Chemistry. Dispersed systems - what is it? Dispersed systems: definition, classification A disperse system is formed by mixing

Heterogeneous or heterogeneous, is considered a system that consists of two or more phases. Each phase has its own interface, which can be mechanically separated.

A heterogeneous system consists of a dispersed (internal) phase and a dispersed (external) medium surrounding the particles of the dispersed phase.

Systems in which liquids are the external phase are called inhomogeneous liquid systems, and systems in which gases are the external phase are called inhomogeneous gas systems. Heterogeneous systems are often called dispersed systems.

The following are distinguished: types of heterogeneous systems: suspensions, emulsions, foams, dusts, fumes, mists.

Suspension is a system consisting of a liquid dispersed phase and a solid dispersed phase (for example, sauces with flour, starch milk, molasses with sugar crystals). Depending on the particle size, suspensions are divided into coarse (particle size more than 100 µm), fine (0.1–100 µm) and colloidal (0.1 µm or less).

Emulsion is a system consisting of a liquid and drops of another liquid distributed in it that do not mix with the first (for example, milk, a mixture of vegetable oil and water). Under the influence of gravity, emulsions separate, but with small droplet sizes (less than 0.4–0.5 μm) or when stabilizers are added, the emulsions become stable, unable to separate over a long period.

An increase in the concentration of the dispersed phase can cause its transition to the dispersed phase, and vice versa. This mutual transition is called phase inversion. There are gas emulsions in which the dispersion medium is liquid and the dispersed phase is gas.

Foam is a system consisting of a liquid dispersed phase and gas bubbles distributed in it (gas dispersed phase) (for example, creams and other whipped products). Foams are similar in properties to emulsions. Emulsions and foams are characterized by phase inversion.

Dusts, fumes, and mists are aerosols.

Aerosols called a dispersed system with a gaseous dispersion medium and a solid or liquid dispersed phase, which consists of particles from quasi-molecular to microscopic sizes that have the property of being suspended for a more or less long time (for example, flour dust formed during sifting, transportation of flour; sugar dust generated during, etc.). Smoke is formed when solid fuel is burned, fog is formed when steam condenses.

In aerosols, the dispersion medium is gas or air, and the dispersed phase in dust and smoke is solids, and in fogs it is liquid. The size of solid dust particles is 3–70 microns, smoke – 0.3–5 microns.

Fog is a system consisting of a gas dispersion medium and liquid droplets distributed in it (liquid dispersed phase). The size of liquid droplets formed as a result of condensation in fog is 0.3–3 μm. A qualitative indicator characterizing the uniformity of aerosol particles in size is the degree of dispersion.

An aerosol is called monodisperse when its constituent particles are of the same size, and polydisperse when it contains particles of different sizes. Monodisperse aerosols practically do not exist in nature. Only some aerosols are close in particle size to monodisperse systems (fungal hyphae, specially produced mists, etc.).

Dispersed, or heterogeneous, systems, depending on the number of dispersed phases, can be single- or multicomponent. For example, a multicomponent system is milk (has two dispersed phases: fat and protein); sauces (dispersed phases are flour, fat, etc.).

It is quite difficult to find a pure substance in nature. In different states they can form mixtures, homogeneous and heterogeneous - dispersed systems and solutions. What are these connections? What types are they? Let's look at these questions in more detail.

Terminology

First you need to understand what disperse systems are. This definition refers to heterogeneous structures, where one substance, as tiny particles, is distributed evenly in the volume of another. The component that is present in smaller quantities is called the dispersed phase. It may contain more than one substance. The component present in larger volume is called the medium. There is an interface between the particles of the phase and it. In this regard, dispersed systems are called heterogeneous - heterogeneous. Both the medium and the phase can be represented by substances in various states of aggregation: liquid, gaseous or solid.

Dispersed systems and their classification

In accordance with the size of the particles included in the phase of substances, suspensions and colloidal structures are distinguished. The former have element sizes of more than 100 nm, and the latter - from 100 to 1 nm. When a substance is crushed into ions or molecules whose size is less than 1 nm, a solution is formed - a homogeneous system. It differs from others in its homogeneity and the absence of an interface between the medium and particles. Colloidal disperse systems are presented in the form of gels and sols. In turn, suspensions are divided into suspensions, emulsions, and aerosols. Solutions can be ionic, molecular-ionic and molecular.

Suspend

These disperse systems include substances with particle sizes greater than 100 nm. These structures are opaque: their individual components can be seen with the naked eye. The medium and phase are easily separated upon settling. What are suspensions? They can be liquid or gaseous. The former are divided into suspensions and emulsions. The latter are structures in which the medium and phase are liquids that are insoluble in each other. These include, for example, lymph, milk, water-based paint and others. A suspension is a structure where the medium is a liquid and the phase is a solid, insoluble substance. Such dispersed systems are well known to many. These include, in particular, “milk of lime,” sea or river silt suspended in water, microscopic living organisms common in the ocean (plankton), and others.

Aerosols

These suspensions are distributed small particles of liquid or solid in a gas. There are fogs, smoke, dust. The first type is the distribution of small liquid droplets in a gas. Dusts and fumes are suspensions of solid components. Moreover, in the former the particles are somewhat larger. Natural aerosols include thunderclouds and fog itself. Smog, consisting of solid and liquid components distributed in gas, hangs over large industrial cities. It should be noted that aerosols as dispersed systems are of great practical importance and perform important tasks in industrial and domestic activities. Examples of positive results from their use include treatment of the respiratory system (inhalation), treatment of fields with chemicals, and spraying paint with a spray bottle.

Colloidal structures

These are dispersed systems in which the phase consists of particles ranging in size from 100 to 1 nm. Such components are not visible to the naked eye. The phase and medium in these structures are separated with difficulty by settling. Sols (colloidal solutions) are found in living cells and in the body as a whole. These fluids include nuclear juice, cytoplasm, lymph, blood and others. These dispersed systems form starch, adhesives, some polymers, and proteins. These structures can be obtained through chemical reactions. For example, during the interaction of solutions of sodium or potassium silicates with acidic compounds, a silicic acid compound is formed. Externally, the colloidal structure is similar to the true one. However, the former differ from the latter by the presence of a “luminous path” - a cone when a beam of light is passed through them. Sols contain larger phase particles than true solutions. Their surface reflects light - and the observer can see a luminous cone in the vessel. There is no such phenomenon in a true solution. A similar effect can also be observed in a movie theater. In this case, the light beam passes not through a liquid, but an aerosol colloid - the air of the hall.

Precipitation of particles

In colloidal solutions, phase particles often do not settle even during long-term storage, which is associated with continuous collisions with solvent molecules under the influence of thermal motion. When approaching each other, they do not stick together, since electric charges of the same name are present on their surfaces. However, under certain circumstances, a coagulation process can occur. It represents the effect of colloidal particles sticking together and precipitating. This process is observed when charges are neutralized on the surface of microscopic elements when an electrolyte is added. In this case, the solution turns into a gel or suspension. In some cases, the coagulation process is observed when heated or in case of changes in the acid-base balance.

Gels

These colloidal disperse systems are gelatinous sediments. They are formed during the coagulation of sols. These structures include numerous polymer gels, cosmetics, confectionery, and medical substances (Bird's Milk cake, marmalade, jelly, jellied meat, gelatin). These also include natural structures: opal, jellyfish bodies, hair, tendons, nervous and muscle tissue, cartilage. The process of development of life on planet Earth can, in fact, be considered the history of the evolution of the colloidal system. Over time, the gel structure is disrupted, and water begins to be released from it. This phenomenon is called syneresis.

Homogeneous systems

Solutions include two or more substances. They are always single-phase, that is, they are a solid, gaseous substance or liquid. But in any case, their structure is homogeneous. This effect is explained by the fact that in one substance another is distributed in the form of ions, atoms or molecules, the size of which is less than 1 nm. In the case when it is necessary to emphasize the difference between a solution and a colloidal structure, it is called true. In the process of crystallization of a liquid alloy of gold and silver, solid structures of different compositions are obtained.

Classification

Ionic mixtures are structures with strong electrolytes (acids, salts, alkalis - NaOH, HC104 and others). Another type is molecular-ion disperse systems. They contain a strong electrolyte (hydrogen sulfide, nitrous acid and others). The last type is molecular solutions. These structures include non-electrolytes - organic substances (sucrose, glucose, alcohol and others). A solvent is a component whose state of aggregation does not change during the formation of a solution. Such an element may, for example, be water. In a solution of table salt, carbon dioxide, sugar, it acts as a solvent. In the case of mixing gases, liquids or solids, the solvent will be the component of which there is more in the compound.

Dispersion systems can be divided according to the particle size of the dispersion phase. If the particle size is less than one nm, these are molecular ionic systems, from one to one hundred nm are colloidal, and more than one hundred nm are coarse. The group of molecularly dispersed systems is represented by solutions. These are homogeneous systems that consist of two or more substances and are single-phase. These include gas, solid or solutions. In turn, these systems can be divided into subgroups:
- Molecular. When organic substances such as glucose combine with non-electrolytes. Such solutions were called true so that they could be distinguished from colloidal ones. These include solutions of glucose, sucrose, alcohol and others.
- Molecular-ionic. In case of interaction between weak electrolytes. This group includes acidic solutions, nitrogenous, hydrogen sulfide and others.
- Ionic. Compound of strong electrolytes. Prominent representatives are solutions of alkalis, salts and some acids.

Colloidal systems

Colloidal systems are microheterogeneous systems in which the sizes of colloidal particles vary from 100 to 1 nm. They may not precipitate for a long time due to the solvation ionic shell and electric charge. When distributed in a medium, colloidal solutions uniformly fill the entire volume and are divided into sols and gels, which in turn are precipitates in the form of jelly. These include albumin solution, gelatin, colloidal silver solutions. Jellied meat, soufflé, puddings are bright colloidal systems found in everyday life.

Coarse systems

Opaque systems or suspensions in which fine particle ingredients are visible to the naked eye. During the settling process, the dispersed phase is easily separated from the dispersed medium. They are divided into suspensions, emulsions, and aerosols. Systems in which a solid with larger particles are placed in a liquid dispersion medium are called suspensions. These include aqueous solutions of starch and clay. Unlike suspensions, emulsions are obtained by mixing two liquids, in which one is distributed in droplets into the other. An example of an emulsion is a mixture of oil and water, droplets of fat in milk. If small solid or liquid particles are distributed in a gas, these are aerosols. Essentially, an aerosol is a suspension in gas. One of the representatives of a liquid-based aerosol is fog - this is a large number of small water droplets suspended in the air. Solid aerosol - smoke or dust - a multiple accumulation of small solid particles also suspended in the air.

General chemistry: textbook / A. V. Zholnin; edited by V. A. Popkova, A. V. Zholnina. - 2012. - 400 pp.: ill.

Chapter 13. PHYSICAL CHEMISTRY OF DISPERSE SYSTEMS

Chapter 13. PHYSICAL CHEMISTRY OF DISPERSE SYSTEMS

Life is a special colloidal system,... this is a special kingdom of natural waters.

IN AND. Vernadsky

13.1 DISPERSE SYSTEMS, THEIR CLASSIFICATIONS, PROPERTIES

Colloidal solutions

The material basis of modern civilization and the very existence of man and the entire biological world is associated with dispersed systems. A person lives and works surrounded by dispersed systems. Air, especially the air of working rooms, is a dispersed system. Many food products, intermediate products and processed products are dispersed systems (milk, meat, bread, butter, margarine). Many medicinal substances are produced in the form of thin suspensions or emulsions, ointments, pastes or creams (protargol, collargol, gelatinol, etc.). All living systems are dispersed. Muscle and nerve cells, fibers, genes, viruses, protoplasm, blood, lymph, cerebrospinal fluid - all these are highly dispersed formations. The processes occurring in them are controlled by physical and chemical laws, which are studied by the physical chemistry of dispersed systems.

Dispersed systems are those in which the substance is in a state of more or less high fragmentation and is evenly distributed in the environment. The science of highly dispersed systems is called colloidal chemistry. Living matter is based on compounds that are in a colloidal state.

A disperse system consists of a dispersion medium and a dispersed phase. There are several classifications of dispersed systems based on various characteristics of dispersed systems.

1. According to the state of aggregation dispersion medium All disperse systems can be reduced to 3 types. Dispersed systems with gaseous

dispersion medium - aerosols(smoke, workspace air, clouds, etc.). Dispersed systems with liquid dispersion medium - lyosols(foams, emulsions - milk, suspensions, dust caught in the respiratory tract; blood, lymph, urine are hydrosols). Dispersed systems with solid dispersion medium - solidozols(pumice, silica gel, alloys).

2. The second classification groups disperse systems depending on the particle size of the dispersed phase. The measure of particle fragmentation is either the transverse particle size - radius (r), or

(radius) of particles (r) is expressed in centimeters, then dispersion D is the number of particles that can be placed closely along the length of one centimeter. Finally, it can be characterized by specific surface area (∑), the units of ∑ are m 2 /g or m 2 /l. Under specific surface understand the surface relationship (S) dispersed phase to its

coefficient of dependence of the specific surface area on the particle shape. The specific surface area is directly proportional to the dispersion (D) and inversely proportional to the transverse particle size (r). With increasing dispersion, i.e. as the particle size decreases, its specific surface area increases.

The second classification groups dispersed systems depending on the particle size of the dispersed phase into the following groups (Table 13.1): coarse systems; colloidal solutions; true solutions.

Colloidal systems can be gaseous, liquid and solid. The most common and studied liquid (lyosols). Colloidal solutions are usually called sols for short. Depending on the nature of the solvent - dispersion medium, i.e. water, alcohol or ether, lyosols are called hydrosols, alcosols or etherosols, respectively. Based on the intensity of interaction between particles of the dispersed phase and the dispersion medium, sols are divided into 2 groups: lyophilic- intensive interaction, as a result of which developed solvation layers are formed, for example, sol of protoplasm, blood, lymph, starch, protein, etc.; lyophobic sols- weak interaction of particles of the dispersed phase with particles of the dispersion medium. Sols of metals, hydroxides, almost all classical colloidal systems. IUDs and surfactant solutions are separated into separate groups.

Table 13.1. Classification of disperse systems by particle size and their properties

Our domestic scientists I.G. made a great contribution to the theory of colloidal solutions. Borschov, P.P. Weimarn, N.P. Peskov, D.I. Mendeleev, B.V. Deryagin, P.A. Rebinder, etc.

Any colloidal solution is a microheterogeneous, multiphase, highly and polydisperse system with a high degree of dispersity. The condition for the formation of a colloidal solution is the insolubility of the substance of one phase in the substance of another, because only between such substances can physical interfaces exist. Based on the strength of interaction between particles of the dispersed phase, freely dispersed and coherently dispersed systems are distinguished. An example of the latter are biological membranes.

The preparation of colloidal solutions is carried out by two methods: dispersing large particles to a colloidal degree of dispersion and condensation - creating conditions under which atoms, molecules or ions are combined into aggregates of a colloidal degree of dispersion.

Hydrosols can be formed by metals, salts that are poorly soluble in water, oxides and hydroxides, and many non-polar organic substances. Substances that are highly soluble in water but poorly soluble in non-polar compounds are not capable of forming hydrosols, but can form organosols.

As stabilizers substances are used that prevent the aggregation of colloidal particles into larger ones and their precipitation. This effect is achieved by: a small excess of one of the reagents from which the dispersed phase substance is obtained, surfactants, including proteins and polysaccharides.

To achieve the dispersion required for colloidal systems (10 -7 -10 -9 m), the following is used:

Mechanical crushing using ball and colloid mills in the presence of a liquid dispersed medium and stabilizer;

Effect of ultrasound (for example, sulfur hydrosol, graphite, metal hydroxides, etc.);

Peptization method, adding a small amount of electrolyte - peptizer;

One of the varieties of the condensation method is the solvent replacement method, which results in a decrease in the solubility of the dispersed phase substance. Molecules of a substance condense into particles of colloidal sizes as a result of the destruction of the solvation layers of molecules in a true solution and the formation of larger particles. The basis of the chemical

Chemical condensation methods involve chemical reactions (oxidation, reduction, hydrolysis, exchange), leading to the formation of poorly soluble substances in the presence of certain stabilizers.

13.2. MOLECULAR-KINETIC PROPERTIES OF COLLOIDAL SOLUTIONS. OSMOSIS.

OSMOTIC PRESSURE

Brownian motion is the thermal movement of particles in colloidal systems, which has a molecular-kinetic nature. It has been established that the movement of colloidal particles is a consequence of random impacts caused to them by molecules of a dispersion medium that are in thermal motion. As a result, the colloidal particle often changes its direction and speed. In 1 s, a colloidal particle can change its direction over 10 20 times.

Diffusion is a spontaneously occurring process of equalizing the concentration of colloidal particles in a solution under the influence of their thermal chaotic movement. The phenomenon of diffusion is irreversible. The diffusion coefficient is numerically equal to the amount of substance diffused through a unit area per unit time with a concentration gradient of 1 (i.e., a change in concentration of 1 mol/cm 3 over a distance of 1 cm). A. Einstein (1906) derived an equation relating the diffusion coefficient to the absolute temperature, viscosity and particle size of the dispersed phase:

Where T- temperature, K; r- particle radius, m; η - viscosity, N s/m 2; to B- Boltzmann constant, 1.38 10 -23; D- diffusion coefficient, m 2 /s.

The diffusion coefficient is directly proportional to temperature and inversely proportional to the viscosity of the medium (η) and the radius of the particles (r). The cause of diffusion, like Brownian motion, is the molecular kinetic movement of solvent and substance particles. It is known that the larger its volume, the smaller the kinetic energy of a moving molecule (Table 13.2).

Using Einstein's equation, you can easily determine the mass of 1 mole of a substance if you know D, T,η and r. From equation (13.1) we can determine r:

Where R- universal gas constant, 8.3 (J/mol-K); N a Avogadro's constant.

Table 13.2. Diffusion coefficient of some substances

When a system is separated from other parts of the system by a partition that is permeable to one component (for example, water) and impermeable to another (for example, a solute), diffusion becomes one-way (osmosis). The force causing osmosis per unit surface area of ​​the membrane is called osmotic pressure. The role of semi-permeable partitions (membranes) can be performed by human, animal and plant tissues (bladder, intestinal walls, cell membranes, etc.). For colloidal solutions, the osmotic pressure is lower than in true solutions. The diffusion process is accompanied by the emergence of a potential difference as a result of different ion mobility and the formation of a concentration gradient (membrane potential).

Sedimentation. The distribution of particles is influenced not only by diffusion, but also by the gravitational field. The kinetic stability of a colloidal system depends on the action of two factors, directed in mutually opposite directions: the force of gravity, under the influence of which the particles settle, and the force under which the particles tend to disperse throughout the entire volume and resist settling.

Optical properties of colloidal solutions. Light scattering. D. Rayleigh equation. It is impossible to distinguish between colloidal and true solutions at first glance. A well-prepared sol is an almost pure transparent liquid. Its microheterogeneity can be detected using special methods. If a sol located in an unlit place is illuminated with a narrow beam, then when viewed from the side one can see a light cone, the apex of which is located at the point where the beam enters the inhomogeneous space. This is the so-called Tyndall cone - a kind of cloudy glow of colloids, observed under side lighting, is called Faraday-Tyndall effect.

The reason for this phenomenon characteristic of colloids is that the size of colloidal particles is less than half the wavelength of light, and diffraction of light is observed; as a result of scattering, the particles glow, turning into an independent light source, and the beam becomes visible.

The theory of light scattering was developed by Rayleigh in 1871, who derived for spherical particles an equation relating the intensity of incident light (I 0) with the intensity of light scattered per unit volume of the system (I p).

Where I, I 0- intensity of scattered and incident light, W/m2; kp - Rayleigh constant, a constant depending on the refractive indices of the substances of the dispersed phase and the dispersion medium, m -3; with v- concentration of sol particles, mol/l; λ - wavelength of incident light, m; r- particle radius, m.

13.3. MICELLAR THEORY OF THE STRUCTURE OF COLLOIDAL PARTICLES

Micelles form the dispersed phase of the sol, and the intermicellar liquid forms a dispersion medium, which includes a solvent, electrolyte ions and non-electrolyte molecules. A micelle consists of an electrically neutral aggregate and an ionic particle. The mass of the colloidal particle is concentrated mainly in the aggregate. The aggregate can have both an amorphous and crystalline structure. According to the Paneth-Fajans rule, ions that are part of the crystal lattice of the aggregate (or are isomorphic with it) are irreversibly adsorbed on the aggregate with the formation of strong bonds with the atoms of the aggregate. An indicator of this is the insolubility of these compounds. They're called potential-determining ions. The aggregate acquires a charge as a result of selective adsorption of ions or ionization of surface molecules. So, the aggregate and potential-determining ions form the core of the micelle and group ions of the opposite sign - counterions - around the core. The aggregate, together with the ionic part of the micelle, forms a double electric layer (adsorption layer). The aggregate together with the adsorption layer is called a granule. The charge of the granule is equal to the sum of the charges of counterions and potential-determining ions. Ionic

part of the micelle consists of two layers: adsorption and diffuse. This completes the formation of an electrically neutral micelle, which is the basis of the colloidal solution. The micelle is depicted as colloidal chemical formula.

Let us consider the structure of hydrosol micelles using the example of the formation of a colloidal solution of barium sulfate under the condition of an excess of BaCl 2:

Sparingly soluble barium sulfate forms a crystalline aggregate consisting of m BaSO 4 molecules. Adsorbed on the surface of the unit n Ba 2+ ions. Associated with the surface of the nucleus is 2(n -x) chloride ions C1 - . The remaining counterions (2x) are located in the diffuse layer:

The structure of the barium sulfate sol micelle obtained with an excess of sodium sulfate is written as:

From the above data it follows, that the sign of the charge of a colloidal particle depends on the conditions for obtaining the colloidal solution.

13.4. ELECTROKINETIC POTENTIAL

COLLOIDAL PARTICLES

Zeta-(ζ )-potential. The magnitude of the charge of the ζ-potential determines the charge of the granule. It is determined by the difference in the sum of the charges of the potential-determining ions and the charges of the counterions located in the adsorption layer. It decreases as the number of counterions in the adsorption layer increases and can become equal to zero if the charge of the counterions is equal to the charge of the nucleus. The particle will be in an isoelectric state. By the value of the ζ-potential one can judge the stability of the dispersed system, its structure and electrokinetic properties.

The ζ potential of different cells in the body varies. Living protoplasm is negatively charged. At pH 7.4, the value of the ζ-potential of erythrocytes is from -7 to -22 mV, in humans it is -16.3 mV. In monocytes it is approximately 2 times lower. The electrokinetic potential is calculated by determining the speed of movement of particles of the dispersed phase during electrophoresis.

The electrophoretic mobility of particles depends on a number of quantities and is calculated using the Helmholtz-Smoluchowski equation:

Where and ef- electrophoretic mobility (electrophoresis speed), m/s; ε is the relative dielectric constant of the solution; ε 0 - electrical constant, 8.9 10 -12 A s/W m; Δφ - potential difference from an external current source, V; ζ - electrokinetic potential, V; η - viscosity of the dispersion medium, N s/m 2; l- distance between electrodes, m; k f- coefficient, the value of which depends on the shape of the colloidal particle.

13.5. ELECTROKINETIC PHENOMENA.

ELECTROPHORESIS. ELECTROPHORESIS

IN MEDICAL AND BIOLOGICAL RESEARCH

Electrokinetic phenomena reflect the relationship that exists between the movement of the phases of a dispersed system relative to each other and the electrical properties of the interface between these phases. There are four types of electrokinetic phenomena - electrophoresis, electroosmosis, flow potential (flow) and sedimentation potential. Electrokinetic phenomena were discovered by F.F. Reiss. He immersed two glass tubes into a piece of wet clay for some distance, poured some quartz sand into them, poured water to the same level and lowered the electrodes (Fig. 13.1).

By passing a direct current, Reiss found that in the anode space the water above the sand layer becomes cloudy due to the appearance of a suspension of clay particles, at the same time the water level in the knee decreases; in the cathode tube the water remains clear, but its level rises. Based on the results of the experiment, we can conclude: clay particles moving towards the positive electrode are negatively charged, and the adjacent layer of water is positively charged, as it moves towards the negative pole.

Rice. 13.1. Electrokinetic phenomena of the movement of dispersed phase particles

in a dispersed system

The phenomenon of movement of charged particles of the dispersed phase relative to particles of the dispersion medium under the influence of an electric field is called electrophoresis. The phenomenon of movement of a liquid relative to a solid phase through a porous solid (membrane) is called electroosmosis. Under the conditions of the described experiment, two electrokinetic phenomena were observed simultaneously - electrophoresis and electroosmosis. The movement of colloidal particles in an electric field is clear evidence that colloidal particles carry a charge on their surface.

A colloidal particle, a micelle, can be considered as a huge complex ion. A colloidal solution undergoes electrolysis under the influence of direct current, colloidal particles are transferred to the anode or cathode (depending on the charge of the colloidal particle). Thus, electrophoresis is the electrolysis of a highly dispersed system.

Later, two phenomena were discovered that were opposite to electrophoresis and electroosmosis. Dorn discovered that when any particles settle in a liquid, for example sand in water, an emf occurs between 2 electrodes inserted into different places in the liquid column, called sedimentation potential (Dorn effect).

When a liquid is forced through a porous partition, on both sides of which there are electrodes, an EMF also appears - flow (percolation) potential.

A colloidal particle moves with a speed proportional to the magnitudeζ -potential. If the system contains a complex mixture, then it can be studied and separated using the electrophoresis method, based on the electrophoretic mobility of particles. This is widely used in biomedical research in the form of macro and micro electrophoresis.

The created electric field causes the movement of particles of the dispersed phase with a speed proportional to the value of the ζ-potential, which can be observed by moving the interface between the test solution and the buffer using optical devices. As a result, the mixture is divided into a number of fractions. When recording, a curve with several peaks is obtained, the height of the peak is a quantitative indicator of the content of each fraction. This method makes it possible to isolate and study individual fractions of blood plasma proteins. Electropherograms of the blood plasma of all people are normally the same. In pathology, they have a characteristic appearance for each disease. They are used to diagnose and treat diseases. Electrophoresis is used to separate amino acids, antibiotics, enzymes, antibodies, etc. Microelectrophoresis involves determining the speed of movement of particles under a microscope; electrophoresis - on paper. The phenomenon of electrophoresis occurs during the migration of leukocytes to inflammatory foci. Immunoelectrophoresis, disk electrophoresis, isotachophoresis, etc. are currently being developed and implemented as treatment methods. They solve many medical and biological problems of both a preparative and analytical nature.

13.6. STABILITY OF COLLOIDAL SOLUTIONS. SEDIMENTATION, AGGREGATION AND CONDENSATION STABILITY OF LYOSOLS. FACTORS AFFECTING SUSTAINABILITY

The question of the stability of colloidal systems is a very important question that directly concerns their very existence. Sedimentation stability- resistance of dispersed system particles to settling under the influence of gravity.

Peskov introduced the concept of aggregative and kinetic stability. Kinetic stability- the ability of the dispersed phase of a colloidal system to be in suspension, not to sediment and to counteract the forces of gravity. Highly dispersed systems are kinetically stable.

Under aggregative stability you need to understand the ability of a dispersed system to maintain its original degree of dispersion. This is only possible with a stabilizer. The consequence of violation of aggregative stability is kinetic instability,

because the aggregates formed from the original particles are released under the influence of gravity (settle or float).

Aggregative and kinetic stability are interrelated. The greater the aggregative stability of the system, the greater its kinetic stability. Stability is determined by the result of the struggle between gravity and Brownian motion. This is an example of the manifestation of the law of unity and struggle of opposites. Factors that determine the stability of systems: Brownian motion, dispersion of particles of the dispersed phase, viscosity and ionic composition of the dispersion medium, etc.

Factors of stability of colloidal solutions: the presence of an electrical charge of colloidal particles. The particles carry the same charge, so when they meet, the particles repel; ability to solvation (hydration) of ions of the diffuse layer. The more hydrated the ions in the diffuse layer, the thicker the overall hydration shell, the more stable the system. The elastic forces of the solvation layers have a wedging effect on dispersed particles and prevent them from approaching each other; adsorption-structuring properties of systems. The third factor is related to the adsorption properties of disperse systems. On the developed surface of the dispersed phase, molecules of surfactants (surfactants) and high molecular weight compounds (HMCs) are easily absorbed. The large sizes of molecules carrying their own solvation layers create adsorption-solvation layers of considerable extent and density on the surface of particles. Such systems are close in stability to lyophilic systems. All these layers have a certain structure, created according to P.A. Rebinder is a structural-mechanical barrier to the convergence of dispersed particles.

13.7. COAGULATION OF SOLS. RULES OF COAGULATION. COAGULATION KINETICS

Sols are thermodynamically unstable systems. Particles of the dispersed phase of sols tend to reduce free surface energy due to a reduction in the specific surface of colloidal particles, which occurs when they combine. The process of colloidal particles combining into larger aggregates, and ultimately precipitating them, is called coagulation.

Coagulation is caused by various factors: mechanical action, temperature changes (boiling and freezing), radiation

tion, foreign substances, especially electrolytes, time (aging), concentration of the dispersed phase.

The most studied process is the coagulation of sols with electrolytes. There are the following rules for coagulation of sols with electrolytes.

1. All electrolytes are capable of causing coagulation of lyophobic sols. The coagulating effect (P) is possessed by ions that have a charge opposite to the charge of the granule (potential-determining ions) and the same sign as the counterions (Hardy's rule). Coagulation of positively charged sols is caused by anions.

2. The coagulating ability of ions (P) depends on the magnitude of their charge. The higher the charge of the ion, the higher its coagulating effect (Schulze rule): PA1 3+ > PCa 2+ > PK + .

Accordingly, for the coagulation threshold we can write:

those. the lower the charge of the ion, the higher the concentration coagulation will occur.

3. For ions of the same charge, the coagulating ability depends on the radius (r) of the solvated ion: the larger the radius, the greater its coagulating effect:

4. Each electrolyte is characterized by the threshold concentration of the coagulation process of the colloidal solution (coagulation threshold), i.e. the smallest concentration, expressed in millimoles, that must be added to one liter of colloidal solution to cause its coagulation. The coagulation threshold or threshold concentration is designated C. The coagulation threshold is a relative characteristic of the stability of a sol with respect to a given electrolyte and is the reciprocal of the coagulating ability:

5. The coagulating effect of organic ions is greater than that of inorganic ions; Coagulation of many lyophobic sols occurs earlier,

This is how their isoelectric state is achieved, at which obvious coagulation begins. This action is called critical. Its value is +30 mV.

The coagulation process for each disperse system occurs at a certain speed. The dependence of the coagulation rate on the concentration of the electrolyte-coagulator is shown in Fig. 13.2.

Rice. 13.2. Dependence of coagulation rate on electrolyte concentration.

Explanations in the text

3 areas and two characteristic points of A&B are identified. The area limited by the OA line (along the concentration axis) is called the area of ​​latent coagulation. Here the coagulation rate is practically zero. This is the sol stability zone. Between points A and B there is an area of ​​slow coagulation, in which the rate of coagulation depends on the concentration of the electrolyte. Point A corresponds to the lowest electrolyte concentration at which obvious coagulation begins (coagulation threshold), and has a critical value. This stage can be judged by external signs: a change in color, the appearance of turbidity. The colloidal system is completely destroyed: the substance of the dispersed phase is released into a precipitate called coagulate. At point B, rapid coagulation begins, that is, all particle collisions are effective and do not depend on the electrolyte concentration. At point B, the ζ-potential is 0. The amount of substance required for coagulation of a colloidal solution depends on whether the electrolyte is added immediately or gradually, in small portions. It is noticed that in the latter case more substance has to be added to cause the same coagulation phenomenon. This phenomenon is used in drug dosing.

If you merge two colloidal solutions with opposite charges, they quickly coagulate. The process is electrostatic in nature. This is used for industrial and waste water treatment. At waterworks, aluminum sulfate or iron (III) chloride is added to the water before sand filters. During their hydrolysis, positively charged sols of metal hydroxides are formed, which cause coagulation of negatively charged particles of microflora, soil, and organic impurities.

In biological systems, coagulation phenomena play a very important role. Whole blood is an emulsion. The formed elements of blood are the dispersed phase, plasma is the dispersion medium. Plasma is a more highly dispersed system. Dispersed phase: proteins, enzymes, hormones. The blood clotting system and anti-clotting system operate in the blood. The first is provided by thrombin, which acts on fibrinogen and causes the formation of fibrin strands (blood clot). Red blood cells sediment at a certain rate (ESR). The coagulation process ensures minimal blood loss and the formation of blood clots in the circulatory system. In pathology, red blood cells adsorb large molecules of gamma globulins and fibrinogens and the ESR increases. The main anti-clotting ability of blood is heparin, a blood anticoagulant. Clinics use coagulograms - a set of tests on the coagulation and anti-coagulation ability of blood (prothrombin content, plasma recalcification time, heparin tolerance, total amount of fibrinogen, etc.), this is important for severe bleeding and the formation of blood clots. Blood clotting must be taken into account when preserving it. Ca 2+ ions are removed with sodium nitrate to precipitate, which increases coagulability. An anticoagulant, heparin, and dicoumarin are used. Polymers used for endoprosthetics of elements of the cardiovascular system must have antithrombogenic or thromboresistant properties.

13.8. STABILIZATION OF COLLOIDAL SYSTEMS (PROTECTION OF COLLOIDAL SOLUTIONS)

Stabilization of colloidal solutions with respect to electrolytes by creating additional adsorption layers on the surface of colloidal particles with increased structural and mechanical properties, adding a small amount of a solution of high

comolecular compounds (gelatin, sodium caseinate, egg albumin, etc.) is called colloidal protection. Protected sols are very resistant to electrolytes. The protected sol acquires all the properties of the adsorbed polymer. The dispersed system becomes lyophilic and therefore stable. The protective effect of an IUD or surfactant is characterized by a protective number. The protective number should be understood as the minimum mass of the IUD (in milligrams) that must be added to 10 ml of the test sol in order to protect it from coagulation when 1 ml of 10% sodium chloride solution is introduced into the system. The degree of protective effect of IUD solutions depends on: the nature of the IUD, the nature of the protected sol, the degree of dispersion, the pH of the medium, and impurities.

The phenomenon of colloidal defense in the body plays a very important role in a number of physiological processes. Various proteins, polysaccharides, and peptides have a protective effect in the body. They adsorb Ca on colloidal particles of such hydrophobic systems of the body as carbonates and calcium phosphates, transforming them into a stable state. Examples of protected sols are blood and urine. If you evaporate 1 liter of urine, collect the resulting precipitate and then try to dissolve it in water, then you will need 14 liters of solvent. Consequently, urine is a colloidal solution in which dispersed particles are protected by albumins, mucins and other proteins. Serum proteins increase the solubility of calcium carbonate by almost 5 times. The increased content of calcium phosphate in milk is due to protein protection, which is impaired with aging.

In the development of atherosclerosis, the leucetine-cholesterol balance plays an important role, when it is disturbed, the ratio between cholesterol, phospholipids and proteins changes, leading to the deposition of cholesterol on the walls of blood vessels, resulting in atherocalcinosis. Large molecular fat-protein components play a large role in protection. On the other hand, the ability of blood to retain high concentrations of carbon and oxygen gases in a dissolved state is also due to the protective effect of proteins. In this case, proteins envelop gas microbubbles and protect them from sticking together.

Protection of colloidal particles used in the manufacture of drugs. It is often necessary to introduce medicinal substances into the body in a colloidal state so that they are evenly distributed in the body and absorbed. Thus, colloidal solutions of silver, mercury, sulfur, protected by protein substances, used

as drugs (protargol, collargol, lisorginone), they not only become insensitive to electrolytes, but can also be evaporated to dryness. The dry residue after treatment with water again turns into a sol.

13.9. PEPTIZATION

Peptization - the reverse process of coagulation, the process of transition of coagulate into sol. Peptization occurs when substances are added to the sediment (coagulate) that promote the transition of the sediment into a sol. They are called Pepti mash. Typically, peptizers are potential-determining ions. For example, a precipitate of iron (III) hydroxide is peptized with iron (III) salts. But the role of a peptizer can also be performed by a solvent (H 2 O). The peptization process is caused by adsorption phenomena. The peptizer facilitates the formation of the electrical double layer structure and the formation of zeta potential.

Consequently, the peptization process is mainly due to the adsorption of potential-determining ions and desorption of counterions, which results in an increase in the ζ-potential of dispersed particles and an increase in the degree of solvation (hydration), the formation of solvation shells around the particles that produce a wedging effect (adsorption peptization).

In addition to adsorption, there are also dissolution peptization. This type covers everything when the peptization process is associated with a chemical reaction of surface molecules of the dispersed phase. It consists of two phases: the formation of a peptizer through a chemical reaction of the introduced peptizer electrolyte with a dispersed particle; adsorption of the resulting peptization agent on the surface of the dispersed phase, leading to the formation of micelles and peptization of the precipitate. A typical example of dissolution peptization is the peptization of metal hydroxides with acids.

The maximum dispersion of sols obtained by adsorption peptization is determined by the degree of dispersity of the primary particles forming sediment flakes. During dissolution peptization, the particle fragmentation boundary can leave the colloid region and reach a molecular degree of dispersion. The process of peptization is of great importance in living organisms, since colloids of cells and biological fluids are constantly exposed to the action of electrolytes in the body.

The action of many detergents, including detergents, is based on the phenomenon of peptization. The colloidal ion of soap is a dipole; it is adsorbed by dirt particles, gives them a charge and promotes their peptization. Dirt in the form of a sol is easily removed from the surface.

13.10. GELS AND JELLES. THIXOTROPY. SYNERESIS

Solutions of IUDs and sols of some hydrophobic colloids are capable of undergoing changes under certain conditions: loss of fluidity, gelation, gelation of solutions occur, and jellies and gels are formed (from the Latin “frozen”).

Jellies (gels)- These are solid, non-fluid, structured systems that arise as a result of the action of molecular adhesion forces between colloidal particles or polymer macromolecules. The forces of intermolecular interaction lead to the formation of a spatial mesh frame; the cells of the spatial mesh are filled with a liquid solution, like a sponge soaked in liquid. The formation of jelly can be represented as the salting out of the IUD or the initial stage of coagulation, the emergence of coagulation structuring.

When the mixture is heated to 45 °C, an aqueous solution of gelatin becomes a homogeneous liquid medium. When cooled to room temperature, the viscosity of the solution increases, the system loses its fluidity, hardens, the consistency of the semi-solid mass retains its shape (can be cut with a knife).

Depending on the nature of the substances that form the jelly or gel, they are distinguished: built from hard particles - fragile (irreversible); formed by flexible macromolecules - elastic (reversible). Brittle ones are formed by colloidal particles (TiO 2, SiO 2). Dried is a hard foam with a large specific surface area. Dried jelly does not swell; drying causes irreversible changes.

Elastic gels are formed by polymers. When dried, they are easily deformed and compressed, resulting in a dry polymer (pyrogel) that retains elasticity. It is capable of swelling in a suitable solvent, the process is reversible and can be repeated many times.

Weak molecular bonds in jellies can be destroyed mechanically (by shaking, pouring, temperature). The rupture of the bond causes the destruction of the structure, the particles acquire the ability

to thermal movement, the system liquefies and becomes fluid. After some time, the structure spontaneously recovers. This can be repeated dozens of times. This reversible transformation is called thixotropy. This isothermal transformation can be represented by the diagram:

Thixotropy is observed in weak solutions of gelatin, cell protoplasm. The reversibility of thixotropy indicates that structuring in the corresponding systems is due to intermolecular (van der Waals) forces - a coagulation-thixotropic structure.

The gels in the body are the brain, skin, and eyeball. The condensation-crystallization type of structure is characterized by a stronger bond of a chemical nature. In this case, the reversibility of thixotropic changes is disrupted (silicic acid gel).

Jelly is a nonequilibrium state of the system, a certain stage of the slowly occurring process of phase separation and the approach of the system to a state of equilibrium. The process comes down to the gradual compression of the jelly frame into a denser compact mass with pressing of the second mobile liquid phase, which is mechanically retained in the spatial mesh of the frame. During storage, individual drops of liquid first appear on the surface of the jellies; over time, they increase and merge into a continuous mass of the liquid phase. This spontaneous process of jelly separation is called syneresis. For fragile jellies, syneresis is the irreversible aggregation of particles, compaction of the entire structure. For IUD jelly, increasing the temperature can stop syneresis and return the jelly to its original position. The separation of clots of coagulated blood, the hardening of bread, and the soaking of confectionery products are examples of syneresis. The tissues of young people are elastic, contain more water; with age, elasticity is lost, less water - this is syneresis.

13.11. QUESTIONS AND TASKS FOR SELF-TEST

PREPARATION FOR CLASSES AND EXAMINATIONS

1. Give the concept of dispersed systems, dispersed phase and dispersion medium.

2. How are dispersed systems classified according to the state of aggregation of the dispersed phase and dispersion medium? Give examples of medical and biological profiles.

3. How are dispersed systems classified according to the strength of intermolecular interaction in them? Give examples of medical and biological profiles.

4.The main part of the artificial kidney apparatus is the dialyzer. What is the principle of the simplest dialyzer? What impurities can be removed from blood through dialysis? What factors determine the speed of dialysis?

5. In what ways can you distinguish between a solution of a low molecular weight substance and a colloidal solution? What properties are these methods based on?

6. In what ways can you distinguish a sol from a coarsely dispersed system? What properties are these methods based on?

7.What methods exist for producing colloidal disperse systems? How are they different from each other?

8.What are the features of the molecular-kinetic and optical properties of colloidal disperse systems? What distinguishes them from true solutions and coarse systems?

9.Give the concept of aggregative, kinetic and condensation stability of dispersed systems. Factors that determine the stability of systems.

10. Show the relationship between the electrokinetic properties of colloidal disperse systems.

11.What electrokinetic phenomena are observed during mechanical mixing of particles of the dispersed phase: a) relative to the dispersion medium; b) relative to particles of the dispersed phase?

12. Explain which of the following preparations belongs to colloidal solutions: a) a preparation of barium sulfate in water, used as a contrast agent for X-ray studies with a particle size of 10 -7 m; b) a silver preparation in water - collargol, used to treat purulent wounds with a particle size of 10 -9 m.

13. The concept of coagulation of sols. Coagulation of lyophilic sols. What are the external signs of coagulation? Indicate possible products of coagulation of sols.

14. Factors causing coagulation of sols. Rules for coagulation of sols with electrolytes. Kinetics of coagulation. Coagulation threshold.

15. As a result of a violation of micro (Ca 2+)- and macro (C 2 O 4 2-)-element and acid-base homeostasis in the gastrointestinal tract, the reaction occurs in the kidneys:

What is the charge of the sol? Which of the indicated ions will have a coagulating effect for particles of this sol: K +, Mg 2+, SO 4 2-, NO 3 -, PO 4 3-, Al 3+?

A calcium oxalate sol is formed. Let's write down the formula of a sol micelle

(13.3.).

The charge of the sol granule is positive, which means that the following ions will have a coagulating effect (k) for the particles of this sol: SO 4 2-, PO 4 3-, NO 3-, according to Hardy’s rule. The higher the charge of the coagulating ion, the stronger its coagulating effect (Schulze's rule). According to Schulze's rule, these anions can be arranged in the following row: C to P0 4 3- > C to SO 4 2- > C to NO 3 - . The lower the charge of the ion, the higher the concentrations coagulation will occur. The coagulation threshold (p) is a relative characteristic of the stability of the sol with respect to a given electrolyte and is the reciprocal of

13.12. TEST TASKS

1. Choose the incorrect statement:

a) condensation methods for producing colloidal solutions include ORR, hydrolysis, and solvent replacement;

b) dispersion methods for producing colloidal solutions include mechanical, ultrasonic, peptization;

c) the optical properties of colloidal systems include opalescence, diffraction, and the Tyndall effect;

d) the molecular kinetic properties of colloidal systems include Brownian motion, light scattering, and change in solution color.

2. Select the incorrect statement:

a) electrophoresis is the movement of a dispersed phase in an electric field relative to a stationary dispersion medium;

b) electroosmosis is the movement in an electric field of a dispersion medium relative to a stationary dispersed phase;

c) the penetration of liquids containing therapeutic ions and molecules through a capillary system under the influence of an electric field is called electrodialysis;

d) electrophoresis is used to separate proteins, nucleic acids and blood cells.

3. A colloidal solution that has lost fluidity is:

a) emulsion;

b)gel;

c) sol;

d) suspension.

4. Blood plasma is:

a) sol;

b)gel;

c) true solution;

d) emulsion.

5. A heterogeneous system consisting of a dispersed phase microcrystal surrounded by solvated stabilizer ions is called:

a) granule;

b) core;

c) unit;

d) micelle.

6. During the formation of a micelle, potential-determining ions are adsorbed according to the rule:

a) Schulze-Hardy;

b) Rebinder;

c)Paneta-Fajanza;

d) Shilova.

7. A micelle granule is an aggregate:

a) together with the adsorption layer;

b) diffusion layer;

c) adsorption and diffusion layers;

d) potential-determining ions.

8. Interfacial potential is the potential between:

a) solid and liquid phases;

b) adsorption and diffuse layers at the sliding boundary;

c) nucleus and counterions;

d) potential-determining ions and counterions.

9. The ability of fine-porous membranes to retain particles of the dispersed phase and freely pass ions and molecules is called:

), 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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