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A dispersed system is a system in which small particles of one or more substances are evenly distributed among the particles of another substance. The dispersed phase is called small particles of a substance that is distributed in the system. A dispersion medium is a substance in which the dispersed phase is distributed. 3 Heterogeneous dispersed system: particles of the dispersed phase have a size greater than 1·10-9 m and constitute a separate phase from the dispersion medium. Homogeneous dispersed system: there is no interface between the dispersed phase and the dispersion medium (true solutions). The sizes of molecules, ions are smaller than 1 10-9 m.

WITH DEGREE OF DISPERSION. TO LASSIIFICATION OF DISPERSIVE SYSTEMS. 4 The degree of dispersion (D) is the reciprocal of the particle size (d) D = 1/d The smaller the particle size, the greater the dispersion of the system Classification according to the degree of dispersion Coarse (d \u003d m) (coarse suspensions, emulsions, powders) . Medium dispersion (d = m) (thin suspensions, smoke, porous bodies). Highly dispersed (d = m) (colloidal systems).

OBTAINING DISPERSIVE SYSTEMS Dispersion methods. This group of methods combines mechanical methods by which solids are crushed, crushed or split. Typical for laboratory, industrial and dispersion processes occurring in nature. In laboratory and industrial conditions, these processes are carried out in crushers, millstones and mills of various designs. The most common are ball mills, in which systems are obtained, with particle sizes ranging from 2 - 3 to 50 - 70 microns. In colloid mills of various designs, finer dispersion is achieved; the principle of operation of such mills is based on the development of breaking forces in a suspension or emulsion under the action of centrifugal force. Suspended large particles experience in this case a significant tearing force and are thus dispersed. High dispersion can be achieved by ultrasonic dispersion. It has been experimentally established that dispersion is directly dependent on the frequency of ultrasonic vibrations. Emulsions obtained by the ultrasonic method are distinguished by the uniformity of the particle sizes of the dispersed phase. five

dispersion methods. The Bredig method is based on the formation of a voltaic arc between dispersible metal electrodes placed in water. The essence of the method lies in the spraying of the metal of the electrode in the arc, as well as in the condensation of metal vapors formed at high temperature. The Svedberg method, which uses a high-voltage oscillatory discharge that causes a spark to jump between the electrodes. This method can be used to obtain not only hydrosols, but also organosols of various metals. During crushing and grinding, materials are destroyed primarily in places of strength defects (macro- and microcracks). Therefore, as the particles are crushed, the strength of the particles increases, which is usually used to create stronger materials. At the same time, an increase in the strength of materials as they are crushed leads to a large energy consumption for further dispersion. The destruction of materials can be facilitated by using the Rehbinder effect - an adsorption decrease in the strength of solids. This effect is to reduce the surface energy with the help of surfactants, which facilitates the deformation and destruction of the solid (liquid metals for the destruction of solid metals). The use of dispersive methods usually fails to achieve a very high dispersion. Systems with particle sizes of the order of - 10 7 cm are obtained by condensation methods. 6 PRODUCTION OF DISPERSIVE SYSTEMS

Condensation methods (physical) Condensation methods are based on the processes of the emergence of a new phase by combining molecules, ions or atoms in a homogeneous medium. These methods can be divided into physical and chemical. Physical condensation - condensation from vapors and replacement of the solvent. (fog formation). The method of replacing the solvent (changing the composition of the medium) is based on such a change in the parameters of the system, in which the chemical potential of the component in the dispersion medium becomes higher than the equilibrium one and the tendency to transition to the equilibrium state leads to the formation of a new phase. Sols of sulfur, phosphorus, arsenic and many organic substances are obtained by this method by pouring alcohol or acetone solutions of these substances into water. 7 OBTAINING DISPERSIVE SYSTEMS

Condensation methods (chemical) Chemical condensation: the substance that forms the dispersed phase appears as a result of a chemical reaction. Thus, any chemical reaction proceeding with the formation of a new phase can be a source of obtaining a colloidal system. 1. Recovery (preparation of gold sol by reduction of gold hydrochloric acid): 2HAuCl 2 + 3H 2 O 2 \u003d 2Au + 8HCl + 3O 2 2. Oxidation (formation of sulfur sol in hydrothermal waters, with oxidizing agents (sulfur dioxide or oxygen)): 2H 2 S + O 2 \u003d 2S + 2H 2 O 3. Hydrolysis 4. Exchange reactions (obtaining arsenic sulfide sol): 2H 3 AsO 3 + 3H 2 S \u003d As 2 S 3 + 6H 2 O so that the concentration of the substance in the solution exceeds the solubility, i.e. the solution must be supersaturated. 8 PRODUCTION OF DISPERSIVE SYSTEMS

METHODS FOR CLEANING COLLOID SOLUTIONS. Sols and solutions of high molecular weight compounds (HMCs) contain low molecular weight compounds as undesirable impurities. They are removed by the following methods. Dialysis is historically the first method of purification. Purification of colloidal solutions through a semi-permeable membrane, which is washed by the solvent. Electrodialysis is the process of cleaning sols from electrolyte impurities in an electric field that accelerates the movement of ions. Ultrafiltration is a cleaning method by forcing a dispersion medium together with low molecular weight impurities through ultrafilters. Microfiltration is the separation by means of filters of microparticles ranging in size from 0.1 to 10 microns. Combined cleaning methods. In addition to individual purification methods - ultrafiltration and electrodialysis - their combination is known: electro-ultrafiltration, used to purify and separate proteins. It is possible to purify and at the same time increase the concentration of the IUD sol or solution using a method called electro-decantation. Electrodecantation occurs when the electrodialyzer is operated without stirring. nine

Since low-molecular impurities (foreign electrolytes) are capable of destroying colloidal systems, the resulting sols in many cases have to be purified. Dispersed systems of natural origin (latexes, crude oil, vaccines, sera, etc.) are also purified. To remove impurities, use: dialysis, electrodialysis, ultrafiltration.

Dialysis- extraction of low molecular weight substances from sols with a pure solvent using a semi-permeable partition (membrane), through which colloidal particles do not pass. Many improved designs of dialyzers have now been proposed to provide a faster cleaning process. The intensification of dialysis is achieved by: increasing the surface of the membranes; reduction of the layer of the liquid to be purified; frequent or continuous change of external fluid; rise in temperature.

Electrodialysis– dialysis accelerated by application of an external electric field. Electrodialysis is due to the migration of ions through the membrane under the action of an applied potential difference of the order of 40 V/cm.

ultrafiltration- electrodialysis under pressure. Essentially, ultrafiltration is not a method for purifying sols, but only a method for concentrating them.

An interesting example of a combination of dialyzer and ultrafiltration is the "artificial kidney" device, designed to temporarily replace kidney function in acute renal failure. The device is surgically connected to the patient's circulatory system. Blood under pressure created by a pulsating pump ("artificial heart") flows in a narrow gap between two membranes, washed from the outside with saline. Due to the large working area of ​​the membranes (~ 15000 cm 2), “slags” are removed from the blood relatively quickly (3-4 hours) - products of metabolism and tissue breakdown (urea, creatine, potassium ions, etc.).

By using membranes with a certain porosity for ultrafilters, it is possible to a certain extent to separate colloidal particles according to their sizes and at the same time to approximately determine their sizes. This method was used to determine the particle sizes of a number of viruses and bacteriophages.

Ultrafiltration is used to purify wastewater from mechanical impurities. This method is used to separate liquid molecules from particles of a colloidal system.

Depending on the dispersion of wastewater, certain types of filter partitions are used. For microfiltration of large amounts of natural water at waterworks, when cleaning mainly from plankton and microorganisms, metal meshes are used, in the case of cleaning from submicron particles and macromolecules, polymer membranes with different pore sizes are used.

Questions and tasks for self-control

1. What does the discipline "Colloid Chemistry" study?

2. What is the difference between colloidal solutions and true ones?

3. On what features is each type of classification of dispersed systems based?

4. What are the methods for obtaining dispersed systems? What is the essence of each method?

5. How can colloidal systems be cleaned? Why do you need to do this?

Chapter 2
THERMODYNAMICS
SURFACE PHENOMENA

In disperse systems, most of all molecules or atoms that make up a substance are located on the interface. These surface molecules differ from the molecules inside the phase in their energy state, which leads to the appearance of excess surface energy. The excess surface energy is equal to the product of the surface tension and the interfacial area:

Any thermodynamic system tends to reduce its surface energy. Excess surface energy can be reduced by:

· reduction of surface tension: adsorption, adhesion, wetting, formation of a double electric layer;

· decrease in surface area: spherical shape of droplets (surface smoothing), association of particles (coagulation, aggregation, coalescence).

There are two general approaches to obtaining disp. systems - dispersion and condensation. The dispersion method is based on the grinding of macroscopic particles to nanosizes (1-100 nm).

Mechanical grinding is not widely used because of the high energy consumption. In laboratory practice, ultrasonic grinding is used. During grinding, two processes compete: dispersion and aggregation of the resulting particles. The ratio of the rates of these processes depends on the duration of grinding, temperature, the nature of the liquid phase, the presence of stabilizers (most often surfactants). By selecting the optimal conditions, it is possible to obtain particles of the required size, however, the particle size distribution is quite wide.

The most interesting is the spontaneous dispersion of solids in the liquid phase. A similar process can be observed for substances having a layered structure. In such structures, there is a strong interaction between the atoms inside the layer and a weak v-d-v interaction between the layers. For example, molybdenum and tungsten sulfides, which have a layered structure, spontaneously disperse in acetonitrile to form nanometer-sized bilayer particles. In this case, the liquid phase penetrates between the layers, increases the interlayer distance, and the interaction between the layers weakens. Under the action of thermal vibrations, the detachment of nanoparticles from the surface of the solid phase occurs.

Condensation methods divided into physical and chemical. The formation of nanoparticles occurs through a series of transition states during the formation of intermediate ensembles, leading to the appearance of a new phase nucleus, its spontaneous growth, and the appearance of a physical phase interface. It is important to ensure a high rate of embryo formation and a low rate of its growth.

Physical methods are widely used to obtain metallic ultrafine particles. These methods are essentially dispersion-condensation. In the first stage, the metal is dispersed to atoms by evaporation. Then, due to supersaturation of the vapors, condensation occurs.

Molecular beam method used to obtain coatings with a thickness of about 10 nm. The starting material in a diaphragm chamber is heated to high temperatures under vacuum. The evaporated particles, passing through the diaphragm, form a molecular beam. The beam intensity and the rate of particle condensation on the substrate can be changed by varying the temperature and vapor pressure above the source material.

Aerosol method consists in the evaporation of the metal in a rarefied atmosphere of an inert gas at a low temperature, followed by condensation of the vapors. This method was used to obtain Au, Fe, Co, Ni, Ag, Al nanoparticles; their oxides, nitrides, sulfides.

Cryochemical synthesis based on the condensation of metal atoms (or metal compounds) at low temperature in an inert matrix.

Chemical condensation. A colloidal solution of gold (red) with a particle size was obtained in 1857 by Faraday. This sol is on display at the British Museum. Its stability is explained by the formation of a DEL at the interface of the solid phase-solution and the occurrence of an electrostatic component of the disjoining pressure.

Often, the synthesis of nanoparticles is carried out in solution during chemical reactions. Reduction reactions are used to obtain metal particles. As a reducing agent, aluminum and borohydrides, hypophosphites, etc. are used. For example, a gold sol with a particle size of 7 nm is obtained by reducing gold chloride with sodium borohydride.

Nanoparticles of salts or metal oxides are obtained in exchange or hydrolysis reactions.

Natural and synthetic surfactants are used as stabilizers.

Mixed composition nanoparticles were synthesized. For example, Cd/ZnS, ZnS/CdSe, TiO 2 /SiO 2 . Such nanoparticles are obtained by deposition of molecules of one type (shell) on a pre-synthesized nanoparticle of another type (core).

The main disadvantage of all methods is the wide size distribution of nanoparticles. One of the methods for controlling the size of nanoparticles is associated with the preparation of nanoparticles in reverse microemulsions. In reverse microemulsions, the dis phase is water, the disperse medium is oil. The droplet size of water (or other polar liquid) can vary widely depending on the conditions of preparation and the nature of the stabilizer. A drop of water plays the role of a reactor in which a new phase is formed. The size of the resulting particle is limited by the size of the drop, the shape of this particle repeats the shape of the drop.

Sol-gel method contains the following stages: 1. preparation of the initial solution, usually containing metal alkoxides M(OR) n , where M is silicon, titanium, zinc, aluminum, tin, cerium, etc., R is alkali or aryl; 2. gel formation due to polymerization reactions; 3. drying; 4. heat treatment. hydrolysis in organic solvents

M(OR) 4 +4H 2 OM(OH) 4 +4ROH.

Then polymerization and gel formation occurs.

mM (OH) n  (MO) 2 + 2mH 2 O.

peptization method. Distinguish between peptization when washing the precipitate, peptization of the precipitate with electrolyte; peptization with surfactants; chemical peptization.

Peptization during washing of the precipitate is reduced to the removal of the electrolyte from the precipitate, which caused coagulation. In this case, the thickness of the DEL increases, and the forces of ion-electrostatic repulsion prevail over the forces of intermolecular attraction.

Precipitation peptization with electrolyte is associated with the ability of one of the electrolyte ions to be adsorbed on particles, which contributes to the formation of DES on particles.

Peptization with surfactants. Surfactant macromolecules are adsorbed on particles or give them a charge (ionic surfactants) or form an adsorption-solvation barrier that prevents particles from sticking together in the sediment.

Chemical peptization occurs when a substance added to the system interacts with sediment matter. In this case, an electrolyte is formed, which forms a DEL on the surface of the particles.

Two methods for obtaining dispersed systems - dispersion and condensation

Dispersion and condensation - methods for obtaining free-dispersed systems: powders, suspensions, sols, emulsions, etc. Under dispersion understand the crushing and grinding of a substance, by condensation - the formation of a heterogeneous dispersed system from a homogeneous one as a result of the association of molecules, atoms or ions into aggregates.

In the world production of various substances and materials, the processes of dispersion and condensation occupy one of the leading places. Billions of tons of raw materials and products are obtained in a free-dispersed state. This ensures the convenience of their transportation and dosage, and also makes it possible to obtain homogeneous materials in the preparation of mixtures.

Examples include crushing and grinding ores, coal, cement production. Dispersion occurs during the combustion of liquid fuels.

Condensation occurs during the formation of fog, during crystallization.

It should be noted that during dispersion and condensation, the formation of dispersed systems is accompanied by the appearance of a new surface, i.e., an increase in the specific surface area of ​​substances and materials, sometimes by thousands or more times. Therefore, obtaining dispersed systems, with some exceptions, requires energy.

During crushing and grinding, materials are destroyed primarily in places of strength defects (macro- and microcracks). Therefore, as the grinding process increases, the strength of the particles increases, which leads to an increase in energy consumption for their further dispersion.

The destruction of materials can be facilitated by using Rebinder effect – adsorption lowering of the perversity of solids. This effect is to reduce the surface energy with the help of surfactants, thereby facilitating the deformation and destruction of the solid. As such surfactants, here called hardness reducers, can be used, for example, liquid metals to destroy solid metals or typical surfactants.

Hardness reducers are characterized by small amounts that cause the Rebinder effect and specificity of action. Additives that wet the material help the medium to penetrate into the places of defects and, with the help of capillary forces, also facilitate the destruction of the solid. Surfactants not only contribute to the destruction of the material, but also stabilize the dispersed state, preventing particles from sticking together.

Systems with the maximum degree of dispersity can only be obtained using condensation methods.

Colloidal solutions can also be obtained chemical condensation method, based on the conduct of chemical reactions, accompanied by the formation of insoluble or poorly soluble substances. For this purpose, various types of reactions are used - decomposition, hydrolysis, redox, etc.

Purification of dispersed systems.

Sols and solutions of high molecular weight compounds (HMCs) contain low molecular weight compounds as undesirable impurities. They are removed by the following methods.

Dialysis. Dialysis was historically the first method of purification. It was proposed by T. Graham (1861). The scheme of the simplest dialyzer is shown in fig. 3 (see appendix). The sol to be purified, or IUD solution, is poured into a vessel, the bottom of which is a membrane that retains colloidal particles or macromolecules and passes solvent molecules and low molecular weight impurities. The external medium in contact with the membrane is a solvent. Low-molecular impurities, the concentration of which in the ash or macromolecular solution is higher, pass through the membrane into the external environment (dialysate). In the figure, the direction of the flow of low-molecular impurities is shown by arrows. Purification continues until the concentrations of impurities in the ash and dialysate become close in magnitude (more precisely, until the chemical potentials in the ash and dialysate are equalized). If you update the solvent, you can almost completely get rid of impurities. This use of dialysis is appropriate when the purpose of purification is to remove all low molecular weight substances passing through the membrane. However, in some cases, the task may turn out to be more difficult - it is necessary to get rid of only a certain part of low-molecular compounds in the system. Then, as an external environment, a solution of those substances that must be stored in the system is used. It is this task that is set when cleaning the blood from low-molecular slags and toxins (salts, urea, etc.).

Ultrafiltration. Ultrafiltration is a cleaning method by forcing a dispersion medium together with low molecular weight impurities through ultrafilters. Ultrafilters are membranes of the same type used for dialysis.

The simplest ultrafiltration plant is shown in Fig. 4 (see appendix). The purified sol or IUD solution is poured into the bag from the ultrafilter. The sol is subjected to an excess pressure compared to atmospheric pressure. It can be created either by an external source (compressed air tank, compressor, etc.) or by a large column of liquid. The dispersion medium is renewed by adding pure solvent to the sol. In order for the cleaning speed to be sufficiently high, the update is carried out as quickly as possible. This is achieved by applying significant overpressures. In order for the membrane to withstand such loads, it is applied to a mechanical support. Grids and plates with holes, glass and ceramic filters serve as such support.

Microfiltration . Microfiltration is the separation by means of filters of microparticles ranging in size from 0.1 to 10 microns. The performance of the microfiltrate is determined by the porosity and thickness of the membrane. To assess porosity, i.e., the ratio of the pore area to the total filter area, a variety of methods are used: punching liquids and gases, measuring the electrical conductivity of membranes, punching systems containing calibrated particles of the dispersed phase, etc.

Microporous filters are made from inorganic substances and polymers. By sintering powders, membranes can be obtained from porcelain, metals and alloys. Polymer membranes for microfiltration are most often made from cellulose and its derivatives.

Electrodialysis. The removal of electrolytes can be accelerated by applying an externally imposed potential difference. This purification method is called electrodialysis. Its use for the purification of various systems with biological objects (solutions of proteins, blood serum, etc.) began as a result of the successful work of Doré (1910). The device of the simplest electrodialyzer is shown in fig. 5 (see attachment). The object to be cleaned (sol, IUD solution) is placed in the middle chamber 1, and the medium is poured into the two side chambers. In the cathode 3 and anode 5 chambers, ions pass through the pores in the membranes under the action of an applied electrical voltage.

Electrodialysis is most appropriate to purify when high electrical voltages can be applied. In most cases, at the initial stage of purification, the systems contain a lot of dissolved salts, and their electrical conductivity is high. Therefore, at high voltage, a significant amount of heat can be released, and irreversible changes can occur in systems with proteins or other biological components. Therefore, it is rational to use electrodialysis as the final cleaning method, using pre-dialysis.

Combined cleaning methods. In addition to individual purification methods - ultrafiltration and electrodialysis - their combination is known: electroultrafiltration, used to purify and separate proteins.

It is possible to purify and at the same time increase the concentration of the IUD sol or solution using a method called electrodecantation. The method was proposed by V. Pauli. Electrodecantation occurs when the electrodialyzer is operated without stirring. Sol particles or macromolecules have their own charge and, under the action of an electric field, move in the direction of one of the electrodes. Since they cannot pass through the membrane, their concentration at one of the membranes increases. As a rule, the density of particles differs from the density of the medium. Therefore, at the site of sol concentration, the density of the system differs from the average value (usually, the density increases with increasing concentration). The concentrated sol flows to the bottom of the electrodialyzer, and circulation occurs in the chamber, which continues until the particles are almost completely removed.

Colloidal solutions and, in particular, solutions of lyophobic colloids, purified and stabilized, despite their thermodynamic instability, can exist indefinitely. The red gold sol solutions prepared by Faraday have not yet undergone any visible changes. These data suggest that colloidal systems can be in metastable equilibrium.

Filtration, dialysis, electrodialysis, and ultrafiltration are used to purify dispersed systems from impurities.

Filtration (lat. Filtrum- felt) is a separation method based on passing the crushed mixture through a porous film. In this case, small particles of df pass through the pores of conventional filters, while large particles are retained. Thus, filtration is also used to remove large particles from the dispersion.

Dialysis (gr. Dyalisis- separation) is a method of removing low molecular weight compounds from dispersed systems and solutions of IUDs using membranes. In the dialyzer, the fluid mixture to be dialyzed is separated from the pure solvent by a suitable membrane (Figure 2.6). DP particles and macromolecules are retained by the membrane, while small molecules and small-sized ions diffuse through the membrane into the solvent and at …
its sufficiently frequent replacement can be almost completely removed from the dialyzable mixture.

The separating ability of membranes with respect to low molecular weight substances is based on the fact that small molecules and ions freely pass through the pores (capillaries) penetrating the membrane or dissolve in the membrane substance.

Various films, both natural and artificial, are used as membranes for dialysis. Natural membranes: bovine or porcine bladder, swim bladder of fish. Artificial: films made of nitrocellulose, cellulose acetate, cellophane, gelatin and other polymers.

There is a wide variety of dialyzers - devices for dialysis. All dialyzers are built according to the general principle. The mixture to be dialyzed (internal fluid) is contained in a vessel in which it is separated from water or other solvent (external fluid) by a membrane (Fig. 2.6). the dialysis rate increases with an increase in the membrane surface, its porosity and pore size, with an increase in temperature, the intensity of mixing of the dialyzed fluid, the rate of change of the external fluid, and decreases with an increase in the membrane thickness.

To increase the rate of dialysis of low molecular weight electrolytes, electrodialysis is used. For this purpose, a constant electric field is created in the dialyzer with a potential drop of 20-250 V / cm and above (Fig. 2.7). Carrying out dialysis in an electric field allows accelerating the purification of dispersed systems by several tens of times.

Ultrafiltration (lat. Ultra- in excess of, filtrum- felt) is used to clean systems containing microparticles (sols, IUD solutions, suspensions of bacteria, viruses). The method is based on forcing the mixture to be separated through filters with pores that pass only molecules and ions of low molecular weight substances. Ultrafiltration can be thought of as pressure dialysis. It is widely used to purify water, proteins, nucleic acids, enzymes, vitamins, etc.