllable modification of polymer membranes by LDDLT plasma flow (antibacterial

48  Nanostructured Polymer Membranes: Volume 1 layer onto PE hollow fiber membrane module) [60], preparation, characterization and performance study of cellulose acetate membranes modified by aliphatic hyperbranched polyester [84], membrane fouling characterization by infrared thermography [85], fouling and its control in membrane distillation [86], and a comprehensive review of surface modified polymer membranes for biofouling mitigation [82], among others. Another important focus in membrane modification is the p ­ ossibility of adapting the membrane for entirely novel functionalities (selective ion retention, photocatalysis capability, molecular imprinting and enzyme immobilization) or improving any process. Along this line, work can be emphasized as surface modification of composite ion exchange membranes by polyaniline [87], fabrication of novel poly(phenylene ether ether sulfone)based nanocomposite membrane modified by Fe2NiO4 nanoparticles and ethanol as organic modifier [88], modification of Nafion membrane using fluorocarbon surfactant for all vanadium redox flow batteries [89], modification of ultrafiltration membranes via interpenetrating polymer networks for removal of boron from aqueous solution [90], among others.

2.6 Conclusions Membrane science and technology is a current topic with many projections for industrial applications. Implementation and new development require the adequate theoretical description and modeling of different membranebased techniques, which must be analyzed in a particular form. However, the general description presented here serves as an introduction to the fundamental aspects of membrane theory and as an illustration of the state of the art of membrane theory. It is clear that fouling is the main problem for the application of this technology and that the effort of researchers is directed towards the modification of the membrane. Thus, many factors need to be considered in the overall process of membrane preparation, such as uniformity, reproducibility, together with precise control over bactericidal and functional groups.

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50  Nanostructured Polymer Membranes: Volume 1 21. E.J. McAdam, S.J. Judd, A review of membrane bioreactor potential for nitrate removal from drinking water. Desalination, 196 (1–3), 135–148, 2006. 22. R. Valadez, F. Castelo, R. Ferreira, A.G. Livingston, A membrane bioreactor for biotransformations of hydrophobic molecules using organic solvent nanofiltration (OSN) membranes. J. Membr. Sci., 317(1–2), 50–64, 2008. 23. A. Zarragoitia, S. Schetrite, M. Alliet, U. Áuregui, C. Albasi, Modelling of submerged membrane bioreactor: Conceptual study about link between activated slugde biokinetics, aeration and fouling process. J. Membr. Sci., 325(2), 612–624, 2008. 24. A. Saxena, B. Tripathi, M. Kumar, V. Shahi, Membrane-based techniques for the separation and purification of proteins: An overview. Adv. Colloid Interf. Sci., 145, 1–22, 2009. 25. T. Käppler, C. Posten, Fractionation of proteins with two-sided electro-ultrafiltration. J. Biotechnol., 128(4), 895–907, 2007. 26. A.D. Enevoldsen, E.B. Hansen, G. Jonsson, Electro-ultrafiltration of industrial enzyme solutions. J. Membr. Sci., 299(1–2), 28–37, 2007. 27. X. Chai, T. Kobayashi, N. Fujii, Ultrasound effect on cross-flow filtration of polyacrylonitrile ultrafiltration membranes. J. Membr. Sci., 148(1), 129–135, 1998. 28. T. Kobayashi, X. Chai, N. Fujii, Ultrasound enhanced cross-flow membrane filtration. Sep. Pur. Technol., 17(1), 31–40, 1999. 29. A. Simon, L. Penpenic, N. Gondrexon, S. Taha, G. Dorange, A comparative study between classical stirred and ultrasonically-assisted dead-end ultrafiltration. Ultrason. Sonochem., 7(4), 183–186, 2000. 30. M.Y. Teng, S.H. Lin, R.S. Juang, Effect of ultrasound on the separation of binary protein mixtures by cross-flow ultrafiltration. Desalination, 1–3, 280–282, 2000. 31. E. Peleka, P. Mavros, D. Zamboulis, K. Matis, Removal of phosphates from water by a hybrid flotation–membrane filtration cell. Desalination, 198(1–3), 198–207, 2006. 32. C. Blöcher, J. Dorda, V. Mavrov, H. Chmiel, N.K. Lazaridis, K.A. Matis, Hybrid flotation–membrane filtration process for the removal of heavy metal ions from wastewater. Water Res., 37(16), 4018–4026, 2003. 33. K.A. Matis, N.K. Lazaridis, A.I. Zouboulis, G.P. Gallios, V. Mavrov, A hybrid flotation–microfiltration process for metal ions recovery. J. Membr. Sci., 247(1–2), 29–35, 2005. 34. M. Palencia, B.L. Rivas, E. Pereira, Polymer-enhanced ultrafiltration: Counterion distribution and its relation with the divalent metal-ion retention properties by sulfonic acid polyelectrolytes. Polym. Bull., 67, 1123–1138, 2011. 35. M. Palencia, B.L. Rivas, E. Pereira, A. Hernández, P. Prádanos, Study of polymer–metal ion–membrane interactions in liquid-phase polymer-based retention (LPR) by continuous diafiltration. J. Membr. Sci., 336, 128–139, 2009.

Membrane Technology and Chemistry  51 36. M. Palencia, B.L. Rivas, Metal ion retention by emulsion liquid membrane coupled to liquid-phase polymer-based retention. Colloid. Polym. Sci., 289, 1695–1709, 2011. 37. M. Palencia, B.L. Rivas, Metal-ion retention properties of water-soluble amphiphilic block copolymer in double emulsion system (w/o/w) stabilized by non-ionic surfactants. J. Colloid Interf. Sci., 363, 682–689, 2011. 38. B. Rivas, M. Palencia, Removal-concentration of pollutant metal-ions by water-soluble polymers in conjuction with double emulsion systems: A new hybrid method of membrane-based separation. Sep. Pur. Technol., 81, 435–443, 2011. 39. G.Z. Trznadel, M. Khayet, Membranes in nuclear science and technology: Membrane modification as a tool for performance improvement, in: Membrane Modification Technology and Applications, N. Hilal, M. Khayet, C.J. Wright (Eds.), pp. 1–20, CRC Press, Taylor & Francis Group, 2012. 40. Z. Xu, X. Huang, L. Wan, Surface Engineering of Polymer Membranes, 333 pp., Zhejiang University Press, Hangzhou and Springer-Verlag GmbH Berlin Heidelberg, 2009. 41. M. Palencia, M. Vera, E. Combatt, Polymer networks based in (4-Vinylbenzyl)N-methyl-D-glucamine supported on microporous polypropylene layers with retention boron capacity. J. Appl. Polym. Sci., 131, 1–7, 2014. 42. X. Teng, J. Dai, J. Su, G. Yin, Modification of Nafion membrane using fluorocarbon surfactant for all vanadium redox flow battery. J. Membr. Sci., 476, 20–29, 2015. 43. K.C. Khulbe, C. Feng, T. Matsuura, The art of surface modification of synthetic polymeric membranes. J. Appl. Polym. Sci., 115, 855–895, 2010. 44. G. Kang, Y. Cao, Application and modification of poly(vinylidene fluoride) (PVDF) membranes—A review. J. Membr. Sci., 463, 145–165, 2014. 45. N. Nady, M.C.R. Franssen, H. Zuilhof, M.S. Mohy, R. Boom, K. Schroën, Modification methods for poly(arylsulfone) membranes: A mini-review focusing on surface modification. Desalination, 275, 1–9, 2011. 46. A.A. Nabe, E. Staude, G. Belfort, Surface modification of polysulfone ultrafiltration membranes and fouling by BSA solutions. J. Membr. Sci., 133, 57–72, 1997. 47. D. Duputell, E. Staude, Heterogeneous modification of ultrafiltration membranes made from poly(vinylidene fluoride) and their characterization. J. Memb. Sci., 78, 45–51, 1993. 48. A.Y. Tremblay, C.M. Tam, M.D. Guiver, Variations in the pore size of charged and noncharged hydrophilic polysulfone membranes. Ind. Eng. Chem. Res., 31, 834–838, 1992. 49. M. Bryjak, H. Hodge, B. Dach, Modification of porous polyacrylonitrile membranes. Angew Makromol. Chem., 260, 25–29, 1998. 50. A.V.R. Reddy, H.R. Patel, Chemically treated polyethersulfone/polyacrylonitrile blend ultrafiltration membranes for better fouling resistance. Desalination, 221, 318–323, 2008.

52  Nanostructured Polymer Membranes: Volume 1 51. C.X. Liu, D.R. Zhang, Y. He, X.S. Zhao, R. Bai, Modification of membrane surface for anti-biofouling performance: Effect of anti-adhesion and antibacteria approaches. J. Memb. Sci., 346, 121–130, 2010. 52. S. Minko, Grafting on solid surfaces: “Grafting to” and “grafting from” methods, in: Polymer Surfaces and Interfaces: Characterization, Modification and Applications, S. Manfred (Ed.), pp. 215–234, Springer. Berlin, 2008. 53. S. Belfer, R. Fainchtain, Y. Purinson, O. Kedem, Surface characterization by FTIR-ATR spectroscopy of polyethersulfone membranes-unmodified, modified and protein fouled. J. Membr. Sci., 172, 113–124, 2000. 54. M. Nyström, P. Järvinen, Modification of polysulfone ultrafiltration membranes with UV irradiation and hydrophilicity increasing agents. J. Membr. Sci., 60, 275–296, 1991. 55. H. Yamagishi, J.V. Crivello, G. Belfort, Evaluation of photochemically modified poly(arylsulfone) ultrafiltration membranes. J. Membr. Sci., 105, 249–259, 1995. 56. Y. Tsujii, K. Ohno, S. Yamamoto, A. Goto, T. Fukuda, Structure and properties of high-density polymer brushes prepared by surface-initiated living radical polymerization. Adv. Polm. Sci., 197, 1–45, 2006. 57. D.S. Wavhal, E.R. Fisher, Membrane surface modification by plasma-induced polymerization of acrylamide for improved surface properties and reduced protein fouling. Langmuir, 19, 79–85, 2003. 58. P. Gröning, M.C. Coen, O.M. Küttel, L. Schlapbach, Influence of gas pressure on the plasma polyethersulphone. Appl. Surf. Sci., 103, 79–89, 1996. 59. D.S. Wavhal, E.R. Fisher, Hydrophilic modification of polyethersulfone membranes by low temperature plasma-induced graft polymerization. J. Membr. Sci., 209, 255–269, 2002. 60. M. Li, Z. Zhao, M. Wang, Y. Zhang, Controllable modification of polymer membranes by LDDLT plasma flow: Antibacterial layer onto PE hollow fiber membrane module. Chem. Eng. J., 265, 16–26, 2015. 61. C. Zhao, J. Xue, F. Ran, S. Sun, Modification of polyethersulfone membranes—A review of methods. Prog. Mater Sci., 58, 76–150, 2013. 62. A.C. Chao, S.S. Shyu, Y.C. Lin, F.L. Mi, Enzymatic grafting of carboxyl groups on to chitosan to confer on chitosan the property of a cationic dye adsorbent. Bioresour. Technol., 91, 157–162, 2004. 63. G. Kumar, P.J. Smith, G.F. Payne, Enzymatic grafting of a natural product onto chitosan to confer water solubility under basic conditions. Biotechnol. Bioeng., 63, 154–165, 1999. 64. T. Chen, G. Kumar, M.T. Harris, P.J. Smith, G.F. Payne, Enzymatic grafting of hexyloxyphenol onto chitosan to alter surface and rheological properties. Biotechnol. Bioeng., 70, 564–573, 2000. 65. G. Kumar, J.F. Bristow, P.J. Smith, G.F. Payne, Enzymatic gelation of the natural polymer chitosan. Polymer, 41, 2157–2168, 2000. 66. L.T. Duarte, C.C. Pereira, A.C. Habert, C.P. Borges, Polyurethane/­ polyethersulphone composite hollow fibers produced by simultaneous ­spinning of two polymer solutions. J. Membr. Sci., 311, 12–22, 2008.

Membrane Technology and Chemistry  53 67. A. Hamza, G. Chowdhury, T. Matsuura, S. Sourirajan, Sulphonated poly(2,6dimethylphenylene oxide)-polyethersulphone composite membranes: Effects of composition of solvent system, used for preparing casting solution, on membrane-surface structure and reverse osmosis performance. J. Membr. Sci., 129, 55–64, 1997. 68. J. Hyun, H. Jang, K. Kim, K. Na, T. Tak, Restriction of biofouling in membrane filtration using a brush-like polymer containing oligoethylene glycol side chains. J. Membr. Sci., 282, 52–59, 2006. 69. T.H. Bae, T.M. Tak, Effect of TiO2 nanoparticles on foulingmitigation of ultrafiltration membranes for activated sludge filtration. J. Membr. Sci., 249, 1–8, 2005. 70. D. Nikolaeva, C. Langner, A. Ghanem, M. Rehim, B. Voit, J. Haack, Hydrogel surface modification of reverse osmosis membranes. J. Membr. Sci., 476, 264–276, 2015. 71. X. Ma, Y. Su, Q. Sun, Y. Wang, Z. Jiang, Preparation of protein-adsorptionresistant poly(ethersulfone) ultrafiltration membranes through surface segregation of amphiphilic comb co-polymer. J. Membr. Sci., 292, 116–124, 2007. 72. Y. Su, C. Li, W. Zhao, Q. Shi, H. Wang, Z. Jiang, S. Zhu, Modification of polyethersulfone ultrafiltration membranes with phosphorylcholine copolymer can remarkably improve the antifouling and permeation properties. J. Membr. Sci., 322, 171–177, 2008. 73. N.A. Rahman, T. Maruyama, H. Matsuyama, Performance of polyethersulfone/Tetronic 1307 hollow fiber membrane for drinking water production. J. Appl. Sci. Environ. Sanit., 3, 1–7, 2008. 74. Z.P. Zhao, W. Wang, S.C. Wang, Formation, charged characteristic and BSA adsorption behavior of carboxymethyl chitosan/PES composite MF membrane. J. Membr. Sci., 217, 151–158, 2003. 75. Z. Zhao, Z. Wang, N. Ye, S. Wang, A novel N, O-carboxymethyl amphoteric chitosan/poly(ethersulfone) composite MF membrane and its charged characteristics. Desalination, 144, 35–39, 2002. 76. J.F. Li, Z.L. Xu, H. Yang, L.Y. Yu, M. Liu, Effect of TiO2 nanoparticles on the surface morphology and performance of microporous PES membrane. Appl. Surf. Sci., 255, 4725–4732, 2009. 77. M.L. Luo, J.Q. Zhao, W. Tang, C.H. Pu, Hydrophilic modification of poly(ether sulfone) ultrafiltration membrane surface by self-assembly of TiO2 nanoparticles. Appl. Surf. Sci., 249, 76–84, 2005. 78. B. Fang, Q. Ling, W. Zhao, Y. Ma, P. Bai, Q. Wei, Modification of polyethersulfone membrane by grafting bovine serum albumin on the surface of polyethersulfone/ poly(acrylonitrile-co-acrylic acid) blended membrane. J. Membr. Sci., 329, 46–55, 2009. 79. Y. Mansourpanah, S.S. Madaeni, A. Rahimpour, A. Farhadian, A.H. Taheri, Formation of appropriate sites on nanofiltration membrane surface for binding TiO2 photo-catalyst: Performance, characterization and fouling-resistant capacity. J. Membr. Sci., 330, 297–306, 2009.

54  Nanostructured Polymer Membranes: Volume 1 80. V. Kochkodan, D.J. Johnson, N. Hilal, Polymeric membranes: Surface modification for minimizing (bio)colloidal fouling. Adv. Colloid Interface Sci., 206, 116–140, 2014. 81. K. Scot, R. Hughes (Eds.), Industrial Membrane Separation Technology, Blackie Academics & Professional, London, 1996. 82. V. Kochkodana, N. Hilal, A comprehensive review on surface modified polymer membranes for biofouling mitigation. Desalination, 356, 187–207, 2015. 83. J.R. Du, S. Peldszus, P.M. Huck, X. Feng, Modification of membrane surfaces via microswelling for fouling control in drinking water treatment. J. Membr. Sci., 475, 488–495, 2015. 84. H. Mahdavi, T. Shahalizade, Preparation, characterization and performance study of cellulose acetate membranes modified by aliphatic hyperbranched polyester. J. Membr. Sci., 473, 256–266, 2015. 85. K.O. Ndukaife, J.C. Ndukaife, A.G.A. Nnanna, Membrane fouling characterization by infrared thermography. Infrared Phys. Technol., 68, 186–192, 2015. 86. L.D. Tijing, Y.C. Woo, J.S. Choi, S. Lee, S.H. Kim, H.K. Shon, Fouling and its control in membrane distillation — A review, J. Membr. Sci., 475, 215–244, 2015. 87. H. Farrokhzad, M.R. Moghbeli, T.V. Gerven, B.V. Bruggen, Surface modification of composite ion exchange membranes by polyaniline. React. Funct. Polym., 86, 161–167, 2015. 88. S. Ansari, A.R. Moghadassi, S.M. Hosseini, Fabrication of novel poly(phenylene ether ether sulfone) based nanocomposite membrane modified by Fe2NiO4 nanoparticles and ethanol as organic modifier. Desalination, 357, 189–196, 2015. 89. X. Teng, J. Dai, J. Su, G. Yin, Modification of Nafion membrane using fluorocarbon surfactant for all vanadium redox flow battery. J. Membr. Sci., 476, 20–29, 2015. 90. M. Palencia, M. Vera, B.L. Rivas, Modification of ultrafiltration membranes via interpenetrating polymer networks for removal of boron from aqueous solution. J. Membr. Sci., 466, 192–199, 2014.

3 Characterization of Membranes Derya Y. Koseoglu-Imer1,2*, Ismail Koyuncu1,2*, Reyhan Sengur-Tasdemir2,3, Serkan Guclu1,2, Recep Kaya1,2, Mehmet Emin Pasaoglu1,2 and Turker Turken1,2 Faculty of Civil Engineering, Department of Environmental Engineering, Istanbul Technical University, Maslak, Istanbul, Turkey 2 Dr. Dincer Topacık National Research Center on Membrane Technologies, Istanbul Technical University, Maslak, Istanbul, Turkey 3 Department of Nanoscience and Nanoengineering, Istanbul Technical University, Maslak, Istanbul, Turkey 1

Abstract

Membrane processes can solve a wide range of separation problems with a specifically designed membrane system. For determination of membrane type, different membranes must be characterized in terms of physical and structural properties. At the selection of membrane process, membrane characterization is the most important step because the membrane processes are designed according to physical and chemical properties of membranes. The ultimate goal of the membrane technology is to find the proper relationship between membrane fabrication– membrane morphology–membrane performances. Different techniques are used for membrane characterization, some of which are mostly prefered by membrane researchers and manufacturers. The most basic way to characterize membrane is to measure and classify its pore size and pore size distribution. The pore sizes are determined by different methods: microscopic, bubble pressure and gas transport, porosimetry, liquid-vapor equilibrium, liquid-solid equilibrium and gas-liquid equilibrium. Another important way is the chemical characterizations of membranes with Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, electron spectroscopy, atomic force microscopy, secondary ion mass spectrometry and surface hydrophilicity and surface energy. This chapter focuses on the physical

*Corresponding authors: [email protected], [email protected]; [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 1, (55–88) © 2017 Scrivener Publishing LLC

55

56  Nanostructured Polymer Membranes: Volume 1 and chemical characterization methods of pressure-driven membrane processes such as reverse osmosis, nanofiltration, ultrafiltration, and microfiltration. Keywords:  Membrane characterization, physical methods, membrane ­morphology, membrane chemical structure

3.1 Introduction The membrane processes have been increasingly used for drinking water and wastewater treatment. These processes have now become an attractive option for water and wastewater treatment and reuse of industrial and municipal wastewaters. Different polymers (e.g., polyethersulfone (PES), polysulfone (PS), cellulose acetate (CA), polyvinylidene fluoride (PVDF), etc.) could be selected as membrane material at membrane fabrication according to their good physical and chemical characteristics such as high filtration performance, good fouling resistance and environmental endurance as well as easy processing. The ultimate goal of the membrane technology is to find the proper relationship between membrane fabrication–membrane morphology–membrane performances. A rational guideline for membrane fabrication conditions is to achieve some specific membrane morphology, which enables the desired separation performance. For this reason, membrane morphology characterization is one of the indispensable components of the membrane research area. Physical and chemical properties of membranes can be characterized with different laboratory techniques. These techniques generally include the following aspects: pore size and porosity, surface and cross-section morphologies, hydrophilicity, mechanical strength, surface area, surface chemistry and charge. Even though these characterization techniques are not always necessary, in most cases, some of the above aspects are needed to characterize a specific type of membrane. It can be said that the ideal characterization method should be easy, accurate, and repeatable and fast and also should give a maximum possible number of data. This chapter focuses on the general physical and chemical characterization methods of pressure-driven membrane processes such as reverse osmosis, nanofiltration, ultrafiltration, and microfiltration.

3.2 Physical Methods for Characterizing Pore Size of Membrane The classical way of characterizing the membrane is to determine pore size and pore size distribution of the membrane. Many characterization

Characterization of Membranes   57 methods like permporometry, thermoporometry, mercury porosimetry, gas adsorption–desorption, nuclear magnetic resonance, gas-liquid porosimetry and liquid-liquid porosimetry, along with several microscopic techniques, both electronic, such as scanning and transmission electron microscopies, and atomic, such as atomic force microscopy, have been used to analyze the pore structure and pore size distribution of the membrane. Each of these methods has different features and principles of operation and needs different theoretical considerations to convert the direct results into pore sizes. In any case, the information given by all these methods must not be considered as competitive but rather complementary, since all results should contribute to a complete picture of pore characteristics [1].

3.2.1 Microscopy Microscopic observation and image processing of micrographs directly give visual information on membrane morphology such as surface pore shape and size, their distributions, pore density, surface porosity, crosssectional structure, etc. However, it has not been possible to obtain an indication of pore length or tortuosity [2]. The microscopy methods, including scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM), are the most direct methods to characterize the membrane pore structure. Many image processing algorithms were developed to characterize the membrane pore structure, such as pore radius, pore density, pore shape, pore length, tortuosity, and effective pore diameter [3]. A SEM image of porous membrane can give detailed visual information about the size, shape and structure of individual pores inside the membrane and at the membrane surface, which is the benefit of this technique over the porometry methods. The main disadvantage of SEM imaging is coating of the sample with conducting materials like Au/Pd or Pt. This thin coating layer can cause differentiation between the coated and original sample’s pore shapes and sizes. Furthermore, the dry conditions required for this microscopy technique can also change the morphology of the membrane surface when compared to sample wetted with solvent before porometry measurements. SEM can be used in various pore size characterization studies to visually inspect pore sizes and shapes. Electron microscopic investigations have revealed that the membranes not only differ with respect to the fine structure of the pores (sponge-, cell-, sphere-, or channel-like) but also in their mesoscopic structures (voids, cavern- or ­finger-like) and with respect to their homogeneity across the membrane [4]. Figure 3.1 shows

58  Nanostructured Polymer Membranes: Volume 1

Figure 3.1  SEM image of a PES flat sheet membrane. (Copyright 2015, with permission from MEM-TEK Research Center)

an example of a SEM image of flat sheet membrane fabricated with PES polymer. Pore size was measured during SEM analysis. The other microscopy method for pore size and shape determination of the membrane sample is atomic force microscopy (AFM) [5]. The AFM technique has proven itself to be a useful and versatile tool in the field of surface characterization. The AFM instrument itself is centered around a probe consisting of a sharp tip mounted on the end of a flexible micro­ cantilever. The topography of the sample can be profiled by faster scanning of this probe tip over a surface. An AFM instrument consists of a sharp probe mounted upon a flexible microcantilever arm [6]. The conductive material coating for SEM is not required for AFM, however the cantilever tip used in this microscopy device has not always been able to pick up surface pore structures conveniently. From a scan of the surface of a porous polymer membrane, the dimensions of individual pores can be measured to allow calculation of the mean surface pore size. Providing that enough pores are visible in the image, the surface pore size distribution can also be calculated. The AFM technique has many advantages over other imaging techniques, including: (1) a reliable 3D image of sample surface at very high resolution (subnanometer level); (2) it does not require any sample preparation or sample coating even for nonconductive samples; and (3) it can image the wet samples and samples inside liquid medium. One important limitation of AFM measurement is tip convolution. When the size of the scanning tip is comparable to the pore size, the tip cannot penetrate inside the pore and determine the exact dimensions. Tip convolution primarily affects dimensions at the pore bottom, and this is defined as the maximum penetration of the tip inside the pore [7].

Characterization of Membranes   59

3.2.2 Bubble Pressure and Gas Transport Both SEM and AFM techniques for pore size determination cannot give detailed information about the subsurface of membrane samples. So the ­seimaging methods cannot truly reflect the membrane samples’ selectivity capabilities. Liquid expulsion, buble point/gas transport methods have also been used to determine the membrane pore size. These porometry measurements can also give information about the pore size distribution (PSD) of membrane surface area [8]. When a pore is immersed in a liquid, the liquid fills the pore due to the surface tension of liquid, to achieve equilibrium with gravity force (see Figure 3.2). The filled pore will empty when an applied pressure exceeds the “capillary pressure.” The applied gas pressure passes through the emptied pores and this flow is measured together with the applied pressure. A method has been thoroughly developed to characterize the membrane pores and it is called the bubble point method. Several more sophisticated characterization methods related to the bubble point measurements have been developed. All of them use Washburn’s equation, which gives the pressure needed to displace one fluid by another through a pore of diameter D as:

P



4 cos (3.1) D

Pore diameter

Capillary force

Capillary force

Pressure

Wetting fluid

Figure 3.2  Schematic of applied forces in a membrane pore. (Copyright 2015, with permission from MEM-TEK research center)

60  Nanostructured Polymer Membranes: Volume 1 where P is the applied pressure, γ is the surface tension of the liquid, and θ is the wetting angle with the solid matrix of the membrane. When the fluid totally contacts and wets the membrane pores, cos θ is assumed as 1, leading to what is commonly called the Cantor equation. Thus, the equation becomes:

D

4 (3.2) P

The largest pore diameter is obtained from the pressure at which the flow of gas begins. This pressure is known as the bubble point pressure. In 1908, Bechhold was the first researcher to use this equation to evaluate pore sizes by measuring the pressure necessary to blow air through a waterfilled membrane [9]. This method is only able to discriminate the maximum pore size present in the pore distribution, corresponding to the minimum pressure necessary to blow the firstly observed air bubble. Furthermore, when the water is used as wetting fluid, the pressure necessary to evaluate pore sizes as, for example, 0.01 µm, can be as high as 145 bar. To avoid these high pressures, Bechhold et al. [9] and Erbe [10] used two different liquids instead of an air-liquid interface, appreciably reducing the surface tension. For example, using isobutyl alcohol-water interface, the measurement of pore sizes 40 times lower is achieved, as compared with the air-water interface. The latest measurement techniques include the use of perfloroethers which have zero contact angle and surface tension of as low as 16 dynes/ cm. Using this type of wetting fluid on the membrane surface enables measurement of very small pores as low as 0.015 µm. Pores are detected by gas flow. First the membrane pores are emptied by the applied gas pressure. The gas, usually air, passes through all open pores and progressively increases the pressure. Pore distribution of membrane can be measured with the gas flow rate. The methods involve a stepwise increase of the pressure and measurement of the gas flow rate through the membrane pores at every pressure step. This method has been called porometry. The flux of the displacing fluid is measured as a function of pressure, and a cumulative distribution of permeability as a function of pore size is determined using Cantor’s equation. This information is then related to a cumulative pore number distribution. Pore size measurement of the membrane sample relies on these steps: Wetting of the membrane sample with a proper wetting fluid Displacement of the fluid by applied gas pressure step by step. (The applied pressure and gas flow through the membrane are recorded; this is the so-called “wet run”)

Characterization of Membranes   61 Measurement of the gas transport as a function of increasing pressure through the membrane after the wet run; this is the so-called “dry run.” The largest pore to be emptied (at the lowest pressure at which flow is sensed) defines the so-called “bubble point.” After all pores have been emptied (up to the highest pressure achievable) during the “wet” run, a second “dry” run is performed on the same sample. From the complete data set, various flow-related pore size parameters, pore size distributions and gas permeability can be calculated. An example of the wet and dry run pore size measurement and pore number distrubution of a flat sheet membrane are given in Figures 3.3 and 3.4, respectively. The cumulative and differential flow distributions have been calculated from both wet and dry flow data. The Hagen-Poiseuille equation determines the flow rate, Q, of a fluid (air) with viscosity, η, and differential pressure, ΔP, through a number of cylindrical pores, N, of radius, r, and length, ℓ:

Q



N r4 P (3.3) 8

3.2.3 Porosimetry The term “porosimetry” includes the measurements of pore size, volume, distribution, density, and other porosity-related characteristics of a material. In this technique, a non-reactive and non-wetting liquid (generally mercury) are used and the mercury cannot penetrate into the pores until 35.000

Pore range

Flowrate, I/minute

30.000 25.000 Maximum pore size

20.000 Wet flow Dry flow

15.000 10.000 5.000

Minimum pore size (bubble point)

0.000 0.800 1.000 1.200 1.400 1.600 1.800 2.000 2.200 2.400 2.600 2.800

Figure 3.3  Wet and dry run measurement of a flat sheet membrane. (Copyright 2015, with permission from MEM-TEK research center)

100.000

18.000

90.000

Cum. pore size dist. 16.000 Diff. pore size dist. 14.000

80.000 70.000

12.000

60.000

10.000

50.000

8.000

40.000

6.000

30.000 20.000

4.000

10.000

2.000

0.000 0.000

0.100

0.200

0.300 0.400 0.500 Pore size, µm

0.600

0.700

Differential pore number, %

Cumulative pore number, %

62  Nanostructured Polymer Membranes: Volume 1

0.000 0.800

Figure 3.4  Pore number distrubution of a flat sheet membrane. (Copyright 2015, with permission from MEM-TEK research center)

the sufficient pressure is applied to force its entrance. The pore size can be determined based on the external pressure needed to force the liquid into a pore against the opposing force of the liquid’s surface tension [11]. Mercury porosimetry has been used for the characterization of various aspects of porous media, including porous membranes and powders. Its working range can be estimated from 30 Ǻ up to 900 Ǻ in diameter. Mercury intrusion porosimetry involves placing the sample in a special sample cup (penetrometer) and surrounding the sample with mercury. Mercury is a non-wetting liquid to most materials and resists entering voids, doing so only when pressure is applied. The pressure at which mercury enters a pore is inversely proportional to the size of the opening to the void. As mercury is forced to enter pores within the sample material, it is depleted from a capillary stem reservoir connected to the sample cup. The incremental volume depleted after each pressure change is determined by measuring the change in capacity of the stem. This intrusion volume is recorded with the corresponding pressure or pore size. By this technique, both pore size and pore size distribution can be determined [12]. The relationship of pressure and pore diameter is given by the Washburn equation. As mercury does not wet the membrane (because of its contact angle greater than 90°, cos θ will be negative), the Washburn equation is modified as follows:

D

4 cos (3.4) P

Characterization of Membranes   63 The contact angle of mercury with polymeric material is often 141.3° (the contact angle of mercury with most solids is between 135° and 142°) and the surface tension at the Hg/air interface is 0.48 N/m. Hence the equation will be:

D



14984 (3.5) P

where D is expressed in nm and P in bar. In this equation, it is assumed that the membranes have capillary pores.

3.2.4 Liquid-vapor Equilibrium Liquid-vapor equilibrium is also called the adsorption-desorption method (Barett-Joyner-Halenda (BJH) method). Gas adsorption is one of the most popular methods and is generally used for the surface characterization and structural properties of porous materials, allowing the determination of their surface area, pore volume, pore size distribution and adsorption energy distribution. Generally, nitrogen is used as adsorbent gas but other adsorbents, such as argon and benzene, are also used. Adsorption–­ isotherm versus relative pressure is drawn and assuming capillary condensation the data are analyzed. The vapor pressure, p, of the adsorbent liquid in the pore of radius rp is given by the following Kelvin equation:

p p0

2 V Cos (3.6) rp RT

where p0 = saturation vapor pressure, γ  = surface tension of the adsorbent liquid, V = molar volume of the adsorbent liquid, R = the universal gas constant, T = absolute temperature, and θ = contact angle. For the liquid nitrogen, when we assume θ = 0°, the equation becomes the following:

rP

4. 1 (3.7) p log p0

64  Nanostructured Polymer Membranes: Volume 1 Assuming that the radius of all pores are smaller than rp at a given relative pressure p/p0, the cumulative pore volume curve versus rp can be drawn. The thickness layer (t) is generally added to rp and is obtained above to calculate more precise pore radius. Depending on the pore structure, actual sorption-desorption curves are different due to hysteresis. It is important to choose which branch of the isotherm is used during the analysis [12].

3.2.5 Liquid-solid Equilibrium (Thermoporometry)

endo>

dQ / dt (mW)

Thermoporometry was first introduced by Brun et al. [13] in 1973, and various porous structures have been examined with this technique. In the thermoporometry measurements of polymeric membranes, generally water is used as liquid imbibed inside the pores, as this system of waterfilled pores has the closest resemblance to the practical situation. During the measurements, a liquid-solid transition take place at least one time, and a volume expansion of water occurs with the change of state [14]. The basic principle of thermoporometry is the reduction of the freezing (or melting) point of liquid, which shows strong curvature of the solid-liquid interface present within small pores. Brun et al. [13] identified a full thermodynamic of this phenomenon. According to Brun et al., the size of a confined ice crystal is inversely proportional to the degree of undercooling (ΔT). In porous matrix, the presence of finely tuned ice thus melts at temperatures below the ambient melting point of the ice. The smallest size of the ice crystals or the smallest pore size can be described from assumptions for thermodynamic descriptions of curved surfaces set properly. Porous substrate analysis of the melting diagram (Figure 3.5) can be monitored in a differential scanning calorimeter (DSC) and the pore size and pore volume are calculated using an equation derived by Brun et al. [13].

B

A

0

T(°C)

Figure 3.5  DSC thermograms for a narrow pore size distribution (A) and for a broad pore size distribution (B). (Reprinted from [14] with permission from Elsevier)

Characterization of Membranes   65 A cylindrical pore having radius r in the equation is filled with water for the solid-liquid transition. Heat effect occurs during the transition, and the pore volume at a certain undercooling (i.e., pore radius) is calculated. In Equation 3.9, the melting heat is given as a function of temperature in this procedure. Similar equations may be formed for the solidification process [14].

r



Wa

0.68 (32.33/ T ) (3.8)

0.155 T 2 11.39 T 332 (3.9)

Here r represent the pore radius (nm), ΔT is the characteristic undercooling (°C), and Wa is the apparent transition energy (J/g).

3.2.6 Gas-liquid Equilibrium (Permporometry) One of the most promising methods is permporometry, where a mixture of non-condensable gas and condensable vapor is fed to a porous membrane and the permeation rate of non-condensable gas is measured [15]. This method is mainly based on the capillary condensation of liquids in micropores. Permporometry is suitable for active pores with diameters ranging from about 1.5 nm to 0.1 μm in porous media which have asymmetric and/ or composite structure. This method is a relatively new technique and controls the blocking of pores by capillary condensation and simultaneous measurement of the gas diffusion flux through the remaining open pores [12].

3.2.6.1 Capillary Condensation Physisorption processes of condensable vapor are generally divided into different stages [16]. In the beginning of the process, it is assumed that only adsorption of vapor molecules into the pore wall is occurring. This adsorption is limited by a layer, which is called the “t-layer,” with a maximum thickness of the order of a few molecules. Capillary condensation is followed at relatively high pressure, i.e., the condensation of a vapor, commencing in the smallest pores. While the pressure gradationally increases, the whole system reaches equilibrium conditions with the filling of wider pores with condensate. In the Kelvin formula (Eq. 3.10), capillary pores start filling at relative pressure depending on the radius of pores.

ln pr

RT

Cos

1 rk1

1 (3.10) rk2

66  Nanostructured Polymer Membranes: Volume 1

2rk

(ii)

(i) Pr1 = 1

Pr2 < 1

Pr3 < Pr2 < 1

v v

2rk

(iii)

(iv)

2rp

t Pr4 = 0

2rp = 2rp + 2t

Figure 3.6  Steps in the desorption process: (i) liquid-filled pore, at saturation pressure, (ii) just before desorption starts, pore is still filled, (iii) just after evaporation is complete, t-layer remains, (iv) after complete desorption. (Reprinted from [17], with permission from Elsevier)

In the Kelvin reaction equation, is generally assumed to be zero. This simplification of Equation 3.10 allows us to directly use a pore radius at a certain pressure [16]. Similar adsorption behavior is shown at desorption. A pore filled by liquid will not empty before the vapor pressure falls below the equilibrium pressure in Equation 3.10. The desorption process is ­schematically shown in Figure 3.6. Because of the hysteresis phenomenon, the adsorption and desorption processes are not defined with the same tortuosity of liquid-gas interface [14]. Relative pressure strongly relates to the thickness of t-layer. This thickness is calculated from the different adsorption experiments which are performed using the same homogeneous material with the porous medium. This approach works very well and therefore an approximation is used to calculate the t-layer thickness directly from permporometry data. Capillary condensation provides the possibility to block a certain size with liquid just by setting the relative pressure. In permporometry, this principle is combined with the measurement of free diffusive transport through the open pores. When the pressure is reduced, pores having a size related to the vapor pressure applied are emptied and become available for gas transport. The distribution of active pores can be found by measuring the gas transport through the membrane upon decreasing the relative pressure. Similar measurements can be done during the adsorption process, but the equilibrium of the adsorption process is difficult to reach and therefore a quantitative analysis of the desorption process is preferred [14].

Characterization of Membranes   67

3.3 Membrane Chemical Structure 3.3.1 Fourier Transform Infrared Spectroscopy (FTIR) Fourier transform infrared (FTIR) spectroscopy is widely used in the structural characterization of membrane surfaces. While the method does not have the capability of providing the precise, atomic-resolution molecular structure, it is exquisitely sensitive to conformational changes occurring in functional transitions or upon intermolecular interactions [18]. Attenuated total reflection Fourier transform infrared spectroscopy (ATRFTIR) allows for infrared (IR) analysis of surfaces. The IR spectrum can provide the determination of vibrational frequencies and transition intensities of most molecules (with the exception of diatomics such as N2 and O2), including characteristic functional group frequencies. Knowledge of vibrational frequencies of functional groups (or reference spectra) allows for chemical identification of at least a class of compounds (e.g., aromatic amides), which may be less apparent in X-ray photoelectron spectroscopy (XPS) spectra [19]. Example images of ATR-FTIR spectra for a variety of membranes are given in Figure 3.7. The IR radiation typically penetrates to 1 μm inside

12 Desal5DL 10

Absorbance

8

Desal51HL

6 4

NTR7450

2 D71

0 1800

1600

1400

1200 1000 Wavenumber / cm–1

800

600

400

Figure 3.7  An example of ATR-FTIR spectrum. Ar-SO2-Ar typical frequencies are ν(S = O asym) = 1325 cm–1, ν(S = O symm) = 1140 cm–1. (Reprinted from [20] with permission from Elsevier)

68  Nanostructured Polymer Membranes: Volume 1 Evanescent wave

IR radiation incident beam

Polymer membrane

IRE

Totally internally reflected light

Figure 3.8  Diagram of ATR-FTIR. (Reprinted from [20] with permission from Elsevier)

the surface. This is a disadvantage because this thickness is deeper than the active layer thickness of most composite membranes, so the top layer is not necessarily isolated [20]. During the analysis of a membrane sample, the sample is pressed against an internal reflection element (IRE) which is a high refractive index material, maybe a block of zinc selenide (Figure 3.8). Total internal reflection can occur at the interface between media with different refractive indices. The ATR-FTIR spectroscopy technique is used to characterize a polymer membrane and to monitor the modification and fouling of membrane. For example, in a membrane sample, ATR-FTIR can show albumin adsorption on membrane surface. In addition, it was also shown that cake and pore fouling by proteins has been examined by using standard infrared spectroscopy [21].

3.3.2 Raman Spectroscopy The main spectroscopic techniques used for detecting vibrations in molecules are based on either infrared absorption or Raman scattering. In Raman scattering, it is possible to obtain chemical structures of membrane surfaces, to identify substances from the characteristic sprectral patterns, which are called fingerprints, and to determine the amount of substances in a sample quantitatively or semi-quantitatively. Besides, in Raman spectroscopy, mapping of chemical compounds and depth profiling of the chemical composition are possible. It requires minimal sample preparation. It is possible to use wet samples without the disturbing effect of water in the spectrum. This property makes Raman spectroscopy superior over infrared spectroscopy. Its analysis depth is 1–5 µm and spatial resolution is between 0.5–1 µm. However, the employment of the Raman technique is not widely used due to problems with sample degradation and fluorescence.

Characterization of Membranes   69

(1)

782

Cleaned Intensity

Laser

1807 1587

Sample

1149 1110 1075

With recent advances in the technology, the instrument has become simplified and some of the problems have been reduced [22–24]. Raman spectroscopy technique usually employs a laser source and s­ cattered light and ­analyzes in terms of wavelength, intensity and polarization. When a ­sample is irradiated by using monochromatic light source, most of the incoming radiation (Rayleigh scattering) is scattered by the sample at the same wavelength. Some of this scattered light is directed to the detector, where Raman spectrum is recorded. In this spectrum, light at the original laser (or Rayleigh) frequency is shown, and features obtained are unique to the sample (Figure 3.9). Lines which are formed by absorption and re-emission of light are called parent lines, whereas lines consisting of not only absorption/­emission but also vibrational excitation or de-excitation are called weaker lines (Raman lines). The difference between Raman lines and parent lines gives the vibrational frequency. The IR and Raman techniques are complimentary to each other in terms of membrane surface functional group or bond analysis since IR-inactive vibrations are generally Raman active. Raman scattering is capable of detecting elestic vibrations of an entire nanoparticle, therefore Raman scattering is good for detecting nanoparticles on the membrane surface [23–26]. Fourier transform raman spectroscopy (FT-Raman) and micro-Raman spectroscopy techniques are applicable to membrane surface characterization. In FT-Raman, it is possible to characterize overall membrane surface; however, the porous asymmetric membrane matrix cannot be analyzed. The micro-Raman system enables the chemical mapping and imaging of a sample surface. In this technique, even though Raman scattering is weak, very small amounts of materials can be detected [23].

2 mW–1s–1 Fouled

(2) (3)

Virgin 2000 1800 1600 1400 1200 1000 800 Wavenumber / cm–1

600

400

200

Detector

Figure 3.9  Schematic of Raman spectroscopy. (Adapted from [27] with permission from Elsevier)

70  Nanostructured Polymer Membranes: Volume 1

3.3.3 Energy-dispersive X-ray Spectroscopy (EDS) 3.3.3.1 Basics of EDS Energy dispersive X-ray spectroscopy is a useful tool to obtain element analysis and elemental mapping of a surface. The scanned area is up to a few micrometers and the scan depth between 0.5 and 5 µm. Generally, the X-ray microanalysis system is added to SEM or environmental scanning electron microscopy (ESEM) and is called energy dispersive X-ray detector (EDX) or energy dispersive spectrometer (EDS). It is a very beneficial addition to SEM, because it provides the opportunity to analyze an observed point in SEM images. Also, electron beam can be focused on desired points, particles, lines or regions to determine an element specification. When an electron beam is focused on the surface, there are some signals emitted like secondary electrons, back-scattered electrons, X-ray photons, etc. All of them can be evaluated to characterize the sample. The EDS system uses X-ray photons to characterize and it is possible to detect elements down to beryllium. EDS only gives elemental information, not chemical information like bonds and functional groups [28]. Overlapping of energy levels emitted from different elements may cause detection of wrong elements, so evaluation of EDS results should be done carefully. In a typical element, K, L, or M energy-level shells produce X-rays. For instance, overlapping energy levels can be seen on Ti-Kα and Ba-L, Mn-Kβ or Fe-Kα, etc. [29].

3.3.3.2 Applications of EDS in Membrane Characterization Energy dispersive spectrometer (EDS) analysis can be helpful for both membrane characterization and foulant characterization. For example, Sile-Yüksel et al. [30] used EDS analysis to determine the location of silver nanoparticles in different polymer membrane matrices. They measured the cross-section elemental mapping, as seen in Figure 3.10, and saw that with increasing solvent viscosity, silver nanoparticles remain deeper inside the membrane, and with decreasing viscosity, silver nanoparticles are much nearer to the membrane surface. This location affects the antifouling characteristics of membrane. Corneal et al. [31] coated tubular ceramic membranes with manganase oxide nanoparticles. They examined the coating layer using SEM-EDS. They observed with the help of EDS analysis that the manganase oxide nanoparticles were not just successfully placed on the surface but also penetrated into the membrane matrix. Hofs et al. [32] fabricated a commercial thin film nanocomposite reverse osmosis membrane and a regular

Characterization of Membranes   71

(a)

(b)

(c)

(d)

(e)

(f)

Figure 3.10  The cross-section SEM-EDS images of nanocomposite membranes. (a) SEM image of 0.09 AgNP/PS membrane; (b) SEM image of 0.09 AgNP/PES membrane; (c) SEM image of 0.09 AgNP/CA membrane; (d) EDS mapping of PS/AgNP membrane; (e) EDS mapping of 0.09 AgNP/PES membrane; (f) EDS mapping of 0.09 AgNP/CA membrane. (Reprinted from [30] with permission from Elsevier)

thin film composite membrane. They used EDS analaysis to determine the nanoparticle content of the membrane and the chemical content of foulants after filtration. They could not observe nanozeolite in membrane using EDS due to the low amount of it. After filtration with tap water, they observed that the iron on membrane surfaces was about 0.2% and 0.4% for nanocomposite and regular thin film composite membrane, respectively. Also, a trace amount of other elements, such as Al, P, and Si, were found on the surface. The EDS technique can also be used to determine the foulant elemental compositon. Soffer et al. [33] used EDS analysis to show colloidal iron fouling on ultrafiltration membrane surface. Long-term fouling of a reverse osmosis membrane was examined by Melián-Martel et al. [34]. They used EDS to determine foulants on membrane and observed Si, S, Al,Fe, Na, Cl and Mg, as seen in Figure 3.11. Also, they conducted EDS analysis on the cross section of membrane sample. They found that the materials containing sulfur penetrate inside membrane matrix.

3.3.4 X-ray Photoelectron Spectroscopy (XPS) In X-ray photoelectron spectroscopy (XPS), the sample surface is subjected to X-ray radiation capable of removing electrons from the inner shells of the atoms (except H and He). The amount of emitted electrons is

72  Nanostructured Polymer Membranes: Volume 1

(a1)

(a2)

Figure 3.11  Fouled membrane surface (a1) and EDS spectrum of this surface (a2). (Reprinted from [34] with permission from Elsevier)

recorded as a function of binding energy. Moreover, the kinetic energy of these photoelectrons is measured in an electron spectrometer and used to identify the elements present by calculating the electron binding energy. Surface depths of 0.5–10 nm can be probed, depending on incident beam angle [35]. The sample is irradiated with low energy, monochromatic aluminum anode X-rays, causing the emission of photoelectrons and Auger electrons. Determination of their number and energies in spectral peaks allows the measurement of the number and kind of atoms at the surface [19]. The XPS provides information about surface layers or thin film structures and is used for many industrial applications, including the polymer surface modification, catalysis, corrosion, adhesion, semiconductor and dielectric materials, electronics packaging, magnetic media, and thin film coatings, used in a number of industries. The greatest advantage of XPS is the shallow probing depth that is required and its ability to characterize the elemental composition of the membrane surface. However, the chemical identity of the polymer (or additives) cannot be deduced based on elemental composition alone [20]. XPS can be used for both qualitative and quantitative analyses of membranes. The first step is taken in characterizing the surface chemistry. On the other hand, many of the more prominent ions from metal or semiconductor surfaces are singly charged atomic ions, which makes the allocation of mass numbers of slightly charged atomic ions slightly easier. The various modules necessary for analysis by XPS are: a source of the primary beam, sample inlet system, an electron energy analyzer and detection system, all contained within a vacuum chamber; and a data system, which is

Characterization of Membranes   73 nowadays an integral part of the system. Energetic X-ray photons, commonly in the range of 1000–2000 eV, are used as a source for core electron ionizations [36].

3.3.5 Electron Spectroscopy 3.3.5.1 Auger Electron Spectroscopy (AES) Atoms that are bombarded with higher electrom beam have undergone inner shell ionization and become excited by the primary electron beam, relax when electrons from higher energy levels drop into the vacant inner shells. When electron fills vacant sites (K-shell) from higher energy level (Lıı shell), the energy difference (excess energy) can be released as X-rays or can be given to a third electron such as the Lııı shell of the same atom; in this case the Auger transition process occurs (Figure 3.12). The emitted electron which has lower energies (100–1000 eV) is called as Auger electron. Auger electrons are the characteristics of parent atom since the characteristic kinetic energy of Auger electrons depend on the binding energies of core levels within the target atom. Except for light elements (hydrogen, helium), the Auger yield is small and this makes the method extremely useful for surface analysis [26, 37]. Using electron beams rather than photons to produce Auger spectra is beneficial for focusing low energy electron beams from field emission sources to a dimension of 10–50 nm. Especially for the analysis of upmost surface layers, AES has great application potential due to its improved lateral spatial resolution. Fine structures of surfaces can be identified better than the XPS method in AES [23]. The lowest detection limit for AES can be between 0.1–1 atom %. The surface sensitivity of AES depends on

LIII LII LI

e–

KLIILIII K (a)

(b)

Figure 3.12  Auger transition illustration (a) excitation; (b) electron. (Reprinted from [37] with permission from Elsevier)

74  Nanostructured Polymer Membranes: Volume 1 the kinetic energy of electrons to be measured. When Auger electrons are detected with SEM, it enables chemical mapping of surfaces. Depth distribution of elements can also be accomplished in AES by sputtering each layer with an energetic ion beam; however, when sample thickness is higher than 2 µm, the technique becomes very time-consuming and resolution is worse due to uneven sputtering. The AES technique requires a vacuum environment and sample must be dried [23, 37]. Utilization of AES is limited by high energy electron beam bombardment, which deteriorates the structure of nonconducting samples such as polymeric membranes. There are literature studies related to the utilization of AES for the surface composition of Pd-based membranes [38].

3.3.5.2 Electron Energy Loss Spectroscopy (EELS) For the analysis of adsorbates on the crystal surface, electron energy loss spectroscopy (EELS) is one of the most important applications of surface vibrational spectroscopy. Bonds such as C-H and C-C have characteristic vibrational frequencies in EELS. In the EELS technique, the sample is placed in ultrahigh vacuum conditions. Highly monoenergetic electron beams are directed toward the surface and the angular distributions and energy spectrum of backscattered electrons are measured. Incident electrons have kinetic energies in the range of eV. Under these conditions, the electrons can penetrate only the outer part of the surface (three or four atomic layers of the crystal) [39, 40]. The method can be used in transmission electron microscopy (TEM). In this case, EELS can reach very high spatial resolutions. The technique allows high resolution elemental mapping and the measurement of local electronic structures for the determination of local chemical bonding within an interface or in a defect. With EELS, besides chemical composition characterization, it is also possible to find specimen thickness, and the mechanical and electronic properties of the materials [26, 40].

3.3.6 Atomic Force Microscopy (AFM) 3.3.6.1 Basics of AFM Atomic force microscopy (AFM) provides the opportunity to examine surface morphology and interaction forces in the nanometric scale. AFM uses a sharp probe as a stylus to obtain data from the surface and three-­ dimension images having nanometric scale resolution are derived from this data. Surface type and different media are not an important limitation in AFM. Researchers can study several surfaces, like conductive or

Characterization of Membranes   75 insulate, soft or hard, bulk or powder and organic or inorganic, in several environments like vacuum, air and liquid. The AFM technique determines geometrical morphology, adhesion, friction, surface impurities, type of different spots of the surface, elasticity, magnetic, different chemical bonds, distribution of surface electrical charges, and polarity of various electrical points [41]. These potential applications also make AFM an all-­purpose device to examine the membrane surfaces, providing high resolution images, surface roughness, surface pore size, and pore size distribution, and measurement of foulant-membrane interaction forces. The capability of analyzing in different media, like liquid or humid air, allows observing the effect of solution pH, ionic strength, etc., on membrane surface m ­ orphology [6]. A basic schematic representation of an AFM system is given in Figure 3.13. A cantilever is attached to a chip. Cantilevers are generally V-shaped, like in Figure 3.13, or they are rectangular diving-board shaped. Generally, a sharp tip used as the probe of interaction forces attaches close to the cantilever’s free end. This probe is contacted to the sample surface using piezocrystal. Any motion in this direction has traditionally been called the z-axis. A laser beam is sent to the backside of the cantilever and reflected laser beam is collected by a position-sensitive photodetector. Any movement of the cantilever will be detected by a position-sensitive photodetector by changing the reflection spot of the laser beam onto it [42]. There are a lot of imaging modes in AFM. The most common modes are contact, non-contact and tapping modes. Contact mode imaging is the

Laser diode Light path

Photo detector

Probe Sample Cantilever

Figure 3.13  Basic schematic presentation of an AFM setup. (Reprinted from [6] with permission from Elsevier)

76  Nanostructured Polymer Membranes: Volume 1 easiest operation mode in AFM, which was used in early instruments. The probe is always contacted to the surface in this mode. There are two submodes in this mode named constant force and variable force. In constant force mode, there is a mechanism to keep the cantilever constant. While the cantilever deviates, z height is modified to return to the set point. Recorded change in z-position is used to obtain a topographical image. In variable force imaging, this mechanism to keep the cantilever constant is shut down. So the z height stays fixed and the deviation is recorded and analyzed to obtain a topographic image. Contact mode is preferred in samples having rigid and flat surfaces. But there are some disadvantages. While the probe passes by mounds on the surface, lateral forces can occur and may damage the probe. Also, some adhesive and frictional forces between probe and sample can cause lateral forces which ends with sample or probe deformation [42]. Tapping mode was developed to cope with restrictions of contact mode imaging. In this mode, a cantilever is oscillated at a value close to its resonant frequency. During oscillation, the probe will contact with the ­surface and then will separate from the surface due to amplitude. As a result, ­lateral forces will be reduced in contact mode. The frequency of ­oscillation will change due to surface properties like mechanical and adhesive. This will cause a shift in the phase signal between the drive frequency and the actual frequency of cantilever. This phenomenon makes it possible to obtain phase images of the surface showing changes in the material properties. But this data is qualitative data and it’s hard to get quantitative data from the phase analysis because of complex parameters, including adhesion, scan speed, load force, topography and the material properties [42]. In non-contact mode, the cantilever is oscillated at a much smaller amplitude than in tapping mode. When the prob gets closer to the surface, some interactions, like van der Waals and electrostatic forces, occur between sample and probe atoms, causing some shift in the frequency of cantilever oscillations. There is much greater surface resolution because the probe does not touch the surface and the interaction between probe and surface is minimized. But in practice there are some limitations. Most hydrophobic surfaces have a water layer on them. During AFM analysis in air medium, these water layers may be thicker than the range of van der Waals forces. This situation will negatively affect image quality and resolution [42].

3.3.6.2 Applications of AFM in Membrane Characterization Atomic force microscopy (AFM) can be used in membrane characterization in three common ways: surface roughness characterization, pore size

Characterization of Membranes   77 measurement, and fouling tendency characterization by measuring interaction forces between foulant and membrane. Pore size measurement is explained in previous sections. In this section surface roughness characterization and fouling tendency measurements will be explained. Surface roughness plays a major role in the fouling of membrane surfaces due to the effect of roughness on the surface area available to foulants. In general, it has been observed that the greater surface roughness caused the greater surface area available for foulants to adhere to, so much effort has been put into reducing fouling by reducing surface roughness. However, the influence of surface roughness is not simple and the amount of surface area available for fouling by particulates can depend upon the interplay between the shape and size of surface topography and the size scale of the foulant particulates themselves and their own surface roughness [6]. Generally, increase in surface roughness means increase in surface area, which results in much more contact area between the membrane surface and foulant molecules. Increasing the contact area means greater adhesive forces between them. So this affects the fouling tendency and cleaning performance. Surface roughness is the diversity in the height of an area and there are statistical analyses of this diversities in height. The most common surface roughness parameters are average roughness (Ra, Sa) and the root mean squared surface roughness (RMS, Rq or Sq). The Rx parameter is a two-dimensional roughness parameter made from a line scan. The Sx paramteres are derived from three-dimensional measurements from surfaces [6]. AFM is a very common tool to determine surface roughness in membranes. There are a lot of studies in the literature using AFM to d ­ etermine roughness. Generally, AFM is known as a microscopy technique to visualize the surface. But besides this ability, it can be used to examine membrane surface and foulant interactions. Some examples found in the literature are given in this section to show the ability of AFM in chemical analysis. For instance, Mi and Elimelech [43] used a silica particle probe to study scaling of dissolved silica in cellulose triacetate and polyamide membranes. They found adhesion forces were larger in cellulose acetate membrane than polyamide membrane. In another study, Guillén-Burrieza et al. [44] used a CaCO3 particle probe to measure the adhesion force between surface and particle to find fouling tendency. An SEM image of a CaCO3 particle probe is shown in Figure 3.14. The PVDF membranes were found to have much more adhesion force than PTFE membranes. Bowen et al. [45] fabricated the sulphonated poly(ether ether) ketone into polysulphone membrane matrix. They found lower adhesion forces, as given in Table 3.1. They used AFM technique to find these forces. Low

78  Nanostructured Polymer Membranes: Volume 1

Figure 3.14  SEM image of a CaCO3 particle probe. (Reprinted from [44] with permission from Elsevier)

Table 3.1  Normalized adhesion forces for a range of colloid probes at a PSU/ SPEEK membrane. (Reprinted from [46] with permission from Elsevier) Mean F/R SPEEK/PSU

(mNm–1) PSU

Silica

0.84 ± 0.35

6.5 ± 0.8

Cellulose

0.51 ± 0.23

2.2 ± 0.32

Materials

Latex

2.1 ± 0.52

14.3 ± 0.8

BSA

1.4 ± 0.64

12.3 ± 1.4

Yeast

3.2 ± 0.85

9.5 ± 1.5

adhesion values means less fouling tendency during filtration. As seen in the literature examples, AFM not only provides data on surface roughness and surface 3D images, but can also be used to measure adhesive forces of several materials.

3.3.7 Secondary Ion Mass Spectrometry (SIMS) Secondary ion mass spectrometry (SIMS) is based on the observation that charged particles (secondary ions) are ejected from a sample surface bombarded by a primary beam of heavy particles. Typically secondary ions have kinetic energies of the order of 20 eV, but different ions have different energy distributions. In particular, the molecular/cluster ions have significantly narrower distributions than atomic ions and they peak at

Characterization of Membranes   79 Unmodified PVDF

Intensity

F

10

20

30

40

50

60

F

10

15

20

m/z

70

80

Modified PVDF (ETNA01PP) 25

30

35

40

45

Figure 3.15  SIMS spectra of unmodified PVDF membrane and the modified PVDF membrane (ETNA01PP). (Reprinted from [50] with permission from Elsevier)

slightly lower energy. Flosch et al. [47] characterized the modifications on the surface of poly(vinylidene difluoride) membranes by XPS and SIMS. Significant differences in surface chemistry were observed between two membranes having hydrophilic and hydrophobic surfaces. The advantage of SIMS compared to XPS is that precise molecular information of polymers can be obtained with SIMS. It is also possible to analyze the molar masses of polymer samples up to 10,000 g/mol. Several sample types can be analyzed (powders, liquids and solids) and special pretreatment is not needed. However, the analyis is performed under ultrahigh vacuum, which means that to ensure stability under vacuum conditions membrane samples must be analyzed in their dry state [48]. This method is widely used because of its great sensitivity to all elements, from hydrogen to uranium. It can detect an impurity in a sample (detection limit below 0.1 ppb), and allows the adsorption, adhesion, corrosion, contamination and biocompability of materials to be studied. Thus, SIMS is used for analyzing the grafting of polymers on the surface of various membranes [49]. An example of analysis is presented in Figure 3.15.

3.3.8 Surface Hydrophilicity and Surface Energy 3.3.8.1 Determination of Hydropilic/Hydrophobic Nature of Membranes Material hydrophilicity is a very important parameter as it affects the solute-membrane interactions. Hydrophobic molecules or particles (proteins, colloids, etc.) are present in the feed side and they cause fouling problems in the membrane applications. The interaction energy between

80  Nanostructured Polymer Membranes: Volume 1 hydrophobic molecules and a hydrophilic surface is lower than in a hydrophobic-hydrophobic system, so it is easier to regenerate the membrane surface by washing in the initial case. The hydrophilic-hydrophobic nature of a membrane may be determined by measuring the contact angle or by using the capillary elevation balance. Water is commonly used for contact angle measurement analysis.

3.3.8.2 Contact Angle Measurement by Drop Profile Analysis 3.3.8.2.1  Sessile Drop Method The basic principle of the sessile drop method consists of placing a drop of liquid on the membrane surface. The wettability of the membrane surface is characterized by the contact angle between the solid surface and the tangent to the liquid surface at the contact point. Generally, this angle is measured inside the liquid, as in Figure 3.16. Placed drop volume on the membrane surface must have predominant wetting effects over gravitational effects. The radius re corresponding to the volume of the sessile drop must be by the following equation: L

re



L .g

(3.11)

where re = the radius of the liquid drop (m), = the surface tension of the liquid (Nm–1), L = the density of the liquid (kg m–3), and L g = the acceleration due to gravity (m s–2). The contact angle may be obtained from a drop on the solid surface. The drop geometrical parameter is identified with the following symbols: h = height of the drop at the apex, re = radius of the drop, and h Liquid (water) drop

re

Membrane

Figure 3.16  Diagram of the sessile drop method to measure contact. (Reprinted from [51] with permission from Elsevier)

Characterization of Membranes   81 the cosine of the angle may be found from the following relationship:

1

b re

1

b re

cos



(3.12)

There are three different situations between the solid and liquid in terms of hydrophilicity-hydrophobicity relations. If = 0 then the solid is considered to be perfectly wettable by the liquid (hydrophilic if the liquid is water), if surface is more or less wettable then < /2, and if surface is more or less not wettable (hydrophobic if the liquid is water) then > /2. In the sessile drop method, the sample must be dried in advance because the presence of water in the membrane structure affects the value of the contact angle. This preconditioning may have the disadvantage of inducing changes in the surface properties of the membrane which may affect the angle measured [51]. 3.3.8.2.2  Captive Bubble Method The captive bubble method can be used when membrane properties do not allow the contact angle to be measured by placing a drop on the surface or if it is not appropriate to dry the sample. The surface to be analyzed faces downward after immersing into the water, as shown in Figure 3.17. An air bubble is trapped on the lower surface of the membrane using a microsyringe. In the same manner as the sessile drop method, the contact angle is calculated by determining the geometrical parameters of the air bubble. In the captive bubble method, preconditioning (membrane drying) of the membrane is not required. However, the captive bubble method is Membrane

Cell

Air bubble

Air injection

Analyzed surface

Liquid (water)

Figure 3.17  Diagram showing the captive bubble method to measure the contact angle between a liquid (water) and a membrane. (Reprinted from [51] with permission from Elsevier)

82  Nanostructured Polymer Membranes: Volume 1 sensitive to parameters such as roughness, porosity, and heterogeneity of the surface [51]. 3.3.8.2.3  Capillary Elevation Balance The last two methods previously mentioned are appropriate for plane surfaces like flat sheet membranes. But hollow fibers have nonplanar structure and another method can be considered which includes measuring the weight gained by a material placed in contact with a liquid as a function of time [51]. Porosity character and the attraction between material and liquid affects the speed of penetration of a liquid by capillarity. The weight (m) is given in Washburn’s differential equation as a function of time (t) by:

dm dt

2 L .R. L .cos

1 . m

4.

(3.13)

where L is the surface tension of the liquid (N m–1), the density of the liquid (kg m–3), L the viscosity of the liquid (Pa s), the advancing contact angle between the liquid and the solid (°), R the geometrical parameter characteristic of the porous structure of the material. By integration, the following is acquired:

m2

L

2 L

R 2

cos

t (3.14)

But this method is theoretically practicable in the case of can be found by the plot of:

2 m2 t L2

< 90. Cos

f ( L ) (3.15)

and so contact angle of liquid can be found. Some researchers used this method to identify surface tension of poly(tetrafluoroethylene), poly(ethylene) and poly(propylene) membranes using hexane, toluene, cyclohexane [52]. It should be emphasized that the contact angle values are linked to measured technique. Furthermore, they also depend on sample preperation, local roughness, porosity, heterogeneity in surface chemical composition, etc. For

Characterization of Membranes   83 instance, more porous membranes have a tendency to be more hydrophilic. Also, measured contact angle values should be correlated with membrane surface charge. To establish a comparative classification of membranes, it is better to use methods which lead to other side parameters being equal [51].

3.3.8.3 Surface Energy 3.3.8.3.1  Characterization of Membrane Charge In the case of charged membranes, the charges are distributed on the membrane surface and on the pore surface. When a charged solute passes through a membrane, the charges located on the pore surface can be important. It is possible to have some information about the net charge of the pore surface, and therefore about the charge distribution inside the electrochemical double layer, by measuring the zcta potential. The zeta potential represents the potential located at the shearing plane between the compact layer attached to the pore wall and the mobile diffuse layer [53]. The presence of charged groups on membrane surface affects the fouling mechanisms of membranes during the filtration operation. The measuring method must be adapted according to charge places whether it is on the surface or inside the pores. In other cases, membrane specificities are taken into account, in particular for ultrafiltration membranes in which the pore radius (≈ 10 nm) and the mean flux are small [51]. The techniques used are electrokinetic and consist of measuring: 1. Electro-osmotic fluxes for microfiltration membranes: solvent flux through membrane pores as a result of the application of an electrical potential difference; and 2. Streaming potentials for ultrafiltration and microfiltration membranes: An electrolyte solution is set in motion with respect to the membrane or pore surface, a double layer formation occurs, and a flux of ions is created, giving rise to the streaming potential. The above-mentioned methods may help to determine the same quantity, which is known as the zeta potential (ζ) [54, 55]. 3.3.8.3.2  Relation between Streaming Potential and Zeta Potential Using the Helmholtz-Smoluchowski equation, the zeta potential calculated from the slope of the plotting curve leads to a straight line, as shown in Figure 3.18.

E P

0

r

Kl

(3.16)

84  Nanostructured Polymer Membranes: Volume 1 Virgin membrane

10

Membrane after adsorption

5

BSA in solution

0 –5

2

3

4

5

6

7

8

pH

–10 –15 –20 –25 –30 (mV)

Figure 3.18  Zeta potential curve. (Reprinted from [51] with permission from Elsevier)

where ΔP = transmembrane pressure, ΔE = the streaming potential (V), = electrical permittivity of vacuum, F·m−1, 0 = relative permittivity of the liquid, dimensionless, r = zeta potential, V, = the dynamic viscosity of electrolyte solution (Pa.s), and Kl = conductivity of electrolyte solution (S.m–1). Zeta potential values are connected with the charge density on the studied membrane surface theoretically. For this calculation, it is necessary to know the exact number of ions present in the electrolyte solution. On the other hand, while determining this value, pH changes make the charge density calculation imprecise [51]. Determining the zeta potential of ­membranes is also of great interest since it enables the characterization of the membrane material in contact with a liquid phase. For instance, membrane zeta potential has been used to correlate the transport of both inorganic and organic solutes through reverse osmosis (RO) and nanofiltration (NF) membranes. In addition, flux performance and fouling behavior of a membrane are also affected by its zeta potential. Zeta potential and electrokinetic charge density of membranes can be inferred from electrokinetic methods such as streaming potential and streaming current. Since these techniques permit consideration of the compensation of the surface charge by the ions of the electrical double layer on the liquid side of the interface, they are particularly useful when investigating problems of practical relevance [56].

Characterization of Membranes   85

3.4 Conclusions Many conditions should be carried out for a proper characterization of membranes. The structure properties of membranes should be known for the determination of filtration performance parameters. The relationships between membrane structure and performance can be determined with the use of different methods and so the prediction of performance is directly related to some active parameters.

References 1. J.I. Calvo, R.I. Peinador, P. Pradanos, A. Bottino, A. Comite, R. Firpo, A. Hernandez, Porosimetric characterization of polysulfone ultrafiltration membranes by image analysis and liquid-liquid displacement technique. Desalination, 357, 84–92, 2015. 2. C.S. Zhao, X.S. Zhou, Y.L. Yue, Determination of pore size and pore size distribution on the surface of hollow-fiber filtration membranes: A review of methods. Desalination, 129, 107–123, 2000. 3. F.H. She, D. Gao, W.M. Gao, D.Y. Wu, Z. Peng, M. Hoang, L.X. Kong, Characterization of membranes with X-ray ultramicroscopy. Desalination, 236, 179–186, 2009. 4. R. Ziel, A. Haus, A. Tulke, Quantification of the pore size distribution (porosity profiles) in microfiltration membranes by SEM, TEM and computer image analysis. J. Membr. Sci., 323, 241–246, 2008. 5. W.R. Bowen, N. Hilal, R.W. Lovitt, P.M. Williams, Atomic force microscope studies of membranes: Surface pore structures of Cyclopore and Anopore membranes. J. Membr. Sci., 110, 233–238, 1996. 6. D. Johnson, N. Hilal, Characterisation and quantification of membrane surface properties using atomic force microscopy: A comprehensive review. Desalination, 356, 149–164, 2015. 7. A.M. El Hadidy, S. Peldszus, M.I. Van Dyke, Development of a pore construction data analysis technique for investigating pore size distribution of ultrafiltration membranes by atomic force microscopy. J. Membr. Sci., 429, 373–383, 2013. 8. M. Mietton-Peuchot, C. Condat, T. Courtois, Use of gas-liquid porometry measurements for selection of microfiltration membranes. J. Membr. Sci., 133, 73–82, 1997. 9. H. Bechhold, M. Schlesinger, K. Silbereisen, L. Maier, W. Nurnberger, Pore diameters of ultrafilters. Kolloid Z., 55, 172–198, 1931. 10. F. Erbe, Blockierungsphenomene bei Ultrafiltern, Kolloid Z., 59, 32–44, 1932. 11. A.B. Abell, K.L. Willis, D.A. Lange, Mercury intrusion porosimetry and image analysis of cement-based materials. J. Colloid Interface Sci., 211, 39–44, 1999.

86  Nanostructured Polymer Membranes: Volume 1 12. K.C. Khulbe, C.Y. Feng, T. Matsuura, in: Membrane Processes Vol.1: Desalination and Water Resources, EOLSS, 2010. 13. M. Brun, A. Lallemand, J.F. Quinson, Ch. Eyraud, Changement d’état liquidesolide dans les milieux poreux I-III. J. Chim. Phys., 70, 973–989, 1973. 14. F.P. Cuperus, D. Bargeman, C.A. Smolders, Critical points in the analysis of membrane pore structure by thermoporometry. J. Membr. Sci., 66, 45–53, 1992. 15. T. Tsuru, T. Hino, T. Yoshioka, M. Asaeda, Permporometry characterization of microporous ceramic membranes. J. Membr. Sci., 186, 257–265, 2001. 16. S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd ed., Academic Press, London, 1982. 17. F.P. Cuperus, D. Bargeman, C.A. Smolders, Permporometry: The determination of the size distribution of active pores in UF membranes. J. Membr. Sci., 71, 57–67, 1992. 18. S.A. Tatulian, Structural characterization of membrane proteins and peptides by FTIR and ATR-FTIR spectroscopy. Methods Mol. Biol., 974, 177–218, 2013. 19. A.E. Childress, J.A. Brant, P. Rempala, D.W. Phipps Jr., P. Kwan, Evaluation of membrane characterization methods, Water Research Foundation Report, 2012. 20. K. Boussu, J. DeBaerdemaeker, C. Dauwe, M. Weber, K.G. Lynn, D. Depla, S. Aldea, I. Vankelecom, C. Vandecasteele, B. Van der Bruggen, Physicochemical characterization of nanofiltration membranes. Chem. Phys. Chem., 8, 370–379, 2007. 21. J.P. Labbe, A. Quemerais, F. Michel, G. Daufin, Fouling of inorganic membranes during whey ultrafiltration: Analytical methodology. J. Membr. Sci., 51, 293–307, 1990. 22. https://depts.washington.edu/ntuf/facility/docs/NTUF-Raman-Tutorial.pdf, url-1, in. 23. M.N. Kallionen, M Nyström, Membrane surface characterization, in: Advanced Membrane Technology and Applications, N.N. Li, A.G. Fane,  W.S.W.  Ho, T. Matsuura (Eds.), pp. 841–877, John Wiley and Sons Inc., 2008. 24. E. Smith, G. Dent, Modern Raman Spectroscopy: A Practical Approach, John Wiley & Sons, Ltd., 2005. 25. Introduction to Raman spectroscopy, Thermo scientific manual, http://www .biotechprofiles.com/companyfiles/madisonnetwork/5bdd0a9f37694d6c9fb6 e62db5049477.pdf, url-2, in. 26. R.W. Kelsall, I.W. Hamley, M. Geoghegan (Eds.), Nanoscale Science and Technology, John Wiley & Sons Ltd, 2005. 27. R. Lamsala, S.G. Harroun, C.L. Brosseau, G.A. Gagnon, Use of surface enhanced Raman spectroscopy for studying fouling on nanofiltration membrane. Sep. Purif. Technol., 96, 7–11, 2012. 28. M.W. Ernstsson, T. Wärnheim, Surface analytical techniques applied to cleaning processes, in: Handbook for Cleaning/Decontamination of Surfaces, I. Johansson, P. Somasundaran (Eds.), 1st ed., vol 2, chap. 4, Elsevier Science, 2007.

Characterization of Membranes   87 29. D.A.R. Brabazon, A. Raffer, Advanced characterization techniques for nanostructures, in: Emerging Nanotechnologies for Manufacturing, J. Ramsden (Ed.), chap. 3, pp. 59–91, Elsevier, 2010. 30. M. Sile-Yuksel, B. Tas, D.Y. Koseoglu-Imer, I. Koyuncu, Effect of silver nanoparticle (AgNP) location in nanocomposite membrane matrix fabricated with different polymer type on antibacterial mechanism. Desalination, 347, 120–130, 2014. 31. L.M. Corneal, S.J. Masten, S.H.R. Davies, V.V. Tarabara, S. Byun, M.J.  Baumann, AFM, SEM and EDS characterization of manganese oxide coated ceramic water filtration membranes. J. Membr. Sci., 360, 292–302, 2010. 32. B. Hofs, R. Schurer, D.J.H. Harmsen, C. Ceccarelli, E.F. Beerendonk, E.R. Cornelissen, Characterization and performance of a commercial thin film nanocomposite seawater reverse osmosis membrane and comparison with a thin film composite. J. Membr. Sci., 446, 68–78, 2013. 33. Y. Soffer, J. Gilron, A. Adin, Streaming potential and SEM-EDX study of UF membranes fouled by colloidal iron. Desalination, 146, 115–121, 2002. 34. N. Melián-Martel, J.J. Sadhwani, S. Malamis, M. Ochsenkühn-Petropoulou, Structural and chemical characterization of long-term reverse osmosis membrane fouling in a full scale desalination plant. Desalination, 305, 44–53, 2012. 35. M. Mulder, Basic Principles of Membrane Technology, 2nd ed., Kluwer Academic Publishers, 1996. 36. Z.K. Xu, L.S. Wan, X.J. Huang, Surface Engineering of Polymer Membranes, Springer, 2009. 37. Y.W. Chung, Auger electron spectroscopy, in: Practical Guide to Surface Science and Spectroscopy, chap. 2, Academic Press, 2001. 38. A. Basile, F. Gallucci, S. Tosti, Synthesis, characterization, and applications of palladium membranes. Membr. Sci. Technol., 13, 255–323, 2008. 39. R.F. Egerton, Electron energy-loss spectroscopy in the TEM. Rep. Prog. Phys., 72, 1–25, 2009. 40. H. Ibach, D.L. Mills, Introduction, in: Electron Energy Loss Spectroscopy and Surface Vibrations, chap. 1, Academic Press, 1982. 41. M. Aliofkhazraei, N. Ali, AFM applications in micro/nanostructured coatings, in: Comprehensive Materials Processing, S. Hashmi (Ed.), vol. 7, pp. 191–192, Elsevier, 2014. 42. N. Hilal, D. Johnson, The use of atomic force microscopy in membrane characterization, in: Comprehensive Membrane Science and Engineering, Vol. 1, E. Drioli, L. Giorno (Eds.), pp. 337–354, Elsevier Kidlington, 2010. 43. B.X. Mi, M. Elimelech, Silica scaling and scaling reversibility in forward osmosis. Desalination, 312, 75–81, 2013. 44. E. Guillén-Burrieza, R. Thomas, B. Mansoor, D.J. Johnson, N. Hilal, H.A. Arafat, Effect of dry-out on the fouling of PVDF and PTFE membranes under conditions simulating intermittent seawater membrane distillation (SWMD). J. Membr. Sci., 438, 126–139, 2013.

88  Nanostructured Polymer Membranes: Volume 1 45. W.R. Bowen, T.A. Doneva, H.B. Yin, Polysulphone-sulphonated poly(ether ether) ketone blend membranes: Systematic synthesis and characterization. J. Membr. Sci., 181, 253–263, 2001. 46. W.R. Bowen, N. Hilal, Investigating membranes and membrane processes with atomic force microscopy, in: Atomic Force Microscopy in Process Engineering, 1st ed., chap. 4, Butterworth-Heinemann, 2009. 47. D. Flosch, H.-D. Lehmann, R. Reichl, O. Inacker, W. Göpel, Surface analysis of poly(vinylidene difluoride) membranes, J. Membr. Sci., 70, 53–63, 1992. 48. N.N. Li, A.G. Fane, W.S.W. Ho, T. Matsuura (Eds.), Advanced Membrane Technology and Applications, John Wiley & Sons, Inc., 2008. 49. E. Drioli, L. Giorno (Eds.), Comprehensive Membrane Science and Engineering, Elsevier, 2010. 50. J. Wei, G.S. Helm, N.C. Walker, X. Hou, Characterization of a non-fouling ultrafiltration membrane. Desalination, 192, 252–261, 2006. 51. C. Causserand, P. Aimar, Characterization of filtration membranes, in: Comprehensive Membrane Science and Engineering, vol. 1, pp. 311–335, Elsevier, 2010. 52. J. Troger, K. Lunkwitz, K. Grundke, W. Bürger, Determination of the surface tension of microporous membranes using wetting kinetics measurements. Colloid Surf. A, 134, 299–304, 1998. 53. C. Combe, E. Molis, P. Lucas, R. Riley, M.M. Clark, The effect of CA membrane properties on adsorptive fouling by humic acid. J. Membr. Sci., 154, 73–87, 1999. 54. J.H. Sung, M.-S. Chun, H.J. Choi, On the behavior of electrokinetic streaming potential during protein filtration with fully and partially retentive nanopores. J. Colloid Interface Sci., 264, 195–202, 2003. 55. A. Szymczyk, N. Fatin-Rouge, P. Fievet, Tangential streaming potential as a tool in modeling of ion transport through nanoporous membranes. J. Colloid Interface Sci., 309, 245–252, 2007. 56. E.I. Mouhoumed, A. Szymczyk, A. Schafer, L. Paugam, Y.H. La, Physicochemical characterization of polyamide NF/RO membranes: Insight from streaming current measurements. J. Membr. Sci., 461, 130–138, 2014.

4 Ceramic and Inorganic Polymer Membranes: Preparation, Characterization and Applications Chiam-Wen Liew1,2* and S. Ramesh1 Centre for Ionics University of Malaya, Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia 2 School of Foundation Studies, Xiamen University Malaysia Campus, Selangor Darul Ehsan, Malaysia 1

Abstract

Low ionic conductivity is the main shortcoming in research pertaining to polymer electrolytes. Several approaches have been employed to improve the ionic conductivity of polymer electrolytes. Addition of nanosized fillers is an alternative way to increase the ionic conductivity. Nanocomposite polymer electrolytes (NCPEs) were prepared using solution casting technique. The effect of silica (SiO2), alumina (Al2O3), titania (TiO2) and zirconia (ZrO2) on polymer membrane containing poly(vinyl alcohol) (PVA) and 1-butyl-3-methylimidazolium bromide (BmImBr) was investigated thoroughly in this work. Incorporation of filler increases the ionic conductivity and electrochemical stability window of the polymer electrolyte. Nanofillers also reduce the glass transition temperature (Tg) and degree of crystallinity of the polymer complexes. NCPE-based electrical double-layer capacitors (EDLCs) also manifest better electrochemical performances compared to EDLC using filler-free system. Among all the nanosized fillers, alumina is an excellent candidate to be applied in polymer electrolytes for use in electrochemical devices. Keywords:  Nanocomposite polymer electrolyte, electrochemical double-layer capacitors, cyclic voltammetry, capacitance, electrochemical stability

*Corresponding author: [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 1, (89–136) © 2017 Scrivener Publishing LLC

89

90  Nanostructured Polymer Membranes: Volume 1

4.1 Introduction 4.1.1 Overview of Polymer Electrolytes Electrolyte and electrodes are important elements in electro­chemistry. Electrolyte is a conducting medium to provide the charge carriers, whereas  electrode is a conductor to provide the contact with the nonmetallic ­electrolyte in a circuit when the electric current is applied across the cell. The electric current of a cell is basically driven from the ion diffusion in the permeable electrolyte through two separated electrodes. Liquid electrolytes are liquid-state electrolytes used to conduct the electricity. However, these conventional liquid electrolytes possess several disadvantages, for example, leakages of corrosive solvent and harmful gas, electrolytic degradation of electrolyte, formation of lithium dendrite growth and poor long-term stability due to the evaporation of the liquid electrolytes, as well as low safety performance because of the use of flammable organic solvent [1, 2]. Other drawbacks are low operating temperature range, difficulty in handling and manufacturing due to the presence of liquid phase in the electrolytes, and low shelf life with high possibility of internal circuit shorting [3, 4]. Solid polymer electrolytes (SPEs) have been of keen interest to replace these harmful liquid electrolytes. These non-aqueous electrolytes not only overcome those shortcomings, but they also exhibit some extra advantages. These advantages are negligible vapor pressure, high automation potential, high ability to be configured in any shape due to high flexibility of polymer matrix, low volatility, high energy density and excellent electrochemical, structural, thermal, photochemical and chemical stabilities with high mechanical strength [5–9]. However, polymer-salt electrolytes exhibit low ionic conductivity due to high crystallinity of polymer system. Therefore this shortcoming becomes the main concern in the development of polymer electrolytes.

4.1.2 Methods to Enhance Ionic Conductivity of Polymer Electrolytes Several attempts have been made to solve the ionic conductivity problem, for example, polymer blending, polymer modifications, dual salt system, addition of plasticizers, inclusion of ionic liquid and adulteration of fillers, as well as different preparation methods. Addition of plasticizers into the polymer system produces gel polymer electrolytes (GPEs), which is a new class of polymer electrolytes. Plasticizers exhibit some distinctive properties, i.e., high dielectric constant, high conductivity, low viscosity, superior

Ceramic and Inorganic Polymer Membranes  91 miscibility with polymer, cost effective and excellent salt-solvating power with improved processability [8, 10–12]. Even though plasticization is a good way to soften the rigid polymers and reduce the glass transition temperature of polymer matrix, this technique faces some limitations. These drawbacks are poor dimensional and mechanical stabilities, slow evaporation due to gel state of polymer electrolyte, low safety performances, narrow potential window, poor interfacial stability and reduction in thermal, electrical and electrochemical stabilities as well [1]. Much effort has been made to solve these problems. A new material has been introduced to replace plasticizer, which is ionic liquid (IL). However, gel polymer electrolytes possess low mechanical stability, so addition of solid plasticizer, filler is an alternative way to improve the mechanical integrity of polymer electrolytes. In this chapter, we will employ ionic liquid and nanosized ceramic fillers in poly(vinyl alcohol) (PVA)-based polymer.

4.1.3 Ionic Liquids Ionic liquids (ILs) are nonvolatile molten salts with a low melting temperature (Tm) ( basic > neutral > weakly acidic [46]. The same research group also studied the effect of concentration and grain size of nanosized alumina [47]. The same polymer was used in this literature; however, LiTFSI salt was replaced with lithium triflate (LiCF3SO3). According to the result, the ionic conductivity is increased when the grain size is smaller. The ionic conductivity of polymer electrolytes doped with nanosized alumina is higher than microsized alumina. A recent study on the dielectric properties of NCPEs using nanoparticles of alumina was also done by Masoud et al. Based on the results, the addition of 1.25 mol of nano-Al2O3 particles enhances the ionic conductivity of polymer electrolyte by a hundred times at room temperature, with the highest conductivity of 8.3 × 10–5 S cm–1 [48]. A battery using NCPEs was fabricated by Polu and Kumar in 2013. Poly(ethylene gycol) (PEG)-magnesium acetate [Mg(CH3COO)2] containing 10 wt% of alumina achieved the maximum ionic conductivity of 3.45 × 10–6 S cm–1. The fabricated battery showed a current density of 13.91 μA/cm2, discharge capacity of 1.721 mA h, power density of 13.14 mW/kg and energy density of 1.84 W h/kg with an open circuit (OCV) of 1.85 V [33].

98  Nanostructured Polymer Membranes: Volume 1

4.2.2 TiO2 Titanium dioxide, also known as titanium (IV) oxide (or more commonly known as titania) (TiO2) is a naturally occurring metal oxide in minerals. There are three different polymorphs of titania: rutile, anatase and brookite. Rutile and anatase crystalline phases have tetragonal structures, whereas brookite has an orthorhombic structure. Anatase and rutile phases of TiO2 are of interest for technical applications [49]. Anatase and rutile phases of TiO2 have the same chemistry but different structures. The octahedron in the anatase phase shares four edges with other octahedrons and hence produces four-fold axis. In contrast, the rutile phase shares only two edges with other octahedrons and forms chains [50]. Anatase phase of nanoscale TiO2 is widely used for optoelectronic and photocatalytic applications, whereas rutile phase is used as white pigment material as it shows good visible light-scattering property and effective UV light absorption [51]. Several synthesis routes have been explored and developed to produce laboratory-scaled TiO2 nanoparticles over the last decades; for example, gas condensation, sol-gel method, flame aerosol synthesis, thermal decomposition, hydrothermal, precipitation and hydrolysis, solvothermal, sonication, ball-milling and chemical vapor deposition (CVD) as well as spray pyrolysis [49, 51]. Among them, sol-gel, sonication and spray pyrolysis are the most common alternative means for high surface area TiO2 nanoparticles production with controlled particle size, structure and morphology [51]. On the other hand, there are only two processes in large-scale production of TiO2 particles. The first method involved in the sulphate process where titanium oxyhydrate, TiO(OH)2, is precipitated during hydrolysis of titanyl sulfate (TiOSO4) and subsequently calcined to TiO2. For the second method, TiO2 is produced through the chloride process in which the precursor, titanium tetrachloride (TiCl4) is oxidized in an oxygen flame to form TiO2 particles [49, 52]. Titania possesses numerous outstanding features, such as improved physical and electrochemical stabilities, enhanced cationic transport number, hydrophilicity, good mechanical integrity, improved sinterability and excellent stability in acidic and oxidative environments with free-flowing structure [50–51, 53–54]. The introduction of TiO2 nanoparticles into the proton conducting polymer electrolytes for fuel cell application is owing to the excellent mechanical and thermal resistances, better water uptake and high ionic exchange capacity (IEC) [34, 55]. Development of mass production of TiO2 has attracted researchers because of its wide variety of applications, including beam splitters, optical and anti-reflection coatings, heterogeneous catalysis, gas sensors, ultraviolet (UV) absorbers, lithium

Ceramic and Inorganic Polymer Membranes  99 batteries, optical, electronic and electrochromic devices [51]. Titania has also been widely used in selfcleaning surface coatings, UV-resistant coatings and paints, solar cells, disinfectant sprays, water treatment agents and topical sunscreens [49, 52]. Moreover, titania also plays an important role in the biological field, such as to improve the biocompatibility of bone implants [49]. Nanosized TiO2 is also used in clean technologies such as environmental remediation, pigments, paints, ceramics, cosmetics and solar energy conversion due to its high photocatalytic activity and chemical stability [50]. Titania-based composite polymer electrolytes are also widely prepared. A new composite polymer electrolyte containing high molecular mass polyethylene oxide (PEO), lithium iodide (LI) and iodine (I2) filled with TiO2 was prepared by Katsaros et al. in 2002. The fabricated solar cell shows its overall conversion efficiency (η) of 0.96% with open circuit voltage (Voc) of 0.67 V, short-circuit current (Jsc) of 2.050 mA/cm2 and fill factor (FF) of 39% under direct solar irradiation [56]. High efficiency quasi-solid-state dyesensitized solar cell (DSSC) with energy conversion efficiency of 4.74% was achieved by adding 2.5 wt% TiO2 into polymer electrolyte containing low molecular mass polysaccharide (agarose), lithium iodide (LI) and iodine (I2) filled with TiO2, as reported in Yang et al. [22]. Nanoscale TiO2 was also dispersed into porous membrane which was comprised of two copolymers, poly(vinylidene fluoride-co-hexafluorophosphate) (PVdF-co-HFP) and poly(ethylene oxide-co-ethylene carbonate) (P(EO-co-EC)), as reported in Jeon et al. [57]. According to this study, the highest ionic conductivity of 5.1 × 10–5 S cm–1 was achieved at 25 °C. The ionic conductivity was slightly higher than the polymer electrolyte without addition of TiO2 nanoparticles [57]. Nafion hybrid composite membrane containing nanosized TiO2 was prepared by Hammami et al. using sol-gel technique. Even though the proton conductivity of composite membrane is lower than that of pristine Nafion membrane, the conductivity of this hybrid composite membrane is boosted at temperatures above 80 °C. They suggested that this abrupt increase in proton conductivity is attributed to the hydrophilic properties of the well-dispersed TiO2 nanoparticles in the polymer matrix. Inclusion of this inorganic filler provides a preferential pathway for proton hopping mechanism and improves the interconnection of the ionic clusters in the polymer membrane, which results in higher proton conductivity [36].

4.2.3 ZrO2 Zirconia, or zirconium dioxide or zirconium (IV) oxide (ZrO2), is a white crystalline solid. The name “zirconium” comes from two old Persian

100  Nanostructured Polymer Membranes: Volume 1 words, “zar” (gold) and “gun” (color), or from the Arabic “zargon” (golden in color), for the aspect of some zircon ores. This zirconia was discovered by a German chemist, Martin Heinrich Klaprothin in the year 1789. The heat treatment of jacinth, a vermillion zirconia gemstone, brought him this idea on zirconia production [40, 58]. Zirconia exists in three well-defined structures, such as monoclinic (m), tetragonal (t) and cubic (c), depending on temperature. Zirconia appears as a monoclinic unit cell at ambient temperature. This phase is stable up to 1170 °C. The phase is transformed into tetragonal and cubic forms at 1170 °C and above 2370 °C, respectively. Unlike alumina, the phase transitions of zirconia are reversible [40, 58]. For the monoclinic crystal system, the zirconium ion has a sevenfold coordination with oxygen ions arranged in a complex structure that resembles a tetrahedral structure. The tetragonal form of zirconia shows that the eight-fold coordinated zirconium ion connects two slightly distorted tetrahedral shapes that are rotated through 90°. In contrast, there is slight difference between tetragonal and cubic phases, where cubic phase has a face-­centered structure. Similarly, the zirconium ion has eight-fold coordination but the oxygen ions are arranged in two equal tetrahedrals. Zirconia is a nontoxic compound with some inherent properties, such as superior chemical inertness, biocompatibility, good hydrophilic properties and high fracture toughness (up to 8–10 MPa m1/2) [59–61]. High ionic conductivity, good chemical and dimensional stabilities, excellent mechanical strength, coupled with a Young’s modulus similar to that of stainless steel alloys, make zirconia a promising additive in the development of NCPEs [58, 62]. It was used for a long time blended with rare earth oxides as pigment for ceramics [58]. Since it is stable in strong alkaline media, it can be used in alkaline polymer electrolyte as a solid plasticizer to improve the electrochemical properties of SPEs [63]. Nowadays, zirconia is also significantly used in oxygen sensors and fuel cell applications because of its high ability to allow oxygen ions to transport freely through the crystal structure of zirconia in the polymer membrane at high temperature [61]. Not only is zirconia used in the preparation of NCPEs, but also in electrode preparation. Uma and coworkers reported that the ionic conductivity of polymer electrolytes is improved an order of magnitude upon addition of ZrO2 inert filler with particle size of 21.5 µm [64]. Three different percentages (5 wt%, 10 wt% and 20 wt%) of commercial zirconium (IV) oxide was dispersed into the Nafion composite membrane through doctor blade technique, as mentioned in Saccà et al. Nafion-ZrO2 composite membranes showed higher water uptake and proton conductivity than unmodified Nafion membrane. Polymer electrolyte membrane fuel cells (PEMFCs) were assembled using the composite membrane. Power density

Ceramic and Inorganic Polymer Membranes  101 values of 604 mW cm−2 were obtained at 0.6 V at 110 °C (100% of relative humidity) for the fuel cell containing 10 wt% of filler [65]. Different techniques were used to synthesize Nafion-ZrO2 composite membranes by Pan et al. in 2010. They employed in-situ hydrolysis of tetrabutylzirconate (TBZ) in Nafion solution through sol-gel process to produce Nafion-ZrO2 nanocomposite membranes [66]. These Nafion-zirconia nanocomposite membranes showed high water retention ability at elevated temperature compared to recast pure Nafion membrane, especially at medium and high relative humidity, and therefore inferred the potential of these membranes for PEMFC application [66]. Nanosized ZrO2 can also be a possible electrode candidate material for electrical double-layer capacitors (EDLCs) [67]. The EDLCs were prepared using different mass fraction of ZrO2 and carbon black. The capacitance of EDLC increases with ZrO2 content. The highest capacitance of 43.20 F g−1 was achieved by adding 60 wt% of ZrO2 into carbon slurry, which was comprised of 30 wt% of carbon black and 10 wt% of polytetrafluoroethylene (PTFE). This EDLC using 2 M potassium chloride (KCl) electrolyte also showed good cyclic performance [67].

4.2.4 SiO2 Silica or silicon dioxide (SiO2) is a white crystalline powder and is naturally originated from quartz and various living organisms. Among all the fillers used in this chapter, SiO2 has the most crystalline forms. Examples of polymorphs of silica are quartz (α and β phases), tridymite (α and β phases), cristobalite (α and β phases), faujasite, keatite, fibrous, seifertite, moganite, coesite, and stishovite [68–75]. However, only α and β phases of quartz, α-tridymite and α-cristobalite are metastable polymorphs [70]. All of these stable polymorphs of silica have different crystal structures. The α-quartz is in rhombohedral (or known as trigonal) shape, whereas β-quartz is in hexagonal shape. On the other hand, α-tridymite and α-cristobalite appear as orthorhombic and tetragonal structures, respectively. Fumed silica is used as inorganic filler in this chapter. Fumed silica is also known as pyrogenic silica because it is produced by a flame hydrolysis. Spherical silica particles (7–40 nm diameter) are produced from a continuous flame hydrolysis technique of silicon tetrachloride in a hydrogenoxygen flame [76]. Two processes are involved in the production of fumed silica. First, silicon tetrachloride (SiCl4) is converted to the gas phase. After that, it reacts with water to yield silica (SiO2) and hydrochloric acid (HCl), as shown below. SiCl4 + H2O

SiO2 + HCl

102  Nanostructured Polymer Membranes: Volume 1 The HCl is easily separated out as it can remain in the gas phase, while the fumed silica is in solid form. The fumed silica is a hydrophilic compound due to the presence of hydroxyl groups on the surface. In addition, the treated grades of fumed silica are produced by reacting fumed silica with organosilicons and they will convert the natural hydrophilic into a hydrophobic behavior. Fumed silica consists of microscopic droplets of amorphous silica fused into branched and three-dimensional secondary particles. These particles form chain-like aggregate of tertiary particles thereafter [76]. Fumed silica was chosen as filler because of its high surface area, hydrophilic feature, low bulk density, high porosity, low refractive index, unique chemical properties and high ability to tailor the mechanical properties to a specific need by modifying its surface functionalities [76–78]. The inherent three-dimensional chains promote the formation of network structure which is conducive to a high ionic mobility of the polymer matrix [79]. There are some applications for fumed silica, including filler for rubbers and plastics, coatings, adhesives and sealants as well as thickening agent. Besides, it is widely used in the manufacturing of cosmetics, pharmaceuticals, pesticides, inks, batteries and abrasives. This filler is also a vital element in electrochemical devices, especially fuel cells. This fumed silica helps in enhancing the water uptake ability, the wettability and the performances of membrane electrode assembly (MEA) at low humidity conditions, as reported by Choi and coworkers [80]. Aravindan and Vickraman prepared a new type of NCPE using PVdFco-HFP as host polymer, LiPF3(CF3CF2)3 (lithium fluoroalkyl phosphate (LiFAP)) as salt and nanosized SiO2 [81]. The NCPE membrane achieved the maximum ionic conductivity of 1.13 mS cm–1 with addition of 2.5 wt% of SiO2 at ambient temperature [81]. The effect of addition of nanosized SiO2 was also investigated by Yang et al. in 2011. Different mass fraction of nanosized SiO2 was studied. They found that the NCPE containing 10 wt% of SiO2 exhibited the highest conductivity of 35 mS cm–1 at room temperature. The direct methanol fuel cell (DMFC) using this proton conductor was then fabricated. The highest peak power density of 19.57 mW cm−2 was achieved at ambient condition using PtRu anode and a mixture of 4 M potassium hydroxide (KOH) and 2 M methanol (CH3OH) solution fuel [82]. Similarly, the effect of an amorphous SiO2 nanofiller on ionic conductivity and crystallinity of PEO-based electrolyte has been investigated by Ketabi and Lian. The ionic conductivity of this polymer-ionic liquid electrolyte containing 1-ethyl-3-methylimidazolium hydrogensulfate (EMIHSO4) and nanosized SiO2 reached 2.15 mS cm–1 at room temperature, which is more than a two-fold increase over the electrolyte without

Ceramic and Inorganic Polymer Membranes  103 filler. The solid EDLC device containing this conducting polymer electrolyte demonstrated an excellent cycle life and high rate response [83].

4.2.5 Supercapacitors Supercapacitor (or also known as ultracapacitor or electrochemical capacitor) is an energy storage-based electrochemical device used as a power source. The energy storage of a supercapacitor arises from the ion accumulation at the electrode-electrolyte interface of active materials through rapid and reversible adsorption and/or desorption of charges carriers [84, 85]. Supercapacitors consist of one pair of electrodes and electrolyte. The electrode can be derived from many materials such as carbon, metal oxide, conducting polymers and so on. On the other hand, the electrolyte can be liquid electrolyte, solid polymer electrolyte, gel polymer electrolyte or composite polymer electrolyte but must be conductive with high ionic mobility. The ion accessibility from electrolyte to the electrode becomes an important parameter to govern the capacitance of supercapacitors.

4.2.5.1 Types of Supercapacitors Supercapacitors are subdivided into three main types, viz. pseudocapacitors, EDLCs and hybrid capacitors. Pseudocapacitors (also recognized as redox capacitors) are the capacitors involved in fast Faradaic processes, such as intercalation, under-potential deposition and redox reaction, occurring at the surface of electrode at an appropriate applied potential [86]. The active materials used as electrode in pseudocapacitors are electroactive conducting polymers (e.g., polypyrrole (PPy) and poly(thiophene) derivaties such as poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(aniline) (PANI)) and nobel metal oxide (e.g., ruthenium oxide (RuO2), nickel(II) oxide (NiO), TiO2, cobalt(II) dicobalt(III) oxide (Co3O4), tin dioxide (SnO2), iridium(IV) oxide (IrO2) and manganese(IV) oxide (MnO2)) [87, 88]. Pseudocapacitors give rise to high capacitance value. But these capacitors bring some disadvantages such as shorter cycle life, poor electrochemical stability, are relatively expensive due to high raw material cost and possess difficulty in processing, which limits their practical applications [89]. On the contrary, EDLCs do not involve any electrochemical Faradic reactions over the potential range of operation. The basic principle of charge storage in these non-Faradaic capacitors is related to the formation of the Helmholtz layer (or well-known as double layer) as a result of electrostatic charge accumulation between the electrode and electrolyte without any c­ hemical reaction. Carbonaceous materials are the common electrode

104  Nanostructured Polymer Membranes: Volume 1 materials for an EDLC. These carbon-based materials are activated carbon powder, carbon black, carbon nanotubes (CNTs), graphite, carbon fiber and carbon aerogel [90]. Carbon-based electrode offers several advantages, for instance, well polarizable, excellent electrochemical properties due to its amphoteric behavior, environmentally friendly, easy processability, easy accessibility of ions, high specific surface area, cost effective, chemically stable in different solution (from acidic to basic), long cycle life, ­maintenance-free operation, and wide operating temperature with excellent performances [85, 91–92]. The low specific capacitance of EDLC has led to the invention of hybrid capacitors which are combinations of pseudocapacitors and EDLC and are a compromise between both capacitors. There are many intensive approaches to hybridize the electrodes. The common way is to add the electrochemical active materials into the carbon-based electrode, which produces composite hybrid capacitor [93]. Asymmetric hybrid capacitor is another class of hybrid capacitors. This capacitor is comprised of a pseudo-capacitive metal oxide/hydroxide electrode and a capacitive carbon electrode such as activated carbon (AC)//Ni(OH)2 and AC//MnO2 [94]. Lithium-insertion electrode with a capacitive carbon electrode like Li4Ti5O12//AC is another brand new hybrid capacitor. However, the depletion of electrolyte in this hybrid capacitor has been identified as a major challenge [94]. So, researchers have come out with a brilliant idea, which is the production of battery-like hybrid capacitor. The lithium cations (Li+) will be intercalated to a cathode compound, such as LiMn2O4 as cathode, while activated carbon will be used as anode material [94]. The EDLC will be fabricated using the most conducting NCPE of each system in this chapter.

4.2.5.2 Advantages of Supercapacitors Supercapacitors have emerged as a new type of electrochemical devices to replace lithium ion batteries and conventional electrolytic capacitors. Supercapacitors not only deliver higher power density than lithium ion secondary batteries but also exhibit higher energy density than conventional dielectric capacitors [85]. EDLC is a great choice as an electrochemical device because of its long charge-discharge cycle lifetime (over 106 cycles), low cost, high power density (up to 10 kW kg–1), high dynamic of charge propagation (short-term pulse), fast energy storage, higher ability to be charged and discharged continuously without degrading, maintenancefree long life operation and short charging time [86, 92, 95–97].

4.2.5.3 Applications of Supercapacitors Supercapacitors have a wide range of applications ranging from lowenergy density to high-energy density applications. They have also been

Ceramic and Inorganic Polymer Membranes  105 widely used as memory backup for biomedical and electronic devices containing complementary metal oxide semiconductors (CMOS), random access memory (RAM) or microprocessor [95, 96]. They can be used as auxiliary power sources in some small portable appliances and digital tele­communication systems such as laptops, computers, alarms, video ­ cassette recorders (VCRs), telephones, typewriters and camcorders [95,  96]. The high-energy density applications of supercapacitor include spaceships, hybrid electric vehicles (HEVs) and pulse laser ­techniques [96, 98].

4.3 Methodology 4.3.1 Materials Nanocomposite polymer electrolytes containing PVA, 1-butyl-3-­ methylimidazolium bromide (BmImBr) and different nanosized ­fillers were prepared in this work. The PVA (Sigma-Aldrich, USA, 99% hydrolyzed with molecular weight of 130000 gmol–1), BmImBr (Merck, Germany), SiO2 (particle size of 70 nm from Sigma-Aldrich, USA), Al2O3 (average particle size of 40–50 nm from Alfa Aesar, UK), TiO2 (particle size < 100 nm from Sigma-Aldrich, USA) and ZrO2 (particle size < 100 nm from Sigma-Aldrich, USA) were used as received.

4.3.2 Sample Preparation Poly(vinyl alcohol)-based NCPEs were prepared by solution casting method. The PVA was initially dissolved in distilled water. Appropriate amount of BmImBr was subsequently mixed into PVA solution. The weight ratio of PVA:BmImBr was kept at 70:30. Different mass ratio of nanofiller was thus doped into the PVA-BmImBr aqueous solution to prepare NCPEs. The resulting solution was placed and heated in a sonicator for 30 minutes at 70 °C to make sure filler was well dispersed in the solution. The solution was then thoroughly stirred and heated at 70 °C for several hours. The solution was eventually cast in a glass Petri dish and dried in an oven overnight at 60 °C to obtain a free-standing polymer electrolyte film. The appearance of the polymer electrolyte from each system is displayed in Figure 4.1. The polymer electrolyte without addition of any filler is designated as filler-free system. On the contrary, the highest conducting polymer electrolyte containing silica, titania, zirconia and alumina are assigned to Si system, Ti system, Zr system and Al system, respectively.

106  Nanostructured Polymer Membranes: Volume 1

(a)

(b)

(c)

(d)

Figure 4.1  Free-standing polymer film with addition of (a) SiO2, (b) TiO2, (c) ZrO2 and (d) Al2O3.

4.3.3 Sample Characterization 4.3.3.1 Ambient Temperature-ionic Conductivity and Temperaturedependent Ionic Conductivity Studies Freshly prepared NCPEs were subjected to AC-impedance spectroscopy for ionic conductivity determination. A digital micrometer screw gauge was used to measure the thickness of the samples. The impedance of the polymer electrolytes was measured using the Hioki 3532–50 LCR HiTester impedance analyzer over the frequency range between 50 Hz and 5 MHz at ambient temperature. Continuous heating was then applied to the samples from room temperature to 120 °C to examine the temperature-dependent conductivity study. The measurement was taken by sandwiching the NCPE between two stainless steel (SS) blocking electrodes at a signal level of 10 mV. The filler-free and highest conducting NCPEs of each system were subjected to the linear sweep voltammetry (LSV) study and EDLC fabrication. The bulk ionic conductivity of polymer electrolytes is determined by using the equation:

Rb A



(4.1)

where is the thickness (cm), Rb is bulk resistance (Ω) and A is the known surface area (cm2) of polymer electrolytes. The semicircle fitting was accomplished to obtain Rb value. Rb of the thin electrolytes film was calculated from extrapolation of the semicircular region on Z real axis (Z ).

4.3.3.2 Differential Scanning Calorimetry (DSC) The DSC analysis was performed using a TA Instrument Universal Analyzer 200 which consists of a DSC Standard Cell FC as main unit and Universal V4.7A software. The entire analysis was done in a nitrogen atmosphere at

Ceramic and Inorganic Polymer Membranes  107 a flow rate of 60 ml min–1. Samples weighing 3–5 mg were hermetically sealed in an aluminum Tzero pan. A tiny hole was punched on top of the pan to eliminate the water and moisture which are released in the heating process. In contrast, an empty aluminum pan was hermetically sealed as reference cell. As a preliminary step, the samples were heated from 25 °C to 110 °C at a heating rate of 20 °C min–1 to remove any trace amount of water and moisture. The heating process was maintained at 110 °C for 2 minutes to ensure the complete evaporation. After that, the samples were rapidly cooled to −80 °C at a cooling rate of 20 °C min–1. Beyond this preliminary step, the samples were heated to 220 °C and maintained for 2 minutes. The samples were cooled to –80 °C thereafter at the pre-set heating rate. This heat-cool process is repeated 3 times to reach the equilibrium state. Crystallization temperature (Tc) was obtained in the cooling cycle. On the other hand, glass transition temperature (Tg) and crystalline melting temperature (Tm) were evaluated using the final scan with the provided software.

4.3.3.3  Linear Sweep Voltammetry (LSV) A CHI600D electrochemical analyzer was used to evaluate LSV responses of the samples. These cells were analyzed at a scan rate of 10 mVs–1 by placing the polymer electrolyte between two SS electrodes in the potential range of ±3V.

4.3.4 Electrode Preparation Activated carbon-based EDLC electrodes were prepared by dip-coating technique. The preparation of carbon slurry was prepared by mixing 80 wt% activated carbon (Kuraray Chemical Co Ltd., Japan) of particle size between 5 and 20 µm, surface area between 1800 and 2000 m2g–1; 5 wt% carbon black (Super P); 5 wt% multiwalled carbon nanotubes (CNTs) (SigmaAldrich, USA) with outer diameter, OD, between 7 and 15 nm and length, L, ranging from 0.5 to 10 μm and 10 wt% poly(vinylidene fluoride) (PVdF) binder (molecular weight of 534000 gmol–1 from Sigma-Aldrich); and dissolving them in 1-methyl-2-pyrrolidone (purity ≥ 99.5 % from Merck, Germany). Activated carbon was initially treated with sodium hydroxide (NaOH) and sulphuric acid (H2SO4) to increase the porosity of carbon. This slurry was stirred thoroughly for several hours at ambient temperature. The carbon slurry was then dip-coated on an aluminum mesh current collector. The coated electrodes were dried in an oven at 110 °C for drying purposes.

108  Nanostructured Polymer Membranes: Volume 1

4.3.5 Electrical Double-layer Capacitors (EDLCs) Fabrication The EDLC cell was assembled in the configuration of electrode/polymer electrolyte/electrode. The EDLC cell configuration was eventually placed in a cell kit for further electrochemical analyses.

4.3.6 Electrical Double-layer Capacitors (EDLCs) Characterization The fabricated EDLC cell was subsequently subjected to cyclic voltammetry (CV), electrochemical impedance spectrometer in the low frequency range and galvanostatic charge-discharge (GCD) instruments.

4.3.6.1 Cyclic Voltammetry (CV) The CV study of EDLC was investigated using CHI600D electrochemical analyzer. The cell was rested for 2 seconds prior to the measurement to achieve the equilibrium state. The EDLC cell was then evaluated at 10 mVs–1 scan rate in the potential range between 0 and 1 V in intervals of 0.001 V. The specific capacitance (Csp) of EDLC was computed using the following equations [99, 100]:

Csp

i (F g 1 ) (4.2) sm



Csp

i (Fcm 2 ) (4.3) sA

where i is the average anodic-cathodic current (A), s is the potential scan rate (Vs–1), m refers to the average mass of active materials and A represents surface area of the electrodes, that is 1 cm–2. The average mass of electrode materials is around 0.01 g.

4.3.6.2 Galvanostatic Charge-discharge Analysis (GCD) The charge-discharge study was carried out using a Neware battery cycler. The EDLC was charged and discharged at current of 1 mA. Then EDLC was allowed to rest for 10 minutes before taking the measurements. The specific discharge capacitance (Csp) was obtained from charge-discharge curves, according to the following relation [99]:

Ceramic and Inorganic Polymer Membranes  109

Csp



I (4.4) dV m dt

where I is the applied current (A), m is the average mass of electrode materials (including the binder and carbon black), dV represents the potential change of a discharging process, excluding the internal resistance drop occurring at the beginning of the cell discharge, and dt is the time interval of discharging process. The dV/dt is determined from the slope of the discharge curve. The mass of the electrode used in this study is 0.01–0.012 g. Energy density E (W h kg–1), power density P (kW kg–1), and Coulombic efficiency η (%) were assessed using the following equations [97]:

Csp (dV )2 1000  2 3600

E P

(4.5)

I dV 1000 (4.6) 2 m td 100 (4.7) tc

where td and tc are the discharging and charging times, respectively.

4.4 Results and Discussion 4.4.1 Ambient Temperature-ionic Conductivity Studies Ionic conductivity is the main parameter in the polymer electrolytes to confirm the ionic transportation within the polymer matrix. The ionic conductivity of polymer electrolytes of each system with different mass fraction of fillers is depicted in Figures 4.2–4.5. Based on all the results in Figures 4.2–4.5, the ionic conductivity of polymer electrolytes is improved upon addition of nanofillers. Among all the fillers, dispersion of SiO2 shows the least increase in ionic conductivity. Upon addition of 4 wt% of SiO2, the ionic conductivity is increased by almost one order of magnitude that is from 3.18 × 10–5 S cm–1 to 2.58 × 10–4 S cm–1 at ambient temperature. Impregnations of 6 wt% of TiO2 and 8 wt% of ZrO2 show more than one order of magnitude of the

Log [σ ( S cm-1)]

110  Nanostructured Polymer Membranes: Volume 1 –3.50 (2.58 ± 0.01) × 10–4 S cm–1 –3.60 –3.70 –3.80 –3.90 –4.00 –4.10 –4.20 –4.30 –4.40 –4.50 (3.18 ± 0.01) × 10–5 S cm–1 –4.60 0 2 4 6 8 Weight percent of SiO2 (wt. %)

10

Figure 4.2  The logarithm of ionic conductivity of polymer electrolyte at different mass fraction of nanosized SiO2. –3.00

(7.04 ± 0.01) × 10–4 S cm–1

Log [σ ( Scm–1)]

–3.20 –3.40 –3.60 –3.80 –4.00 –4.20 –4.40 –4.60

(3.18 ± 0.01) × 10–5 S cm–1 0

2

4 6 Weight percent of ZrO2 (wt. %)

8

10

Log [σ ( Scm–1)]

Figure 4.3  The logarithm of ionic conductivity of polymer electrolyte at different mass fraction of nanosized ZrO2. –3.00 –3.20 –3.40 –3.60 –3.80 –4.00 –4.20 –4.40 –4.60

(7.45 ± 0.01) × 10–4 S cm–1

(3.18 ± 0.01) × 10–5 S cm–1 0

2

4 6 Weight percent of TiO2 (wt. %)

8

10

Figure 4.4  The logarithm of ionic conductivity of polymer electrolyte at different mass fraction of nanosized TiO2.

increase in ionic conductivity, where their maximum ionic conductivities are 7.45 × 10–4 S cm–1 and 7.04 × 10–4 S cm–1, respectively. Doping of 8 wt% of Al2O3 achieves the highest conductivity of 2.30 mS cm–1 which is almost higher two orders of magnitude than filler-free polymer electrolyte.

log [σ ( Scm–1)]

Ceramic and Inorganic Polymer Membranes  111 –2.60 –2.80 –3.00 –3.20 –3.40 –3.60 –3.80 –4.00 –4.20 –4.40 –4.60

(2.30 ± 0.01)mS cm–1

(3.18 ± 0.01) × 10–5 S cm–1 0

2

4 6 Weight percent of Al2O3 (wt. %)

8

10

Figure 4.5  The logarithm of ionic conductivity of polymer electrolyte at different mass fraction of nanosized Al2O3. CH2

CH

CH2

CH

OH

O O

Si O

H

O

n

OH

O Si O

H

O

O Si

O

O

Figure 4.6  Formation of three-dimensional network between polymer backbone and the surface of SiO2 nanoparticles.

A general trend was observed in all the samples. The ionic conductivity of polymer electrolytes increases with mass loadings of fillers, up to a maximum level. The increase in ionic conductivity is due to new formation of three-dimensional (3D) network. The hydroxyl groups (designated as Me-OH) would be formed on the surface of filler particles when hydrophilic metal oxides are immersed in water [40]. Silanol (Si-OH), Ti-OH, Zr-OH and Al-OH bonds are produced in respective system. This surface hydroxyl group could interact with the polar group of side chain of PVA through Lewis acid-base interactive bonding and form 3D network, as demonstrated in Figure 4.6. This Lewis acid-base interaction plays some important roles in enhancing the ionic conductivity. The crosslinkages within the 3D polymeric network will hinder the reorganization of polymer chain and prevent the crystallization and recrystallization of polymer matrix. Hence, these crosslinking agents reduce the degree of crystallinity of macromolecules [33,  83]. This Lewis acid-base interaction also promotes the ionic

112  Nanostructured Polymer Membranes: Volume 1

where SiO2

charge carrier

Figure 4.7  Model representation of an effective ionic conducting pathway through the space charge layer of the neighboring SiO2 grains at the boundaries.

dissociation contributing to rapid ionic hopping mechanism. Apart from that, this 3D network could provide a conducting interfacial layer or space charge layer when the nanoparticles are getting closer to contact with their neighbors [47, 83]. So, there is an extra pathway for the ion to migrate in the vicinity of filler grains, as illustrated in Figure 4.7 [101]. In addition, filler can act as solid plasticizer. The porous morphology of filler absorbs and retains the electrolyte solution in the polymer network [83]. As a result, this retention raises the mobility of charge carriers which are transported from one site to another vacant site. Beyond the optimum level, there is a drop in ionic conductivity. This is suggestive of excessive filler grains in the polymer membrane, which block the conducting pathway for ionic migration. The ionic conductivity of polymer electrolytes is in this order: Al system > Ti system > Zr system > Si system > Filler– free system. Among all the fillers, polymer electrolytes containing Al2O3 achieve the highest conductivity at room temperature. We suggest that this is related to the chemical structure of the filler. Two aluminum cations are coordinated with three oxygen anions, while only one metal ion is coordinated to two oxygen anions for other fillers. Therefore, more hydroxyl groups can be formed on the surface of the nanoparticles. As a result, the formation of 3D polymeric entanglements is more favored. So, the mobile charge carriers are easier to be transported from one site to another vacant site in this bulky network. Hence, the dispersion of Al2O3 into the polymer electrolytes has higher ionic conductivity than that of other fillers.

4.4.2 Temperature-dependent–ionic Conductivity Studies A temperature-dependent conductivity study is done to investigate the mechanism pertaining to mobile conducting charge carriers. Figure 4.8

Ceramic and Inorganic Polymer Membranes  113 –2.00

y = –0.6449x – 0.4834 R2 = 0.9954

Log σ [(Scm–1)]

–2.50

y = –0.7118x – 0.7307 R2 = 0.9875

–3.00 y = –0.721x – 0.7513 R2 = 0.9863 –3.50

y = –1.0677x – 1.8396 R2 = 0.9921

–4.00 –4.50 –5.00 2.5

y = –1.3861x – 2.8442 R2 = 0.9854 2.6

2.7

Filler-free system

2.8

2.9 3 3.1 3.2 3.3 1000/T (K–1) Si system Zr system Ti system Al system

3.4

Figure 4.8  Arrhenius plot of filler-free polymer electrolyte and filler-doped NCPEs.

depicts the conductivity of polymer electrolytes from ambient temperature to 120 °C. All the plots show linear relationships with regression value approaching unity (~0.99). This variation infers that the polymer electrolytes exhibit Arrhenius behavior. The ionic conductivity is expressed as follows in this thermally activated principle:

A exp

Ea  kT

(4.8)

where A is a constant which is proportional to the amount of charge carriers, Ea is activation energy, k is Boltzmann constant, that is 8.6173 × 10–5 eV K–1, and T represents the absolute temperature in K. This theory implies the ionic hopping mechanism in the polymer electrolytes. The ionic conductivity increases with temperature due to high vibrational mode of macromolecules. High vibrational mode of macromolecules could weaken the interaction between the polar group of PVA and charge carriers at elevated temperature. Therefore, high mobility of charge carriers could be easily decoupled from the polymer segments with increasing the temperature. The decoupling process could lead to the formation of vacant sites in the polymer backbone. Thus, ions from adjacent sites tend to occupy these neighboring voids and re-coordinate with the polymer backbone. This continuous process is known as hopping mechanism. The parameters obtained from Arrhenius plot are determined and listed in Table 4.1. The Al system shows the highest conductivity, whereas the filler-free system illustrates the lowest conductivity, as shown in Figure 4.8. The ionic

114  Nanostructured Polymer Membranes: Volume 1 Table 4.1  Activation energy, Ea, and pre-exponential constant, A, of polymer electrolytes obtained from Arrhenius plots. Activation energy, Ea (eV)

Log A

Pre-exponential constant, A

Filler-free system

0.2751

–2.8442

1.43 × 10–3

Si system

0.2119

–1.8396

0.0145

Zr system

0.1431

–0.7513

0.18

Ti system

0.1413

–0.7307

0.19

Al system

0.1280

–0.4834

0.33

Sample

conductivity of other filler systems is arranged in the following order: Ti system > Zr system > Si system, as discussed in the previous study. Two obvious trends are observed in Table 4.1, which are that the activation energy of each polymer system is inversely proportional to ionic conductivity while pre-exponential constant is directly proportional to the conductivity. These two observations can be used to explain the conductivity trend. Activation energy is defined as the minimum energy used for ion to hop from one site to another adjacent empty site. The lower the activation energy, the higher the conductivity of the polymer electrolyte. The p ­ olymer electrolyte possessing lower activation energy has higher ionic m ­ obility and then produces rapid ionic migration, which is in accordance with higher ionic conductivity. This idea is also supported by the evaluation of the pre-exponential constant, A, which is proportional to the amount of charge carriers. The most conductive Al system shows the highest value in A. Since the charge carriers in Al system require lower energy for transportation, the coordinative bonding between polymer backbone and charge carriers is weaker. This weaker interaction could ease the ionic dissociation in the polymer electrolyte and hence promote the ionic hopping mechanism by providing more mobile charge carriers. So we can conclude that the highest conductivity of Al system is ascribed to the highest amount of charge carriers and ionic mobility.

4.4.3 Differential Scanning Calorimetry (DSC) Differential scanning calorimetry is an important tool to study the thermal behavior of the polymer electrolytes. The DSC curves of all the polymer systems are shown in Figures 4.9–4.11.

Ceramic and Inorganic Polymer Membranes  115 Exo

9.06 °C

Heat flow (W/g)

12.15 °C 14.11 °C 16.32 °C

Endo

18.12 °C

–20 –15 –10 –5 0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 Temperature (°C)

Filler-free system

Si system

Zr system

Ti system

Al system

Exo

Figure 4.9  Glass transition temperature (Tg) of polymer electrolytes.

Heat flow (W/g)

167.90 °C

175.45 °C

Endo

178.23 °C 179.1 °C 184.13 °C 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 215 220 Temperature (°C) Filler-free system

Si system

Zr system

Ti system

Al system

Figure 4.10  Crystalline melting temperature (Tm) of polymer electrolytes.

The initial drop in heat flow is known as glass transition temperature (Tg). This change in heat flow is then followed by an endothermic peak, which is designated as crystalline melting temperature (Tm). Both of these temperatures are obtained in the heating scan within the temperature regime. On the contrary, there is only one exothermic peak in the cooling scan, i.e., crystallization temperature (Tc). These temperatures are shifted

Exo

116  Nanostructured Polymer Membranes: Volume 1

Heat flow (W/g)

127.25 °C 133.66 °C 135.39 °C

Endo

140.91 °C

147.57 °C

90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 Temperature (°C) Filler-free system

Si system

Zr system

Ti system

Al system

Figure 4.11  Crystallization temperature (Tc) of polymer electrolytes.

to lower temperature as the conductivity of polymer electrolytes increases. The Tg is very important in determining the flexibility of polymer chains, which contribute to high ionic conductivity. The glassy-like properties of polymer matrix will be changed to a rubbery state in an amorphous region at a certain temperature (so-called Tg) upon heating. Conductive polymer electrolytes could theoretically have low Tg. This is in good agreement with the results obtained in DSC. The Tg is lowered when we added the fillers into the polymer complexes. Among all the filler-based polymer systems, Al system portrays the lowest Tg. Incorporation of nanofillers could soften the polymer backbone at low Tg. The softening in polymer chains enhances the flexibility of polymer matrix and hence promotes the detachment of the charge carriers. Consequently, more mobile charge carriers would be transported in the polymer electrolytes, resulting in higher ionic conductivity. We propose that the reduced crystallinity in the polymer electrolyte can be one of the attributors in higher ionic conductivity of filler-doped polymer electrolytes in Section 7.1.4.2. The downward shift in Tm and Tc might be ascribed to the lesser crystalline region in the polymer electrolytes. The extent of the peaks is related to the crystallinity of the polymer electrolytes. The smaller the surface area, the lesser the crystallinity of the polymer electrolyte. Again, the most conducting Al system illustrates the lowest Tm and Tc with small peak intensity. The relative degree of crystallinity is also calculated from the melting endotherm using the equation below and summarized in Table 4.2:

Ceramic and Inorganic Polymer Membranes  117 Table 4.2  Heat of fusion and relative crystallinity of polymer electrolytes obtained from melting endotherm. Sample



Heat of Fusion (Jg–1)

Relative crystallinity (%)

Filler-free system

719.7

100

Si system

358.7

50

Zr system

287.6

0.40

Ti system

217.1

0.30

Al system

203.3

0.28

Xc

Hm 100%  Hm

(4.9)

where ΔHm denotes the heat of fusion of sample and H m is the heat of fusion of pure PVA obtained from the DSC result (719.7 Jg–1 in this work). The heat of fusion is area under the curve of melting peak, which can be determined using Universal V4.7A software. This result proves that addition of nanosized fillers can reduce the degree of crystallinity in the polymer matrix. The degree of crystallinity of polymer electrolytes is reduced almost 50% upon addition of hydrophilic silica. Abrupt decreases in relative crystallinity are observed when we added zirconia, titania and alumina into the polymer systems. The relative crystallinity of Al system is expected to be the lowest, as this system reached the highest ionic conductivity. We can conclude that the higher ionic conductivity in the filler-added polymer electrolytes is mainly due to the lower crystallinity of polymer matrix.

4.4.4 Linear Sweep Voltammetry (LSV) Linear sweep voltammetry is studied to investigate the electrochemical stability window of the polymer electrolytes. This study is very important to check whether the polymer electrolyte is suitable to be applied in the electrochemical device or not. Figures 4.12–4.16 exemplify the LSV responses of filler-free polymer electrolyte and NCPEs. The filler-free system portrays a short potential window of 1.2 V, ranging from –0.7 V to 0.5 V, as can been seen in Figure 4.12. We found that the potential window of NCPEs has been almost doubled, up to 2–2.3 V. The

118  Nanostructured Polymer Membranes: Volume 1 0.03 0.02

0.5 V

–0.7 V

Current, I ( A)

0.01 0 –0.01

–4

–3

–2

–1

0

1

2

3

4

3

4

–0.02 –0.03 –0.04 –0.05 –0.06 Cell potential, E (V)

Figure 4.12  LSV response of filler-free system.

2.5 1.5

1.1 V

–1.0 V

Current, I (mA)

0.5 –0.5 –4

–3

–2

–1

0

1

2

–1.5 –2.5 –3.5 –4.5 Cell potential, E (V)

Figure 4.13  LSV response of Si system.

Si and Zr systems possess lower potential window compared to Ti and Al systems. The electrochemical window of Si and Zr systems are 2.1 V and 2 V, respectively, where their cathodic potential limits are –1 V. On the contrary, the respective anodic potential limits of Si and Zr systems are 1.1 V and 1 V. The Ti and Al systems demonstrate the same anodic potential limit at 1.2 V but different cathodic potential window. The Ti system can be

Ceramic and Inorganic Polymer Membranes  119 0.8 0.6 0.4

1.0 V

–1.0 V

Current, I (mA)

0.2 0 –0.2

–4

–3

–2

–1

0

1

2

3

4

2

3

4

–0.4 –0.6 –0.8 –1 –1.2 Cell potential, E (V)

Figure 4.14  LSV response of Zr system. 4

Current, I (mA)

0 –2 –4

1.2 V

–1.1 V

2 –3

–2

–1

0

1

–4 –6 –8 –10 –12 –14 –16 –18 –20 Cell potential, E (V)

Figure 4.15  LSV response of Ti system.

charged up to 2.3 V, whereas the most conductive Al system can be operated up to 2.2 V. Based on the results, we can deduct that filler can widen the electrochemical stability window.

4.4.5 Cyclic Voltammetry (CV) Cyclic voltammetry is a useful technique to determine the electrochemical behavior of an EDLC. The fabricated EDLCs were charged and discharged

120  Nanostructured Polymer Membranes: Volume 1 6 4

1.2 V

–1.0 V

2

Current, I (mA)

0 –2

–4

–3

–2

–1

0

1

2

3

4

–4 –6 –8 –10 –12 –14 Cell potential, E (V)

Figure 4.16  LSV response of Al system.

110

Current, i ( A)

90 70 50 30 10 –10 –30

0

0.2

0.4

0.6

0.8

1

Cell potential, E (V)

Figure 4.17  Cyclic voltammogram of EDCL containing filler-free system.

in the potential range from 0 to 1 V. Figures 4.17–4.21 represent CV of EDLC containing filler-free polymer electrolyte and NCPEs. The capacitance of EDLC was evaluated using CV curve. The principle of energy storage in an EDLC is based on the formation of electrical double layer at the electrode-electrolyte boundary, previously mentioned in Section 7.2.5.1.

Ceramic and Inorganic Polymer Membranes  121 1 0.8

Current, i (mA)

0.6 0.4 0.2 0 –0.2 –0.4 –0.6 0

0.2

0.4

0.6

0.8

1

Cell potential, E (V)

Figure 4.18  Cyclic voltammogram of EDCL containing Si system.

1.2 1

Current, i (mA)

0.8 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 0

0.2

0.4

0.6

0.8

1

Cell potential, E (V)

Figure 4.19  Cyclic voltammogram of EDCL containing Zr system.

No redox reaction is observed in all EDLCs, revealing non-Faradic reaction in the EDLC. The abrupt increase in current at the high potential is suggestive of the degradation of polymer electrolytes. All EDLCs demonstrate CV approaching ideal rectangular shape. However, the surface area of the rectangular shape of filler-based EDLCs is much bigger than that of the polymer electrolyte without addition of filler. This infers that impregnation of filler can improve the electrochemical properties of EDLC. The specific capacitance of EDCL containing filler-free system

122  Nanostructured Polymer Membranes: Volume 1 1.2 1

Current, i (mA)

0.8 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 0

0.2

0.4

0.6

0.8

1

Cell potential, E (V)

Figure 4.20  Cyclic voltammogram of EDCL containing Ti system.

2.5 2

Current, i (mA)

1.5 1 0.5 0 –0.5 –1 –1.5 0

0.2

0.4

0.6

0.8

1

Cell potential, E (V)

Figure 4.21  Cyclic voltammogram of EDCL containing Al system.

is 0.15 F g–1, which is equivalent to 1.74 mF cm–2. Nanofiller-based EDLCS exhibit more than a 10-fold increase in specific capacitance. The specific capacitances of EDLCs containing Si system and Zr system are 2.88 F g–1 (equivalent to 34.54 mFcm–2) and 4.18 F g–1 (equivalent to 38.85 mF cm–2), respectively. A slightly higher specific capacitance is attained by EDLC containing Ti system in comparison to the previous two systems. This system shows the capacitance of 4.20 F g–1 (equivalent to 40.85 mF cm–2).

Ceramic and Inorganic Polymer Membranes  123 Electrolyte

Electrical double layer

Electrode

where charge carrier

pore

Figure 4.22  Schematic diagrams of the formation of electrical double layer at the electrode-electrolyte interface.

On the other hand, the capacitance of EDLC containing the most conducting Al system is increased significantly to 7.55 F g–1 or equivalent to 90.54 mF cm–2. There is a strong relationship between the ionic conductivity of polymer electrolyte and specific capacitance of EDLC, i.e., the capacity of EDLC would be higher if conductivity of the polymer electrolyte is higher. This can be proven in the CV findings as the specific capacitance of EDLC is in this order: Filler-free system < Si system < Zr system < Ti system < Al system. So, we can conclude that increment of capacitance in filler-based EDLC is mainly due to its high conductivity of polymer electrolytes. High ionic conductivity of polymer electrolytes denotes high ion transportation within the macromolecules. The mobile charge carriers with high mobility will migrate in the polymer complexes during charging process when an external electric field is connected to an EDLC, as shown in Figure 4.22. There will be more charge carriers with high mobility transported in the polymer matrix if the polymer electrolyte is conductive. These mobile charge carriers could be adsorbed on the pores of carbon electrode. As a result, these ions would build up an electrical double layer at the electrodeelectrolyte interface, as illustrated in Figure 4.22. Another feature that we must take note of is that the operational current used in NCPEs-based EDLCs for the charging and discharging processes are relatively higher than EDLC containing filler-free system. We suggest that this is related to the rapid ion adsorption. So, higher energy is required to drive the electrons moving from an electrode to opposite electrode. Therefore, higher current is needed for charging and discharging processes. Among all the NCPE-based EDLCs, Al system is a promising candidate to be a conducting medium in EDLC as it achieves the highest capacitance value with a good CV curve.

124  Nanostructured Polymer Membranes: Volume 1

4.4.6 Galvanostatic Charge-discharge Analysis (GCD) Galvanostatic charge-discharge analysis is employed to study the electrochemical properties of EDLC upon charging and discharging processes. Figures 4.23–4.26 demonstrate the GCDs of NCPE-based EDLCs over the first 5 cycles of charging and discharging. Some parameters, such as specific discharge capacitance, coulombic efficiency, energy density and power density of EDLCs, were calculated and tabulated in Table 4.3. All the figures depict symmetrical shape, except Figure 4.24 (which is Zr system-based EDLC). Even though EDLC assembling with Zr system has higher specific discharge capacitance than that of Si system, it has poor electrochemical properties with low coulombic efficiency, as this system requires longer time for the ion accumulation and shorter time for the ion desorption. Therefore, it is not a good choice to be applied in EDLC application. The initial drop in the cell potential in the discharge cycle is known as internal resistance or ohmic loss. This resistance originates from interfacial resistance between electrolyte and electrode, interfacial resistance between current collector and active material, and resistances of electrolyte, active materials and connector [100, 102, 103]. The resistance between electrode 1.2

Cell potential, E (V)

1

0.8

0.6

0.4

0.2

0

0

20

40

60

80

100

120

140

160

180

200

220

Time (s)

Figure 4.23  Galvanostatic charge-discharge performances of EDLC containing Si system over first 5 cycles.

Ceramic and Inorganic Polymer Membranes  125 1.2

Cell potential, E (V)

1

0.8

0.6

0.4

0.2

0

0

100

200

300

400

500

600

700

Time (s)

Figure 4.24  Galvanostatic charge-discharge performances of EDLC containing Zr system over first 5 cycles. 1.2

Cell potential, E (V)

1

0.8

0.6

0.4

0.2

0

0

100

200

300

400

500

600

700

Time (s)

Figure 4.25  Galvanostatic charge-discharge performances of EDLC containing Ti system over first 5 cycles.

126  Nanostructured Polymer Membranes: Volume 1 1.2

Cell potential, E (V)

1 0.8 0.6 0.4 0.2 0 0

200

400

600

800

1000

Time (s)

Figure 4.26  Galvanostatic charge-discharge performances of EDLC containing Al system over first 5 cycles.

Table 4.3  The specific capacitance, coulombic efficiency, energy density and power density of NCPE-based EDLCs. Specific discharge capacitance, Csp (F g–1)

Coulombic efficiency, η (%)

Si system

2.58

86

0.18

34.94

Zr system

4.16

40

0.36

36.76

Ti system

4.34

76

0.42

38.41

Al system

8.62

81

0.95

41.15

System

Power Energy density, density, E (W h kg–1) P (W kg–1)

and electrolyte arises from the charge transfer resistance and bulk resistance of polymer electrolyte [104]. It is noteworthy that this internal resistance of the cell is decreased as the ionic conductivity of polymer electrolyte increases. A similar observation has also been observed in specific discharge capacitance, energy density and power density of EDLC. High conductive polymer electrolyte has higher capacitance, energy density and power density compared with that of less conductive polymer electrolyte.

Ceramic and Inorganic Polymer Membranes  127 So, we can conclude that the capacitance, energy density and power density of an EDLC is strongly related to ionic transportation in the polymer electrolyte. The result is reliable, as the specific discharge capacitance obtained in GCD is almost the same as the CV findings. The fabricated EDLC using Si system reveals the lowest value in capacitance (2.58 Fg–1), energy density (0.18 W h kg–1) and power density (43.93 W kg–1) and discloses that Si system is not suitable to be applied in EDLC. These parameters are boosted drastically in both Zr and Ti systems. The Zr system displays capacitance of 4.16 Fg–1 and energy density of 0.36 W h kg–1 with power density of 36.76 W kg–1, whereas Ti system portrays the capacitance of 4.34 Fg–1 and energy density of 0.42W h kg–1 with power density of 38.41 W kg–1. Significant increases in capacitance, energy density and power density are also observed in the most conducting Al system. Among all the NCPEs, Al system is the most talented candidate as a polymer electrolyte in EDLC as it achieves the highest specific discharge capacitance of 8.62 Fg–1, energy density of 0.95 W h kg–1 and power density of 41.15 W kg–1.

4.5 Conclusions Nanocomposite polymer electrolytes containing silica, zirconia, titania and alumina were prepared and analyzed in this work. The effect of adding filler into the polymer electrolyte was also investigated throughout the work. Incorporation of filler not only increases the ionic conductivity of the polymer electrolyte, but also improves the electrochemical stability window. Electrochemical properties can also be enhanced by doping with fillers. All the polymer electrolytes obey Arrhenius theory and indicate the presence of ionic hopping mechanism. Addition of nanosized fillers can lower the Tg and crystallinity of the polymer matrix, as proven in the thermal studies. NCPE-based EDLCs also exhibit excellent electrochemical properties by showing higher capacitance than EDLC using filler-free system. The higher the conductivity of polymer electrolyte, the higher the specific capacitance of the EDLC is, along with lower internal resistance. The capacitance values of all NCPE-based EDLCs obtained in the chargedischarge curve are in good agreement with the CV results. Alumina-based polymer electrolyte is a good candidate to be applied in electrochemical devices. This system achieves the maximum ionic conductivity of 2.30 mS  cm–1 at ambient temperature, while the fabricated EDLC using this system also achieves the maximum specific capacitance of 7.55 F g–1 (or equivalent to 90.54 mFcm–2), energy density of 0.95 W h kg–1 and power density of 41.15 W kg–1, as shown in the charge-discharge curve.

128  Nanostructured Polymer Membranes: Volume 1

Acknowledgment One of the authors, Chiam-Wen Liew, gratefully acknowledges the Skim Bright Sparks Universiti Malaya (SBSUM) for the scholarship awarded.

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130  Nanostructured Polymer Membranes: Volume 1 28. G.A. Giffin, M. Piga, S. Lavina, M.A. Navarra, A. D’Epifanio, B. Scrosati, V.D. Noto, Characterization of sulfated-zirconia/Nafion composite membranes for proton exchange membrane fuel cells. J. Power Sources, 198, 66–75, 2012. 29. S. Jung, D.W. Kim, S.D. Lee, M. Cheong, D.Q. Nguyen, B.W. Cho, H.S. Kim, Fillers for solid-state polymer electrolytes: Highlight. Bull. Korean Chem. Soc., 30(10), 2355–2361, 2009. 30. V.D. Noto, M. Bettiol, F. Bassetto, N. Boaretto, E. Negro, S. Lavina, F. Bertasi, Hybrid inorganic-organic nanocomposite polymer electrolytes based on Nafion and fluorinated TiO2 for PEMFCs. Inter. J. Hydrogen Energy, 37(7), 6169–6181, 2012. 31. P. Zapata, J.H. Lee, J.C. Meredith, Composite proton exchange membranes from zirconium-based solid acids and PVDF/acrylic polyelectrolyte blends. J. Appl. Polym. Sci., 124(S1), E241-E 250, 2012. 32. S.J. Lue, D.T. Lee, J.Y. Chen, C.H. Chiu, C.C. Hu, Y.C. Jean, J.Y. Lai, Diffusivity enhancement of water vapor in poly(vinyl alcohol)-fumed silica nano-composite membranes: Correlation with polymer crystallinity and free-volume properties. J. Membr. Sci., 325(2), 831–839, 2008. 33. A.R. Polu, R. Kumar, Effect of Al2O3 ceramic filler on PEG–based composite polymer electrolytes for magnesium batteries. Adv. Mater. Lett., 4(7), 543–547, 2013. 34. J.H. Tian, P.F. Gao, Z.Y. Zhang, W.H. Luo, Z.Q. Shan, Preparation and performance evaluation of a Nafion–TiO2 composite membrane for PEMFCs. Inter. J. Hydrogen Energy, 33(20), 5686–5690, 2008. 35. K.M. Kim, N.G. Park, K.S. Ryu, K.S. Chang, Characterization of poly(vinylidenefluoride-co-hexafluoropropylene)-based polymer electrolytes filled with TiO2 nanoparticles. Polymer, 43(14), 3951–3957, 2002. 36. R. Hammami, Z. Ahamed, K. Charradi, Z. Beji, I.B. Assaker, J.B. Naceur, B. Auvity, G. Squadrito R. Chtourou, Elaboration and characterization of hybrid polymer electrolytes Nafion–TiO2 for PEMFCs. Inter. J. Hydrogen Energy, 38(26), 11583–11590, 2013. 37. D. Saikia, Y.W. Chen-Yang, Y.T. Chen, Y.K. Li, S.I. Lin, 7Li NMR spectroscopy and ion conduction mechanism of composite gel polymer electrolyte: A comparative study with variation of salt and plasticizer with filler. Electrochim. Acta, 54(4), 1218–1227, 2009. 38. P. Raghava, X. Zhao, J.K. Kim, J. Manuel, G.S. Chauhan, J.H. Ahn, C. Nah, Ionic conductivity and electrochemical properties of nanocomposite polymer electrolytes based on electrospun poly(vinylidenefluoride-cohexafluoropropylene) with nano-sized ceramic fillers. Electrochim. Acta, 54(2), 228–234, 2008. 39. R.F. Joel, Polymer Science and Technology, 2nd ed., 283 pp., Prentice Hall, US, 2003. 40. C. Piconi, S.G. Condo, T. Kosmač, Alumina- and zirconia-based ceramics for load-bearing applications, in: Advanced Ceramics for Dentistry, J. Shen (Ed.), 1st ed., pp. 219–253, Elsevier, USA, 2014.

Ceramic and Inorganic Polymer Membranes  131 41. M. Aghayan, I. Hussainova, M. Gasik, M. Kutuzov, M. Friman, Coupled thermal analysis of novel alumina nanofibers with ultrahigh aspect ratio. Thermochim. Acta, 574, 140–144, 2013. 42. M. Cao, Q. Yan, X. Li, Y. Mi, Effect of plate-like alumina on the properties of alumina ceramics prepared by gel-casting. Mater. Sci. Eng. A, 589, 97–100, 2014. 43. X. Li, P. Wu, D. Zhu, Properties of porous alumina ceramics prepared by technique combining cold-drying and sintering. Int. J. Refract. Met. Hard Mater., 41, 437–441, 2013. 44. G. Yamamoto, K. Shirasu, T. Hashida, T. Takagi, J.W. Suk, J. An, R.D. Piner, R.S. Ruoff, Nanotube fracture during the failure of carbon nanotube/alumina composites. Carbon, 49(12), 3709–3716, 2011. 45. S. Zuzjaková, P. Zeman, S. Kos, Non-isothermal kinetics of phase transformations in magnetron sputtered alumina films with metastable structure. Thermochim. Acta, 572, 85–93, 2013. 46. P.A.R.D. Jayathilaka, M.A.K.L. Dissanayake, I. Albinsson, B.E. Mellander, Effect of nano-porous Al2O3 on thermal, dielectric and transport properties of the (PEO)9LiTFSI polymer electrolyte system. Electrochim. Acta, 47(20), 3257–3268, 2002. 47. M.A.K.L. Dissanayake, P.A.R.D. Jayathilaka, R.S.P. Bokalawala, I. Albinsson, B.E. Mellander, Effect of concentration and grain size of alumina filler on the ionic conductivity enhancement of the (PEO)9LiCF3SO3:Al2O3 composite polymer electrolyte. J. Power Sources, 119–121, 409–414, 2003. 48. E.M. Masoud, A.A. El-Bellihi, W.A. Bayoumy, M.A. Mousa, Organicinorganic composite polymer electrolyte based on PEO–LiClO4 and nanoAl2O3 filler for lithium polymer batteries: Dielectric and transport properties. J. Alloys Compd., 575, 223–228, 2013. 49. R. Dittmann, E. Wintermantel, T. Graule, Sintering of nano-sized titania particles and the effect of chlorine impurities. J. Eur. Ceram. Soc., 33(15–16), 3257–3264, 2013. 50. K.M. Kim, J.M. Ko, N.G. Park, K.S. Ryu, S.H. Chang, Characterization of poly(vinylidenefluoride-co-hexafluoropropylene)-based polymer electrolyte filled with rutile TiO2 nanoparticles. Solid State Ionics, 161(1–2), 121–131, 2003. 51. S. Arunmetha, P. Manivasakan, A. Karthik, N.R.D. Babu, S.R. Srither, V. Rajendran, Effect of processing methods on physicochemical properties of titania nanoparticles produced from natural rutile sand. Adv. Powder Technol., 24(6), 972–979, 2013. 52. M. Mehta, V. Raman, R.O. Fox, On the role of gas-phase and surface chemistry in the production of titania nanoparticles in turbulent flames. Chem. Eng. Sci., 104, 1003–1018, 2013. 53. S.Y. Huang, P. Ganesan, B.N. Popov, Electrocatalytic activity and stability of niobium-doped titanium oxide supported platinum catalyst for polymer electrolyte membrane fuel cells. Appl. Catal. B, 96(1–2), 224–231, 2010.

132  Nanostructured Polymer Membranes: Volume 1 54. C.C. Yang, W.C. Chien, Y.J. Li, Direct methanol fuel cell based on poly(vinyl alcohol)/titanium oxide nanotubes/poly(styrene sulfonic acid) (PVA/nt–TiO2/ PSSA) composite polymer membrane. J. Power Sources, 195(11), 3407–3415, 2010. 55. H. Ekström, B. Wickman, M. Gustavsson, P. Hanarp, L. Eurenius, E. Olsson, G. Lindbergh, Nanometer-thick films of titanium oxide acting as electrolyte in the polymer electrolyte fuel cell. Electrochim. Acta, 52(12), 4239–4245, 2007. 56. G. Katsaros, T. Stergiopoulos, I.M. Arabatzis, K.G. Papadokostaki, P. Falaras, A solvent-free composite polymer/inorganic oxide electrolyte for high efficiency solid-state dye-sensitized solar cells. J. Photochem. Photobiol. A, 149(1–3), 191–198, 2002. 57. J.D. Jeon, M.J. Kim, S.Y. Kwak, Effect of addition of TiO2 nanoparticles on mechanical properties and ionic conductivity of solvent-free polymer electrolytes based on porous P(VdF–HFP)/P(EO–EC) membranes. J. Power Sources, 162(2), 1304–1311, 2006. 58. C. Piconi, G. Maccauro, Zirconia as a ceramic biomaterial. Biomater., 20(1), 1–25, 1999. 59. I. Danilenko, F. Glazunov, T. Konstantinova, G. Volkova, V. Burkhovetski, Effect of oxide nanofillers on fabrication, structure, and properties of zirconia-based composites. J. Euro. Ceram. Soc., 33(12), 2321–2325, 2013. 60. S. Pundir, N. Chauhan, J. Narang, C.S. Pundir, Amperometric choline biosensor based on multiwalled carbon nanotubes/zirconium oxide nanoparticles electrodeposited on glassy carbon electrode. Anal. Biochem., 427(1), 26–32, 2012. 61. R. Vinodh, M. Purushothaman, D. Sangeetha, Novel quaternized polysulfone/ZrO2 composite membranes for solid alkaline fuel cell applications. Inter. J. Hydrogen Energy, 36(12), 7291–7302, 2011. 62. Z. Özkurt, U. İseri, E. Kazazoglu, Zirconia ceramic post systems: A literature review and a case report. Dent. Mater. J., 29(3), 233–245, 2010. 63. C.C. Yang, Study of alkaline nanocomposite polymer electrolytes based on PVA–ZrO2–KOH. Mater. Sci. Eng. B, 131(1–3), 256–262, 2006. 64. T. Uma, T. Mahalingam, U. Stimming, Mixed phase solid polymer electrolytes based on poly(methylmethacrylate) systems. Mater. Chem. Phys., 82(2), 478–483, 2003. 65. A. Saccà, I. Gatto, A. Carbone, R. Pedicini, E. Passalacqua, ZrO2–Nafion composite membranes for polymer electrolyte fuel cells (PEFCs) at intermediate temperature. J. Power Sources, 163(1), 47–51, 2006. 66. J. Pan, H. Zhang, W. Chen, M. Pan, Nafion-zirconia nanocomposite membranes formed via in situ sol-gel process. Inter. J. Hydrogen Energy, 35(7), 2796–2801, 2010. 67. M. Nasibi, M.A. Golozar, G. Rashed, Nano zirconium oxide/carbon black as a new electrode material for electrochemical double layer capacitors. J. Power Sources, 206, 108–110, 2012.

Ceramic and Inorganic Polymer Membranes  133 68. J.A. Hriljac, M.M. Eddy, A.K. Cheetham, J.A. Donohue, G.J. Ray, Powder neutron diffraction and 29Si MAS NMR studies of siliceous zeolite-Y. J. Solid State Chem., 106(1), 66–72, 1993. 69. K. Kihara, M. Takeo, I. Moritaka, Structural change of orthorhombic-tridymite with temperature: A study based on second-order thermal-vibrational parameters. Z. Kristallogr., 177(1–2), 27–38, 1986. 70. J. Kuriplacha, W. Anwand, G. Brauer, W. Skorupa, Positron characteristics of various SiO2 polymorphs. Appl. Surf. Sci., 194(1–4), 84–88, 2002. 71. L. Levien, C.T. Prewitt, High-pressure crystal structure and compressibility of coesite. Am. Mineral., 66, 324–333, 1981. 72. L.G. Liu, Bulk moduli of SiO2 polymorphs: Quartz, coesite and stishovite. Mech. Mater., 14(4), 283–290, 1993. 73. G. Miehe, H. Graetsch, Crystal structure of moganite: A new structure type for silica. Eur. J. Mineral., 4(4), 693–706, 1992. 74. J.R. Smyth, R.J. Swope, A.R. Pawley, H in rutile-type compounds: II. Crystal chemistry of Al substitution in H-bearing stishovite. Am. Mineral., 80, 454–456, 1995. 75. A. Weiss, A. Weiss, Über siliciumchalkogenide. VI. zur kenntnis der faserigen siliciumdioxyd-modifikation. Z. Anorg. Allg. Chem., 276(1–2), 95–112, 1954. 76. S.A. Khan, G.L. Baker, S. Colson, Composite polymer electrolytes using fumed silica fillers: Rheology and ionic conductivity. Chem. Mater., 6(12), 2359–2363, 1994. 77. P. Junlabhut, S. Boonruang, W. Pecharapa, Optical absorptivity enhancement of SiO2 thin film by Ti and Ag additive. Energy Procedia, 34, 734–739, 2013. 78. M. Osinska, M. Walkowiak, A. Zalewska, T. Jesionowski, Study of the role of ceramic filler in composite gel electrolytes based on microporous polymer membranes. J. Membr. Sci., 326(2), 582–588, 2009. 79. S. Ahmad, S.A. Agnihotry, Nanocomposite electrolytes with fumed silica in poly(methyl methacrylate): Thermal, rheological and conductivity studies. J. Power Sources, 140(1), 151–156, 2005. 80. I. Choi, K.G. Lee, S.H. Ahn, D.H. Kim, O.J. Kwon, J.J. Kim, Sonochemical synthesis of Pt-deposited SiO2 nanocomposite and its catalytic application for polymer electrolyte membrane fuel cell under low-humidity conditions. Catal. Commun., 21, 86–90, 2012. 81. V. Aravindan, P. Vickraman, Polyvinylidenefluoride-hexafluoropropylene based nanocomposite polymer electrolytes (NCPE) complexed with LiPF3(CF3CF2)3. Eur. Polym. J., 43(12), 5121–5127, 2007. 82. C.C. Yang, Y.J. Li, T.H. Liou, Preparation of novel poly(vinyl alcohol)/SiO2 nanocomposite membranes by a sol-gel process and their application on alkaline DMFCs. Desalination, 276(1–3), 366–372, 2011. 83. S. Ketabi, K. Lian, Effect of SiO2 on conductivity and structural properties of PEO–EMIHSO4 polymer electrolyte and enabled solid electrochemical capacitors. Electrochim. Acta, 103, 174–178, 2013.

134  Nanostructured Polymer Membranes: Volume 1 84. E. Frackowiak, Carbon materials for supercapacitor application. Phys. Chem. Chem. Phys., 9(15), 1774–1785, 2007. 85. Z.S. Wu, K. Parvez, X. Feng, K. Müllen, Graphene-based in-plane microsupercapacitors with high power and energy densities. Nat. Commun., 4, 1–8, 2013. 86. N.A. Choudhury, S. Sampath, A.K. Shukla, Hydrogel-polymer electrolytes for electrochemical capacitors: An overview. Energy Environ. Sci., 2(1), 55–67, 2009. 87. S.A. Hashmi, H.M. Upadhyaya, Polypyrrole and poly(3–methyl thiophene)based solid state redox supercapcitors using ion conducting polymer e­ lectrolyte. Solid State Ionics, 152–153, 883–889, 2002. 88. C. Peng, S. Zhang, D. Jewell, G.Z. Chen, Carbon nanotube and conducting polymer composites for supercapacitors. Prog. Nat. Sci., 18(7), 777–788, 2008. 89. Y. Shao, Z. Yi, F. Lu, F. Deng, B. Li, Poorly crystalline Ru0.4Sn0.6O2 nanocomposites coated on Ti substrate with high pseudocapacitance for electrochemical supercapacitors. Adv. Chem. Eng. Sci., 2(1), 118–122, 2012. 90. S.A. Hashmi, R.J. Latham, R.G. Linford, W.S. Schlindwein, Studies on all solid state electric double layer capacitors using proton and lithium ion conducting polymer electrolytes. J. Chem. Soc., Faraday Trans., 93(23), 4177–4182, 1997. 91. B. Fang, L. Binder, A novel carbon electrode material for highly improved EDLC performance. J. Phys. Chem. B, 110(15), 7877–7882, 2006. 92. E. Frackowiak, F. Béguin, Carbon materials for the electrochemical storage of energy in capacitors. Carbon, 39(6), 937–950, 2001. 93. L. Deng, G. Zhang, L. Kang, Z. Lei, C. Liu, Z.H. Liu, Graphene/VO2 hybrid material for high performance electrochemical capacitor. Electrochim. Acta, 112, 448–457, 2013. 94. J. Jiang, G. Tan, S. Peng, D. Qian, J. Liu, D. Luo, Y. Liu, Electrochemical performance of carbon-coated Li3V2(PO4)3 as a cathode material for asymmetric hybrid capacitors. Electrochim. Acta, 107, 59–65, 2013. 95. M. Endo, T. Takeda, Y.J. Kim, K. Koshiba, K. Ishii, High power electric double layer capacitor (EDLC’s): From operating principle to pore size control in advanced activated carbons. Carbon Sci., 1(3–4), 117–128, 2001. 96. G.P. Pandey, S.A. Hashmi, Y. Kumar, Performance studies of activated charcoal based electrical double layer capacitors with ionic liquid gel polymer electrolytes. Energy Fuels, 24, 6644–6652, 2010. 97. H. Yu, J. Wu, L. Fan, Y. Lin, K. Xu, Z. Tang, C. Cheng, S. Tang, J. Lin, M. Huang, Z. Lan, A novel redox-mediated gel polymer electrolyte for highperformance supercapcitor. J. Power Sources, 198, 402–407, 2012. 98. M.J. Jung, E. Jeong, S. Kim, S.I. Lee, J.S. Yoo, Y.S. Lee, Fluorination effect of activated carbon electrodes on the electrochemical performance of electric double layer capacitors. J. Fluorine Chem., 132(12), 1127–1133, 2011. 99. F.E. Amitha, A.L.M. Reddy, S. Ramaprabhu, A non-aqueous electrolytebased asymmetric supercapacitor with polymer and metal oxide/multiwalled carbon nanotube electrodes. J. Nanopart. Res., 11(3), 725–729, 2009.

Ceramic and Inorganic Polymer Membranes  135 100. A.K. Arof, M.Z. Kufian, M.F. Syukur, M.F. Aziz, A.E. Abdelrahman, S.R.  Majid, Electrical double layer capacitor using poly(methyl ­methacrylate)–C4BO8Li gel polymer electrolyte and carbonaceous material from shells of mata kucing (Dimocarpus longan) fruit. Electrochim. Acta, 74, 39–45, 2012. 101. S. Ramesh, C.W. Liew, Exploration on nano-composite fumed silica-based composite polymer electrolytes with doping of ionic liquid. J. Non-Cryst. Solids, 358(5), 931–940, 2012. 102. S. Mitra, A.K. Shukla, S. Sampath, Electrochemical capacitors with plasticized gel-polymer electrolytes. J. Power Sources, 101(2), 213–219, 2001. 103. J.P. Zheng, Resistance distribution in electrochemical capacitors with a bipolar structure. J. Power Sources, 137(1), 158–162, 2004.  104. C.W. Liew, S. Ramesh, A.K. Arof, Good prospect of ionic liquid basedpoly(vinyl alcohol) polymer electrolytes for supercapacitors with excellent electrical, electrochemical and thermal properties. Int. J. Hydrogen Energy, 39(6), 2953–2963, 2013.

5 Supramolecular Membranes: Synthesis and Characterizations Cher Hon Lau*, Matthew Hill and Kristina Konstas Commonwealth Scientific and Industrial Research Organisation, Clayton Victoria, Australia

Abstract

This chapter contains an overview of supramolecular membranes, primarily focusing on polymeric membranes and mixed matrix membranes for gas separation applications. Other topics discussed are membranes that are synthesized from self-assembly, hydrogen bonding, π-π stacking of block copolymer systems, small molecules, and nanoparticles. The first recorded experiment for gas transport in polymeric membranes was performed by Thomas Graham in 1829, which resulted in the formulation of two relationships in the years to come; gas diffusion and gas effusion, both becoming known as “Graham’s laws.” Extensive research has been performed and published, resulting in membranes being fabricated from a variety of organic or inorganic materials. The majority of commercially available membranes are made from synthetic organic polymers, due to the polymers ability to be controlled and tailored specifically. Improvements to the polymer matrices can be made with the addition of additives, which are known as mixed matrix membranes. Additives can introduce supplementary voids to increase the fractional free volume via intrinsic particle porosity or through repulsive interactions between polymer chains and particle surface. The surplus of void spaces can also act as adsorption sites for gas molecules, facilitating further gas transport. Polymeric membranes are susceptible to dense chain packing, reducing the fractional free volume and gas transport pathways. This chapter summarizes the recent research in utilizing supramolecular science for gas membrane separations. Keywords:  Metal organic frameworks, porous aromatic frameworks, self-­ assembled membranes

*Corresponding author: [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 1, (137–202) © 2017 Scrivener Publishing LLC

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5.1 Overview The Materials for Energy, Water and Environment team (MEWE) at the CSIRO, Australia’s national science agency, have developed a long-term research profile that centers around the ability to control void space, and exploit this control for a plethora of applications. These areas have included aging in metal alloys [1–5], gas storage [6–20], medical imaging [21], water purification [22], sensing of toxins [23] and minerals processing [24, 25]. However, the highest profile research has typically been undertaken within the fields of supramolecular chemistry and polymeric separation membranes [26–42]. Supramolecular chemistry has typically been focused within the inorganic field, with our understanding of porous silicas [43] leading to breakthroughs with electrochemical energy storage [44, 45]. This work was followed by the exploration of further control within micropores with the motivation of investigating molecular sieving within membranes. The MEWE team have in particular developed a strong interest in metal organic frameworks (MOFs), which, when combined with polymeric membranes, have been found to exhibit superior performances in a number of settings. Our expertise in supramolecular membranes extends from prediction and modeling of potential candidate materials [46, 47], to synthesis of new supramolecular compounds [6, 48–50], preparation [51, 52] and performance testing of candidate membranes [39, 41, 53, 54], through to detailed analysis of the void space created using techniques such as positron annihilation lifetime spectroscopy (PALS) [26–42] and solid-state nuclear magnetic resonance (SS-NMR) [3]. Drawing on this background, in the following we present an overview of what we believe to be the most prospective group of membranes for future gas separations—mixed matrix membranes containing MOFs. We highlight both the control over MOF chemistry for use in separations, and also their incorporation into membranes and the resulting potential for membrane-based separations.

5.2 Supramolecular Materials Supramolecular chemistry is broadly defined as the chemistry of multicomponent molecular assemblies in which the component units are held together by a variety of weaker (noncovalent) interactions—like hydrogen bonding, metal coordination, π-electron stacking or van der Waals forces. These systems rely on self-assembly and a distinct feature using weak,

Supramolecular Membranes  139 noncovalent interactions is that such interactions are normally readily reversible so that the final product is in thermodynamic equilibrium with the components, giving them a built-in capacity for error correction. This section discusses porous materials which utilize the fundamental features associated with supramolecular chemistry for use in membranes.

5.2.1 Porous Materials Membranes are commonly solid materials, such as polymers, that must have good chemical stability, efficiency, longevity and can be readily processed into defect-free thin films with high surface area; to ensure good chemical separation but also high throughput. New approaches have been designed and investigated to improve the membranes performance; this involves the incorporation of porous composite materials. The addition of porous composite materials to polymers provides chemical flexibility and improved separation by tuning the pore sizes, pore continuity and pore density. There is a broad range of porous materials; from natural to artificial, inorganic to organic and from crystalline to amorphous. However, there are only two main categories of porous solid materials; inorganic and organic carbon-based materials. Naturally found crystalline zeolites are considered inorganic and are a major part of the many industry processes. Activated carbons are amorphous organic porous materials with higher porosity then zeolites. A new advanced category of porous materials has emerged, inorganic-organic hybrid material, otherwise known as metalorganic frameworks (MOFs) and zeolitic-like metal-organic frameworks (ZMOF). Figure 5.1 presents the general current classification of porous solids. The hybrid inorganic-organic materials have created much research interest, none more so then for separations [55–58]. Porous solids Organic

Inorganic Organic-inorganic hybrid

Polymers

MOFs

Zeolites

Figure 5.1  Schematic representation of the general classification of porous solids (top, an example is given in each case: polymers for the porous organic solids; zeolites for porous inorganic solids; and MOFs for the porous organic-inorganic hybrid solids). (Reproduced from [55])

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5.2.1.1  Metal-organic Frameworks Metal-organic frameworks are a class of supramolecular coordination polymers that emerged in the literature over two decades ago when they could be identified by single-crystal X-ray crystallography [55, 59–62]. As mentioned, they can be best described as a hybrid material, consisting of metal and organic building blocks which self-assemble to give an infinite, uniformed framework with periodic porosity on the sub-nanometer scale. MOFs are connected by coordination bonds, noncovalent bonds and other weak interactions (H-bonds, π-electron stacking or van der Waals interactions). They can provide one-, two-, and three- dimensional crystalline ­networks with porosity and high surface area. One of the simplest and most studied MOF is the Zn4O cluster connected by 1,4-benzenedicarboxylate (BDC) linkers, known as MOF-5 (Figure 5.2) [63]. MOF-5 has a cubic structure with an ordered three-dimensional (3D) porous system and a calculated BET surface area of 3800 m2g–1 [64]. Highly complexed MOFs can be constructed from two or more linkers and in some cases improve the surface area and pore sizes [10, 65]. MOF UMCM-2 contains the Zn4O clusters and two distinct linkers (1,3,5-benzenetribenzoate and thieno-3,2-­ bithiophene-2,5-dicarboxylate) giving an improved calculated BET surface of 5300 m2g–1 [66]. The connectivity of the organic linker can be linear, ditopic, tritopic or tetratopic. The MOFs architecture can be designed from either

Figure 5.2  Construction of the MOF-5 framework. Top, the Zn4(O)(O12C6) cluster. Left, as a ball stick model (Zn, blue; O, green; C, grey). Middle, the same with the Zn4(O) tetrahedron indicated in green. Right, the same but now with the ZnO4 tetrahedra indicated in blue. Bottom, one of the cavities in the Zn4(O)(BDC)3, MOF-5 framework. Eight clusters (only seven visible) constitute a unit cell and enclose a large cavity, indicated by a yellow sphere of diameter 18.5 Å in contact with 72 C atoms (grey). (Reproduced with permission from [63])

Supramolecular Membranes  141

Edgedirected

Facedirected

Figure 5.3  A supramolecular cube can be constructed from a linear ditopic and 90° tritopic building blocks using a edge-directed method (left) or from six tetratopic panels joined by twelve 90° ditopic building blocks using a face- or panel-based approach (right). (Reproduced with permission from reference [67])

“edge-directed self-assembly” or a “face-directed self-assembly” approach. For example, a supramolecular cube can be constructed from linear ditopic and 90° tritopic building blocks to give an edge-directed self-assembly or from six tetratopic panels joined by twelve 90° ditopic building blocks using a face-directed approach (Figure 5.3) [67]. While the connectivity of the ligand building block is important, the metal building block plays an integral part in the overall MOFs structure. The most common metal ion used in the synthesis of MOFs are transition metals, particularly Zn(II) and Cu(II). The metal building block ­commonly used in MOF synthesis is the well-known paddlewheel dimetallic unit, consisting of the two metal centers bridged by four ligands to give the unit M4O(L)4. As shown in Figure 5.4, paddlewheel complexes can theoretically lead to 3D cubic grids, 2D square lattices, 1D linear wires and supramolecular squares [67]. With all sites used, an octahedral geometry results. With the capping of either the axial or equatorial sites, alternative geometries are formed. HKUST-1 was one of the earliest MOFs reported using a Cu2 paddlewheel configuration with benzenetricarboxylate ­linkers to give a 3D system with hexagonal-shaped 18 Å windows (Figure 5.5) [68]. As the metal building block determines the structure, the length of the building blocks determines the pore size within the structure. O’Keeffe and Yaghi have published a comprehensive review on the deconstruction and topological analysis of MOFs [69, 70]. 5.2.1.1.1 Synthesis The MOF structures are obtained by a self-assembling process starting from metal ions that assemble together with linker molecules. MOFs are successfully synthesized from solvothermal reactions with metal and organic building blocks dissolved in organic solvents and heated up to 130  °C. Mixed solvents can be used to control the solution polarity and the rate

142  Nanostructured Polymer Membranes: Volume 1

Open Open O O O O Open Open O O O Open O Open

(a)

Octahedral

Open Open O O O O Cap Cap M M O O O O Open

Square

Open

(b)

Open Cap Cap

O

O M O

(c)

O O

O Cap O O Cap

M

Open

trans-capped square

Figure 5.4  Paddlewheel dimetallic building blocks can be used to make 3D MOFs (a), 2D MOFs (b), 1D wires (c). (Reproduced with permission from [67])

Figure 5.5  [Cu3(TMA)2(H2O)3]n viewed along the cell body diagonal [111], showing a hexagonal shaped 18 Å window. (Reproduced with permission from [68])

of product crystallization. Solvents are almost always incorporated in the as-synthesized MOF structures and act as space-filling molecules. These can be used for generating open metal sites which are ideal for increasing gas adsorption capacity. Figure 5.6 summarizes the synthetic methods,

Supramolecular Membranes  143

Conventional heating

Electrochemistry

Microwave-assisted Mechanochemistry heating

Sonochemistry

Conventional methods

Conventional autoclave

Highthroughput methods

High-throughput autoclave

Room temperature

Size 1 nm – 1 mm

Elevated temperature

Morphology Thin films

Solvothermal conditions

Membranes

Temperature

Composites

Figure 5.6  Overview of synthesis methods, possible reaction temperatures, and final reaction products in MOF synthesis. (Reproduced with permission from [71])

possible reaction temperatures and possible final reaction products [71]. In addition to conventional heating used for solvothermal reactions, MOFs can be synthesized using electrochemistry, mechanochemistry and ultrasonic methods. The syntheses of MOFs typically favor the thermodynamic product because the coordination-driven self-assembly of metal-ligand bonds is substitutionally labile. If a rigid ligand coordinates in an improper orientation, the ligand can dissociate during the assembly and rectify the structural defect en route to a thermodynamic product (Figure 5.7) [67]. Furthermore, MOFs have also been shown in which the metal ions can be post-synthetically exchanged with alternative metal ions. MOF U ­ iO-66 is a zirconium-based MOF which is particularly stable to water vapor, although it exhibits mid-range CO2 adsorption capacity (1.7  mmol  g–1 at 1 atm 298 K) [72]. Recently reported was the post-­ synthetic exchange of zirconium ions with titanium (Ti) ions to deliver a heterobimetallic MOF; UiO-66(Zr/Ti) with a CO2 uptake of 4 mmol g–1 at 1 atm 298 K [8].

144  Nanostructured Polymer Membranes: Volume 1 Incorrect orientation

Dissociation

Thermodynamic product

Random reorientation

Figure 5.7  The self-assembly of a square may proceed through incorrectly oriented intermediate species. As these fragments associate and dissociate in solution, they will eventually funnel to the thermodynamically favored square, automatically healing any defects. (Reproduced with permission from [67])

5.2.1.1.2  Functional Exploration and Applications The fact that MOFs are thermally stable and porous in nature makes them prime candidates for gas storage applications, which has been the main application of interest. The first study of hydrogen adsorption was reported in 2003 for MOF-5 [73], confirming the potential of MOFs for gas storage. Since then, hundreds of MOFs have been studied for hydrogen and other gas adsorption applications [60, 74–77]. It has been shown that MOFs can reversibly absorb carbon dioxide gas, making them promising materials for the selective capture of carbon from the atmosphere and flue gas. The large quadrupole moment of carbon dioxide molecules causes them to interact with the framework, increasing the uptake of the gas over other inert adsorbents such as zeolites. Other applications MOFs have shown to have value in are biological applications such as drug sensing, delivery or as miniature devices [78–80]. While extensive efforts have gone into the synthesis of MOFs and gas storage applications, screening for industrial applications and MOF material processing are also increasingly developing areas. Interest has also been generated for MOFs being processed into thin films for practical membrane applications, although the body of literature is mostly on proof of principle thus far. 5.2.1.1.3  MOF Thin Films There are two classes of MOF thin films; polycrystalline films and selfassembled monolayers (SAMs) [81]. Polycrystalline films can be seen as an assembly of more or less randomly orientated MOF crystals or particles

Supramolecular Membranes  145

Metal precursor

Organic linker

SAM

Figure 5.8  The step-by-step approach for the growth of the SURMOFs on a SAMfunctionalized substrate. The approach involves repeated cycles of immersion in solutions of the metal precursor and solutions of organic ligand. Between steps the material is rinsed with solvent. (Reproduced with permission from [83])

that rest on a surface. The film thickness is related to the size of the MOF particles but generally often lies in the micrometer range. The second class, SAMs (also known as SURMOFs) consist of layer-by-layer adsorption of components from the liquid phase to a surface containing seed crystals or functional groups to give ultrathin MOF multilayers that are perfectly orientated and ordered. MOF, HKUST-1 was the first example of this to be made into a thin film on a functionalized substrate at room temperature [82]. The organic linkers and metal precursors are placed in separate ethanolic solutions and the substrate is sequentially immersed in the two solutions (Figure 5.8). Polycrystalline thin films are made from direct synthesis where a substrate is used bare with the appropriate mother growth solution for the given MOF, heat treated as required for solvothermal synthesis. Most recently, HKUST-1 membranes have been synthesized from solid mesoporous free-standing copper hydroxide nanostrand thin films as the metal source immersed into an ethanol-water solution of 1,3,5-­benzenetribenzoic acid [84]. This synthetic method produces 5 µm thick HKUST-1 membrane in 2 hours [85], the thinnest reported for gas separation experiments. MOF polycrystalline thin films are also synthesized by dip coating or slow diffusion of reactants. Dip coating requires repeated immersion of a substrate into freshly prepared mother solutions. This technique works well for MOFs that crystallize as soon as the linker and metal salt are mixed together. Kubo et al. were the first to exploit this technique with a ­pyrazolate-based MOF; [Cu2(pzdc)2(pyz)] [86]. The slow diffusion of reactants across an interface has also proven to be successful for the growth of MOF thin films. The method involves the metal and organic linker to cross a porous membrane and crystallize at the interface [87]. This method was applied for

146  Nanostructured Polymer Membranes: Volume 1 Nylon membrane

Zinc nitrate solution (a)

Zn2+

Hmim

ZIF-8 film

ZIF-8 film

Hmim solution (b)

Figure 5.9  (a) Diffusion cell for ZIF-8 film preparation and (b) the schematic formation of ZIF-8 films on both sides of the nylon support via contra-diffusion of Zn2+ and Hmim through the pores of the nylon support [88].

the fabrication of a zeolitic imidazole framework (ZIF) film (discussed in Section 5.2.2) on a flexible porous nylon membrane (Figure 5.9) [88]. The ZIF film was observed to grow mostly on the metal ion side of the membrane with a continuous film thickness of 18 µm.

5.2.1.2  Zeolitic Imidazole Frameworks (ZIFs) Zeolites are a group of microporous inorganic materials known as “molecular sieves,” as they have the ability to selectively sort molecules based primarily on a size exclusion process due to their pore structure. Zeolites are widely used in industry for water purification, adsorbents, catalysts and gas separations. They are naturally found but can also be synthesized to incorporate a range of small inorganic and organic species. The frameworks are constructed from tetrahedral groups (e.g., AlO4, SiO4, PO4, BeO4, GaO4, GeO4 and ZnO4) linked to each other by sharing all of the oxygen atoms to produce in excess of 150 different types of frameworks [89]. There are inherent limitations to inorganic zeolites; the connectivity within the material can often lead to blockage by larger diameter molecules, a problem overcome in zeolitic imidazole frameworks (ZIFs). ZIFs are a subset class of zeolites in which the tetrahedral building blocks can be replaced by common transition metals such as Zn, Co or Cu and the metal nodes are connected by organic imidazole linkers to give highly microporous structures analogous to zeolites networks [90]. ZIFs are MOF-like which can be designed to have specific pore sizes and network complexity that can enable alternative routes for molecule transportation if a blockage results [46]. Pore window diameters are within the range of 2–5 Å, owing to the metal-imidazolate-metal bridging angle of 145° [91]. The structures of ZIFs have been well documented since the pioneering work of You and

Supramolecular Membranes  147

ZIF-8 sod

ZIF-10 mer

ZIF-11 rho

Figure 5.10  The single crystal X-ray structures of ZIFs. (Left and Center) In each row, the net is shown as a stick diagram (left) and as a tiling (center). (Right) The largest cage in each ZIF is shown with ZnN4 tetrahedra in blue. (Reproduced with permission from [91])

co-workers in 2002 [60, 89, 92, 93]. Imidazolate linkers are five-membered rings that serve as the bridging unit between the metal center and the coordinating N atom in the ring. The single X-ray structures of ZIF-8, ZIF-10 and ZIF-11 are illustrated in Figure 5.10 [89]. 5.2.1.2.1  Synthetic Methods Traditional solvothermal methods are used to synthesize ZIFs and the crystalline materials precipitate after 24–96 h of heating. There have been recent advancements in the area of ZIF thin films being grown on hollow-fiber supports or on inorganic flat sheet materials. The thin film growth can be quickly achieved through seeded, microwave-induced or contra-diffusion growth [51]. Most recently patterned growth of ZIF thin films has been shown to grow on a porous polymeric substrate by selective modification of the surface chemistry using plasma polymerization [51] as well as using lithography to selectively crosslink ZIF with sol-gel upon exposure to X-ray radiation to give a micropattern thin film of ZIF/Sol-gel (Figure 5.11) [52]. 5.2.1.2.2  Functional Exploration and Applications As previously mentioned, zeolites have found applications as adsorbents, membranes and catalysts; as for ZIFs, much attention thus far has been focused on their use as adsorbent materials for mobile energy storage applications [94]. With ZIFs containing pore windows on a smaller scale

148  Nanostructured Polymer Membranes: Volume 1

(a)

(b)

(c)

(d)

(e)

(f)

Figure 5.11  Fabrication process: phenyltriethoxysilane (PhTES) solution is drop-cast onto a silicon wafer (a) and heated until set (b). ZIF-9 powder is spread across the PhTES surface (c), heated to adhere the crystals (d) then repeated to achieve a dense coverage. The surface is then exposed to lithographic X-ray beam (e) and rinsed with ethanol, etching unexposed areas to leave behind the patterned surface (f). (Reproduced with permission from [52])

to many other networks (2–5 Å) and having similar kinetic diameters of many simple gas molecules, these materials are prime candidates to act as size-exclusive molecular sieves for membrane applications. The current research finding will be discussed in Section 5.3.

5.2.2 Porous Organic Materials (POMs) Porous organic materials can be constructed by noncovalent and covalent bonds and represent a large class of materials. Covalently linked organic porous materials are well represented by numerous systems, such as hypercrosslinked polymers (HCPs), polymers of intrinsic microporosity (PIMs),

Supramolecular Membranes  149 covalent organic frameworks (COFs), porous aromatic frameworks (PAFs) and conjugated microporous polymers (CMPs). These systems feature high porosity, lightweight elements and strong covalent linkages. These systems are designed at the molecular level and synthetically controlled, which makes them prime candidates for use in membrane gas separation applications, whereby pores can be tuned for a specific size. Membranebased gas separation generally exhibits high gas selectivity in the kinetic separation process, which is achieved by the different diffusion rates of the membrane with or without composites.

5.2.2.1  Covalent Organic Frameworks (COFs) Covalent organic frameworks (COFs) provide an alternative porous crystalline material to MOFs or ZIFs, and their molecular framework is made up of light elements (carbon, boron, hydrogen and oxygen) which in return reduce the relative molecular weight of the material attributing to their low mass densities, high thermal stabilities and permanent porosity. As the name suggests, the networks assemble using strong covalent bonds instead of the weaker coordinatively linked ionic bonds found in porous crystalline MOFs or ZIFs. Covalent system polymers enable elaborate integration of organic building blocks into an ordered structure with atomic precision. COFs were first discovered by Côté and Yaghi in 2005 whilst searching for new porous materials [95]. The first COF synthesized, named COF-1, exploited reversible boronate ester formation, which was obtained from the molecular dehydration of 1,4-benzenediboronic acid to converge to form a layered hexagonal framework [95] (Figure 5.12). Previous attempts to prepare COFs generally focused on synthesizing porous organic polymers with non-ordered structures or one-dimensional (1D) crystalline structures [96, 97]. Since this breakthrough paper by Côté and Yaghi, the synthesis of COFS has progressed significantly, and they are showing great potential for functional exploration. An extensive review has been published by Jiang and coworkers [98], describing the basic design concepts, synthetic strategies, structural studies and the frontiers of functional exploration. The design and synthesis of COFs have two key issues: the structure of the organic building block and the synthetic method involving the r­ eaction media and conditions. The organic building block plays a major role in the successful formation of a crystalline and ordered COFs. The building block should contain reactive groups that trigger dynamic covalent bond formation and be conformationally rigid. The formation of the COF should be a reversible reaction and there are no irreversible side reactions. Several reversible reactions have been explored and the successful reactive groups

150  Nanostructured Polymer Membranes: Volume 1

O B HO

HO

B

B

B O

OH

–H2O

O B

B O

O B

O B

B O

B O

O B

O B

O B

B O

O B B O

B O

O B

B O

15 Å

OH

O B

Diboronic acid BDBA

B O

B O

O B

B O

O B

O B

B O

B O

O B

B O

O B

B O

COF-1

Figure 5.12  Condensation reaction of 1,4-benzenediboronic acid to produce COF-1 [95].

—B(OH)2 +

—B(OH)2 +

—B(OH)2

—B(OH)2

OH

+

OH —B(OH)2

O

+

O —CN

+

—CN

+

—CN

B

O B O B

O

Boronate ester linkage O B O Boroxine linkage O B O Boroxine linkage N N

N

Triazine linkage CHO

HN–NH2 O

+

+

—NH2

CHO

N--Imine linkage

HN–N O Hydrazone linkage

Figure 5.13  Schematic representation of the dynamic reactions for the preparation of COFs [98].

include; borantes, dialcohols, nitriles, aldehydes and imines. The condensation reaction of boronic acid into six-membered B3O3 rings has been heavily investigated, with the most successful COFs forming from such a system. A schematic representation of the chemical reactions for the formation of COFs is presented in Figure 5.13 [98].

Supramolecular Membranes  151 The rigid conformation of the building block enables the topological design of the COF, this giving a 2- or 3-dimensional (2D or 3D) structure. The rigid nature and distinct bonding direction of arenes makes aromatic π systems suitable building blocks along with the abundance of aromatic systems. The self-condensation of tetrahedral buildings blocks will form 3D COFs along with its co-condensation with linear or triangular building blocks. For example, the self-condensation of the tetrahedral tetra(4-­ dihydroxyborylphenyl)methane (TBPM) or (4-dihydroxyborylphenyl)silane (TBPS) form COF-102 and COF-103, respectively [99]. Both COF-102 and COF-103 adopt the same ctn topology and a pore size cavity of 5.66 and 5.98 Å, respectively, from the nearest hydrogen atoms. Hexagonal honeycomb architectures with 2D topology are assembled using tri-substituted derivatives with di-subsituted building blocks. The simplest 2D COFs are COF-5, COF-6, COF-8 and COF-10, shown in Figure 5.14 [95, 100]. With tetragonal 2D architectures, tetra-substituted

Figure 5.14  Co-condensation of boronic acids building locks to give 2D COFs (COF-6, COF-8, COF-10). (Reproduced with permission from [100])

152  Nanostructured Polymer Membranes: Volume 1 building blocks are required with the addition of di-substituted units. Prime examples of tetra-substituted building blocks are porphyrin and phthalocyanine derivative systems [101, 102]. In all cases, the length of the building blocks determines the pore size of the structure. The 2D COFs consist of planar sheets that stack using π-π interaction to form layered structures, giving 1D channels. The channel size is dependent on the length and size of the building blocks utilized, making 1D channels a precise molecular design. The 3D COFs are synthesized from tetrahedral units or by the appropriate combinations of tetrahedral and triangular units. 5.2.2.1.1  Synthetic Methods The reaction media and conditions (temperature and pressure) should be taken into account for the formation of thermodynamically stable crystalline structures. Regulating the thermodynamic equilibrium during the covalent bond formation is the key to forming the ordered network. Solvothermal methodology is the typical synthetic condition used for preparing COFs [98]. This involves combining degassed and dried building blocks and solvents, sealed in a vessel and heated to a designated temperature for a certain reaction time. Solvent combinations and molar ratio of building blocks are important factors in preparing porous crystalline frameworks. Other synthetic methods evolved include microwave- or ionothermal-assisted synthesis. Cooper and coworkers developed protocols for the rapid synthesis of boronated ester linked COFs under microwave conditions [103]. Ionothermal synthesis involves the use of elevated temperatures to form molten metal salts, for example, ZnCl2. The metal salt acts both as the solvent and the catalyst for the reaction. This method generally works for the cyclotrimerization of aromatic nitrile building blocks with high thermal stability [104, 105]. There have been recent advancements towards the synthesis of COF films. These have been explored using substrates such as metal surfaces and graphene sheets [106]. Dichtel and coworkers have reported the synthesis and characterization of layered 2D COF films onto single layer graphene (SLG) surfaces supported by several different substrate materials: polycrystalline Cu films on Si wafers (SLG/Cu), fused SiO2 (SLG/SiO2) and SiC (SLG/SiC) under solvothermal conditions [107, 108] (Figure 5.15). 5.2.2.1.2  Functional Exploration and Applications Due to the porous nature of COFs and the intelligent design of pore sizes, adsorption and storage of gases, such as hydrogen, carbon dioxide and methane, have been widely explored as potential applications for these materials [109]. The pore environments within a COF can be modified to enhance the capacity and, most importantly, the selectivity of a specific gas,

Supramolecular Membranes  153 OH

HO

OH HO

B

OH

+

HO

HHTP

OH OH

HO

B

OH

PBBA

Substrate supported graphene mesitylene / dioxane / 90 °C

Substrate supported graphene

COF-5 2.7 nm

COF-5 powder

25-400 nm COF-5 film

Figure 5.15  Solvothermal condensation of HHTP and PBBA in the presence of a substrate-supported SLG surface provides COF-5 as both a film on the graphene surface, as well as a powder precipitated in the bottom of the reaction vessel. (Reproduced with permission from [108])

making them attractive candidates for gas storage applications. This also makes them attractive candidates for membrane separation applications, whereby the materials can be designed to have certain pore sizes that act as a molecular sieve. The crystalline features of COFs can be utilized as nanocomposites within polymer membranes to increase permeability and selectivity. This will be discussed in Section 5.3.

5.2.3 Cages Porous organic cage (POC) molecules are discrete microporous solids that have the unique feature of being solution processable, which gives these materials an advantage over other porous materials discussed when casted into thin films. Discrete organic cage molecules self-organize into ­intrinsically porous materials and tend to pack efficiently in the solid state to maximize intermolecular interactions and reorient their structures in response to molecular and/or physical stimuli [110, 111]. POCs can be subdivided into two types: 1) extrinsically porous, which means that the molecules themselves do not possess any predefined voids, clefts, cavities

154  Nanostructured Polymer Membranes: Volume 1 Extrinsic porosity A

Intrinsic porosity A Amorphous material

Crystallization from

C

Porous clathrate

Amorphous material - permanent porous Crystallization from

Permanent porous crystal

B

C

Porous clathrate

Permanent porous crystal

B Denser packing

Densest packing - permanent porous

Figure 5.16  Schemes to describe extrinsic (left) and intrinsic (right) porous cage materials. (Scheme is modified from original diagrams from [112])

or internal free molecular volume (Figure 5.16, left side), and 2) i­ ntrinsically porous, meaning rigid molecules which contain shape-persistent cavities or clefts (Figure 5.16, right side). Depending on the vertex functionality and synthetic conditions, POCs can either pack in a crystalline or amorphous manner. They are synthesized using a modular building block approach where organic units are covalently linked into rigid 3D nanoporous cages with predetermined geometries. These discrete molecules facilitate precise control of c­avity dimensions and cage functionality through the astute selection of the organic building block. These materials have the potential for size- and shape-selective guest binding or molecular separations. Since POCs are a new class of material compared to MOFs and COFs there are few examples reported [6, 112]. Doonan and coworkers have reported the solution processable and permanently porous shape-persistent organic cage material, as presented in Figure 5.17, that is constructed entirely from thermodynamic robust carbon-carbon bonds. This distorted triangular prism has internal vertical and horizontal diameters of 13.5 Å and 12 Å respectively [6]. Cooper and coworkers have described a series of porous tetrahedral imine-linked cage molecules synthesized by the [4 + 6] condensation of 1,3,5-triformylbenzene with three different vicinal diamines (Figure 5.18) [110], which adsorb small gas molecules, such as nitrogen, hydrogen, methane and carbon dioxide. The cages feature four triangular windows with effective diameters of 5.8 Å, 6.1 Å and 5.8 Å, respectively [110].

5.2.4 PAFs Porous aromatic frameworks (PAFs) are a series of porous organic polymer networks with exceptional high surfaces and gas uptakes [113]. Both

3.1 nm

Supramolecular Membranes  155

Figure 5.17  A space-filled representation diagram of the cage structure produced by Doonan and coworkers. (Reproduced from [6])

1

2

3

Figure 5.18  Structures for cages synthesized by the [4 + 6] condensation of 1,3,5-triformylbenzene with three different vicinal diamines. Hydrogen atoms are omitted for clarity, carbon and nitrogen atoms are colored grey and blue, respectively. Methyl and cyclohexyl groups on the vertices are shown in green and red respectively. (Reproduced with permission from [110])

PAFs  [114] and functionalized PAFs [14, 115] have BET surface area capacities of up to 7000 m2 g–1 and are, making these materials superior for thermal stability. PAFs are tetrahedral, based on a diamond structure and are connected to four neighboring atoms, and the linkers are generally comprised of aromatic rings which increase the internal surface areas (Figure 5.19). These systems are generally synthesized through Yamamoto homocoupling of a tetrahedral monomer, tetrakis(4-bromophenyl)methane [117] or tetrakis(4-cyanophenyl)methane [118] but have also been synthesized using silane tetrahedral or adamantine cage building blocks [119]. The ­connecting linkers can be lengthened by the addition of phenyl rings, increasing the free volume and the cavity diameter size from 5.2 (1 ­phenyl linker) to 12.4 Å (2 phenyl linkers) to 20.8 Å (3 phenyl linkers) and 28.6 Å (4 phenyl linkers) (Figure 5.20) [120]. As shown by various groups like Zhou et al. and Hill et al., PAFs are amorphous materials but have great

156  Nanostructured Polymer Membranes: Volume 1

(a)

(b)

(c)

(d)

Figure 5.19  Structures of (a) diamond shape framework; (b) Structure model of P1; C structure model of P2; d0 structure model of P3. P refers to phenyl rings. (Diagram reproduced with permission from [116])

(a) PAF-301

(b) PAF-302

(c) PAF-303

(d) PAF-304

Figure 5.20  The blue polyhedron represents the tetrahedral bonded carbon atom and the yellow spheres denote the pore sizes in the 3D PAFs; (a) one phenyl linker; (b) two phenyl linkers; (c) three phenyl linkers and (d) four phenyl linkers. (Reproduced from [120])

Supramolecular Membranes  157 potential for being functionalized, which improves the properties of these materials. Results have shown the gas uptake performance of functionalized PAFs improves along with the selectivity, making them prime candidates for use in membrane applications. Currently this area is being explored and will be briefly discussed in this chapter.

5.3 Supramolecular Membranes Supramolecular chemistry describes chemical systems comprising a number of assembled molecular subunits or components arranged in spatial organizations using noncovalent bonding like hydrogen bonding, metal coordination, and hydrophobicity. However, it remains difficult to spatially organize molecular subunits via supramolecular chemistry in the sub-nano range. Hence, membranes fabricated via supramolecular chemistry are rarely reported for gas separations, and are more common for liquid separation or purification, filtration membranes. However, ­supramolecular chemistry concepts like molecular recognition, self-assembly and host-guest chemistry are underlying principles for molecular transport in most membranes. For example, akin to molecular recognition, polar ether oxygen groups in polyethylene oxide (PEO) membranes preferentially interact with acidic gas molecules like carbon dioxide, SOx, Nox, etc., via dipole-quadrapolar. Fractional free volume (FFV) content in super-glassy polymer membranes is known to act as adsorption sites for gas ­molecules—similar to host-guest chemistry. The first part of this section will focus on supramolecular chemistry concepts in polymeric membranes, followed by a short discussion on how metal coordination and host-guest chemistry play important roles in mixed matrix membranes. We will also discuss membranes that are synthesized from the self-assembly, hydrogen bonding, or π-π stacking of block copolymer systems, small molecules, and nanoparticles.

5.3.1 Concepts of Supramolecular Chemistry in Polymeric Membranes Glassy polymers like 1,2-disubstituted polyacetylenes [121], and p ­ olymers with intrinsic microporosity (PIMs) [122, 123] exhibit exceptional gas transport properties and reverse selectivity for the separation of condensable hydrocarbons or gases from permanent gases. The unique ­permeation properties of these glassy polymers are ascribed to the large excess free volume yielded from stiff main chains, bulky substituents, and low cohesive

158  Nanostructured Polymer Membranes: Volume 1 energy structure [122–125]. Collectively, these materials are known as super-glassy polymers. The large excess free volume in super-glassy polymers can behave like pores or cavities of porous materials like zeolites, cyclodextrins, metal organic frameworks (MOFs), porous aromatic frameworks (PAFs), etc. These pores can facilitate adsorption and hydrogen bonding with penetrant molecules—a host-guest relationship. Here we shall discuss two classes of super-glassy polymers in detail—poly(dialkylacetylenes) and polymers with intrinsic microporosity (PIMs).

5.3.1.1 Poly(dialkylacetylenes) Figure 5.21 shows the chemical structures of some polyacetylenes discussed in this section. The poly(dialkylacetylene), poly(4-methyl-2-pentyne) (PMP), synthesized by Masuda et al. [126] has very unusual gas permeation properties [127]. The chemical structure of PMP consists of alternating double bonds along the main chain, a methyl group on the carbon and an isopropyl side-group. These moieties inhibit dense chain packing that consequently creates large FFV content, i.e., gas adsorption sites that are accessible by penetrant gas molecules. The gas permeabilities in PMP at 25 °C decrease as a function of the critical volume of gas penetrants in the following order: n-C4H10 > CO2 > H2 > C3H8 > C2H6 > CH4 > He > O2 > N2. The gas permeabilities in Table 5.1 show the suitability of PMP membranes for separating condensable gases from permanent gases. Other forms of poly(dialkylacetylenes), including poly(5-methyl-2-hexyne) (P5M2H) and poly(6-methyl-2-heptyne) (P6M2H), also display similar trends in gas permeability [128]. This unusual gas permeation behavior is ascribed to a very large fractional free volume (FFV) content [129]. The permeabilities of condensable gases, like CO2, and higher hydrocarbons and permanent gases decrease as temperature is increased; thus, the activation energies of permeation in PMP are negative. This atypical behavior of glassy polymers is also observed in poly(1-trimethylsilyl-2-pentyne) (PTMSP). PTMGP

PTMSP

PMP

P5M2H

CH3

CH3 C H3C

CH3 C Ge CH3

n CH3

C H3C

C

C Si CH3

n CH3

H3C

P6M2H

CH3 C C H

n

C

CH3 C

n

C

CH2

CH3 H3C

C H

C

n

CH2 CH2 CH3

Figure 5.21  Chemical structures of common poly(dialkylacetylenes).

H3C

C H

CH3

Supramolecular Membranes  159 5.3.1.1.1  Polytrimethylsilylpropyne (PTMSP) One of the most permeable polymers known, PTMSP is an amorphous glassy polymer [133]. The ultrahigh permeability of PTMSP is attributed to an exceptionally high fraction free volume content resulting from poor polymer chain packing owing to the stiff chain and pendant groups. The preferential transport of condensable gases or vapors over permanent gas molecules is due to competitive sorption [134]. By comparison (Table 5.1), the gas permeability in the most gas permeable rubbery polymer, polydimethylsiloxane (PDMS), is only one-tenth of the permeability of PTMSP. The ultrahigh gas permeability of PTMSP leads to low ­selectivity for gas separations. Using various chemical, e.g., chemical crosslinking [39, 135, 136], and physical approaches, e.g., aging [137, 138], researchers have modified the structure of PTMSP to increase the gas selectivity whilst maintaining reasonably high gas permeabilities. For most gas separation applications, PTMSP membranes operate as size-selective membranes. Meanwhile, when PTMSP membranes are used to isolate hydrocarbons from mixtures containing condensable hydrocarbon vapors and p ­ ermanent gases, these membranes operate in the reverse-selective mode [139]. This can be attributed to a significant reduction in gas permeability in the presence of vapors. The vapors condense within regions of high free volume of the polymer; surface diffusion of the condensed component becomes dominant [131]. In addition, PTMSP is also attractive for the separation and recovery of fluoro-compounds like CF4, C2F6, and C3F8 from permanent gases for the microelectronics industry. Hydrocarbons are generally more soluble in PTMSP than their fluorocarbon counterparts [131], and this is attributed to the differences in penetrant condensability. Despite its unique property of high hydrocarbon/gas selectivity and permeability, PTMSP has apparently found no industrial applications. This is due in part to PTMSP being highly soluble in liquid hydrocarbons [39, 135]. The timedependent nature of these materials, i.e., physical aging, has also limited their use. In fact, polymer aging appears to be the nemesis of super-glassy polymers in industrial applications. The substitution of silicon with germanium in this family of 1,2-disubstituted poly(acetylenes) can overcome this problem whilst maintaining high gas permeability [136, 140]. 5.3.1.1.2  Poly(1-trimethylgermyl-2-pentyne) (PTMGP) Langsam and Savoca first replaced the silicon heteroatom in the side group of 1,2-disubstituted poly(acetylenes) with a germanium atom to form poly(1-trimethylgermyl-propyne) (PTMGP) [140]. Because of germanium’s higher atomic weight and larger ionic radius, the Ge-C bond is more stable than the Si-C bond. Hence, in corrosive environments,

2650

460

240

–

–

–

PMP [127]

P5M2H [128]

P6M2H [128]

PTMSP [131]

PTMGP [132]

PDMS [130]

890

5520

15000

370

750

5640

H2

800

3130

9000

130

245

2460

O2

400

2030

6600

51

93

1250

N2

b

a

Measured at 1 atm. Gas permeability measured in Barrer 10–10 cm3 (STP)-cm/cm2 sec cm Hg.

He

Polymer

3800

13100

27000

390

900

9090

CO2

1200

5070

15000

112

190

2690

CH4

3300

–

31000

160

280

3730

C2H6

Table 5.1  Gas permeabilities of PDMS [130], silicon-based glassy polymers [131], and PTMGP [132].

4100

–

38000

605

1700

4160

C3H8

–

19950

80000a

–

–

–

n-C4H10

160  Nanostructured Polymer Membranes: Volume 1

Supramolecular Membranes  161 germanium-containing polymers are more stable than their silicon analogues [141]. Kwak and Masuda reported that the synthesis of PTGMP in solvents like carbon disulfide, cyclohexane, carbon tetrachloride, and tetrahydrofurane yielded PTMGP that is resistant to aliphatic and aromatic hydrocarbons [142]. Yave et al. also investigated the synthesis of PTMGP using various polymerization conditions and reported that the geometric structure of PTMGP macromolecules (isomeric content) and ­supramolecular organization of the PTMGP chains play significant roles in determining the gas permeability [132]. Figure 5.22 shows that gas permeation properties of PTMGP depend rather strongly on the cis/trans ratio in the polymer chain backbone; apparently this ratio alters polymer chain packing and, thus, the total free volume content. It appears that permeability is maximum at about 80% trans content. However, the replacement of silicon with germanium, a rare element, makes PTMGP an expensive material that has precluded commercial interest in these materials as gas separation membranes. 5.3.1.1.3  Other 1,2-Disubstituted Polyacetylenes Other than substituting with germanium, researchers have also characterized more permeable polyacetylenes that contain bulky spherical substituents like trifluoromethyl groups [143]. However, the gas permeation 20000 18000 16000 Permeability (barrer)

14000 n-C4H10

12000 10000 8000

CO2

6000

H2

4000

CH4 O2 N2

2000 0

10/90

15/85

20/80

60/40

Cis/trans content in PTMGP

Figure 5.22  Pure gas permeability of poly(1-trimethylgermyl-2-pentyne) (PTMSP) containing different cis/trans contents.

162  Nanostructured Polymer Membranes: Volume 1 properties of the latter class of polymers are lower than that of PTMSP. Hu et al. synthesized a wide range of indan-containing poly(diphenylacetylene) polymers which display gas permeabilities higher than those reported for PTMSP despite the absence of bulky spherical groups [144]. The methyl groups at the indan moiety contribute to high levels of excess free volume that leads to the higher gas permeabilities. Table 5.2 shows that gas permeability enhancements in fluorinated indan-containing poly(diphenylacetylene) polymers are quite pronounced, whereby inhibitive polymer chain packing ascribed to the intermolecular repulsive fluorine atoms with high electron density enhanced free volume content. The bulkiness and electronegativity of the substituents on the aromatic phenyl ring also influenced gas permeability, i.e., with increased bulkiness or electronegativity, gas permeability decreases [144]. By far, these indan-containing poly(diphenylacetylenes) are the most permeable reverse-selective glassy polymers. Another widely studied form of polyacetylene is poly(p-tert-butyl diphenylacetylene) (PptBDPA) [145]. The biphenyl groups incorporate thermal stability into these polymers while the moderately, bulky Table 5.2  Comparison of gas permeability coefficients of indan-containing poly(diphenylacetylenes) with PTMSP. The gas permeability coefficients were measured at 25 °C and are in the units of Barrer 10–10 cm3 (STP)-cm/cm2 sec cm Hg. Polymer

Substituent on phenyl

2a

He

H2

O2

N2

CO2

CH4

–

11200

24800

14400

11600

38700

29100

2b

p-F

15800

33300

17900

15600

47000

34300

2c

m-F

12800

27000

14300

12000

35200

27800

2e

p,m-F2

17800

36800

18700

16600

44200

35000

2f

m,m-F2

13700

28500

15200

13100

36100

29100

2g

p-Cl

9300

20600

11700

9200

35200

22700

2h

p-Br

9200

20700

11400

9200

32800

22400

2l

p-Me

5000

10800

5700

4100

16900

10100

2k

none

10500

24100

12900

10400

36400

25200

2m

p-SiMe3

530

1100

450

170

2000

470

9700

24800

14800

11500

47000

29900

PTMSP None

Supramolecular Membranes  163 and spherical tert-butyl group enhances polymer solubility and gas ­permeability. Compared to other forms of polyacetylenes, PptBDPA is less permeable to gases.

5.3.1.2 Polymers with Intrinsic Microporosity (PIMs) Polymers with intrinsic microporosity (PIMs) possess a unique structural characteristic that differentiates them from other polymers—a bulky polymer backbone with no rotational freedom (Figure 5.23) [123]. Like 1,2-disubstituted polyacetylenes, PIMs are amorphous glassy polymers that are soluble in solvents [122]. The microporous networks in PIMs consist of assemblies of planar functional units as rigid spirocyclic molecular scaffolds formed via dibenzodioxanes that are resultant of aromatic nucleophilic substitutions between catechols and fluorinated compounds. The absence of single bonds in the polymer backbone further inhibits backbone rotation [122]. Networking in PIMs is attributed to more than 2 spirocyclic linkages attached to each planar functional unit. Some PIMs could possibly offer greater microporosity than PTMSP, while providing better chemical and thermal stability [122]. 5.3.1.2.1 PIM-1 Figure 5.23 shows the chemical structure of PIM-1, first synthesized by Budd et al. [122]. It is the most commonly studied PIM for gas separation. Polar groups in PIMs can enhance both the intermolecular interactions (between gas penetrants and polymer) and sorption. The rigid and contorted polymer chain traps sufficient free volume in membrane films. The microporosity in PIMs provides more sorption sites for gas transport, thus the solubility coefficients of PIMs are extraordinarily high when compared

(a)

(b)

Figure 5.23  Chemical structures of (a) PIM-1 and (b) PIM-7, common PIMs. PIM-1s are usually synthesized at 65 °C in the presence of K2CO3 and DMF.

164  Nanostructured Polymer Membranes: Volume 1 to other polymers [122]. The large free volume content also leads to high diffusivity coefficients. Coupled with high gas solubilities and diffusivities, the gas separation performance of PIMs falls beyond Robeson’s upper bound line (Figure 5.24) [146]. Table 5.3 shows that CO2 permeability in PIM-1 is the highest amongst all other permanent gases. Similar to ether oxygens in PEO, the electronegative nitrogen atom of polar nitrile groups in PIM-1 preferentially interacts with the electrophilic carbon atom in CO2; hence CO2 transport in PIM-1 is governed by its solubility coefficients. Xe gas, a strong adsorbent, has the lowest diffusivity coefficients amongst permanent gases; resulting in 100.0

O2 / N2 selectivity

CO2 / CH4 selectivity

10.0

1.0

0.1

(a)

1.0

10.0

100.0

10.0

1.0

1,000.0 10,000.0

O2 permeability / barrer

0.1

1.0

10.0

100.0

1,000.0 10,000.0

CO2 permeability / barrer

(b)

Figure 5.24  The gas permeability and selectivity of PIMs exceed most other polymers and lie above Robeson’s upper bound line. represents PIM-1 while represents PIM-7 [146].

Table 5.3  Gas diffusivity (D), solubility (S) and permeability (P) coefficients of PIM-1 and PIM-7 at 30 °C. Diffusivity coefficients are measured in 10–8 cm2 s–1, solubility coefficients are measured in 10–3 cm3 cm–3 cmHg–1, and permeability coefficients are measured in 10–10 cm3 [STP] cm cm–2 s–1 cmHg–1. PIM-1

O2

N2

He

Ar

Xe

H2

CO2

CH4

P

370

92

660

200

55

1300

2300

125

D

81

22

2700

40

0.46

1700

26

6.8

S

46

42

2.4

50

1200

7.6

880

180

PIM-7

O2

N2

He

Ar

Xe

H2

CO2

CH4

P

190

42

440

100

26

860

1100

62

D

62

16

900

30

0.37

1100

21

5.1

S

31

26

5

33

700

8

520

120

Supramolecular Membranes  165 low Xe permeability. Strongly adsorbed penetrants have desorption difficulties and may block transport of gases. Similarly to PTMSP, PIM-1 is particularly permeable to hydrocarbon vapors. Meanwhile, as PIM-7 does not have any polar moieties in its chemical structure, permanent gas permeabilities in PIM-7 are lower than those of PIM-1.

5.3.2 Supramolecular Concepts in Nanocomposite Membranes In the same way as the FFV content in super-glassy polymers, the incorporation of porous or nonporous additives, like metal organic frameworks, porous aromatic frameworks, zeolites, or fumed silica, can introduce more voids via intrinsic particle porosity or the repulsive interactions between polymer chains and particle surface. As mentioned previously, these voids can act as hosts to accommodate guest molecules via adsorption. Additive incorporation into polymer matrices remains one of the most common ways in which supramolecular chemistry is observed in membranes. For example, Merkel et al. reported that the incorporation of nonporous fumed silica nanoparticles into a PTMSP polymer matrix enhanced gas permeability [54]. It was proposed that the introduction of silica nanoparticles disrupted PTMSP chain packing, thus increasing free volume content for molecular transport. Using transmission electron microscopy (TEM) and positron annihilation lifetime spectroscopy (PALS), De Sitter et al. observed that the free volume content in PTMSP-based nanocomposites was larger than that of pristine PTMSP materials, and additional interstitial mesopores were observed in PTMSP nanocomposites [147]. These interstitial cavities were primarily located between agglomerated silica nanoparticles and were responsible for the enhanced permeability and reduced vapor/gas selectivity. These mesopores are accessible only to the smaller gas penetrants. The lack of competitive sorption by larger, condensable molecules in these mesopores permits smaller gas penetrants to permeate across the material. Merkel et al. reported that the inclusion of nonporous fumed silica nanoparticles into a PMP polymer matrix simultaneously enhanced vapor/gas selectivity and permeability [42]. Both TEM and PALS analysis indicated that the inclusion of silica did not alter the free volume content in the PMP-silica nanocomposite, while lesser interstitial cavities were formed and nanoparticle dispersion was better in PMP than PTMSP [129, 147]. The isolated mesopores maintained or improved the vapor/gas selectivity in PMP-nanocomposites. The reason behind better nanoparticle dispersion in PMP (as seen in Figure 5.25) remains unclear. In this section, we shall also look at how metal coordination plays a role in mixed matrix membranes via the incorporation of metal organic

166  Nanostructured Polymer Membranes: Volume 1

PTMSP

PMP

Figure 5.25  Graphical representation of the membrane structure in PTMSP and PMP nanocomposites [129, 147].

frameworks into polymer matrices, and a revolutionary work with supramolecular chemistry at its roots that will change the way gas separation membranes are thought of and fabricated.

5.3.2.1 Metal Organic Frameworks (MOFs) in Polymer Membranes Metal Organic Frameworks (MOFs) are highly porous compounds with metal ions coordinated with organic ligands to form rigid, multidimensional structures. The pores of these materials are extremely stable and can be used to store guest molecules ranging from solvents to gases. MOFs have been discussed in Section 5.1. The focus of this section is to discuss ­supramolecular effects of MOFs in polymer-MOF nanocomposites. The incorporation of MOFs into polymers can enhance the gas permeability and selectivity of polymeric membranes. For example, our group recently incorporated UiO-66, a Zr-based MOF, and titanium (Ti) exchanged UiO66 (Ti/Zr) into PIM-1. Figure 5.26 shows that CO2 permeability of our membranes is enhanced by 25% with the incorporation of UiO-66 MOFs. By replacing the Zr ions in UiO-66 with Ti ions, the gas permeability of the resulting PIM-1 membrane increased by 160%, while slightly improving CO2/N2 selectivity. The key in our recent work is that by using a simple post-synthetic modification on MOFs, we have shown that the CO2 permeability of a super-glassy polymer membrane can be enhanced by 160%. The most common class of MOFs used in polymer-MOF nanocomposites is zeolitic imidazolate frameworks (ZIFs). Topologically similar to zeolites, ZIFs comprise transition metal ions that are tetrahedrally coordinated by organic imidazole linkers, and have high permanent porosity. ZIFs are

Supramolecular Membranes  167

Figure 5.26  CO2 permeability of PIM-1 is increased by 160% with the incorporation of Ti-exchanged UiO-66 MOFs.

also chemically and thermally stable when compared to other MOFs. The high porosity in ZIFs and other classes of MOFs are well-suited to ­support gas storage and gas separations via host-guest chemistry. Molecular recognition, particularly affinity towards carbon dioxide (CO2), in ZIFs can be tuned via ligand functionalization. Using this unique property of ZIFs, different ZIFs have been incorporated into polymeric membranes to enhance their gas permeability and gas selectivity. For example, to improve the dispersion of ZIF-8 (Zn-based) in high molecular weight polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), and polystyrene (PS), Smarsly et al. [148] electrospun polymer/ZIF-8 nanocomposites t­argeting gas adsorption and separation. The two-step gas adsorption isotherms of these “MOF textiles” showed that the porosity of ZIF-8 remains accessible to gas molecules despite their embedment within various polymers. Meanwhile, desorption curves similar to adsorption curves indicate that the release of the guest molecules are achieved through pressure release—a clear sign that the gas molecules do not interact strongly with ZIF-8. Other approaches to improve ZIF dispersion in polymer matrices include the use of excess ligands during synthesis [149], deliberately missing the drying procedure of ZIF synthesis to reduce agglomeration [150], reducing ZIF particle size [151], and matching the hydrophobicity of ZIFs with polymer choices [152]. Other than ZIFs, there are many works reporting the using of other MOFs like UiO-66, UiO-67 (zirconium-based MOF) in a fluoropolyimide [153];

168  Nanostructured Polymer Membranes: Volume 1 Rigidification C O Al Zn Cu

(a)

(b)

Blockage (c)

Figure 5.27  Schematic diagram of MOF-polyimide interactions: (a) MIL-53(al)/ polyimide, (b) MOF-5/polyimide and (c) Cu3(BTC)2/polyimide [157].

Mg2(dobdc) in PDMS; crosslinked PEO and polyimide [154]; CAU-1-NH2 in poly(methyl methacrylate) [155]; Cu-HFS-BIPY [156], MIL-53 (Al), MOF-5, and Cu3(BTC)2 in polyimides [157]; Cu-BDC in poly(vinylacetate); MIL-series MOFs in polysulfones [156]. A ­potential drawback in MOF/ polymer membranes is that open metal sites in MOFs may facilitate strong interactions between MOFs and polymers. For example, interactions between carbonyl groups in polyimides can i­ nteract with unsaturated metal ions. Functionalized ligands used during MOF s­ ynthesis may promote MOF dispersion in the polymer matrix, but could also cause polymer chain rigidfication that consequently lowers the gas permeances of resultant MOF/polymer membranes. Thermal treatment of p ­ olymer films is a common practice to remove remnant solvent molecules or to reset thermal history. Although MOFs possess good thermal s­ tability, the thermal treatment process usually imparts some form of flexibility to polymer chains, which may end up filling the pores of MOFs. This will result in lower gas permeances. These possible scenarios are summed up in Figure 5.27.

5.3.2.2 Porous Aromatic Frameworks (PAFs) in Super-Glassy Polymers From the previous section, it is clear that although MOFs can drastically enhance the gas permeabilities of polymeric membranes and influence post-fabrication treatment approaches; MOF-polymer interactions, and MOF-polymer compatibility can also potentially offset the advantages that MOFs can offer. To overcome the disadvantages of a MOF-polymer combination, we have recently incorporated another class of porous materials into a polymer, and observe finely dispersed nanoparticles in polymers (Figure 5.28) and, more importantly, gas permeabilities were enhanced by 98%, while solvent fluxes were enhanced by 700%.

Supramolecular Membranes  169 2 Ni (COD)2 1 (a)

PAF-1 Time

C

PAF-1 Si Time

(b)

(c)

(d)

Figure 5.28  (a) PAF-1 particles are synthesized via a Yamamoto coupling reaction. (b) Schematic of super-glassy polymer/PAF-1 intermixing. Typically PTMSP, PMP and PIM-1 densify to give a non-permeable conformation (top right), but with the addition of PAF-1, the original permeable structure is maintained (bottom right). (c) TEM micrograph of 50 nm PTMSP/PAF-1 film shows that PAF-1 particles are surrounded by Si, indicating the fine dispersion of PAF-1 within the PTMSP matrix. The noisy Si X-ray signal within the PAF-1 particle could be due to stray scattering of exciting Si fluorescence. (d) TEM and EDX analysis of PAF-1 particles immersed in PTMSP solution show that Si is well-intercalated within the PAF-1 particles. These particles were washed thoroughly with chloroform prior to imaging.

More importantly, we discovered that this particular porous particle, specifically, PAF-1, could also solve the age-old problem of polymer aging in super-glassy polymers. Polymer aging and gas permeabilities were tuned using PAF-1s, their functionalized analogues, and PAF-1s decorated with C60 fullerene nanoparticles. The main mechanism behind tailoring aging in various super-glassy polymers is the rigidification of main polymer chains by confining the bulky side chains or chemical moieties within the pores of PAF-1 via noncovalent interactions. Supramolecular chemistry plays an important role here in tailoring aging, and in gas permeation. With an aim of gas permeability enhancements, we first incorporated PAF-1 into super-glassy polymer matrices like PTMSP, PMP, and PIM-1. The CO2 permeability was enhanced by 140% in PTMSP and by 320% in PIM-1. The discrepancy in CO2 permeability enhancement is ascribed to the difference in chemical structure of the polymers. In PTMSP, the

170  Nanostructured Polymer Membranes: Volume 1 saturated carbons at the bulky trimethylsilyl side group are attracted to the saturated carbons of the PAF-1 particle. This physical attraction between saturated carbons facilitates the intercalation of PTMSP chains within the 4.8 nm interparticle pores and 1.1 nm intraparticle pores of PAF-1 ­particles, and is verified by solid-state nuclear magnetic resonance spectroscopy and the location of these bulky side chains is pinpointed using positron annihilation lifetime spectroscopy (PALS). With embedded side chains, the interchain distance between largely linear PTMSP main chains will be relatively smaller when compared to the contorted and bulky main chains of PIM-1; hence the smaller magnitude in permeability enhancements. In the case of PIM-1, bulky dimethyl moieties are intercalated within the pores of PAF-1, while the bulky nitrile moieties are not. As these bulky side groups are located on different components of PIM-1, the intercalation of the bulky dimethyl groups in PAF-1 pins down and restricts mobility of the spirobiisindane portion; whilst the portion comprising the tetrafluoronitrile building block remains flexible. Consequently, aging continues to take place in PIM-1. However, pore shrinkage in PIM-1 is beneficial for CO2 transport as the optimal pore size for CO2 sorption is reached. It is interesting to note that throughout these experiments, we have not observed any forms of covalent bonding between PAF-1 pores and the bulky side chains or moieties of PTMSP and PIM-1. It is believed that hydrogen bonding between saturated carbon moieties and aromatic rings in PAF-1 provide strong interaction between the polymer chains and PAF-1 particles, thus leading to revolutionary membranes for the separation industry. Figures 5.29 and 5.30 show the gas separation, liquid ­separation, and filtration capabilities of our PTMSP/PAF-1 membranes. The main supramolecular concepts in these membranes include host-guest chemistry for separation mechanisms, and possibly hydrogen bonding between bulky chemical moieties and PAF-1. These results could be a harbinger to ­facilitate the revisitation of super-glassy polymer as separation membranes that provide high flux and good selectivity.

5.4 Membrane Fabrication Using Supramolecular Chemistry In this section, we shall look at how common concepts of ­supramolecular chemistry are used in membranes to achieve separations. Concepts like molecular recognition, host-guest chemistry, and self-assembly are common in membranes. Molecular recognition in polymeric membranes can be simply described using the affinity of the membrane towards targeted gas molecules. For example, the CO2 affinity in rubbery polymer membranes

Supramolecular Membranes  171 100

30 104

80

25

(i)

103 104

100 80 60 40

103

(ii)

100

104

80

20

15

10

Robeson’s upp bound Model of PIM-1 Model of PIM-1/PAF-1 PIM-1 PIM-1/PAF-1 PTMSP PTMSP/PAF-1 PTMSP/ZIF-8 PMP PMP/PAF-1

103

60 40

20 CO2/N2 selectivity

40

Absolute gas permeabilities (barrer)

Relative gas permeabilities (%)

60

(iii) 0

60

102 120 180 240 Aging (days)

PTMSP CO2 PTMSP N2 PTMSP/PAF-1 CO2 PTMSP/PAF-1 N2 PTMSP/ZIF-8 CO2

(a)

0

60 120 180 240 Aging (days)

PMP CO2 PMP N2 PMP/PAF-1 CO2 PMP/PAF-1 N2

5

PIM-1 CO2 PIM-1 N2 PIM-1/PAF-1 CO2 PIM-1/PAF-1 N2

(b)

103

104 CO2 permeability (barrer)

(c)

Figure 5.29  The (a) relative and (b) absolute gas permeabilities of (i) PTMSP-based, (ii) PMP-based, and (iii) PIM-1-based nanocomposites. The solid and empty symbols represent CO2, and N2 permeabilities of membranes, respectively. Squares, circles, and triangles represent pristine super-glassy polymers, super-glassy polymers with PAF-1, and super-glassy polymers with metal organic frameworks, respectively. Lines are drawn to guide the eye. The deviation of these permeability measurements is within ±10%. (c) The CO2 permeability and CO2/N2 selectivity of PTMSP- (red), PMP- (green), and PIM-1-based (blue) nanocomposites are plotted on Robeson’s upper bound. The squares represent pure super-glassy polymer films, circles represent super-glassy polymer/PAF-1 films, and triangles represent super-glassy polymer/metal organic framework films. Our model accurately predicts the aging trends in different systems. Ultimately, PAF-1-based nanocomposites perform better over time without any CO2 permeability loss. The arrows provide a visual guide to show the relationship between CO2 permeability and selectivity over time (aging is tracked from the bottom of each arrow).

2.5

100

IPA permeance (L m–2 h–1 bar–1)

90 Dye rejection rate (%)

2.0 1.5 1.0 0.5 0.0

(a)

80 70 60 50 40 30 20 10 0

As-cast As-cast Aged Aged PTMSP PTMSP/PAF-1 PTMSP PTMSP/PAF-1 Membrane type

(b)

As-cast As-cast Aged Aged PTMSP PTMSP/PAF-1 PTMSP PTMSP/PAF-1 Membrane type

Figure 5.30  (a) Isopropanol permeance and (b) dye rejection rate of PTMSP and PTMSP/ PAF-1 membranes before and after polymer aging.

172  Nanostructured Polymer Membranes: Volume 1 fabricated from polydimethylsiloxane (PDMS) and polyethylene oxide (PEO) are derived from the interactions between CO2 molecules and polar functional groups like siloxanes in PDMS and ether oxygens in PEO. The O2 affinity in polymers is achieved via cobalt-coordinated polymers [158], while doping polymer matrices with silver ions yields membranes suitable for olefin separations [159].

5.4.1 Molecular Recognition – CO2 Affinity Polydimethylsiloxane (PDMS) is a polymer with unique properties owing to the inorganic siloxane backbone and organic methyl groups attached to the silicon. The fundamental structural properties of PDMS include low intermolecular forces between the methyl groups, unique flexibility in the siloxane backbone and high bonding energy of the siloxane bond. Such properties are attributable to high gas permeabilities and high selectivities for condensable gases in PDMS membranes [160]. Prior to using PDMS as membranes, crosslinking must be carried out first [130]. Merkel et al. reported the gas sorption, diffusion and permeation properties of crosslinked poly(dimethylsiloxane). Gas permeabilities of fluorinated gases in PDMS are approximately an order of magnitude lower than those of their hydrocarbon analogs. This is primarily due to the lower solubility and diffusivity coefficients of perfluorinated carbons in PDMS [130]. Besides using crosslinked PDMS for gas separation membranes, PDMS-based membranes can also exist in the forms of nanocomposites [161], block copolymers [162], and coatings [163–165]. Kim et al. fabricated a gas separation membrane consisting of singlewalled carbon nanotubes embedded in a poly(imide siloxane) copolymer [161]. The poly(imide siloxane) component was synthesized using an ­aromatic dianhydride, an aromatic diamine, and amine-terminated PDMS. The purpose of PDMS in this membrane was to facilitate good adherence of the polymer matrix to the carbon nanotubes. Park et al. used a sol-gel reaction to synthesize an imide-siloxane block copolymer/silica hybrid membrane that displayed increasing He, CO2, O2 and N2 gas permeabilities with increasing PDMS content [162]. The increase in silica content in these membranes retarded the decrease in gas selectivities at an elevated temperature, while plasticization of the organic matrix was restricted. ­Dal-Cin et al. applied PDMS as a caulking layer to alleviate structural defects in a polyetherimide-polyethylene glycol interpenetrating network (PEI-PEG-IPN) membrane [166]. Owing to its high gas diffusivities, PDMS is an ideal choice as a caulking material as it does not hinder the intrinsic gas transportation and, consequently, gas separation capabilities of the membrane material.

Supramolecular Membranes  173 Polyethylene oxide (PEO) is polyethylene with high concentrations of polar ether oxygens separating each pair of carbon atoms. Polar ether ­oxygen moieties are thought to be the only polar moieties that can improve CO2/H2 selectivity [167]. This is attributed to the dipole-quadrapole interaction between the polar ether oxygen (EO) units in PEO and the acidic CO2 [41, 167, 168]. The CO2 affinity of the ether oxygen moiety depends on the nature of the monomeric unit, polymerization degree, end group effect and ether oxygen concentration [169]. For example, EO units in the propylene oxide analog are more CO2-philic than its counterparts in the form of ethylene oxide and tetrahydrofuran (THF) [169]. The CO2-affinity properties of polyether-based polymers can be tuned via modifications to the entropy and enthalpy of mixing. The entropy of mixing of a chain molecule increases with chain flexibility while the enthalpy of mixing is dependent on the number and strength of solute-solute, solvent-solvent, and solutesolvent interactions. Solute-solute and solvent-solvent interactions must be weak while solute-solvent interactions must be strong to favor CO2 mixing. With low steric factors and glass transition temperatures, PEO is one of the candidates for CO2-philic rubbery membranes. The CO2 permeability of semicrystalline PEO can reach 12 Barrer and a CO2/H2 selectivity of 6.7, while the estimated CO2 permeability and CO2/H2 selectivity of amorphous PEO is expected to be 143 Barrer and 7, respectively [170]. Polymer crystallinity reduces chain molecule flexibility and solute-solvent interactions that consequently reduce CO2 permeabil­ olecular ity. Lin et al. utilized photochemical reactions to crosslink low m weight PEO-based acrylates to fabricate rubbery membranes with low crystallinity [41, 167, 168]. The CO2 permeability of these rubbery membranes reached 400 Barrer while achieving a CO2/H2 selectivity of 30 at −20 °C. These highly amorphous membranes are considered to be s­ tate-of-the-art CO2/H2 selective membranes. Besides crosslinking low molecular weight PEO-based monomers to form rubbery membranes [171–173], other alternatives include the synthesis of PEO copolymers [174–178], physical blends of PEO-based additives into membranes [179–181], and integration of nanoparticles into PEO-based polymers via chemical bonds [182–184]. Common PEOcopolymers that are frequently studied in other works consist of hard and soft segments. Hard segments can comprise polyamides, polyimides or other glassy polymers, e.g., poly(butylene-terephtalate) (PBT). A commercially known polyamide-PEO block copolymer is Pebax MH 1657 (poly(amide-b-ethylene oxide)). Car et al. reported that the CO2 permeability of this commercial PEO-based copolymer reached 73 Barrer with a CO2/H2 selectivity of 9.1 [180]. The CO2 transport properties of Pebax

174  Nanostructured Polymer Membranes: Volume 1 can be described using the solution-diffusion model [185]. In another work, Car studied PEO-PBT block copolymers, also commercially known as Polyactive, that possessed CO2 permeabilities reaching up to 120 Barrer while achieving a CO2/H2 selectivity of 11 [174]. The CO2 permeabilities of these PEO-based copolymers were enhanced by blending low molecular weight PEG additives [174, 179]. Membranes containing physically blended low molecular weight PEG additives are unsuitable for industrial gas separation applications as the presence of water causes leaching, i.e., removal of these PEG additives. Mixed-matrix PEO-based membranes can overcome the shortcomings of physically blended materials. Patel et al. reported that the incorporation of functionalized fumed silica (FS) nanoparticles (up to 14 nm in diameter) reduced CO2 permeability while improving CO2/light gas ­selectivity [183]. The chemically bonded FS nanoparticles reduced chain mobility and subsequently penetrant diffusivity. However, as the chain length of the PEObased oligomer increased, penetrant diffusivity and s­olubility increased. Consequently, CO2 permeability increased as well. Wilder et al. crosslinked PEG with dibenzylidene sorbitol, a low molecular weight organic molecule ­ etworks (IPNs) con[186], while Mu et al. fabricated interpenetrating n sisting of poly(ethylene oxide) and polyhedral oligomeric silsesquioxane (POSS) nanoparticles [187]. Although the gas permeation and separation properties of these nanohybrid materials are not reported in these works, the reported physicothermal and rheological properties of these ­nanomaterials are aptly suitable for gas separation. Mixed matrix membranes can also be fabricated using catalyzed sol-gel processes using low to mid-range molecular weight organic monomers and inorganic molecules [188–191]. The gas separation properties of these organic-inorganic materials (OIMs) can be tailored by tuning synthesis conditions [192]. Organic-inorganic membranes (OIMs) synthesized from simple fabrication processes have shown high CO2 affinity that leads to good CO2 permeability and CO2/light gas separation [189]. Acid-catalyzed sol-gel methods yield OIMs that possess distinctive phase separations at the molecular level between the inorganic and organic networks [193, 194]. In our recent work [190], we have shown that OIMs fabricated using polyether diamines and silica (PEDS) display comparable CO2/H2 separation properties to the membranes synthesized from crosslinked organic PEO-acrylate monomers [168]. At 35 °C, the CO2 permeability of this base PEDS OIM reached 370 Barrer with a CO2/H2 selectivity of 9 [190]. At the expense of good mechanical properties, CO2 permeabilities of these OIMs can be enhanced with the addition of more polyether diamines [190]. To enhance CO2 permeability while inhibiting the side effects of excessive polyether content,

Supramolecular Membranes  175 we propose to graft short polyether side-chains onto the OIMs. The addition of PEG side-chains can minimize chain mobility in the main chains, thus, retaining mechanical strength while enhancing the interaction of the resultant nanohybrid material with highly condensable gas penetrants.

5.4.2 Host-guest Chemistry Host-guest chemistry is the underlying principle of facilitated transport membranes. The first facilitated transport membrane for CO2 separation was introduced by Ward and Robb in 1967 [195]. Aqueous bicarbonatecarbonate solution was immobilized as a film onto a porous cellulose acetate film to achieve a CO2/O2 separation of 1500. CO2 is preferentially transported from the high pressure feed side where the CO2 is consumed on the surface of the film to form a bicarbonate that diffuses across the film and decomposes to CO2 at the low pressure permeate side. Due to solution loss in the porous supports, these supported liquid membranes (SLM) are unsuitable for industrial applications. However, the work of Ward and Robb laid the foundation for SLM using ionic liquids, where the vapor pressure of ionic liquids can be negligible. Ionic liquids are simply molten salts for a certain temperature range (usually at or slightly higher than room temperature) that primarily consist of ions and short-lived ion pairs [196], hence they are usually known as room temperature ionic liquids (RTILs). Compared to conventional organic solvents, ionic liquids possess negligible vapor pressure, good thermal stability, tunable viscosity and miscibility with water and ­solvents, and good extractability for various organic compounds and metal ions [197]. The unique properties of ionic liquids are due to their chemical structures that usually consist of bulky, asymmetrical organic cations such as imidazolium, pyrrolidinium, pyridinium, ammonium or phosphonium, and organic/inorganic anions like tetrafluoroborate and bromide [198]. Proposed applications range from organic synthesis, analytical chemistry, to catalysis in clean technologies, and, most importantly, to gas separation [199, 200], since the properties of ionic liquids can be tuned via molecular chemistry, i.e, changing the structures of the anions and cations; thus allowing specific and selective gas sorption. An important fundamental study on gas sorption in RTILs is the work by Anthony et al. [201], who found high solubility of carbon dioxide and water in the ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6], while the hydrogen and nitrogen solubilities are negligible. Yokozeki and Shiflett reported that the CO2/H2 selectivity of [bmim][PF6] ranged from 30–300, and is suitable as “solid” sorbents [202].

176  Nanostructured Polymer Membranes: Volume 1 Extremely high pressures are required for CO2 capture from light gases like O2, N2 and H2 using [bmim][PF6] to avoid the limitations of interphase mass transfer at low pressures. Hence, to use RTILs in industrial gas separations, the gas selectivity of RTILs must be significantly improved so that the high cost of gas compression is worthwhile. RTILs have been targeted for acidic gas removal from permanent gas mixtures. The use of RTILs in the form of polymeric membranes for gas separation can be categorized into 1) s­ upported RTIL (SILM) membranes, whereby RTILs are deposited into a porous membrane, 2) polymerized ionic liquid membranes (PIL), and 3) polymeric blends with RTILs. Scovazzo et al. [203] fabricated a SILM using a water-stable ion, bis(trisfluoromethanesulfonyl)amide [Tf2N]–, and reported the use of membrane for CO2 separation from N2 and CH4 gas mixtures. Regardless of relative humidity, the bis(trisfluoromethanesulfonyl)amide [Tf2N]–based water-stable ionic liquids have the highest CO2 permeability (up to 1,500 Barrer). At a constant testing pressure, the gas permeability of this ionic liquid is higher in humid conditions. SILMs are highly selective with high permeabilities that are comparable to polymeric membranes; however, they are generally rather thick, so fluxes may be low. Ilconich et al. reported the impregnation of commercial polysulfones (HT Tuffryn ) and polyethersulfone (Supor ) polymeric porous substrates with the ionic liquid, 1-n-hexyl-3-methylimidazolium bis(trisfluoromethanesulfonyl) imide [hmim][Tf2N] [204]. As gas diffusivities in liquid phases are generally higher than in solid phases, gas permeabilities in SILMs are relatively higher than solid phase polymerized ionic liquid (PIL) membranes. In this work, it is reported that RTILs can plasticize polymeric matrices. The CO2 and He permeability of [hmim][Tf2N]-SILMs can reach 1,200 and 270 Barrer at 100 °C, respectively. Because of size similarities, He permeability is expected to be similar to H2 permeability. However, at 135 °C (similar to the operating temperature in power plants), gas permeability is significantly reduced. The reduction in gas permeability can be due to the densification of the porous polymeric matrix above its glass transition temperature. Polymer densification leads to pore collapse, consequently forcing the ionic liquid out of the pores, thus reducing gas diffusion. Bara et al. photopolymerized 1-alkyl-3-methylimidazolium RTILs via functional groups that have low CO2 interactivity, e.g., styrene and ­acrylates to form dense polymeric membranes for CO2 separation from N2 and CH4 [205]. The gas permeabilities of such membranes increase as a function of alkyl length. Although similar behaviors were observed for acrylate-based systems, the styrene-based RTILs are preferred as they can be easily prepared using commercially available starting materials.

Supramolecular Membranes  177 Poly RTILs contain large ions where one end is tethered to the cation in the backbone and the other end is free to interact with penetrant molecules. Another controllable parameter in RTILs is the alkyl chain length. Longer alkyl chain lengths may yield larger free volume content, thus enhancing N2 and CH4 permeabilities. Meanwhile, the repulsion effects between the immobilized ions and ion-phobic alkyl chains may also create extra free volume content. Longer alkyl chains dilute the concentration of polar ions hence decreasing the N2 and CO2 solubility coefficients. However, CH4 solubility coefficients remain nearly unchanged, as CH4 prefers to dissolve in nonpolar alkyl chains [205]. The CH4 solubility coefficient decreases only when methyl groups are next to the imidazolium ring, i.e., when the alkyl chain did not provide sufficient nonpolar moieties to enhance CH4 ­sorption. The CO2/N2 separation performance of these first generation RTILs are slightly above Robeson’s upper bound line [146]. Overall the gas permeability of this PIL is still lower than that of SILMs. Bara et al. improved the gas permeabilities of PILs by polymerizing polymerizable RTIL monomers in the presence of nonpolymerizable, free RTILs [206, 207]. The CO2 permeability of neat poly RTIL films can be enhanced by 400% with the introduction of 20 mol% of free cations. Although this concept is akin to physical blending, the attractive forces between the cations and the anions hold the free RTILs in place. Varying the organic functionality attached to the imidazolium cation can tune the gas permeability of PIL-RTIL composite membranes [208]. The organic functionality can simultaneously enhance CO2 permeability by 100–250% while maintaining CO2/N2 and CO2/CH4 selectivity. The “free,” i.e., nonpolarizable, RTILs act as “non-volatile” polarizers located between the polymer chains that improve CO2 permeability and maintain CO2/light gas selectivity. PIL-RTIL composites containing ether-based cations have the most promising combination of enhanced permeability and selectivity. However, siloxane-based cations have the most significant enhancements in gas permeability at the expense of lower selectivities. Another approach to obtain RTIL-polymer blends is to physically mix RTILs and polymers. Chen et al. have demonstrated, for the first time, a polymer blend comprising poly(vinylidene fluoride) (PVDF) and a roomtemperature ionic liquid (RTIL) that show a high CO2 permeability of 1778 barrers with CO2/H2 and CO2/N2 selectivity of 12.9 and 41.1, respectively [209]. The low viscosity RTIL, 1-ethyl-3-methylimidazolium tetracyanoborate ([emim][B(CN)4]) possesses a high CO2 solubility, and PVDF provides mechanical strength to the blend membranes. Compared to miscible ionic liquid based blends, where molecular level interactions may restrain chain flexibility and reduce gas permeability, Chen et al. showed that

178  Nanostructured Polymer Membranes: Volume 1 heterogeneous PVDF/RTIL blend systems possess far superior gas transport properties. Their gas transport and separation capabilities fall within the attractive region demarcated by the “2008 Robeson upper limit” for CO2/H2 and CO2/N2 gas pairs, and are also very stable at considerably high pressures. The disadvantages of SLM can be overcome by fixating carrier sites (FSC) in facilitated transport membranes. Such membranes combine the advantages of SLMs and a solid polymeric membrane (stability). Hagg et al. added NH4F to a polyvinylamine membrane with fixed site carriers for CO2 separations [210]. The CO2/CH4 selectivity of this membrane is as high as 1143. The hypothesis behind the ultrahigh CO2/CH4 selectivity is ­attributed to hydrogen-bonded water molecules on the fluoride ion ­providing enhanced basicity that consequently provided more CO2 affinity. Using sodium salts, oxysalts, and polyoxometalates, Zhang and Wang used Hagg’s work as a basis to investigate the role of hydrogen bonding promoting facilitated transport within membranes, and also reported the utilization of salting-out effects to maximize CO2 capture [211]. By comparing the CO2/N2 selectivity of membranes with NaF, NaCl, and NaBr salts, the distinctively enhanced selectivity of NaF can be ascribed to hydrogen bonding strength between the anions of metal salts and CO2 molecules. The best permselectivity is yielded from the most electronegative anion with the smallest atomic radius that provides high hydrogen bonding strength. The amount of hydrogen bonds also influences CO2 transport in ­glutaraldehyde-crosslinked polyallylamine membranes loaded with oxysalts. Hydrogen bonded water molecules could possibly change structure, or even influence matrix structure (salting out) [212]. Polyoxometalates (POMs) were used to investigate the structural effects of POMs on membrane stability.

5.4.3 Self-assembled Membranes Self-assembly is an important concept in biological systems, however, our focus in this section emphasizes synthetic self-assembled membranes. Selfassembled membranes can replace traditional lithographic approaches that are expensive and require specific hardware for membrane fabrication, or even mimic or replace fragile biological membranes. Self-assembled membranes can be fabricated using polymer systems, particles, small molecules with molecular weights lower than 1.5 kD, and hybrid materials that combine inorganic, bio- and polymer materials. Polymeric systems and nanoparticles provide the most straightforward self-assembly approach, while small molecules can be harnessed to fabricate ultrathin or multifunctional membranes. Hybrid systems are highly promising as they combine the advantages of

Supramolecular Membranes  179 v­ arious systems. Self-assembled membranes can be used for chiral separations, water separations, nano-, micro- and ultra-filtration, delivery vessels, controlled-release materials, sensors, catalysts, and tissue-engineering scaffolds. In the following sections, we will discuss the approaches of using polymers, small molecules, and nanoparticles for self-assembled membranes.

5.4.4 Self-assembled Polymers as Membranes The most common self-assembly polymeric membranes are fabricated using phase separating block copolymers, where two incompatible polymer blocks are used to create desired architectures. These covalently bonded polymer blocks each have their own properties and are distinctively different in polarity. Due to the covalent bonding, these polymer blocks are separated at the nanoscale. The morphologies (spherical, cylindrical, gyroidal, and lamellar) of these polymers can be tailored according to the length of the polymer blocks. Most importantly, these polymers can self-­assemble into lamellar, cylindrical, gyroidal, or compartmentalized membranes when placed in a selective solvent at low concentration. The phase separation creates well-defined pore-like structures, except that the pores of these structures are occupied by polymer, e.g., perpendicularly aligned cylinders. The polymer in these cylinders can be swelled [213] or removed [214] by introducing a selective solvent or a combination of compatible and noncompatible solvents. Other than using solvents to derive self-­assembled membranes, Ndoni and coworkers have shown that thermally driven crosslinking reactions between block copolymers of 1,2-polybutadiene-bpolydimethylsiloxane can yield membranes with different morphologies. The initial lamellar morphology in this block copolymer can be transformed and permanently frozen into gyroidal morphology or hexagonal symmetry at crosslinking temperatures of 140 °C and 85 °C. Using collective osmotic shock to burst polymer spheres in a controlled fashion, Sivaniah and coworkers fabricated a polystyrene-b-polymethylmethacrylate membrane with interconnected spherical voids to form pillar-like structures [215]. Quemener et al. fabricated a responsive self-regulating membrane that is suitable for water purification [216]. The pores of this membrane decreased and the water flow resistance decreased under increased pressure. An ABA triblock copolymer—poly(styrene-co-acrylonitrile)-b-poly(ethylene oxide)-b-poly(styrene-co-acrylonitrile) (PSAN-b-PEO-b-PSAN)—was used to generate flower-like micelles consisting of a compressible hydrophilic soft PEO corona (26 vol%) and a hard hydrophobic core of PSAN that maintains the micelle’s structural integrity (Figure 5.31). A porous film was formed through the self-assembly of 50 nm micelle formations upon solvent

180  Nanostructured Polymer Membranes: Volume 1 evaporation in a spin-casted film. Nanoporosity in the film is ascribed to free volume between micelles. The self-healing properties of this membrane are attributed to the formation of novel copolymer bridges when the PEO corona is exposed to different pressures. More importantly, the shape of these PEO coronas can be tailored according to water pressure, even after several cycles of testing (Figure 5.32).

(a) Solvent concentration

(b) Nanoporosity

(c)

Figure 5.31  (a) Chemical structure of the ABA triblock copolymer used by Quemener et al. [216]. (b) During solvent evaporation, high concentration of block copolymer will assemble into micelles to form dynamically responsive membranes. (c) An SEM image of the membrane film.

Supramolecular Membranes  181 (a)

(b)

t0 (backside)

(c)

(d)

0.07 bar

0.8 bar

1.5 bar

P m

(e)

100 80 60 40 20 0

m

(f)

120 100 80 60 40 20 0

Figure 5.32  (a–d) SEM images of membrane during the self-healing process in the presence of pressure over extended periods of time; (e) and (f) are false-color reconstruction of the SEM images in (a–d).

5.4.5 Self-assembled Molecules and Nanoparticles as Membranes Schmidt et al. used a bottom-up approach to form supramolecular nanofibers inside a scaffold to prepare stable polymer-microfiber/­­ supramolecular-nanofiber composites for filter applications [217]. This in-situ process was accomplished by dipping a nonwoven fabric into a solution of 1,3,5-­benentrisamides (BTAs) at elevated temperatures. At these temperatures, BTAs are dissolved in the solution and can soak and ­penetrate throughout the pores of the fabric. Upon removal from the ­solution and solvent e­ vaporation, the small molecules self-assemble into supramolecular nanofibers that strongly self-fixate to the polymer microfibers (Figure 5.33). Despite the presence of additional material (supramolecular nanofibers) within the nonwoven fabric, the BrunauerEmmett-Teller (BET) surface areas of these nanocomposite materials are about 400% higher than the BET s­ urface area of the polymer microfibers. The average filtration efficiencies of these microfiber-nanofiber composites increased with higher content of supramolecular nanofibers. These materials were tested using an aerosol containing 6000 particles of ISO fine test dust per cm3. As there is better control over nanofiber formation, this approach is an appealing and beneficial alternative to traditional techniques like melt blowing, centrifugal melt spinning and electrospinning.

182  Nanostructured Polymer Membranes: Volume 1 Immersion

Self-assembly upon cooling

at elevated temperatures

and solvent evaporation

Nonwoven

Soaked nonwoven

Ø 100–500 nm

Ø ~ 13 m

Microfiber-nanofiber composite

(a)

100

Filtration efficiency / %

D: 7.0 wt% 80

C: 6.4 wt%

60 40

B: 4.4 wt%

20

A: 2.8 wt% nonwoven

(b)

0 0.2

0.4

0.6 Particle size / m

0.8

1.0

Figure 5.33  (a) Schematic representation of self-assembled supramolecular nanofibers (orange fibers) within a nonwoven microfiber substrate (blue fibers) [217]. (b) Average filtration efficiencies of microfiber-nanofiber nanocomposites of Schmidt et al. The filtration efficiency was measured using a test aerosol containing 6000 particles cm–3 of ISO fine dust over a testing period of 30 seconds.

Rotello and coworkers presented a versatile strategy to construct tuneable porous membranes by impregnating glass filters with a nanoparticledendrimer network assembly [218]. Membrane porosity is controlled by varying dendrimer generation, thus providing size selectivity, while membrane surface functionality is tailored via a simple post modification process in accordance with charge and membrane hydrophobicity. This membrane consists of multilayered gold nanoparticle-dendrimer film coated on an amine-functionalized glass microfiber filter and is suitable for size-selective separation of both small molecules and biomacromolecules. In the presence of carbon disulfide, dithiocarbamate groups are formed over a reaction between CS2 and the amine groups of the dendrimer. The dithiocarbamate groups are prevented from crosslinking with the gold nanoparticles in excess CS2 and a high molar ratio of dendrimer to nanoparticle. Amine functionality of the dendrimers is restored via acid exposure without breaking up dithiocarbamate-gold bonds. These regenerated amine groups are then utilized for post-functionalization to incorporate positive and negative charges to the membrane. Subsequently, these

Supramolecular Membranes  183 positive and negative charges were utilized to chemioselectively separate cationic and anionic species through ion transport. Wang et al. capitalized on the ability of a MnO2 and V2O5 nanowire macro-membranes with nanopores to form hydrogen bonds with water molecules to separate water and oil on a macroscale [219]. The separation properties of these membranes can be turned “on” and “off ” by pretreating these membranes with ethanol and water, respectively. Oxygen atoms exposed on the membranes serve as hydrogen bond acceptors with water molecules acting as donors. Consequently, a thin water film will form as the initial water molecules bond onto the oxygen atoms and will also act as hydrogen bond acceptors for other water molecules, i.e., a hydrogen bond network consisting of water molecules. After pretreatment with water, only pure water is obtained in the permeate stream, while other immiscible solvents like oil and cyclohexane remain in the feed side. When ethanol is used as a pretreatment solvent, the hydrogen atoms in ethanol will also form hydrogen bonds with the oxygen atoms; exposing the lipophilic ethyl group on the nanowire surface; thus allowing cyclohexane to pass through the nanowire membranes as well (Figure 5.34). By lengthening the polyethylene glycol chain in 5,5 -bis(1-ethynyl-7polyethylene glycol-N,N -bis(ethylpropyl) perylene-3,4,9,10-tetracarboxy­ lic diimide)-2,2 -bipyridine) (PP2b), Rybtchinski et al. prepared a ­dissolvable, and reusable size-selective nanostructured supramolecular membrane from fibrous assemblies in water [220]. Upon solvent evaporation and filtration over commercial microfiltration syringes, three-­ dimensional supramolecular networks were formed within cellulose acetate membranes that are suitable for inexpensive and fast water separations. Unlike the work of Schmidt et al., the supramolecules did not infiltrate the cellulose acetate fibers, as this could severely impede solvent flux. Rather, the three-dimensional PP2b supramolecular, fibrous film was deposited onto the cellulose acetate membrane. By tuning supramolecular film thickness, a 12 μm thin film could be used to remove gold nanoparticles from water where the gold nanoparticles are located in the filtrates; or a 45 μm thick film can be used to perform size-selective chromatography in the sub-5  nm region. Although some gold nanoparticles remain trapped within the PP2b matrix after filtration, the PP2b supramolecular membrane can still be recycled by disassembling this film in a mixture of water and ethanol. PP2b is then retrieved from this mixture when dichloromethane is added to the aforementioned suspension; hence allowing the recycling of both the retentate (gold nanoparticles) and the PP2b material. Figure 5.35 summarizes this work and describes the recycling process of the membrane.

184  Nanostructured Polymer Membranes: Volume 1 Transmittance (%)

100 (4) (3)

Nanoscale-pore size

H2O film

(2)

90

H2O layer

4000

H2O

530

65

70 60

(1) Membrane

70

80

3403

60 600

539

550

500

3200 2400 1600 800 Wavenumber (cm-1)

(b)

(5) (6) Intrinsic structure

(7)

H2O

(110) H2O

Hydration Drying

MnO2

b,c:

O Mn

(110) Hydration

Al

co

ho yin liza tio g

Dr

O V

(a)

(10) (8)

Recycling process

a:

V2O5

(c)

Hydration surface

n

Alcoholization EtOH

Hydration Alcoholization

EtOH

(11)

(9)

(d)

Figure 5.34  (a) Schematic diagram for the switch effect of the membrane. (1) The as-obtained macroscopic membrane. (2) The randomly arranged NWs. (3) A higher magnification of (2), by which innumerous nanopores were formed. (4) Water films formed in the nanopores based on H-bond. (5) Hydration surface of the NWs based on H-bond. (6) Intrinsic structure of the MnO2 NWs. (7–10) The mechanism of the recycle process of the membrane between hydration and alcoholization. (7) The crystal structure of the MnO2 showing a (110) plane on which a lot of oxygen atoms are exposed to the surface to provide binding sites of H-bond. (8) A hydration surface. (9) An alcoholization surface. (10) Water could pass through the membrane smoothly while oil could not when the ‘switch’ was closed. (11) Oil could pass through the membrane smoothly when the ‘switch’ was open. (b) The IR spectrum of the membrane. (c–d) The crystal structure of the V2O5 from different views. (Adapted from Wang et al. [219]).

5.5 Conclusions In the preceding we have outlined the state of the art with regards to the preparation of periodic nonporous materials for molecular sieving applications. These materials include the inorganic metal-organic frameworks, and zeolitic imidazolate frameworks, through to the organic covalent organic frameworks and porous aromatic frameworks, along with their molecular counterparts, porous organic cages. This family of ­supramolecular

Supramolecular Membranes  185 Fabrication and use Supramolecular PP2b solution

5. Reassembling

1. Fliter preparation

Commercial microfiltration membrane

Large nanoparticles in aqueous phase H2O/EtOH (4:6, v/v)

2. Filtration

Nanoparticles in H2O

DCM

4. Extraction

Small nanoparticles in filtrate 3. Disassembling

Recycling

Figure 5.35  A scheme showing the fabrication, use, and recycling of supramolecular membrane of Rybtchinski et al. [220].

materials are attractive due to the large flux afforded by the open pore topologies, and the high selectivities possible by molecular sieving in uniform pores or by surface chemical functionalization. A detailed analysis of the incorporation of these materials into membranes has also been reported. The most attractive route to exploiting these materials is within mixed matrix membranes where non-selective transport pathways at grain boundaries can be minimized. Enhancement of interfacial structures between the two phases can be achieved by employing nanoparticulate versions of these supramolecular materials, yet the molecular counterparts remain attractive due to the potential for molecular-scale intimate mixing during solvent casting, avoiding grain boundaries altogether.

186  Nanostructured Polymer Membranes: Volume 1 Prospective membranes employing these approaches exhibit attractive properties such as increased permeability, some selectivity enhancements and in particular the cessation of aging within super-glassy polymer membranes—an age-old problem in the field. Future research in supramolecular membranes will focus on new periodic nanoporous materials that are engineered not just for improved gas transport properties, but for better incorporation within polymeric thin film membranes. Commercial application of these membranes will require further development of these composite materials in architectures commonly employed within commercial settings. These include large-scale flat sheets and asymmetric hollow fibers. Attainment of these prototypes, in line with the attractive performance enhancements offered by the addition of these supramolecular materials, is a prospective route to the next generation of polymer separation membranes.

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200  Nanostructured Polymer Membranes: Volume 1 196. T. Welton, Room-temperature ionic liquids. solvents for synthesis and catalysis. Chem. Rev., 99(8), 2071–2084, 1999. 197. R.D. Rogers, K.R. Seddon, Ionic liquids—Solvents of the future?. Science, 302(5646), 792–793, 2003. 198. D. Han, K.H. Row, Recent applications of ionic liquids in separation. Molecules, 15, 21, 2010. 199. J.F. Brennecke, B.E. Gurkan, Ionic liquids for CO2 capture and emission reduction. J. Phys. Chem. Lett., 1(24), 3459–3464, 2010. 200. E.D. Bates, R.D. Mayton, I. Ntai, J.H. Davis, CO2 capture by a task-specific ionic liquid. J. Am. Chem. Soc., 124(6), 926–927, 2002. 201. J.L. Anthony, E.J. Maginn, J.F. Brennecke, Solubilities and thermodynamic properties of gases in the ionic liquid 1-N-butyl-3-methylimidazolium hexafluorophosphate. J. Phys. Chem. B, 106(29), 7315–7320, 2002. 202. A. Yokozeki, M.B. Shiflett, Hydrogen purification using room-temperature ionic liquids. Appl. Energy, 84(3), 351–361, 2007. 203. P. Scovazzo, J. Kieft, D.A. Finan, C. Koval, D. DuBois, R. Noble, Gas separations using non-hexafluorophosphate [PF6] anion supported ionic liquid membranes. J. Membr. Sci., 238(1–2), 57–63, 2004. 204. J. Ilconich, C. Myers, H. Pennline, D. Luebke, Experimental investigation of the permeability and selectivity of supported ionic liquid membranes for CO2/He separation at temperatures up to 125 °C. J. Membr. Sci., 298(1–2), 41–47, 2007. 205. J.E. Bara, S. Lessmann, C.J. Gabriel, E.S. Hatakeyama, R.D. Noble, D.L. Gin, Synthesis and performance of polymerizable room-temperature ionic liquids as gas separation membranes. Ind. Eng. Chem. Res., 46(16), 5397–5404, 2007. 206. J.E. Bara, E.S. Hatakeyama, D.L. Gin, R.D. Noble, Improving CO2 permeability in polymerized room-temperature ionic liqiud gas separation membranes through the formation of a solid composite with a room-temperature ionic liquid. Polym. Advan. Technol., 19(10), 1415–1420, 2008. 207. J.E. Bara, D.L. Gin, R.D. Noble, Effect of anion on gas separation performance of polymer- room-temperature ionic liquid composite membranes. Ind. Eng. Chem. Res., 47(24), 9919–9924, 2008. 208. J.E. Bara, R.D. Noble, D.L. Gin, Effect of “free” cation substituent on gas separation performance of polymer-room-temperature ionic liquid composite membrane. Ind. Eng. Chem. Res., 48(9) 4607–4610, 2009. 209. H.Z. Chen, P. Li, T.S. Chung, PVDF/ionic liquid polymer blends with superior separation performance for removing CO2 from hydrogen and flue gas. Int. J. Hydrogen Energ., 37(16), 11796–11804, 2012. 210. M.B. Hagg, T.J. Kim, B. Li, Membrane for separating CO2 and process for the production thereof, US Patent 20080156188 A1, assigned to NTNU Technology Transfer AS, 2008. 211. L. Zhang, R. Wang, Salting-out effect on facilitated transport membranes for CO2 separation: From fluoride salt to polyoxometalates. RSC Adv., 2(25), 9551–9554, 2012.

Supramolecular Membranes  201 212. F.A. Long, W.F. McDevit, Activity coefficients of nonelectrolyte solutes in aqueous salt solutions. Chem. Rev., 51(1), 119–169, 1952. 213. M. Ulbricht, Advanced functional polymer membranes. Polymer, 47(7), 2217–2262, 2006. 214. K.V. Peinemann, V. Abetz, P.F.W. Simon, Asymmetric superstructure formed in a block copolymer via phase separation. Nat. Mater., 6(12), 992–996, 2007. 215. P. Zavala-Rivera, K. Channon, V. Nguyen, E. Sivaniah, D. Kabra, R.H. Friend, S.K. Nataraj, S.A. Al-Muhtaseb, A. Hexemer, M.E. Calvo, H. Miguez, Collective osmotic shock in ordered materials. Nat. Mater., 11(1), 53–57, 2012. 216. P. Tyagi, A. Deratani, D. Bouyer, D. Cot, V. Gence, M. Barboiu, T.N.T. Phan, D. Bertin, D. Gigmes, D. Quemener, Dynamic Interactive Membranes with Pressure-Driven Tunable Porosity and Self-Healing Ability. Angew. Chem. Int. Ed., 51(29), 7166–7170, 2012. 217. H. Misslitz, K. Kreger, H.W. Schmidt, Supramolecular nanofiber webs in nonwoven scaffolds as potential filter media. Small, 9(12), 2053–2058, 2013. 218. M.H. Park, C. Subramani, S. Rana, V.M. Rotello, Chemoselective nanoporous membranes via chemically directed assembly of nanoparticles and dendrimers. Adv. Mater., 24(43), 5862–5866, 2012. 219. Y.H. Long, J.F. Wang, P.P. Xiang, G.L. Xu, B. Hu, S. Zhu, W.C. Lu, X.Q. Zhuang, W.J. Xun, Hydrogen bond nanoscale networks showing switchable transport performance. Sci. Rep., 2, 612, 2013. 220. E. Krieg, W. Haim, E. Shirman, E. Shimoni, B. Rybtchinski, A recyclable supramolecular membrane for size-selective separation of nanoparticles. Nat. Nanotechnol., 6(3), 141–146, 2011.

6 Organic Membranes and Polymers for the Removal of Pollutants Bernabé L. Rivas1*, Julio Sánchez1 and Manuel Palencia2 Polymer Department, Faculty of Chemistry, University of Concepción, Concepción, Chile 2 Department of Chemistry, Faculty of Natural and Exact Sciences, University of Valle, Cali, Colombia 1

Abstract

Fundamental aspects of membranes as well as membrane processes are presented in this chapter, including membranes as electro-ultrafiltration, ultrafiltration ­coupled with ultrasound, flotation coupled with microfiltration, liquid-phase polymer-based retention (LPR), and surfactant liquid membrane coupled with liquid-phase polymer-based retention. As a result of their simplicity of operation and relatively low energy consumption, it has been indicated that these membrane separation processes are a very attractive alternative for aqueous effluent treatment. The emphasis of this chapter is on LPR for the removal of hazardous inorganic species such as heavy metal ions and arsenic. This technique is understood to occur by a mechanism in two steps: formation of polymer-metal ions species and separation by size exclusion. But, the process is visualized to be carried out in two stages: first a charge stage related to the retention of metal ions in solution and then a discharge stage related to the release of metal ions and regeneration of the polymer. Regeneration of polymer can be carried out by different techniques (protolysis, transcomplexation, thermal or electrochemical methods). There is also a section of the chapter that focuses on the use of poly(ethyleneimine) and poly(ethyleneimine epichlorohydrin) to remove Cu2+, Cd2+, Co2+, Ni2+, Zn2+, Pb2+, and Cr3+ under different experimental conditions. Keywords:  Water-soluble polymers, membranes, ultrafiltration, environment, metal ions, arsenic *Corresponding author: [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 1, (203–236) © 2017 Scrivener Publishing LLC

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204  Nanostructured Polymer Membranes: Volume 1

6.1 Membranes: Fundamental Aspects A membrane is defined by the International Union of Pure and Applied Chemistry (IUPAC) as “a structure with lateral dimensions much greater than its thickness, through which mass transfer may occur under a variety of driving forces” [1]. In consequence, a membrane is a thin planar structure or interphase that separates two phases and permits mass transfer between the phases. Commonly, membranes can be classified into two main groups: (1) biological membranes and (2) artificial or synthetic membranes. Thus, biological membranes include cell membranes (i.e., lipid bilayer systems) and physiological membranes (e.g., the meninges that surround the brain, peritoneum or pericardium) [2]. On the other hand, artificial membranes are made from different materials by different technological processes (i.e., materials used for separation process such as reverse osmosis, microfiltration, ultrafiltration, pervaporation, dialysis, emulsion liquid membranes, membrane-based solvent extraction, membrane reactors, gas permeation, supported liquid membranes, electrodialysis) [2–8]. Commonly for separation processes, synthetic membranes are fabricated in two main geometries: flat sheet membranes and cylindrical membranes. These membranes can be fabricated from both ceramic and polymeric materials. However, though ceramic membranes have numerous advantages over polymeric membranes (such as higher chemical and thermal stability), the polymer membranes are the main type of membranes in the market because polymeric materials are easier to process and less expensive [2, 3]. Thus, different polymers are used to make membranes, for instance, polyehtersulfone, polysulfone, polypropylene, cellulose, polyurethanes, polyacrylonitrile, poly(vinylidene fluoride), polyimide, polyamide, and others [9]. In addition, membranes can be classified according to their structure as asymmetric and symmetric membranes. Asymmetric membranes consist of two layers, the top one is a very thin dense layer, commonly called the skin layer or active layer, and determines the permeation properties; whereas the bottom one is a porous sublayer, commonly called the support layer, and is a passive layer which provides mechanical strength to the membrane [2, 3]. Membranes may have a porous or nonporous structure. A porous membrane is structural and functionally analogous to a conventional filter. This kind of membrane has a rigid and hollow structure (i.e., interconnected and randomly distributed pores with a diameter of the order of 0.01 to 10 μm) [1–3]. Thus, separation process by porous membrane is determined by a size exclusion mechanism, whereby all particles larger than the largest pores are rejected by the membrane (retentate stream) and particles smaller than the largest pores, but larger than the smallest pores are

Organic Membranes and Polymers for Removal of Pollutants  205 partially rejected, according to the operation conditions and pore size distribution of the membrane; besides, particles much smaller than the smallest pores will pass through the membrane (permeate stream). In general, solutes separation by porous membranes is mainly a function of molecular size and pore size distribution [2, 3, 5, 6]. Nonporous membranes or dense membranes (e.g., liquid membranes) consist of a dense film through which solutes or permeants are dissolved in the membrane phase and transported by diffusion under the driving force. The separation of various components of a mixture is related directly to their relative transport rate within the membrane, which is determined by their diffusivity and solubility in the membrane material [2, 3]. At the present time, membrane separation processes have been accepted as a unit operation for a variety of separations in industries. These membrane processes can be driven by pressure, concentration, or electric field across the membrane. Applications of synthetic membranes are very numerous and depend on membrane process, membrane properties, and the finality of separation. Thus, for reverse osmosis (RO), membranes should support very high pressures and these can be used for sea water and brackish water desalination, wastewater treatment, petroleum industry, recovery of plating chemicals from wastewaters, and process waters in the electroplating and metal-finishing industry. Ultrafiltration membranes are used in electrodialysis pretreatment, electrophoretic paint, cheese whey treatment, juice clarification, recovery of textile sizing agents, separation of oil/water emulsion, water treatment, and reverse osmosis pretreatment. Microfiltration membranes are used in purification of fluids in the semiconductor manufacturing industry, clarification and biological stabilization in the beverage industry, sterilization processes in the pharmaceutical industries, and different stages during chemical analysis. For special separations, specific membranes are required. Whereas in ultrafiltration, microfiltration, and reverse osmosis membranes are porous structures, in other applications, porous structures are not needed. An example of this are gas separations (oxygen/nitrogen separation, helium recovery, removal of acid gases from light hydrocarbons, biogas processing, etc.) [1–10].

6.1.1 Membrane Transport Theory Previously, it was indicated that membrane can be classified to be porous and nonporous. Current models to describe the separation mechanism in the membranes are based on this initial classification. Two models are typically used and these are the solution-diffusion model and size exclusion model (or pore-flow model) [2]. In the first one, permeants are dissolved in

206  Nanostructured Polymer Membranes: Volume 1 the membrane material and then are diffused through the membrane down a concentration gradient. Thus, this mechanism imply two steps which are directly associated with the nature of permeants. In the second model, permeants are transported by a driving force which produces a convective flow through pores. In this case, separation occurs because some of the permeants are retained whereas other permeants pass through the membrane pores. A schematic illustration of these models is shown in the Figure 6.1. The basis of the solution-diffusion model is the diffusion process. By diffusion, matter is transported from one part of a system to another by a concentration gradient. The individual permeant molecules in the membrane phase are in constant random molecular motion without any preferred direction of motion, but if a concentration gradient of permeate molecules is formed then a net transport of matter will occur from the high concentration region to the low concentration region. As a consequence, solution-diffusion model is described by Fick’s law, which states:

Ji



Di

dCi (6.1) dx

where Ji is the flux of component i and dCi/dx is the concentration gradient Ci of component i. The term Di is the diffusion coefficient and is a measure of the mobility of the individual molecules. The minus sign shows that the direction of diffusion is down the concentration gradient [2, 3, 10]. Reverse osmosis, pervaporation and gas permeation in polymer films are described by solution-diffusion model [2, 11]. In all cases the diffusion (a)

(b)

Membrane phase

Membrane phase

Figure 6.1  Membrane transport mechanism: (a) size exclusion model and (b) solutiondiffusion model.

Organic Membranes and Polymers for Removal of Pollutants  207 of molecules occurs in a dense polymer membrane; besides, it is established that pressure, temperature, and composition of the fluids on either side of the membrane determine the concentration of permeant at the membrane phase in equilibrium with the fluid phase. The basis of the size exclusion model is the pressure-driven convective flow and usually is used to describe the particle flow in a capillary or porous medium. Size exclusion model is described by Darcy’s law, which states:

Ji

KCi

dP (6.2) dx

where dp/dx is the pressure gradient existing in the porous medium, Ci is the concentration of component i in the medium and K is a coefficient associated with the nature of the medium. Ultrafiltration and microfiltration separation methods are described by size exclusion model [2, 3]. In the pore-flow model, the simplest representation for a membrane is one in which the membrane pores are assumed to be parallel cylindrical pores and perpendicular to the membrane surface. Thus, the length of each one of the cylindrical pores is equal or almost equal to the membrane thickness. Therefore, an ideal membrane that has these characteristics can be denominated a “Hagen-Poiseuille membrane” since the volume flux through these pores can be described by the Hagen-Poiseuille equation [3]. Assuming that all the pores have the same radius, that fouling and concentration polarization are not present, that the flow through the pores is laminar, incompressible, Newtonian and time independent, the Hagen-Poiseuille equation is given by:

Ji

rp2 P 8

x

(6.3)

where Ji is the flow rate through the membrane, rp is the channel radius (in this case, the mean pore radius), ∆P is the applied pressure (∆P must exactly correspond to the net driving force, and thus the osmotic pressure, ∆π, is important, and therefore the effective pressure should be ∆P − ∆π), η is the viscosity of the fluid permeating the membrane, ∆x is the length of the channel and ε is the surface porosity of the membrane, which is defined to be:

np r 2 Am

(6.4)

where Am is the membrane area and np is the number of pores.

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6.1.2  Pressure-driven Membrane Methods In their general form, pressure-driven membrane methods separate a feed stream into two effluent streams: the permeate, which is the portion of the fluid that has passed through the membrane, and the retentate or concentrate, which is the portion containing the constituents that have been rejected by the membrane [1, 10, 11]. Thus, three streams can be identified to be feed, permeate, and retentate. The main pressure-driven membrane methods are reverse osmosis, ultrafiltration and microfiltration. However, some authors include nanofiltration. The main difference between these methods is the pore size of the membrane and, consequently, the applied pressure to produce a permeate stream (see Table 6.1). Reverse osmosis is defined by IUPAC as a pressure-driven separation process in which applied transmembrane pressure causes selective movement of solvent against its osmotic pressure difference. Also, microfiltration, ultrafiltration, and nanofiltration are defined by IUPAC to be pressure-driven membrane-based separation processes in which particles and dissolved macromolecules larger than 100 nm, between 2 and 100 nm, and molecules smaller than about 2 nm are rejected, respectively [1]. For pressure-driven membrane separation methods there are mainly two types of filtration according to flow direction [3]: Conventional filtration or dead-end filtration consists of the flow through a membrane module in which the only outlet for upstream fluid is through the membrane, and in which the flow is perpendicular to membrane surface. Tangential filtration or cross-flow filtration consists of the passage of the flow through a membrane module in which the fluid on the upstream side of the membrane moves parallel

Table 6.1  Typical values of pore size and applied pressure for main pressuredriven membrane methods [1]. Membrane method

Pore size range (nm)

Applied pressure range (kPa)

Reverse osmosis

0.45

0.30

0.79

0.40

0.82

–

–

–

Porosity %

Table 9.6  Materials, composition of hollow fiber liquid membranes.

[117] [117]

CO2 absorption with MEA and AMP

[115, 116]

[100]

[114]

[113]

[112]

[111]

[111]

[110]

CO2 absorption with MEA and AMP

H2S and CO2 absorption with Na2CO3 solution

CO2 absorption with pure water

CO2 absorption with PG, MEA and MDEA solutions

CO2 absorption with water, DEA and NaOH solutions

CO2 absorption with CORAL 20 solution

CO2 absorption with MEA solution

CO2 absorption with MEA solution

H2S absorption with NaOH solution

[110]

[109]

SO2 absorption with NaOH, Na2CO3, Na2SO3 and NaHCO3 solutions H2S absorption with NaOH solution

Ref.

Process

Liquid Membranes  353

354  Nanostructured Polymer Membranes: Volume 1

9.3.6  Bulk Hybrid Liquid Membranes The use of liquid membranes containing specific metal ion carriers offers an alternative method to the solvent extraction processes for selective separation and concentration of the metal ions from aqueous dilute solutions. A variety of types of liquid membranes exist, i.e., bulk liquid membranes (BLMs), emulsionized liquid membranes (ELMs), supported liquid membranes (SLMs), and dual module hollow fiber (DMHF) membranes [118]. Bulk liquid membranes consist of aqueous feed and stripping phases separated by a water-immiscible liquid membrane (thickness≥ 0.1 cm). They usually demonstrate stable transport properties [119]. During recent years a significant increase in the applications of liquid membranes for the separation of cobalt from nickel has been observed. Tertiary amines are widely used as ion carriers in the liquid membranes.

9.3.6.1 Instruction Liquid membrane technology has great potential for the removal of heavy metals from aqueous dilute solutions. The liquid membrane is a layer of an organic solvent separating two aqueous solutions. The main advantages of the liquid membranes, if compared with the traditional solvent extraction, are the relatively small volume of the organic phase, simultaneous extraction and back-extraction within a single technological stage, higher extraction degrees of target compounds from dilute solutions, more effective separation of elements with similar properties, easy scale-up, and low capital and operating costs [120–122]. During the three decades from 1940 to 1960,69 papers related to mixed surfactant systems were published; in the 1970s, the number was 129; and in the 1980s there were 436 papers. Consulting SciFinder Scholar, in the period from 1990 up to the first half of 2007, about 2,700 references are found containing the search term “mixed surfactant system.” In addition, when “surfactant mixture” is entered into the SciFinder Scholar, since 1990 more than 25,000 references are found containing the search term “surfactant mixture” [123]. Application of a pressure difference, an electric field, or temperature considerably intensifies the process, but these special methods are beyond the scope of this overview.

9.3.6.2  Theoretical Aspects of Bulk Hybrid Liquid Membranes Liquid membranes can be classified into different types: bulk, emulsionized, supported, and hollow fiber liquid membranes. Bulk liquid membranes

Liquid Membranes  355 consist of aqueous feed and stripping phases, separated by a water-­ immiscible liquid membrane. They demonstrate stable transport properties. During recent years the application of different liquid membrane technologies to remove chromium (VI) has been widely studied [124]. A number of reviews and articles have appeared on related subjects. Theoretical aspects of the hollow-fiber liquid membrane (HFCLM) selective separation processes have been covered in the reviews and articles [125–128]. The concept of bulk hybrid liquid membrane (BHLM) transport is simple. As shown in Figure 9.5, a solution of an extracting reagent (carrier phase, E) flows between two membranes, which separate the carrier phase from the feed (F) and receiving (R) phases [129]. A solute of interest diffuses to the F/E interface, which is extracted from the feed phase by a LM as a result of the thermodynamic conditions at the F/E interface; the solute-carrier complex diffuses to the E/R interface and is simultaneously stripped by the receiving phase due to the different thermodynamic conditions at the E/R interface. The theory for BHLM was developed for flat thin uncharged symmetric membranes without variation in porosity and pore sizes across the membrane thickness. Concentration profiles with hydrophobic membranes are demonstrated in Figure 9.6, while those containing hydrophilic or ionexchange membranes are in Figure 9.7 [130].

Counter-transport Feed compartment F

Carrier compartment E

M1

nHL

Mn+ nH+

MLn

nHL

Co-transport

Strip compartment R M2

MLn

Mn+

Mn+

nH+

nX–

(a)

(b)

Carrier compartment E

Strip compartment R M2

nC nC

Mn+ M(CX)n

M(CX)n

nX–

(c) nHL

Mn+ nH+

Feed compartment F M1

MLn

nHL MLn

Mn+

Mn+

nH+

nX– (d)

nC nC

M(CX)n

Mn+ nX–

M(CX)n

Figure 9.5  Schematic transport models of a bulk organic hybrid liquid membrane (BOHLM) system with (a, c) hydrophobic membranes and (b, d) hydrophilic or ion exchange membranes.

356  Nanostructured Polymer Membranes: Volume 1 Compartment F Stirring

hfe

F/E membrane hmf

[M]e1

hef

Stirring

her

hmr

Compartment R

hre

Stirring

[M]e2

[M]F Concentration

E/R membrane

Compartment E

[M]E

[M]f1 [M]e3

[M]e4 [M]r1 [M]R

Distance H

Figure 9.6  Schematic concentration profile of each Ti(IV) chemical species, transported through the BOHLM system with hydrophobic membranes. Compartment F Stirring

hfe

F/E membrane hmf

Compartment E hef

Stirring

E/R membrane her

hmr

Compartment R hre

Stirring

[M]e1

Concentration

[M]F

[M]E

[M]f1

[M]e4

[M]f2 [M]r3

[M]r1 [M]R

Distance H

Figure 9.7  Schematic concentration profile of each Ti(IV) chemical species, transported through the BHLM with hydrophilic or ion-exchange membranes.

9.3.6.3  Pertraction in a Multi-membrane Hybrid System Similarly to conventional liquid membrane transport (Figure 9.8), pertraction of carboxylic acids in the multi-membrane hybrid system (MHS) (Figure 9.8b) occurs as a coupled process involving the subsequent steps of extraction, diffusion, and re-extraction. The difference is introduced by the

Liquid Membranes  357 s

s IEM LM

LM

s

LM

(a)

IEM LM IEM s

(b)

Figure 9.8  Scheme of conventional agitated bulk liquid membrane (a), multi-membrane hybrid system, and respective concentration profiles (b).

presence of additional reactive membranes reacting with carboxylic acids or a stripping agent dissolved in the external aqueous solutions. A strongly basic membrane can exchange anions or react with undissociated acid molecules [131], whereas a strongly acidic membrane can lower the dissociation of acids and enhance the sorption (extraction) of the weaker ones [132]. Formally, the overall interfacial processes, as mediated by functionalized membranes, can still be regarded as known indirect extraction (or re-extraction) processes coupled to the solution-diffusion transport in an organic liquid membrane phase. A theoretical model of transport in a simple agitated bulk liquid membrane (BLM) was formulated by Reusch and Cussler [133]. The corresponding concentration profile is schematically shown in Figure 9.8a. In the case of a multi membrane hybrid system the processes specific for the polymer membranes should be additionally taken in to account (Figure 9.8b). Thus, the interfacial steps from Figure  9.8a are split by the additional reactiondiffusion or solution-diffusion transport of acids across the strongly basic or acidic polymer membranes, respectively. It means that qualitative and quantitative description of transport in the simple MHS (without any carrier) can be made according to the Reusch-Cussler model. It can be regarded as Fickian transport through a liquid membrane with boundary conditions determined by the sorption equilibrium 9.1 at the respective interface:



J

Dkd C /l mol/cm2s 

(9.1)

where D and kd are the diffusion and distribution coefficients, ΔC is the concentration difference and l the membrane thickness. According to

358  Nanostructured Polymer Membranes: Volume 1 Equation 9.1, the transport of organic molecules through organic phase will depend on the effective partition coefficients (kd) of the acid between a liquid membrane and an aqueous phase contained in a hydrophilic ionexchange membrane. This coefficient can be controlled by hydrophobicity of solutes and liquid membrane or their own solubility parameters. Moreover, the mass transfer in all membranes constituting the system will be controlled by specific diffusion (D) coefficients, which can be correlated with an organic phase viscosity. The applications of emulsionized liquid membranes for the separation of cobalt from nickel using trialkylamine [134], tri-n-octylamine [61, 135] and triisooctylamine [136, 137] as mobile carriers have been reported. The separation of cobalt, copper and nickel by polymer inclusion membranes containing tri-noctylamine and triisooctylamine was investigated [137]. However, practical importance of the liquid membranes is not sufficiently high at present. The main issue for all types of supported liquid membranes is the loss of the membrane solvent and carrier. The emulsionized liquid membrane process is much more complex than the bulk liquid membrane. The slow transport of metals is a disadvantage of polymeric liquid membranes [138]. Therefore, the development of new membrane extraction processes for the separation of cobalt from nickel is of important environmental and economic interest.

9.3.6.4 Applications Liquid membrane technology has great potential for the removal of heavy metals from aqueous dilute solutions. A liquid membrane is a layer of an organic solvent separating two aqueous solutions. The addition into the membrane of mobile carrier species can increase the membrane’s solute permeability and selectivity by carrier-facilitated transport. The main advantages of liquid membranes compared with the traditional solvent extraction include relatively small volume of the organic phase, simultaneous extraction and back-extraction in a single technological step, higher extraction degrees of target compounds from dilute solutions, more effective separation of elements with similar properties, easy scale-up, low capital and operating costs [139, 140]. Liquid membranes provide greater selectivity and permeability than the solid ion-exchange membranes. Data on electrodialysis of liquid membranes are rather low compared with membrane extraction. One of the first works in this field was conducted by Kislik et al. [141], who used electrodialysis through bulk liquid membranes to concentrate rhenium from industrial solutions. The development of a theoretical model for the

Liquid Membranes  359 +

1

4

2

6

3

7

5

–

3

2

1

Figure 9.9  Electrodialysis cell with liquid membrane (1 – platinum electrodes; 2 – sulphuric acid solutions; 3 – solid anion-exchange membranes; 4 – feed solution; 5 – cellophane films; 6 – liquid membrane; 7 – strip solution).

description of the transport of cations through thick neutral carrier membranes during electrodialysis can be found in references [142] and [143]. Application of a direct electric field significantly intensifies the transport of ions through the liquid membranes and facilitates the stripping of metals from the organic phase [144–147]. The experiments were carried out in an electrodialysis cell (Figure 9.9). It consists of five Teflon compartments. The liquid membrane was separated from the aqueous solutions by two vertical cellophane films.

9.3.6.5 Summary In comparison with liquid-liquid extraction (LLX), supported liquid membrane (SLM), and emulsionized liquid membrane (ELM), BHLM has the potential to provide many economic and operational advantages, such as: low carrier losses, long membrane lifetime, “once-through” continuous operation, compact equipment, application of different driving forces (chemical potential, pressure, temperature gradients between different compartments, electric field, etc.), no need of surface formation, impregnation, gravity gradients, high membrane capacity, etc. Commercially available membrane modules and equipment may be used in the BHLM. One more advantage of the BHLM system is realized from theoretical simulations. At feed-side resistances not controlling the solute transport (as in the case with titanium) the fluxes of the solute to the strip are approaching those in the SLM systems, but without the many drawbacks of the latter. Currently, the only commercial application of the LM technologies is waste treatment (water and degasification), where low concentration solutes are removed from large volumes of effluents. Solvent extraction and ion exchange are often not economically convenient in these cases. Due to the complexation reaction and low quantity of complexing agent-carrier

360  Nanostructured Polymer Membranes: Volume 1 required, the BHLM technologies are suited for high recoveries of dilute solutes. Despite promising technological performance, few BHLM techniques have been commercialized. This is due to a number of economic and technical factors. The main drawbacks of the BHLM systems are the poisoning of the carrier by irreversible reactions, low diffusion rate of the large organic molecules, losses of the LM and contamination of the treated and product aqueous solutions by the LM organic components, the membrane walls binding or fouling due to the emulsion, and gel formation on their surfaces. Long-term stability of complexing carriers in the LM, stability of membrane walls, and their long-term permeability are problems that limit the commercialization of the BHLM technologies. To optimize the BHLM processes and improve separation and transport properties, it is necessary to develop complexation chemistry and new selective carriers, or to improve existing ones. Such improvements and reducing the price of membrane film or hollow-fiber production will definitely speed up industrial application of the BHLM technologies, especially for gas, pharmaceutical and bioreactor applications.

9.3.7  Bulk Aqueous Hybrid Liquid Membranes The bulk aqueous hybrid liquid membrane (BAHLM) system has been reported for selective separation of solutes such as metal ions and acids [148, 149]. The term bulk organic hybrid liquid membrane (BOHLM) includes all bulk liquid membrane processes incorporating liquid-liquid extraction (LLX) and membrane separation in one continuously working module. Bulk aqueous hybrid liquid membrane (BAHLM) is one of the BOHLMs, which utilizes an extracting reagent (carrier) solution, immiscible with water, circulating or flowing between membranes as barriers.

9.3.7.1 Introduction Recent studies on mixed surfactant systems were systematically overviewed, paying special attention to synergism observed in micellization as well as adsorbed film formation upon mixing of a few nonionic surfactants with a variety of surfactants (such as anionics including bile salts and a hybrid type surfactant, cationics including a Gemini-type surfactant, different types of nonionics and a zwitterionic surfactant used as a membrane solubilizer) in addition to various combinations of anionics. A liquid membrane is a layer of an organic solvent separating two aqueous solutions. Compounds, promoting the transport of substances from one aqueous solution to another,

Liquid Membranes  361 may be dissolved in the organic phase. Liquid ­membranes offer a lot of advantages over conventional separation technologies such as easy operation, low capital and operating costs, continuous operation, high selectivity, high fluxes, combination of extraction and stripping processes into a single stage, uphill transport against concentration gradients and small amounts of extractants [120, 121, 139]. However, the practical importance of liquid membranes is at present rather low [122].

9.3.7.2 Theoretical Aspects of Bulk Aqueous Hybrid Liquid Membranes As can be seen from the scheme in Figure 9.10, the technological concept of BAHLM transport is simple: an aqueous solution of a carrier, E, flows between two membrane barriers, which separate the carrier from the feed, F, and strip, R, aqueous solutions [148]. The BAHLM systems with ion-exchange membranes, based on Donnan dialysis [150, 151], will be considered below. Donnan dialysis is a continuously operating ion-exchange process. Water flux through the IEM behaves in an anomalous manner [152, 153]. Nondiffusible ionic species, fixed in the IEM pores, affect the anomalous osmosis. The anomalous osmosis may be positive, with the flux in the direction of the activity gradient, or negative, with the flux in the opposite direction to the activity gradient. For the BAHLM systems, Donnan membrane potential [154–158], osmotic pressure gradient [159], and possibly pressure gradient [160, 161], have to be added as driving forces. Therefore, the theory should take into account

1

F

1

M2

H

2

E H

1

M1

2

R

2

Figure 9.10  Schematic diagram of the three-aqueous phase (BAHLM) module: F, E, and R are the compartments of the feed, LM, and receiving (strip) solutions, respectively; M1 and M2 are the ion-exchange membranes; 1 and 2 are the inlet and outlet of the feed, LM, and strip solutions, respectively.

362  Nanostructured Polymer Membranes: Volume 1 Compartment F/E F membrane Stirring

hfe

Compartment E

hmf

hef

Stirring

E/R membrane her

Compartment R

hmr

hr

Stirring

[M]e1 Concentration

[M]E

[M]F

[M]e2

[M]f2 [M]f3

[M]r1

[M]f1

[M]r2 [M]R

[M]r3

(a)

Concentration

Stirring

hfe

hf/e

Stirring

hr

he/r

Stirring

[M]e1 [M]E

[M]F

[M]R [M]r1 [M]f1

(b)

Distance H

Figure 9.11  Simulation of the BAHLM transport: (a) concentration profiles in a regular scheme and (b) concentration profiles in a simplified scheme.

both diffusive and convective transport. As can be seen in Figure 9.11, the physicochemical aspects of the BAHLM processes are complicated. According to the transport model equations, developed for the BAHL membrane systems [149], the transport selectivity of two metal species (SM /SM ) is determined by the relation 9.2: 1



2

SM1 /M2

K M1 (QF0 )M1

K M2 (QF0 )M2

K M1 [M1 ]0F

K M2 [M2 ]0F

(9.2) 

where subscripts M1 and M2 refer to the two metal species; KM and KM are 1 2 the total overall mass transfer coefficients, (QF0)M and (QF0)M are the initial 1 2 quantities of two metals in the treated feed phase, and [M1]F0 and [M2]F0 are the initial concentrations of two metals in the same phase.

Liquid Membranes  363

9.3.7.3 Applications Applications of the BAHLM processes are mainly in metal separation, waste water treatment, biotechnologies, drug recovery-separation, organic compounds, and gas separation. The recent applications of different types of the BAHLM systems are summarized below. Metal separation-concentration Metal ions separation for hydrometallurgical and waste water treatment applications has attracted considerable interest. Selective separations of alkali, alkali earth, rare earth, heavy metal ions, precious metals, etc., are studied by many authors using all of the above-described techniques. Biotechnological products recovery-separation Recovery and separation of carboxylic and amino acids from fermentation broth have been tested using layered BLM, rotating, creeping, ­spiral-type FLM, HFLM, HLM, and MHS-PV techniques of the BAHLM processes. Pharmaceutical products recovery-separation Selected papers from the last 15 years have presented results of separation of drugs, antibiotics, and enantiomers by layered BLM, hollow-fiber LM, rotating film techniques. Organic compounds separation and organic pollutants recovery from wastewaters The BAHLM techniques have been considered for the separation of ethylene, benzene, propanol, olefin, aromatic amines from organic liquid mixtures of volatile organic compounds (VOC) and phenol from wastewater. Fermentation or enzymatic conversion-recovery separation (bioreactors) The BAHLM systems, integrating reaction, separation, and concentration functions in one apparatus (bioreactor), have attracted great interest in the last few years. Bioreactors combine the use of specific biocatalyst for the desired chemical reactions, with repeated or continuous application of it under very specific conditions. Such techniques were termed hybrid membrane reactors. In biotechnology and pharmacology, these applications are termed hybrid membrane bioreactors or simply bioreactors. Analytical applications The BAHLM techniques were intensively used in analytical chemistry for separation and preconcentration of metals, organic acids, and organic and pharmaceutical compounds.

364  Nanostructured Polymer Membranes: Volume 1

9.3.7.4 Summary Bulk aqueous hybrid liquid membrane (BAHLM) is an environmentally friendly and economical technology due to operation at ambient temperature, low energy requirements, low chemical consumption, and no formation of parasitic byproducts. They are particularly attractive for waste water treatment due to its high selectivity, which allows transfer of small amounts (50–1000 ppm) of toxic metals (instead of large volumes of solutions) and their concentration in the strip solutions. In some cases, those metal concentrates could be selectively separated. Many directions for the application of the BAHLM systems deserve further development. These directions include the separations of mixtures after degradation of organic compounds, products of fermentation, biological mixtures (hormones, peptides, etc.), drug conversion and selective separation, catalytic reaction enhancement and selective separation, wastewater and water treatment, etc.

9.3.8  Liquid Membranes in Gas Separation 9.3.8.1 Introduction Dating back to the 1960s, liquid membrane (LM) separation technology has aroused the interest of many scholars because of its unique advantage [162]. Compared with solid polymer separation membrane, the mass transfer rate of the liquid membrane separation process had shown absolute advantages because the diffusion coefficients of molecular solutes in liquid membranes are higher by three to four orders of magnitude in comparison to solid membranes. Based on this point, the gradual application of liquid membrane in gas separations has been an intensively studied separation process. Generally, liquid membranes with and without supports can be differentiated. For those not employing supports, the so-called bulk liquid membranes (BLMs) and emulsionized liquid membranes (ELMs) are found. The liquid membranes employing a support can be subdivided into immobilized liquid membranes (ILMs), supported liquid membranes (SLMs) and contained liquid membranes (CLMs) [46]. The ELM is mainly used in the directional movement and separation of metal ions in solution, while the SLM is mostly applied in the field of gas separation. Investigated applications of liquid membranes contain the separation or concentration of ions, the separation of liquid feeds and the separation of gases or vapors. This section reviews liquid membranes for gas separations. It summarizes the separation mechanism of liquid membrane, materials for liquid membranes and specific application of liquid membranes for gas separation.

Liquid Membranes  365

9.3.8.2  Separation Mechanism The process of liquid membrane in gas separation is mainly explained by  the dissolve-diffusion mass transfer mechanism [163]. Including the mass transfer steps in the respective feed and permeate phases, a gas molecule is transported across the membrane in the following seven steps: 1. Convective transport of the molecule towards the membrane. 2. Diffusion of the molecule through the boundary layer at the feed-membrane interface. 3. Absorption into the membrane phase. 4. Diffusion through the liquid membrane. 5. Desorption into the permeate phase. 6. Diffusion of the molecule through the boundary layer at the permeate-membrane interface. 7. Convective transport of the molecule into the permeate phase. The actual solution-diffusion mechanism is given by steps 3–5 only. Given the assumption of similar diffusivities of two gases in a liquid, the selectivity of a liquid membrane is based on the sorption selectivity between the two gases in the feed phase. In case this sorption selectivity is very low or lacking, carrier species may be employed (Figure 9.12). A molecule of the preferred gas is reversibly bound by a carrier and transported across the membrane either via diffusion of the carrier-­molecule complex or via a hopping mechanism of the molecule from one  carrier to another [164, 165]. At the permeate-membrane interface, the molecule dissociates from the carrier and is desorbed into the permeate phase. This transport mechanism is often called facilitated transport. A/B

B Feed A + B at p1 Absorption Reversible chemical r reaction Carrier ± A

Carrier - diffusion Carrier + A

Carrier

Carrier mediated (facilitated) transport

P1 >> P2 Desorption

A Permeate at p2

Figure 9.12  Facilitated transport ILM.

Mesoporous support

Mesoporous support

A

366  Nanostructured Polymer Membranes: Volume 1 In the case of facilitated transport, the selectivity of a separation is mainly influenced by the availability of free carrier molecules. For one thing, the solubility of carrier molecules in the membrane phase is limited. For another, the mobility of the carrier molecules and the formed complexes determine the number of free carrier molecules at the feed-membrane interface. As long as free carrier molecules are available, the flux across the membrane increases non-linearly with increasing driving force across the membrane. From the point where diffusion of unsaturated carriers towards the feedmembrane interphase and diffusion of saturated carriers from the feedmembrane interphase become the limiting steps in the solution-diffusion process, i.e., the chemisorption and diffusion process, the flux increases due to the seizure of facilitated transport. However, superimposed physical diffusion might lead to a linear increase with increasing driving force. At given transport of undesired species by means of physical diffusion, the selectivity of a membrane also increases non-linearly up to the point of full carrier saturation at the feed-membrane interface. Due to the very low diffusive flux of undesired species at low concentration difference across the membrane, selectivity shows maximum values within the range from zero to full carrier saturation concentration difference across the membrane. From the point of full carrier saturation at the feed-membrane interphase, selectivity might either show constant values (ratio of physical diffusion between desired and undesired species stays constant) or even decrease independent of the applied gas-concentration difference across the membrane (enhanced diffusive flux of undesired species in contrast to slightly or non-enhanced flux of the desired species). A different approach to increase or obtain a sorption selectivity of the membrane liquid is given by the use of homogeneous catalysts (Figure 9.13).

A/B

B Feed A / B at p1 Absorption Selective solubility A Diffusion

(Reverse diffusion)

Chemical reaction on a catalyst

P1 >> P2

Catalytic active membrane Desorption

Mesoporous support

Mesoporous support

A +A

Product 2A

Product 2A / Reactand A Permeate at p2

Figure 9.13  ILM with simultaneous gas or vapor separation and catalytic reaction.

Liquid Membranes  367 In contrast to a carrier, the catalyst increases the solubility of a gas inside the membrane liquid due to conversion of the gas to a product, which diffuses towards the permeate phase. However, in this case a back-diffusion of reaction products cannot be excluded since the partial pressure for the product in the gas phases is very low at both sides of the membrane. The combination of the unit operations homogeneous catalysis and gas separation into one clearly leads to process integration. At present, homogeneous catalysis inside an ILM has only been investigated by Carlin et al. [166, 167].

9.3.8.3  Materials for LM During the process of liquid membrane in gas separation, the properties of the membrane material have a great influence on gas selectivity and separation capacity. Thus, the materials of LM, such as liquids and carriers as well as the supports, are briefly reviewed in the following sections, respectively. The liquids and carriers are responsible for LM properties in terms of permeability and selectivity, while the choice of the support merely affects the permeability by its porosity. 9.3.8.3.1  Supports for LM In Table 9.7, details on the supports of the reported configuration in the literature are presented. The thickness of the liquid membrane most often corresponds to the thickness of the support and lies between 25 and 380 μm. Exceptions to this are given by the configurations of Gan et al. [20]. An additional layer of ionic liquid is placed on top of the fully wetted support to avoid gas permeance through the solid polymeric support. This example shows that the gas permeability of the polymeric support itself must be negligible if liquid membrane permeability is to be determined and highly selective membrane configurations are to be achieved. The pore width of the supports lies between 0.005 and 13μm. Porosity of the supports ranges from 0.4 and 0.83, while tortuosity—if determined—ranges from 1.0 to 3.05. 9.3.8.3.2  Liquids and Carriers for LM Table 9.8 shows the different liquids and carriers employed in the LM configurations of the last many years. In gas separations, the long-term stability of a LM configuration is mainly dependent on the volatility of the membrane liquid. Hence, low to non-volatile liquids such as glycerol or triethylene glycol (TEG) are most suitable to avoid a breakdown of the membrane function due to evaporation. Ionic liquids (ILs) or liquid molten salts (MS) do show even lower to non-measurable vapor pressures. While molten salts were already used many years ago [44], ILs have only gained importance in the last several years [21, 23].

FS HF FS FS

NA

Pall, HT Tuffryn (PS), Supor (PES)

NA

NA

Membrana/Hollow fiber membrane mat, Celgard/X40–200

Sterlitech Corp., YMHLSP1905 Polymer

Membrana/Hollow fiber membrane mat, Celgard/X30–240

Whatman

Millipore/Durapore

Pall

PTFE support for ILM which is sandwiched between two hydrophobic PTFE supports

PS, PES

Hydrophilic PES

Hydrophilic CA, Hydrophobic PVDF

PP

NA

PP

PAA

Hydrophilic PVDF

PES

FS

FS

FS

FS

FS

FS

FS

Whatman

PAA

Shape

Supplier/type

Support material

Table 9.7  Supports for liquid membranes.

152

100

60

250

167 + 118

NA

52 (CA), 45 (PVDF)

150

152 (PS), 145 (PES)

35

60

Thickness [μm]

368  Nanostructured Polymer Membranes: Volume 1

PTFE: NA, PVDF: Millipore/Durapel hydrophobic

PTFE: Toyo Roshi Kaisha Ltd. Japan, PVDF: Millipore/Durapel hydrophobic

NA

PVDF: Millipore, PP: Celgard/2500

Celgard/3401

NA

Millipore/Type AA-WP

Millipore

Millipore/Durapore VVLP

Millipore/AA WP Type

Millipore/Durapore VVLP

PP: Celgard/3401, PTMSP: NA

Celgard/3500

Hydrophilic PTFE (ILM) supported on hydrophobic PVDF support

Hydrophilic PTFE (ILM) supported on hydrophobic PVDF support

Hydrophilized PVDF

Hydrophilized PVDF, PP

PP

Hydrophilic supports: PAN, CUN, Hydrophilized PS fibers

Hydrophilic CAN

Hydrophilic PVDF

Hydrophilic PVDF

Hydrophilic CAN

Hydrophilic PVDF

PP (ILM)/PTMSP (SLM)

PP

FS

FS

FS

FS

FS

FS

FS

HF

FS

FS

FS

FS

FS

25 (Continued)

PP: 25, PTMSP: NA

100

150

100

100

150

PAN: 50, CUN: 10, PS: 40 and 275

25

PVDF: 100, PP: 25

100

45–70

35–55

Liquid Membranes  369

Supplier/type NA Celgard/2500 Pall Steel mesh: Pall, Zirconia cloth: Zircar Ceramics 0.1 0.1

0.2 0.22 0.22 (CA), 0.15 (PVDF) 0.1 0.04 NA 0.04 0.02

Support material

NA

PP

Stainless steel woven wire mesh

Stainless steel woven wire mesh Woven zirconia cloth

PAA

PTFE support for ILM which is sandwiched between two hydrophobic PTFE supports

PES, PS

Hydrophilic PES

Hydrophilic CA, hydrophobic PVDF

PAA

PP

NA

PP

PAA

Table 9.7  Cont.

NA

0.4

NA

0.4

NA

NA

0.8

0.75–0.85

0.71

NA

FS

FS

FS

FS

Shape

1.0

NA

NA

NA

1

NA

NA

NA

NA

1

Wire mesh: 200, Ceramic cloth: 380

200

25

NA

Thickness [μm]

370  Nanostructured Polymer Membranes: Volume 1

0.83

1.00 1.00 0.10 PVDF: 0.10 1.00 PVDF: 0.10, Celgard: 0.08, PAN: 70000 MWCO, PS: 0.70 1.00 NA

Hydrophilic PTFE support (ILM) supported on PVDF support

Hydrophilic PTFE support (ILM) supported on PVDF support

Hydrophilized PVDF

Hydrophilized PVDF, PP

Hydrophilic PTFE support (ILM) supported on PVDF support

Hydrophilized PVDF

Hydrophilized PP, PAN fibers Hydrophilized PS fibers

Hydrophilic PTFE support (ILM) supported on PVDF support

Porous LiAlO2

NA

0.83

PAN: NA, PS: 0.3–0.4

PVDF: 0.7, PP: 0.45,

0.83

PVDF: 0.7, PP: 0.45

0.7

0.83

0.8

0.20

PES

NA

0.10

Hydrophilic PVDF

NA

NA

(Continued)

PAN: NA, PS: NA

PVDF: 2.58, PP: 2.54,

NA

PVDF: 2.58, PP: 2.54

2.58

NA

NA

NA

NA

Liquid Membranes  371

Supplier/type PVDF: 0.10, PP: 0.04 0.80 0.10 NA 0.80 0.10 NA PAN: NA CUN: 0.005, PS: 0.10 and 0.20 Celgard: 0.05 × 0.125, PTMSP: NA 0.075 × 0.25 NA 0.04 4.0–13.0 4.0–13.0

Support material

Hydrophilic PVDF Hydrophilized PP

Hydrophilic CAN

Hydrophilic PVDF

Hydrophilic PVDF

Hydrophilic CAN

Hydrophilic PVDF

PP

Hydrophilic supports: PAN,

CUN, hydrophilized PS fibers

PP (ILM)/PTMSP (SLM)

PP

NA

PP

Stainless steel woven wire mesh

Stainless steel woven wire mesh, woven zirconia cloth

Table 9.7  Cont.

NA

NA

0.45

NA

0.45

PP: 0.5, PTMSP: NA

CUN: 0.6–0.7, PS: 0.3–0.4 and 0.7–0.8

PAN: NA

0.5

0.7

NA

0.63

0.7

0.82

PVDF: 0.7, PP: 0.45

Shape

NA

NA

2.1

NA

1.75

PP: 1.25, PTMSP: NA

NA

1.25

3.23–3.35

3.05

2.61

2.58

3.05

PVDF: 2.58, PP: 2.62

Thickness [μm]

372  Nanostructured Polymer Membranes: Volume 1

Liquid

[Ag][(1-hexene)Tf2N], [Ag][(1-pentene) Tf2N], [Ag][(1-soprene)Tf2N]

Glycerol

TEG

[C3NH2mim][CF3SO3], [C3NH2mim][Tf2N], [C4mim][Tf2N]

H2O

[hmim][Tf2N]

IL [pmim][I]

ILs: [C2-mim][Tf2N], [C2-mim][CF3SO3], [C2-mim][dca], [thtdp][Cl]

LiCl·H2O

[emim][BF4], [bmim][BF4], [hmim][BF4], [bmim][PF6],[bmim][Tf2N]

Glycerol

Separation task

C6H12/C6H14, C5H10/C5H12

C4H8/C4H10

C3H6/C3H8

CO2/CH4

CO2/air

CO2/He

CO2/CH4/H2, watervapor/air, b ­ enzenevapor/cyclohexane vapor

CO2/N2 and CO2/CH4

H2O vapor/air/VOC

SO2/N2, SO2/CH4, SO2/CO2

CO2/N2

Table 9.8  Separation tasks, liquids and carriers of liquid membranes.

(Continued)

Sodium carbonate, sodium glycinate

None

None

None

(K2CO3)

None

DEA

Functionalized liquid,except [C4mim][Tf2N] - no carrier

AgBF4 or AgNO3

AgNO3

Functionalized liquid

Carrier

Liquid Membranes  373

Liquid

TEG, DGA

PAMAM

TEG, PEG

MS: 0.78 Li2SO4, 0.135 K2SO4 and 0.085 Na2SO4

Glycerol

H2O

Glycerol

H2O

MS: LiNO3 and ZnCl2

MS: anhydrous LiNO3, dry NaNO3

PEG

PEG

PEG-400

H2O

Separation task

CO2/CH4

CO2/N2, CO2/O2

Water vapor/air

SO2/air

CO2/N2

CO2/air

CO2/N2

SO2/N2

NH3/N2, NH3/H2

O2/N2 (air)

CO2/C2H6

SO2/N2

CO2/CH4

CO2/CH4

Table 9.8  Cont.

DEA, MEA

DEA, DIPA

DEA

DEA

Functionalized liquid

Functionalized liquid

None

Na2CO3

PEG, DBC, K2CO3

Glycine-Na, EDA Glycine-Na2CO3,

None

None

(Glycerol)

(KHCO3), (DEA)

Carrier

374  Nanostructured Polymer Membranes: Volume 1

Liquid Membranes  375

9.3.9  Common Gas Separation Applications The number of gases for which suitable carriers are currently available is small and most effort has been devoted to the cleanup of acid gases. Many studies on facilitated transport of gases such as O2, CO2, H2S, SO2, N2, and H2 through liquid membranes are reported in some of the literature.

9.3.9.1  Carbon Dioxide Separation from Various Gas Streams As the most important greenhouse gas, CO2 has attracted worldwide interest in order to prevent global warming. Nowadays, the separation of CO2 from gas streams, especially flue gas or stack gas from power stations, is more and more urgent. Thus, many methods of CO2 separation are being developed to separate and concentrate CO2. Among these methods, facilitated liquid transport membranes have been attracting attention since they can obtain high selectivity compared to polymeric membranes. Qiu et al. [168] prepared poly(amide-6-b-ethyleneoxide) (Pebax1657)/1butyl-3-methylimidazo-liumbis [trifluoromethyl) sulfonyl]-imide ([Bmim] [Tf2N]) blend membranes with different [Bmim] [Tf2N] contents via solution casting and solvent evaporation method. The permeation properties of the blend membranes for CO2, N2, CH4 and H2 were studied. Results showed that, with the addition of [Bmim] [Tf2N], the CO2 permeability increased and reached up to approximately 286 Barrier at 40 wt% [Bmim] [Tf2N], which was nearly double that of pristine Pebax 1657 membrane. The increase of CO2 permeability may be attributed to the high intrinsic permeability of [Bmim] [Tf2N], the increase of fractional free of volume (FFV) and plasticization effect. Shimoyama et al.  [169] measured permeabilities of carbon dioxide through glycol derivative supported liquid membranes at 30–70 °C with a sweep gas method. The supported liquid membrane was prepared by the impregnation of the porous membrane with glycol derivative solvent. The effects of the glycol derivative species and temperatures on CO2 permeability were investigated. Results showed that the stabilities of the supported liquid membranes using monoethyleneglycol, diethyleneglycol monomethyl ether and diethyleneglycol monoethylether are low compared with those of triethyleneglycol, triethyleneglycol monomethyl ether and triethyleneglycol monobutylether. These parathion coefficients of carbon dioxide and nitrogen through the membranes with triethyleneglycol monomethyl ether give the separation coefficient higher than those with triethylene glycolmonobutylether. McDanel et al. [170] reported that the amine functional groups in room temperature ionic liquid (RTIL)-based epoxy-amine ion gel membranes allow for the fixed-site-carrier facilitated transport of CO2. These

376  Nanostructured Polymer Membranes: Volume 1 membranes were tested under humidified mixed-gas (CO2/N2) feeds in order to evaluate the effects of relative humidity, CO2 partial pressure, and hydrophobicity of the added free RTIL on CO2/N2 separation performance. Changes in relative humidity were found to have little effecting CO2/N2 separation performance at constant CO2 partial pressure. However, a comparison to dry-gas measurements showed the presence of water vapors necessary to observe facilitatedCO2 transport in these systems. Increased CO2 permeability and CO2/N2 selectivity were observed for these epoxy-amine ion gel membranes with decreasing CO2 partial pressure, as expected for materials operating via the fixed-site-carrier facilitated transport mechanism.

9.3.9.2  Sulfur Dioxide Separation from Various Gas Streams The combustion of fossil fuels (e.g., petroleum, coal and natural gas) generates huge amounts of acidic gases such as sulfur dioxide (SO2) and the selective separation of them is an environmental issue of significant concern. Sulfur dioxide emissions have to be controlled and minimized in order to reduce environmental risks. One of the most attractive approaches for the separation of sulfur dioxide separation from various gas streams is the selective absorption into a liquid membrane. Jiang et al. [171] investigated the permeabilities and selectivities of gases such as carbon dioxide, sulfur dioxide, nitrogen and methane in six imidazolium-based ionic liquids ([emim][BF4], [bmim][BF4], [bmim][PF6], [hmim][BF4], [bmim][Tf2N] and [emim][CF3SO3]) supported on polyether sulfone microfiltration membranes in a single gas feed system using nitrogen as the environment and reference component at temperature from 25 to 45 °C and pressure of N2 from 100 to 400 k Pa. It has been found that SO2 has the highest permeability in the tested supported ionic liquid membranes, being an order of magnitude higher than that of CO2, and about two to three orders of magnitude larger than those of N2 and CH4. The observed selectivity of SO2 over the two ordinary gas components was also striking. It has been shown experimentally that the dissolution and transport of gas components in the supported ionic liquid membranes, as well as the nature of ionic liquids, play important roles in the gas permeation. Huang et al. [172] investigated the facilitated separation of SO2 in a series of supported ionic liquid membranes (SILMs) impregnated with carboxylate-based ILs (including monocarboxylates and dicarboxylates) was systematically under humidified condition. The effects of transmembrane pressure difference and temperature on the permeability of acidic gases and permselectivity of gas pairs were studied. The results showed that, when the anions of dicarboxylate-based ILs are half deprotonated,

Liquid Membranes  377 they are efficient solvents for the selective separation of SO2. Besides, triethylbutyl ammoniumdimalonate ([N2224] [dimalonate]) could facilitate the permeation of SO2 with a permeability of 7208 Barriers under the transmembrane pressure difference of 0.05 bar, and the perm-selectivities of SO2/N2, SO2/CH4 and SO2/CO2 are 585, 271 and 18, respectively.

9.3.9.3  Hydrogen Separation from Various Gas Streams The recovery and valorization of valuable compounds that often appear in the residual streams of industrial processes is a relevant strategy among the various alternatives explored in the attempt to reduce the environmental impacts of the industrial activities. Such is the case of hydrogen (H2), which is present in high concentration in the flue gases of some industrial processes and which is a high-quality clean energy carrier. Liquid membrane technology has tremendously proven potential to compete against traditional gas separation process due to its low energy consumption, simplicity of operation and modularity. Because of this, it is worth mentioning its industrial reliability in application of hydrogen separation from various gas streams. Zarca et al.  [173] provided fundamental knowledge on the transport properties of H2 through supported ionic liquid membranes (SILMs) prepared with an imidazolium-based room-temperature ionic liquid (RTIL) combined with either a chloride or a chlorocuprate (I) anion. Results showed that H2 is more permeable than CO and N2 through these RTILs due to its higher diffusivity. The H2/N2 and H2/CO selectivities through the chloride-based RTIL are 11 and 6, respectively. Leelachaikul et al. [173] reported the use of a perfluorooctanol-based liquid membrane to separate the gases. The membrane shows an excellent separation performance at room temperature with an average H2/O2 selectivity of 100 with a H2 flux of 2.9 × 10–10 mol·m–2·s–1·Pa–1(1030 barriers) for a 50/50 mixed gas feed. The separation was tested under various operating conditions, and it was found that higher temperatures and flow rates did not considerably affect the performance of the membrane, but decreased its stability. The diffusivity and solubility of H2 in the perfluorooctanol membrane were measured by a time-lag technique, resulting in diffusivities of 1.4 × 10–4 cm2 ·s–1 and solubilities of 4.4 × 10–4 mol·m3·Pa–1.

9.3.9.4  Olefin Separation Olefins such as ethylene and propylene are very important commodities because they are the main feeds of major polymeric petrochemical plants. The product of an olefin plant which is mainly composed of either ethylene

378  Nanostructured Polymer Membranes: Volume 1 or propylene is often associated with paraffins like ethane and propane and some other impurities. The existence of these gases in olefin streams leads to a decrease in the productivity of polyolefin reactors. Therefore, separation of these gases is of great interest. Liquid membrane separation technology is one of the most attractive alternatives for olefin/paraffin separation. Low energy consumption and reasonable apparatus size are the preliminary benefits of this technology. Azizi et al.  [174] reported the separation of propylene/propane gas mixtures using a supported liquid membrane. N-methyl-2-pyrrolidone (NMP) and silver nitrate were used as the solvent and carrier, respectively. Gas permeation tests were conducted at different salt concentrations (0, 1.8, 5.4 wt%), feed pressures (1.2, 1.7, 2.2 bar), and propylene concentration (0.3, 0.5 and 0.7 mol/mol). The results showed that the permeation fluxes of both gases in pure NMP were high (i.e., 3.79 × 10−4 mol/m2·s), and the corresponding selectivity was around unity for different cases. Introduction of the silver salt into the solvent improved the permeation flux of propylene and its selectivity up to 8 ×10−4 mol/m2·s and 4.5, respectively, and caused the propane permeation flux to decay to some extent. Moreover, the permeation flux of propylene increased as the previously mentioned variables increased.

9.3.9.5  Conclusion and Outlook As a new technology, liquid membrane in gas separation has the characteristics of simple operation and low energy consumption. Though it can effectively separate mixed gas, there are still issues to be resolved during its development. Progresses are expected in the following aspects: 1. Enhancing the stability of liquid membrane, reducing the effective thickness and improving the penetration rate are essential to make liquid membrane based separation technologies available for industrial applications. 2. It is necessary to develop new membrane materials with high permeability, high selectivity, corrosion resistance and low cost, in order to make the liquid membrane separation more competitive. 3. It is important to optimize the system and process by combining liquid membrane separation technology with other separation methods to develop new types of integrating separation technology.

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9.4 Conclusion Liquid membrane separation processes have been developed in biochemical processing, industrial wastewater treatment, gas separations, food and beverage production, and pharmaceutical applications. Current research, development and scale-up efforts in the fields are focusing on: 1. Desalination of sea water, removal of toxic and/or objectionable pollutants from industrial and municipal ­ aqueous wastewaters, or from contaminated water-sources. 2. Concentration and sterilization of liquid foodstuffs and beverages without flavor/fragrance deterioration, to permit packaging and long-term storage in “fresh” condition without refrigeration. 3. Production of ultra-pure, submicron-particle-free water, liquid reagents, and gases employed in the processing of electronic materials and devices. 4. Removal of hazardous pollutants from gaseous effluents. 5. Large-scale separation of air gases. 6. Large-scale separation and purification of industrial chemicals and biochemicals. 7. Recovery of trace minerals from hydrometallurgical process streams and natural water sources. 8. Processing, separation, and purification of gaseous and l­ iquid fuels. However, much research and development efforts are still needed to further commercialize the technology for large scale industrial operations.

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Liquid Membranes  387 109. J. Ren, R. Wang, H.Y. Zhang, Z. Li, D.T. Liang, J.H. Tay, Effect of PVDF dope rheology on the structure of hollow fiber membranes used for CO2 capture. J. Membr. Sci., 281(1), 334–344, 2006. 110. K. Li, W.K. Teo, Use of permeation and absorption methods for CO2 removal in hollow fibre membrane modules. Sep. Purif. Technol., 13(1), 79–88, 1998. 111. N. Nishikawa, M. Ishibashi, H. Ohta, et al. CO2 removal by hollow-fiber gasliquid contactor. Energy Convers. Manage., 36(6), 415–418, 1995. 112. P.H.M. Feron, A.E. Jansen, CO2 separation with polyolefin membrane contactors and dedicated absorption liquids: Performances and prospects. Sep. Purif. Technol., 27(3), 231–242, 2002. 113. H.A. Rangwala, Absorption of carbon dioxide into aqueous solutions using hollow fiber membrane contactors. J. Membr. Sci., 112(2), 229–240, 1996. 114. A. Bottino, G. Capannelli, A. Comite, et al. CO2 removal from a gas stream by membrane contactor. Sep. Purif. Technol., 59(1), 85–90, 2008. 115. D. Wang, W.K. Teo, K. Li, Removal of H2S to ultra-low concentrations using an asymmetric hollow fibre membrane module. Sep. Purif. Technol., 27(1), 33–40, 2002. 116. D. Wang, W.K. Teo, K. Li, Selective removal of trace H2S from gas streams containing CO2 using hollow fibre membrane modules/contractors. Sep. Purif. Technol., 35(2), 125–131, 2004. 117. D. Demontigny, P. Tontiwachwuthikul, A. Chakma, Using polypropylene and polytetrafluoroethylene membranes in a membrane contactor for CO2 absorption. J. Membr. Sci., 277(1), 99–107, 2006. 118. R.A. Bartsch, J. Way (Eds.), Chemical Separation with Liquid Membranes, ACS symposium series 642, American Chemical Society, Washington, DC, 1996. 119. T. Zh. Sadyrbaeva, Separation of cobalt(II) from nickel(II) by a hybrid liquid membrane electrodialysis process using anion exchange carriers. Desalination, 365, 167–175, 2015. 120. S. Yu. Ivakhno, E.V. Yurtov, Membrane Extraction (in Russian), VINITI, Moscow, 1990. 121. T. Araki, H. Tsukube, Liquid Membranes: Chemical Applications, CRC Press, Boca Raton, Florida, 1990. 122. M. Cox, Liquid–liquid extraction and liquid membranes in the perspective of the twenty-first century, in: Solvent Extraction and Liquid Membranes: Fundamentals and Applications in New Materials, M. Aguilar, J.L. Cortina (Eds.), pp. 2–18, CRC Press, NewYork, 2006. 123. G. Sugiharal, S. Nagadome, S.-W. Oh, J.-S. Ko, A review of recent studies on aqueous binary mixed surfactant systems. J. Oleo Sci., 57(2), 61–92, 2008. 124. L. Sadoun, F. Hassaine-Sadi, Purification-concentration process. Studies on the transport mechanism of a chromium(VI)-sulfuric acid-tri-n-octylamine (TOA)–ammonium carbonate system. Desalination, 167, 159–163, 2004. 125. M. Teramoto, H. Matsuyama, N. Ohnishi, Development of a spiral-type flowing liquid membrane module with high stability and its application to the recovery of chromium and zinc. Sep. Sci. Technol., 24, 981–999, 1989.

388  Nanostructured Polymer Membranes: Volume 1 126. M. Teramoto, H. Matsuyama, H. Takaya, S. Asano, Development of spiraltype supported liquid membrane module for separation and concentration of metal ions. Sep. Sci. Technol., 22, 2175–2201, 1987. 127. H. Matsuyama, J. Boku, M. Teramoto, Separation and concentration of heavy metal ions by spiral type flowing liquid membrane module. Water Treat., 5, 237–252, 1990. 128. M. Teramoto, et al. Selectivity in the extraction of metals by liquid membranes, Proc. ISEC-86, 1, 545–560, 1987. 129. V. Kislik, A. Eyal, Hybrid liquid membrane (HLM) and supported liquid membrane (SLM) based transport of titanium(IV). J. Membr. Sci., 111, 273–281, 1996. 130. V. Kislik, A. Eyal, Hybrid liquid membrane (HLM) system in separation technologies. J. Membr. Sci., 111, 259–272, 1996. 131. R. Wódzki, J. Nowaczyk, Membrane transport of organics. II. Permeation of some carboxylic acids through strongly basic polymer membrane. J. Appl. Polym. Sci., 71, 2179–2190, 1999. 132. R. Wódzki, J. Nowaczyk, Membrane transport of organics. I. Sorption and permeation of carboxylic acids in perfluorosulfonic and perfluorocarboxylic polymer membranes. J. Appl. Polym. Sci., 63, 355–362, 1997. 133. C.F. Reusch, E.L. Cussler, Selective membrane transport. AIChE J., 19, 736–741, 1973. 134. J. Fang, M. Li, Z. Xu, Separation of cobalt from a nickel-hydrometallurgical effluent using an emulsion liquid membrane. Sep. Sci. Technol., 38, 3553–3574, 2003. 135. R.A. Kumbasar, Extraction and concentration of cobalt from acidic leach solutions containing Co–Ni by emulsion liquid membrane using TOA as extractant. J. Ind. Eng. Chem., 16, 448–454, 2010. 136. R.A. Kumbasar, Selective extraction of cobalt from strong acidic solutions containing cobalt and nickel through emulsion liquid membrane using TIOA as carrier. J. Ind. Eng. Chem., 18, 2076–2082, 2012. 137. R.A. Kumbasar, Separation and concentration of cobalt from zinc plant acidic thiocyanate leach solutions containing cobalt and nickel by an emulsion liquid membrane using triisooctylamine as carrier. J. Membr. Sci., 333, 118–124, 2009. 138. B. Pospiech, W. Walkowiak, Separation of copper(II), cobalt(II) and nickel(II) from chloride solutions by polymer inclusion membranes. Sep. Purif. Technol., 57, 461–465, 2007. 139. R.D. Noble, J.D. Way (Eds.), Liquid Membranes: Theory and Application, ACS symposium series 347, American Chemical Society, Washington, 1987. 140. V.S. Kislik (Ed.), Liquid Membranes: Principles and Applications in Chemical Separations and Wastewater Treatment, Elsevier, Oxford, 2010. 141. B.A. Purin, Electrochemical extraction as the method of the purification of metals using liquid membranes (article in Russian). Izv. Akad. Nauk Latv. SSR., 5, 31–368, 1971.

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10 Recent Progress in Separation Technology Based on Ionic Liquid Membranes M.J. Salar-García1*, V.M. Ortiz-Martínez1*, A. Pérez de los Ríos2 and F.J. Hernández-Fernández1 Department of Chemical and Environmental Engineering, Polytechnic University of Cartagena, Campus Muralla del Mar, Cartagena, Murcia, Spain 2 Department of Chemical Engineering, University of Murcia, Campus de Espinardo, Murcia, Spain 1

Abstract

Ionic liquids (ILs) are considered as a green alternative to conventional organic solvents due to their unique properties and thus their applications in chemical processes have multiplied within the last years, with a growing interest in the scientific community. In the field of separation technology, a wide range of new processes based on ionic liquids has been developed for the selective separation of chemical species, namely organic compounds, metal ions, biomolecules and mixed gases. The use of ionic liquids as liquid membrane phase could greatly reduce environmental impact thanks to their great features such as negligible vapor pressure and the possibility of minimizing their solubility by the adequate selection of their chemical structure. Furthermore, ionic liquids of low toxicity could be designed and synthesized. This chapter provides an overview of the recent advances in separation technology using ionic liquid-based membranes. It has been divided into sections according to the way ionic liquids are used in separation technology, specifically bulk ionic liquid membranes, emulsion liquid membranes and immobilized ionic liquid membranes, which in turn comprises supported membranes, polymer inclusion membranes, polymeric membranes, membranes based on the gelation of ionic liquids and other novel techniques. Moreover, this chapter covers relevant issues in ionic liquid-based separation technology such as methods of preparation, mechanisms of transport, stability and fields of application.

*Corresponding authors: [email protected]; [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 1, (391–418) © 2017 Scrivener Publishing LLC

391

392  Nanostructured Polymer Membranes: Volume 1 Keywords:  Separation technology, ionic liquids, liquid membranes, green ­solvents, ion transport

10.1 Introduction In recent years, many researchers have focused their interest on membrane science for different extraction processes. The increasing generation of contaminated wastes in industrial processes has encouraged the development of new membrane technology to regulate the concentration of pollutants in these effluents. Liquid membrane processes are based on membrane separation and liquid-liquid extraction (LLX) in a single step. The main principle of extraction processes lies in the use of a fixed or mobile reagent solution phase, which is often immiscible with water, placed between liquid or gaseous feeding and stripping phases. The most common systems consist of an organic membrane and aqueous feeding and stripping phases. Depending on the application, a porous material may be included as support or as separator (supported/polymer liquid membranes). However, in other cases such as emulsion liquid membranes or layered bulk membranes no support is required [1]. The incorporation of ionic liquids (ILs) has been one of the great advances in membrane technology. These novel materials are salts that remain liquid at room temperature, offering advantageous properties as solvents such as near-zero vapor pressure and high conductivity. In addition, their structure can be adapted to a specific process. These are only some of the unique properties that make ionic liquids an alternative to organic solvents in many chemical separation and purification processes [2, 3]. The use of ionic liquids as liquid phase in several types of membrane systems adds many advantages to membrane extraction processes. The selection of adequate chemical structures and the high viscosity typical of ionic liquids allow the solubility of membranes to be reduced, avoiding losses of liquid and contributing to their stability. Moreover, ionic liquids can increase the conductivity and selectivity of membranes [4, 5]. This chapter focuses on the recent advances in the use of ionic liquid membranes in separation technology and has been divided into sections according to the way ionic liquids are used as part of liquid membranes. Thus, this chapter covers the main applications of bulk ionic liquid membranes, emulsion ionic liquid membranes and immobilized ionic liquid membranes in separation technology, the latter including supported membranes, polymer inclusion membranes, polymeric membranes, membranes based on the gelation of ionic liquids and other recent techniques.

Separation Technology Based on Ionic Liquid Membranes  393 This chapter also focuses on key aspects of ionic liquid-based membrane technology such as transport, stability of membranes, relevant properties of ionic liquids for their use in technology separation, and main fields of application, among others.

10.2 Ionic Liquid Properties The special properties of ionic liquids have gained the attention of the scientific community, being one of the most promising materials investigated in the last decades for a wide range of chemical applications. Ionic liquids (ILs) are organic salts that usually remain liquid at temperatures lower than 100 °C due to the weak coordination of their ions. They are nonflammables and their vapor pressure is near-zero. These salts show high thermal and electrochemical stability, remarkable solvating potential and the possibility of being reused. All these features make ILs an alternative to conventional organic solvents. Their thermal stability and almost negligible vapor pressure make them suitable for high-temperature processes. However, the most interesting characteristic of ILs is that their properties, such as hydrophobicity, melting point, density, polarity and viscosity, can be modified by changing and combining different cations and anions. Thus, the ideal ionic liquid for a specific process can be designed [6, 7]. The high coulomb forces between anions and cations are the cause of their negligible vapor pressure. On the other hand, the length of alkyl chains and the degree of symmetry greatly affect the melting point of ionic liquids, so that the longer the alkyl-chains and the higher the degree of symmetry, the higher the melting point of ionic liquids [8]. Ionic liquids can be classified into two main groups: i) simple salts and ii) binary salts. Simple salts contain a single cation-anion pair, such as ethyl ammonium nitrate [EtNH3][NO3]. It was the first compound defined as ionic liquid in 1914 by Paul Walden [6, 9, 10]. However, ionic liquids did not gain much attention until binary salts appeared. Binary ionic liquids are made of different ionic species (e.g. aluminium (III) chloride mixed with N-alkylpyridinium), and their properties depend on the mole fraction of each species [11]. Ionic liquids can also be grouped according to their cations, mainly: i) alkylammonium; 2) dialkylimidazolium; 3) phosphonium; 4) N-alkylpyridinium (Figure 10.1) [12]. Cations are usually bulky and asymmetric organic compounds containing alkyl chains, while anions are smaller, typically of inorganic nature. The most popular anions used in simple ionic liquids are tetrafluoroborate (BF4–), hexafluorophosphate (PF6–),

394  Nanostructured Polymer Membranes: Volume 1 [NRxH4–x]+

[PRxH4–x]+

Alkylammonium

Phosphonium +

+ R

N

N

N

R’

R Dialkylimidazolium

N-alkylpyridinium

Figure 10.1  Main classification of ionic liquid cations.

Table 10.1  IL cations and anions most studied. Cations

Anions

Imidazolium

Tetrafluoro­borate

Tetraalkyl­ ammonium

Hexafluoro­ phosphate

Tetraalkyl­ phophonium

Bis(trifluoro­ methylsulfonyl) imide

Pyridinium

Triflate

Pyrrolidinium

Chloride

Cl–

trifluoromethanesulfonate (TFO–), and bis(trifluoromethyl)sulfonyl imide (NTf2–) and chloride (Cl–). In the case of cations, the most studied ones are imidazolium, tetralkylammonium, tetralkylphosphonium, pyridinium and pyrrolidinium (see Table 10.1). As mentioned above, a huge variety of ionic liquids can be designed by changing the structure of the cation  (e.g.  by

Separation Technology Based on Ionic Liquid Membranes  395 varying the length of the chain, the substituents on the ring and/or on the chain) and by modifying the anion/cation pair [8, 13, 14]. The synthesis process of ionic liquids is usually performed in two main steps: 1) the formation of the desired cation and 2) anion exchange. The first step entails the synthesis of the cation by several methods, such as the protonation of an amine by an acid or the quaternization reaction of an amine with a halo-alkane. The second step is based on generating Lewis acid-based ionic liquids by the reaction of halide salts with Lewis acids or by the metathesis of anions [9]. Due to these unique properties, ionic liquids are regarded as good candidates to replace organic solvents in many industrial processes. Among them, their selectivity toward certain compounds makes ionic liquids especially suitable for extraction and separation processes. The following sections focus on the application of ionic liquid membranes in separation technology.

10.3 Bulk Ionic Liquid Membranes Bulk liquid membranes (BLMs) typically consist of an organic liquid phase separating two aqueous or gaseous phases—a feed phase and a receiving or stripping phase—that transports the targeted solute selectively from one phase to another. BLMs are non-supported membranes often set in a U-tube configuration (see Figure 10.2), although they can also be arranged in other ways, including rotating disc contactors, creeping film contactors and with immobilized interfaces in a hollow fiber [15]. These separation methods imply low operational cost and technical simplicity and independency of the transport equilibrium limitation. The application of liquid membranes has been widely investigated in separation processes, mainly for the separation of organic compound and metal ions.

Feed phase

Receiving or stripping phase

Bulk ionic liquid membrane

Figure 10.2  U-shape liquid membrane configuration.

396  Nanostructured Polymer Membranes: Volume 1 Bulk ionic liquid membranes (BILMs) employ ILs as liquid phase, offering great adaptability to a specific separation task by increasing the selectivity of the liquid membrane since, in the presence of ILs, the properties of BLMs are tunable by selecting the appropriate combination of cations and anions. Moreover, the use of ionic liquids allows the disadvantages of traditional organic membrane solvents, typical of hydrocarbons and chloroalkanes, to be avoided. While such solvents are often flammable, volatile and toxic, which limits their application on an industrial scale, ionic liquids have almost negligible vapor pressure and low flammability [16]. Ionic liquids as bulk membranes have been investigated for the separation of a wide range of organic compounds. Several studies have proven that the imidazolium group provides selective transport with sufficient stability. The polarity of ILs has been reported as a crucial factor for high affinity and selectivity for a specific organic compound. Polar cations that include ether or hydroxyl functional groups can lead to a selective increase in the affinity for organic compounds such as 1-butanol, 1-propanol, cyclohexanol, 1,4-dioxane, cyclohexanone, morpholine and methylmorpholine, although they can also reduce the selective transport observed [17]. Bulk ionic liquids can employ metal ions as mediators for the transport of a given organic compound. The use of 1-methyl-3-octyl imidazolium chloride as BILM in combination with silver ion Ag+ as carrier has been successfully studied for the separation of toluene from n-heptane, offering high selectivity toward this hydrocarbon, although presenting low permeate rates [18]. In this case, factors such as the concentration of the feed phase, the stirring speed of the membrane phase and temperature may have a strong effect on the permeation of the compound to be separated. Other authors have studied the separation of propylene from propane with BILMs based on 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim+] [BF4–], and the extractive distillation of benzene/cyclohexane, 1-hexene/ n-hexane and, 2-propanol/water with 1-hexyl-3-methyl-imidazolium and 1-butyl-1-methylpyrrolidinium bis(trifluorpmethylsulfonyl)imides, [hmim+][NTf2–] and [bmpyr+][NTf2–], respectively [19, 20]. Imidazoliumbased ionic liquids, especially chlorinated dialkylimidazolium ionic liquids, have also shown good efficiency when separating organic acids such as taurine, 2-aminoethanesulfonic acid (98.5% in one-step process) [21]. Other ionic liquid groups have been employed for phenol removal, for example, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide, [bmim+][NTf2–], which has high hydrogen-bond basicity. Also in such cases, stirring speed can favor the separation process and the hydrophobic nature of ILs increases the stability of the membrane and facilitates its recovery and reusability [16].

Separation Technology Based on Ionic Liquid Membranes  397 The separation of biomolecules by means of ionic liquids can be performed in several ways, including direct extraction, ionic liquid-based aqueous two-phase systems (ILATPs) and the use of ionic liquids in combination with other agents. These separation methods have been applied for the extraction of proteins, enzymes and amino acids. The extraction of hemoglobin has been carried out by imidazolium-based ionic liquids, which is possible thanks to the covalent bond between the cationic ionic liquid moiety and the iron atom of the heme group [23]. Talking specifically of BILMs, this type of membrane has been used for the separation of amino acid esters. 1-Octyl-3-methylimidazolium hexafluorophosphate, [omim+][PF6–], has been shown to be able to separate benzyl, phenylalanine methyl and phenylglycine methyl esters, while the extraction yield of respective amino acids is almost negligible since amino acids have strong polar character and their carboxyl groups may form hydrogen bonds with water. This fact suggests the potential application of ionic liquids in the selective separation of amino acid esters from amino acids. For such compounds, BILMs have been reported to be more selective when compared to supported ionic liquids [24]. The extraction of lactoferrin, a protein present in bovine whey, has also been studied by the use of bulk ionic liquid membranes in U-tube configurations between two aqueous phases, although the efficiencies obtained were not high. Again, hydrophobic imidazoliumbased Ils, such as 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, [bmim+][NTf2–], have been reported to offer selective extraction of this protein in a aqueous solution with a maximum separation yield of 20%, possibly due to specific interaction with the iron atoms contained in proteins. Conditions such as low protein concentration, neutral pH and low ionic strength are among the most favorable conditions to improve separation rate.

10.4 Emulsified Ionic Liquid Membranes Emulsion liquid membranes (ELMs) consist of a double emulsion. The first step is emulsion between small drops of an aqueous receiving phase (inner phase) within an organic membrane phase (intermediate phase), such as water-in-oil emulsion. The aqueous phase is slowly added to a fixed volume of the organic phase and both are homogenized at high speed. Then, this emulsion is slowly re-dispersed into the aqueous source phase (outer phase) to obtain larger bubbles and prevent the breakdown of the liquid membrane [25, 26]. Figure 10.3 shows a scheme of emulsion liquid membranes.

398  Nanostructured Polymer Membranes: Volume 1 Outer aqueous phase Droplet of aqueous phase Emulsion 2: Dispersion of emulsion 1

Emulsion 1

Organic liquid membrane phase

Figure 10.3  Scheme of emulsion liquid membranes.

Table 10.2  Advantages and disadvantages of EIMs. Advantages Low cost Small quantities of organic solvent Good stability Mass transfer rates higher than other liquids membrane

Disadvantages Reduction of extraction efficiency and selectivity due to: Swelling processes Leakage processes Coalescence

The extraction process by ELMs normally consists of the transference of the solute of interest from the outer to the inner phase across the membrane phase. However, if the molecule of interest is originally within the internal phase, its transport could also take place in the opposite direction. The separation of the solute is performed in a one-step process due to the differences in size between the inner (smaller) and the outer phase (larger). The use of appropriate carriers allows the selectivity of the membrane to be enhanced. In addition, the target solute may be concentrated against its concentration gradient [27] or reacts with encapsulating reagents, enzymes or cells inside the aqueous drops within the inner phase, generating several by-products [27, 28]. The use of EIMs in separation processes has some advantages and drawbacks as listed in Table 10.2 [26, 29]. These disadvantages can be partially alleviated by embedding the emulsion in porous solid supports, by adding suitable carriers and emulsifiers and through the optimization of the conditions of the preparation of the emulsion and its final composition [25]. In 1968, Li [30] developed a separation method of hydrocarbons based on emulsion liquid membranes to overcome the limitations posed by other

Separation Technology Based on Ionic Liquid Membranes  399 separation techniques such as ion exchange, ultracentrifugation or chromatography. These techniques usually require pretreatment, their upscaling is complex and their cost high. Although the first application of ELMs can be found in the fractionation of hydrocarbons, the use of these liquid membranes has extended to a broader range of applications: i) the recovery of heavy metal ions, ii) wastewater treatment, iii) replacing catalysts in heterogeneous reactions, iv) treatment of disorders in blood stream and v) the extraction of products from fermentation [25]. As mentioned before, ionic liquids are a promising alternative to organic solvents due to their potential as extractants. In the case of ELMs, the amount of ionic liquid required is very low since the organic phase is very small. Venkatesan and Begum [31] performed the extraction of benzimidazole by using emulsion liquid membranes based on tri-n-octyl methyl ammonium chloride as organic phase and chloride acid as aqueous phase. They optimized the operating conditions such as emulsification time, the concentration of the carrier used and the concentration of the inner phase, and studied their effect on the extraction process. The maximum yield of benzimidazole extraction was 97.5% after only 12 minutes. Emulsion ionic liquid membranes have also been applied in the extraction of heavy metals. For instance, Lende et al. [32] investigated the removal of Pb(II) ions from aqueous solution. In this case, the aqueous phase of the ELM consisted of sulphuric acid as aqueous phase and the organic phase contained Span-80 as surfactant, D2EHPA as extractant and n-hexane as diluent. The addition of the ionic liquid 1-octyl-3-methylimidazoliumhexafluorophosphine to the organic phase was investigated, and several parameters of the process, such as pH, emulsification speed or treat ratio, were optimized. The results offered a total extraction of Pb(II) with the ionic liquid mentioned versus a extraction yield of 97% in the absence of such ionic liquid. Moreover, the use of this ionic liquid provides more stability to the membrane by reducing the coalescence of the inner phase drops [32]. Other heavy metals, such as chromium (Cr), have also been extracted from wastewater by using a double ionic liquids-based ELM. Goyal et al. [33] used 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [bmim+][NTf2–], to stabilize the membrane phase and tri-n-octylmethylammonium chloride as carrier. They achieved an extraction yield of chromium of approximately 97% and confirmed the capacity of [bmim+][NTf2–] to stabilize the membrane [33]. Another important application of EILMs is their use for gas separation, improving the mass transfer and increasing CO2 absorption [34]. Kamarudin et al. [35] included methyldiethanolamine and piperazine as

400  Nanostructured Polymer Membranes: Volume 1 extractant in sodium hydroxide solution, and Span-80 as surfactant in the organic solution. They achieved a separation yield of 54% of the CO2 present in a mixture of CO2/CH4by using an EILM made of 8% v/v of Span-80, 8% v/v of methyldiethanolamine and 6% v/v of piperazine [35]. As seen before, emulsion liquid membrane is a promising technology for many separation processes, especially for heavy metals removal, wastewater treatment and gas separation. However, the success of its application depends on several key factors such as the adequate selection of the emulsification method and the emulsion formulation, factors that largely determine the stability of these membranes.

10.5 Immobilized Ionic Liquid Membranes The immobilization of ILs can increase efficiency, facilitate recycling and widen the range of applications of these compounds with unique chemical and physical properties. Ionic liquid can be immobilized as liquid phase in membrane materials for their application in separation processes, offering continuous operation with a minimal amount of ionic liquid as active phase. There are several ways of immobilizing ionic liquids, supported ionic liquid membranes (SILMs) on porous support materials being one of the first methods developed [36]. Ionic liquids can also be immobilized by casting method through the physical occlusion in organic polymers, which is known as ionic liquid-based polymer inclusion membranes (PIMILs) [37]. More recently, polymeric ionic liquid membranes (PILMs) consist of polymerizing IL monomers [38]. This technique allows the stability of the membrane to be greatly increased. Polymeric ionic liquid membranes offer higher stability than polymerionic liquid inclusion membranes, although the latter are more stable when compared with supported ionic liquid membranes. In the case of SILMs, the ionic liquid contained in the polymeric membrane can be released to the surrounding phases by action of differential migration, which in turn depends on the solubility degree of such ILs in the phases that the membrane is in contact with. In this context, the proper selection of the anion and the cation present in the ionic liquid plays a vital role in the solubility of ionic liquids. Alternatively, the gelation of ionic liquids is another immobilization method, through which the ionic liquid membrane phase is gelled to avoid its release from the membrane to the media. Finally, non-dispersive solvent extraction (NDSX) and pseudo-emulsion hollow fiber strip dispersion (PEHFSD) are novel separation techniques that use microporous materials as support. The application of each type of technology will be discussed in detail in the following sections.

Separation Technology Based on Ionic Liquid Membranes  401

10.5.1 Supported Ionic Liquid Membranes Supported ionic liquid membranes (SILMs) are porous materials impregnated with ionic liquids (solvent). SILMs have drawn great attention in recent years because of their ease of preparation and versatility. In a SILM, the ionic liquid is responsible for improving the selectivity towards a certain compound and the porous support provides mechanical stability. Due to the high viscosity of ILs, the method of preparation of SILMs can be decisive for the performance of the membrane. There are three general methods for preparing SILMs: i) direct immersion, in which the support material is in contact with the ionic liquid until it is soaped up, ii) pressure method, which consists of making the ionic liquid flow into the pores of the support membrane by applying pressure (e.g., nitrogen) in an ultrafiltration system and iii) vacuum method, in which the support material is submerged in ionic liquid and vacuum is applied to release all air occluded in the pores of the membranes so that they can be occupied by the ionic liquid of interest [20, 39]. In comparison, immobilization method by pressure offers higher stability to membranes [40]. On the other hand, given that ionic liquids mainly remain immobilized on the external layer of the membrane, IL viscosity affects the final stability of SLIMs. Very high viscosity hinders the penetration of pores, this being one of the most important factors when it comes to the immobilization of ionic liquids. The SILMs have been widely study in separation technology, receiving growing attention in the last decades due to their favorable features. Among them, the selective separation of organic compounds has been one of the most studied applications for supported ionic liquid membranes. In the case of organic compounds, the transport mechanism through SILMs has been explained according to the solution-diffusion model. When a supported ionic liquid is placed between a feed and a receiving phase, the driving force is due to the concentration gradient existing between both phases and the movement of organic species through the membrane phase is assumed to be given by Fick’s first law (see Figure 10.4). As in the case of bulk ionic liquids, imidazolium-based SLIMs using different organic polymers have been investigated for the selective separation of compounds such as 4-dioxane, 1-propanol, 1-butanol, cyclohexanol, cyclohexanone, morpholine and methylmorpholine. In such cases, the combination of the selected ionic liquid with the support material is a key factor to achieve good selectivity for a specific solute. For instance, the use of the anion hexafluorophosphate (e.g., [bmim+][PF6−] immobilized in polyvinylidene fluoride) has proven to achieve high selectivity toward secondary amines [4].

402  Nanostructured Polymer Membranes: Volume 1 SILM Feed phase CF

Receiving phase C1

C2

Interface 1

CR

Interface 2

Figure 10.4  Transport of organic compounds through SILMs (C: concentration of species).

Furthermore, SILMs have been studied for the selective separation of products and substrates in transesterification reactions such as the biosynthesis of organic esters that is often carried out in the perfumery and flavor industries. In this type of reaction, a mixture of vinyl ester, organic acid, alkyl esters (flavor esters) and alcohol is produced as a result of the transesterification from vinyl esters and alcohols, which is commonly performed in an enzyme-catalyzed process. Several works have demonstrated the possibilities of exploiting SILMs for the selective separation of such products [41]. Among the ionic liquids studied for the separation of transesterification reactions, those containing dialkylimidazolium cations with a short alkyl chain length and sulfate anions with a short alkyl substituent have been found to show higher selectivity. With respect to cations, the use of dialkylimidazolium-based supported membranes would offer higher efficiencies as compared to dialkylpyridinium-based ones. Moreover, in comparison with membranes based on conventional organic solvents, SLIMs allow the transport of organic acids in the absence of enzymes, which, in the former case, are responsible for the esterification of the organic acids present in the feed phase to facilitate their transport to the receiving phase [42]. Another field of application of SILMs is the selective separation of aromatic hydrocarbons from aliphatic hydrocarbons. The separation of benzene, toluene and p-xylene from n-heptane has been studied by using SILMs based on the cations 1-butyl-3-methylimidazolium, [bmim+], 1-hexyl-3-methylimidazolium, [hmim+] and 1-octyl-3-methylimidazolium, [omim+], in combination with the anion [PF6−], respectively, and diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethyl)sulfonylimide, [Et2MeMoEtN+][NTf2−]. It was found that aromatic hydrocarbons

Separation Technology Based on Ionic Liquid Membranes  403 were successfully transported through membranes based on these ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim+] [PF6−], showing the maximum selectivity towards n-heptane when used in the liquid membrane phase [43]. As for other organic compounds, the application of SILMs has been reported in the separation of pharmaceutical compounds from diluted buffered solutions, e.g. the concentration of penicillin G [43, 44]. Supported ionic liquid membranes have also been widely studied for the separation of mixed gases. Their potential industrial application in low ­ pressure processes, such as the treatment of biomethane from anaerobic digesters and CO2capture, has encouraged basic research to study the selectivity toward pairs of gases, e.g. CO2/CH4 and CO2/N2. 1-Ethyl-3-methylimidazolium tetrafluoroborate, [emim+][BF4−], and 1-ethyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimida, [emim+] [NTf2−], are examples of ionic liquids that have shown good selectivity for the separation of these gases [45]. Ionic liquids containing functional groups that facilitate CO2 transport, such as N-aminopropyl3-methylimidazolium bis(trifluoromethyl)sulfonylimide, [NH2pmim+] [NTf2−] and N-aminopropyl-3-methylimidazolium trifluoromethanesulfonate, [NH2pmim+][CF3SO3−], are of specific interest to this type of application [46]. They could have potential application in the pretreatment of gaseous mixtures coming from fermentation processes, which are primarily composed of H2, CO2 and N2, for their use in fuel cells. It has been reported that SILMs based on imidazolium groups such as [bmim+][PF6−] offer higher permeability to CO2 than to N2 and H2, which would make possible the enrichment of biohydrogen to obtain the high hydrogen concentrations required in fuel cells. This separation method could also be applied for hydrogen production from syngas generated through the gasification of carbon sources [47]. Other interesting reported applications are the selective separation of CO2 from binary mixtures such as CO2/He and of CO2/ CH4 and H2S/CH4 from ternary mixtures such as CO2/H2S/CH4 [48, 49]. On the other hand, although the nature of the ionic liquid selected is regarded as the most determining factor, some operating conditions have been established as preferable to favor the separation performance in a certain way. The content of water vapor in the gas stream, temperature and transmembrane pressure can differently affect both gas permeability and selectivity of SILMs. Several works that have studied the influence of these variables have reported that water vapor can simultaneously increase gas permeability and diminish selectivity as result of the formation of water clusters inside the membrane. Besides, permeability can increase with temperature and decrease with transmembrane pressure [50, 51].

404  Nanostructured Polymer Membranes: Volume 1 The removal of metal ions from aqueous solutions is another interesting field of application of SILMs. Ionic liquid membranes based on methyltrioctylammonium chloride, [mtoa+][Cl–] or Aliquat 336, can offer selective extraction of the metal ions Fe(III), Zn(II), Cd(II) and Cu(II). The influence of the nature and composition of the stripping phase can greatly affect the extraction efficiency, often measured in terms of reaching pertraction factor (PFs), that is, the ratio between the concentration of the target metal ion in the receiving and the feed phase. For example, a solution of Milli-Q water and Na2CO3 allows the removal of Zn(II) and Fe(III), with high reaching pertraction factors (PFs) for Zn(II) (up to 370). The use of NH3 as receiving phase has also been studied for the recovery of Cd(II) (PF of 15). Thus, the target metal ion can be changed by modifying the receiving composition [52]. The transport of other types of small cations such as sodium (Na+) and chloride (Cl−) can be of interest. SILMs based on 1-n-alkyl-3-methylimidazolium have been shown to be particularly suited for this type of extraction method. In this case, the mechanism of transport through SILMs is mainly dominated by the mobility of water microenvironments within the ionic liquid phase [53].

10.5.2 Polymer Ionic Liquid Inclusion Membranes Polymer inclusion membranes (PIMs) are a specific variety of supported liquid membranes (SLMs). They are a new type of membranes that contain a plasticizer, commonly a polymer such as polyvinyl chloride (PVC). They have been studied in processes for the separation of heavy metals, biomolecules and inorganic anions [54, 55]. Polymer inclusion membranes contain a selective carrier to remove specific solutes, in the case of ionic liquid membranes the carrier is its own ionic liquid (PIMILs). The extraction mechanism is similar to conventional liquid-liquid extraction but avoiding the use of organic solvents. The main advantage of this method is that the extraction and the stripping steps are performed continuously. Since the carrier is immobilized into a polymer matrix, PIMILs show better mechanical properties and chemical resistance than supported liquid membranes. These properties and their easy method of synthesis have made them a promising technology for many separation processes. Due to the selectivity exhibited by many ionic liquids towards several heavy metals, PIMILs are one of the most attractive separation techniques for removing heavy metals from industrial wastes. Guo et al. [56] synthetized a new PIMIL using poly(vinylidene fluoride) (PVDF)

Separation Technology Based on Ionic Liquid Membranes  405 as support, 1-alkyl-3-methylimidazolium hexafluorophosphate or tetrafluoroborate-based plasticizers (ILPs) ([Cnmim+][PF6–] or [BF4–]) and trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate as carrier, to transport chromium (VI), one of the key pollutants among heavy metals. The membrane prepared with 1-octyl-3-methylimidazolium tetrafluoroborate, [omim+][BF4–], showed a maximum permeability coefficient (P) of 19.6 µm.s–1. They also demonstrated an increase of the permeability of 13 times when Cyphos IL 104 is used instead of Aliquat 336 (N-methyl-N,N,N-trioctylammonium chloride) as carrier. The new membrane also showed higher long-term stability and higher resistance to high flux than other SLMs and PIMs [56]. The transport of other heavy metals, such as zinc (II), iron (II) and iron (III), have been investigated as well as by using ionic liquid-based PIMs. Regel-Rosocka et al. [57] included trihexyl(tetradecyl) phosphonium chloride ([P6,6,6,14+][Cl–] or Cyphos IL 101) and trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (Cyphos IL 104 or [P6,6,6,14+] [TMPPhos–]) as carrier in the structure of PIMs. The synthetized membrane removed around 100% of Zn(II) and Fe(III) from different acidic media. Trihexyl(tetradecyl)phophonium chloride-based membranes, [P6,6,6,14+][Cl–], exhibited a high efficiency for Fe(III) removal but less efficiency for Zn(II) when both are in the same mixture. These results show that both metals can be totally extracted from independent solutions and separated from the same feed [57]. Apart from heavy metals, the transport of different cations across ionic liquids-based PIMs has also been analyzed. Gizli et al. [58] synthetized and characterized the capability of tetradecyl(trihexyl)phosphoniumdecanoate-based membranes, [P6,6,6,14+][C9COO–], for ion transport. They mixed the ionic liquid with dioctyl phthalate (DOP) to be used as plasticizers and carriers. They achieved a Na+ diffusion of five orders of magnitude. Another important application of these types of membranes is the separation of biobutanol, which has a great potential as biofuel. This is produced during the fermentation of alcohols and ketones from biomass. Matsumoto et al. [59] synthetized PVC-Cyphos IL-101, 102 ([P6,6,6,14+] [Br–]), 104 and Aliquat 336-based PIMs for the pervaporation of 1-butanol and isopropyl alcohol (IA). Among all the ionic liquids investigated, Aliquat 336 showed the best performance in terms of 1-butanol/IA separation and the maximum flux of 1-butanol produced was around 26 g.m–2. Matsumoto et al. [60] also developed PVC-Aliquat 336-, Cyphos IL-101and 102-based PIMs for the separation of lactate. In this case, Aliquat 336 also exhibited the highest mass transference, being a promising alternative for industrial-scale lactate separation. Similar types of membranes using

406  Nanostructured Polymer Membranes: Volume 1 the same ionic liquid (Aliquat 336) were elaborated by Pratiwi et al. [61] for the separation of succinic acid.

10.5.3 Polymeric Ionic Liquid Membranes Polymeric ionic liquid membranes (PILMs) have attracted the interest of many researchers since they combine the unique properties of ionic liquids and the general features of polymers such as their capability of forming film structures [62]. This type of liquid membranes has also drawn much attention in recent years due to their wide range of application in different scientific fields such as CO2 separation [62–64], anion exchange [65], direct methanol fuel cells [66] and polymer electrolytes [62, 67]. Carlisle et al. [63] prepared membranes based on main-chain poly(imidazolium) for oxygen dioxide separation. They synthetized three imidazolium ionene membranes: i) poly(imidazolium) bromide salt-based membrane, ii) poly(imidazolium) bis(trifluoromethylsulfonyl)imide saltbased membrane and iii) a composite film made of poly(imidazolium) bis(trifluoromethylsulfonyl)imide salt-based membrane and a room temperature IL. They achieved high CO2 permeabilities using poly(imidazolium) bis(trifluoromethylsulfonyl)imide salt-based membrane and the composite one, similar to those obtained with poly roomtemperature ionic liquids. Tomé et al. [68] investigated the permeation of different gases (CO2, CH4 and N2) in different polymeric ionic liquid membranes. They used cations such as imidazolium, ammonium, pyrrolidinium, pyridinium and cholinium, all of them combined with bis(trifluoromethylsulfonyl) imide. Among the membranes synthetized, pyrrolidinium, ammonium and cholinium-based membranes showed the lowest permeabilities and diffusivities. However, they present the highest CO2/CH4 and CO2/ N2 permselectivities compared to imidazolium and pyridinium-based membranes. Guo et al. [65] developed new anion exchange membranes using copolymer of 1-allyl-3-methylimidazolium chloride, [amim+][Cl–] and two types of polymers: methyl metracrylate and butyl methacrylate. The membranes prepared showed high hydroxyl ionic conductivity at 30 °C and low methanol permeability. Their thermal and chemical stability were higher than in the case of alkyl quaternary ammonium functionalized polymers. All these properties show the potential of these materials for their use in alkaline fuel cells. Neves et al. [66] included different ionic liquid cations in Nafion 112 membranes. The cations selected were ammonium and imidazolium, combined with different counter anions. The materials prepared showed a

Separation Technology Based on Ionic Liquid Membranes  407 low methanol and gas transport compared with Nafion 112 membranes. Li et al. [62] prepared polymeric guanidinium-based membranes with different anions by copolymerization and anion exchange reaction. These novel materials were characterized and 1g2-MA-BF4 showed the best thermal stability and the best ionic conductivity. All of these research works show the great potential of polymeric ionic liquid membranes in separation technology.

10.5.4 Membranes Based on Gelation of Ionic Liquids Much progress has been made to improve the long-term stability of immobilized ionic liquid membranes. The most important is the use of novel supporting materials [69] or the gelation of ionic liquids [70]. This section focuses on gelled supported ionic liquid membranes. In this case, the ionic liquid membrane phase is gelled to avoid its release from the membrane to the media. In 1992, Neplenbroek et al. [70] developed a new procedure to improve the stability of SLMs. They managed to increase mechanical stability and long-term permeability by filling the pores of a supporting material with a homogenous gel structure, concluding that the diffusion rate of carrier molecules increases with decreasing polymer content. The most important application of this type of membrane is CO2 gas separation. Voss et al. [71] prepared gelled 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [hmim+][NTf2–], by using a low molecular gelator (12-hydroxystearic acid). The membrane obtained had good mechanical stability and maintained similar gas transport properties to the original ionic liquid. This gel was able to remain 98% wt of ionic liquid so the CO2 permeability was similar to that obtained with common supported liquid membranes. However, CO2 gas permeability in this gelled SILM was two orders of magnitude higher than that showed by similar ionic liquids supported in solid polymers. These preliminary gelled ionic liquid membranes showed similar gas separation properties as traditional supported membranes and an increase of the mechanical stability of approximately 23%. The CO2/N2 selectivity of the ionic liquid gelled membrane was similar to the neat ionic liquid used. The gel showed high mechanical stability and a slight reduction of CO2 gas selectivity. Due to their unique properties, gelled ionic liquids are promising materials to be used in membranes for gas separation processes [71]. This work demonstrated that gelation is a simple and promising method to control the mechanical properties of ionic liquid-based membranes without sacrificing separation performance.

408  Nanostructured Polymer Membranes: Volume 1 A few years later, in 2013 Nguyen et al. [72] tested two imidazoliumbased ionic liquids to prepare gelled SILMs by using aspartame as low ­molecular-molecular weight gelator for selective separation of CO2 from CO2/ N2 mixtures. 1-Hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [hmim+][NTf2–], and 1-ethyl-3-methylimidazolium bis(trifluoro­ methylsulfonyl)imide, [emim+][NTf2–], were successfully gelled and deposited onto a hydrophobic porous support made of polytetrafluorethylene (PTFE). These membranes showed high sol-gel transition temperatures (ca. 135 °C) suitable for flue gas applications. The analysis related the loading of the gelator to high CO2 permeability and good mechanical stability. In the case of [emim+][NTf2–], 80 mg/mL of gelator provided high CO2 permeability, high CO2/N2 selectivity, good mechanical properties and the resulting gels can be coated in composite membranes [72]. Other recent applications of gelled SILMs are the separation of butanol from acetone-butanol-ethanol aqueous mixtures. Plaza et al. [73] used 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim+][PF6–], to prepare gelled membranes on polytetrafluoroethylene-based support material. The evaporation process was performed by using the gelled SLIM and the same support without IL. Plaza et al. confirmed that the use of gelled [bmim+][PF6–] membranes increase the selectivity of butanol/ethanol in pervaporation experiments compared to the evaporation process.

10.5.5 Non-dispersive Solvent Extraction (NDSX) and Pseudo-emulsion Hollow Fiber Strip Dispersion (PEHFSD) Based on Ionic Liquids There are other types of SILMs that use a solid microporous separator between aqueous and organic phases. Non-dispersive solvent extraction (NDSX) and pseudo-emulsion hollow fiber strip dispersion (PEHFSD) belong to this group [2]. Non-dispersive solvent extraction (NDSX) is an alternative procedure to traditional processes based on solvent extraction. This novel method overcomes the disadvantages of conventional techniques and provides several advantages such as the incorporation of a porous material to stabilize the aqueous-organic phase, preventing the mixture of both phases. The two fluids circulate independently, avoiding flooding or loading-related problems [74]. Non-dispersive solvent extraction based on ionic liquids consists of two hollow fibers units: in the first unit, the feeding solution moves along one side of the fiber whereas the ionic liquid (carrier) moves along the other side. The pores of the fiber wall put in contact both solutions. Then, the ionic liquid goes to the second unit where the stripping process takes place [2]. Finally, the ionic liquid is reused and the process is

Separation Technology Based on Ionic Liquid Membranes  409 Feeding inlet

Feeding outlet

Unit 1 extraction

Ionic liquid

Ionic liquid

Unit 2 stripping

Stripping phase

Figure 10.5  Scheme of a non-dispersive solvent extraction ionic liquid membrane process using two hollow fiber contactors.

begun once again in the first unit. Another possibility is that extraction and the stripping of the solute take place at the same time in a single fiber unit. Luis et al. [75] used ionic liquid-based non-dispersive solvent extraction for removing sulfur dioxide from a gas effluent generated in roasting processes. They selected 1-etyl-3-methylimidazolium ethylsulfate as absorbent phase and ceramic hollow fiber units. Figure 10.5 shows a scheme of batch mode of a NDSX process using ionic liquids as carrier [74]. On the other hand, pseudo-emulsion hollow fiber strip dispersion (PEHFSD) based on ionic liquids consists of only one membrane unit where the extraction and the stripping processes take place, the process being similar to non-dispersive solvent extraction. This system also includes two homogenization tanks for the feeding phase and the pseudo-emulsion. The pseudo-emulsion phase is made of a strip solution dispersed into the organic phase and mixed with an ionic liquid. Pseudo-emulsion and feeding phase are separately pumped into the unit. When the mixing process finishes, the strip and the organic phases are separated automatically. Both non-dispersive solvent extraction and pseudo-emulsion hollow fiber strip share the same procedure and can be performed either in countercurrent or concurrent flows of the two phases (feeding phase and carrier) [2, 75]. This type of extraction process has been applied for recovering heavy metals such as chromium (III) [76] from an alkaline mixture and iron (III) [77] and cadmium (II) [78] from acid mixtures. These works show that

410  Nanostructured Polymer Membranes: Volume 1 PEHFSD based on ionic liquids is a feasible technology for the separation and concentration of many other heavy metals. Furthermore, this technique avoids the saturation of the carrier, which is continuously recirculated, increasing the performance of the process.

10.6 Green Aspect of Ionic Liquids Growing concern about environmental pollution has contributed significantly to increasing the interest of researchers in developing pollution-free processes based on supercritical fluids, biodegradable materials or ionic liquids, among other options. All these new processes have helped to develop the concept of Green Chemistry, which involves twelve principles established by Paul Anastas and John Warner as: 1) prevention; 2) atom economy; 3) less hazardous chemical syntheses; 4) designing safer chemical; 5) safer solvents and auxiliaries; 6) design for energy efficiency; 7) use of renewable feedstocks; 8) reducing derivatives; 9) catalysis; 10) design for degradation; 11) real-time analysis for pollution prevention and 12) inherently safer chemistry for accident prevention [79]. The attractive properties of ionic liquids fulfill all the requirements of green chemistry set for green solvents. In contrast to conventional organic solvents, ionic liquids are not volatile, which reduces their atmospheric emissions. Most of them can be recycled, avoiding the production of hazardous wastes. Ionic liquids show a wide range of toxicity since many types of cations and anions can combine to form ionic liquids [80]. Thus, their toxicity can be reduced by using cations with short chains of alkyl substituent. Moreover, the bioaccumulation of a broad variety of ionic liquids has been studied, being lower than those produced by conventional organic solvents. Whatever the case, a good selection of the cation and the anion would allow environmentally friendly ionic liquids to be obtain [81, 82]. Because of all these reasons, ionic liquids are considered as the green alternative to organic solvents for many applications. However, the high cost of ionic liquids has hindered the commercialization of the processes based on this technology. Their high prize is caused by the expensive materials and energetic techniques needed for their synthesis [83]. Major efforts have been made to overcome this economic issue by selecting low cost anions and cations and optimizing the preparation processes using energy efficient techniques. In addition, ionic liquid offer high reusability and ease of separation [84]. Hydrophobic ionic liquids form a separated phase in water and it is easy to separate them, for example, by decantation. However, hydrophilic or hydrophilic-hydrophobic ionic liquids tend to form micelles in water, and their separation process is more difficult. In

Separation Technology Based on Ionic Liquid Membranes  411 recent years, many methods have been proposed to recover these types of ionic liquids such as adsorption, membrane-based methods or by inducing physical separation. All of these processes allow ionic liquids to be reused in many cycles and implicitly reduce the operational costs [83]. These facts support the idea of ionic liquids as green solvents whose applications are increasing rapidly in all research areas.

10.7 Conclusions Ionic liquid membranes have been applied to a broad range of separation processes since these compounds offer task-specific properties and therefore appropriate ionic liquids can be designed for a given separation process. The main advantages of using ionic liquid as liquid phase are nearzero vapor pressure, thermal and chemical stability and, specifically in the context of separation technology, their high ion conductivity and high solvent power. Ionic liquid membranes can be classified according to the way they are employed as liquid phase in different membrane configurations for their application in separation technology. Furthermore, as discussed in previous sections, depending on the functional group present in the ionic liquid, this type of separation process provides effective ways for the selectivity and transport of a remarkable variety of ion species, such as metal ions, including heavy metal ions, organic compounds, biomolecules and mixed gases, with potential industrial implementation. Among the membrane configurations studied, namely, bulk ionic liquids, emulsion ionic liquid membranes and supported/polymer ionic liquid membranes, the last option can increase separation efficiency, facilitate recycling and widen the range of applications through the immobilization of ILs. Morover, the continuous appearance of new ionic liquids offer promising prospects for new fields of application in separation technology and the optimization of the existing processes.

Acknowledgments This work was partially supported by the Spanish Ministry of Science and Innovation (MICINN) and by the FEDER (Fondo Europeo de Desarrollo Regional), ref. CICYT ENE2011–25188 and by the SENECA Foundation 18975/JLI/2013 grants. M.J. Salar-García and V.M. Ortiz-Martínez thank the Ministry of Economy and competitiveness and the Ministry of Education for supporting their doctoral theses (Refs. BES-2012-09.55350 and FPU12/05444 respectively).

412  Nanostructured Polymer Membranes: Volume 1

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11 Membrane Distillation Mohammadali Baghbanzadeh, Christopher Q. Lan*, Dipak Rana and Takeshi Matsuura Department of Chemical and Biological Engineering, University of Ottawa, Ottawa, Ontario, Canada

Abstract

Membrane Distillation (MD) is a thermal-driven membrane-based separation process with great potential in applications such as desalination, wastewater treatment, and separation of volatile components from liquid mixtures. Since the formation of the MD concept in the 1960s, extensive studies have been devoted to both the understanding of the fundamental principles and improving the economic competitiveness of MD. This chapter strives to provide a comprehensive coverage of both the fundamentals and recent developments associated with the application, process design, and membrane fabrication in this field. Keywords:  Separation, distillation membrane, membrane characteristic, t­ ransport phenomena

11.1 Introduction Membrane distillation (MD), introduced in the late 1960s [1], is considered as a low cost and energy saving alternative to conventional separation processes such as distillation and reverse osmosis (RO). MD is a membrane-based thermal-driven separation process that uses vapor pressure difference across the membrane as the driving force for mass transfer. The major obstacles hindering the commercial application of MD include the unavailability of appropriate MD membranes with required characteristics at reasonable costs [2] and the huge process thermal energy demand that result in a process which is not economically appealing in comparison with *Corresponding author: [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 1, (419–456) © 2017 Scrivener Publishing LLC

419

420  Nanostructured Polymer Membranes: Volume 1 the conventional separation processes such as RO [3]. Extensive efforts have been focused on the development of novel MD membrane based upon a better understanding of mass and heat transfer principles of MD [4] with significant advances in many different fronts. In an MD process, liquid molecules are evaporated at the liquid-vapor interface and only vapor molecules are permitted to pass through the porous and hydrophobic medium. Finally, concentrated solution would be collected on the permeate side. The following are considered characteristic of the MD process [5]: 1. A porous membrane is needed. 2. The membrane should not be wetted by process liquid. 3. Capillary condensation should not take place inside the membrane pores. 4. Liquid is not permitted to pass through the membrane and only vapor should be transported across the membrane. 5. The membrane must be neutral to the vapor equilibrium of the different components in the process liquid. 6. At least one side of the membrane should be in direct contact with the process liquid. 7. For each component in the process liquid, the membrane operation driving force is a partial pressure gradient in the vapor phase across the membrane. In an MD process, the feed does not need to be heated up to the boiling temperature of the volatile component. Therefore, the process will work at low operating temperatures compared to conventional processes. In addition, the operating pressure in an MD arrangement is much less than that of pressure-driven membrane processes such as RO, microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF), resulting in a separation process consuming less electric power, requiring materials of less mechanical strength, and being less sensitive to membrane fouling compared to the pressure-driven processes. Furthermore, MD permeate can theoretically approach a selectivity of 100%, which is similar to conventional thermal distillation processes and much more superior to pressure-driven membrane-based processes. Table 11.1 compares the advantages and disadvantages of MD and RO for desalination.

11.2 Applications of Membrane Distillation Technology Membrane distillation has the potential of being used in water desalination, solution degassing, treatment of industrial effluents, purification of

Membrane Distillation  421 Table 11.1  Comparison between MD and RO for desalination. Advantages

Disadvantages

RO

1. Established commercial processes; 2. Good membrane durability; 3. Near zero thermal energy consumption; 4. Compact systems with small footprint.

1. High operating pressure; 2. Need for mechanically strong systems; 3. Lower product quality compared to that of MD; 4. Limited water recovery; 5. Environmental liabilities due to rejection of large volumes of concentrated brines; 5. Membrane fouling a significant concern; 6. Rigid pre-treatment of feed required.

MD

1. No transmembrane pressure required; 2. Small power consumption; 3. Less tendency to fouling; 4. Use of low-grade energy; 5. Close to 100% salt rejection leading to high quality water products; 6. Works well with concentrated brines.

1. High operating temperature in comparison with RO; 2. Consumes large quantities of thermal energy; 3. Membrane durability a hindrance; 4. Relatively small flux in ­comparison to RO; 5. Economically uncompetitive as stand-alone desalination process at present.

pharmaceuticals, processing of foods, removal of organic compounds and heavy metals from aqueous solutions [6–10] and radioactive wastes [11], and concentrating diluted non-volatile acids such as sulfuric acid and phosphoric acid [12]. In addition, MD could be employed for recovery of volatile compounds from aqueous solutions. By applying a hydrophobic membrane, components more volatile than water would be transported through the membrane pores and consequently the other side would be enriched by those components. Such a process has the potential to be used for integrated systems. For instance, in a membrane bioreactor, by continuous removal of ethanol from broth during fermentation, MD could enhance the process yield [13]. Another example of MD application is HCl recovery from industrial effluents [14]. For instance, HCl might be used as the pickling liquor for removing surface oxides before electroplating and spent pickling liquors should be refined from the harmful heavy metals.

422  Nanostructured Polymer Membranes: Volume 1 While neutralization methods are traditionally used, the MD process can be employed for acid recovery. In this process, water vapor and gaseous acid are transferred through the membrane pores to the permeate side. Vapor is condensed and gaseous acid is then dissolved [15]. The MD process could potentially be used for water treatment in place of RO provided to having high performance MD membranes which are comparable to the conventional RO membranes in terms of permeability and durability. Furthermore, a novel MD process which demands much less thermal energy input compared to the conventional MD processes needs to be developed. The main disadvantages of RO technology in water treatment are huge electrical energy consumption, limited water recovery and environmental liabilities associated with the rejection of large volumes of concentrated brines [16]. Furthermore, osmotic pressure increases significantly with the increase of the brine concentration and, therefore, operation pressure and electrical energy consumption would increase dramatically with the increase of water recovery. In addition, fouling and scaling are important challenges in RO.

11.3 Different Kinds of Membrane Distillation Configurations There are several configurations of MD systems, which are different based on the structure of the permeate side.

11.3.1 Direct Contact Membrane Distillation (DCMD) Direct contact membrane distillation is the simplest MD configuration. The membrane is in direct contact with the liquid phase and has the ability of producing a high flux. Hot feed is in direct contact with the hot side of the membrane surface and vapor molecules pass through the membrane toward the permeate side and condensation takes place inside the module. The application of this configuration is in desalination and concentration of aqueous solutions [17, 18]. Because of its simple structure and high flux, DCMD was exposed to a large amount of laboratory research. The main disadvantage of this configuration is its low energy efficiency [4] and that is a great obstacle in its commercialization. As a result of a higher heat transfer coefficient on the permeate side, DCMD has the highest heat conduction loss among the other configurations, that

Membrane Distillation  423 Pore

Permeate

Feed

Membrane

Figure 11.1  Direct contact membrane distillation (DCMD) [4].

will decrease its thermal efficiency [19, 20]. Figure 11.1 shows a schematic of the DCMD concept.

11.3.2 Air Gap Membrane Distillation (AGMD) In an air gap membrane distillation arrangement, there is a layer of air between the membrane and condensation surface. Hot side is similar to that of DCMD, but as well as the membrane, vapor molecules pass the stagnant air to reach the cold surface inside the module. This configuration has a high energy efficiency with a relatively low flux. AGMD can be used particularly where the available energy is small [21]. Due to its greater mass transfer and thermal resistances, air gap controls the heat and mass transfer. The air gap is usually thicker than the membrane and its thermal conductivity is smaller, thus more heat energy in this configuration will be used to evaporate water in comparison with DCMD. Latent heat of vapor can be recovered by the condenser, if a low temperature feed as the cooling stream is used to condense vapor. Because of a low temperature difference across the membrane, this configuration has a low flux and larger surface area is required [18, 21, 22]. Figure 11.2 shows a schematic of the AGMD concept.

11.3.2.1  Memstill and Aquastill Memstill (Figure 11.3) is a novel configuration based on the energyefficient AGMD idea. Desalination takes place in a countercurrent flow configuration. Cold feed solution flows through a condenser with nonpermeable walls, which results in an increment in its temperature. Then

424  Nanostructured Polymer Membranes: Volume 1

Cold feed

Hot feed

Pore

Permeate

Cooling plate

Membrane

Figure 11.2  Air gap membrane distillation (AGMD) [4]. Fuel or waste/solar heat T between 50–100°C

Waste heat

90°C

87°C

T

high

water vapour

25°C

(a)

T low

brine Sea Fresh water 28°C water 28°C Condensor array Membrane array

Seawater Brine

(b)

Heat transfer

Distilled water

Water vapour

Figure 11.3  Memstill configuration [4, 24].

it is heated up using a source of energy and enters an evaporator whose walls consist of a microporous hydrophobic membrane. The condenser and membrane could be either tubular or flat sheet. It has been reported that depending upon the cost of thermal energy provided, using Memstill for desalination would reduce the product cost to 0.26–0.50 $/m3 of water produced [20]. The low cost is due to the cheaper plant materials for the module compared to high pressure RO. Furthermore, there is the possibility of using low grade sources of heat, such as waste heat, to provide thermal energy for desalination. Memstill has the lowest ­thermal energy

Membrane Distillation  425 required among the other ­configurations (56–100 kWh/m3), which would result in the highest gain output ratio (GOR) that has ever been reported (11.2) [4]. Feed temperature should be within 80–90 °C and electrical energy required is almost 0.75 kWh/m3 [23].

11.3.3 Permeate Gap Membrane Distillation (PGMD) Permeate gap membrane distillation is a configuration between AGMD and DCMD. The air gap is inherently open to atmospheric air at ambient pressure. By closing the bottom distillate outlet and allowing the permeate to fill up the air gap, the permeate will be discharged from the top; such a configuration is called PGMD. In fact, stagnant air in AGMD is replaced by stagnant liquid in PGMD. An important advantage of this configuration compared to DCMD is the separation of permeate channel from the cooling stream. It results in the use of other fluids for cooling purpose. For instance, cold feed solution could be employed as the coolant and it would be preheated before entering the hot side. Actually, it provides the possibility of internal heat recovery. There is a thin foil between the permeate gap and coolant in this configuration that adds an extra thermal resistance to the system and reduces the effective temperature difference across the membrane. The PGMD arrangement has higher permeate flux in contrast to AGMD [25] due to a faster condensation that takes place when vapor is directly mixed with colder liquid, and also a faster heat loss rate towards the polymeric film (foil) that is in contact with the coolant stream. Furthermore, at low feed temperatures, PGMD has lower specific thermal energy consumption compared to AGMD. A schematic of PGMD is given in Figure 11.4.

11.3.4 Sweep Gas Membrane Distillation (SGMD) In SGMD, there is an inert gas (stripping gas) instead of the air gap in AGMD to carry the vapor that leaves the membrane. Vapor is condensed in an external condenser. Unlike AGMD, the gas barrier for reducing heat loss is not stationary, which results in an enhancement in the mass transfer coefficient. This configuration is used to remove volatiles from an aqueous solution [26, 27]. Due to the greater driving force originating from the reduced vapor pressure on the permeate side, SGMD has higher mass transfer in comparison with AGMD and lower heat loss across the membrane than DCMD, but employing an external condenser and blower would increase its capital and operating costs. Figure 11.5 presents a schematic of the SGMD concept.

426  Nanostructured Polymer Membranes: Volume 1

Cold feed

Permeate

Hot feed

Pore

Cooling plate

Membrane

Figure 11.4  Permeate gap membrane distillation (PGMD).

Pore

Sweeping gas

Feed

Membrane

Figure 11.5  Sweep gap membrane distillation (SGMD) [4].

11.3.5 Vacuum Membrane Distillation (VMD) In this configuration, the permeate side is vapor under reduced pressure, which could be condensed in separate equipment such as an external condenser. In this arrangement, pressure is maintained below the equilibrium vapor pressure to improve mass transfer. VMD is beneficial for removing volatiles from an aqueous solution [28, 29]. It is characterized by first: vaporization of the more volatile components at the liquid-vapor interface and second: diffusion of the vapor through the membrane pores according to a Knudsen mechanism. To maintain a vapor pressure difference across the membrane, the vapor permeate should be removed continuously from the vacuum chamber. By considering the fact that vapor

Membrane Distillation  427

Permeate

Feed

Pore

Vacuum

Membrane

Figure 11.6  Vacuum membrane distillation (VMD) [4].

pressure on the cold side can be reduced to near zero, VMD could provide the highest driving force at the same feed temperature compared to the other MD configurations. VMD is a suitable approach for reducing heat loss and reaching higher vapor flow rates. It should be mentioned that the possibility of liquid penetration into the membrane pores in this configuration is higher than the other arrangements and a membrane with a smaller mean pore size should be applied [6]. A schematic of VMD configuration is shown in Figure 11.6.

11.3.5.1  Vacuum Gap Membrane Distillation (VGMD) Vacuum gap membrane distillation (Figure 11.7) is a type of AGMD configuration in which a slight vacuum pressure is applied on the permeate side. In other words, VGMD is an arrangement between AGMD and VMD. It is different from VMD in the way that product water is condensed inside the module at the highest possible temperature. To create the vacuum pressure, an air ejector or a vacuum pump could be used [25]. By using a slight vacuum, electrical energy consumption would be significantly decreased compared to the conventional VMD process which works at a very high vacuum pressure, moreover, there would not be a need for a continuous evacuation in VGMD. Therefore, to prevent vacuum loss due to the existence of non-condensable gases in feed solution, it is necessary to remove them first. To this end, degassing could be proposed as the preliminary step before the VGMD process. To degas the feed solution, hollow fiber membranes are typically employed under a vacuum pressure [30, 31]. Higher vacuum pressure means a greater level of degassing and, consequently, a better VGMD performance. The performance of VGMD

Cold feed

Permeate

Hot feed

Pore

Slight vacuum

Membrane

Cooling plate

428  Nanostructured Polymer Membranes: Volume 1

Figure 11.7  Vacuum gap membrane distillation (VGMD).

is greatly dependent on the vacuum pressure which is used on the permeate side. Higher vacuum would result in a better performance, however, it would result in more energy consumption and less effective internal heat recovery [25]. At a very slight vacuum of 0.9 bar, VGMD was demonstrated to have a performance between PGMD and AGMD in terms of permeate flux and specific thermal energy consumption [25]. It is predicted that the VGMD function could be enhanced by increasing the vacuum pressure.

11.3.5.2 Memsys Memsys (Figure 11.8) is a novel configuration of VGMD that employs an internal heat recycling concept to reduce thermal energy consumption. It is known as vacuum multi-effect membrane distillation (VMEMD), which combines the advantages of multi-effect and vacuum concepts to make a multistage setup integrated into a compact plate and frame module. This configuration has been successfully commercialized [32]. A Memsys module consists of a steam raiser, multiple stages and a condenser. Each stage has several membrane and foil frames. Temperature difference between steam raiser and condenser provides a driving force for the entire process. Foils are made from metal-coated polypropylene (PP) and act as the condensation plates. Membranes are made from polytetrafluoroethylene (PTFE) and serve as vapor channels. Feed solution flows in the space between foils and membrane frames. A vacuum pump is employed to generate vacuum pressure throughout the module; however, vacuum pressure varies from 0.1 to 0.3 bar in different stages [32]. Thermal energy consumption of Memsys process is within 175–350 kWh/m3, which corresponds to a GOR of 3.6. Its electrical energy requirement is in the range from 0.75 to 1.75 kWh/m3, and feed solution temperature falls between 60–100 °C, while the coolant temperature is less than 40 °C [4].

Membrane Distillation  429 Membrane Boiling seawater Brine Condensor foil

p1, T1

p2, T2

p3, T3

p4, T4

Evaporator

Cooling water Condensor

Preheated feed

Distillate

Figure 11.8  Memsys configuration [4, 33]. Solution-diffusion flow Diffusion

Poiseuille flow

rv

Transition flow

ri

Liquid-vapor interface

Solution

Feed liquid

Knudsen flow

δ Membrane Permeate vapor

Feed liquid (a)

(b)

rk δ Membrane

Permeate vapor

Figure 11.9  (a) Pervaporation (PV), (b) Vacuum Membrane Distillation (VMD), modified according to Khayet and Matsuura [34].

11.3.5.3  Differences between VMD and Pervaporation (PV) Both VMD and PV are membrane-based separation processes in which hot solution is in direct contact with membrane on the upstream side, while a vacuum pressure is employed on the downstream side of the membrane. Membrane plays a significant role in the differences between these two types of processes. In VMD configuration, porous and hydrophobic membranes are applied which function like a support for vapor-liquid interface and do not influence the separation performance; but in pervaporation, dense and selective membranes are employed and the separation process is affected by the solubility and diffusivity of the components in the ­membrane. In VMD, vapor molecules cross the pores of the membrane, but in PV, diffusion is the dominant mechanism and vapor molecules are diffused through the membrane. In other words, VMD uses porous membranes, while nonporous membranes are applied in PV. Therefore, VMD usually achieves higher flux than that of PV. In Figure 11.9 the differences between VMD and PV are stated schematically [34]. Table 11.2 summarizes the differences between various configurations of MD.

Applicable module

1. Plate and Frame

1. Plate and Frame

1. Plate and Frame 2. Spiral Wound

1. Plate and Frame 2. Hollow Fiber

Configuration

DCMD

AGMD

PGMD

SGMD 1. Larger flux than AGMD due to ­turbulence on permeate side; 2. Smaller conductive heat loss than DCMD.

1. Simple structure; 2. Larger driving force and hence flux than AGMD at same feed/coolant temperature; 3. Less conductive heat loss than DCMD; 4. Capable of latent heat recovery.

1. Simple structure; 2. Higher energy efficiency than DCMD; 3. Capable of latent heat recovery.

1. Simple structure; 2. High flux; 3. Broad availability of research data.

Advantages

Table 11.2  Comparison between different MD configurations.

Need external condenser/air blower

1. Smaller driving force and hence flux than DCMD at same feed/coolant temperature; 2. Lower energy efficiency than AGMD due to conductive heat loss.

Smaller flux than DCMD due to smaller driving force at same feed/coolant temperature

1. Low energy efficiency due to ­conductive heat loss; 2. Coolant must be the same as the permeate.

Disadvantages

430  Nanostructured Polymer Membranes: Volume 1

1. Plate and Frame 2. Hollow Fiber 3. Spiral Wound

1. Plate and Frame

VMD

VGMD 1. Larger flux than AGMD due to larger driving force; 2. Lower thermal energy consumption than AGMD; 3. No need of external condenser; 4. No need of continuous evacuation, less power consumption than VMD.

1. Greatest driving force among the MD configurations because the pressure on permeate side could be reduced to almost zero; 2. High flux; 3. Absence of conductive heat loss. 1. Need degassing of feed; 2. Maintenance of vacuum a challenge, especially at large scale; 3. Complex system demanding careful sealing.

1. Require membranes of large liquid entry pressure; 2. Need external condenser/vacuum pump; 3. Challenge of maintaining vacuum at large scale; 4. Increase water pH due to CO2 removal.

Membrane Distillation  431

432  Nanostructured Polymer Membranes: Volume 1

11.4 Distillation Membranes 11.4.1 MD Modules There are four major membrane modules which are plate and frame, hollow fiber, tubular and spiral wound [6].

11.4.1.1  Plate and Frame In this module, membrane and spacers are placed between two plates. It is suitable for flat sheet membranes and can be applied for DCMD, AGMD, SGMD, and VMD configurations. Spacers are used to increase turbulence and reduce temperature polarization for enhancing the flow dynamics. Packing density in this module is within 100–400 m2/m3 [35]. Its effective area for the same volume is relatively smaller than the other modules, and a membrane support is required, but multiple layers of flat sheet membranes could be used to increase the effective area. On the other hand, the simplicity of its construction, cleaning and replacing damaged membranes make the module appropriate for laboratory applications to test the influence of membrane properties and process variables on the membrane performance. Indeed, this type of module is most commonly used in laboratory experiments. Figure 11.10 shows a schematic of plate and frame module and flat sheet membrane.

11.4.1.2  Hollow Fiber In this module, thousands of hollow fibers are bundled and sealed into a housing. It is possible for the feed solution to flow inside or outside the fibers while the permeate product would be collected from the other side. According to the cooling fluid arrangement, presence of air gap, sweeping

Membrane

Active layer

Support layer (a)

(b)

Figure 11.10  (a) Plate and frame module; (b) Supported flat sheet membrane [4].

Membrane Distillation  433 gas or negative pressure on the permeate side, DCMD, AGMD, SGMD or VMD configurations could be achieved. It has a very high packing density (about 3000 m2/m3) [36] and its energy consumption is very low, which are two main advantages of this module that further result in the process being potentially applicable in industry. Although, its high tendency to fouling and difficulty in cleaning and replacing the broken hollow fibers are considered as the main disadvantages of this module. The broken hollow fibers could be detected by a liquid decay test [37–39]. Low efficient distribution of feed solution in the shell side may cause high degrees of temperature polarization. To reduce this effect in hollow fiber modules, cross-flow patterns have been developed [40]. A schematic of hollow fiber module and hollow fiber membrane are given in Figure 11.11.

11.4.1.3 Tubular In this module, there are two cylindrical chambers for hot and cold fluids and a tube shaped membrane is placed between the chambers. Its ease of cleaning, high effective area and low tendency to fouling make it a good option for commercial applications. However, having a high operating cost is considered to be a disadvantage of this module. Tubular modules could be used in DCMD, AGMD, and VMD configurations.

11.4.1.4  Spiral Wound In this module, flat sheet membranes and spacers are enveloped and rolled around a perforated central collection tube. Feed flows in an axial direction through the membrane surface, and the permeate product moves radially to the center where it is accumulated by the collection tube. In this module a feed spacer mesh is used to keep the membranes separated; therefore, feed water will flow between the sheets, and then a separate

Hollow fibre membrane Membrane wall

(a)

(b)

Figure 11.11  (a) Hollow fiber module; (b) Hollow fiber membrane [4].

434  Nanostructured Polymer Membranes: Volume 1 permeate carrier is obtained on the opposite side of the membrane sheets to carry the permeate to the perforated tube for the collection of the product. Generating turbulence, lowering the boundary layers thickness close to the membrane surface, and helping reduce scaling and fouling possibilities are the main concerns that need to be taken into account in a feed spacer design. This module has average packing density, tendency to fouling, and energy consumption [6].

11.4.2 Applicable Membranes for MD Flat sheet (Figure 11.10b) and hollow fiber (Figure 11.11b) are the two most common types of membrane in different applications. As shown in Figure 11.10b, a supported flat sheet membrane is made up of two layers; a thin active layer and a porous support layer. The support layer provides sufficient mechanical strength and enables the active layer to be constructed as thin as possible to reduce the mass transfer resistance. It should be pointed out that unsupported flat sheet membranes are also common in MD. PP, PTFE, and polyvinylidenefluoride (PVDF) are materials which are used for the fabrication of flat sheet membranes, while PP, PVDF and PVDF-PTFE composite are mainly used for preparing hollow fiber membranes. Hollow fiber membranes have larger specific surface area than flat sheet ones, but they typically have low flux due to their poor flow dynamics and high degree of temperature polarization [41, 42]. However, high-flux hollow fiber membranes (as high as that of flat sheet membranes) have been recently developed, such as hollow fiber membranes with a spongelike structure and thin walls, dual-layer hydrophilic-hydrophobic fibers with a very thin effective hydrophobic PVDF layer (50 µm) [4].

11.4.2.1  Nanocomposite Membranes Much attention has been paid to nanocomposite membranes in different membrane-based processes such as MF, UF, NF, RO, and FO [43–52]. However, using nanomaterials in developing the MD membranes is considered to be a new concept and only a few studies have been reported in the literature in this direction. Carbon nanotubes [53], CaCO3 [16, 54, 55], CuO [16], and SiO2 [56–58] are the nanomaterials which were used for making the MD nanocomposite membranes. The use of hydrophobic nanomaterials was more often among those studies [54–57]. Interestingly, in spite of the nature of the MD membranes which demands highly hydrophobic surfaces, Baghbanzadeh et al. demonstrated that hydrophilic

Membrane Distillation  435 nanoparticles such as CaCO3 [16], CuO [16], and SiO2 [58] have a great potential in enhancing the membrane performance by increasing the permeability through enlarging the surface pore size, porosity, and reducing the thickness of the sponge-like layer which is considered to be the major contributor to the mass transfer resistance across the membrane. They showed that hydrophilic silica nanoparticles could increase the thickness of the finger-like layer significantly, and since such a membrane might be mechanically fragile in long-term MD operation, nanocomposite membrane could be stabilized by a nonwoven polyester backing material for longer durability [58]. A quite large VMD pure water flux of 12.7 kg/m2h was reported in their work when a feed temperature of 27.5 °C and a vacuum pressure of 1.2 kPa were used [58]. It is worth pointing out that the hydrophilic nanomaterials incorporated membranes demonstrated nearly perfect salt rejection in those studies [16, 58].

11.4.3 Membrane Characteristics in MD Hydrophobic microporous membranes are employed in MD processes. Generally, membranes in MD applications should have high thermal and chemical resistance at relatively high temperatures against basic and acidic solutions. In the following, the major characteristics of the membranes in an MD process are presented.

11.4.3.1  Liquid Entry Pressure (Wetting Pressure) Liquid entry pressure (LEP) is the highest pressure that can be applied in MD without penetration of the liquid feed into the membrane pores. If the pressure exceeds this amount, liquid would enter the hydrophobic membrane. It depends upon maximum pore size and membrane hydrophobicity. Feed concentration would influence the LEP as well. For example, organic solutes usually reduce the LEP. Gostoli and Sarti [59] reported that by increasing the concentration of ethanol, LEP would linearly decrease. Generally, LEP is related to the nature of organic matter, its concentration, and feed temperature [60]. The Laplace equation (Eq. 11.1) can be used to determine LEP [61].

P

Pf

Pp

2 B l cos (11.1) rmax

Where Pf and Pp are the hydraulic pressure on the feed and permeate side, respectively, B is a geometric pore coefficient (equal to 1 for cylindrical

436  Nanostructured Polymer Membranes: Volume 1 pore), γl is liquid surface tension, θ and rmax are contact angle (between the solution and membrane surface) and maximum pore size, respectively. For an aqueous solution of NaCl, surface tension depends on the salt concentration (cf) and can be calculated by Equation 11.2 [62].

l

l ,water

1.467c f (11.2)

Where γl,water is pure water surface tension and equals to 72 mN/m at 25 °C. Considering Equation 11.1, a membrane with a high contact angle (equivalent to high hydrophobicity), small pore size, low surface energy (equivalent to high hydrophobicity), and high surface tension is greatly desired in MD for reaching an appropriate LEP for the feed solution.

11.4.3.2  Membrane Thickness Permeate flux is inversely related to the membrane thickness. It means a thicker membrane results in a lower permeate flux. It is due to an increase in mass transfer resistance when the membrane becomes thicker, however, heat loss would decrease in this case. According to Laganà et al. [63], the optimum membrane thickness is in the range from 30 to 60 μm. Among the MD configurations, membrane thickness has the smallest impact in AGMP, because mass transfer is controlled by stagnant air gap thickness in the arrangement.

11.4.3.3  Porosity and Tortuosity Porosity ( ) is defined as the ratio of the membrane pores volume to the total volume of the membrane. It varies between 30% and 85% in different membranes [64]. Higher porosity means higher thermal resistance (lower conductive heat loss) and permeability of the MD membranes due to a larger evaporation surface area, and results in an increment in the heat efficiency and flux. Although mechanical strength decreases by increasing the porosity that brings about a membrane with a high tendency to crack formation under mild pressures. The Smolder–Franken equation is used for calculating the porosity as follows [65]:

1

m

(11.3)

pol

where ρm and ρpol are the densities of membrane and polymer material, respectively.

Membrane Distillation  437 Tortuosity is a parameter for determining the deviation of the pore structure from a cylindrical shape. Higher tortuosity means a more complicated transportation path and accordingly a smaller permeate flux. Macki-Meares suggested the following relationship between porosity and tortuosity [66]:

(2

)2

(11.4)

11.4.3.4  Mean Pore Size and Pore Size Distribution Usually MD membranes have a pore size in the range from 0.1 to 1 μm [17,  64]. Permeate flux increases with membrane pore size. Since pore size distribution in a membrane is not uniform, the mean pore size is usually used to characterize the membrane. The membrane pore size should be optimized in such a way that it provides an appropriate flux while not allowing the membrane to become wet. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) can be used to determine surface morphology of an MD membrane [16, 56, 57, 58, 67–70]. These analyses are able to estimate the porosity, pore size, and pore size distribution of a membrane. The SEM technique is used to study the top and bottom surface as well as the cross section of the membrane, while the sample needs to be gold/carbon-sputtered before analysis. However, AFM can be used without sample preparation at ambient pressure and temperature. To measure the maximum and mean pore size besides the pore size distribution, bubble point with gas permeation method can be employed. The procedure has been explored elsewhere [34].

11.4.3.5  Thermal Conductivity The MD process demands membranes with low thermal conductivity because sensible heat transfer increases with thermal conductivity, which would further result in a reduction in the permeate flux as a result of the reduced interface temperature gradient [4]. Thermal conductivity of a membrane (km) consists of two contributors, i.e., polymer and gas thermal conductivities (ks and kg, respectively). Since the thermal conductivities of  air and water vapor are close to each other, it could be assumed that gas inside the pores only consists of a single component, and its thermal conductivity at a temperature close to 40 °C could be calculated by Equation 11.5 [21].

438  Nanostructured Polymer Membranes: Volume 1

kg



1.5 10

3

T (11.5)

Finally, thermal conductivity of an MD membrane can be better estimated  by a volume average of the polymer and gas resistances by using Equation 11.6 [71]:

km



1

1 kg

ks

(11.6)

where ks depends upon the temperature, degree of crystallinity and the shape of the crystal. Thermal conductivities of most hydrophobic polymers are close to each other. Equation 11.7 has been suggested to calculate the thermal conductivity of PTFE [6].

ks

4.86 10 4 T 0.253 (11.7)

11.4.3.6  Membrane Fabrication Among the materials used in membrane fabrication, PTFE has the highest hydrophobicity, good thermal and chemical stability, and oxidation resistance. But due to its relatively high thermal conductivity, it has fairly high heat loss due to conduction. However, PVDF and PP membranes also show appropriate hydrophobicity, thermal and chemical resistance and mechanical strength. New materials such as carbon nanotubes and fluorinated copolymers have been recently developed in membrane fabrication [4]. For membrane fabrication, different methods such as sintering, stretching, and phase inversion are mostly used. Sintering is used to prepare PTFE membranes. Polymeric powder is pressed into a film or plate and sintered below the melting point. The membranes which are prepared by this method have a porosity within 10–40% and their typical pore size is in the range from 0.2 to 20 μm [4]. A stretching process could be used to manufacture PP and PTFE membranes. In this method, polymeric layers are formed by extrusion from a polymeric powder at temperatures near the melting point that is combined with a rapid drawdown. The pore size of the membranes fabricated in this way is within 0.2–20 μm and their porosity is around 90% [4]. The PVDF membranes could be made by applying the phase inversion method. In this process, a polymeric solution is prepared using an appropriate solvent

Membrane Distillation  439 and cast as a thin layer on nonwoven polyester, PP backing material or PP scrim backing supports to make a supported membrane or on a glass plate to generate an unsupported membrane. Afterwards, the cast membrane is immersed in a nonsolvent medium, such as water, and the solution is converted into two phases (a solid polymer-rich phase and a liquid-rich phase). The pore size of such a membrane is within 0.2–20 μm, while its porosity is approximately 80% [4].

11.5 Transport Phenomena in MD In an MD process, heat and mass transfers take place in the same direction from the hot to cold side. Feed solution temperature drops by convection mechanism through the feed side boundary layer from Tf to Tf,m at the membrane hot surface. Simultaneously, part of liquid feed molecules are evaporated and transferred across the membrane to the cold permeate side. Along with the mass transfer through the membrane, heat is transported by conduction (sensible heat) and carried by the vapor molecules (latent heat) to the other side. In the cold side, depending upon the MD configuration which is used, temperature decreases across the permeate boundary layer from Tp,m at the membrane cold surface to a bulk temperature of Tp at the cold plate or stream. Therefore, the driving force for the transport phenomena is the vapor pressure difference between Tf,m and Tp,m.

11.5.1 Mass Transfer in MD Mass transfer in MD includes three steps. First of all, liquid feed solution vaporizes at the liquid-gas interface. Second, vapor molecules go through the membrane pores towards the cold interface as a result of vapor pressure difference, and finally, according to the MD configurations, vapor molecules will be condensed in a liquid stream, stagnant or moving gas, or reduced pressure space. Thus, there are two main factors that control mass transfer. The first is the vapor pressure difference and the second is the membrane permeability. Mass transfer in MD is limited by mass transfer across the membrane if fluid dynamics on the hot and cold sides of the membrane shows a good condition with an appropriate turbulency. A global transport equation for mass transfer through the membrane is given by Equation 11.8 that correlates mass flux to vapor pressure difference across the membrane by introducing a membrane coefficient

440  Nanostructured Polymer Membranes: Volume 1 (Cm) that is related to the dominant mechanism for mass transfer in the ­membrane [6].

J

Cm ( Pf

Pp ) (11.8)

Where Pf and Pp are the vapor pressure at the membrane feed and permeate interfaces, respectively, and could be calculated by the Antoine equation [72]. The following should be taken into consideration regarding the mass transfer in the membrane [4]: 1. Since the porosity of a membrane is always less than 100%, the effective area for mass transfer is less than the total area of the membrane. 2. Since the membrane pores do not go straight within the membrane matrix, i.e., a tortuosity of less than unity, vapor molecules travel a longer path than the membrane thickness. 3. Resistance to diffusion increases by the inner walls of the pores due to a decrease in momentum of the vapor molecules. Regarding the presence or absence of air molecules inside the membrane pores, there are three basic mechanisms which would determine the mass transfer through the membranes: Knudsen diffusion (K), Poiseuille or viscous flow (P), and Molecular diffusion (M). However, a combination of them could also take place, which is known as transition mechanism. When Knudsen is applicable, the molecule-pore wall collisions are dominant over molecule-molecule collisions. Molecular diffusion would be the case when the vapor molecules collide with each other, while a Knudsen/ Molecular mechanism happens when the vapor molecules collide with each other, and also diffuse through the air film. In Poiseuille flow (viscous flow), the gas molecules act as a continuous fluid driven by a pressure gradient [6]. To determine which mechanism plays the major role in mass transfer across the membrane, the Knudsen number is defined by Equation 11.9 as follows [4]:

Kn

d

(11.9)

where is the mean free path of the molecules (the average distance which is traveled by a molecule between consecutive collisions), while d is the

Membrane Distillation  441 mean pore size of the membrane. is obtained from the kinetic theory and using Equation 11.10 [6]:

kBT



2 Pde2

(11.10)

where KB, T, and P are Boltzmann constant, absolute temperature, and average pressure within the membrane pores, respectively. A large value of Kn means that the mean free path of vapor molecules is large in comparison with the membrane mean pore size, while a small amount of Kn proves that membrane has a large mean pore size. Table 11.3 shows different situations which may take place in mass transfer through a membrane in different MD configurations. In AGMD configuration, mass transfer through the stagnant air is ­controlled by molecular diffusion and vapor molecules are transferred from the cold surface of membrane to the condensation film by molecular diffusion. The flux in an air gap layer is calculated by Equation 11.11.    N Air

Gap

D P PAir RT lAir Gap Plm

C Air

Gap

Gap

PAir

Gap (11.11)

The total vapor flux in an AGMD can be estimated via Equation 11.12 as follows:



N Vap

AGMP

1 Cm

1

1 C Air

Gap

PTotal (11.12)

By considering Equation 11.8 and Table 11.3, membrane coefficient is a function of temperature, pressure, membrane structure, and diffusing species, while the driving force (Pf-Pp) is a function of liquid solution temperature and composition at hot and cold membrane surfaces. Furthermore, it could be concluded that in an MD process, flux is enhanced by increasing the pore size and membrane porosity, and by reducing the membrane tortuosity and thickness. Although, when the thickness of membrane decreases, sensible heat loss increases. It verifies that an optimum thickness for the membrane should be found out.

Knudsen diffusion

Transition (Knudsen/Poiseuille)

Poiseuille diffusion

0.01 < Kn < 10.0

Kn < 0.01

Molecular diffusion

Kn < 0.01

Kn > 10.0

Transition (Knudsen/Molecular)

0.01 < Kn < 1.0

VMD

Knudsen diffusion

Kn > 1.0

DCMD, PGMD, AGMD, and SGMD

Mechanism

Kn

MD configuration

CP

CT

CKn

CM

CT

CKn

Cm

1

12

2 8RT 3 MW

8RT MW

r 4 Pave 1 8 RT

RT

2 1 3 RT

12

2 8RT 3 MW

8RT MW

PD r 2 RT Pair

RT

1

2 1 3 RT

Table 11.3  Dominant mechanisms in mass transfer across the membrane in different MD configurations.

r3

1 2

r3

r3

1 2

r3

1

1

r Pave 8

4

PD 2 r Pair

1

1

1

1

442  Nanostructured Polymer Membranes: Volume 1

Membrane Distillation  443

11.5.2 Heat Transfer in MD Heat transfer from bulk feed to the permeate side involves two steps. First of all, heat is transferred from the hot to cold side through the membrane as sensible heat and latent heat. Sensible heat is conducted across the membrane, while the latent heat is carried by the vapor molecules through the membrane pores. As a result of sensible and latent heat transfer, a temperature difference between the boundary layer and bulk liquid in the hot channel is generated that would result in the second step. In this phase, heat is transferred from the bulk feed to the hot side boundary layer by convection. It is worth mentioning that the heat transfer in both steps is equal. Heat transfer via convection in the feed boundary layer is given by Equation 11.13:

Qf



h f (Tf

Tf ,m ) (11.13)

where hf represents heat transfer coefficient at the feed boundary layer. Heat transfer across the membrane via conduction and vapor molecules movement is presented by Equation 11.14:

Qm



km

(T f ,m Tp ,m ) J H v (11.14)

where km, , J, and ΔHv are membrane thermal conductivity, membrane thickness, permeate flux, and heat of vaporization, respectively.    Q f

Qm

km

(T f ,m Tp ,m ) J H v

h f (Tf

T f ,m ) (11.15)

It should be pointed out that the above equations are used for the heat transfer analysis at the hot side and across the membrane for all of the MD configurations except VMD, where heat conduction in the membrane is not applicable. Surface temperatures of both sides of the membrane (Tf,m and Tp,m) cannot be obtained experimentally; however, a mathematical iterative model according to Equation 11.16 has been proposed by Termpiyakul et al. [73] for their estimation.

444  Nanostructured Polymer Membranes: Volume 1



T f ,m

Tf

Tp ,m

Tp

J Hv

km (Tf ,m Tp ,m )/

m

hf J Hv

km (Tf ,m Tp ,m )/ hp

m

(11.16)

Heat transfer on the permeate side of DCMD, SGMD, and VMD can be simulated by Equation 11.17.

Qp



hp (Tp ,m Tp ) (11.17)

According to Equation 11.18, heat transfer on the cold side of AGMD configuration includes two parts. The first is related to heat transfer across the stagnant air gap that occurs by conduction and vapor molecules transportation, and the second is for condensation in the permeate boundary layer. It is worth mentioning that for a PGMD configuration, since the vapor molecules become liquid as soon as they pass through the membrane, the second term related to latent heat in Equation 11.18 will be ignored [74].    QAG

kAG (Tp ,m Tfilm ) J H v lAG

Qp

hp (T film Tp ) (11.18)

where heat transfer coefficient (hp) in the condensate film on a vertical plate is obtained via Equation 11.19 [75].



hp

2 2 3

k 3film

2

g Hv

L(Tfilm Tp )

1 4

(11.19)

If the air gap is thicker than 5 mm, free convection would take place between two vertical plates across the air gap region and heat coefficient could then be estimated by Equation 11.20 [6].

Membrane Distillation  445

   Nu

c(Pr Gr )n

l L

1 9

10

5

Gr 10

c

0.07 1 n 3 (11.20) c 0.2 1 n 4

7

, 104

Gr 105

Table 11.4 summarizes the heat transfer in different MD arrangements.

11.5.2.1  Thermal Efficiency and Heat Loss In an MD process, thermal efficiency (Π) is defined as the ratio of latent heat of vaporization to the total (latent and sensible) heat [6]. Usually, thermal efficiency is enhanced by increasing the feed temperature, feed flow rate, and membrane thickness. On the other hand, it would decrease with an increase in feed concentration [76]. Equations 11.21 and 11.22 are used to calculate the thermal efficiency of a DCMD and AGMD process [18], respectively.

km





1

J Hv (T f ,m Tp ,m ) J H v

Ta 2.1 (Tf Tp )c p Ta 2.1

(11.21)

ka l (11.22)

Where and are obtained experimentally for an air gap distance less than 5 mm, and Ta is the average MD temperature between 30 and 80 °C [77]. The cp and ka are specific heat and air gap thermal conductivity, respectively. According to Martínez-Díez et al. [78], heat loss reduces by increasing the feed temperature and flow rate. Three sources of heat loss in an MD configuration are: 1) presence of air inside the membrane pores, 2) heat loss across the membrane via conduction, and 3) heat loss due to temperature polarization. To minimize heat loss in MD, degassing of feed solution, increasing the membrane thickness, setting up an air gap between the membrane and the condensation surface, and working in a turbulent regime have been suggested [6]. Using heat recovery in MD would result in better performance for the process, however, it would need an external heat exchanger and a membrane with larger area that increases the capital costs.

h f (T f

h f (T f

h f (T f

h f (T f

h f (T f

DCMD

PGMD

AGMD

SGMD

VMD

MD configuration

Qf

Tf ,m )

Tf ,m )

Tf ,m )

Tf ,m )

Tf ,m )

km

km

km

km

J Hv

(T f ,m Tp ,m ) J H v

(T f ,m Tp ,m ) J H v

(T f ,m Tp ,m ) J H v

(T f ,m Tp ,m ) J H v

Qm

J Hv

hA (Tp ,m T film ) J H v

kAG (Tp ,m T film ) J H v lAG

kPG (Tp ,m Tp ) lPG

–

QGap

hp (T film Tp )

hp (T film Tp )

hp (T film Tp )

QGap

hp , DCMD (Tp ,m Tp )

Qp

Table 11.4  Heat transfer in different regions of an MD configuration (if there is no resistance in cooling surface).

–

–

If l > 5 mm

If l > 5 mm

–

Natural convection

446  Nanostructured Polymer Membranes: Volume 1

Membrane Distillation  447

11.5.3 Temperature and Concentration Polarization In an MD process, the feed solution is evaporated at the membrane hot surface that would create a heat transfer boundary layer on the hot side. In addition, by condensing the vapor molecules in the other side, the permeate heat transfer boundary layer would also be established on the cold side. The presence of boundary layers would bring about a temperature difference between the liquid-vapor interface and bulk fluids that is known as temperature polarization. It is investigated by Equation 11.23 as follows:

Tm, f Tf

Tm, p Tp

(11.23)

For a VMD configuration, temperature polarization is characterized based upon Equation 11.24 [17].

Tf Tf

Tm, f Tp

(11.24)

Temperature polarization represents the effect of heat transfer boundary layers on total heat transfer resistance of the system [6]. It means by reducing the boundary layer resistances, the temperature difference between the liquid-vapor interface and bulk fluids becomes smaller and approaches 1. In addition, if the heat transfer is controlled by the boundary layer resistances, the system demonstrates a high degree of temperature polarization and approaches zero. According to Alkhudhiri et al. [6], for DCMD arrangement, is within 0.4–0.7. It is worth mentioning that the temperature polarization is more important at high concentration, high temperature, and low feed velocity, when the boundary layer resistances become greater. Typically, by making the fluids more turbulent using a spacer, temperature polarization would be reduced. Concentration polarization is defined according to Equation 11.25 as the ratio of solute concentration on the membrane surface to that of the bulk feed, and is symbolized by Ф.

Cm (11.25) Cf

448  Nanostructured Polymer Membranes: Volume 1 Solute concentration on the membrane surface is obtained via Equation 11.26 [6]:

Cm

C f exp

j (11.26) K

where, and K are the liquid density and mass transfer coefficient, respectively. Since the accumulated solute on the membrane surface generates a diffusive flow back to the feed [6], concentration polarization and fouling should be taken into account for modeling purposes, and vapor flux cannot only be estimated by Knudsen, molecular, and Poiseuille flow mechanisms, due to the differences between the properties of boundary layer at the membrane surface and the bulk solution.

11.5.4 Fouling Fouling is related to the creation of an additional layer on the membrane surface from the components which are present in the liquid solution. The additional layer could be due to biological fouling (by bacteria) or scaling as a result of high concentration solutions. Fouling and scaling would result in the blockage of the membrane pores, which would further reduce the effective area of the membrane, and finally decrease the permeate flux. Furthermore, the additional film acts as an extra resistance against heat transfer. Moreover, it may cause a pressure drop and severe temperature polarization effect. Equation 11.27 is used to estimate the heat transfer in the additional layer that is generated as a result of fouling. Heat is transported through the layer via conduction.

Q fouling

k fouling fouling

(T f , fouling T f ,m ) (11.27)

Fouling highly depends upon membrane and feed solution properties, module geometry, and operating conditions [79]. It can be inhibited by using pretreatment methods or membrane cleaning. In addition, working at low temperature with high feed flow rate would bring about a decrease in tendency to fouling.

Membrane Distillation  449

11.5.5 Operating Parameters There are several parameters that influence vapor flux in an MD configuration, some of which are presented below.

11.5.5.1  Feed Temperature Since vapor pressure increases exponentially with temperature according to the Antoine equation, considering Equation 11.8, by increasing the feed temperature, permeate flux would exponentially increase. Srisurichan et al. [66] reported that using a feed solution of high temperature would increase the mass transfer coefficient in the membrane. Furthermore, it has been reported that by increasing the feed temperature, temperature polarization would decrease [71].

11.5.5.2  Permeate Temperature Permeate flux decreases with an increase in permeate temperature; however, because vapor pressure variation with temperature is insignificant at low temperatures, the cold side temperature has smaller effects on permeate flux compared to the feed temperature [6].

11.5.5.3  Feed Concentration By increasing the feed concentration, vapor pressure decreases, temperature polarization increases, and feed viscosity would exponentially increase, which results in a reduction in driving force and, eventually, a decrease in the permeate flux [6].

11.5.5.4  Feed Flow Rate By increasing the feed flow rate or its velocity, turbulence increases, which reduces hydrodynamic boundary layer thickness and, subsequently, temperature polarization, eventually resulting in an increment in convective heat transfer coefficient. Therefore, permeate flux would increase. The effects of feed flow rate on permeate flux are more significant at higher temperatures [6].

11.5.5.5  Air Gap Thickness Permeate flux declines linearly by increasing the air gap distance in AGMD module [17]. Temperature gradient across the gap decreases by reducing the air gap thickness that results in the permeate flux enhancement.

450  Nanostructured Polymer Membranes: Volume 1

11.5.5.6  Membrane Properties Permeate flux increases with membrane porosity, while it is inversely proportional to the membrane tortuosity and thickness. In addition, membranes with larger pore size show a higher flux, and the presence of backing material is favorable in mechanically stabilizing the membrane, although it has a negative effect on vapor flux through the membrane.

11.6 Conclusion Taking into consideration recent developments in the processes presented in this chapter, membrane distillation (MD) is still an emerging technology that has not yet been totally commercialized. This is due to the expensive, low performance MD membranes and the huge thermal energy consumption throughout the process, which would bring about a more expensive final product compared to the other membrane-based processes such as reverse osmosis. To overcome these problems, thermal energy consumption in MD needs to be optimized, while thermal energy input should be replaced by large sources of available cheap energy such as industrial waste heat. Moreover, novel MD membranes need to be developed which are appropriate in terms of permeability, selectivity, chemical and mechanical stability, and durability to further reduce the cost of the final product. To this end, nanocomposite membranes have been demonstrated to be suitable in improving the performance of the MD membranes. Nonetheless, Memsys has successfully developed a commercial MD module for salt water desalination [32]. The module uses novel concepts such as using a low grade heat for the steam raiser stage (feed temperature lies between 50 and 80 °C), applying approximately high vacuum to the entire system (0.1–0.3 bar), and employing an internal heat recycling system to reduce the amount of thermal energy consumption.

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454  Nanostructured Polymer Membranes: Volume 1 54. D. Hou, J. Wang, X. Sun, Z. Ji, Z. Luan, Preparation and properties of PVDF composite hollow fiber membranes for desalination through direct contact membrane distillation. J. Membr. Sci., 405, 185, 2012. 55. D. Hou, G. Dai, H. Fan, J. Wang, C. Zhao, H. Huang, Effects of calcium carbonate nano-particles on the properties of PVDF/nonwoven fabric flat-sheet composite membranes for direct contact membrane distillation. Desalination, 347, 25, 2014. 56. J.E. Efome, M. Baghbanzadeh, D. Rana, T. Matsuura, C.Q. Lan, Effects of superhydrophobic SiO2 nanoparticles on the performance of PVDF flat sheet membranes for vacuum membrane distillation. Desalination, 373, 47, 2015. 57. J.E. Efome, D. Rana, T. Matsuura, C.Q. Lan, Enhanced performance of poly(vinylidene fluoride) nanocomposite membrane by nanofiber coating: A membrane for sustainable desalination through membrane distillation. Water, 89, 39, 2016. 58. M. Baghbanzadeh, D. Rana, C.Q. Lan, T. Matsuura, Effects of hydrophilic silica nanoparticles and backing material in improving the structure and performance of VMD PVDF membranes. Sep. Purif. Technol., 157, 60, 2016. 59. C. Gostoli, G.C. Sarti, Separation of liquid mixtures by membrane distillation. J. Membr. Sci., 41, 211, 1989. 60. M.C. García-Payo, M.A. Izquierdo-Gil, C. Fernández-Pineda, Wetting study of hydrophobic membranes via liquid entry pressure measurements with aqueous alcohol solutions. J. Colloid Interface Sci., 230, 420, 2000. 61. A.C.M. Franken, J.A.M. Nolten, M.H.V. Mulder, D. Bargeman, C.A. Smolders, Wetting criteria for the applicability of membrane distillation. J. Membr. Sci., 33, 315, 1987. 62. J. Zhang, N. Dow, M. Duke, E. Ostarcevic, J. Li, S. Gray, Identification of material and physical features of membrane distillation membranes for high performance desalination. J. Membr. Sci., 349, 295, 2010. 63. F. Laganà, G. Barbieri, E. Drioli, Direct contact membrane distillation: Modelling and concentration experiments. J. Membr. Sci., 166, 1, 2000. 64. M.S. El-Bourawi, Z. Ding, R. Ma, M. Khayet, A framework for better understanding membrane distillation separation process. J. Membr. Sci., 285, 4, 2006. 65. M. Khayet, T. Matsuura, Preparation and characterization of polyvinylidene fluoride membranes for membrane distillation. Ind. Eng. Chem. Res., 40, 5710, 2001. 66. S. Srisurichan, R. Jiraratananon, A.G. Fane, Mass transfer mechanisms and transport resistances in direct contact membrane distillation process. J. Membr. Sci., 277, 186, 2006. 67. Z. Chen, D. Rana, T. Matsuura, Y. Yang, C.Q. Lan, Study on the structure and vacuum membrane distillation performance of PVDF composite membranes: I. Influence of blending. Sep. Purif. Technol., 133, 303, 2014. 68. Z. Chen, D. Rana, T. Matsuura, D. Meng, C.Q. Lan, Study on structure and vacuum membrane distillation performance of PVDF membranes: II. Influence of molecular weight. Chem. Eng. J., 276, 174, 2015.

Membrane Distillation  455 69. J.C. Mierzwa, C.D. Vecitis, J. Carvalho, V. Arieta, M. Verlage, Anion dopant effects on the structure and performance of polyethersulfone membranes. J. Membr. Sci., 421, 91, 2012. 70. Y. Yang, D. Rana, T. Matsuura, S. Zheng, C.Q. Lan, Criteria for the selection of a support material to fabricate coated membranes for a life support device. RSC Adv., 4, 38711, 2014. 71. J. Phattaranawik, R. Jiraratananon, A.G. Fane, Heat transport and membrane distillation coefficients in direct contact membrane distillation. J. Membr. Sci., 212, 177, 2003. 72. F.A. Banat, J. Simandl, Theoretical and experimental-study in membrane distillation. Desalination, 95, 39, 1994. 73. P. Termpiyakul, R. Jiraratananon, S. Srisurichan, Heat and mass transfer characteristics of a direct contact membrane distillation process for desalination. Desalination, 177, 133, 2005. 74. A. Cipollina, M.G. Di Sparti, A. Tamburini, G. Micale, Development of a membrane distillation module for solar energy seawater desalination. Chem. Eng. Res. Des., 90, 2101, 2012. 75. S. Kimura, S. Nakao, S. Shimatani, Transport phenomena in membrane distillation. J. Membr. Sci., 33, 285, 1987. 76. S. Al-Obaidani, E. Curcio, F. Macedonio, G. Di Profio, H. Al-Hinai, E. Drioli, Potential of membrane distillation in seawater desalination: Thermal efficiency, sensitivity study and cost estimation. J. Membr. Sci., 323, 85, 2008. 77. G.L. Liu, C. Zhu, C.S. Cheung, C.W. Leung, Theoretical and experimental studies on air gap membrane distillation. Heat Mass Transfer, 34, 329, 1998. 78. L. Martínez-Díez, F.J. Florido-Díaz, M.I. Vázquez-González, Study of evaporation efficiency in membrane distillation. Desalination, 126, 193, 1999. 79. S. Shirazi, C. Lin, D. Chen, Inorganic fouling of pressure-driven membrane processes—A critical review. Desalination, 250, 236, 2010.

12 Alginate-based Films and Membranes: Preparation, Characterization and Applications Jiwei Li and Jinmei He* School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, China

Abstract

This chapter provides a comprehensive overview of general properties, recent developments, and applications of alginate-based films and membranes. Sodium alginate (SA) is water soluble, nontoxic, biocompatible, biodegradable, reproducible, and can yield coherent films or membranes upon casting or solvent evaporation. However, films made from SA alone have limited applications owing to their brittleness, poor water resistance, weak mechanical performance, as well as lack of specific functional properties. Various strategies, such as addition of cross-linking agents or plasticizers, blending, compositing, and controlling the drying process, have been explored to improve the properties of these films. In this way SA-based films with improved properties and functions have been extensively used in v­ arious fields such as pharmaceutical, medical and packaging applications, and environmental applications for wastewater treatment and separation. Further scientific and technological developments are still needed to develop more multifunctional SA-based films with high feasibility of being used in industrial processes. Keywords:  Alginate, gelation, plasticizing, blending, compositing, drying, application

12.1 Introduction Sodium alginate (SA) is a very important polysaccharide in nature, which is mainly composed of (1,4)-linked β-D-mannuronate (M) and α-L-guluronate

Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 1, (457–490) © 2017 Scrivener Publishing LLC

457

458  Nanostructured Polymer Membranes: Volume 1 COOO

OH

-OOC OH

O

OH

M

O O

OH M

O

HO M

O O

OH O O

HO M

OH -OOC HO O

COOG

G

HO -OOC

OH

O

-OOC

O

O

O

O

COOG

G

OH

OH

OH

O

OH

O

O

O

O HO

COO-

OH

-OOC HO O O HO

O

OH

COOG

M

Figure 12.1  Chemical structures of G-block, M-block, and alternating block in alginate [1].

(G) residues, and can be arranged in the form of homopolymeric sequences (MMM or GGG) or alternating sequences (MGMG) along the polymeric chain (Figure 12.1) [1, 2]. The stiffness of the three blocks decreases in the order GGG > MMM > MG [2, 3]. The physical and chemical properties of alginates strictly depend on their composition and the sequence of guluronic and mannuronic residues in the polymeric chain [3–5]. The molecular weights of commercially available sodium alginate range between 32,000 and 400,000 g/mol [1]. Basically, increasing the molecular weight of alginate can improve the physical properties of resultant gels [6, 7]. However, an alginate solution formed from high molecular weight polymer becomes greatly viscous, which is usually undesirable in processing. Sodium alginate is slowly soluble in cold water, forming viscous, colloidal solution. It is insoluble in alcohol and hydroalcoholic solutions in which alcohol content is greater than 30% by weight [8]. Furthermore, sodium alginate rich in guluronic acid is more soluble in water than mannuronic acid-rich sodium alginate [9]. Aqueous sodium alginate solution can undergo sol-to-gel transformation in the presence of cross-linking cations. Polyvalent cations can replace the binding sites of the residing sodium ions in the chain segments and

Alginate-based Films and Membranes  459

COO-

Ca2+ HO O

HO O

O

O HO

HO -OOC

Ca2+

O

OH O OH

O COOOH O

O

O OH

-OOC

Figure 12.2  Formation of an alginate gel by calcium cations, resulting in “egg-box” calcium linked junctions [10].

produce a cross-linked “egg-box” model (Figure 12.2) [10]. The formation of the junction, and eventually of the hydrogel, upon addition of the divalent ion can be described as a cooperative process with an unfavorable binding of the first ion and a more favorable binding of the following ones (zipper mechanism) [4, 11]. In general, divalent cations (Cd2+, Co2+, Cu2+, Mn2+, Ni2+, Pb2+ and Zn2+) are suitable cross-linking agents but not monovalent cations or Mg2+. Furthermore, the divalent cations are believed to bind preferably to guluronate blocks of the alginate chains, as the structure of the guluronate blocks allows a high degree of coordination of the divalent ions [12–14]. Therefore, the composition of the guluronic segments (molecular weight and M/G ratio) will largely affect the quality of the matrices formed and alginates with high G contents yield stronger gels. Other chemical or physical cross-linking methods, including covalent cross-linking, thermal gelation, and cell cross-linking, have also been developed for alginate gels [5, 10, 15]. And the physicochemical properties of the alginate gels are dependent on the type of cross-linking, crosslinking density, molecular weight and composition of the alginate [16–19].

12.2 Recent Development Sodium alginate can dissolve readily in water to form homogeneous filmforming solutions, which upon casting or solvent evaporation can yield

460  Nanostructured Polymer Membranes: Volume 1 coherent films. Nevertheless, pure SA films normally produce unfavorable final product properties such as brittleness, poor moisture barriers, no specific functional properties, and low mechanical properties, etc., which restrict its widespread applications [20–22]. Furthermore, manufacture of SA films is a complex operation and depends on cross-linking method, drying condition, and plasticization [12, 22, 23]. Accordingly, many strategies have been explored to overcome these drawbacks, such as the addition of cross-linking agents [23–25] or plasticizers [26, 27], blending [28, 29], compositing [19, 30], and controlling the drying process [31, 32].

12.2.1 Cross-linking Since SA films are hydrophilic and soluble matrices, the cross-linking process with polyvalent cations has been used to improve their water barrier properties, mechanical resistance, cohesiveness and rigidity [12, 27]. The cross-linking process and properties of SA films are strongly dependent on the method used for the introduction of the cross-linking ion. Direct mix SA with cross-linking agent and cast as a “mixing” film has been long known. However, gel formation of alginate with cross-linking ions is so instantaneous that it prevents casting to make films in some cases [33–35]. Due to the fast cross-linking process between alginate and cross-linking ions, localized gelling areas are produced, compromising the uniformity and quality of films [35–37]. In order to achieve a homogeneous gel, the internal gelation method has been established as it can control, introduce or release the cross-linking ion from insoluble calcium salt in an acidified medium [4, 38, 39]. However, internal gelation has the disadvantage of being difficult to apply on a large scale [14, 38]. Conversely, external gelation is the most relevant method of alginate cross-linking, from both the industrial and the biotechnological points of view, as it can be simply obtained by dropping alginate-based matrices in a cross-linking solution. Nevertheless, in external gelation, the fast cross-linking process and the high swelling state lead to polymer folding lumps, which give rise to the non-uniformity of films. Some previous studies have been made to improve the external gelation technique [37, 40, 41]. Sikareepaisan et al. combined the benefits of “direct mixing” and “direct immersion” into a two-step cross-linking procedure [37]. The alginate films prepared by the two-step cross-linking procedure had homogeneous surface and improved dimensional stability. In addition, Al-Remawi used sucrose as a crosslinking modifier for the preparation of calcium alginate films via external gelation and the ­prepared film was more uniform without the appearance of localized gelling areas at the surface [40]. Moreover, monovalent cations

Alginate-based Films and Membranes  461 (such as Na+ and K+) have also been used to adjust the external gelling properties of alginate [42, 43]. For example, Goh et al. reported that the added non-gelling Na+ increased the tensile strength of the films crosslinked by Ca2+ but it exerted the opposite effect on those cross-linked by Cu2+ or Zn2+ [42]. In addition, adding sodium chloride to both the gelling bath and polymer solution can increase the homogeneity of Ca-alginate matrices. Recently, Li et al. prepared cross-linked alginate films by external gelation in CaCl2 water solutions using different proportions of ethanol as co-solvent [11]. The addition of ethanol into the CaCl2 water solution was found to improve the visual appearance, thickness, surface homogeneity, and mechanical properties of the films, which can be attributed to the reduced swelling degree of films during the cross-linking process (Figure 12.3). They provided a green, efficient, and simple technique for the preparation of calcium alginate films with excellent surface appearance and mechanical properties for pharmaceutical products. However, divalent cations cross-linking process leads to an increase in the structural cohesion of the films, resulting in a decrease of permeability to gases, water vapor and solutes as well as the rate of release of active agents [18, 44]. In addition to being cross-linked by divalent metal ions, mostly Ca2+ ions, SA can also be cross-linked by other chemical cross-linking [45, 46] and thermal gelation [47, 48]. For example, SA can be cross-linked by glutaraldehyde (GA) with the reaction between the aldehyde groups of GA and hydroxyl groups of SA [15, 49]. In that case, the cross-linking occurs covalently, and it is not affected by pH of medium as in the case of its ionic Step.2 Crosslinking

CaCl2/H2O Higher swelling

Step.1 Casting and drying

Ca-SAE0 Drying

Ca-SAE40 Drying

CaCl2/H2O/C2H5OH Lower swelling Calcium

Sodium alginate

Sodium alginate film

Distilled water

Ethanol/water mixture

Figure 12.3  Schematic of preparation for calcium alginate films and a schematic illustration for the swelling state of the films cross-linked in different systems [11].

462  Nanostructured Polymer Membranes: Volume 1 cross-linking with metal ions [46, 50]. Furthermore, the cross-linking of SA with GA can be controlled much more easily than that with Ca2+ ion, and higher cross-link length can be obtained with GA in comparison to that with Ca2+ ion [51, 52].

12.2.2 Plasticizing It is well known that the formation of biopolymer films requires the addition of plasticizers to overcome their brittleness and improve their processing behavior [53–55]. For SA films, the addition of plasticizer enhances flexibility, decreases brittleness, as well as avoids shrinking during handling and storage [56–58]. In hydrophilic SA films, water can act as a plasticizer that disrupts hydrogen bonds between polymer chains [26, 59]. Thus, physical and barrier properties of SA films usually depend upon the surrounding relative humidity. As effective plasticizers for biopolymer films, polyols such as polyethylene glycol [26, 60], glycerol [61, 62], xylitol [58, 63], sorbitol [26, 58], fructose [58] and sucrose [33, 64] have been used in the SA-based films. Among these plasticizers, glycerol is most usually used and it can impart film flexibility and enhance the sensory, mechanical and barrier properties [58, 65, 66]. However, because polyols possess hydroxyl groups that can form hydrogen bonds with water, the mechanical and barrier properties of plasticized films depend on the moisture content of the storage conditions [54]. Although the main purpose of plasticizers is to modify the mechanical properties and processing behavior, they can also affect the film’s drug permeation and release properties [54, 67]. Moreover, above a critical concentration the plasticizer can exceed the compatibility limit with the biopolymer and phase separation with plasticizer exclusion is usually observed [53]. The work of Avella et al. demonstrated that only at specific mass ratio between the SA and glycerol is it possible to obtain the correct plasticizing effect induced by glycerol [65]. In another study, Silva et al. manufactured composite biofilms of alginate and LM-pectin cross-linked with calcium ions by a two-step contact with Ca2+: initially a low-structured pre-film is formatted which is further cross-linked in a second contact with a more concentrated Ca2+ solution containing plasticizer [27]. The results showed that increasing the glycerol concentration of the cross-linking solution increased film solubility in water, moisture content, volumetric swelling and flexibility and decreased the resistance to tensile stress. And as a compromise between film mechanical resistance and flexibility, keeping low solubility and swelling in water, the use of 5–10% glycerol in the finishing cross-linking step is recommended. Furthermore, small plasticizers have a propensity to

Alginate-based Films and Membranes  463 diffuse to the film surface, especially during extended storage, causing film embrittlement that impacts the mechanical properties [18, 53]. Therefore, an equilibrium between the degree of cross-linking with Ca2+ (necessary to reduce the solubility in water but induces brittleness) and the addition of plasticizers for better workability is important.

12.2.3 Blending It is well known that blending is an effective and convenient method to improve the performance of polymer materials [68, 69]. Accordingly, blending is also being used to improve the properties of SA-based films and membranes. Zhang et al. first prepared ion exchange membranes by coagulating a blend of 6.4 wt% cellulose cuoxam and 3 wt% aqueous alginate solution [70]. It was found that the mechanical properties of alginic acid membranes in water were significantly improved by blending with cellulose cuoxam. Then the same group developed Ca2+ cross-linked cellulose/alginate blend films [71] as well as regenerated cellulose/alginic acid blend membranes [72]. Compared with pure alginate or alginic acid membranes, the mechanical properties of the blend films were significantly improved by introducing cellulose or regenerated cellulose. Bacterial cellulose (BC), a natural biopolymer synthesized in abundance by different strains of bacteria, displays high water content, high wet strength and chemical purity [73, 74]. Due to these special properties, BC has been blended with SA to prepare films for different applications. For instance, Phisalaphong et al. synthesized BS/SA-based nanoporous membranes with chemical stability, high mechanical strength, high water adsorption capacity, and high water vapor transmission rate, which can be used in membrane separation processes [75]. Sodium carboxymethyl cellulose (NACMC) is a watersoluble cellulose derivative and has been used to blend with SA since they are compatible polymers due to the formation of hydrogen bonds [49, 76]. For example, a study by Zhang et al. found that the addition of NACMC and cobalt ions to alginate gel significantly improved the inner and outer surface properties of membranes and the activity of the enzymes [76]. Recently, Ibrahim et al. prepared carboxymethyl cellulose (CMC)/SA blends induced by gamma irradiation [77]. It was found that the gel fraction, mechanical and thermal properties increase with increasing irradiation dose up to 20 kGy, while the swelling of CMC/SA blend films tends to increase with increasing SA content and reduce with increasing irradiation doses. Additionally, ternary blend films composed of pullulan/alginate/ carboxymethyl cellulose blend films have also been developed and Fourier

464  Nanostructured Polymer Membranes: Volume 1 transform infrared spectroscopy (FTIR) results indicated that blending pullulan with alginate and CMC resulted in weaker hydrogen bonds acting on –OH groups compared to the pure pullulan [78]. Carrageenan is water-soluble galactose polymers extracted from red seaweed, which can form excellent gels or films because of a negative charge per disaccharide [79, 80]. With regard to the film-forming properties of SA and carrageenan, SA/carrageenan blend films have been described in the literature [81, 82]. Other polysaccharides, such as cashew tree gum [83], pectin [20, 27, 84], starch [85, 86], konjac glucomannan [87, 88] and chitosan [46, 89, 90], have also been blended with SA to prepare films for different applications. SA is a polyelectrolyte with negative charges on its. Chitosan is the N-deacetylated derivative of chitin, a cationic polysaccharide composed of d-glucosamine and N-acetyl d-glucosamine residues with 1,4-linkages [91, 92]. Thus, SA can interact with positively charged chitosan to form polyelectrolyte complexes (PEC) that have interesting characteristics for film applications [90, 93, 94]. For example, Zhang satisfactorily prepared O-carboxymethylated chitosan/alginate blend membranes by coagulating a mixture of O-carboxymethylated chitosan (CM-chitosan) and alginate in aqueous solution with 5 wt% CaCl2, and then by treating with 1 wt% HCl aqueous solution [95]. The tensile strength in the wet state and thermostability of the blend membranes were significantly superior to that of alginic acid membrane, and cellulose/alginate blend membranes, owing to a strong electrostatic interaction caused by -NH2 groups of CM-chitosan with -COOH groups of alginic acid. Polyelectrolyte layer-by-layer (LbL) deposition methods have also been used to construct chitosan/alginate multilayer films [25, 96, 97]. For example, Yuan et al. constructed a chitosan/alginate multilayer film via LbL self-assembly. The surface composition of the self-assembled multilayer film can be simply tailored through pH control during the assembly process, which can provide a simple novel approach to building high performance biointerfaces through pH control for potential applications in highly sensitive immunosensors [96]. Since polyvinyl alcohol (PVA) hydrogels alone possess high mechanical properties, while alginates advantage are sufficient physical and biological properties, PVA/SA hydrogel blend films have been developed [98, 99]. Gopishetty et al. developed biocompatible stimuli-responsive thin porous hydrogel films via phase separation of a PVA and SA intermolecular complex [100]. The pore diameter of these films can be tuned by changes in pH. Moreover, these membranes are mechanically robust and can be transferred onto the surface of porous substrates. Interpenetrating polymer network (IPN) is a combination of two polymers in network form, at least one of which is synthesized and/or cross-linked in the immediate presence

Alginate-based Films and Membranes  465 of the other [101, 102]. A thin, flexible and smooth SA/PVA-based IPN hydrogel membrane for transdermal delivery of prazosin hydrochloride has been achieved by Kulkarni et al. [103]. The SA/PVA-based IPN membranes extended drug release up to 24 h, while SA and PVA membranes discharged the drug quickly. Polyethylene glycol (PEG) is a biocompatible polymer with excellent biocompatibility and nontoxicity. Immiscible SA/PEG blend films with enhanced thermal stability have been developed [104]. Moreover, SA/PEG films with drug-eluting properties have also been reported [105]. Collagen, a well known fibrillar protein obtained from connective tissues of animals, has a good film-forming property, excellent biodegradability and biocompatibility [106, 107]. Currently, collagen/SA films have been achieved [106, 107]. Gelatin is a single-stranded protein obtained from collagen by hydrolytic degradation [108, 109]. Films of alginate and gelatin, cross-linked with Ca2+, with ciprofloxacin hydrochloride as model drug incorporated in different concentrations, were obtained by a casting/ solvent evaporation method [110]. This SA/gelatin film was potentially useful in drug delivery systems. SA/gelation blend films for cardiac tissue engineering have also been reported [111]. In another study, Liu et al. prepared SA/gelation semi-interpenetrating polymer network (semi-IPN) membranes with good electro-responsive properties [112].

12.2.4 Compositing Compositing is another effective approach for improving the properties of SA-based films or membranes. Cellulose is the most abundant biopolymer on the planet and is a renewable, strong, lightweight, biocompatible, chemically robust material [113, 114]. Several SA-based composite films have been developed by adding cellulose origined reinforcements (fillers) to enhance their performance and applicability [115–117]. For instance, the reduction on water sensitivity of alginate bionanocomposite film has been achieved by incorporation of cellulose nanoparticles (CN) obtained from sulfuric acid hydrolysis into alginate biopolymer [118]. In addition, incorporating nanocrystalline cellulose (NCC) can significantly improve the tensile strength, water vapor permeability, and thermal stability of the SA-based nanocomposite films [119]. Azeredo et al. reported that the cellulose whiskers from different origins (cotton fiber or coconut husk fiber) improved the overall tensile properties (except by elongation) and the water vapor barrier of alginate/ acerola puree films plasticized with corn syrup [116]. Recently, Chen et al. prepared cellulose nanocrystals (CNs)/alginate nanocomposite coatings

466  Nanostructured Polymer Membranes: Volume 1 from aqueous suspensions by electrophoretic deposition (EPD) technique [117]. Electrochemical testing demonstrated that a significant degree of corrosion protection of stainless steel could be achieved when CNs containing alginate coatings were present. Silk fibroin (SF) is a typical protein polymer produced by the silkworm, Bombyx mori [120, 121]. It presents unique tensile strength and elasticity, good thermal stability, hygroscopicity, microbial resistance, biodegradability as well as biocompatibility [121, 122]. Accordingly, the incorporation of SF improved the tensile strength of SA films and also provided a new property to the alginate, that is, resistance to tear [123]. Furthermore, the SF/ SA blend films with self-assembled SF globules can be utilized to modulate structural and mechanical properties of the final material and may be used in designing high performance wound dressings [124]. Recently, the formation of nanocomposites by adding low amounts of clay (1–5%) to the polymer matrix has become a novel practical way for improving the properties of SA-based films [125–128]. Montmorillonite (MMT) is the most commonly used natural clay and has been successfully applied in numerous nanocomposite systems [129, 130]. Its distinctive advantages of high surface area, large aspect ratio (50–1000) and platelet thickness of 1 nm make it suitable for reinforcement purposes [126, 130]. To date, MMT-based clay materials in combination with SA have provided nanocomposite films with improved physical, transport and mechanical properties, which are vital for their pharmaceutical and biomedical uses [19, 131, 132]. For example, Mishra et al. prepared methyl cellulose (MC)/ SA/MMT clay-based bionanocomposite films for wound healing, which were able to significantly inhibit (10 mg/ml) the growth of Enterococcus faecium and Pseudomonas aeruginosa found in the presence of wound [127]. Rectorite (REC) has a layered structure similar to that of montmorillonite and exhibits good mechanical and thermal properties as well as high resistance to ultraviolet rays, etc. [125, 133]. Deng et al. prepared sodium alginate/Na+rectorite-graft-itaconic acid/acrylamide (SA/Na+REC-g-IA/ AM) hydrogel film via solution polymerization [125]. A slower and more continuous release of salicylic acid for SA/Na+REC-g-IA/AM composite hydrogel film was achieved, in comparison with SA film. They also prepared novel superabsorbent composites based on sodium alginate-graftacrylic acid (SA-g-AA) and Na-rectorite (Na-REC), i.e., SA-g-AA/Na-REC [133]. Compared to SA-g-PAA, the absorption, swelling rate, salt-resistant properties, and thermal stability of SA-g-AA/Na-REC composites improved greatly. Graphene and graphene oxide (GO), new types of carbon nanomaterials, have also attracted interest in SA-based films research [134–136].

Alginate-based Films and Membranes  467 Yadav et al. prepared graphene oxide/carboxymethylcellulose/alginate (GO/CMC/Alg) composite with a simple solution mixing evaporation method [136]. Compared to a CMC/Alg blend, the incorporation of 1 wt% graphene oxide improved the tensile strength and Young’s modulus by 40% and 1128%, respectively. In order to improve the miscibility between GO and SA, Nie et al. modified the face of GO [135]. And it was found that the modified GO/SA composite films have better mechanical properties and thermal stability than that of unmodified GO/SA composite films. Recently, bioactive glasses have also been combined with sodium alginate to improve the physical properties and versatility of the biomaterials [137]. It was found that the incorporation of bioactive glass nanoparticles (NBG) into Ga3+ cross-linked alginate films significantly improved their mechanical properties when compared with films fabricated with micronsized bioactive glass particles.

12.2.5 Drying The SA-based films or membranes are commonly fabricated by means of a solvent evaporation technique and gelation process. In order to form a more stable structure that is easier to handle and store, after the gelation process the wet SA films need to be dried. According to the results available from the literature, alginate films are dried using temperature at 25 °C, 35 °C, 40 °C, 50 °C, 60 °C, and 80 °C [11, 138–142]. And the drying time can vary according to the drying temperature, frame volume, solution concentration, and relative humidity of the condition [31, 32, 142]. It is believed that the integrity, thickness, morphology, moisture content, water vapor permeability, drug release properties, crystallinity, and the mechanical properties of alginate films were affected by different drying methods, temperature, and time, matrix content and relative humidity and conditions [32, 142–144]. For example, Ashikin et al. verified that the plasticity of film decreased with an increase in drying temperature from 40 to 60 °C following heat-induced polymer-polymer interaction [142]. A further increase in drying temperature to 80 °C greatly promoted film plasticity through air bubble formation and reduced alginate molecular weight in film. Silva et al. reported that increasing the drying temperature decreased the drying time [32]. And compared to other drying conditions, films oven dried at 60 °C were thinner, had lower moisture content, and were less flexible. Generalized two-dimensional (2D) correlation spectroscopy is a powerful tool for the detailed analysis of various spectroscopic data by external perturbation, e.g., temperature dependent, time dependent, pH dependent, concentration dependent, etc. [145, 146]. Recently, Xiao et al. studied the drying process

468  Nanostructured Polymer Membranes: Volume 1 of SA films by two-dimensional correlation ATR-FTIR spectroscopy [146]. As the drying process continued, the absorbance bands at around 1127 and 1035 cm–1 significantly shifted to lower wavenumbers (1120 and 1027 cm–1, respectively). This result suggested that oxygen atoms at the 2nd and 3rd positions in the pyranose ring might have hydrogen bonded with water or alginate chains. Currently, although a considerable amount of research has been developed on preparation and characterization of SA films, the influence of the drying process is still poorly studied and understood.

12.3 Applications SA-based films or membranes are extensively used in various domains such as in pharmaceutical, medical, packaging and environmental applications.

12.3.1 Pharmaceutical and Medical Applications Because of their intrinsic properties, such as natural abundance, relatively low material and production costs, high water absorption, ion exchange capability, biocompatibility, hemostatic property and nonimmunogenicity [1, 3, 5, 10], alginate-based films have been widely used in pharmaceutical and medical applications. As wound dressings, alginate hydrogels can retain and create a moist environment around the wound to promote wound healing [147, 148]. Also, several reports have suggested that certain alginate dressings can enhance wound healing by stimulating monocytes to produce elevated levels of cytokines [148–150]. Alginates, however, do not have inherent antimicrobial properties which are important when used as wound dressings to provide a barrier against the growth of unwanted microorganisms. Therefore, additives such as sodium ampicillin [98], nitrofurazone [99], simvastatin [151] and ciprofloxacin hydrochloride [152] have been used in alginate film-based wound healing dressings to render them antimicrobial. Furthermore, all natural medicinal extracts, such as essential oils [153], asiaticoside [37], chitosan [154] and aloe [155, 156], have also been applied in alginate films for wound care. Recently, Arockianathan et al. prepared composite films of alginate (AL) and sago starch (SG) impregnated with silver nanoparticles (AgNP) with and without antibiotic gentamicin (G) by solvent casting method [157]. These AL–SG–AgNP composite films were used as wound dressing material in experiments in rats and the results have shown a faster healing pattern in the wounds treated with AL–SG–AgNP and AL–SG–AgNP–G composites compared to untreated control.

Alginate-based Films and Membranes  469 Due to the low strength and elasticity of wound dressing films made from SA alone, SA-based blending hydrogel wound dressing membranes have been developed [98, 152, 158, 159]. For example, Han et al. prepared ciprofloxacin hydrochloride sustained-release alginate/chitosan bilayer composite membrane as novel wound dressings (Figure 12.4) [152]. This novel wound dressing could avoid providing substrate for microbial breeding, reduce the risk of wound infection, decrease the frequency of changing, improve compliance and could work as a drug reservoir for sustained antimicrobial efficacy. Recently, Thu et al. developed an alginate-based bilayer hydrocolloid slow-release wound dressing film that is composed of an upper layer impregnated with a model drug (ibuprofen) and a drugfree lower layer that acts as a rate controlling membrane [158]. The bilayer shows a slow drug release profile in vitro and exhibits accelerated wound healing compared to the controls in a wounded mouse model. Moreover, they used gelatin to enhance the drug dispersion in alginate bilayer film via the formation of crystalline microaggregates [160]. SA-based films can be proposed as a material for drug delivery system because of their ability to enhance the efficacy and paracellular transport as well as prolong the release time of drugs [161–164]. For example, Gilhotra et al. prepared surface cross-linked alginate/chitosan film for ocular drug delivery with a drug release period of 24 h [89]. In addition, SA-based films have been used in other pharmaceutical and medical application fields such as cell culture [165, 166], biosensing [167, 168], and hemodialysis [169]. Nagai first cultured V. convallaria on the alginate gel membrane and the cells can be patterned by lift-off of the calcium alginate membrane [166]. In another study, it was reported that alginate film can support the survival and growth of human retinal pigment epithelium cells and induce the cells

Prevention of bacterial invasion

Controlled water vapor evaporation

Chitosan layer Drug reservoir Calcium alginate layer

Ciprofloxacin Hydrochloride

Drug release Drainage of wound exudates

Figure 12.4  Design of the Alg/Chs bi-layer composite membrane [152].

470  Nanostructured Polymer Membranes: Volume 1 to reorganize into tissue-like structures [170]. Jian et al. incorporated cytochrome c (Cyt c) in three-dimensional porous calcium alginate (CA) and formed the electroactive porous Cyt c/CA composite films [171]. These porous films can provide a favorable microenvironment for the protein to directly transfer electrons with the underlying electrode, showing the possible future application for biosensors and biocatalysis. More recently, Amri et al. characterized the butanediol/alginate ester as a candidate of hemodialysis membrane [169]. The results showed that the increase in 1,4-butanediol molar ratio causes hemodialysis ratio to improve, and causes protein adsorption and platelet adhesion at the membrane surface to decrease. In the case of protein adsorption and platelet adhesion, the membrane with the molar ratio of 1.0 has better hemocompatibility properties.

12.3.2 Packaging Applications Sodium alginate is an ideal material for biodegradable packaging materials as well as edible films and coatings. However, naturally limited mechanical performance, low gas barrier properties and high water sensitivity limit its application [172, 173]. Many researchers have attempted to improve these shortcomings of SA films. For example, Liu et al. prepared a novel chitosan/benzalkonium chloride (C/BC) complex by ionic gelation using tripolyphosphate as a coagulant, and a biocomposite obtained through the adsorption of C−BC complex on microfibrillated cellulose, MFC/C−BC, was then incorporated into a sodium alginate film [174]. The MFC/C−BC biocomposite incorporated sodium alginate film exhibited remarkable antibacterial activity and high tensile strength. The work by Wang et al. suggested that a film with high tensile strength could be obtained by mixing sodium alginate and gelatin, while the inclusion of whey protein isolate to alginate negatively impacts the tensile strength of dry films [175]. Recently, Harper et al. found that the addition of various proteins (gelatin, soy protein isolate (SPI) and heated/unheated whey protein isolate (WPI)), at two different concentration levels (1% and 2%), significantly decreased the force to puncture the “wet” alginate/protein composite films compared to the control alginate film [176]. The increasing demand for safe and minimally processed foodstuff has intensified investigations on active packaging [177–179]. Antimicrobial packaging is one of the most promising concepts of active packaging, which can prevent product deterioration, extending the shelf life and maintaining the safety and sensory quality of several food products [180–182]. However, the non-antibacterial properties of sodium alginate films negatively impact their application for food packaging. Thus, antibacterial agents such as

Alginate-based Films and Membranes  471 sodium dehydroacetate [183], rosemary extract [183], nisin [184], garlic oil [138], ginseng extract [185, 186], potassium sorbate [23, 140], natamycin [94], oregano essential oil [18], grapefruit seed extract or grapefruit essential oil [187], lemongrass oil [86], silver [188–190], silver-montmorillonite nanoparticle [191], and zinc oxide nanoparticles [192] have been incorporated into SA films to prepare antimicrobial films. These antimicrobial agents are released continuously from the packaging film, thus providing a fair protection for food from bacterial contamination. As the activity of antimicrobial films is based on the diffusion of the active agents from the film matrix to food surfaces, the release rate of the active agents from the film to the food surface should be the basic concern, thus helping to design proper antimicrobial films to be used in food preservation [20, 93]. Silva et al. modeled the natamycin release from alginate/ chitosan active films and found that the release kinetics of the antimicrobial is very slow in water, being markedly hindered in the alginate/chitosan composite films, probably due to electrostatic interactions between chitosan and natamycin [93]. Zactiti et al. found that an increase of the crosslinking degree decreased the permeability of potassium sorbate in sodium alginate films [23]. Recently, research showed that alginate/clay nanocomposite films can be a vehicle to release active compounds slower when they are used in contact with food products [193]. Furthermore, the addition of antimicrobial agents may cause changes in the polymeric structure of the film, affecting its mechanical properties and effectiveness as a barrier [180, 181, 189]. For example, Bierhalz et al. investigated the influence of natamycin loading methods on the physical characteristics of alginate active films prepared by three different approaches: the conventional loading method, where natamycin is added directly to the polymeric aqueous film-forming solution; the immersion procedure, by which a previously prepared film is contacted with natamycin solution; and by the supercritical solvent impregnation method, with loading tests performed in CO2 containing natamycin, with and without addition of cosolvent (ethanol, 10% molar) [194]. It was found that the conventional method led to films heterogeneities with high surface roughness, and the immersion technique evinced several disadvantages like low incorporation yields, and negative influence on water vapor permeability and the swelling degree of the film. The supercritical method showed that longer contact times and the use of ethanol as a cosolvent increased the natamycin loading yields and led to homogeneous films, whereas the SSI (CO2+ etoh) process produced visually attractive and translucent films. Although many efforts have been made to improve the mechanical and functional properties of alginate-based coatings or edible films, few

472  Nanostructured Polymer Membranes: Volume 1 studies have focused on optical properties such as color or transparency. Recently, Acevedo et al. designed an alginate film based principally on color changes by using a red, green, blue (RGB) color model [195]. Results showed that edible films made using the optimal surface concentration would not mask microbial contamination and have good physical properties (water vapor transmission and swelling) compared with other surface concentrations.

12.3.3 Environmental Applications Membrane technology is one of the most widely adopted technologies in the area of water reclamation and wastewater treatment [196–198]. Since SA is a biopolymer carrying carboxyl groups capable of forming complexes with metal ions, much attention has been paid to the use of SA films for removing heavy metal ions from water solution [199–201]. For example, humic acid immobilized sodium alginate and hydroxyl ethyl cellulose (GAHA/SA-HEC) blending porous composite membranes have been proven to be an effective and economical adsorbent for Cd(II) ions removal [202]. The prepared GA-HA/SA-HEC can be used more conveniently and it is proposed to take advantage of the high metal ion uptake capacity of the HA, low cost biomaterials of SA and good film-forming property of HEC. Moreover, the removal of Fe(III), Cu(II), and Cd(II) ions from aqueous solutions by alginic acid/cellulose composite membranes has been achieved [203]. Recently, the application of alginates, algae or algae-based materials for sorption of phenolic compounds was reported [204]. The good antifouling properties, simple synthetic process, and low cost of SA are promising features for separation [205–207]. Accordingly, preparation of SA membrane with high separation performance has received the utmost attention in recent years [46, 208–210]. Unfortunately, a very high hydrophilicity of alginate resulting from both of its carboxyl and hydroxyl groups, leads to a significant swelling of the membrane in an aqueous solution, followed by a remarkable decline of selectivity and mechanical strength [15, 208, 211–213]. Various strategies have been explored to improve the properties of alginate films. For example, some studies have used different divalent cations [214], glutaraldehyde [15, 215], and phosphoric acid [216] to cross-link SA-based films for improving its mechanical strength. Additionally, mixed matrix membranes of alginate membranes have been developed [217–219]. For example, Dong et al. prepared SA/PVA blend composite membrane coated on polysulfone hollow fiber ultrafiltration membrane for pervaporation dehydration [220]. Huang et al. prepared a novel SA/CS dense composite membrane for the

Alginate-based Films and Membranes  473 dehydration of isopropanol and ethanol [221]. Suratago et al. developed a bacterial cellulose/alginate nanocomposite membrane for separation of ethanol–water mixtures via impregnation by diffusion of sodium alginate and cross-­linking with calcium chloride solution [222]. The polyion complex composite membranes consisting of chitosan and sodium alginate were prepared for the separation of methanol/methyl tert butyl ether mixtures via pervaporation process [223]. Attempts were also made to incorporate particulate fillers to improve the performance of SA separation membranes [224–226]. For instance, Kariduraganavar et al. developed a novel pervaporation membrane by incorporating a known amount of zeolite into the SA polymer solution for the dehydration of isopropanol, ethyl alcohol, tetrahydrofuran, and 1,4-dioxane [227]. In another study, organophilic bentonite filled SA/carboxymethyl cellulose blend membranes were used for evaporative separation of benzene from its mixtures with cyclohexane [206]. Recently, Zhao et al. synthesized poly(acrylic acid)−Fe3O4 nanoparticles (PAA−Fe3O4) with a size of 50 nm by a one-pot method and then blended it with sodium alginate matrix to prepare an ultrathin hybrid active layer of composite membrane for pervaporation dehydration of ethanol solution [228]. Compared with the membrane blended with Fe3O4 nanoparticles, the membrane blended with PAA−Fe3O4 nanoparticles displayed a much higher separation factor. Furthermore, the hybrid membrane possessed reliable long-term operation stability.

12.4 Conclusion This chapter has demonstrated that with regard to the excellent filmforming properties of alginate, many original and new film and membrane materials have been developed. Furthermore, many strategies, such as the addition of cross-linking agents or plasticizers, blending, compositing, and controlling the drying process, have been explored to improve the properties of these films. SA-based films or membranes have been extensively utilized in various domains such as in pharmaceutical, medical and packaging applications as well as environmental applications for wastewater treatment and separation. However, current SA-based films are still unable to meet all the design parameters and functional properties simultaneously. Moreover, most of the SA-based membranes were prepared by casting methods with limited industrial application. Thus, further scientific and technological developments are still needed to develop more multifunctional SA-based films with prominent commercial applications.

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Index Advanced photolithography, 304 Advantages of the ELM system, 332 Air gap membrane distillation (AGMD), 423–425, 427–428, 430–433, 441, 442, 444–446 Alginate, 1, 14, 15, 457–473 Alumina (Al2O3), 89, 93, 95–96, 105, 111–112, 131 Antibiotics, 297 Antoine equation, 440, 449 Apparent rejection coefficient, 31 Apparent sieving coefficient, 31 Arrhenius equation, 113–114, 127 Arsenic, 203, 221–228 maximum retention capacity of, 226–228 pH effect on retention of, 223–225 polymer effect on retention of, 225–226 Artificial membrane, 28, 29, 41, 42 Asymmetric membrane, 2, 29 Atomic force microscopes (AFMs), 309, 314 Atomic force spectroscopy (AFM), 57 Bacterial cellulose, 14 Bacterial colonization, 313 BAHLM processes, 363 BET surface area, 140, 155, 181 Biological membranes, 7, 28 Biomaterials, 303, 315 Biomedical devices, 293–294, 313

Biomedicine, 294 Blending, 457, 460, 463–464, 469, 472–473 Block copolymers, 295 Blocking pore, 35–37 Brilluoin light scattering (BLS), 308–309 Bubble point, 59–61 Bulging test, 308 Bulk liquid membrane, 2, 12 Burn wound infection, 296 Cake filtration, 35, 37 Cantilevers, 300 Capacitance, 89, 101, 103–104, 108, 120–124, 126 Carbon dioxide permeance, 295, 307 plasticization, 296, 310 Carbon nanotubes (CNTs), 296–297, 312 Catheter, 313, 315–316 Cellulose, 6, 8, 14, 15 Ceramic, 1, 14 Chemical vapor deposition (CVD), 301, 303 initiated CVD (iCVD), 301–302 oxidized CVD (oCVD), 301–302 plasma-enhanced CVD (PECVD), 301–302 vapor phase polymerization (VPP), 301–302

Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 1, (491–500) © 2017 Scrivener Publishing LLC

491

492 Index Chitin-, 14 Chitosan, 14 CO2, 306–307 /N2 selectivity, 295, 311 capture, 311 permeance, 311 separation, 307, 311 CO2 separation, 375 Complete blocking, 35, 36 Concentration method, 214 Concentration polarization, 34, 37, 447–448 Concentrations, 298, 305, 316 Conductive polymer, 296, 316 Contact angle, 314 Contents of emulsion liquid membranes, 336 Conventional filtration, 208 Counter-current flow, defined, 209 Covalent organic frameworks, 149–153 Cross-flow filtration, 208 Crosslinking, 457–463, 471, 473 Crystallinity, 89–90, 94, 102, 111, 116–117, 127, 130 Cyclic voltammetry (CV), 89, 97, 108, 119–120, 127 Darcy’s law, 32 Dead-end filtration, 208 Desalination, 419–424, 450 Desorption rate, 305 Development and scale-up efforts in the liquid membrane, 379 Developments on liquid membranes, 330 Dielectric permittivity, 310 spectroscopy, 310 Differential scanning calorimetry (DSC), 106, 114, 116 Diffusion coefficient, 295, 315 Diffusivity, 305, 307 Dip coating, 300

Direct contact membrane distillation (DCMD), 422–423, 425, 430, 432–433, 442, 444–447 Distillation, 311 DNA, 314 Drug, 463, 465–468, 471, 473 Drug delivery, 294, 313–315 Drying, 457, 460, 465, 473 Elastic modulus, 296, 308–309 Electrical double-layer capacitors ((EDLCs), 89, 101, 103, 108, 119, 121–124, 126–127 Electrode, 90, 94–95, 100–104, 107–109, 120, 123–124, 132, 134 Electron spectroscopy (ES) auger ES, 73 electron energy loss spectroscopy (EELS), 74 Electro-ultrafiltration, 210 Ellipsometry, 309 ELM process, 331 Elongation ultimate ε, 308 Elution method, 213 Emulsion liquid membranes, 333 Emulsions used LMs for heavy metals removal, 342 Energy dispersive X-Ray spectroscopy (EDS), 70 Energy harvesting, 294 Enrichment method, 214 Environment, 203 Environmental pollution, 294 Equilibrium at the surfaces, 305–306 Fick’s law, 305 Filler, 89–95, 97, 99–103, 105–106, 109–123, 127, 129–133 Film layer models, 38 Flotation coupled with microfiltration, 210 Flux, 31, 35–37, 43, 241

Index  493 Focused Ion Beam (FIB), 304, 309 Foreign body response, 313 Forwarded osmosis (FO), 313 Fouling, 34–37, 211–212 Fourier transform infrared spectroscopy (FTIR), 67 attenuated total reflection FTIR, 67–68 Fractional free volume content, 138, 157, 158 Free carrier molecules, 366 Free volume, 296, 306–307 Freezing interfacial polymerization, 296 Fuel cell technologies, 294, 311 Gain output ratio (GOR), 425, 428 Galvanostatic charge-discharge (GCD), 108, 124, 127 Gas flow, 305–306, 311 permeability, 293–294, 296–297, 302–304, 310 permselectivity, 310 purification, 293–294, 310 separation, 296, 299, 304, 307, 310 transport properties, 293–294 Gas adsorption, 3 Gas permeation, 240, 245 Gas transport, 59–60 Gas-liquid equilibrium (permporometry) capillary condensation, 65 Gel layer model, 38, 39 Gene therapy, 314 Glass transition temperature, 294, 296 Glass transition temperature (Tg), 89, 94, 107, 115 Glassy polymers, 294–295, 297, 310–311 Graphene, 298

H2 and Olefin separation, 377 Hagen-Poiseuille’s membrane flux, 32 surface porosity, 33 Healthcare, 294 Heat transfer, 420, 422, 424, 438, 443–449 Henry’s law, 295 Hermia’s model, 35–37 Hollow fiber, 395, 408–409 Hollow fiber liquid membranes, 350 Host-guest chemistry, 175 Hybrid separation method, 209–211 electro-ultrafiltration, 210 flotation coupled with microfiltration, 210 liquid-phase polymer-based retention (LPR), 211 membrane bioreactor (MBR), 209–210 surfactant liquid membrane coupled with liquid-phase polymer-based retention, 211 ultrafiltration coupled to ultrasound, 210 Hydrogel, 313–314 Hydrogen bonding, 157 Hydrogen purification, 311 Hydrophilic, 434–435 Hydrophobic, 420–421, 424, 429, 434–436, 438 Hydrostatic pressure, 312 Hyperbranched polymers, 315 Imidazolium, 396–397, 399, 401–409 Immobilized Liquid Membranes, 334, 345 Implant, 313 Information and communications technology, 294 Inorganic membrane, 41, 42 Inorganic polymer membranes, 14

494 Index Instruction of bulk hybrid liquid membranes, 354 Integrated gasification combined cycle (IGCC), 311 Interfacial polymerization, 296, 312 Intermediate blocking, 35, 37 Ionic conductivity, 89–90, 92, 97, 99–100, 102, 106, 109–114, 116–117, 123, 126–128, 130–133 Ionic liquid, 1, 4, 9, 12, 13 Ionic liquid membranes bulk ionic liquid membranes, 395–397 emulsified ionic liquid membranes, 397–400 gelled ionic liquid membranes, 407–408 polymer ionic liquid inclusion membranes, 404–406 polymeric ionic liquid membranes, 406–607 supported ionic liquid membranes, 401–404 Ionic liquids, 91–92, 129, 175, 263, 268, 270, 272, 393–395, 410–411 Irreversible fouling, 212 Islam model, 305 Knudsen diffusion, 241 Knudsen number, 441 L-3,4-dihydroxyphenylalanine (DOPA), 314 Langmuir-Blodgett (LB) deposition, 301 Laplace equation, 436 Laser, 303–304 Layer-by-layer assembly (LbL), 300–301, 307, 312–314, 316 Linear sweep voltammetry (LSV), 106–107, 117–119 Liquid electrolytes, 4

Liquid membrane chemical nature, 42 configuration definition, 40 mechanisms of transport, 40 Liquid membrane and development, 330 Liquid membrane configurations, 344 Liquid membrane technology, 358 Liquid membranes, 1, 9, 11–13 Liquid membranes based separation processes, 330 Liquid-phase polymer-based retention (LPR), 211, 212–213 removal of inorganic species by, 217–228 theory and fundamental aspects of, 213–216 Liquid-solid equilibrium (thermoporometry), 64 Liquid-vapor equilibrium, 63 Lithography, 304 electron beam, 304 ion beam, 304 soft, 304 Long-term stability, 293–294, 309 Lung cells, 295 Lysine (K) peptide, 314 Manning’s zone distribution model, 216 Mass transfer, 419, 423, 425–426, 434–436, 439–442, 448–449 Mass transport definition, 29 pore-flow model, 33 solution-diffusion model, 33 Materials, composition of hollow liquid membranes, 353 Matrimid, 247 Matrimid , 310 MD module hollow fiber, 427, 430–434 plate and frame, 428, 430–432

Index  495 spiral wound, 430–434 tubular, 424, 432–433 Mechanical properties, 294–296, 298, 307, 309 Medicine, 294, 313 Membrane cellulose acetate, 304, 312 fouling, 304 freestanding nanoscale, 300 multilayer, 300 polyamide thin film composite (TFC), 312 polymeric, 294, 297, 299, 301, 303–304, 306–307, 311 thickness, 298, 306, 313 Membrane bioreactor (MBR), 209–210 Membrane distillation, 1, 13, 14 Membrane material, 351 Membrane modification blending, 46 chemical treatment, 42 coating, 46 combined methods, 47 composite, 46 current research, 47 grafting, 43 initiation technique, 44, 45 physical methods, 46 Membrane performance durability, 421–422, 435, 450 permeability, 422, 435, 437, 440, 450 selectivity, 420, 429, 450 stability, 438, 450 Membrane stability, 392–393, 396, 398–408 Membranes, 203, 238, 240 defined, 204 dense, 245, 246 fundamental aspects, 204–205 hybrid separation method, 209–211 mixed matrix, 239, 258 polymeric, 239 porous, 246 pressure-driven methods, 208–209

supported ionic liquid, 239, 263 transport theory, 205–207 Memsys, 428–429, 450 Metal ions, 203 maximum retention capacity of, 221 pH effect on, 219–221 Metal organic frameworks, 138–149, 166 Microelectromechanical systems (MEMS), 297 Microenvironment, 404 Microfiltration, 2, 6, 7 Microscopic acoustic, 297 Microwave, 303 Mixed matrix, 297 materials, 294 membranes (MMMs), 297 Molecular dynamics simulations (MDS), 307 Molecular recognition, 172 Molecular sieves, 294, 297 Molecular sieving, 241 Molten salt membranes, 348 Nano, 457, 460, 467, 468, 473 Nanocomposite membrane, 435, 450 Nanocomposite polymer electrolytes (NCPEs), 89, 92, 95, 97, 100, 105–106, 113, 117, 120, 123, 127 Nanofiltration, 2 Nanoidentation, 309 Nanomembranes, 1, 9, 10, 294–296, 300–301, 309, 313 biodegradable, 296 giant, 300 multicomposite, 300 nanostrucuterd polymer, 294 polyelectrolyte, 298 polymer, 296–298, 301, 303, 306–307, 309–310, 312, 315–316

496 Index Nanoparticle, 293–294, 297 gold, 297, 300 magnetic, 300 magnetite, 298 Nanopatterning, 303 Nanopores, 302 Oligopeptide, 314 Optical microcavity, 294 Organic membrane, 41, 42 Organic membranes, 1, 2, 7 Organic-inorganic membranes, 174 Osmotic pressure, 312 Outlook for liquid membrane, 378 Oxygen plasma, 299 Packaging, 457, 468 Perfluorocyclobutyl (PFCB), 311 Permeability, 241, 244, 252–256, 260, 261, 264–267, 270–271, 273, 297, 305–307, 310–311 definition, 31, 32 porous membrane, 32 Permeance, 242, 305–307 Permeate gap membrane distillation (PGMD), 425–426, 428, 430, 442, 444, 446 Permeation properties of membranes polyimides, 247, 249, 250 polysulfone, 252, 253 polymer blends, 254, 255–257 mixed matrix, 264–267 supported ionic liquid, 270, 271, 273 matrimid blends, 255 Pertraction factor, 404 pertraction in a multi-membrane hybrid system, 357 pH sensing, 316 pH effect on metal-ion removal, 219–221 on retention of As(V), 223–225 Physical aging, 294, 310 Piezoelectric, 316

Plasma, 302–304, 313 Plasma surface treatment, 2 Plasmonics, 294 Plasticizing, 457, 462 Pollutants, 1, 7 applications for removal of, 216–217 Poly (1-trimethylgermyl-2pentyne), 159 poly(3,4 ethylenedioxythiophene)polystyrene sulfonic acid (PEDOT:PSS), 296 Poly(3-thiophene methyl acetate) (P3TMA), 296 Poly(acrylic acid) (PAA), 295 Poly(allylammine)/sulfonate polystyrene (PAH/PSS), 297 Poly(dialkylacetylenes), 158 Poly(diallyldimethylammonium bromide), 298 Poly(diallyldimethyl-ammonium chloride) (PDMADMAC), 296 Poly(dimethyldiallylammonium chloride) (PDDA), 300 Poly(ethylene oxide)-poly(butyleneterephthalate) (PEO-PBT), 295 Poly(lactic acid) (PLA), 296 Poly(l-trimethylsilyl-1-propyne) (PTMSP), 307 Poly(methylmethacrylate) (PMMA), 297, 308 Poly(tetramethylenesuccinate) (PE44), 296 Poly(vinyl alcohol) (PVA), 89, 91, 105, 111, 113, 117, 128, 132 Poly(vinylacetate) (PVAc), 297 Polyamide (PA), 297, 312 Polyaniline (PANI), 296, 316 Polydimethylsiloxane, 172 Polydimethysiloxane (PDMS), 295, 299, 305–306, 309, 311, 313–314 Polyelectrolyte multilayers (PEM), 300, 314

Index  497 Polyethylene glycol, 15 Polyethylene glycol (PEG), 295, 313–314 Polyethylene oxide, 157, 173 Polyimides, 246 Polymer aging, 169 Polymer blends, 253 immiscible, 254 miscible, 254 Polymers with intrinsic microporosity, 157, 163 Polypropylene (PP), 428, 438–439 Polypyrrole (PPy), 296, 303 Polysaccharide, 296, 314–315 Polystyrene (PS), 296–297, 316 Polysulfone, 251 Polysulfone (PSF), 310 Polytetrafluoroethylene (PTFE), 428, 434, 438–439 Polytrimethylsilylpropyne, 159 Polyurethane (TPU), 296 Polyvinyl alcohol (PVA), 297, 299, 316 Polyvinylidenefluoride (PVDF), 434, 438–439 Porosimetry, 61 mercury porosimetry, 62 Porous aromatic frameworks, 154, 168 Porous membrane, 32 Porous organic cages, 153 Pressure-driven mechanism, 306 Profilometry, 309 Protein adsorption, 313 Protonation, 395 PVDF membranes, 351 Quartz crystal microbalance (QCM), 316 Radius of gyration, 295 Raman spectroscopy, 68 fourier transform (FT-Raman), 69 micro, 69 Refractive index, 310 Regenarative medicine, 313

Removal of inorganic species by LPR arsenic and, 221–228 pH effect on metal-ion removal, 219–221 sorption-desorption process, 221–222 Reverse osmosis (RO), 2–4, 7, 297, 312, 419–422, 424, 435, 450 Reverse osmosis, defined, 208 Reverse selective membranes, 159, 162 Reversible fouling, 212 Rheological properties, 295 Roberson’s upper bound, 242, 245 Rubbery polymers, 293, 295 Sacrificial layer, 299 Salt rejection, 312 Scanning electron microscopes (SEM), 309 Scanning electron microscopy (SEM), 57, 70 Secondary ion mass spectroscopy (SIMS), 78 Selectivity, 242, 249, 252, 264, 270, 274, 297, 304, 306–307, 310–311, 313, 392, 395, 396, 398, 401–404, 407–408 Self-assembled block copolymers, 179 Self-assembled membranes, 178 Self-assembled molecules and nanoparticles, 181 Self-assembled monolayer, 303 Sensors, 293–294, 315 ammonia, 316 bio/chemical, 295 chemical, 296, 315 chemomechanical microcantilever, 315 gas, 315 humidity, 294, 315 photothermal, 297 pressure, 315 strain, 315

498 Index Separation hybrid methods, 41 Separation mechanism, 365 Separation technology, 1, 12 Series resistance model, 37 Silica (SiO2), 89, 93, 95, 101–102, 105–106, 109–110, 112, 133 Silver nanowires, 297 SO2 separation, 376 Sol-gel, 147, 172, 174 Solubility, 295, 305 Solution diffusion mechanism, 240 Solution-diffusion model, 304 Sorption rate, 305 Sorption-desorption process, 221–222 Spin coating, 298–300 Standard blocking, 35, 36 Stents, 313 Strain-induced elastic buckling instability for mechanical measurements (SIEBIMM), 308 Sulfate chitosane, 314 dextrane, 314 Supercapacitors, 95, 103–104, 134–135 Supported ionic liquid membranes, 263, 270, 273, 276 Supported liquid membrane for VOCs separation, 349 Supramolecular membranes, 1, 5, 157 Surface anitmicrobial, 313 antifouling, 313 graft polymerization, 303 patterning, 303 roughness, 303–304 treatments, 293–294, 303 Surface energy, 79, 83 streaming potential, 83 zeta potential, 83

Surface hydrophilicity, 79 capillary elevation balance, 82 captive bubble method, 81 sessile drop method, 80 Surfactant liquid membrane coupled with liquid-phase polymerbased retention, 211 Sweep gas membrane distillation (SGMD), 425–426, 430, 432, 442, 444, 446 Symmetric membrane, 29 Syring pump, 308 Tangential filtration, 208 Temperature polarization, 432–434, 445, 447–449 Tensile stress ultimate σ, 307, 308 The advantages of BHLM, 359 The drawbacks of the BHLM systems, 360 The hydrophile-lipophile balance, 333 The stability of an immobilized liquid membrane, 346 The theory for BHLM, 355 The thermal stability of the membranes, 352 Theoretical aspects of BAHLM, 361 Theoretical aspects of bulk hybrid liquid membranes, 355 Thermal conductivity, 423, 438, 443, 445 Thermal energy, 419, 421–422, 424–425, 428, 431, 450 Thickness characteristics Lc, 305 Tissue engineering, 294 Titania (TiO2), 89, 93, 95, 98–99, 106, 110, 128, 130, 132 Transmission electron microscopes (TEM), 309 Transportation process of the BOHLM system, 356

Index  499 Ultrafiltration, 2–4, 7, 203 coupled to ultrasound, 210 Ultrathin films, 295 Upper bound, 297, 307, 311 UV, 303 Vacuum membrane, 14 Vacuum membrane distillation (VMD), 426–427, 429, 431–433, 435, 442–444, 446–447 Various hydrophobic polymers, 351 Visceral pleural defect, 296 Washing method, 213 waste water treatment, in Gas separation, 364 Water, 457–465, 467–468, 471–473 brackish, 312 desalination, 294, 312 flux, 312–313

purification, 293, 297, 313 seawater, 312 Water-soluble polymers, 7, 203 membrane separation with, 213 structures of, 219 Wound, 466, 468–469 X-ray photoelectron spectroscopy (XPS), 71 Yamamoto homocoupling, 155 Young’s modulus, 295, 298, 307 Zeolites, 6, 8, 10, 297, 312 materials, 294 membranes (MMMs), 297 NaA, 297 silicate-1, 297 Zirconia (ZrO2), 89, 93, 95, 100–101, 105–106, 109, 110, 132 ε-lysine, 313 π–π stacking, 517