Characteristics of disperse systems and their types. Disperse system What disperse systems do you know?

7.1.Basic concepts and definitions. Topic structure 3

7.1.1. Classification of solutions 3

7.1.2.Structure of topic 4

7.2. Dispersed systems (mixtures) their types 5

7.2.1.Coarse-dispersed systems 6

7.2.2. Fine-dispersed systems (colloidal solutions) 6

7.2.3. Highly dispersed systems (true solutions) 9

7.3. Concentration, ways of expressing it 10

7.3.1. Solubility of substances. 10

7.3.2. Methods of expressing the concentration of solutions. eleven

7.3.2.1. Interest 12

7.3.2.2.Molar 12

7.3.2.3.Normal 12

7.3.2.4.Molar 12

7.3.2.5.Mole fraction 12

7.4.Physical laws of solutions 13

7.4.1.Raoult's Law 13

7.4.1.1.Change in freezing temperatures 14

7.4.1.2.Change in boiling points 15

7.4.2.Henry's Law 15

7.4.3 Van't Hoff's law. Osmotic pressure 15

7.4.4. Ideal and real solutions. 16

7.4.4.1.Activity - concentration for real systems 17

7.5.Theory of solutions 17

7.5.1.Physical theory 18

7.5.2.Chemical theory 18

7.6.Theory of electrolytic dissociation 19

7.6.1.Electrolyte solutions 20

7.6.1.1.Dissociation constant 20

7.6.1.2.Degree of dissociation. Strong and weak electrolytes 24

7.6.1.3. Ostwald's law of breeding 27

7.6.2. Electrolytic dissociation of water 27

7.6.2.1. Ionic product of water 28

7.6.2.2. Hydrogen index. Acidity and basicity of solutions 29

7.6.2.3.Acid-base indicators 29

7.7. Ion exchange reactions. 31

7.7.1.Formation of a weak electrolyte 32

7.7.2. Gas release 34

7.7.3. Formation of precipitation 34

7.7.3.1. Condition for the formation of sediment. Solubility product 34

7.7.4. Hydrolysis of salts 36

7.7.4.1. Equilibrium shift during hydrolysis 38

1. Basic concepts and definitions. Topic structure

Dispersed systems or mixtures are multicomponent systems in which one or more substances are uniformly distributed in the form of particles in the medium of another substance.

In disperse systems, a dispersed phase is distinguished - a finely divided substance and a dispersion medium - a homogeneous substance in which the dispersed phase is distributed. For example, in turbid water containing clay, the dispersed phase is solid clay particles, and the dispersion medium is water; in fog, the dispersed phase is liquid particles, the dispersion medium is air; in smoke the dispersed phase is solid particles of coal, the dispersion medium is air; in milk - dispersed phase - fat particles, dispersion medium - liquid, etc. Dispersed systems can be either homogeneous or heterogeneous.

A homogeneous disperse system is a solution.

1. Classification of solutions

Based on the size of dissolved substances, all multicomponent solutions are divided into:

coarse systems (mixtures);

finely dispersed systems (colloidal solutions);

highly dispersed systems (true solutions).

According to their phase state, solutions are:

Based on the composition of dissolved substances, liquid solutions are considered as:

electrolytes;

non-electrolytes.

1. Topic structure

1. Dispersed systems (mixtures) their types

Dispersed system - a mixture of two or more substances that are completely or practically immiscible and do not react chemically with each other. The first of the substances ( dispersed phase) finely distributed in the second ( dispersion medium). The phases are separated from each other by an interface and can be separated from each other physically (centrifuge, separate, etc.).

The main types of disperse systems: aerosols, suspensions, emulsions, sols, gels, powders, fibrous materials such as felt, foams, latexes, composites, microporous materials; in nature - rocks, soils, precipitation.

By kinetic properties dispersed phase disperse systems can be divided into two classes:

Freely dispersed systems in which the dispersed phase is mobile;

Connectedly dispersed systems in which the dispersion medium is solid, and the particles of their dispersed phase are interconnected and cannot move freely.

By particle size dispersed phase is distinguished coarse systems(suspensions) with a particle size greater than 500 nm and finely dispersed(colloidal solutions or colloids) with particle sizes from 1 to 500 nm.

Table 7.1. A variety of disperse systems.

), which are completely or practically immiscible and do not react chemically with each other. The first of the substances ( dispersed phase) finely distributed in the second ( dispersion medium). If there are several phases, they can be separated from each other physically (centrifuge, separate, etc.).

Typically dispersed systems are colloidal solutions, sols. Dispersed systems also include the case of a solid dispersed medium in which the dispersed phase is located.

Systems with dispersed phase particles of equal size are called monodisperse, and systems with particles of unequal size are called polydisperse. As a rule, the real systems around us are polydisperse.

Based on particle size, freely dispersed systems are divided into:

Ultramicroheterogeneous systems are also called colloidal or sols. Depending on the nature of the dispersion medium, sols are divided into solid sols, aerosols (sols with a gaseous dispersion medium) and lyosols (sols with a liquid dispersion medium). Microheterogeneous systems include suspensions, emulsions, foams and powders. The most common coarse systems are solid-gas systems, such as sand.

According to the classification of M. M. Dubinin, coherently dispersed systems (porous bodies) are divided into:

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See what “Dispersed system” is in other dictionaries:

disperse system- dispersed system: A system consisting of two or more phases (bodies) with a highly developed interface between them. [GOST R 51109 97, article 5.6] Source... Dictionary-reference book of terms of normative and technical documentation

disperse system- A system consisting of two or more phases (bodies) with a highly developed interface between them. [GOST R 51109 97] [GOST R 12.4.233 2007] Topics: industrial cleanliness, personal protective equipment... Technical Translator's Guide

disperse system- – a heterogeneous system consisting of two or more phases, characterized by a highly developed interface between them. General chemistry: textbook / A. V. Zholnin ... Chemical terms

disperse system- ▲ mechanical mixture fine dispersed system a heterogeneous system in which particles of one phase (dispersed) are distributed in another homogeneous phase (dispersion medium). foam (pieces of foam). foam. foam, sya. foam. frothy. foamy... ... Ideographic Dictionary of the Russian Language

disperse system- dispersinė sistema statusas T sritis chemija apibrėžtis Sistema, susidedanti iš dispersinės fazės ir dispersinės terpės (aplinkos). atitikmenys: engl. disperse system; dispersion rus. dispersion; disperse system ryšiai: sinonimas – dispersija … Chemijos terminų aiškinamasis žodynas

disperse system- dispersinė sistema statusas T sritis fizika atitikmenys: engl. disperse system vok. disperses System, n rus. disperse system, n pranc. système dispersé, m … Fizikos terminų žodynas

disperse system- a heterogeneous system of two or more phases with a highly developed interface between them. In a dispersed system, at least one of the phases (it is called dispersed) is included in the form of small particles in another... ... Encyclopedic Dictionary of Metallurgy

Physico mechanical system, consisting of a dispersed phase and a dispersion medium. There are coarse and highly dispersed (colloidal) systems.

Dispersed are called heterogeneous systems in which one substance in the form of very small particles is evenly distributed in the volume of another.

The substance that is present in smaller quantities and distributed in the volume of another is called dispersed phase. It may consist of several substances.

The substance present in larger quantities, in the volume of which the dispersed phase is distributed, is called dispersion medium. There is an interface between it and the particles of the dispersed phase; therefore, dispersed systems are called heterogeneous (inhomogeneous).

Both the dispersion medium and the dispersed phase can be represented by substances in different states of aggregation - solid, liquid and gaseous.

Depending on the combination of the aggregate state of the dispersion medium and the dispersed phase, 8 types of such systems can be distinguished.

Based on the size of the particles of substances that make up the dispersed phase, dispersed systems are divided into coarse(suspensions) with particle sizes greater than 100 nm and finely dispersed(colloidal solutions or colloidal systems) with particle sizes from 100 to 1 nm. If the substance is fragmented into molecules or ions less than 1 nm in size, a homogeneous system is formed - solution. It is uniform (homogeneous), there is no interface between the particles of the dispersed phase and the medium.

Even a quick acquaintance with dispersed systems and solutions shows how important they are in everyday life and in nature (see table).

Table. Examples of dispersed systems

Judge for yourself: without the Nile silt the great civilization of Ancient Egypt would not have taken place; without water, air, rocks and minerals, the living planet - our common home - the Earth would not exist at all; without cells there would be no living organisms, etc.

If all particles of the dispersed phase have the same size, then such systems are called monodisperse (Fig. 1, a and b). Particles of the dispersed phase of unequal size form polydisperse systems (Fig. 1, c).

Rice. 1. Freely dispersed systems: corpuscular - (a-c), fibrous - (d) and film-dispersed - (e); a, b – monodisperse; c – polydisperse system.

Dispersed systems can be freely dispersed(Fig. 1) and cohesively dispersed(Fig. 2, a - c) depending on the absence or presence of interaction between particles of the dispersed phase. Freely dispersed systems include aerosols, diluted suspensions and emulsions. They are fluid; in these systems, particles of the dispersed phase have no contacts, participate in random thermal movement, and move freely under the influence of gravity. Cohesively dispersed systems are solid; they arise when particles of the dispersed phase come into contact, leading to the formation of a structure in the form of a framework or network. This structure limits the fluidity of the dispersed system and gives it the ability to retain its shape. Powders, concentrated emulsions and suspensions (pastes), foams, gels are examples of cohesive disperse systems. A continuous mass of substance can be penetrated by pores and capillaries, forming capillary-dispersed systems (leather, cardboard, fabrics, wood).

Rice. 3. Cohesively dispersed (a-c) and capillary-dispersed (d, e) systems: gel (a), coagulant with a dense (b) and loose arched (c) structure.

Dispersed systems, in accordance with their intermediate position between the world of molecules and large bodies, can be obtained in two ways: methods of dispersion, i.e. grinding of large bodies, and methods of condensation of molecular or ionic dissolved substances.

The interaction of phases of dispersed systems means the processes of solvation (hydration in the case of aqueous systems), i.e., the formation of solvation (hydrate) shells from molecules of the dispersion medium around particles of the dispersed phase. Accordingly, according to the intensity of interaction between the substances of the dispersed phase and the dispersion medium (only for systems with a liquid dispersion medium), according to the proposal of G. Freundlich, the following dispersed systems are distinguished:

− Lyophilic (hydrophilic, if the DS is water): micellar solutions of surfactants, critical emulsions, aqueous solutions of some natural IUDs, for example, proteins (gelatin, egg white), polysaccharides (starch). They are characterized by a strong interaction of DF particles with DS molecules. In the limiting case, complete dissolution is observed. Lyophilic disperse systems are formed spontaneously due to the solvation process. Thermodynamically aggregatively stable.

− Lyophobic (hydrophobic, if the DS is water): emulsions, suspensions, sols. They are characterized by weak interaction of DF particles with DS molecules. They do not form spontaneously; work is required to form them. Thermodynamically aggregatively unstable (i.e., they tend to spontaneous aggregation of particles of the dispersed phase), their relative stability (the so-called metastability) is due to kinetic factors (i.e., low aggregation rate).

3. Weigh.

Suspend – these are dispersed systems in which the phase particle size is more than 100 nm. These are opaque systems, individual particles of which can be seen with the naked eye. The dispersed phase and the dispersed medium are easily separated by settling and filtration. Such systems are divided into:

1. Emulsions ( both the medium and the phase are liquids insoluble in each other). An emulsion can be prepared from water and oil by shaking the mixture for a long time. These are well-known milk, lymph, water-based paints, etc.

2. Suspensions (medium is a liquid, phase is a solid insoluble in it). To prepare a suspension, you need to grind the substance to a fine powder, pour it into the liquid and shake well. Over time, the particle will fall to the bottom of the vessel. Obviously, the smaller the particles, the longer the suspension will persist. These are construction solutions, river and sea silt suspended in water, a living suspension of microscopic living organisms in sea water - plankton, which feeds giants - whales, etc.

3. Aerosols suspensions in a gas (for example, in air) of small particles of liquids or solids. There are dusts, smokes, and fogs. The first two types of aerosols are suspensions of solid particles in gas (larger particles in dust), the latter is a suspension of liquid droplets in gas. For example: fog, thunderclouds - a suspension of water droplets in the air, smoke - small solid particles. And the smog hanging over the world's largest cities is also an aerosol with a solid and liquid dispersed phase. Residents of settlements near cement factories suffer from the finest cement dust always hanging in the air, which is formed during the grinding of cement raw materials and the product of its firing - clinker. Smoke from factory chimneys, smog, tiny droplets of saliva flying out of the mouth of a flu patient are also harmful aerosols. Aerosols play an important role in nature, everyday life and human production activities. Accumulation of clouds, treatment of fields with chemicals, application paint coatings using a spray, treatment of the respiratory tract (inhalation) are examples of those phenomena and processes where aerosols are beneficial. Aerosols are fogs over the sea surf, near waterfalls and fountains; the rainbow that appears in them gives a person joy and aesthetic pleasure.

For chemistry highest value have dispersed systems in which the medium is water and liquid solutions.

Natural water always contains dissolved substances. Natural aqueous solutions participate in soil formation processes and supply plants with nutrients. Complex life processes occurring in human and animal bodies also occur in solutions. Many technological processes in chemical and other industries, for example, the production of acids, metals, paper, soda, fertilizers, occur in solutions.

4. Colloidal systems.

Colloidal systems (translated from Greek “kolla” - glue, “eidos” - glue-like type) – These are dispersed systems in which the phase particle size is from 100 to 1 nm. These particles are not visible to the naked eye, and the dispersed phase and dispersed medium in such systems are difficult to separate by settling.

You know from your general biology course that particles of this size can be detected using an ultramicroscope, which uses the principle of light scattering. Thanks to this, the colloidal particle in it appears as a bright dot against a dark background.

They are divided into sols (colloidal solutions) and gels (jelly).

1. Colloidal solutions, or sols. This is the majority of the fluids of a living cell (cytoplasm, nuclear juice - karyoplasm, contents of organelles and vacuoles). And the living organism as a whole (blood, lymph, tissue fluid, digestive juices, etc.) Such systems form adhesives, starch, proteins, and some polymers.

Colloidal solutions can be obtained as a result of chemical reactions; for example, when solutions of potassium or sodium silicates (“soluble glass”) react with acid solutions, a colloidal solution of silicic acid is formed. A sol is also formed during the hydrolysis of iron (III) chloride in hot water.

A characteristic property of colloidal solutions is their transparency. Colloidal solutions are similar in appearance to true solutions. They are distinguished from the latter by the “luminous path” that is formed - a cone when a beam of light is passed through them. This phenomenon is called the Tyndall effect. The particles of the dispersed phase of the sol, larger than in the true solution, reflect light from their surface, and the observer sees a luminous cone in the vessel with the colloidal solution. It is not formed in a true solution. You can observe a similar effect, but only for an aerosol and not a liquid colloid, in the forest and in cinemas when a beam of light from a movie camera passes through the air of the cinema hall.

Passing a beam of light through solutions:

a – true sodium chloride solution;

b – colloidal solution of iron (III) hydroxide.

Particles of the dispersed phase of colloidal solutions often do not settle even during long-term storage due to continuous collisions with solvent molecules due to thermal movement. They do not stick together when approaching each other due to the presence of electric charges of the same name on their surface. This is explained by the fact that substances in a colloidal, i.e., finely divided, state have a large surface area. Either positively or negatively charged ions are adsorbed on this surface. For example, silicic acid adsorbs negative ions SiO 3 2-, of which there are many in solution due to the dissociation of sodium silicate:

Particles with like charges repel each other and therefore do not stick together.

But under certain conditions, a coagulation process can occur. When some colloidal solutions are boiled, desorption of charged ions occurs, i.e. colloidal particles lose their charge. They begin to enlarge and settle. The same thing is observed when adding any electrolyte. In this case, the colloidal particle attracts an oppositely charged ion and its charge is neutralized.

Coagulation - the phenomenon of colloidal particles sticking together and precipitating - is observed when the charges of these particles are neutralized when an electrolyte is added to the colloidal solution. In this case, the solution turns into a suspension or gel. Some organic colloids coagulate when heated (glue, egg white) or when the acid-base environment of the solution changes.

2. Gels or jellies are gelatinous sediments formed during the coagulation of sols. These include a large number of polymer gels, so well known to you confectionery, cosmetic and medical gels (gelatin, jellied meat, marmalade, bread, meat, jam, jelly, marmalade, jelly, cheese, cottage cheese, curdled milk, Bird's Milk cake) and of course, an endless variety of natural gels: minerals (opal), jellyfish bodies, cartilage, tendons, hair, muscle and nervous tissue, etc. The history of development on Earth can simultaneously be considered the history of the evolution of the colloidal state of matter. Over time, the structure of the gels is disrupted (flakes off) - water is released from them. This phenomenon is called syneresis .

Jellies − these are structured systems with the properties of elastic solids. The gelatinous state of a substance can be considered as intermediate between the liquid and solid states.

Jellies of high-molecular substances can be obtained mainly in two ways: the method of forming jellies from polymer solutions and the method of swelling of dry high-molecular substances in appropriate liquids.

The process of transition of a polymer solution or sol into jelly is called jelly formation . Gelatinization is associated with an increase in viscosity and a slowdown in Brownian motion and consists in the unification of particles of the dispersed phase in the form of a network or cells and the binding of all the solvent.

The process of jelly formation is significantly influenced by the nature of dissolved substances, the shape of their particles, concentration, temperature, process time and impurities of other substances, especially electrolytes .

Based on their properties, jellies are divided into two large groups:

a) elastic, or reversible, obtained from high-molecular substances;

b) brittle, or irreversible, obtained from inorganic hydrophobic sols.

As already mentioned, jellies of high-molecular substances can be obtained not only by the method of gelatinization of solutions, but also by the method of swelling of dry substances. Limited swelling ends with the formation of jelly and does not go into dissolution, and with unlimited swelling, jelly is an intermediate stage on the way to dissolution.

Jellies are characterized by a number of properties of solids: they retain their shape, have elastic properties and elasticity. However, their mechanical properties are determined by concentration and temperature.

When heated, the jellies transform into a viscous flow state. This process is called melting. It is reversible, since upon cooling the solution again forms a jelly.

Many jellies are capable of liquefying and turning into solutions under mechanical influence (stirring, shaking). This process is reversible, since at rest after some time the solution forms a jelly. The property of jellies to repeatedly liquefy isothermally under mechanical stress and form a jelly at rest is called thixotropy . For example, chocolate mass, margarine, and dough are capable of thixotropic changes.

Having a huge amount of water in its composition, jellies, in addition to the properties of solid bodies, also have the properties of a liquid body. Various physical and chemical processes can occur in them: diffusion, chemical reactions between substances.

Freshly prepared jellies undergo changes over time, as the process of structuring in the jellies continues. At the same time, droplets of liquid begin to appear on the surface of the jelly, which, merging, form a liquid medium. The resulting dispersion medium is a dilute polymer solution, and the dispersed phase is a gelatinous fraction. The studio calls this spontaneous process of dividing the jelly into phases, accompanied by a change in volume syneresis ( soaking).

Syneresis is considered as a continuation of the processes that determine the formation of jelly. The rate of syneresis of different jellies is different and depends mainly on temperature and concentration.

Syneresis in jellies formed by polymers is partially reversible. Sometimes heating is enough to return the jelly that has undergone syneresis to its original state. In culinary practice, this method is used, for example, to refresh porridges, purees, and stale bread. If chemical processes occur during storage of jellies, then syneresis becomes more complicated and its reversibility is lost, and the jelly ages. In this case, the jelly loses its ability to retain bound water (staling bread). The practical significance of syneresis is quite large. Most often, syneresis is undesirable in everyday life and industry. This is the staling of bread, the soaking of marmalade, jelly, caramel, and fruit jams.

5. Solutions of high molecular weight substances.

Polymers, like low-molecular substances, depending on the conditions for obtaining a solution (the nature of the polymer and solvent, temperature, etc.) can form both colloidal and true solutions. In this regard, it is customary to talk about the colloidal or true state of a substance in solution. We will not touch upon colloidal polymer-solvent systems. Let us consider only solutions of molecular-type polymers. It should be noted that due to the large size of the molecules and the peculiarities of their structure, IUD solutions have a number of specific properties:

1. Equilibrium processes in IUD solutions are established slowly.

2. The process of dissolution of the IUD is usually preceded by a swelling process.

3. Polymer solutions do not obey the laws of ideal solutions, i.e. Raoult's and Van't Hoff's laws.

4. When polymer solutions flow, anisotropy of properties occurs (unequal physical properties solution in different directions) due to the orientation of molecules in the direction of flow.

5. High viscosity of IUD solutions.

6. Due to their large size, polymer molecules tend to associate in solutions. The lifetime of polymer associates is longer than that of low molecular weight substances.

The process of dissolution of the BMC occurs spontaneously, but over a long period of time, and is often preceded by swelling of the polymer in the solvent. Polymers whose macromolecules have a symmetrical shape can go into solution without first swelling. For example, hemoglobin, liver starch - glycogen almost do not swell when dissolved, and solutions of these substances do not have high viscosity even at relatively high concentrations. While substances with highly asymmetric elongated molecules swell very strongly when dissolved (gelatin, cellulose, natural and synthetic rubbers).

Swelling is an increase in the mass and volume of the polymer due to the penetration of solvent molecules into the spatial structure of the IMC.

There are two types of swelling: unlimited, ending with complete dissolution of the IUD (for example, swelling of gelatin in water, rubber in benzene, nitrocellulose in acetone) and limited, leading to the formation of a swollen polymer - jelly (for example, swelling of cellulose in water, gelatin in cold water, vulcanized rubber in benzene).

In the world around us, pure substances are extremely rare; basically, most substances on earth and in the atmosphere are various mixtures containing more than two components. Particles ranging in size from approximately 1 nm (several molecular sizes) to 10 µm are called dispersed(Latin dispergo – scatter, spray). Various systems (inorganic, organic, polymer, protein), in which at least one of the substances is in the form of such particles, are called dispersed. Dispersed - these are heterogeneous systems consisting of two or more phases with a highly developed interface between them or a mixture consisting of at least two substances that are completely or practically immiscible with each other and do not react with each other chemically. One of the phases - the dispersed phase - consists of very small particles distributed in another phase - the dispersion medium.

Dispersed system

According to their state of aggregation, dispersed particles can be solid, liquid, gaseous, and in many cases have a complex structure. Dispersion media are also gaseous, liquid and solid. Most of the real bodies of the world around us exist in the form of dispersed systems: sea water, soils and soils, tissues of living organisms, many technical materials, food products, etc.

Classification of disperse systems

Despite numerous attempts to propose a unified classification of these systems, it is still missing. The reason is that in any classification, not all properties of disperse systems are taken as a criterion, but only one of them. Let us consider the most common classifications of colloidal and microheterogeneous systems.

In any field of knowledge, when one has to deal with complex objects and phenomena, in order to facilitate and establish certain patterns, it is advisable to classify them according to certain criteria. This also applies to the field of dispersed systems; At different times, different classification principles were proposed for them. Based on the intensity of interaction between the substances of the dispersion medium and the dispersed phase, lyophilic and lyophobic colloids are distinguished. Other techniques for classifying disperse systems are briefly outlined below.

Classification by presence or absence of interactionbetween particles of the dispersed phase. According to this classification, dispersed systems are divided into freely dispersed and coherently dispersed; the classification is applicable to colloidal solutions and solutions of high molecular weight compounds.

Freely dispersed systems include typical colloidal solutions, suspensions, suspensions, and various solutions of high-molecular compounds that have fluidity, like ordinary liquids and solutions.

Cohesively dispersed systems include the so-called structured systems, in which, as a result of the interaction between particles, a spatial openwork mesh-framework arises, and the system as a whole acquires the property of a semi-solid body. For example, sols of some substances and solutions of high-molecular compounds, when the temperature decreases or with an increase in concentration above a certain limit, without undergoing any external changes, they lose fluidity - they gelatinize (gelatinize), and pass into a gel (jelly) state. This also includes concentrated pastes and amorphous precipitates.

Classification by dispersion. The physical properties of a substance do not depend on the size of the body, but at a high degree of grinding they become a function of dispersion. For example, metal sols have different colors depending on the degree of grinding. Thus, colloidal solutions of gold of extremely high dispersion have a purple color, less dispersed ones have a blue color, and even less dispersed ones have a green color. There is reason to believe that other properties of sols of the same substance change as they are ground: A natural criterion for classifying colloidal systems by dispersity suggests itself, i.e., division of the region of the colloidal state (10 -5 -10 -7 cm) into a number of narrower intervals. Such a classification was proposed at one time, but it turned out to be useless, since colloidal systems are almost always polydisperse; monodisperse are very rare. In addition, the degree of dispersion can change over time, i.e., it depends on the age of the system.

PLAN:

1. Management…………………………………………………………………………………..2

2. Main types of disperse systems…………………………………...2

3. Formation of dispersed systems……………………………………4

4. Stability of dispersed systems.................................................... .............5

5. Classifications of dispersed systems…………………………………...8

6. Structure formation in dispersed systems and in polymer solutions……………………………………………………………….16

7. Properties of disperse systems and determination of particle size……….23

8. List of used literature. ……………………………24

INTRODUCTION

DISPERSE SYSTEMS- heterogeneous systems of two or more phases with a highly developed interface between them. Typically, one of the phases forms a continuous dispersion medium, in the volume of which the dispersed phase (or several dispersed phases) is distributed in the form of small crystals, solid amorphous particles, drops or bubbles. D. s. may have a more complex structure, for example, represent a two-phase formation, each of the phases of which, being continuous, penetrates into the volume of the other phase. Such systems include solids penetrated by a branched system of channels-pores filled with gas or liquid, some microheterogeneous polymer compositions, etc. There are frequent cases when the dispersion medium “degenerates” to thinnest layers(films) separating particles of the dispersed phase.

Main types of disperse systems.

According to dispersity, i.e., the size of particles of the dispersed phase or the ratio of the total area of ​​the interphase surface to the volume (or mass) of the dispersed phase (specific surface), D. s. conditionally divided into coarsely dispersed and finely (highly) dispersed. The latter, according to tradition, are called. colloidal-dispersed or simply colloidal systems. In coarsely dispersed systems, particles have sizes from 1 micron and above (specific surface no more than 1 m2/g), in colloidal systems - from 1 nm to 1 micron (specific surface reaches hundreds of m2/g). Dispersity is assessed by an average indicator (average particle size, specific surface area) or disperse composition (see analysis of variance). Fine-porous bodies are characterized by porosity, a concept similar to dispersion. In freely dispersed systems, there is no cohesion between particles of the dispersed phase; each particle is kinetically independent and, at sufficiently small sizes, participates in intense Brownian motion. Structured (cohesively dispersed) systems are characterized by the presence of disordered spaces. network (framework) formed by particles of the dispersed phase (see Structure formation in disperse systems). A special group consists of highly concentrated dynamic particles, in which the particles are in “constrained” conditions, such as, for example, in periodic periods. colloidal structures. Fur. The properties of freely dispersed systems are determined by Chap. arr. the properties of the dispersion medium, and of coherently dispersed systems - also the properties and number of contacts between particles of the dispersed phase (see Rheology). Based on the state of aggregation of the dispersion medium and the dispersed phase, the following is distinguished. basic types of dispersion systems: 1) aerodispersed (gas-dispersed) systems with a gas dispersion medium: aerosols (smoke, dust, mists), powders, fibrous materials such as felt. 2) Systems with liquid dispersion medium; dispersed phase m.b. solid (coarse suspensions and pastes, highly dispersed sols and gels), liquid (coarsely dispersed emulsions, highly dispersed microemulsions and latexes) or gas (coarsely dispersed gas emulsions and foams). 3) Systems with a solid dispersion medium: glassy or crystalline. bodies containing small solid particles, liquid droplets or gas bubbles, for example, ruby ​​glasses, opal-type minerals, various microporous materials. Separate groups of D. s. make up plural metallic alloys, rocks, complex compositional and other multiphase systems. Lyophilic and lyophobic D. s. with a liquid dispersion medium differ depending on how close or different the dispersed phase and the dispersion medium are in their properties (see. Lyophilicity and lyophobicity). In lyophilic D. s. intermolecular interactions on both sides of the separating phase, the surfaces differ slightly, so the beats. free surface energy (for a liquid - surface tension) is extremely low (usually hundredths of mJ/m2), the interphase boundary (surface layer) may. blurred and often comparable in thickness to the particle size of the dispersed phase. Lyophilic D. s. thermodynamically equilibrium, they are always highly dispersed, form spontaneously and, if the conditions for their occurrence are preserved, can exist indefinitely. Typical lyophilic D. s. - microemulsions, certain polymer-polymer mixtures, micellar surfactant systems, D.s. with liquid crystal dispersed phases. To lyophilic D. s. often also include minerals of the montmorillonite group that swell and spontaneously disperse in an aqueous environment, for example, bentonite clays. It should be noted that in the past “lyophilic colloids” were called. solutions of polymers, i.e. fundamentally homogeneous. systems. However, in modern terminology, the concept “colloid” refers only to microheterogeneous systems; it is not used in relation to homogeneous (single-phase) systems. In lyophobic D. s. intermolecular interaction in a dispersion medium and in a dispersed phase are significantly different; beat free surface energy (surface tension) is high - from several. units to several hundreds (and thousands) mJ/m2; the phase boundary is expressed quite clearly. Lyophobic D. s. thermodynamically nonequilibrium; large excess of free surface energy determines the occurrence of transition processes in them to a more energetically favorable state. In isothermal conditions, coagulation is possible - the convergence and association of particles that retain their original shape and size into dense aggregates, as well as the enlargement of primary particles due to coalescence - the merging of droplets or gas bubbles, collective recrystallization (in the case of a crystalline dispersed phase) or isothermal. distillation (molecular transfer) of the dispersed phase from small particles to large ones (in the case of dispersion systems with a liquid dispersion medium, the latter process is called recondensation). Unstabilized and, therefore, unstable lyophobic D. s. continuously change their disperse composition in the direction of particle enlargement until complete separation into macrophases. However, stabilized lyophobic D. s. can remain dispersed for a long time. time.

Formation of disperse systems.

Possibly in two ways: dispersion and condensation. Dispersion of macrophases with the formation of lyophilic D.s. occurs spontaneously - the energy of thermal motion is sufficient for this. This process is carried out at values ​​of surface tension s below a certain critical value. values ​​scr = bkT/d2, where d is the particle size of the dispersed phase, T is abs. t-ra, k is Boltzmann's constant, b is a dimensionless coefficient taking values ​​of approximately 10-30. Formation of lyophobic D. s. by dispersing a stable macrophase requires significant energy. costs determined by the total surface area of ​​dispersed phase particles. In real conditions, the formation of surfaces during grinding of solids or during atomization and emulsification of liquids accounts for only a small part (fractions of a percent) of the energy supplied to the system; the rest is spent on side processes and disperses in the surrounding space (see Dispersion). Condensation path of formation of D. s. associated with the nucleation of a new phase (or new phases) in a supersaturated metastable initial phase - the future dispersion medium. For a highly dispersed system to arise, it is necessary that the number of nuclei of the new phase be sufficiently large and their growth rate not too high. In addition, the presence of factors is required that limit the possibility of excessive growth and adhesion of particles of the dispersed phase. Transition of initially stable homog. systems into a metastable state can occur as a result of changes in thermodynamics. state parameters (pressure, temperature, composition). This is how, for example, natural and artificial aerosols are formed (fog - from supercooled water vapor, smoke - from steam-gas mixtures released during incomplete combustion of fuel), certain polymer systems - from solutions when the “thermodynamic quality” of the solution deteriorates. sols, organosols of metals by condensation of metal vapors together with organic vapors. liquids or when passing the former through a layer of org. liquids, colloidal dispersed polycrystalline. bodies (metal alloys, certain types of rocks and artificial inorganic materials). D.'s formation is also possible. as a result of chem. districts in homog. environment, if the product of the solution under given conditions is in a state of aggregation different from the “mother” phase, or is practically insoluble in it. Examples of such systems are aerosols with solid particles of NH4Cl (formed by the interaction of gaseous NH3 and HCl), aerosols with droplet-liquid particles of H2SO4 (by the interaction of SO3 and water vapor). In nature and technology. processes often form hydrosols different composition during the hydrolysis of salts and other compounds that are unstable to the action of water. Oxidation-reduction solutions are used to obtain Au and Ag sols, decomposition of Na2S2O3 dil. sulfuric or hydrochloric acid - to obtain a hydrosol of elemental sulfur. Chem. or thermochemical decomposition of carbonates, org. porophores (porogens, foaming agents) and other compounds. with the release of gaseous substances in initially liquid media is the basis of the industry. produced by pl. foam materials.

Stability of dispersed systems.

The stability of dispersed systems is characterized by constancy of dispersion (particle size distribution) and concentration of the dispersed phase (number of particles per unit volume). Naib. complex in theory. aspect and is important in practice. regarding the problem of stability of aerosols and liquid lyophobic D.s. A distinction is made between sedimentation stability and coagulation resistance (aggregative stability). Colloidal systems with gas and liquid dispersion media are sedimentation stable, in which Brownian motion of particles prevents sedimentation; coarse systems with the same density of their constituent phases; systems, the rate of sedimentation in which can be neglected due to the high viscosity of the medium. In aggregatively stable D. s. directly contacts between particles do not occur, the particles retain their individuality. If the aggregative stability of D. is violated. particles, approaching each other in the process of Brownian motion, join irreversibly or the rate of aggregation becomes significantly greater than the rate of disaggregation. Direct point ("atomic") contacts arise between solid particles, which can then turn into phase (cohesive) contacts, and the contact of drops and bubbles is accompanied by their coalescence and a rapid reduction in the total area of ​​the interphase surface. For such systems, the loss of aggregative stability also means the loss of sedimentation stability. In aggregation-stable systems, the dispersed composition may change due to isothermal conditions. distillation - they say. transfer of the dispersed phase from small particles to larger ones. This process is due to the dependence of the saturated vapor pressure (or the concentration of the saturated solution) on the curvature of the phase interface (see Capillary phenomena). Aggregative stability and long-term existence of lyophobic D. s. with the preservation of their properties is ensured by stabilization. For highly dispersed systems with a liquid dispersion medium, the introduction of aqueous stabilizers (electrolytes, surfactants, polymers) is used. In the Deryagin-Landau-Verwey-Overbeck stability theory (DLFO theory) basic. the role is played by ion-electrostatic. stabilization factor. Stabilization is provided electrostatically. repulsion of diffuse parts of double electric. layer, which is formed by the adsorption of electrolyte ions on the surface of particles. At a certain distance between particles, the repulsion of diffuse layers determines the presence of a minimum on the potential. curve (distant, or secondary, minimum; see figure). Although this minimum is relatively shallow, it can prevent particles attracted by intermolecular interaction forces from further approaching each other. The near, or primary, minimum corresponds to strong adhesion of particles, in which case the energy of thermal motion is not enough to separate them. When approaching a distance corresponding to this minimum, the particles combine into aggregates, the formation of which leads to the loss of aggregative stability by the system. In this case, the stability of the system to coagulation is determined by the height of the energy. barrier.

Dependence of the interaction energy E between particles on the distance R: 1 and 2 are the near and far minima, respectively.

When introduced into D. s. as a surfactant stabilizer, a stabilization factor could be "thermodynamic elasticity" of films of the medium separating particles. Stabilization is ensured by the fact that when particles, for example, droplets or gas bubbles, come closer together, the stretching and thinning of the surfactant-containing layer separating them occurs, and, as a result, adsorption is disrupted. balance. The restoration of this equilibrium leads to an increase in the stability of the layer of medium separating the particles. Hydrodynamic resistance to the displacement of a liquid dispersion medium from the layer between approaching particles is one of the kinetic. factors of stabilization of D. s. It is especially effective in systems with a highly viscous dispersion medium, and when the latter vitrifies, it makes the system unlimitedly resistant to particle aggregation and coalescence. Structural-mech. the stabilization factor, according to P. A. Rebinder, arises when polymolecular protective layers are formed at the interphase boundary from micelle-forming surfactants, high-molecular compounds, and sometimes thin continuous or discrete phase films. The interfacial protective layer must have the ability to resist deformation and destruction, sufficient mobility to “heal” defects that have arisen in it and, most importantly, be lyophilized from the outside. side facing the dispersion medium. If the protective layer is not lyophilic enough, it, while protecting particles from coalescence, will not be able to prevent coagulation. Structural-mech. the barrier is essentially a complex stabilization factor, which includes thermodynamic, kinetic. and structural components. It is universal and capable of providing high aggregative stability of any D.S. with a liquid dispersion medium, including highly concentrated ones, max. important in practice respect. Basic St. D. s. are determined by surface phenomena: adsorption, the formation of a double electrical layer and the electrokinetic phenomena caused by it, contact interactions of particles of the dispersed phase. The particle size is determined by the optical (light scattering, etc.) and molecular kinetic. Saints (diffusion, thermophoresis, osmosis, etc.). D. s. ubiquitous in nature. These are rocks, soils, soils, atm. and hydrosphere precipitation, grows. and animal tissues. D. s. widely used in technology. processes; in the form of D. s. Most industrial products are produced. products and household items. Highly dispersed tech. materials (filled plastics, dispersion-strengthened composite materials) are extremely durable. On highly developed surfaces, heterogeneity occurs intensively. and heterogeneous-catalytic. chem. processes. The doctrine of D. s. and surface phenomena in them constitute the essence of colloidal chemistry. Self-sufficient. a section of colloidal chemistry - physical and chemical mechanics - studies the laws of structure formation and mechanics. Holy structured D. s. and materials in their connection with physical-chemical. phenomena at interphase boundaries.

Classifications of dispersed systems.

According to the degree of fragmentation (dispersity), systems are divided into the following classes: coarse, the particle size of which is more than 10 -5 m; finely dispersed (microheterogeneous) with particle sizes from 10 -5 to 10 -7 m; colloidal-disperse (ultramicro-heterogeneous) with particles ranging in size from 10 -7 to 10 -9 m. If we focus on the two main components of disperse systems, then one of them should be assigned the role of a dispersion medium, and the other - the role of a dispersed phase. In this case, all disperse systems can be classified according to the phase states of aggregation.

This classification was proposed by Ostwald and is widely used to this day. A disadvantage of the classification should be considered the impossibility of classifying dispersed systems prepared with a solid or liquid dispersed phase into any class if the particle size is several nanometers. An example of such a classification is given in table. 1.

Academician P.A. Rehbinder proposed a more advanced classification of disperse systems according to the aggregative states of the phases. He divided all dispersed systems into two classes: freely dispersed systems and continuous (or coherently dispersed) systems (Tables 2 and 3). In freely dispersed systems, the dispersed phase does not form continuous rigid structures (grids, trusses or frames). These systems are called sols. In continuous (cohesively dispersed) systems, particles of the dispersed phase form rigid spatial structures (grids, frames, trusses). Such systems resist shear deformation. They are called gels.

According to Rehbinder's classification, a dispersed system is designated by a fraction in which the dispersed phase is placed in the numerator and the dispersion medium in the denominator. For example: T 1 / F 2. Index 1 denotes the dispersed phase, and index 2 denotes the dispersion medium.

Colloidal chemistry studies the properties of both fine and coarse systems; both free- and cohesively dispersed systems.

The inclusion in one science of such a large number of diverse systems, different both in the nature of the phases and in the particle sizes and aggregative state of the phases, is based on the fact that they all have common properties - heterogeneity and fundamental thermodynamic instability. Ultramicroheterogeneous systems with free particles occupy a central place in colloidal chemistry. These are the so-called colloidal systems.

Table 1

Classification of disperse systems according to the phase states of aggregation.

Colloidal systems are unusually labile, i.e. unstable. For many of them, adding a tiny amount of electrolyte is enough to cause precipitation. The reason for such an easy change in the state of colloidal systems is associated with the variability of the degree of their dispersity. There are two types of stability of any fragmented system - kinetic and aggregative.

table 2

Examples of freely dispersed systems

This classification is based on the state of aggregation of the phases of the dispersed system.

The concept of aggregative stability, which was first introduced by N.P. Peskov, implies the absence of aggregation, i.e. reducing the degree of dispersion of the colloidal system during storage. To determine kinetic stability, it is necessary to study the conditions for the release of dispersed particles in a gravitational or centrifugal field. The rate of such release depends on the intensity of the Brownian motion of particles, i.e. on the degree of dispersion of the system and the difference in density of the dispersion medium and dispersed phase, as well as on the viscosity of the medium.

Table 3

Cohesively dispersed systems

If they want to determine the aggregative stability of a system, then they examine the conditions of constancy (or, on the contrary, inconstancy) of the degree of dispersion of the system. One of the most striking and characteristic differences between a colloidal system, both from a true solution and from coarsely dispersed systems, is that their degree of dispersion is an extremely variable value and can vary depending on a wide variety of reasons.

This classification is based on the state of aggregation of the phase interface.

Based on the above, let us define colloidal systems.

Colloidal systems are two- or multiphase systems in which one phase is in the form of individual small particles distributed in another phase. Such ultramicroheterogeneous systems with a certain (colloidal) dispersion exhibit the ability to undergo intense Brownian motion and have high kinetic stability.

Having a highly developed phase interface and, consequently, a huge excess of free surface energy, these systems are fundamentally thermodynamically unstable, which is expressed in the aggregation of particles, i.e. in the absence of aggregative stability. However, these properties do not exhaust all the features by which colloidal systems differ from other systems. So, for example, at first glance it seems incomprehensible why colloidal particles, performing energetic movements and colliding with each other, do not always stick together into larger aggregates and do not precipitate, as would be expected based on the second law of thermodynamics, since in this case The total surface would decrease, and with it the free energy.

It turns out that in many cases the stability of such systems is associated with the presence of a stabilizer layer on the surface of colloidal particles. Thus, a necessary condition for creating stable colloidal systems is the presence of a third component - a stabilizer. Stabilizers of colloidal systems can be electrolytes or some other substances that do not have an electrolyte nature, for example, high molecular weight compounds (HMCs) or surfactants. The mechanism of stabilization by electrolytes and nonelectrolytes is significantly different.

The influence of electrolytes on the stability of colloidal systems is complex. In some cases, insignificant additions of electrolyte can lead to disruption of the stability of the system. In others, the introduction of an electrolyte helps increase stability.

The formation of adsorption layers of stabilizers such as surfactants becomes especially important in the presence of two-dimensional structures with enhanced structural and mechanical properties. In many cases, stabilization is achieved by covering only 40-60% of the surface of colloidal particles with a monolayer, when the protective layer is discontinuous (in the form of islands). Maximum stability is achieved, naturally, with the formation of a completely saturated monomolecular layer. The structural and mechanical properties of adsorption layers largely determine the behavior of colloidal systems. These layers can be formed or modified by small amounts of any dissolved substances, therefore it becomes possible to regulate a number of properties of colloidal systems, which is widely used in various practical applications.

Colloidal systems consisting of particles of a dispersed substance capable of freely moving in a liquid dispersion medium together with molecules or ions of a third component (stabilizer) adsorbed on their surface are called lyosols, and the particles themselves, which have a complex structure, are called micelles.

Based on the nature of the interaction of colloidal particles with the dispersion medium, lyosols can be divided into lyophilic and lyophobic. This classification was first proposed by the German colloid scientist Freundlich. He divided all systems into two classes - lyophilic and lyophobic. In accordance with the concepts developed by Freundlich, systems are called lyophobic if the particles of the dispersed phase do not interact with the dispersion medium, do not solvate or dissolve in it. Lyophilic systems are systems whose dispersed phase particles interact intensively with the dispersion medium.

Lyophobic systems include sols precious metals, sols of metalloids (sulfur, selenium, tellurium), dispersions of polymers in water (for example, polystyrene, fluorolone), sols of arsenic, antimony, cadmium, mercury sulfides, sols of iron and aluminum hydroxides, etc. These systems are characterized by so-called kinetic stability and aggregative instability and require stabilization. Freundlich classified solutions formed by the dissolution of natural or synthetic IUDs as lyophilic colloidal systems. These are solutions of proteins, starch, pectins, gums, cellulose ethers and various resins, both natural and synthetic.

Thus, IUD solutions were previously considered as lyophilic colloidal systems. They were considered two-phase dispersed systems and thus the essence of the Freundlich classification was reduced to molecular interactions between the dispersed phase and the dispersion medium. It was on this basis that a division was made into lyophilic and lyophobic systems. Lyophilic systems were considered two- or multiphase, thermodynamically unstable, and not subject to the Gibbs phase rule. But this idea turned out to be wrong. In fact, it has now been reliably established that IUD solutions are true solutions, i.e. single-phase systems, homogeneous, thermodynamically stable and subject to the Gibbs phase rule. It was believed that reversibility is characteristic property lyophilic colloidal systems, but this is not so, because in this case the IUD solutions are not dispersed systems.

In this regard, Academician V.A. Kargin, back in 1948, drew attention to the fact that Freundlich’s classification was completely incorrect and even moreover, harmful.

In order not to change the meaning of these terms, P.A. Rebinder proposed formalizing the concepts of lyophilic and lyophobic colloidal systems. He divided dispersed multi- or two-phase systems into two classes, based on the value of the specific interphase energy (surface tension).

Dispersed systems with a sufficiently high interphase tension (s 12), greater than a certain boundary value s m, were classified as lyophobic systems:

s 12 > s m . (1)

These systems are characterized by large interphase free energy, therefore the phase boundary is sharply expressed: the system is aggregatively unstable and requires the introduction of a stabilizer. The dispersion of such systems is arbitrary.

Lyophilic systems are two-phase colloidal systems with low, although positive, interphase free energy less than or equal to the cutoff value,

s 12 ≤ s m. (2)

These are systems with very low interphase energy, they are thermodynamically stable and form spontaneously. Their dispersion is quite definite and is in the colloidal region.

The fact that disperse systems are classified according to the value of free surface energy shows that colloidal phenomena are closely related to the properties of the phase interface.

Lyophilic systems include:

1) so-called critical emulsions, formed as a result of a decrease in surface tension when heated to a temperature close to the temperature of unlimited mixing, or as a result of the addition of very large quantities of surfactants;

2) associative colloidal systems formed in an aqueous environment by substances such as soaps, some dyes and tanning agents, and in a non-aqueous environment by some surfactants. Such substances in dilute solutions are in a molecular state; with increasing concentration, aggregation of molecules occurs with the formation of particles of colloidal size, i.e. micelles are formed. The concentration of a substance in a solution at which a transition from a true solution to a colloidal one occurs is usually called the critical micelle concentration (CMC).

Dispersed systems can be classified according to the specific surface area and porosity of the dispersed phase.

In those processes in which two contacting phases participate, the properties of the interface, or boundary layer, separating one phase from the other are of great importance. The molecules that make up such layers have special properties. If we consider the monolithic phase, then the number of molecules forming the surface layer can be neglected in comparison with the huge number of molecules in the volume of the body. We can assume that the energy reserve of the system is proportional to the mass contained in the volume of the body.

When grinding a solid body, the number of molecules in the surface layer increases and reaches maximum value in colloidal disperse systems. Therefore, the processes occurring in disperse systems are determined by the properties of the surface layers at the interface. The formation of foams, emulsions, mists, flotation processes, wetting and dispersion, sorption technology and many, many others are based on the properties of interphase surfaces in disperse systems.

Specific surface area is the ratio of the surface of a body to its volume or mass:

A beat = A/V or A beat = A/Vr, (3)

where A beat, A are the specific and total surface, respectively; r is the density of the substance, V is the volume of the body.

For cubic particles

A beat = 6a 2 /a 3 = 6a -1

A beat = 6a 2 /a 3 r = 6/ar (m 2 /kg). (4)

For spherical particles

A beat = 4 r 2 /(4/3 r 3) (m -1),

A beat = 3/r (m -1),

A beat = 3/rr (m 2 /kg). (5)

If you take a cube of a substance, divide its three sides into 10 parts and draw planes in three directions, you will get smaller cubes. Such a process can be considered as a simulation of the dispersion process. The change in specific surface area during the dispersion process is shown in table. 4.

Table 4

Dependence of specific surface area on dispersion

Fibers, threads and films play an important role in textile colloidal systems. The specific surface area of ​​such systems can be calculated using the formulas:

for film

A beat = 2l 2 /l 2 a = 2/a, (6)

where a is the thickness of the film, l is its width and length;

for cylinder (fiber, thread)

A beat = 2lr /r 2 l = 2/r, (7)

where r is the radius of the cylinder, l is its length.

Cohesive-disperse systems - porous bodies - along with the external specific surface, can be characterized by the size (radius) of the pores, their volume and internal specific surface. A convenient classification of pores by size was proposed by M.M. Dubinin. In accordance with this classification, all porous bodies can be divided into three classes (depending on adsorption properties): microporous bodies with a pore radius of 2·10 -9 m, mesoporous (transition porous) - (2/50) · 10 -9 m, macroporous 50·10 -9 m.

Microporous bodies in Lately divided into ultra- and supermicroporous. This classification very approximately reflects the entire spectrum of possible pore sizes (from macropores through mesopores and micropores to subatomic “pores” in the form of spaces between macrocrystals in polymers or point defects in crystals). In this regard, it should be noted that any classification cannot fully cover the entire variety of dispersed systems existing in nature and technological practice.

Structure formation in disperse systems and in polymer solutions.

With an increase in the concentration of the dispersed phase in dispersed systems (or the concentration of dissolved polymers), the formation of such aggregates of particles (or associates of macromolecules) is possible that cause the flow of such systems to deviate from the laws of Newton and Poiseuille. Such liquids are called abnormally viscous, and the concentration at which a qualitative change in the properties of the system occurs is called the critical concentration of structure formation. When a critical concentration of the dispersed phase is reached in a dispersed system, a spatial structure spontaneously arises from particles interacting with each other.

The formation of a strong structure, called crystalline, results from direct contact between particles, i.e. such a contact at which the phase boundary between particles disappears. This process is observed during the formation of a dispersed system by the condensation method, when individual crystals grow together: during the hardening of concrete, during the formation of paper or non-woven material, the formation of spatial networks during polymerization, etc. The interaction of particles through a thin layer of the liquid phase leads to the formation of coagulation contacts. After destruction, these contacts are reversibly restored. This property is called "thixotropy". Such contacts are possible in pigment pastes, in ceramic masses, in solutions and dispersions of polymers. The ability to reversibly restore the structure after removing the load is based on the action of sizing agents and thickeners in printing inks when coloring textile materials, as well as gluing fibers with latex when producing nonwoven materials, preserving the shape of ceramic products, holding varnishes, paints and enamels on vertical walls, etc. .

Coagulation structures are characterized by relatively low interaction energies and in most cases arise with a partial decrease in the stability of dispersed systems. In such structures, the average distance between particles corresponds to the equilibrium thickness of liquid films and is characterized by the first or second minimum in the potential energy curves of pair interaction of particles.

In accordance with the method of formation of coagulation structures, particles can be located at distances H 1 » 10 -9 m or H 2 » 10 -7 m.

The interaction energy in the first potential minimum is two orders of magnitude higher than the interaction energy in the second potential minimum (potential well). In practice, structure formation with fixation of particles in the second potential minimum is more common.

The volume fraction of the dispersed phase at which the formation of a coagulation structure occurs depends on the shape of the particles. Asymmetric particles can form a structure at a much lower concentration than spherical ones. The asymmetric shape of the particles is characteristic of iron and aluminum hydroxides, clay and some pigments. The strength of a structure is characterized by the stress required to destroy the spatial structure.

Structured liquids do not obey Newton's and Poiseuille's laws of flow. There are two types of structured liquid: with a liquid-like structure and with a solid-like structure.

Liquids with a liquid-like structure are characterized by rheological flow curves that do not have a critical shear stress, and there are two linear sections of pseudo-Newtonian flow.

Solid structures must be destroyed before flow begins. In other words, such a structure before destruction has the properties of a solid body.

The area of ​​colloidal chemistry that deals with the study of the patterns of formation and destruction of structure in dispersed systems and in polymer solutions is called “rheology”. In rheology, they operate with such concepts as deformation, i.e. relative displacement of part of the system without violating its integrity. The deformation can be elastic and residual. With elastic deformation, the shape of the body is restored after the stress is removed.

In Fig. Figure 2.30 shows a diagram of a uniform displacement of a cube with an edge length l, conditionally isolated from the system under study, under the action of a shear stress P. The measure of the shift is the ratio of the displacement x to the original length of the cube edge l, i.e. height at which displacement occurs

x/l = tga = g, (2.4.52)

where a is the displacement angle of the structure element.

A measure of the strain rate is the displacement rate gradient:

Rheology operates with three idealized relationships between P and g(or) to describe three structural properties (elasticity, viscosity and plasticity) and uses combinations of these relationships to describe more complex processes occurring in structured disperse systems.

Elastic deformation (or elasticity) is proportional to shear stress:

where E is Young's modulus.

Equation (2.4.54) is called Hooke's law. The dependence, which is described by equation (2.4.54) for an ideal elastic body, is shown in Fig. 2.31, a. The physical model of Hooke's ideal elastic body is usually depicted in the form of a spiral spring, attached to one end and stretched at the other.

A measure of elasticity is Young's modulus, defined as ctga of the dependence shown in Fig. 2.31, a. This dependence for an ideal body is linear. The physical meaning of elastic deformation is a change in interatomic distances when stress is created and the body strives to return atoms to the original equilibrium state, characterized by a minimum of free energy. In this regard, an ideal elastic body restores its shape and size almost instantly after the stress is removed. To restore the original size and shape in real elastic bodies, some insignificant time is required.

Viscous flow is described by Newton's equation (2.4.1, a) in the form. The diagram of the viscous flow model and the dependence of the displacement velocity gradient on stress are shown in Fig. 2.31, b. The viscosity of a liquid is defined as сtgb. A mechanical model of an ideal viscous Newtonian fluid is a piston in a cylinder, between which flow is possible.

The physical model of viscous flow is associated with a thermally activated process of restructuring of interacting

friend of molecules. Naturally, under the action of stress, some bonds between liquid molecules are broken, while others are formed again. In a truly viscous Newtonian fluid, the viscosity coefficient remains constant from very small loads up to stresses at which laminar flow becomes turbulent. In some cases, when studying viscous flow, a value is used that is the inverse of viscosity, which is called fluidity.

Plasticity, or plastic flow, is not a linear function of stress. As a model of plastic deformation, use solid, lying on a plane (Fig. 2.31, c) and held in place by the forces of dry friction up to a certain voltage that can overcome this dry (Coulomb) friction. Such a flow is possible, for example, in pigment pastes, when there is a sequential destruction and restoration of contacts between particles that are fixed in space through a certain layer of the liquid phase. In the event that a crystalline structure is formed in the system through direct contact between particles, the flow will begin only after the irreversible destruction of such contacts and the critical stress will correspond to their strength.

Of course, in the practical application of structure formation

and destruction (for example, when the structure in polymer-thickened printing inks is destroyed during mixing and during its application to fabric and when the structure is restored in the design that is applied to the fabric, or when applying a polymer solution - a sizing agent - to threads), can simultaneously Various types of deformations also appear: elastic deformation, then viscous or plastic flow and subsequent structuring.

If external stress in the system is spent on overcoming elastic deformation and viscous flow, then use the model proposed by Maxwell, from series-connected elements of the Hooke and Newton models (Fig. 2.32, a). In such systems, a typical manifestation of stress relaxation is described by the equation

P 0 (t) = P 0 exp(t/t p), (2.4.55)

where P 0 = E 0 g 0 - initial voltage; t r = h/E - relaxation time.

At t >t p, Maxwell's model corresponds to a liquid-like flow. The relaxation phenomenon is due to the fact that a certain time is required for the restructuring of the structure at a relatively low voltage. Therefore, with a short-term (instantaneous) application of voltage, gradually decreasing internal stresses arise in the system. It is possible that the release of internal stress will be realized at t®¥. A liquid described by the Maxwell model is characterized by irreversible deformation.

Thus, the properties of the system (solid or liquid) depend on the relaxation time, determined by the intersection of the tangent to the initial section of the deformation curve with the abscissa axis (see Fig. 2.32, a).

If a system experiences an increase in deformation over time at constant stress and a complete decrease in deformation within a certain time after the load is removed, then such systems are described by the Kelvin-Voigt model, consisting of elements of the Hooke and Newton models connected in parallel (Fig. 2.33). This model is typical for a mechanically reversible solid structured body. For such a structure, the equation with P = const is usually used

g(t) = P 0 / E . (2.4.56)

This equation describes the ascending branch of the curve in Figure 2.33. The descending branch (at P = 0) is described by the equation

g = g m exp(- t/t p) . (2.4.57)

The most accurate model for describing the behavior of real systems should be considered a model of elements of the Newton and Coulomb models connected in parallel, proposed by Bingham. The model diagram and deformation curve are shown in Fig. 2.34.

At stresses less than the yield stress P t, the system has elastic properties. After reaching this stress, plastic flow begins, to describe which Bingham proposed the equation

Such a viscoplastic flow is characteristic of many coagulation structures - pigmented melts and solutions of polymers, printing inks, clay solutions, concentrated emulsions, etc. Often an increase in stress leads to additional destruction of the structure. In such systems, we should talk about the “effective” viscosity h ef, which decreases with increasing stress to a certain limiting value corresponding to the complete destruction of the structure in the system.

Properties of disperse systems and determination of particle size.

The section “Properties of colloidal systems” includes consideration of diffusion, Brownian motion, osmosis, sedimentation, light scattering and absorption, and are also considered general principles determining the most important characteristic of systems - the average particle size. Particles in dispersed systems usually have a size distribution, so students’ knowledge of how to determine the parameters of these distributions will allow them to correctly understand that the properties of colloidal systems are a function of not only the degree of fragmentation (dispersity) of the crushed (dispersed) phase, but also its particle size distribution.

This fact is manifested in those industrial disperse systems that are used in the production and refinement of textile materials, for example, when using dispersed and sulfur dyes or pigment dispersions when printing patterns on fabrics and dyeing fibers in the mass. During storage in disperse systems (paints) based on pigments or in a colored mass of fiber-forming polymer, a coarse fraction is released or an uneven distribution of particles in the polymer mass occurs, which can change the shade or even color of the colored fibers, since the intensity of light reflection and its scattering depend on particle size.

Determining the particle or droplet size of the emulsion is also important for creating an effective process for emulsifying natural fibers during their processing or during the auxiliary processing of synthetic fibers. The interaction of fibers with particles, for example, polymer latexes used for gluing fibers in nonwoven materials or in finishing fabrics, depends on their size, therefore, when considering the theoretical aspects of particle adhesion to fibers, coagulation (aggregation of particles) and heterocoagulation (deposition of particles on fibers) , the ability to determine particle sizes must undoubtedly be one of the most important skills that students will develop after studying this section of the textbook.

Assessing the dispersity of heterogeneous systems is a complex task due to the diversity of the shape of their particles, polydispersity, and possible aggregation of primary particles. Therefore, a certain average value is usually determined and an error of 10-20% is considered acceptable.

List of used literature:

1. Rebinder P.A., Surface phenomena in dispersed systems.

2. Colloidal chemistry, El. works, M., 1978; Deryagin B.V., "Advances in Chemistry", 1979, v. 48, v. 4, p. 675-721

3. Urev N.B., Highly concentrated disperse systems, M., 1980

4. Coagulation contacts in dispersed systems, M., 1982

5. Capillary chemistry, ed. K. Tamaru, trans. from Japanese, M., 1983

6. Shchukin E. D., Pertsov A. V., Amelina E. A., Colloid Chemistry, M., 1982

7. Also lit. under articles Colloidal chemistry. Surface phenomena. Physico-chemical mechanics. L. A. Shits. E. D. Shchukin.