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Colloid chemistry of nanodisperse systems. Brief lecture notes: educational manual
 9786010430426

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AL-FARABI KAZAKH NATIONAL UNIVERSITY

A. O. Adilbekova K. B. Musabekov

COLLOID CHEMISTRY OF NANODISPERSE SYSTEMS BRIEF LECTURE NOTES Educational manual

Almaty «Qazaq University» 2017

UDC 544 (075) LBC 24.6 я 73 А 20 Recommended for publication by the decision of the Faculty of Chemistry and Chemical Technology Academic Council, and Editorial and Publishing Council of al-Farabi Kazakh National University (Protocol №2 dated 03.11.2017)

Reviewers Candidate of Chemical sciences, Associate professor Zh.B. Ospanova

А 20

Adilbekova A.O. Colloid chemistry of nanodisperse systems. Brief lecture notes: educational manual / A.O. Adilbekova, K.B. Musabekov. – Almaty: Qazaq university, 2017. – 124 p. ISBN 978-601-04-3042-6 The manual is written on the base of lecture courses of the disciplines «Colloid Chemistry of nanodisperse systems», «Colloidal chemical bases of nanotechnology» and «Bases of nanotechnology». The educational manual represents a set of lecture notes devoted to colloidal chemical fundamentals of nanotechnology and nanochemistry. It is shown the connection between Colloid Science and Nanoscience, historical background of Nanotechnology. The lecture material contains the basic concepts, definitions, classifications regarding to nanoparticles, nanodisperse systems and their properties. The textbook will be in interest of bachelor students of specialties «Chemistry», «Chemical Technology of Organic Substances», «Chemical Technology of Inorganic Substances», master students, PhD students of Faculty of Chemistry and Chemical Technology. Published in authorial release.

UDC 544 (075) LBC 24.6 я 73 ISBN 978-601-04-3042-6

© Adilbekova A.O., Musabekov K.B., 2017 © Al-Farabi KazNU, 2017

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Many of the cells are very tiny, but they are very active; they manufacture various substances; they walk around; they wiggle; and they do all kinds of marvelous things – all on a very small scale. Also, they store information. Consider the possibility that we too can make a thing very small which does what we want – that we can manufacture an object that maneuvers at that level. (From the lecture «There’s Plenty of Room at the Bottom», delivered by Richard P. Feynman at the annual meeting of the American Physical Society at the California Institute of Technology; Pasadena, December 29, 1959).

INTRODUCTION Colloid Science describes the features of substances in colloidal state and studies the coarse and fine disperse materials. According to their sizes the colloid systems occupy the intermediate area between molecules (atoms, ions) and macroscopic objects (macrophases). The size of dispersed particles approximately equals to 1 nm – 100 μm. Among fine disperse systems there are particles with nanosizes (1-100 nm) which possess by unique physicochemical properties due to their small sizes. The uniqueness of nanosized systems is explained by the influence of so called «size effect». Nowadays, the new branch of chemistry concerning the special properties associated with assemblies of atoms or molecules of nanoscale (1-100 nm) is referred as Nanochemistry. Therefore, Colloid Science and Nanochemistry study the fine disperse systems (ultradisperse systems) or nanosystems. The size of the nanoparticles should be in the range of approximately 1 nm to 100 nm at least one of three dimensions. So, one of the parameters of nanosystems is that should be measured in nanometers. Generally, Colloid Chemistry as an area of science has emerged due to fine disperse systems. In second half of the 19th century Italian scientist F. Selmi noticed abnormal properties of some solutions of highly dispersed systems according to the modern classification, sols, i.e. highly dispersed solid particles distributed in 3

a liquid dispersion medium. Later Scottish chemist Thomas Graham named these solutions as colloids, believing that the glue is a typical example of such solutions (in Greek colla means glue, hence Colloid Chemistry). During the next years through in the framework of the colloidal chemical science the researches of ultrafine systems were carried out and examined their application to establish the connection of the fundamental chemistry with Colloid Chemistry. It is advisable to recall the main stages of the development of Colloid Chemistry after the pioneering researches of M. Faraday (1857), when the stable colloidal solutions of highly dispersed gold were obtained at first time. It is precisely these systems with a particle size of several nanometers have been the focus of many physicists and chemists of the second half of the 19th and early 20th centuries. Experimental studies of colloidal solutions led to the discovery of the Colloid Chemistry basics: Brownian motion and diffusion of colloidal particles (A. Einstein), heterogeneous nature of colloid solutions (R. Zsigmondy), sedimentation-equilibrium of highly dispersed systems in the gravity field (J. Perrin), sedimentation in a centrifuge (T. Svedberg), light scattering (J. Rayleigh), coagulation of sols with electrolytes (G. Schulze and B. Hardy). Investigations of the properties of colloidal solutions of various substances allowed to establish the fundamental principle of universality of the colloidal state. Leading scientists in the field of Colloid Chemistry of this period perfectly understood the fundamental importance of a systematic study of highly dispersed systems, however, the experimental methods of that time still did not have an enough resolution to clarify the structure of the ultrafine particles. Perhaps this fact contributed to a bias of research peak towards of disperse systems with larger particles (foam, emulsion, suspension, aerosol) and structured systems (gels, coagulated and crystallized structure). Thus, Colloid Chemistry is a pioneer in the investigation and synthesis of fine and nanosized particles. The prefix nano came with the introduction of the SI system of units. Nano - is 10-9 m, i.e. one billionth part of meter. The prefix nano comes from a Greek word which means «dwarf». If stick to the original meaning of the nano prefix, then a nanoparticle implies a system with a very small particle size. 4

Long before «nanoboom», back in 1950, nanoparticle of metals with diameters less than 100 nm were obtained in the nuclear industry of the USSR. They were used for membrane of diffusion separation of uranium isotopes. At first time the concept of nano was coined by Japanese scientist Norio Taniguchi in 1974 to evaluate the accuracy of the processed surfaces, but for a while this notion was forgotten. It gained new life with the invention of a scanning electron microscope in 1981. At present, sometimes it is forgotten the colloidal chemical origin of nanodisperse systems. There was a kind of assimilation of Colloid Chemistry in nanoterminology. But there is not a significant difference between the nanoparticles and dispersed phase particles of fine disperse systems. They are conventional objects of Colloid Chemistry. The appearance in the second half of the 20th century the highresolution methods for studying the structure of the substance have allowed to carry out the systematic studies of the structure and properties of ultrafine colloidal systems. Over the last 30-35 years in this area it was done a lot, some fundamentally new classes of ultrafine systems were obtained and studied. Fine disperse systems as objects of Colloid Chemistry are widely represented in the various spheres of human activity. Such systems arise during the production cycles, they are part of the manufactured products and they form under natural conditions, in plant world and living organisms. They are accompanied by processes associated with production activities. Nanoparticles are artificially prepared, used separately or are involved in a variety of materials to impart special properties which they did not possess previously. This is one (but not only) difference of nanoparticles from the fine disperse system particles. Most plants have a root system and soil is permeated with pores. The roots form capillary channels which are fine disperse systems. The channels of the root system are moving into the stem and leaves of plants, such capillary channels permeate the whole plant. The rise of water in plants occurs under the action of capillary forces. Simultaneously the pumping water from soil takes place. Cellulose, polysaccharides relate to the highly dispersed plant polymers. 5

Cellulose is the main part of the plant cells. The unit cell of these fibers are microfibrils containing several hundreds of macromolecules ranging in size from 2 to 20 nm The air around us is substantially aerosols with dispersed phase from billions of fine particles. The vehicles generate hundred million tons of waste into the atmosphere per year. Before the age of nanotechnology people have repeatedly used the objects and processes related to the nanoworld without knowing it. For example, the biochemical reactions between the macromolecules, which are the basis of vital activity of the whole biosphere and human as a part of it; fermentation processes in the preparation of wine, beer, cheese and bread in presence of a biocatalysts – enzymes that are nanosized; obtaining of photo images by photochemical reactions with participating of silver nanoparticles. These are «intuitive» examples of nanotechnologies. However, without a proper understanding of the physical chemistry of nanoprocesses and a strong scientific basis the development and use of nanoobjects are impossible. The contemporary history of nanoparadigm emerged after famous lecture of Richard Feynman, Nobel Laureate in Physics: «There's Plenty of Room at the Bottom» at an American Physical Society meeting in 1959. Feynman considered the possibility of direct manipulation of individual atoms as a more powerful form of synthetic chemistry than those used at that time. Nowadays the opportunities of nanotechnology allow: − to create entirely new materials; − to reduce the size of products; − to adjust the properties of materials at atomic and molecular level; − to impart new functions to materials; − to use effectively the biological structures; − to solve problems that cannot be solved within the framework of traditional technologies; − to develop new devices and products; − to reduce labor intensity of production and consumption of materials; − to reduce pollution of the environment; 6

− to create new methods and technologies of air and water purification; Table 1 shows some features of nanosystems in comparison with classical fine disperse systems which are objects of Colloid Chemistry. Table 1 Some distinctive features of nanosystems Fine disperse systems as an object of Colloid Chemistry Preparation in the form of colloids, by means of dispersion methods and another conventional methods Limited range Limited opportunities for studying Fine disperse systems accompany the technological processes in various industries In the flora and fauna, in the soil processes, in the Earth's atmosphere and space

Features of nanosystems New special methods of preparation

A wide range of synthesized nanoparticles reaching ten thousands New modern methods of investigation A special technology of nanosystem preparation, their widespread use Impart the special properties to processes and materials that do not have natural analogues

In recent years, the research of nanosystems ceased to be the prerogative of Сolloid Сhemistry. The number of such systems have increased significantly, the modern methods of research are introduced, the properties of nanosystems are widely studied. Here are some definitions used in the field of nanotechnology: − Nanochemistry studies the synthesis and features of physical and chemical properties of nanoparticles and nanosystems. − Nanoparticle is an aggregate of atoms (molecules) bonded together with a radius between 1 and 100 nm. It typically consists of 10–105 atoms. Nanoparticles form dispersed phase of disperse system and the interface with the surrounding dispersion medium. − Nanostructures are nanoparticle aggregation at presence of connections between them and maintaining the peculiarities of nanoparticles.

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− Nanocomposites are number of nanoparticles characterizing by a significant interaction between them.

− Nanosystems сonsist of nanoparticles and their surrounding medium (gas, liquid, solid body). Colloid Chemistry of nanoparticles is a major area of Colloid Science which considers the specific features of nanoparticles on the basis of the general laws of Colloid Chemistry. The versatility of the nanoparticles is in the fact that the study of their properties and applications occurs at the intersection of Materials Science, Physics, Chemistry, Medicine and other sciences. Nanoparticles have a special electrical, magnetic, catalytic and other properties. They allow to create materials with unique strength and plasticity, help minimize the sizes of computers and other electronic devices, develop a fundamentally new medication etc. The discipline «Colloid chemistry of nanodispersed systems» studies the peculiarities of nanoparticles and nanosystems, their fabrication, properties. The lecture course is based on new contemporary materials accessible through review of scientific papers, internet resources and modern textbooks on Colloid Chemistry (Zimon A.D., Pavlov A.N. «Colloid Chemistry of nanoparticles», Shchukin E.D., Pertsov A.V., Amelina E.A., Zelenev A.Z. «Colloid and Surface Chemistry», Summ B.D. «Bases of Colloid Chemistry», Show D.J. Introduction to Colloid and Surface Chemistry etc.).

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Lecture 1 GENERAL INTRODUCTION TO THE DISCIPLINE «COLLOID CHEMISTRY OF NANODISPERSE SYSTEMS». CONNECTION BETWEEN COLLOID SCIENCE AND NANOCHEMISTRY. NANOTECHNOLOGIES. HISTORY OF NANOTECHNOLOGIES The discipline «Colloid chemistry of nanodisperse systems» represents the interdisciplinary course including fundamentals of Colloid Science and Nanochemistry. This course describes nanosystems and their properties, methods of nanomaterial preparation based on knowledge of previous courses devoted to Colloid Chemistry. The Colloid Chemistry and Nanochemistry have common objects of research – nanoparticles and nanosystems. A new area of science referred to as the Chemistry and Technology of Nanosystems, also known as Nanochemistry, has emerged. Colloid Chemistry studies the substances in the dispersed state or disperse systems. The systems of interest in colloid chemistry include coarse disperse systems (with sizes of 1 μm or larger and with surface area less than 1 m2/g) and fine disperse systems. Fine disperse systems are ultramicroheterogenious colloidal systems with fine particles down to 1 nm in diameter and with surface areas reaching 1000 m2/g (nanosystems). Nanochemistry studies the nanoparticles and nanosystems. From the point of Colloid Science view nanoparticles and nanosystems relate to the fine disperse systems. Today the new field of Colloid Science has emerged which called as Colloid chemistry of nanoparticles and nanodisperse systems. Nanoparticles are characterized by at least one dimension in the nanometer range and they are objects of nanochemistry research. A nanometer (nm) is one billionth of a meter, or 10–9 m. One nanometer is approximately the length equivalent to 10 hydrogen or 5 silicon atoms aligned in a line. Nanoscience is a relatively new area of knowledges which studies the fundamental properties of substances (nanomaterials, nano9

systems) in nanometer scale. Nanoparticle and nanosystems possess with outstanding electrical, optical, magnetic and mechanical properties are rapidly being developed for use in information technology, bioengineering, medicine, industry and energy and environmental applications due to nanotechnologies. The term «nanotechnology» was invented at the first time by Professor Norio Taniguchi at the University of Tokyo in 1974 for estimation of accuracy of processed surfaces. The original definition of N. Taniguchi, translated into English: «Nanotechnology is the production technology to get the extra high accuracy and ultra fine dimensions, i.e. the preciseness and fineness on the order of 1 nm (nanometer), 10-9 meter in length». After invention of scanning tunneling microscope in 1981 the term «nanotechnology» found a new development. Because it has given the opportunity to assemble artificially the Norio Taniguchi nanomaterials and nanodevices from single atoms (1912 –1999) and molecules. was a professor of Tokyo Nanotechnology studies the manipulating University of matter at the atomic and molecular scale. Science. He Nanotechnology literally means any technology coined the term on a nanoscale that has applications in the real nanotechnology world. Nanotechnology encompasses the producin 1974 tion and application of physical, chemical, and biological systems at scales ranging from individual atoms or molecules to submicron dimensions, as well as the integration of the resulting nanostructures into larger systems. Nanotechnology is the art and science of manipulating matter at the nanoscale (down to 1/100,000 the width of a human hair – to imagine brightly at the philistine level) to create new and unique materials and products with enormous potential to change society. The one of contemporary definitions of nanotechnology according to NASA’s definition: «Nanotechnology is the creation of functional materials, devices and systems through control of matter on the nanometer length scale (1-100 nanometers), and exploitation 10

of novel phenomena and properties (physical, chemical, biological, mechanical, electrical...) at that length scale». Although nanotechnology is a rather new area of study, nanomaterials are known to be used for centuries. For example, the Chinese used gold nanoparticles as an inorganic dye to impart red color for their ceramic porcelains more than thousand years ago. Roman glass artifacts contained metal nanoparticles, which provided beautiful colours. In medivial times, nanoparticles were used for decoration of cathedral windows (Fig. 1).

a)

b)

Figure 1. Examples of ancient nanotechnologies a) decoration of Canterbury cathedral, window (Great Britain) b) extraordinary cup was probably made in Rome in the 4th century AD (British Museum)

Michael Faraday was the first who conducted the systematic studies on the properties of metal colloids, in particular, gold (Fig. 2). In 1857, during his lecture at the Royal Society of London, Faraday presented ‘gold reduced in exceedingly fine particles, which becoming diffused, produce a ruby-red fluid … the various preparations of gold, whether ruby, green, violet or blue … consist of that substance in a metallic divided state’. 11

Michael Faraday, (1791-1867)

Figure 2. Gold sol prepared by Faraday

Table 2 summarizes the outline of the historical background relating to nanoparticles (nanotechnology). Table 2 Chronological table of nanotechnology Year

Description

1 1200–1300 BC 290–325 AD 1618 1676

1857

2 Discovery of soluble gold Lycurgus cup First book on colloidal gold Book published on drinkable gold that contains metallic gold in neutral media Publication of a complete treatise on colloidal gold Synthesis of colloidal gold

1902

Surface plasmon resonance (SPR)

1908

Scattering and absorption of electromagnetic fields by a nanosphere Transmission electron microscope (TEM )

1718

1931

12

Country/people, scientists 3 Egypt and China Alexandria or Rome F. Antonii J. von Lowenstern-Kunckel (Germany) Hans Heinrich Helcher M. Faraday (The Royal Institution of Great Britain) R. W. Wood (Johns Hopkins University, USA) G. Mie (University of Gottingen, Germany) M. Knoll and E. Ruska (Technical University of Berlin, Germany)

1 1937

2 Scanning electron microscope (SEM )

1959

Feynman’s Lecture on «There’ s Plenty of Room at the Bottom»

1960 1960

Microelectromechanical systems (MEMS ) Successful oscillation of a laser

1962

The Kubo effect

1965

Moore’ s Law

1969

The Honda–Fujishima effect

1972

Amorphous heterostructure photodiode created with bottom-up process Concept of nanotechnology proposed

1974

3 M. von Ardenne (Forschungs laboratorium fur Elektronenphysik, Germany) R. P. Feynman (California Institute of Technology, Pasadena, CA,USA) I. Igarashi (Toyota Central R&D Labs, Japan) T. H. Maiman (Hughes Research Laboratories, USA) R. Kubo (University of Tokyo, Japan) G. Moore (Fairchild Semiconductor Inc., USA) A. Fujishima and K. Honda (University of Tokyo, Japan) E. Maruyama (Hitachi Co. Ltd., Japan) N. Taniguchi (Tokyo University of Science, Japan) M. Endo (Shinshu University, Japan) D. E. Carlson and C. R. Wronski (RCA, USA) Nobel Prize) K. von Klitzing (University of Wzburg, Germany) G. Binnig and H. Rohrer (IBM Zurich Research Lab., Switzerland) G. Binnig (IBM Zurich Research Lab., Switzerland) S. Chu (Bell Lab., USA)

1976

Carbon nanofiber

1976

Amorphous silicon solar cells

1980

Quantum hall effect

1981

Scanning tunneling microscope (STM ) (Nobel Prize)

1986

Atomic force microscope (AFM )

1986

Three-dimensional space manipulation of atoms demonstrated (Nobel Prize) «Vehicles of creation: the arrival of E.Drexler, Massachusetts the nanotechnological era» Institute of Technology, USA

1986

13

1 1987

1990 1991 1992

2 Gold nanoparticle catalysis

Atoms controlled with scanning tunneling microscope (STM) Carbon nanotubes discovered

3 M. Haruta (Industrial Research Institute of Osaka, Japan) D. M. Eigler (IBM, USA) S. Iijima (NEC Co., Japan)

1995

Japan’s National Project on Ultimate Manipulation of Atoms and Molecules Nano-imprinting

1996

Nano sheets

2001

National Nanotechnology Initiative (NNI) 21st Century Nanotechnology USA Research and Development Act Nanosciences and Europe Nanotechnologies: An action plan

2003 2005

Japan

S. Y. Chou (University of Minnesota, USA) T. Sasaki (National Institute for Research in Inorganic Materials, Japan) USA

As it seen in Table 2 in 1959 Richard Feynman delivered the lecture on «There’ s Plenty of Room at the Bottom». For the first time, he emphasized importance of smallsized products with the use of atoms as building particles. Therefore, this lecture is referred to as the origin of the nanotechnological paradigm. Richard Feynman (1918-1988), American theoretical physicist known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics and the physics of the superfluidity of supercooled liquid helium, as well as in particle physics for which he proposed the parton model. He is regarded as a founder of nanotechnology for his speech on top-down nanotechnology called «There's Plenty of Room at the Bottom»

Some ideas of Richard Feynman were developed by E. Drexler (Massachusetts Institute of Technology, USA ) in his book «Vehicles of creation: the arrival of the nanotechnological era» published in 1986. E. Drexler introduced imagination about molecular robotics 14

based on biological models. It was mentioned on strategy of «bottom-up» in contrary to «topdown» method. In the second half of 1980s to the early 1990s a number of researches and publications on nanotechnology increases extremely, which created a significant influence on further development of nanotechnologies over the world. After 2000s different countries initiated Eric Drexler, is an their National programs (USA, Europe counAmerican engineer tries, Japan, China, South Korea, Russia) on best known development of nanotechnologies for their for popularizing introduction to economy, industry, medicine, the potential of molecular national security and other fields. nanotechnology More and more attention is given to nanotechnology development in Kazakhstan within the framework of state scientific programs. For example, in Kazakh National University the National Nanotechnological Laboratory was opened in 2008 where scientists can carry out their investigations, prepare the qualified specialists, develop «breakthrough» projects in this field. Revision questions: 1. What are objects of study of Colloid Chemistry and Nanochemistry? 2. What are fine disperse systems? 3. Why one can say that Colloid Chemistry and Nanochemistry have common objects of research? 4. What are nanoparticles? 5. What does mean prefix «nano»? 6. Who invented the nanotechnology term for the first time? 7. Give the definition of nanotechnology. 8. Describe the early background of nanotechnology. 9. Describe the history of nanotechnology. 10. Why R.Feynman is regarded as one of founder of nanoscience? 11. How can you characterize the state of nanotechnology in Kazakhstan?

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Lecture 2 NANOPARTICLES AS A NEW OBJECT OF COLLOID CHEMISTRY. FEATURES OF NANOPARTICLES. CLASSIFICATION OF NANOPARTICLES. SPECIFIC AREA OF NANOPARTICLES Nanoparticles are the most fundamental components in the fabrication of a nanomaterials and nanostructures. The size of nanoparticles spans the range between 1 nm and 100 nm. They are far smaller than the world of everyday objects that are described by Newton’s laws of motion and Coulomb’s law of electrostatic interaction, but bigger than an atom or a simple molecule that are governed by quantum mechanics. They have new properties and they are new object of science. Nanoparticle research is currently an area of intense scientific interest due to a wide variety of potential applications in different fields (electronics, biomedicine, optics and others). Today Colloid Chemistry of nanoparticles exists as a new division of Colloid Science which considers features of nanoparticles based on colloidal chemical regularities. From point of view of Colloid Science fine disperse systems consist of disperse phase particles (including nanoparticles) distributed in a dispersion medium. Disperse phase nanoparticles can be distinguished by dimensions on onedimensional, two-dimensional and three-dimensional nanoparticles (Fig. 3).

Figure 3. Scheme of three-dimensional, two-dimensional and one-dimensional nanoparticles

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For three-dimensional particle all three dimensions are relate to the nanoscale less than 100 nm. These nanoparticles have very small curvature radius. Quantum dots (tiny particles of semiconductor material), nanoshells, nanorings, microcapsules, precipitates, colloid solutions (sols) and aerosols, microemulsions, nucleus particles of first type (crystals, droplets, gas bubbles), spherical micelles of surfactants can be regarded as three-dimensional particles. Two-dimensional nanoparticles have two dimensions in nanoscale (other one dimension is extended). They are thin fibers, nanowires, capillaries and porous, cylindric micelles of surfactants and nanotubes. One-dimensional nanoparticles have one dimension in nanoscale (other two dimensions are not confined to the nanoscale). Thin films, coatings, surfactant mono- and polylayers at the interfaces, including Langmuir-Blodgett films, lamellar micelles of surfactants are related to one-dimensional nanoparticles. Nanoparticles can be not continuous. They consist of crystal grains, filaments or include hollows. The assemblage of nanoparticles (clusters, quantum dots) of certain size with the presence of functional connections form the nanostructures or nanostructured systems. They can be considered as bulk nanomaterials. Bulk nanomaterials are materials that are not confined to the nanoscale in any dimension. These materials are characterized by three arbitrary dimensions above 100 nm. With respect to the presence of features at the nanoscale, bulk nanomaterials can contain dispersions of nanoparticles, bundles of nanowires, and nanotubes as well as multinanolayers. It should be noted that there is another classification of nanoparticles in scientific papers by numbers of dimensions not confined to the nanoscale 3-D (bulk), 2-D (quantum well), 1-D (quantum wire), and 0-D (quantum dot) systems. The classification of the dispersed particles by their dimensions determines the colloidal chemical properties of nanoparticles. Dimensions significantly affect the dependence linking the physical parameters: − coefficient of degree at distance in Newton's law of gravitation and the Coulomb electrostatic interaction in the case of 17

three-dimensional space is equal to two, in the case of one-dimensional – to one. − the dependence of heat capacity of solids at constant volume Cv on temperature T at sufficiently low temperatures for onedimensional structures has the form C ∼ f(T2), and for the three-dimensional (crystals of selenium, HF, MgSiO4) has a linear relationship C ∼ f(T). Thus, there is a specific, often nonlinear dependence of nanoparticle properties on dimension. Nanoparticles can be amorphous or crystalline, metallic, ceramic or polymeric, integrated in a surrounding matrix material, deposited on a substrate. Medium surrounding the nanoparticles can be gaseous, air and liquid. Gaseous dispersion medium can be divided on protective gases, reagent gases and gas carriers. Protective gaseous medium (nitrogen, argon) defenses the nanoparticles from air action at technological processes, basically from oxygen. Gas-reagents are used at semiconducting devices for imparting of necessary electrophysical properties. Changes of semiconductor properties are achieved due to diffusion. Gases-carriers are applied for protective coatings (oxygen and others) for processing of surface layer (chloric hydrogen). Water and organic solvents, as well as liquids, such as freon relate to the liquid protective medium. Nanodisperse systems have huge interface and additional excess of surface energy. The area of interfaces mainly determines the properties of disperse systems. For nanoparticle the interface (Sint) reaches a considerable size, and consist of the specific surface area (Ssp), and additional area (Sad) charactering the structure of the particles, that is:

𝑆𝑆𝑖𝑖𝑛𝑛𝑡𝑡 = 𝑆𝑆𝑠𝑠𝑝𝑝 + 𝑆𝑆𝑎𝑎𝑑𝑑.

(1)

Specific surface area is a total area of all the particles, often per 1 kg of the dispersed phase, equal to:

𝑆𝑆𝑠𝑠𝑝𝑝 = 6⁄𝜌𝜌𝑑𝑑,

where d – diameter of particles, ρ – density of particle material. 18

(2)

Considering the dispersion is D = 1/d, specific surface, expressed in terms of dispersion, is:

𝑆𝑆𝑠𝑠𝑝𝑝 = 6𝐷𝐷⁄𝜌𝜌.

(3)

Thus, Ssp is determined in accordance with the formula (2) by diameter of the nanoparticles. One can estimate the surface of the nanoparticles as a dispersed phase of a disperse system according to equation (2). For nanoparticles composed of homogeneous mass (without pore) with diameter 10 nm (10-8 m) and a density of 3000 kg/m3, surface area Ssp calculated through the formula (2) is 2·105 m2/kg. The specific surface area of nanoparticles of low-melting compound nanoparticles without pores is presented in Table 3. Table 3 Specific area of nanoparticles Nanoparticles

Density ρ, 103 kg/m3

Carbide (B4C), 2.3-2.5 Boron nitride (BN) Titanium nitride (TiN), 5.0-5.4 Vanadium oxide (V2O3) Wolfram carbide (WC) 15.8

Specific area, Ssp, m2/kg for nanoparticle of for nanoparticle 100 nm of 10 nm (1.8-1.9)·104 (1.8-1.9)·105 (1.1-12)·104

(1.1-1.2)·105

4·103

4·104

From Table 3 it is seen that the nanoparticle specific surface area can be equal to 105 m2/kg. For comparison, the specific area of sugar particles is 5 m2/kg, powdered sugar about 500 m2/kg, and for flour – the sizes vary between 300 – 700 m2/kg. The increase of density from 2.3 to 15.8·103 kg/m2 and the growth of particle sizes (from 10 to 100 nm) results in a decrease of the specific area of the nanoparticles. Nanoparticle specific area is determined by many factors: the dispersion, shape, structure and phase state of nanoparticle surface, degree of their aggregation. It should be noted that crystalline nanoparticles consist of grains which increase the specific surface area. Specific surface of iron hydroxide (Fe(OH)3) nanoparticles of the 19

spherical form equals to 7.8·104 m2/kg, and needle shape – 1.2·105 m2/kg. The specific area of the oxide film with a two-dimensional structure equals to 7·104 m2/kg. Nanoparticles of framing carbon structure – fullerenes and nanotubes have an additional specific area (Sad). Let us explain the meaning of Sad on example of carbon framing structure, in particular, fullerenes and nanotubes. Fullerenes was named in honour of the architect Fuller, who invented such structures for use in architecture. Fullerenes have a skeleton structure, very similar to soccer ball and consist of a «patch» – pentagons and hexagons. When peaks of these polygons consist of carbon atoms, then the fullerene C60 has the most stable form. Fullerene C60 is the best-known form of carbon nanoparticle consisting of symmetric twenty hexagons. In addition to hexagons, there are pentagons that border only with hexagons. Thus, each hexagon has three common sides with adjacent hexagons and three of them with pentagons (Fig. 4). Fullerenes are allotropic molecular forms of carbon. In solid state the fullerene C60 is a crystal with densely packed cubic structure, forming the nanoparticles. The density of crystalline C60 under normal conditions is 1.69·103 kg/m3, i.e. significantly less than for nanoparticles represented in Table 3. The structure of fullerene is characterized by a cavity inside the peculiar carbon «ball». The surface of fullerenes forms a threedimensional system. Furthermore, additional surface Sad (1) happens due to the two-dimensional internal cavity of fullerenes. Thus, fullerenes combine three-dimensional and two-dimensional surfaces (Fig. 6) due to the faces of hexa- and pentagons i.e. form a three-dimensional-two-dimensional dispersed phase. Consequently, interfacial surface reaches a huge quantity – more than 106 m2/kg, i.e. million square meters (or square kilometre) per one kilogram of product.

Figure 4. Fullerens: C60, C70, C90

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Various modifications of fullerenes whose number of atoms varied from 36 to 540 have been studied (Fig. 4). If some substance is introduced into the fullerene, then its properties will dramatically change and even an insulator turn into a superconductor. In 1991 the Japanese professor Sumio Injima discovered long carbon cylinders known as nanotubes. Nanotubes (Fig. 5) are composed of more than one million carbon atoms and presented as cylinders with a diameter of 0.5 nm and a length of several tens of microns, i.e. not nanosized quantities. Nanotubes can be related to the two-dimensional disperse systems (Fig. 3). The huge interfacial surface area is achieved due to not nanosized length dimension of tubes, about hundreds of thousands m2/kg. On the walls of a nanotube the carbon atoms are located at the peaks of regular hexagons.

Figure 5. Structure of carbon nanotube

In addition, the nanotubes have internal porosity, pore size reaches a few nm. They form an additional surface. Specific area can reach km2 per kg of substance mass as that of fullerenes. The volume of the inner cavity may include other substances, adding the unique properties of the nanotubes. Another allotrope form of carbon nanoparticles called as a graphene. Graphene consists of a single layer of carbon atoms arranged in an hexagonal lattice. Graphene is a twodimensional structure in the form of a sheet (not a spiral) (Fig. 6).

Figure 6. Structure of graphene

21

Thus, fullerenes and nanotubes can form nanostructures with a huge area of the interface. Fullerenes and carbon nanotubes consist of the same crystal structures but differ in a shape. Depending on the packaging of the elementary units and structure sizes fullerenes and carbon nanotubes form the interface and can be considered as disperse systems. Nanoscale particles constitute a special form of so called fractal structure (fractals are plurality with irregularly branched structure). Their elementary cell are clusters (with a diameter not more than 20 nm) woven (Fig. 7) in threads with length which can reach to tens of micrometers, i.e. not nanosized values.

Figure 7. Structure of fractal cluster: model consisting of 106 of the primary particles (clusters)

For this reason, clusters can be referred to two-dimensional disperse systems. Clusters are systems of interconnected atoms or molecules. If these bonds are realized by the Van der Waals forces, the clusters are not capable to form an interface. Increase of the number of molecules in clusters containing ions, their interaction can exceed the Van der Waals forces, that leads to the formation of nanoparticles as disperse systems. Clusters efficiently are formed in a supersaturated vapour when the gas escapes from the nozzle; they can form crystallization centres as an intermediate stage of a microdroplets formation. Fractal cluster is defined as a system with a minimum particle size ro as a structure element and the maximum rf as the size of the 22

cluster. The number of particles N(r) in a bulk with linear dimensions r (ro ≤ r ≤ rf ) equals to: N(r) ~ (r/ro)n,

(4)

where n – parameter called as a fractal dimension. Fractal dimension is an indicator of imperfection of the system. It should satisfy inequality n < a, where a – the size of the space which a fractal cluster forms (a = 2 for surface, a = 3 for a bulk). At the expense of the special structure the cluster has a huge interface, and it can be up to 1 km2 per kilogram, as well as nanotubes. So, one of the main essential feature of nanoparticles is their considerable interfacial area due to both nanoparticle sizes and their structures. Revision questions: 1. What are nanoparticles? 2. Classify the nanoparticles by dimensions. 3. What are peculiarities of nanoparticles? 4. Give the examples of one-dimensional, two-dimensional and three-dimensional nanoparticles. 5. How dimension can influence on a nanoparticle property? 6. To what kind of nanoparticles do nanotubes relate by their dimension? 7. How one can characterize the gaseous media surrounding the nanoparticles? 8. How can you characterize the interfacial area of nanoparticles? 9. How can you describe the specific area of nanoparticles? Give examples. 10. What is the feature of fullerene C60? 11. How do you understand the fractal structure of nanoparticles? Give example. 12. How can you explain the influence of structure on specific area?

23

Lecture 3 METHODS OF PREPARATION OF NANOPARTICLES. TOP-DOWN AND BOTTOM-UP METHODS. TWO STAGE METHODS. SPECIAL METHODS There are two general approaches to nanoparticle preparation: «top-down» and «bottom-up» methods. «Bottom-up» (condensation methods in terms of colloid chemistry) allows obtaining bigger particles from small particles (atoms, molecules, ions). «Top-down» (dispersion) methods are applied for preparation of nanoparticles from bigger particles (Fig. 8). In addition, nanoparticles can be prepared by combination and special methods for nanoparticle synthesis.

Figure 8. Methods of nanoparticle preparation

Dispersion methods implies the preparation of colloids/nanoparticles from bulk materials. At top-down process the mechanical milling are used (colloidal mills, ball mills, dispersers, centrifugal milling, ultrasound milling by means of cavitations of solids and liquids, electrosound dispersion etc). Modern dispersion methods allow to produce solid nanoparticles, nanodroplets with monodispersion distribution of particles by sizes. 24

Condensation methods. These methods allow obtaining the highly dispersed and ultrafine particles, therefore, they are widely used in nanotechnology. Here are given characteristics of the physical and chemical condensation methods. Physical condensation methods. The base of this method for dispersed particles obtaining is the separation of the new phase particles from vapour (at condensation) or from a liquid (at crystallization). The formation of dispersed particles occurs because of a first-order phase transition. A necessary condition for physical condensation is the deviation of an initial homogeneous system (vapour or liquid) from the state at some particular temperature or pressure. For example, changes of temperature and pressure are used in aerosol production. Thus, fog forms in a system containing saturated water vapour at temperature decreasing. In the Wilson chamber the formation of fog takes place at adiabatic extension of air saturated with water vapour; that causes supercooling of a system and formation of water droplets. Similar processes of condensation take place in the case of air containing saturated vapours of substances as phosphorus oxide (V), zinc oxide, sulphur, arsenic, etc. In this case, at temperature decreasing solid particles form a smoke. Method of solvent replacement. This method is widely used for the preparation of solid particles of a colloidal solution (sol). The method implies that a solution of a substance with constant stirring is poured into a liquid in which this substance is practically insoluble. The resulting supersaturation leads to the formation of dispersed particles. Their size is regulated by supersaturation: the larger supersaturation, the smaller particles. By the method of solvent replacement hydrosols of sulphur, phosphorus, rosin and other substances can be obtained because of they are well soluble in organic liquids, but practically insoluble in water. The organic substances used in the method of solvent replacement must have unlimited solubility in water and a sufficiently high vapour pressure at room temperature. For these purposes usually acetone, ethyl alcohol and isopropyl alcohol and similar solvents are used. The solvent replacement method is used to solve many contemporary nanotechnological problems. For example, by means of this method in 1986 at first time a compound with high-temperature conductivity was obtained. 25

Chemical condensation methods. Chemical methods leading to the formation of dispersed particles with a particular composition and size. Chemical reactions with formation of insoluble substances. This method is widely used for the preparation of colloidal solutions (sols or nanosols). It was used by M. Faraday at first time (Table 1, Fig. 2) to synthesize the colloidal gold nanoparticles with a size of 5-20 nm. The principle of the method is a supersaturation which can be achieved as a result of a chemical reaction with the formation of a new phase nucleus of insoluble compound. The reaction of oxidation, reduction, ion exchange and hydrolysis can be used for chemical condensation method. For example, the formation of gold sol: HAuCl4 +2K2C03+H2O = Au(OH)3+2CO2+4KCl 2Au(OH)3+K2CO3 = 2KAuO2+3H2O+CO2 2KAuO2+3HCOH+K2CO3 = 2Au+3HCOOK+KHCO3+H2O Another example is preparation of Prussian blue solution and formation a ferric hydroxide sol: 4FeCl3+3K4[Fe(CN)6] = Fe4[Fe(CN)6]3+12KCl FeCl3+3H2O = Fe(OH)3+3HCl As a result of chemical condensation, the micelle of ferric hydroxide (nanosized particle) form (Fig. 9): {m Fe(OH)3 n Fe3+ 3(n-x)Cl–}3x Cl– The size of particles depends on conditions of formation of disperse systems, a ratio of the rates of simultaneous formation and growth of new phase nuclei. In order to obtain the highly dispersed particles, the rate of nucleation of a new phase must be greater than the rate of their growth. These conditions can be reached when a concentration of solution of one of the components is added to the dilute solution of another component under strong mixing. Thus, 26

regulating the rate of formation and growth of the nucleus of a new phase, one can change purposefully the dispersion of system obtained. At low solubility of the compounds obtained during the reaction, large supersaturations and low particle growth rates can be achieved, that leads to the formation of highly dispersed systems.

Figure 9. Dispersed phase nanoparticle obtained by hydrolysis of ferric hydroxide

Two-stage physical (combination) methods are widely used to produce metal nanoparticles. The first stage is dispersion of metal up to atomic dimensions with formation of a vapour; the second stage consists in the subsequent condensation of these vapour and formation of nanoparticles. There are several ways of this techniques. Method of molecular beams. The initial material is placed in a vacuum chamber with a narrow hole (diaphragm). After heating to sufficiently high temperature, the substance evaporates. Passing through the diaphragm, the evaporated particles form a molecular beam. It is directed to a substrate on the surface of which condensation of the vapour takes place. After that dispersed particles or a thin coating with a thickness about 10 nm are formed. Aerosol method. Metal evaporates in a rarefied atmosphere of an inert gas. At temperature decreasing the vapours condense and 27

dispersed metal particles are formed with sizes from 1-3 to 100 nm. The aerosol method is used to produce metal nanoparticles (iron, cobalt, nickel, copper, silver, gold, aluminium) and their compounds (oxides, nitrides, sulphides). Spray drying method. In the first stage, the solution of a substance (for example, salt) is dispersed to small droplets in a stream of heated gas (air). At average temperatures of gas, the solvent evaporates, and a dispersed powder of salt particles are formed as a process product. At sufficiently high temperatures along with the evaporation of the solvent, thermal decomposition of the salt until oxide powder occurs. Cryochemical synthesis. The main feature of this method is that at first the metal is vaporized in a stream of inert gas (argon or xenon) at intense heating. Then cathode is sputtering by means of electric fracturing or other method. Then the metal vapour condenses on the surface of a substrate at low and ultra-low temperatures in a large excess (by thousands times) of an inert gas. As a result, nanoparticles form on a substrate. Very low temperatures in combination with strong dilution prevent diffusion of nanoparticles along the surface of the substrate, so they keep their small size. Plasma method. In an inert atmosphere (or with an admixture of hydrogen) electric arc is created. Evaporated material is used as an anode. Very high temperature (up to 7000 K) is created in a stream of vapour emanating from an anode. Outside the arc the temperature of the gas is much lower. As the result, a very high supersaturation of the metallic vapour is reached, that leads to metal vapour condensation up to nanoparticles. Sol-gel method. This method is used for separation of fine solid particles or nanoparticles from a colloidal solution (sols) under certain conditions, the dispersed particles stick with each other and form a spatial gel structure. As the result of the fast drying of the gel a powder of fine particles is formed. A method of supercritical drying consists in heating of wet gel in a closed apparatus, so that the pressure and temperature exceed the critical values of temperature and pressure of the liquid, which is in the pores of the gel. As the result, a wet gel is converted into a highly porous airgel with very small (up to 2 nm) pores. Overall, supercritical drying of gels allows to obtain gas nanodispersed particles (pores). 28

Synthesis of nanoparticles in microreactors. For many technological processes (for example, in microelectronics) the particles have a small size distribution and they are monodisperse. To obtain monodisperse nanoparticles with desirable sizes the chemical synthesis is carried out in microreactors or so called nanoreactors. As nanoreactors can be applied the disperse systems: − microemulsions − micellar systems − highly porous substances (for example, zeolites) Microemulsions are transparent liquid (or slightly opalescent) fine disperse and thermodynamically stable systems. Microemulsions are distinguished as direct (oi-in-water) and reverse (water-in-oil) microemulsions. The size of the water droplets can vary within wide limits from 1-3 to 100 nm depending on the preparation terms of microemulsion and nature of surface active stabilizers. A microdroplet in this case can be considered as a microreactor in which a new phase is formed. The size of particle obtained takes the size and the shape of microdroplets. By this method nanoparticles of spherical form is prepared and also filamentary nanoparticles of metals, metal oxides and sparingly soluble salts can be obtained. The method of ultrafine particle preparation in micellar systems is called as a template synthesis. The method has some advantages. Depending on the concentration of surfactant solution, the micelles have different shapes: spherical (at low concentrations) and cylindrical (at high concentrations). Due to this, the simulate synthesis allows obtaining particles of different dimension: three-dimensional particles in spherical micelles, two-dimensional particles (nanofibers). Another advantage of the method is simple purification the particles from the templates (surface-active substances). In addition, synthesis of nanoparticles in nanopores is widely used due to the possibility of obtaining particles of very small dimensions and a narrow pore size distribution. Using this method, for example, gold nanoparticles of about 2 nm in zeolite pores were synthesized. Revision questions: 1. What are «top-down» and «bottom-up» methods? 2. What dispersion methods are used for nanoparticle production?

29

3. The basic principles of physical condensation methods. 4. Describe the chemical condensation method. 5. What is an aerosol method? What particles can be prepared by this method? 6. Characterize the two stages methods. 7. Describe the features of cryochemical synthesis. 8. Condition that influence on the size of particles. 9. Give the examples of nanoreactors. 10. Consider advantages of template synthesis.

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Lecture 4 SIZE EFFECT. INFLUENCE OF SIZE EFFECT ON PHYSICAL AND CHEMICAL PROPERTIES OF NANOPARTICLES The position of the atoms on a surface is geometrically and physically different from their position in a bulk of body. On a surface the atomic reconstruction and another order of atoms arrangement take place. The surface composition does not correspond to the stoichiometric composition of chemical compounds in a bulk. The depth of this discrepancy occupies a several interatomic layers. In addition, there are ledges, cavities and other irregularities for atoms on a surface of crystal grains of nanoparticle, causing additional differences between the surface and bulk properties of nanoparticles. The size effect defines the relationship between surface and bulk properties of nanoparticles, depending on a size of particles. Quantitatively, the size effect can be represented as a ratio between surface of nanoparticles (for spherical particles 𝑆𝑆𝑠𝑠𝑢𝑢𝑟𝑟𝑓𝑓 = 𝜋𝜋𝑟𝑟2) and their volume (𝑉𝑉 = 4/3𝜋𝜋𝑟𝑟3), i.e Ssurf / V value is inversely proportional to the radius or diameter of the particles, i.e. 1/r or 1/d. A ratio of the number of atoms (molecules) on a surface of nanoparticles (Ns) to the number of atoms in a bulk (Nb) can be represented as: 𝛽𝛽 = 𝑁𝑁s/𝑁𝑁b,

(5)

where 𝛽𝛽 determines the fraction of atoms (molecules) surface in relation to their number in a bulk of nanoparticles. In accordance with the formula for spherical nanoparticles, the 𝛽𝛽 is changed as shown in Table 4: Table 4 The number of atoms in a bulk β (fraction of surface atoms), %

106 4

31

105 9

104 19

103 40

102 86

Decreasing the quantities of atoms in a bulk from 106 to 102 (decrease the size of nanoparticles) causes increase of surface atom fraction to 86 %, i.e. decreasing the size of particles dramatically results in increasing of the fraction of surface atoms compared to bulk. The number of atoms in a bulk equalled to 104 corresponds to lower size of nanoparticles (2-3 nm) which can form the dispersed phase particle. In addition, the effect of size can be represented as a fraction of surface layer ΔV in the total volume of particles (V); this fraction for nanoparticles with diameters a and thickness of a surface layer h equals to: .

(6)

The fraction of (surface) atoms β, %

When the thickness of surface layer of h equal to 3-4 atoms of (0.5-1.5 nm), and at average size of nanoparticles of 10-20 nm, then about 50 % of the total mass of nanoparticles locate on the surface layer. In general, the dependence of the fraction of surface atoms on bulk atoms is shown in Fig. 10.

The number of atoms in the bulk Nb

Figure 10. Dependence of the fraction of surface atoms (β) and the diameter of the nanoparticles (a) on the number of atoms in a bulk (Nb)

32

Size effect is determined by quantitative ratio and qualitative feature of the surface of nanoparticles. Properties of atoms on surface differ from the properties of atoms in a bulk. Surface atoms connect the nanoparticles with the surrounding media. The atoms in a bulk are surrounded by same type of atoms. Due to the formation of unsaturated bonds on surface of nanoparticles the atomic recombination can occur and there is a different arrangement of atoms compared to bulk atoms. Atoms and molecules of a medium can be adsorbed on the surface of nanoparticles. So, the first distinctive feature of nanoparticles is the size effect. The peculiarity of fine disperse particles and nanoparticles is that their properties depend on both chemical composition of material and the particle size. The size effects observed in disperse systems can be divided into two large groups. 1. Effects associated with the curvature of the surface of a liquid or gaseous dispersed particle: a) the dependence of surface tension of liquid on a radius of the droplet or gas bubble in the liquid; b) the dependence of the saturated vapour pressure (pr) on the radius and the sign of curvature (convex or concave) of liquid surface at the interface with the gas; c) the dependence of the capillary pressure (pc) on the radius of liquid surface; 2. Changes of physical and chemical properties due to small size of dispersed particles: a) crystal structure and the degree of symmetry of the crystal lattice; b) thermodynamic parameters: heat capacity, melting point (crystallization), Debye temperature; c) mechanical properties: strength and plasticity; d) magnetic and electrical properties; e) chemical properties, e.g. catalytic activity. Size effects are especially significant for fine disperse systems, i.e. in the range of nanosize. This is one of the reasons for the great importance of nanosystems in contemporary science and high technologies. 33

It should be noted that size effects of nanoparticles have a quantum nature in some cases. Let us consider several size effects characteristic for fine disperse particles. Thermodynamic properties. Decrease of melting point. Decrease of solid dispersed particle size leads to a gradual decrease of melting point (Tm) of various substances. For metal particles the size effect manifests strongly in the range of sizes d < 50 nm. For example, for gold particles the temperature difference ΔT = Tm,0 – Tm,d is noticeable at d < 20 nm (Tm,0, Tm,d – melting point of macroscopic sample and fine disperse sample, respectively) (Fig. 11). In the interval d < 5 nm, the decrease of the melting point is hundreds of degrees; at d = 2 nm we have ΔT = 1000 degrees. A significant decrease of the melting point was also observed for lead, bismuth, tin, and indium.

Figure 11. Dependence of the melting temperature (Tm.d) of nanodispersed particles of gold on their size (d); the dotted line shows the change of the melting temperature of a macroscopic sample of gold

Thus, the size of nanoparticles can be regarded as a kind of temperature analogue. Therefore, when using the phase rule for fine disperse systems, it is necessary to enter an additional variable (degree of freedom). Decrease of melting point of fine disperse particles is clearly explained by the following simplified model of the melting process. It is assumed that when the heat input, the crystal begins to melt when the amplitude (δ0) of the oscillations of atoms in the crystal lattice is commensurable with the interatomic distance (b). The 34

melting point of macroscopic crystals corresponds at this scheme to the condition δ0 ~ b. For atoms on the surface of crystal, the amplitude of the oscillations is δd > δ0. Reducing the particle size leads to an increase in the relative fraction of surface atoms in a given particle. At d = 3 nm, this fraction already reaches 50%. As a result, condition δd ≥ b is achieved at a lower melting point. For the theoretical description of size effects for melting the following equations are used: ,

(7)

,

(8)

,

(9)

where Sm.0, Sm.d – the entropy of melting of a macroscopic and highly dispersed sample of the substance respectively, J/(mole·K); Hm,0, Hm,d – the enthalpy of melting of a macroscopic and highly dispersed sample respectively, J/mole; R – universal gas constant; 2d0 – the minimum particle size at which there are no structural differences between a solid and a liquid: at the size d0 the differences in the enthalpy and entropy of the macroscopic phase and the dispersed particle equal to zero. Consider relatively large interval, when d/d0 >> 1, i.e. d > 5-10 nm. In this case: . Then at expanding the exponent of equation (9) in a row, it is sufficient to consider only the first two terms of the series: .

35

(10)

Equation (10) has a deep physical meaning. It shows that the change of the thermodynamic parameter (in this case, the melting enthalpy) is inversely proportional to the size of the dispersed particle d: ΔH ~ 1/d. Such nature of size effects causes by the fact that the quantity 1/d = S/V, i.e. represents the ratio of a surface area of a particle S to its volume V; thus, the thermodynamic parameter is determined by the particle dispersion. Therefore, the smaller the size d, the greater is the fraction of surface atoms in a dispersed particle. This principle of changing the thermodynamic properties of dispersed particles (inversely proportional to their size d or radius r) is quite general in Colloid Chemistry. An example of such connection is given by Thomson (Kelvin) law for saturated vapour pressure above the curved surface of a liquid and the Gibbs-Ostwald equation. The nature of the dependence of the melting temperature of dispersed particles on their size is essentially related to the aggregate state of the dispersion medium, which surrounds the particle. The effect of lowering the melting point with decreasing particle size is observed for aerosol disperse systems when the dispersion medium is a gas. In many technological processes the dispersed particles are distributed in thin pores of another solid phase. In such systems, both positive size effect and negative one are observed. For example, indium nanoparticles melt in the pores inside the iron at lower temperature (Tm,d < Tm,0). Inside the aluminium matrix there is an effect opposite in sign: indium particles melt at higher temperatures than macroscopic samples (Tm,d > Tm,0). Supercooling of droplets. The size of dispersed particles has a strong influence on the crystallization process also. Highly dispersed droplets of different liquids can keep a liquid state for a long time at strong supercooling. For liquid metals, the maximum supercooling (ΔTmax) of drops with a diameter of 2-100 μm is shown in Table 5. Table 5 Metal Mercury Gallium Tin Lead Silver

Temperature, oC 77 76 118 80 227

Metal Gold Copper Manganese Nickel Platinum

36

Temperature, oC 230 236 308 309 370

A significant supercooling is observed for droplets of other substances, for example water, organic liquids and melts of salts (degrees) (Table 6): Table 6 Compound Carbon tetrachloride Octane Decane Hexadecane Lithium fluoride Sodium fluoride Sodium chloride Potassium bromide Cesium bromide

Temperature, °C 50 30 29 14 232 281 168 168 161

Heat capacity. The size effect of metallic nanoparticles on a heat capacity is manifested sharply at very low temperatures. For example, for palladium clusters the temperature dependence of the heat capacity deviates from the corresponding dependence c=f(T) for macroscopic samples. These differences increase as the number of palladium atoms in the cluster decreases. Thus, the reduction of the size of dispersed particles affects their thermal properties similarly to a decrease of temperature. Mechanical properties. The main mechanical property of solids - their strength. A quantitative characteristic of the strength of materials is the limiting stress (P), at which the sample (rod) ruptures under uniaxial tension. The limiting stress is defined by the equation pc = fc/ S,

(11)

where fc is the tensile force causing the rupture; S – cross-sectional area of the sample. For samples with a sufficiently large cross section (diameter d > 0,1 mm) the strength depends only on the chemical nature of the substance. But for thin samples with diameter corresponding to the size of dispersed particles, obvious size effect is shown. It consists in the fact that the limiting stress increases as the diameter (d) of the rods, fibers, particles, etc., decreases. The dependence of the strength 37

(pc) of glass threads on their diameter (d) illustrates this size effect (Table 7): Table 7 d, mkm pc, N/m2

22.0 220

16.0 1070

12.5 1460

8.0 2070

2.5 5600

The data presented show that the strength of thin threads increases sharply with their diameter decreasing. The size dependence pc = f(d) in this region is defined by the following equation pc = pc,min + β/d,

(12)

where β is a constant, which depends on the chemical nature of the material, N / m. Thus, as for thermodynamic properties, there is an inverse relationship between the mechanical property of a dispersed particle (thread) and its size. The strength of nanotubes is about 10 times higher than that of steel, although their ductility is 6 times smaller. The main physical reason for increasing strength at decreasing the diameter of the sample and the grain size is as follows. The limiting stress depends not only on the chemical nature of the substance, but also on the various structural defects. The so called linear defects, edge and screw dislocations, have the strongest effect on mechanical properties. The probability of finding in the sample of a dangerous defect responsible for its destruction; the lower probability, the smaller the diameter of the sample. The average distance between dislocations in crystals is about 10 nm. Therefore, strength growth is characteristic for samples of smaller sizes. This state is the basis for the production of high-strength composite materials. They are based on very thin fibers (filiform) which have increased strength. Another remarkable mechanical property of nanoscale samples of some substances is their great plasticity. It was established that composite copper-niobium and copper-chromium wires with a diameter of 15-20 Nm with very strong cooling (in liquid helium) is deformed plastically, limiting elongation reaches 10 %. Massive sam38

ples of the same composition under these conditions are very fragile. Another example of the high strength of nanosized samples is carbon nanotubes. Magnetic properties. The size effect favors the significant decrease of the Curie point which shows the transition temperature from the ferromagnetic state to the paramagnetic state. For iron, cobalt, nickel nanoparticles smaller than 10 nm, the Curie point is lower by hundreds of degrees than for macroscopic samples. The measurement of the magnetization of such type small particles is accompanied with a large spread of data (as at the measurement of the melting point). Therefore, Mossbauer spectroscopy is used to determine the Curie point of the nanoparticles. Magnetic phase transitions also proceed in a larger (20-50 nm) dispersed particles. However, to carry out such transitions, the additional energy (in comparison with nanoparticles) needs. The source of this energy is internal stresses and defects that arise at obtaining the particles by means of topochemical reactions. Palladium clusters have clear magnetic size effects. Macroscopic samples of palladium exhibit paramagnetic capability and their magnetic susceptibility is almost independent on temperature up to the temperature of liquid helium. The magnetic susceptibility of giant palladium clusters by several times smaller than that of a macroscopic metal, but they remain paramagnetic. With a significant reduction of cluster sizes (up to tens of palladium atoms), they become diamagnetic. The size of dispersed particles also effects on the coercive field (Hc, A/m). This value characterizes the demagnetizing field at which the residual magnetization of the sample equals to zero. A coercive field represents an important characteristic of ferromagnetic materials. At Hc < 100 A/m, materials are considered as magnetically soft, at H > 100 A/m – magnetically rigid. The coercive field of iron nanoclusters (d < 4 nm) is almost zero. Such low values of the coercive field is caused by thermal fluctuations. For small particles, their energy is sufficient to destroy the ordered orientation of the magnetic domains and to transfer the crystal to the paramagnetic state. At room temperature the value of the coercive field of iron is maximal for crystals of 20-25 nm. Therefore, nanocrystalline ferromagnets can be using to obtain storage devices with large memory. 39

Another important magnetic property of highly dispersed particles is a low magnetic anisotropy. In polycrystalline materials the minimum magnetic anisotropy of ferromagnets is observed at grain sizes of 10-20 nm. Losses due to remagnetization of such nanomaterials are small. The nanodispersed magnetized particles with a diameter of about 10 nm are used for the preparation of ferromagnetic liquids. Ferromagnetic liquids are colloidal solutions in which the dispersed phase is nanomagnetic particles and the dispersion medium is a liquid, for example water or kerosene. When an external magnetic field is applied, the nanoparticles begin to move and activate the surrounding liquid. The prospect of industrial use of this effect is very high (for example, for cooling powerful transformers in electrical engineering and for magnetic enrichment of ores). Catalytic properties. Nanodispersed solid particles of metals and metal oxides have a high catalytic activity which allows producing various chemical reactions at relatively low temperatures and pressures. Let us give some examples illustrating the catalytic properties of highly dispersed particles. 1. Gold nanoparticles with size of 3-5 nm have highly specific catalytic activity. The catalytic activity is connected with the transition of the crystalline structure of gold from the face-centered cubic in larger particles to the icosahedral structure of nanoparticles. The most important characteristics of these nanocatalysts (activity, selectivity, temperature) depend on the substrate material to which they are applied. In addition, even traces of moisture are very much affected. Nanosized gold particles effectively catalyze the oxidation of carbon monoxide at low (-70 oC) temperatures. However, they have a very high selectivity at reduction of nitrogen oxides at room temperature, if the gold particles are deposited on the surface of the alumina oxide. 2. A highly dispersed catalyst based on non-stoichiometric cerium oxide significantly decreases the temperature of sulfur oxide (IV) reduction under the influence of carbon monoxide. In addition, this catalyst has a higher stability than conventional cerium oxide catalysts with respect to the poisoning of the water vapor and carbon dioxide involved in the reaction 40

3. In addition to ultrafine particles, thin catalyst layers with thickness of several nanometers have a high catalytic activity. For example, for MoSi nanolayers the high catalytic activity is found under hydrodesulfurization reaction. The high degree of ordering of nanostructure of crystalline MoSi favors the maximum catalytic activity. The number of MoSi monolayers adjusts the selectivity of the catalyst. Catalytic selectivity can also be controlled by changing the structure of MoSi nanoparticles, that opens up a very promising direction in the heterogeneous catalysis of organic reactions. Also, the extremely small nanoparticles (clusters) have the high catalytic activity. For example, giant palladium clusters consisting of several hundred (about 600) atoms exhibit the high selective catalytic activity in many organic reactions (oxidation with oxygen and nitrobenzene, carbonylation, hydrogenation, acetylation of carbonyl compounds, isomerization of olefins, etc.). At the same time, effective catalysis proceeds in relatively mild conditions – at temperatures and pressures, much less than at using conventional industrial catalysts. The high catalytic activity of metal clusters is explained by their structure. As a rule, their surface has a rather complex structure: it has very sharp protrusions and flat areas. Local positive charges in a cluster are distributing differently. As a result, there are strong Lewis centers (at the vertices of the polyhedral structure) on the surface of the cluster cation, weaker Lewis centers (on the edges), and positively charged metal atoms with the lowest electrophilicity (on the faces). Biological properties. The high chemical activity of nanoparticles allows using them in many biological and medical processes. One of the most urgent areas is protection against certain types of biological weapons. For example, heat-resistant anthrax pathogens are effectively destroyed on air at room temperature by spraying nanoparticles of magnesium oxide. The smaller the size of the nanoparticles, the stronger effect. Revision questions: 1. What is the size effect? 2. Which is affected by the decrease in particle size in the size effect? 3. Why conceptions of a surface and bulk have conditional character for nanoparticles?

41

4. Explain the change in a fraction of surface atoms (β) and diameter of nanoparticles (a), depending on the number of atoms in a bulk (Nb). 5. What properties depend on size effect? Give the examples. Give examples of influence of size effect on thermodynamic properties. 6. How does the size effect influence on mechanical properties? 7. Describe the size effect in relation of catalytic properties. 8. What do you know about magnetic properties of nanoparticles? 9. Give the examples of size effect with respect to biological properties.

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Lecture 5 SURFACE PROPERTIES OF NANOPARTICLES. ADSORPTION Adsorption is an increase of substance concentration at the interface (phases F1 and F2), (Fig. 12). The substances adsorbed on the surface are called as adsorbate (Fig. 12, 1), surface adsorbing the substances is an adsorbent (Fig. 12, 2). Adsorption is a spontaneous process. Excessive adsorption or Gibbs adsorption (Г) shows the change of adsorbate concentration as a result of adsorption. According to the fundamental Gibbs adsorption equation a spontaneous decrease of the surface tension σ occurs due to the chemical potential µ change; for a one-component system this dependence is defined as follows: Г = − 𝑑𝑑𝜎𝜎/ 𝑑𝑑μ.

(13)

The chemical potential is a factor of the intensity of any physicochemical process. The process proceeds spontaneously from a larger chemical potential to a smaller one. The sign «-» shows the surface tension decreasing as a result of adsorption. The value of excess Gibbs adsorption is determined by the mass of the adsorbent the per unit of the interface (kg/m2). Due to the great specific surface area and the excess surface energy, nanoparticles can significantly adsorb.

Figure 12. Scheme of the adsorption process: a) initial moment; b) the equilibrium state

43

Adsorption on the surface nanoparticles has its own characteristics which are determined by the size effect, the crystal structure of nanoparticle surface and the predominance of chemical adsorption over the physical one. The size effect is determined by the number of adsorbed particles n0 with density on the crystalline surfaces which equals to: na = n0 ( - Ea / KT),

(14)

where, Ea is the adsorption energy or adsorption potential, n0 density of atoms on the nanoparticle surface. The adsorption potential characterizes the reversible isothermal work of the adsorption forces and equals to: Ea = RT ln p/ps,

(15)

where, p is the pressure of an adsorbent over the surface of the adsorbent, ps is the pressure corresponding to the condensation of the adsorbent to form a liquid on the surface of the adsorbent. So, for nanoparticle of the diamond, the adsorption potential, depending on size of nanoparticles, changes as shown in Table 8: Table 8 Particle size, nm Adsorption potential, J/g

130 141.2

8 384

The particle size reduces by 16.2 times and the adsorption potential increases by 27 times. The increase of the adsorption potential with decreasing of particle size is explained by the size effect, by the increase of the fraction of atoms on the surface as compared to their amount in the bulk. The adsorption energy is directly related to the interaction of adsorbate with the adsorbent. This relationship can be observed at the adsorption of hydrogen on nanoparticles of titanium alloys (TiNi, TiFe, TiPb). Adsorption of hydrogen on the surface of titanium alloys is characterized by the following parameters (Table 9): 44

Table 9 Alloys

Adsorption energy, EW

Binding energy, EW

TiNi TiFe TiPb

-0.88 -0.75 -0.34

-3.25 -3.12 -2.72

Adsorption thickness, nm 0.046 0.048 0.052

layer

As follows from the data presented in Table 9 the adsorption of hydrogen is preferable for TiNi, TiFe compared to TiPb, where the adsorption energy is proportional to the binding energy between the adsorbent and the adsorbent. The presence of such bond characterizes chemical adsorption (chemisorptions). For nanoparticles, as for all adsorbents, is characterized by physical and chemical adsorption. Physical adsorption proceeds due to intermolecular interaction. When physical adsorption is realized the atoms are mobile and can «roll» over the surface at a distance exceeding the interatomic distances. A distinctive feature of chemical adsorption, or chemisorption, is the irreversibility of the process and a significant thermal effect. Chemisorption on the surface of nanoparticles is determined by the size effect (Lecture 4), the structural properties of nanoparticle, including the crystalline nature. The ability of nanomaterial to realize chemisorption is much higher than that for macroscopic bodies. In addition to the size effect, adsorption is affected by the structural and electronic properties of nanoparticles. These factors are closely interrelated and their effect on adsorption is sometimes difficult to assess. In addition, there are substantial number of unsaturated atoms on nanoparticle surface which capable to form chemical bonds. The nature of the unusual adsorption capacity of nanomaterials can be considered on the example of some chemical interactions. Research the adsorption of CO on Pb, Cu, Ni nanoparticle deposited on graphite showed that the transition of metal-dielectric is accompanied by the enhancement of the CO-Me bond. Nanoparticles of Pb and Ni (with size less than 3 nm) exhibit high reactivity with respect to O2 and H2S. In this range of particle sizes, the metal properties of palladium and nickel are weakened. Direct evidence of the electronic effect of adsorption was obtained by adsorption of gases on single crystals of metal, coated by monolayer palladium. The electronic structure of palladium promotes 45

the formation of chemisorption bonds. Adsorption depends on the structure of the metallic nanoparticles. Effective chemisorption is observed when oxygen is adsorbed on platinum nanocrystals consisting of grains and transition sites. Oxygen molecules are firmly associated with the transition sites due to chemisorption between the crystal faces. The ratio between the transition sites is determined by the nanocrystal sizes. If the size is less than 10 nm, the transition sites predominate, and when the size increases to 60 nm, the faces predominate. The dependence of adsorption on the concentration is characterized by adsorption isotherms which show the increase of adsorption with increasing of adsorbate concentration. Adsorption can be monomolecular when monolayer of adsorbate is formed on the surface of adsorbent, and adsorption is polymolecular one, when two or more monolayer of adsorbed substances is formed on adsorbent surface. Adsorption of polymers on nanoscale objects has a number of features (Fig. 13). At horizontal adsorption (Fig. 13, a), a strong bondings of polymer with the surface of nanoparticle dispersed phase is formed, and the adsorbed layer has molecular dimensions. At vertical adsorption (Fig. 13, b), a bulk layer weakly interacted with the adsorbent is formed, and the polymer molecules can easily lose their bond with the adsorbent (desorption process). «Loops» (Fig. 13, c) correspond to an intermediate variant of the adsorption interaction.

Figure 13. Various forms of the adsorption state of polymers on nanoscale surfaces: a – horizontal, b – vertical, c – formation of «loops»

Absorption processes can change the nanoparticle surface properties: impart them hydrophobicity or hydrophilicity, electrical conductivity, magnetic susceptibility, etc. Thus, due to the adsorption of water vapor, the surface property of diamond particles can vary from hydrophobic modification to 46

hydrophilic one. The presence of a large number of fine pores (their size can vary from 0.4 to 0.7 nm) imparts to nanoporous materials a considerable adsorption capacity. The specific surface of such adsorbents can reach 1.10 m/kg. High activity of nanoporous materials is not explained by only a simple increase of the specific surface. The relatively large number of atoms on the surface and on the near-surface layers with high curvature can radically change the adsorption capabilities of the catalyst. By means of nanoparticles the ion-exchange adsorption can occur. Ion-exchange adsorption is a reversible process of equivalent exchange by ions between electrolyte solutions and solids called as an ionite. Cation-exchange and anion-exchange adsorption are distinguished. The scheme of cation-exchange adsorption can be given as follows: Cat1 +[…]- + Cat2+ Cat2+[…]- + Cat1 + ion exchanger

solution

ion exchanger

solution

Intensive ion-exchange adsorption is observed on crystalline of nanoparticles because the process occurs not only on the crystal surface, but also in the crystal cavity. The relative large intergranular distance (0.7-0.8 nm) makes it possible to carry out the cation exchange on a large surface. Adsorption processes are the basis of any catalysis, that is, changes (mainly increase) of the rate of chemical reactions. Catalysis occurs through several stages: the adsorption of reacting substances by nanoparticles, the migration of adsorbed molecules into the interior of the catalyst, the formation and desorption of reaction products. Mainly, adsorption processes determine the rate of catalysis. The process of catalytic oxidation from CO to CO2 has importance. In turn, the oxidation process is determined by the rate of adsorption. The high catalytic activity is characteristic of nanoparticles. Electronic and geometric effects are explained by small particle sizes. The number of atoms in composition of nanoparticles is small, the distance between the energy levels is: δ = Ef /N, 47

(16)

where Ef is the Fermi energy (the energy value below that all states of the system obey to the quantum statistics, in one state there cannot be more than one particle); N is the number of atoms on the surface of particles, which characterizes the size effect. Using formula (16), it is possible to estimate the size of nanoparticles used as catalysts. Nanoparticles are used as an adsorbent for water purification. The adsorption capacity of nanoparticles of aluminium for heavy metals is 10-80 mg per gram of adsorbent, for halogens 10-45 mg per gram of absorbent, for organic substances up to 150 mg per gram of absorbent, for oil emulsions up to 250 mg per gram of absorbent. The degree of extraction of microorganisms is 80-99 %, and degree of extraction of petroleum products is 99 %. Adsorption by means of nanoparticles significantly improves the efficiency of catalysis. Most catalysts are nanosystems. In case of heterogeneous catalysis, the active substance is mounted on a carrier in the form of nanoparticles to increase the specific surface area of the catalyst and the efficiency of catalysis. In homogeneous catalysis, the molecules of the active substance are nanosized. They can be sorbed on the nanoparticles and desorbed slowly. Revision question: 1. What is the adsorption? 2. How does the adsorption process proceed? 3. Describe the Gibbs equation. 4. Give the examples of size effect with respect to adsorption. 5. What is the difference between chemisorption and physical adsorption? 6. What peculiarities of nanoparticle adsorption do you know? 7. What do monomolecular and polymolecular adsorption mean? 8. What do you know about polymer adsorption? 9. What are features of ion exchange by nanoparticles? 10. Why does the nanoparticle size influence on catalytical processes?

48

Lecture 6 SURFACE PROPERTIES OF NANOPARTICLES. ADHESION OF NANOPARTICLES. ADHESION AND WETTING OF NANODROPLETS Adhesion (cohesion) is the interaction between heterogeneous condensed phases at their molecular contact. An adhesive is a substance that sticks to surface or a substrate. There is an adhesion of particles, liquids, films and structured (elastically viscous plastic) bodies. In the case of nanosystems, an adhesion of nanoparticles and films based on them will be considered. The schemes of components and assemblies of radio engineering devices (conductors, insulators, capacitors, transistors, connecting elements) are formed as a result of adhesion of nanoparticles which provides miniaturization of these devices. Adhesion in some cases contributes to the stabilization of nanoparticles. Due to the high surface activity, the nanoparticles cannot exist separately; adhesion maintains the nanoparticles on the surface. Adhesion of nanoparticles, in contrast to the adhesion of macroparticles, mainly depends on the method of formation of adhesion interaction, namely: – application of nanoparticles with subsequent pressing of their layers (Fig. 14, a, b); – formation of layers on the surface and inside the solid during the formation of the nanoparticles (Fig. 14, c, d); In the first case the adhesion is formed by the applying of nanoparticles or their formation due to condensation, and in the second case by the molecular build-up inside of substrate, when nanoparticles arise as a result of external action (pressure, friction, etc.).

Figure 14. The location of nanoparticles at adhesion

49

Under formation of a film from nanoparticles, the nucleation begins initially by condensation of steam or crystallization from the solution. Then, the growth of nucleus without their number increasing until the formation of nanoparticles. Subsequently, they can aggregate by coagulation of particles, or coalescence leading to the fusion of droplets and by crystallization. Moreover, there is an autohesion of particles, that is, the interaction between particles that occurs spontaneously due to excess surface energy, diffusion and other processes, as well as under the influence of external factors (pressure, electromagnetic field, etc.) It is possible the using of nanoparticles on the surface of larger particles, the adherent layer shields their surface and gives them the specific properties. So, nanoparticles of polystyrene screens the surface of the mica. Because of pressing, and under influence of high temperature, particles are sintered, when the phase boundary between the nanoparticles disappears, and the adhered film turns into a monolith. The disappearance of phase contacts can occur due to a chemical reaction, in particular, by the reduction of oxide films (Ni, Cu, Al). For NiO2, this process proceeds as follows: 2NiO2= 2NiO + O2. A strong fixing of the nanoparticles and the formation of film on the surface due to adhesion causes the stabilization of the adhesivesubstrate system. Thus, particles of crystalline iron settled by means of low-temperature plasma of heptane were fixed at a pressure of 50 MPa. The height of the adhered film was 50 nm. The structure formed inside the body due to the tribo-effect and mechanic activation of nanoparticles has the form of flakes of irregular shape. When polishing the surface of a film of silicon oxide from nanoparticles they are pressed into a rough surface. Adhesion of nanoparticles depends on the conditions of their contact with the surface and further interaction. The metals introduced into the polymeric nanoparticles contribute to the formation of a chemical bond between metal atoms and the individual functional groups of polymers and enhance the adhesive strength. The adherent layer of nanoparticles (Al2O3, TiO2) 50

increases strength of the products by four times. Nanoparticles of metals as an adhesives change the electrical properties from the dielectric to the conductors, consequently, the electrical resistance decreases from 1011-1014 to 10-5-10-4 Ohm·m. This decreasing causes a change of the electrostatic interaction between the substrate and the adhesive. At the formation of composites based on copper nanoparticles (20-30 nm) and formaldehyde and epoxy resines the systems with properties close to metallic have been obtained. The composites have electrical conductivity as well as composites based on polyesters filled with nickel nanoparticles. They can be used as electrodes for carrying out an electrochemical reaction, including reactions in alkaline media. The above examples indicate a change not only the surface properties of the adhered films, but also their bulk characteristics. The features of the adhesion of nanoscale particles also are characterized by size effect and by excess of surface energy. The value of adhesion is determined by the adhesion force characterizing the bonding between the particle (adhesive) and the surface (substrate). The size effect can be expressed by the relative force of adhesion Frel ad – a ratio of the adhesion force of one nanoparticle Fad per unit of volume V: .

(17)

The adhesion force Fad of nanoparticles is negligible and is estimated by tens of nanonewtons, that is, 10-9 N. The force of adhesion of nanoparticle with diameter of 10 nm and macroparticle with diameter of 10 µm or 10·103 nm is presented in Table 10. Table 10 Diameter of particles, nm Force of adhesion, nN Relative adhesion force, Frel ad

10 10-9 1015

10·103 107 102

The adhesion force of nanosize particles by 16 orders smaller than the non-nanosized particles with diameter of 10 μm. However, 51

the relative force of adhesion, taking into account the volume for nanoparticles, by 13 orders higher than for non-nanosized particles. The area of contact of nanoparticle with the surface is calculated in fractions of nm. Thus, due to the size effect the relative force of nanoparticle adhesion is significant and is determined by an additional excess of surface energy. There are difficulties of experimental measurements of nanoparticle adhesion, especially autohesion, that is, interactions between nanoparticles. There is the influence of micro-roughness on the atomic level on the adhesion. For this reason, the adhesion of nanoparticles is determined by calculation, modeling, and indirectly in comparison with friction force. The force of adhesion Fa can be calculated from the theory of JKR (Johnson-Kendall-Roberts) according to the equation: ,

(18)

where A is the Hamaker constant, r is the radius of nanoparticles, and h is the distance between nanoparticles and the surface (under the conditions of intermolecular interaction due to van der Waals forces, this distance can be equal to 0.165 nm). The Hamaker constant is obtained by calculation, for example, its value for nanoparticles with a diameter of about 100 nm varies within the range (40-70) 1021 J. In addition, the Hamaker constant is defined experimentally. According to formula (18), the adhesion force can be approximately estimated and for a nanoparticle with a radius of 10 nm; it equals to 3.10-9 N, i.е. it is insignificant. The theory of JKR does not take into account the deformation in the area of contact of particles with the surface. The theory of Derjaguin-Muller-Toporov (DMT) taking into account the deformation of the contact area of particles with the surface and the adhesion force is: Fad = k·π·r·W,

(19)

where W – the equilibrium work of the adhesion, W = σ + σs – σint = =2(σs*σint)0.5 where, σ, σs is the surface tension of the substrate and 52

the nanoparticles respectively, σint – the interfacial tension of the substrate-nanoparticle; k – coefficient of proportionality, k – according to theory of JKR equals to 3/2, and according to the theory of DMT – 2, r is the radius of nanoparticle. The adhesion force of nanoparticles can be calculated from the value of the frictional force. If the adhesion force (Fadh) prevents the detachment of particles and is directed perpendicularly to the surface of the substrate, then the friction force acts in parallel and counteracts the mutual movement of the particles along the surface. The relationship between the frictional force and the adhesion force of individual particles can be represented by the following formula: Ffr = µ·Fadh,

(20)

where µ – is the coefficient of external friction. By analogy with friction of powders, the value of the coefficient of external friction can be obtained by measuring the breakaway force Fbr that goes to overcome friction. The friction force is equal and opposite to the Fbr force causing the motion of nanoparticle tangentially to the surface. Considering the equality of the breakaway force and the frictional force, instead of equation (20), we can write: Fbr = µ·Fadh.

(21)

Thus, it is not possible to determine experimentally the adhesion of nanoscale particles. But the force of adhesion can be defined according to (21) through the force and the coefficient of external friction which can be experimentally measured. The coefficient of external friction with respect to the nanosized particles on the surface of ceramic products varies between 0.150.25. The breakaway force Fbr is measured using an indexer model with probe tip radius corresponds to the size of the nanoparticles (Fig. 15). The indexer can move along a plane, concave or convex surface. To determine the relief of nanoparticle layer and nanofilm, it is possible to move the index on a surface of different shapes, then, on it, the friction force is defined as follows: Fconcfr ˃ Fplfr ˃ Fconvfr , 53

where Fconcfr , Fplfr, Fconxfr is the frictional force on a concave, plane and convex surface, in other words, under identical conditions, the frictional force on a concave surface is maximal, but on the convex surface is minimum, but the majority of studies have been carried out with reference to a plane surface.

Figure 15. Scheme of the indexer: 1 − probe with a rounding 2 radius r; 3 − adhesion on surfaces; a − plane, b − concave, c − convex

In addition, adhesion was determined for the ratio of not a single probe of the indexer, but their sets. Hundreds of villi were used. Each villus has a radius of curvature corresponding to the radius of nanoparticle. The force of adhesion of the considerable number of villi was determined experimentally, that allowed modeling the adhesion of a multitude of nanoparticles. Then, the force of adhesion was calculated for a single villus. The idea of using of villi for modeling the measurement of adhesion of nanoparticles is borrowed from geckos, capable of moving along vertical walls and even over the ceiling. Studies using a scanning tunneling microscope showed that the surface of each of the fingers of the animal is covered with a midge of villi with a length of 30 to 130 microns. «Microvilli» have heads («hats») with a diameter of tens of nm. The force of adhesion per vortex is measured in nanonewton. It is not possible to measure directly this tiny force of adhesion. Modeling the adhesion of individual villi by measuring the adhesion force of the number of villi one can evaluate the adhesion force with respect to one villi, that is, essentially to separate nanoparicle. The adhesion force depends on the ability of the surface to be wetted, i.e. hydrophilicity or hydrophobicity. 54

The number of villi were pressed to the aluminum surface modified: hydrophilic (contact angle equals to 42.5°) and hydrophobic (contact angle equals to 145) °. The adhesion depending on the force of the pressure changes as shown in Table 11: Table 11 Clamping Force, µN (10-6N) The force of adhesion on the hydrophilic surface, nN The force of adhesion on the hydrophobic surface, nN

1000 20

2000 35

4000 47

5

7

16

As one would expect, hydrophilization of the surface contributes to the enhancement of the adhesion of nanoparticles. As with nonnanosized particles, adhesion of nanoscale particles depends on the properties of the particle surface, the pressing force of particles and other factors (humidity, temperature, etc.). So, the data obtained by various ways (calculation, modeling, as well as by measuring the adhesion interaction of a large number of nanoobjects) showed that the nanoparticle adhesion strength is negligible and amounts to tens of nN (nano-newtons, i.e 10-9 N). Nevertheless, a strong holding of nanoparticles on the surface favors the adhesion. Adhesion depends on the mass of the particles. For this reason, it is necessary to evaluate the adhesion interaction with respect to a unit volume of adherent particles. The relative adhesion force of the particle with a diameter of 10 μm taking into account its mass equals to 103 nN. The relative adhesion force of nanoparticle of 10 nm equals to 10-15 nN, that is, by thirteen orders of magnitude higher than the absolute adhesion force. For this reason, nanosized particles have significant adhesion and are firmly held on different surfaces. Nanodroplet adhesion and wetting. Adhesion and wetting of nanodroplets on surfaces are specific and differ from similar processes for macroscopic drops. Wetting as a result of the adhesion of droplets is determined by the area of their contact with the surface, it is quantitatively characterized by the contact angle. The line along which all three interfaces (L, G or L1 and S) intersect is referred to as the line of wetting; a closed line of wetting 55

forms the perimeter of wetting. The angle between the liquid-gas and solid-liquid interfaces, θ, is referred to as the contact angle (Fig. 16). With respect to water droplets the surfaces are called as hydrophilic (when the contact angle of wetting varies from 0o to 90o) and hydrophobic (when the contact angle of wetting exceeds 90°). For other liquids the oleophilic (0° 0, the linear tension decreases, and it increases for æ < 0. According to theoretical estimation, the equilibrium state of the drops is achieved at a relatively low value of the linear tension of 10-11-10-10N. The contact angles θn formed by drops of sufficiently small size should be calculated with the inclusion of linear tension in the Young equation: cos θ 1 = cos θ o + æ /R σlg ,

(25)

where, θ o is the equilibrium contact angle of nonnanosized droplets larger than 100 nm, æ -linear tension; σlg surface tension of the liquid at the boundary with the gas; r is the radius of the base of the drop. In accordance with the formula (25) cos θn ˃ cos θ0

Figure 19. Structure of nanodroplets: 1 – equilibrium film, 2 – bulk, 3 – transition region

58

(26)

According to the condition (26), on the lyophilic (hydrophilic) surface, when 0 ≤ cos θo ≤ 1 increases, which means that on the hydrophilic surface the wetting with nanosized drops increases. On lyophobic (hydrophobic) surfaces, when -1 ≤ cos θ o < 1 and cos θ 1 ˃ cos θ o wetting decreases for analogous conditions. Thus, on lyophobic surfaces with a decrease of the droplet size the wetting deteriorates (the contact angle increases, see Fig. 17), and on the lyophilic surfaces, on the contrary, decrease of droplet size improves the wetting (contact angle decreases). In addition to contact wetting (by separate drops), nanoparticles are characterized by immersion wetting (over the surface of the solid body). The quantitative determination of immersion wetting is difficult. For this reason, the thermal effect of immersion wetting is measured. It is determined by the enthalpy (ΔH). The dependence of the contact angle value of the wetting of silica powder particles with a diameter of 12 nm on the enthalpy of immersion wetting is shown below in Table 12. Table 12 cos θ (Δ H), J/m2

141 -0.09

118 +0.04

94 +0.02

72 -0.03

With increasing of contact angle, i.e. with decreasing of the cosine of the contact angle, the wetting process transfer from exothermic to endothermic (the negative value is getting positive), and then again it becomes exothermic process. So, like the adhesion of nanoparticles, wetting of nanodroplets of solid surfaces is significantly different from similar processes for macroobjects. A certain regularity of wetting change is observed depending on the size of droplets, as well as the possibility of changing the properties (hydrophobization or hydrophilization) surfaces contacting with nanodroplets. Revision questions: 1. What is the adhesion? 2. What is the autohesion? Describe the external factors that affect of this process.

59

3. 4. 5. 6. 7. 8.

What is the connection between adhesion and size effect? How is the force of adhesion defined? Describe the theory of Johnson-Kendall-Roberts. Describe the equation of Derjaguin-Muller-Toporov. What are hydrophilic and hydrophobic surfaces? Describe the Young equation for nonnanosized droplets and for nanosized droplets. 9. What is the linear tension æ for nanodroplets? 10. What is the immersion wetting?

60

Lecture 7 MOLECULAR KINETIC PROPERTIES OF NANOPARTICLES. BROWNIAN MOTION. DIFFUSION. OSMOSIS Brownian motion. Nanoparticles as fine disperse systems are characterized by the molecular kinetic phenomena such as Brownian motion, diffusion and osmosis. These phenomena are caused by molecular nature of gas and liquid dispersion media and kinetic energy of molecules. Brownian motion is a continuous, chaotic motion of high dispersed particles (including nanoparticles), suspended in gases or liquids because of action of the molecules of the dispersion medium. The nature of this phenomenon: molecules of dispersion medium act (impact) due of kinetic energy on particles of dispersion phase (nanoparticles). These impacts (1020 per second) unequal by energy lead to a resultant force which cause the particles motion. The true way of particle is changing constantly, and it cannot be traced. Therefore, the shift of the particles which characterize the change of the particle coordinate in time is determined. Mean displacement xavr., i.e. the average value of the coordinate x of the particle for particular time τ is equal to: (27) where, R – universal gas constant; T – absolute temperature; NA – Avagadro’s number; τ – time; η – viscosity of disperse media; r – radius of particles of dispersed phase. According to formula (27) value

characterize the molecular-

kinetic properties of dispersion medium. In addition to the translational motion of the particles, the rotational motion is also observed for nanoparticles. It is associated with the geometric inhomogeneity 61

of nanoparticles, especially for crystalline and two-dimensional nanoparticles. The mean square displacement of translational Brownian motion is equal to: Δxavr.2 = 2Dτ, (28) where, D is the diffusion coefficient of translational motion. The standard angular displacement: Δφavr.2 = 2Dr.τ,

(29)

where, Dr. is the diffusion coefficient of rotational motion. Diffusion coefficient D and Dr. characterize the translational and rotational Brownian motion in liquid and gas medium. Most nanoparticles have an irregular shape: protrusions and recesses on their surface contribute to the formation of rotational Brownian motion. It is determined by the angle θ which mean-square value is equal: θ2 = 2kTBτ,

(30)

where θ – the mean square angle of the rotational Brownian motion with respect to the selected axis in time; k – Boltzmann constant; B – rotational mobility, which depends on the mass of the nanoparticle and equals to: B = I/πρd3,

(31)

where I – the number of revolutions of the rotary motion; ρ – density of nanoparticles; d – diameter of nanoparticles. According to calculations, the average number of revolutions (up to 28) due to Brownian motion is achieved within 5 minutes. In rough nanoparticles, in addition to their linear movement, the flicker effect is observed, which is inaccessible to the human eye. In a gaseous medium the nanoparticles can have the same translational energy as molecules, which allows to estimate the speed of Brownian motion ( ): (32) 62

,

(33)

where m – mass of nanoparticles, 3kT – average energy of particles in a gaseous medium. Thus, Brownian motion of the nanoparticles is not differed from the fine disperse systems, studied in Colloid Chemistry. But there are some features of nanoparticle Brownian motion considering the size, shape and dimensions of nanoparticles. Diffusion. Diffusion is a spontaneous process of substance distribution (ions, atoms, molecules, highly dispersed particles, including nanoparticles) from the area of higher concentration to an area of lower concentrations. Unlike Brownian motion, the diffusion is manifested not only in gas and liquids, but also in solids. For a stationary process with one-way diffusion (in one direction), the mass of the diffused substance is characterized by Fick's law: , where D – diffusion coefficient;

(34)

– concentration gradient (dc –

the difference between the concentrations at a distance x from the object;

– for this reason, the minus sign is placed in the

right-hand side of the equation (since diffusion is always positive); S – area flow section; τ − diffusion time; the diffusion coefficient (m2/s) quantitatively determines the diffusion efficiency when the gradient of concentration, area of flow section and time are equal to one. The diffusion coefficient of a nanoparticle depends on the medium in which the diffusion takes place. The approximate value of the diffusion coefficient: in gases 10-4 (at normal temperature and pressure), in liquids – 10-9 and in solids – less than 10-14 m2/s. In liquids, the diffusion coefficient depends on the type of diffusion (Table 13): Table 13 Type of diffusion Diffusion coefficient, m2/s

ionic 10-8

63

molecular 10-9

nanoparticles 10-10

The larger the diffusion coefficient, the more effective the diffusion (see equation 34). In liquids and solids, the diffused particles (molecules, ions, atoms) transition occurs from one stable state to another. The temperature dependence of the diffusion coefficient is determined by the Arrhenius equation as follows: (35) where D0 – an apparent diffusion coefficient, numerically equal to the diffusion coefficient at an infinitely large temperature; Ea – the activation energy of the diffusion process; k – Boltzmann constant; T – temperature. The diffusion of nanoparticles manifests in different cases: in the case of nanoparticles as dispersed phase of dispersion medium; at alloying, that is, the injection of some substances into nanoparticles for giving of certain properties; diffusion of nanoparticles inside the crystalline nanoparticles. The types of diffusion of crystalline nanoparticles at the interface with different surfaces are shown in Fig. 18.

Figure 18. Scheme of different types of diffusion applied to crystalline nanoparticles (c – crystal grain of a nanoparticle, s – surface contacting with nanoparticles); types of diffusion: 1 – bulk, 2 – grain boundary, 3 – surface, 4 – boundary

Four different types of diffusion for crystalline nanoparticles are present: 1 – bulk, in the bulk of a crystal of a polycrystalline body; 2 – grain-boundary between grains of crystals; 3 – surface over the crystal surface; 4 – boundary diffusion an the interface of the crystal 64

k and the contacting surface s. Surface diffusion occurs as a result of the movement of atoms or molecules along the crystal surface within the molecular layer. The different diffusion coefficients for a copper nanoparticle (diameter 10 nm, 293 K) is given in Table 14. Table 14 The value of the diffusion coefficients of copper and other components in the composition of nanoparticles Diffusible Diffusion coefficient, m2/s substance D1 bulk D2 grain-bound Cu 4,0·10-40 4,0·10-24 * -33 Ag 8,0·10 8,0·10-20 Au* 1,6·10-34 2,6·10-26 * -30 Bi 8,8·10 * Note: for copper nanoparticles containing Ag*, Au*, Bi*

D3 surface 2,6·10-20 4,8·10-17 4,7·10-22 2,3·10-19

As it is seen, the maximum diffusion coefficient is observed for surface diffusion, and the minimum for bulk diffusion, that is, D3 > D2 > D1 and for copper the coefficient of surface diffusion is by 20 orders of magnitude higher than the bulk diffusion, and for other types of diffusion it does not exceed by 10 orders of magnitude. Increasing of the temperature leads to increase the values of diffusion coefficients. The surface diffusion is more intense, and the less intensive one is the bulk diffusion. The activation energy of the diffusion process is quantitatively determined by the tangent of the slope of the straight line (36, Fig. 19): . (36) Diffusion processes play an important role in technology in particular at alloying – the introduction of substances into metal alloys to change the structure of alloys and give them certain physical, chemical and mechanical properties. Boron adsorbed on the surface of nickel increases the plasticity of nickel alloys due to grainboundary diffusion. Diffusion of nanoparticles is accompanied with adsorption. Adsorption is associated with the deposition of metal nanoparticles on polymer surfaces and determine the possibility of creating nanocomposites. 65

Figure 19. The dependence of diffusion coefficient on reverse temperature

Osmosis. Molecular-kinetic phenomena include osmosis. Osmosis is the phenomenon when the molecules of solvent 1 (dispersion medium) tend to pass through semipermeable membrane 2 into solution 3 (disperse system) or from more dilute solution 1 into more concentrated one 3. (Fig. 20, a). The motion of the dispersion medium (solvent) is indicated by arrows in Fig. 20. Solution 3 can be a colloidal solution, i.e., sol of nanoparticles. As a result of osmosis there is a pressure π, called as osmotic pressure. Reverse osmosis (Fig. 20, b) – the transition of water or other solvents through the semipermeable membrane from concentrated solution to less concentrated solution as a result of pressure P which exceeds the difference of osmotic pressures of both solutions (P > π). In this case, the membrane passes the solvent, but does not pass some of the substances dissolved in a solvent.

Figure 20. Scheme of osmosis (a) and reverse osmosis (b): 1 – solvent, dispersion medium; 2 – solution, including colloid solutions; 3 – membrane; 4 – movement of a solvent; δ – is the diameter of the nanoscale pores of a membrane

66

The osmotic pressure (π) for an ordinary (molar) solution according to Van’t-Hoff equation is equal to: ,

(37)

but for a nanoparticle as a colloidal solution: ,

(38)

where C – concentration of solution; 𝜗𝜗𝑚𝑚 − mass concentration of nanoparticle kg/m3; r – radius of a nanoparticle; ρ – nanoparticle density; NA – Avogadro's number. The dependence of the osmotic pressure on the sizes of nanoparticle: 𝜋𝜋~1/𝑟𝑟3, that is, the smaller the particle size, the greater the osmotic pressure. For a nanoparticle commensurate with the molecular size at C = 10-1 kmol/m3, 𝜗𝜗𝑚𝑚 reaches 6×1025 particles per m3, and the osmotic pressure is 2.4·105 Pa (or approximately 2.4 atm.) that is great value. For a nanoparticle with a size of 100 nm, the osmotic pressure is about 10-3 atm, that is very insignificant. The intensity of osmosis and reverse osmosis depends on the ratio between the channel diameters of the membrane δ (Fig. 20, a) and the thickness of the diffusion layer λ (Fig. 21, lecture 8). When δ >> λ, the diameter of the channels is much larger than the diameter of the ionic atmosphere, the efficiency of the process does not reduced. At δ < λ decrease in the cross section of the channels and loss of the membrane capacity is observed. Simultaneously with the movement of the solvent, it is possible the motion of the diffusion layer ions that can also lead to a decrease of the productivity of the membranes. The membranes can have cation-exchange and ion-exchange properties. Nanoporous polymeric and inorganic materials are used for reverse osmosis. They contain ions that can be replaced by ions in the solution in contact with the membrane, for example, OH groups. Revision question: 1. What molecular kinetic properties do you know?

67

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

What is the reason of Brownian motion? What is the mean square displacement? What factors influence on the mean square displacement? What features of nanoparticle Brownian motion you can characterize? Write the equations of translational and rotational average square displacements. What is a diffusion? Write the Fick’s law. What is the diffusion coefficient? What types of diffusion for nanoparticles do you know? Why does surface diffusion coefficient exceed the grain-boundary and bulk diffusion coefficients? How does the temperature influence on diffusion? How one can calculate the activation energy through diffusion coefficient? What is the osmosis? What is the reverse osmosis? Write the Van’t-Hoff equation for nanoparticles and compare the osmotic pressure for ordinary solutions and sols?

68

Lecture 8 ELECTROKINETIC PROPERTIES OF NANOPARTICLES Electrokinetic phenomena as electrophoresis and electroosmosis take place in relation to nanoparticles. An electrophoresis is the motion of particles of a dispersed phase under the action of an external electric field. Electroosmosis is a directed motion of dispersion medium particles under the action of the applied potential difference. These electrokinetic phenomena have some peculiarities with respect to nanoparticles. The first feature is related to the commensurability of a nanoparticle diameter with the dimensions of the electrical double layer (EDL), the second one is the electroosmosis in nanoscale capillaries, the third one is the possibility of nanoparticle mixture separation. Let us consider the first peculiarity (Fig. 21). The thickness of the adsorption layer δ is determined by the size of the ions, which are much smaller than the dimensions of nanoparticles. The thickness of diffusion layer λ is by tens times larger than adsorption layer and it is commensurate with the sizes of nanoparticles. Such terms effect on nanoparticle electrophoresis. The electrophoretic mobility 𝑢𝑢𝐸𝐸 equals to the rate of electrophoresis ν which is calculated per unit of external electric field E: .

(39)

In its turn, the electrophoretic mobility is: ,

(40)

where ε – is the relative dielectric constant of a medium; ε0 – electrical constant; ζ – is the electrokinetic or zeta-potential, and η – is the viscosity of the dispersion medium. The zeta-potential (Fig. 21) is determined on the A-A boundary between the adsorption and diffusion layers of EDL. The A-A sur69

face is called as the slip boundary, and the potential on slip plane is called as a zeta-potential. The possibility of nanoparticle electrophoresis is determined by the value of the zeta-potential and its intensity is determined by the rate and electrophoretic mobility. At the isoelectric point (IEP) when the zeta potential is equal to zero, there is no electrophoresis. The IEP depends on the pH of the medium. So, for titanium-oxygen nanostructures the IEP corresponds to pH = 5,9. In a solution of NaCl (10-4-10-3 M) in HCl the IEP for SiO2 nanoparticles is observed at pH = 2. Adding of 0.1 mole of CsCl into the solution causes the displacement of the IEP (pH=3.3). Such shift is explained by the specific adsorption of Cs on SiO2. pH in the IEP for nanoparticles of aluminium shifts to the alkaline region as the oxygen-containing layers grow on the nanoparticle surface (Table 15).

1 – is a layer of potential-determining ions, 2 – counter-ions of the adsorption layer, 3 – counter-ions of the diffusion layer: δ, λ – thickness of the adsorption and diffuse layers; φ – surface potential; ζ – is the electrokinetic potential (zeta-potential) Figure 21. The structure of the electrical double layer: a) simplified scheme on the plane surface; b) in a relation to nanoparticle; c) electroosmosis

70

Table 15 The number of layers, n pH (of the isoelectric point)

0,1 4,7

1 6,2-6,3

4 8,4-8,5

Peculiarities of electrophoresis of nanoscale objects are determined by the dependence of electrophoresis on the size of nanoparticles. Dependence of the electrokinetic potential of a diamond on the size of the particle is represented in Table 16. Table 16 The particle size, nm ζ –potential, mV

130 -6,5

8 -78,6

With decreasing of the nanoparticle size, the substantial growth of the zeta potential is observed that is associated with an increase of the specific surface. The growth of ζ-potential means the increase of the electrophoresis rate. The information given above corresponds to highly dispersed systems. When the surface of diamond nanoparticles (3-5 nm) with the specific surface area (2,5-4,5)·105 m2/kg is saturated by oxygencontaining groups: -COOH, -CHO, -OH, the zeta-potential is equal to -5 mV and the thickness electric double layer is commensurable with the nanoparticle sizes. In the above examples, the ζ-potential is negative. Its sign and value are determined by the structure of the electric double layer which in its turn depends on the pH of the dispersion medium. Let us consider an electrophoresis applied to carbon nanotubes. It allows separating impurities of carbon and catalytic materials and to obtain a clean product. Under the action of an electric field due to a double electric layer in a liquid medium, the nanotubes will be oriented perpendicular to the plane of the electrode in accordance with the amount and sign of the zeta potential. The results of the experiment for nanotubes in the form of a film of 2 μm long and 5 nm thick are presented as follows: in a 2-propanol solution, the nanotubes are charged positively, i.e. ζ > 0, and moved to the cathode. The force resulting from electrophoresis acts on nanotubes depending on their size, for example, such force is much higher for 71

nanotubes of 1 μm in length than for 10 μm nanoobjects. This is the basis for the process of separating nanotubes by their size. Using of electrophoresis allows to obtain nanotubes in pure form without impurities – as a nanostructured carbon material. In addition, it is possible to assemble structures from carbon nanotubes on the surface of various objects, as well as their separation, depending on the geometric and electrical properties. The rate of electrophoresis depends on the dielectric permittivity of the nanotubes. Thus, for metallized nanotubes with high dielectric permittivity the process of electrophoresis proceeds more efficiently than for semiconductors with a lower dielectric permittivity. Such effect makes it possible to separate nanotubes according to various electrical properties. Electrophoresis of nanotubes can be used: − at assembling of complex structures on the surface of various objects; − for their separation according to geometric properties (lengths); − for their separation according to electrical properties (for a constant potential difference of external electric field, the nanotubes orients along the lines of electric field strength). Electroosmosis in a nanoscale capillary. The process is determined by the relationship between the radius of the capillary r and the thickness of the diffusion layer λ (Fig. 21, a). In the case of r/λ1, the electric double layer is fixed on the walls of the capillaries and the structure corresponds to the Fig. 21, a. For a sufficiently small thickness of the diffusion layer in comparison with the radius of the nanocapillary when r/λ1 under the action of an external electric field, the EDL is induced (Fig. 21 a) and the linear rate of the electroosmosis does not depend on the radius of the capillary. Using formula (42) one can calculate the electrophoretic mobility u which is determined by the ratio of the rate of the electroosmosis per unit of external electric field strength, u=vL/E. Electrophoretic mobility is one of the most important parameters characterizing the electroosmosis and electrophoresis. It can vary depending on the contact time of nanoparticles with the solution. Similar process is observed for nanoparticles of Al2O3 at pH=4,5. When the EDL overlaps and r/λ 0, and the second process prevails. For this reason, the entropy of the surfactant adsorption process increases and leads to the spontaneous transition of surfactant from solution on the surface. Adsorption layers of surfactants at the interface forms one-dimensional nanosized structures (Fig. 32, а) or thin films of surfactants. Structural-mechanical barrier preventing the coagulation of nanoparticles is formed due to the adsorption layer of surfactant. Self-organization is possible by means of the local concentrating of surfactant molecules with the formation of «island» nanoscale structures. Often such structures (in the form of meniscus) occur near the line of three-phase contact. The essence of the «island» structures is the uneven distribution of the surfactant in the adsorption layer, their high concentration in this layer, that causes the formation of singular micelles. 97

The formation of structures from surfactant molecules can be carried out by Langmuir – Blodgett technique (Fig. 33). The adsorption layers of insoluble surfactants can be transferred from the airsolution interface onto the solid substrate, forming the so-called Langmuir-Blodgett films. The deposition of adsorption layers takes place on a slide that moves perpendicular to the air – solution interface containing the adsorption layer. Film transfer usually occurs at constant and rather high value of the two-dimensional pressure (Fig. 34).

1 – Frame, 2 – Barriers, 3 – Trough top, 4 – Surface pressure sensor, 5 – Dipping mechanism (Langmuir-Blodgett option), 6 – Interface unit Figure 33. The components of Langmuir-Blodgett trough

Figure 34. The deposition of surfactant adsorption layers on a slide

The method consists of transferring the condensed films from the surface of the liquid to the surface of slides of metal, glass and 98

other materials. «Direct» (Fig. 35 a, by the hydrophilic part of the surfactant molecules) or «reverse» (Fig. 35 b, by the hydrophobic part) orientations on solid surface and hydrophilization or hydrophobization of the surface, respectively, depends on the position of the surfactant.

Figure 35. a) Hydrophilization and b) hydrophobization of the surface by Langmuir – Blodgett technique

If the slide is moved by turns in both upward and downward directions, polymolecular films is formed (Fig. 36). In these films the neighboring monolayers alternately come into contact with their hydrocarbon chains and polar groups, as shown in Fig. 36, b. If the slide always moves into one direction, the «polar» films (with uncompensated dipole moment of surfactant molecules in the neighboring monolayers) may be formed. In such layers the polar groups of one monolayer come into contact with hydrocarbon chains of the other. If the slide is hydrophobic, and moves in the downward direction, the X-type films are formed (Fig. 36, a). The deposition on a hydrophilic slide moving in the upward direction results in the formation of Z-type films (Fig. 36, c). The Langmuir-Blodgett technique allows one to form structures with the predetermined molecular arrangement, in which the neighboring monolayers have the desired composition. For example, by using water insoluble organic acids with sufficiently long chain length, such as stearic, the composition of deposited layers can be modified by changing the electrolyte content of the substrate solution by introducing polyvalent ions that form insoluble salts with the acids. The method of the Langmuir-Blodgett is quite widespread for obtaining self-organized nanoparticles at interfaces. An ordered 99

monolayer of nanoparticles can be formed on vertical surfaces by removing the slide from the colloidal solution and after evaporation of the solvent. Hydrophobic nanoparticles can be formed at the interface air-water and then transferred on a solid substrate.

a) – X-type, b) – Y-type c) – Z-type films Figure 36. Different types of Langmuir-Blodgett films deposited on a solid surface

Using the method of Langmuir-Blodgett allows to change the size and shape of nanoparticles from several tens to hundreds nanometers and form various structures of nanosized films. Ordered structures can be transferred from the surface of the liquid on the solid substrate without noticeable disruption of the crystalline structure of the nanolayer. Various additives are used to impart strength to the nanolayer. In addition, a layer-by-layer assembly of nanoparticle layers is possible, that allows to create the various structures of nanolayers. Among the perspective applications of Langmuir-Blodgett method one can name their use as monochromators and analyzers of soft long wave X-ray and neutron radiation and for synthesis of lighttransmitting, electrically conducting thin films on the surfaces of solids utilized in novel electronic devices.

100

Revision questions: 1. What are self-assembling systems? 2. Give the examples of self-assembling colloidal systems. 3. What are colloidal clusters? Give the examples. 4. What self-assembling structures based on surfactants do you know? 5. Explain the amphiphilic structure of surfactants. 6. How does the entropy of surfactant adsorption process change? 7. What is the Langmuir – Blodgett technique? 8. Describe the Langmuir – Blodgett films.

101

Lecture 13 SURFACTANT MICELLES AS SELF-ASSEMBLING SYSTEMS. TYPES OF MICELLES Surfactant micelles relates to structured self-assembling nanosystems. They are formed due to the colloidal surfactants. The hydrophobic part of colloidal surfactants is a hydrocarbon radical CnH2n+1, CnH2n-1, CnH2n+1C6H9 and others types of radicals, containing from 8 to 18 carbon atoms. Let us consider in detail the distribution of surfactant molecules in solution (Fig. 38). Part of the surfactant molecules adsorb at the interface of liquid gas (water – air). There is a dynamic equilibrium between the surfactant molecules on the adsorption layer of the surface (1) and molecules in solution (2). Part of the surfactant molecules in solution can form micelles (3). There is the balance between colloidal surfactant molecules in solution and molecules within the micelles shown in the Fig. 40 by arrows. The process of micellization from surfactant molecules dissolved in a solvent can be represented as follows: nMr ↔ Mn

(62)

,

(63)

where Mr is the molecular weight of the surfactant molecule, n is the number of surfactant molecules in the micelle (or micellar number), Mn – micellar weight or weight of micelle. Micelles are aggregates (of spherical form and in the form of plates) consisting of tens of surfactant molecules, in which polar groups contact with water, and hydrophobic radicals are located inside, forming a nonpolar core. The state of surfactants in solution depends on their concentration. True solutions is formed at low concentrations (10-4-10-2 M). 102

Above a particular concentration (or more precisely above a very narrow concentration range) called as a critical micellization concentration (CMC), micelles are formed, and they are in thermodynamic equilibrium with surfactant molecules in solution.

Figure 37. The position of colloidal surfactants: 1 – on the surface; 2 and 3 – as a true and colloidal solutions; 4 – hydrocarbon nonpolar core; 5 – hydrophilic polar groups

Micelle formation is a thermodynamic favourable process and ΔG of micellization is negative due to contributing of entropy. Thus, micelles relate to lyophilic systems. The lyophilic processes form spontaneously. It is known that the processes can occur spontaneously, if they accompanied by ΔG < 0. Consequently, lyophilic systems are characterized by very low value of interfacial tension between dispersed phase and dispersion medium. The critical value of interfacial tension at which the spontaneous dispersion proceeds and the thermodynamic stability is provided is calculated according to Shchukin-Rehbinder equation: ,

(64)

where σcr is an interfacial tension, kB – Boltzmann constant, T – temperature, d – average size of particle. The systems with interfacial tension σ ≤ σcr relate to lyophilic systems. The value of critical interfacial tension depending on particle size is in the range of 0.01 – 10-4 mJ/m2. The bigger size of particles, the less value of critical interfacial tension. But lyophilic systems are highly dispersed systems subjected to Brownian motion. 103

Micellization causes the change of physical chemical properties of surfactant solution (Fig. 38). Study the physical chemical properties of surfactant solutions allows to determine CMC value.

Figure 38. The change in some physical chemical properties of sodium dodecylsulphate (SDS) as a fuction of SDS concentration. æ – specific electroconductivity, π – osmotic pressure, τ – turbidity, σ – surface tension, λ – equivalent electroconductivity of SDS solution

The dependence of CMC value on number of CH2-groups is calculated according to formulas: lg CMC = A - Bn,

(65)

where A and B are constants for this particular type of surfactant (A depends on nature and number of hydrophilic groups, temperature), B characterize the energy of dissolving per one CH2 group, (B ≈ lg 2), n is number of carbon atoms in a hydrocarbon chain. (65) is used for water solutions of surfactants. For nonpolar media the following equation is applied: lg CMC = A + Bn.

(66)

The CMC value in some particular solvent is a characteristic of the surfactant but depends on several other factors, because micellization is influenced by thermal and electrostatic forces. Therefore, 104

the CMC values depends on many factors like the carbon chain (tail) length, chain branching, number of C=C bonds, various types of additives, temperature and pressure. CMC value decreases with increasing the hydrocarbon chain-length. An addition of strong electrolyte to ionic surfactant tends to decrease the CMC because reduces the repulsion between the charged groups at the surface of the micelle by the screening action of the added ions. Therefore, the CMC is lowered. For these systems one can use the following formula: lg CMC = A’ – B’n – klgc,

(67)

where A’ and B’ have the same meaning as A and B constant, k is the constant, c is the concentration of counter-ions of indifferent electrolyte. Micellization is opposed by thermal agitation and CMC's would thus be expected to increase with increasing temperature. This is usually, but not always, in the case of nonionic surfactants CMC value decreases due to the dehydratation of oxyethylated groups under the influence of temperature. Determination of the CMC is important in order to understand the self-organizing behaviors of surfactants in solution. When surfactant concentration is above CMC, the excess of surfactant form micelles. Direct and reverse micelles can form. If in direct micelles (Fig. 37) the hydrophilic part of the surfactant molecules faces the dispersion medium (water), then in reverse micelles – the hydrophobic part. At significant content of surfactants, the spherical, cylindric, lamellar types of micelles can form up to complex structure as liquid crystals and gels (Fig. 39). The change of forms of micelles is referred as polymorphism. The formation of micelles is a spontaneous process. Several types of micelles with sizes up to tens of nanometers are possible. Micellar solutions of surfactants can be regarded as a model of easily reproducible system to study the nanosized particles and the structure of the electrical double layer (Fig. 21). Micelles have a perfect spherical shape and their radius equals to units of nanometers. Due to a slight size difference they are characterized by relatively narrow distribution. 105

Figure 39. Polymorphism of surfactant micelles. а – molecules of surfactants in true solution; b – direct spherical micelle; c – direct cylindric micelle; d – hexagonal location of direct spherical micelle; e – lamellar micelles; f – hexagonal location of reverse cylindric micelles

For the border of micelle – intermicelar medium the classical theory of EDL (electrical double layer) is fair (Fig. 21). This micellar solution is represented in the form of uniform spherical particles dispersed in a liquid containing ions. Surface-active ions forming the core of the micelle represent the inner part of EDL. Micelles promote the synthesis of clusters that under certain terms can form a disperse system. Thus, to obtain Ag2S nanoparticles it is necessary to use two types of reverse micelles, comprising Na2S and Ag. Coalescence (flowing of drops) occurs as a result of collision of the micelles and the exchange of incident components (Na2S and Ag) takes place that lead to the formation of Ag2S. To release the nanoparticles from the micelles the solution is destroyed by thiols, then subject to heat in benzene, filter and evaporate. This is the way to get monodisperse nanoclusters with size up to 10 nm. These clusters form ordered nanostructures (the colloidal crystals) by planting on substrate. By this method twodimensional and three-dimensional colloidal crystals are obtained: metallic clusters of Ag, Co, Au, and an oxide of metal clusters. The appearance of micelles is possible in the oil phase; it is determined by the interaction of polar groups of the surfactant between the cores of reverse micelles or by the interaction of polar groups of surfactants with nonpolar solvent. It should be noted that the processes of self-organization of surfactants also can be observed in the gels.

106

Revision questions: 1. Why do surfactant micelles relate to self-assembling systems? 2. What is the surfactant micelles? 3. What is the micellar number? 4. What is the critical concentration of micellization. 5. Describes and explain the change of physical chemical properties of SDS solution depending on surfactant concentration? 6. What factors influence on micellization? 7. What is the polymorphism of micelles? 8. How micellar systems can be used for preparation of nanoparticles of metals?

107

Lecture 14 MICROEMULSIONS AS SELF-ASSEMBLING SYSTEMS. DIFFERENCES BETWEEN EMULSIONS AND MICROEMULSIONS. WINSOR CLASSIFICATION OF MICROEMULSIONS. APPLICATION OF MICROEMULSIONS AT TEMPLATE SYNTHESIS. PERIODIC COLLOIDAL STRUCTURES Microemulsions relate to self-assembly systems. Microemulsions are disperse system of the type L/L, i.e. they are consisting of two immiscible liquids. Macroemulsions are coarse disperse systems in which one liquid phase is water (W) and the another one represents the water-insoluble liquid, called as an oil (O) (liquid fat, mineral oil, etc). Depending on the composition of the dispersed phase and the dispersion medium they can be direct or reverse emulsion. Direct emulsion of oil-in-water (o/w) type is a dispersion of oil droplet in water, dispersed phase in direct emulsion is an oil (Fig. 40, a). Reverse emulsion of waterin-oil type (w/o) is the dispersion of water in oil, they represent water drops distributed in the oil medium (Fig. 40, b).

Figure 40. Oil-in-water (a) and water-in-oil (b) emulsions

Microemulsions should not be regarded as emulsions with very small droplet size; microemusions and ordinary coarse disperse emulsions are fundamentally different. Whereas emulsions are inherently unstable systems in which the droplets eventually undergo coalescence, microemulsions are thermodynamically stable with a 108

very high degree of dynamics with regard to internal structure (Table 18, Fig. 41). As a thermodynamically stable phase on surfactant self-assemblies, it has much in common with other surfactant phases, micellar solutions and liquid crystalline phases. Table 18 Characteristic differences between emulsions and microemulsions Emulsion – Unstable, will eventually separate – Relatively large droplets (1-10 µm) – Relatively static system – Moderately large internal surface, moderate amount of surfactant needed – Small oil/water curvature

Microemulsion – Thermodynamically stable – Small aggregates (~10 nm) – Highly dynamic system – High internal surface, high amount of surfactant needed – The oil/water interfacial film can be highly curved

Figure 41. Scheme illustrating the size of emulsion and microemulsion droplet, amount of surfactants and oil/water interfacial film curvature

Microemulsions can be prepared by controlled addition of lower alkanols (butanol, pentanol and hexanol) to ordinary milky emulsions to produce transparent solutions comprising dispersions of either water-in-oil (w/o) or oil-in-water (o/w) in nanometer or colloidal dispersions (~ 100 nm). The lower alkanols are called cosurfactants, they lower the interfacial tension between oil and water sufficiently low for almost spontaneous formation of the said microheterogeneous systems. The miscibility of oil, water and amphiphile (surfactant plus cosurfactant) depends on the overall composition which is system specific. A well-known classification of microemulsions is that of Winsor who identified four general types of phase equilibria (Fig. 42): 109

− Type I: the surfactant is preferentially soluble in water and oil-in-water (o/w) microemulsions form (Winsor I). The surfactant-rich water phase coexists with the oil phase where surfactant is only present as monomers at small concentration. − Type II: the surfactant is mainly in the oil phase and water-inoil (w/o) microemulsions form. The surfactant-rich oil phase coexists with the surfactant-poor aqueous phase (Winsor II). − Type III: a three-phase system where a surfactant-rich middlephase coexists with both excess water and oil surfactant-poor phases (Winsor III or middle-phase bicontinuous microemulsion). − Type IV: a single-phase (isotropic) micellar solution, that forms upon addition of a sufficient quantity of amphiphile (surfactant plus alcohol).

Figure 42. Schematic illustration of the four Winsor systems (light: organic phase, dark: aqueous phase). From left to right: I – (o/w) microemulsion system, II – (w/o) microemulsion system, III – three phase microemulsion system (bicontinuous middle phase), and IV – one phase microemulsion (or bicontinuous system)

Microemulsions were not really recognized until the work of Hoar and Schulman in 1943, who reported a spontaneous emulsion of water and oil on addition of a strong surface-active agent. There has been much debate about the word «microemulsion» to describe such systems. They are termed a «micellar emulsion» or «swollen micelles» or «nanoemulsions». Microemulsions were probably discovered well before the studies of Schulmann: Australian housewives have used since the beginning of last century water/eucalyptus oil/soap flake/white spirit mixtures to wash wool, and the first commercial microemulsions were probably the liquid waxes discovered by Rodawald in 1928. 110

Interest in microemulsions really stepped up in the late 1970's and early 1980's when it was recognized that such systems could improve oil recovery and when oil prices reached levels where tertiary recovery methods became profit earning. Nowadays, by means of microemulsion it is possible to extract oil from wells, to produce the regeneration and to improve the quality of lubricants, effectively use of pesticides and other biological substances, to improve the properties of cosmetic products, etc. Recently, microemulsion, particularly of water-in-oil type (w/o) have become of interest as a medium for fabrication of nanoparticles. Fine dispersed water droplets represent an ideal microreactors (template) for synthesis of nanoparticles due to the size of the drops which control the size of the grown nanoparticles. Microemulsions are used to produce the monodisperse nanoparticles. Thus, the synthesis of nanoparticles of Pt, Pd, Rh, Ir was carried out by reduction of the corresponding salts in the water droplets. Microemulsions are widely used to obtain a variety of nanoparticles as well as individual metals and their compounds. The preparation of silver, gold, platinum, cobalt and iron nanoparticles have been started in 1990-s by means of microemulsion. Nanoparticles are formed as a result of the reduction of the appropriate metal salts with sodium borohydride or hydrazine or due to the ion exchange reactions provided by my mixing of microemulsions. Functional surfactant bis (2-ethylhexyl) sulfosuccinate of copper Cu(АОТ)2 are used at nanoparticle fabrication. This surfactant plays a dual role: it serves as a stabilizer of water droplets and, on the other hand, it is a source of copper ions in microemulsion droplets. For preparation of nanoparticles the w/o microemulsion stabilized by Cu(АОТ)2 and Na(AOT) in isooctane was mixed with a microemulsion of droplets of sodium borohydride solution stabilized with Na(AOT). After mixing of microemulsions the exchange of substances between droplets and the reduction of copper occurs. Due to the size limitation provided by microemulsion drops nanosized copper particles are formed (Fig. 43). Microemulsions are differed by the size and shape of droplets. The change in the size, structure and shape of the nanodrops can be quantitatively estimated using the following relation: 111

,

(68)

where [Н 2 0 ] and [surfactant] are concentrations of water and surfactant, correspondingly. the value w < 4 means that the microemulsion contains only spherical droplets, the size of which is proportional to w.

Figure 43. Fabrication of copper nanoparticles by means of template synthesis based on microemulsion

Further increase of water content (4 < w < 5,5) leads to a change in the form of microdrops: they are of spheroidal shape. Accordingly, at the reduction of copper the nanoparticles of spherical shape with diameters of 8.2 nm and 12 nm and nanoparticles of cylinder form with a diameter of 12 nm and a length of 18.5 nm are formed. When the relative water content is in the range of 5.5 < w < 11 the microemulsion has a structure of bi-continuous phase and spherical 112

nanoparticles with diameters of 6.7 and 9.5 nm and nanorods with a length of 22.6 nm and a diameter of 9.5 nm are formed. The subsequent increase of water content (w>11) leads to the transformation of the microemulsion structure in which only the rods with a length from 300 to 1500 nm and a diameter of 10 to 30 nm are formed. According to data of electronic microscopy, the nanoparticles synthesized in microemulsion (including long rods noted above) are characterized by defect-free surface that indicates the high quality of nanoparticles obtained by means of microemulsions. A periodic spatial structure can form as a result of chemical reactions. They are can be considered as nanoparticles. Similar process occurs at the formation of the so called rings of Liesegang. They are formed due to the formation and precipitation of solid nanosized particles in the medium of gel (Fig. 44). They are formed due to the mutual diffusion of the two reagents. Rings of Liesegang are formed in the medium of agar, gelatine, polyacrylamide and other gels as a precipitate of nanoparticle of halides and chromates of heavy metals. The process of nanoparticles formation is associated with crystallization: first, due to the supersaturation, and then as a result of diffusion and achievement of the resaturation limit.

a)

b)

Figure 44. a) Liesegang rings – Silver-chromate precipitate pattern in a layer of gelatine, b) Liesegang rings of Magnesium hydroxide in Agar gel. Made by diffusing Ammonium hydroxide into an Agar gel containing Magnesium chloride

Thus, major feature of evolutionary processes in colloidal systems is that they often lead to the emergence of ordered structures of 113

various sizes in nanoscale. These structures can be both two-dimensional and three-dimensional particles, and their size can vary from a few nanometers to hundreds of nanometers. Such structures are fairly common. They are in applied and research interest, including that they are convenient models of a number of biophysical and biochemical systems. Revision questions: 1. Why do microemulsions relate to self-assembling systems? 2. Characterize the differences between emulsions and microemulsions. 3. How are microemulsions classified according to Winsor classification? 4. Give the examples of application of microemulsions at template synthesis. 5. Periodic colloidal structures. What are rings of Liesegang?

114

Lecture 15 SOME ASPECTS OF APPLICATION OF NANOPARTICLES AND NANODISPERSE SYSTEMS. THE BASIC DIRECTIONS OF NANOTECHNOLOGICAL DEVELOPMENTS BASED ON COLLOIDAL CHEMICAL PROPERTIES OF NANOPARTICLE There are a numerous and diverse aspects of the nanoparticles use in various branches of industry, agriculture, medicine and other spheres of human activity. The new directions of nanoparticle application are implementing constantly. Features of use of nanoparticles according to Table 19 consist in: – classification of industries associated with nanotechnology; – presentation of nanoparticle features on the basis of their colloidal-chemical properties; – innovative ideas about the role of nanoparticles. Table 19 presented according to handbook of Zimon A.D., Pavlov A.N. «Colloid Chemistry of nanoparticles» [1] show the classification of applications of nanodisperse system relating to their colloidal chemical characteristics in an innovative way. The areas and results of nanoparticle application are not limited by data of Table 19. Since there are new practical results and field of applications of nanosystems published in considerable number today. Table 19 The general directions of nanotechnologies development on the basis of colloid-chemical properties of nanodispersed systems Application Practical implementation area 1 2 Nanoelectronics The miniaturization of the apparatus, nanoscale lasers, light-emitting diodes instead of incandescent lamps

115

Properties of nanoparticles 3 The size effect, adhesion of nanoparticles, quantum effects

1

2 Transistors at the molecular level Microelectromechanical systems, combination of mechanical nanoelements, sensors and electronics based on silicon Microwave Electronics, Nanoscale heterostructures in radiolocation, communication systems Laser technology, highly efficient lasers based on heterostructures with nanoscale layers Molecular Electronics

3 Nanosynthesis Optical properties of nanoparticles

The size effect, adhesion of nanoparticles, quantum effects

Surface tension of nanoparticles, surface properties of nanosystem.

The use of nanoparticles as a unit cell, nanoparticles as objects of Colloid Chemistry Quantum Computers The size of circuit elements about 100 nm, size effect. Medicine and Targeted delivery of medicines, Two-dimensional structure of pharmaceutics genetic engineering, nanoparticles, the excess of bactericidal preparations. surface energy, synthesis of Improvement of diseases nanoparticles, size effect. diagnosis, transplantation of tissues and regenerative medicine nanoparticles, transportation possibility by various systems of the body. Nanotubes as a basis for drug Dimensions and structure of delivery nanoparticles and nanotubes. Chemical, food, Reduction of friction. Adhesion and friction. processing Nanocatalysis in the oil and gas Dispersion, size effect of industry industry. nanoparticles, catalytical Use of wastes of food industry. properties. Transparent nanofabric and Dimensions and structure of coating with thickness 30 nm. nanoparticles and nanotubes. Recycling, Dispersion of waste. development of products intensifying the recycling processes, creating products for biofertilizers.

116

1

Construction materials

2 High precision of surface treatment in a rocket and aircraft, the ability to control the size of the processed products to the nanometer scale. Nanoadditives to lubricants, development of lubricants based on nanoparticles. Creation of ultrastrong, elastic, plastic materials (alloys, ceramics, protective coatings).

Nanoparticles synthesis at the molecular level. Fabrication of nanofilms.

Thermal insulation materials

Fibrous materials

Nanoparticles dispersion in polymers. Highly porous materials capable to retain gaseous substances (air or other medium). Reflection of light and other types of radiation. Nanofibres

3 Surface tension of nanoparticles, surface properties of nanosystem, mechanical properties, size effect. Surface properties of nanoparticles and their adhesion. Structural and mechanical properties of nanoparticles, strength, elasticity, plasticity, fluidity, hardness and parameters determining them. Preparation of synthetic nanoparticles with desirable properties. Compacting of nanoparticles, «bottom up» synthesis. The process of dispersion. Features of structured nanosystems.

Optical phenomena.

Anti-ultraviolet rays, antibacterial action, resistance to moisture Mechanical Composites based on polymers, Heat-resistant, the power and engineering and ceramics, matrix materials. magnetocapacitance materials construction with the inclusion of industry nanoparticles in a nanosized matrix, size effect. Paints and films with the Prevention of adhesion and addition of nanoparticles. contamination of surfaces, including biological overgrowth of the bottoms of ships. Waterproofing of the surface. Nanocomponents, development Based on fullerenes and of nanoscale gears, rotors, nanotubes. turbines.

117

1

Energetics, including nuclear power

Ecology

2 Graphene as an open nanotube. Self-organization, strengthening of nanostructures. Foam structure based on nanoparticles. Molecular electronics. The development of new types of energy - solar cells on Earth and in space. Creation of engines at the molecular level. Intensification of processes in the nuclear industry. Increase in the level of nuclear fuel processing. Air and water purification, Environmental monitoring. Membrane technology. Nanofilters from graphite. Atmospheric monitoring.

Space, soldiery Military outfit.

Creating of miniature robots. Increasing the combustion rate and thrust of jet engines.

Bulletproof vests.

3 Two-dimensional flat carbon tube, diffusion, viscosity.

Large specific surface area, the optical properties of nanoparticles. Fuel hydrogen elements adsorption of hydrogen. Diffusion enrichment of uranium. Large specific surface area. Nanoparticles in an aerosolized state as a way of monitoring the environment. Large specific surface area. Sensors based on nanoparticles. Anti-adhesive and non-wetted materials as personal protective equipment. Synthesis of nanomaterials. Nanoparticles increase the combustion rate due to the large specific surface area and the size effect The addition of nanoparticles for strength increasing.

As it is seen from the Table 19 the nanodevices and nanosystems possible for a variety of industrial, consumer, pharmaceutical, and biomedical applications. A variety of devices and products have been produced, and many of them are in commercial use. Other applications of nanosystems include devices for Earth observation, space science, and missile defense applications, picosatellites for space applications, fuel cells, and many hydraulic, pneumatic, and being pursued for use in magnetic storage systems, where they are being developed for supercompact and ultrahigh-recordingdensity magnetic disk drives. 118

Nanoelectronic and mechanical systems (NEMS) are produced by nanomachining in a typical top-down approach and bottom-up approach, largely relying on nanochemistry. Examples of NEMS include microcantilevers with integrated sharp nanotips for scanning tunneling microscopy (STM) and atomic force microscopy (AFM), quantum corrals formed using STM by placing atoms one by one, AFM cantilever arrays for data storage, AFM tips for nanolithography, dip-pen lithography for printing molecules, nanowires, carbon nanotubes, quantum wires (QWRs), quantum boxes (QBs), quantum-dot transistors, nanotube-based sensors, biological (DNA) motors, molecular gears formed by attaching benzene molecules to the outer walls of carbon nanotubes, devices incorporating nm-thick films [e.g., in giant magnetoresistive (GMR) read/write magnetic heads and magnetic media] for magnetic rigid disk drives and magnetic tape drives, nanopatterned magnetic rigid disks, and nanoparticles (e.g., nanoparticles in magnetic tape substrates and magnetic particles in magnetic tape coatings). Nanoelectronics can be used to build computer memory using individual molecules or nanotubes to store bits of information, molecular switches, molecular or nanotube transistors, nanotube flat-panel displays, nanotube integrated circuits, fast logic gates, switches, nanoscopic lasers, and nanotubes as electrodes in fuel cells. Bio nano- and microelectromechanical systems (BioNEMS/ MEMS) are increasingly used in commercial and defense applications. They are used for chemical and biochemical analyses (biosensors) in medical diagnostics (e.g., DNA, RNA, proteins, cells, blood pressure and assays, and toxin identification), tissue engineering, and implantable pharmaceutical drug delivery. BioNEMS/MEMS are also being developed for minimal invasive surgery, including endoscopic surgery, laser angioplasty, and microscopic surgery. Other applications include implantable drugdelivery devices (micro/nanoparticles with drug molecules encapsulated in functionalized shells for site-specific targeting applications) and a silicon capsule with a nanoporous membrane filled with drugs for long-term delivery. Nanoscience and nanotechnology continue to move forward in production of nanodevices and nanocompounds are widely used in many spheres of industry such as agriculture, medicine, cosmetics, food industry, ecology and other fields of human activity. 119

Here we consider only some of them concerning to colloidal chemical properties of nanoparticles and nanodispersed systems. Obviously, there is an increasing need for a multidisciplinary, system-oriented approach to the manufacture of nanodevices. This can only be achieved through the cross-fertilization of ideas from different disciplines and the systematic flow of information and scientists. Therefore, there are many researches and publications devoted to the application of nanotechnologies. Revision questions: 1. Provide some examples of nanotechnology use for solving everyday life challenges. 2. Give examples of nanoparticle implementations in areas such as nanoelectronics and construction materials. 3. What do you think about development of nanotechnologies in Kazakhstan and in the world?

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REFERENCES 1. Зимон А.Д., Павлов А.Н. Коллоидная химия наночастиц. – М.: Научный мир, 2012. – 224 с. 2. Shchukin E.D., Pertsov A.V., Amelina E.A., Zelenev A.Z. Colloid and surface chemistry. – Elsevier, 2001. – 747 p. 3. https://www.eolss.net/Sample-Chapters/C05/E6-152-01.pdf 4. http://www.uio.no/studier/emner/matnat/kjemi/KJM5100/h08/undervisning smateriale/kjm5100_2008_nano_intro.pdf 5. http://shodhganga.inflibnet.ac.in/bitstream/10603/31392/9/09_chapter%201.pdf 6. https://application.wiley-vch.de/books/sample/3527331972_c01.pdf 7. Сумм Б.Д. Основы коллоидной химии. – М.: Академия, 2007. – 238 с. 8. Taniguchi N. On the Basic Concept of «Nano-Technology» // Proc. Intl. Conf. Prod. Eng. Tokyo. Part II. – Japan Society of Precision Engineering, 1974. 9. http://friendlyscienceguy.blogspot.com/2013/05/nanotechnology-industryof-very-small.html 10. https://en.wikipedia.org/wiki/Richard_Feynman 11. https://www.intechopen.com/books/recent-advances-in-grapheneresearch/graphene-based-materials-functionalization-with-naturalpolymeric-biomolecules 12. Show D.J. Introduction to colloid and surface chemistry 4th. – 2003 – 306 p. 13. https://en.wikipedia.org/wiki/Langmuir–Blodgett_trough 14. http://www.biolinscientific.com/measurements/langmuir-and-langmuir-blodgett 15. https://csc-iiith.vlabs.ac.in/exp14/index.html 16. Назаров В.В., Гродский А.С., Моргунов А.Ф., Шабанова Н.А., Кривощеков А.Ф., Колосов А.Ю. Практикум и задачник по коллоидной химии. Поверхностные явления и дисперсные системы / В.В.Назаров, А.С. Гродский. – М.: ИКЦ «Академкнига», 2007. – 374 с. 17. Holmberg K., Jonsson B., Kronberg B., Lindman B. Surfactant and polymers in aqueous solutions. – John Wiley & Sons, Ltd, 2003. – 285 p. 18. Bidyut K. Paul, Satya P. Moulik. Uses and applications of microemulsions // Current Science. – 2001. – Vol. 80, № 8. – p. 990-1001. 19. http://www.chm.bris.ac.uk/eastoe/Surf_Chem/3%20Microemulsions.pdf 20. http://slideplayer.com/slide/5734806/19/images/11/Comparison+of+OM,T EM+and+SEM.jpg 21. Schulenburg M. Nanotechnology – Innovation for Tomorrow’s World (European Commission Research Directorate General, Brussels 2004). 22. http://home.ufam.edu.br/berti/nanomateriais/Introduction%20to%20Nanot echnology.pdf 23. Madou M. Fundamentals of Microfabrication: The Science of Miniaturization, 2nd edn. CRC, Boca Raton 2002. 24. https://en.wikipedia.org/wiki/Liesegang_rings 25. Mansurov Z.A., Mofa N.N. Nanochemistry, Bases and Applied Aspects: educational manual. – Almaty: Qazaq University, 2017. – 274 p.

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CONTENTS Introduction .............................................................................................. 3 Lecture 1. General introduction to the discipline «Colloid chemistry of nanodisperse systems». Connection between Colloid Science and Nanochemistry. Nanotechnologies. History of nanotechnologies ....... 9 Lecture 2. Nanoparticles as a new object of Colloid Chemistry. Features of nanoparticles. Classification of nanoparticles. Specific area of nanoparticles. ................................................................... 16 Lecture 3. Methods of preparation of nanoparticles. Top-down and bottom-up methods. Two stage methods. Special methods ................. 24 Lecture 4. Size effect. Influence of size effect on physical and chemical properties of nanoparticles ................................................... 31 Lecture 5. Surface properties of nanoparticles. Adsorption ....................... 43 Lecture 6. Surface properties of nanoparticles. Adhesion of nanoparticles. Adhesion and wetting of nanodroplets. ......................... 49 Lecture 7. Molecular kinetic properties of nanoparticles. Brownian motion. Diffusion. Osmosis. ..................................................... 61 Lecture 8. Electrokinetic properties of nanoparticles. ............................... 69 Lecture 9. Optical properties of nanodisperse systems. ............................ 75 Lecture 10. Optical techniques for studying the nanodisperse systems. Comparison of resolving power of optical and electron microscopes. Transmission electron microscope. Scanning electron microscope. Ultramicroscopy. ...................................................................................... 82 Lecture 11. Bulk properties of nanoparticles. Stability of nanodisperse systems............................................................................. 87 Lecture 12. Self-assembling systems. Mono- and polymolecular layers of surfactants. Thin films................................................................. 95 Lecture 13. Surfactant micelles as self-assembling systems. Types of micelles ....................................................................................... 102

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Lecture 14. Microemulsions as self-assembling systems. Differences between emulsions and microemulsions. Winsor classification of microemulsions. Application of microemulsions at template synthesis. Periodic colloidal structures ..... 108 Lecture 15. Some aspects of application of nanoparticles and nanodispersed systems. The basic directions of nanotechnological development based on colloidal chemical properties of nanoparticles. ..... 115 References ................................................................................................. 121

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Еducational issue

Adilbekova Akbota Orazbakeevna Musabekov Kuanyshbek Bituovich COLLOID CHEMISTRY OF NANODISPERSE SYSTEMS BRIEF LECTURE NOTES

Educational manual Typesetting and cover design G. Кaliyeva Cover design used photos from sites www.goodwp.com

IB №11542

Signed for publishing 14.12.2017. Format 60x84 1/16. Offset paper. Digital printing. Volume 7,75 printer’s sheet. 100 copies. Order №6510. Publishing house «Qazaq university» Al-Farabi Kazakh National University KazNU, 71 Al-Farabi, 050040, Almaty Printed in the printing office of the «Kazakh University» publishing house.

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