Sols purification methods: dialysis, electrodialysis, ultrafiltration. Preparation, properties and purification of colloidal solutions Methods for purification of colloidal solutions

When colloidal solutions are obtained by one method or another, especially with the help of chemical reactions, it is almost impossible to accurately predict the required exact amount of reagents. For this reason, an excessive excess of electrolytes may be present in the resulting sols, which reduces the stability of colloidal solutions. To obtain highly stable systems and to study their properties, sols are purified both from electrolytes and from all kinds of other low molecular impurities.

Purification of colloidal solutions can be carried out either by dialysis or ultrafiltration.

Dialysis consists in the 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. Periodically or continuously changing the solvent in the device for dialysis - dialyzer, it is possible to almost completely remove impurities of electrolytes and low molecular weight non-electrolytes from the colloidal solution.

The disadvantage of the method is the long duration of the cleaning process (weeks, months).

Electrodialysis is a dialysis process accelerated by the application of electric current. The device for its implementation is called an electrodialyzer. The simplest electrodialyzer is a vessel divided by two membranes into three chambers. The colloidal solution to be purified is poured into the middle chamber. Electrodes from the source are placed in the side chambers direct current and provide the supply and removal of the solvent (water). Under the influence electric field there is a transfer of cations from the middle chamber to the cathode chamber, anions - to the anode. The solution in the middle chamber can be cleared of dissolved salts within a short time (minutes, hours).

ultrafiltration- filtration of a colloidal solution through a semi-permeable membrane that passes a dispersion medium with low molecular weight impurities and retains particles of the dispersed phase or macromolecules. To accelerate the process of ultrafiltration, it is carried out with a pressure drop on both sides of the membrane: under vacuum (vacuum) or under elevated pressure. Vacuum is created by evacuating air from a vessel located under the filter, increased pressure is created by forcing air into a vessel located above the filter. To prevent rupture of the membrane, it is placed on a hard porous plate. Ultrafiltration allows you to quickly separate electrolytes and other impurities (low molecular organic compounds) than it does with dialysis. During ultrafiltration, a high degree of purification of the sol is achieved, periodically diluting the latter with water. In the final step, the colloidal .beta.-solution can be concentrated by suction of the dispersion medium. Ultrafiltration can be used in combination with electrodialysis (electroultrafiltration), which greatly accelerates the removal of salts from the colloidal solution.

Since the pores of ordinary filter paper easily pass colloidal particles, special filters (cellophane, parchment, asbestos, ceramic filters, etc.) are used as membranes in ultrafiltration. The use of a membrane with a certain pore size makes it possible to separate colloidal particles into fractions by size and approximately determine these sizes. So the sizes of some viruses and bacteriophages were found. All this suggests that ultrafiltration is not only a method for purifying colloidal solutions, but can be used for the purposes of dispersion analysis and preparative separation of disperse systems.

When obtaining colloidal solutions by one method or another, especially with the help of chemical reactions, it is almost impossible to accurately predict the required quantitative ratio of the reagents. For this reason, an excessive excess of electrolytes may be present in the resulting sols, which reduces the stability of colloidal solutions. To obtain highly stable systems and to study their properties, sols are purified both from electrolytes and from all kinds of other low molecular impurities.

Purification of colloidal solutions can be carried out either by dialysis or ultrafiltration.

Dialysis consists in extracting low molecular weight substances from sols with a pure solvent using a semi-permeable partition (membrane), through which colloidal particles do not pass. Periodically or continuously changing the solvent in the device for dialysis - dialyzer, it is possible to almost completely remove impurities of electrolytes and low molecular weight non-electrolytes from the colloidal solution.

The disadvantage of the method is the long duration of the cleaning process (weeks, months).

Electrodialysis is a process of dialysis accelerated by the application of electric current. The device for its implementation is called an electrodialyzer. The simplest electrodialyzer is a vessel divided by two membranes into three chambers. The colloidal solution to be purified is poured into the middle chamber. Electrodes from a direct current source are placed in the side chambers and the solvent (water) is supplied and removed. Under the action of an electric field, cations are transferred from the middle chamber to the cathode chamber, and anions - to the anode chamber. The solution in the middle chamber can be cleared of dissolved salts within a short time (minutes, hours).

Compensatory dialysis And vividialysis- methods developed for the study of biological fluids, which are colloidal systems. The principle of the method of compensatory dialysis is that in the dialyzer, instead of a pure solvent, solutions of determined low molecular weight substances of various concentrations are used. For example, to determine non-protein, i.e. free, sugar in the blood serum, it is dialyzed against an isotonic saline solution containing various concentrations of sugar. In that solution

The method of vividialization (vividiffusion) is close to this method for in vivo determination of low molecular weight constituent parts. For analysis, glass cannulas are inserted into the ends of the cut blood vessel, the branched parts of which are interconnected by tubes of semipermeable material, and the entire system is placed in a vessel filled with saline or water. In this way, it was found that, in addition to free glucose, there are free amino acids in the blood.

The principle of compensatory vividialization was used to create an apparatus called "artificial kidney". With the help of an “artificial kidney”, it is possible to purify the blood from metabolic products, temporarily replacing the function of a diseased kidney in case of such indications as acute renal failure as a result of poisoning, severe burns, etc.

ultrafiltration- filtration of a colloidal solution through a semi-permeable membrane that passes a dispersion medium with low molecular weight impurities and retains particles of the dispersed phase or macromolecules. To accelerate the process of ultrafiltration, it is carried out with a pressure drop on both sides of the membrane: under vacuum (vacuum) or under high pressure. Vacuum is created by evacuating air from a vessel located under the filter, increased pressure is created by forcing air into a vessel located above the filter. To prevent rupture of the membrane, it is placed on a hard porous plate. Ultrafiltration allows you to quickly separate electrolytes and other impurities (low molecular weight organic compounds) from a colloidal solution than does dialysis. During ultrafiltration, a high degree of purification of the sol is achieved, periodically diluting the latter with water. In the final step, the colloidal solution can be concentrated by suction of the dispersion medium. Ultrafiltration can be used in combination with electrodialysis (electroultrafiltration), which greatly accelerates the removal of salts from the colloidal solution.

Since the pores of ordinary filter paper easily pass colloidal particles, special filters (cellophane, parchment, asbestos, ceramic filters, etc.) are used as membranes in ultrafiltration. The use of a membrane with a certain pore size makes it possible to separate colloidal particles into fractions by size and approximately determine these sizes. So the sizes of some viruses and bacteriophages were found. All this suggests that ultrafiltration is not only a method for purifying colloidal solutions, but can be used for the purposes of dispersion analysis and preparative separation of disperse systems.

Dialysis- the most important of them. The essence of the method: two vessels separated by a semi-permeable membrane (collodion, cellophane, parchment, polysiloxane, PVC, polyethylene). In one vessel - a colloidal solution to be purified, in the other - a pure solvent. Due to diffusion, all ions from the colloidal solution that are able to pass through the holes of the membrane will pass into the solvent, while larger colloidal particles will remain in the solution. Advantage of the method: simplicity and low cost. Disadvantage: dialysis time - several days. The speed can be increased due to temperature, but very slightly.

But the speed can be increased due to the directed movement of ions in an electric field. The dialyzer is equipped with an additional chamber with electrodes (constant voltage). The dialysis time will be several hours or even minutes. This method is widely used in biochemistry, pharmacy, medicine, water purification and food production.

Another type of dialysis is often used - compensatory. The essence of the method of compensatory dialysis (vividialysis) is that the dispersed system is washed not with a pure solvent, but with solutions with different concentrations of a certain substance (or substances). For example: determination of sugar in blood serum. The blood serum is washed with isotonic sugar solution. The concentration of sugar in the external solution will not change if it is equal to the concentration of sugar in the blood. The work of an artificial kidney is based on vividialization ( hemodialysis). An artificial kidney is used to release blood from metabolic products, correct electrolyte-water and acid-base balance in acute and chronic renal failure, as well as to remove dialyzable toxic substances in case of poisoning and excess water in edema.

One of the most promising applications of dialysis is the prolongation of action. medicines. The duration of controlled release is in the range from 2 days to several years, ensuring a uniform supply of the drug. The usual way drugs are administered – either by injection or in pill form – dramatically increases their concentration in the body, which can cause unwanted side effects. So, drugs containing hormones, with the traditional "impulse" input, can cause endocrine disorders. Therefore, drugs coated with a membrane layer are used. A short time after ingestion, the rate of drug entry into the body becomes constant and can be set by the thickness of the membrane.

Ultrafiltration - this is a baromembrane process, which consists in the fact that the liquid is not filtered spontaneously, but under pressure is “squeezed” through a semi-permeable partition. This method is sometimes called dry dialysis, in the sense that there is no solvent on the other side of the membrane. The size of openings (pores) of ultrafiltration membranes ranges from 5 nm to 0.05–0.1 µm. As a material for the manufacture of ultrafiltration membranes, polymeric substances are mainly used - cellulose acetate, polysulfone, polyamide, polyimide, etc. Most membranes consist of a thin selective layer several tens of microns thick and a porous substrate that provides mechanical strength. Most modern polymeric membranes are resistant to microorganisms and chemical compounds in a wide pH range, have high selectivity and productivity, and allow short-term exposure to strong oxidizing agents: free chlorine, ozone. For the production of ultrafiltration membranes, inorganic (ceramic and cermet) materials based on Al 2 O 3 , TiO 2 , and ZnO oxides are also used. Ceramic membranes are characterized by durability, high physical, chemical and bacterial resistance, which allows them to work in the most severe conditions. In industry, ultrafiltration cleans wastewater, separate products of microbiological synthesis, concentrate biologically active substances. IN Lately ultrafiltration is used to cleanse the blood of toxins and remove excess fluid from the body.

Ultracentrifugation- a method for separating and studying particles smaller than 100 nm in the field of centrifugal forces, i.e. during fast circular motion. It allows you to separate mixtures of particles into fractions or individual components, find their molecular weight, etc.
It is carried out using ultracentrifuges. Distinguish the so-called analytical centrifugation (used in the analysis of solutions), the investigated volumes - from 0.01 to 2 ml with a mass of particles from a few micrograms to mg; and preparative centrifugation (used to isolate components from complex mixtures), the volume of liquid and the mass of the test sample can be. several orders of magnitude higher than with analytical ultracentrifugation. Centrifugal accelerations in ultracentrifuges reach 500,000 g. The first analytical ultracentrifuge was created by T. Svedberg (1923; 5000g).

5. Molecular-kinetic properties of colloidal systems do not fundamentally differ from the properties of true solutions. They are also characterized by diffusion, osmosis, etc., but all these phenomena have their own peculiarities. Diffusion– because colloidal particles are much larger in size and mass than molecules and ions, then the speed of their thermal motion is less, therefore, the diffusion rate is also many times less. A colloidal particle advances by 1 cm in a day, sometimes weeks; in true solutions - for hours.

osmotic pressure. It is known that P=CRT. But the concentration of particles in colloidal solutions is small even at a high mass fraction of the solute, so the osmotic pressure in colloidal solutions is low. (In a 1% sugar solution it is 79.46 kPa, in a 1% gelatin solution it is 1 kPa, and in a colloidal solution of arsenic sulfide it is only 0.0034 kPa.) Not surprisingly, such an osmotic pressure is difficult to detect. Plus, it's not permanent. The osmotic pressure of biopolymers is significantly affected by the temperature and pH of the solution. Temperature - because dissociation increases, therefore, the number of particles in the solution increases. The effect of pH is associated with a change in the ratio between positively and negatively charged groups. At the isoelectric point, the osmotic pressure will be minimal; when the pH shifts to the acidic or alkaline side from the IEP, it will increase. Osmotic pressure of the blood is calculated by the cryoscopic method by determining the depression (freezing point of the solution), which for blood is 0.56-0.58°C below zero. The osmotic pressure of the blood is approximately 7.6 atm. The osmotic pressure of blood depends mainly on the low molecular weight compounds dissolved in it, mainly salts. About 60% of this pressure is created by NaCl. Osmotic pressure in the blood, lymph, tissue fluid, tissues is approximately the same and is constant. Even in cases where a significant amount of water or salt enters the blood, the osmotic pressure does not undergo significant changes. With excessive intake of water into the blood, water is quickly excreted by the kidneys and passes into tissues and cells, which restores the initial value of osmotic pressure. If the concentration of salts in the blood rises, then water from the tissue fluid passes into the vascular bed, and the kidneys begin to excrete salts intensively. Digestion products of proteins, fats and carbohydrates, absorbed into the blood and lymph, as well as low molecular weight products of cellular metabolism, can change the osmotic pressure within a small range. Maintaining a constant osmotic pressure plays an extremely important role in the life of cells.

Part of the osmotic pressure of the blood, which depends on the content of large molecular compounds (proteins) in solution, is called oncotic pressure. Although the concentration of proteins in plasma is quite high, the total number of molecules due to their large molecular weight is relatively small. Therefore, oncotic pressure does not exceed 30 mm Hg. Oncotic pressure is more dependent on albumins (80% of oncotic pressure is created by albumins), which is associated with their relatively low molecular weight and a large number of molecules in plasma. Oncotic pressure plays an important role in the regulation of water metabolism. Proteins are well hydrated and retain water in the bloodstream. The greater the oncotic pressure, the more water is retained in the vascular bed and the less it passes into the tissues and vice versa. Oncotic pressure affects the formation of tissue fluid, lymph, urine and water absorption in the intestine. Therefore, blood-substituting solutions must contain biopolymers capable of retaining water. With a decrease in the concentration of protein in the plasma, edema develops, since water ceases to be retained in the vascular bed and passes into the tissues.

Sedimentation– because particles are affected not only by diffusion, but also by the gravitational field; under the action of gravity, particles with sufficient mass can settle (sediment). The settling rate of particles depends on their mass (ceteris paribus). In a blood test, determine suspension stability of blood(erythrocyte sedimentation rate - ESR). Blood is a suspension, or suspension, since its formed elements are in suspension in the plasma. The suspension of erythrocytes in plasma is maintained by the hydrophilic nature of their surface, as well as by the fact that erythrocytes (like other formed elements) carry a negative charge, due to which they repel each other. If the negative charge of the formed elements decreases, which may be due to the adsorption of such positively charged proteins as fibrinogen, γ-globulins, paraproteins, etc., then the electrostatic “spread” between erythrocytes decreases. At the same time, erythrocytes, sticking together with each other, form the so-called coin columns. Such "coin columns", stuck in the capillaries, interfere with the normal blood supply to tissues and organs. If the blood is placed in a test tube, having previously added substances that prevent clotting, then after a while you can see that the blood is divided into two layers: the upper one consists of plasma, and the lower one is formed elements, mainly erythrocytes.

Special properties of colloidal systems. For colloidal systems, the characteristic optical property is the scattering of light, and in this they differ significantly from the properties of true solutions. The phenomenon of light scattering (opalescence) discovered by Faraday (1857) and Tyndale (1864). They observed the formation of a luminous cone when a beam of light was passed through a colloidal solution under side illumination. According to Rayleigh's theory of light scattering, when a light wave passes through colloidal systems, an electromagnetic field causes polarization of dispersed particles. The resulting dipoles are sources of new radiation.

Rayleigh equation:

Where: I o is the intensity of the incident light, V is the volume of particles, K is the ratio of the refractive indices of the dispersed phase and the dispersed medium, is the concentration of the dispersed phase, is the wavelength.

Because the intensity is inversely proportional to the wavelength to the fourth power, which means that during the passage of a beam of white light, the shortest waves (i.e. blue and violet) should predominantly scatter. Therefore, systems with an uncolored substance of the dispersed phase under side illumination are characterized by blue opalescence. This explains the blue color of burning gas, tobacco smoke, the sky, and skimmed milk. On the contrary, in transmitted light we observe red shades associated with the loss of the blue part of the spectrum. That is why the red color was chosen as a danger signal - it does not dissipate and therefore is visible far away. Colloidal solutions can also absorb a certain part of the spectrum. For example, highly dispersed gold sols absorb the green part of the spectrum and are colored red. As the particle size increases, the color of the solution shifts to the cold region. The color of a number of minerals is associated with the phenomena of absorption and scattering of light. precious stones and gems (amethyst, sapphire, ruby).

Nephelometry– method of analysis based on the phenomenon of light scattering. Instruments designed to determine the concentration and size of particles (according to the Rayleigh equation) are called nephelometers. Usually, in these devices, the intensity of light scattered by the standard and the test solution is compared. Nephelometers determine turbidity, i.e. the concentration of colloidal particles in various solutions in water treatment or in the production of juices and wine ...

Ultramicroscopy. In a conventional microscope, colloidal particles are invisible. But if colloidal systems are illuminated with side light against a dark background, then luminous dots can be seen, because each particle becomes a source of scattered light. An instrument that allows one to see colloidal particles against a dark background under side illumination is called an ultramicroscope. Particles up to 3 nm in size are visible. Such a microscope was constructed in 1903 by Siedentopf and Zsigmondy. It was with his help that they confirmed the theory of Brownian motion and determined the Avogadro number. But we must understand that we do not see the particles themselves, but the reflections from them on the screen. Therefore, the concentration of particles can be determined, but their size or shape cannot be determined.

Electrokinetic Phenomena in colloidal systems, this is a group of properties that reflect the relationship that exists between the movement of particles 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: electroosmosis, electrophoresis, flow potential and sedimentation potential.

Electroosmosis is the movement of the liquid phase relative to the stationary solid phase under the action of an electric current (1808, Moscow State University, Reiss). When direct current was passed through a U-shaped tube filled with quartz sand and water, the water in the knee with the negative electrode (cathode) rose higher, while in the other it fell. Those. the liquid phase moved under the action of an electric current.

electrophoresis– movement of the solid phase relative to the stationary liquid phase under the action of an electric current. When direct current (100V) was passed through a device consisting of two water-filled glass tubes immersed in wet clay, Reise found that clay particles detached from the surface of the clay and move upward (against gravity!) To the positive pole (anode). Those. the solid phase moved under the action of an electric field.

flow potential is the opposite of electroosmosis. Quincke in 1859 discovered that when water is filtered through a porous membrane, a potential difference arises between its two sides. Quincke suggested that the surface of a solid body is charged with one sign, and the adjacent liquid layer with another. This idea later led to the discovery amazing phenomenon on the phase interface - a double electric layer. Settling potential is the opposite of electrophoresis. Quartz sand was poured into a tall cylinder filled with water. During sedimentation of quartz particles in water, the potential difference between the electrodes located at different heights was recorded.

Electrophoresis discovered by Professor Reiss, as well as other electrokinetic phenomena, served as the basis for creating methods for studying the double electric layer on the surface of colloidal particles and studying the structure of colloidal particles in general. According to modern concepts, on the surface of any body as a result of ODS, dissociation processes, selective ion adsorption, etc. an electric double layer (DES) is formed - two layers of oppositely charged ions located in spaces in close proximity to each other. DES consists of two parts: internal - dense and external - diffuse. dense layer make up potential-determining ions, firmly bound to a solid surface, and part of the counterions, attracted due to electrostatic attraction and specific adsorption forces. This inner part of the DEL is called the adsorption layer. The sum of the charges of the potential-determining ions and prothioions in the adsorption layer is not equal to zero, and there are usually fewer counterions. A certain amount of counterions, missing to compensate for the charges of potential-determining ions, is located in the outer, diffuse layer. The diffuse layer is formed by counterions that are attracted to the surface from the solution due to electrostatic interaction, but are very weakly connected to the surface. When the solution moves, a gap occurs between the adsorption layer (firmly fixed on the surface) and the diffuse layer (ions located in the solution layer). We have a directed movement of charged particles - electricity. Conversely, in an electric field, the granules (solid phase) move in one direction, and the counterions of the diffuse layer (liquid phase) move in the other direction, i.e. there is a movement of the phases of colloidal systems.

For example: If a solution of silver nitrate is added dropwise to a solution of potassium iodide (i.e., it is in excess), then silver iodide does not precipitate; the solution contains few silver ions needed for crystal growth. And small crystals will not connect either, because they have the same charge. Those. the crystallization process that has begun does not lead to the formation of a precipitate if there is an electrolyte-stabilizer in the solution. A colloidal solution of silver iodide is formed with particles, the structure of which is usually expressed by special "micellar" formulas:

( m nI - (n-x)K + ) x - xK +, where m is the core, i.e. a small crystal of sparingly soluble silver iodide;

m nI - (n-x)K + - adsorption layer, consisting of potential-determining iodine ions, which were selectively adsorbed on the crystal (they were in excess in solution) and a certain amount of potassium counterions, strongly associated with iodine ions; xK + - mobile diffusion layer of potassium ions; ( m nI - (n-x)K + ) x - is a granule of a colloidal particle that will move independently in an electric field. The charge of the granule determines the magnitude and charge of the (zeta)-potential (electro-kinetic potential) on the surface of the colloidal particle.

In biosystems, DES can also arise due to selective adsorption or ionization of surface functional groups. Adsorption occurs mainly on polysaccharides, lipids, cholesterol, and on proteins, DES usually occurs due to the dissociation of the carboxyl and amino groups. It is known that amino acids, depending on the pH of the medium, exist in solutions in the form of neutral bi-ions, cationic or anionic forms of the protein.

The potential decreases as the number of counterions in the adsorption layer increases and can become zero if the total charge of the counterions becomes equal to the charge of the potential-determining ions (isoelectric state). This can happen with an increase in the concentration of counterions in the solution. The more - potential, the more stable is the CS, because the presence of a charge prevents particles from sticking together.

The value of the potential cannot be measured, it can be calculated using the Helholtz-Smoluchovsky equation:

Where is the viscosity of the medium, is the dielectric constant of the medium, is the distance between the electrodes, U is the electrophoresis rate, E is the potential difference.

Application of electrokinetic phenomena. Seventy years after Reiss discovered electrokinetic phenomena (back in the 19th century), electroosmosis was put into practice for drying peat, and then for drying wood. Since the 60s of the 20th century, electroosmosis has been used for drying and strengthening soils during the construction of buildings, to combat landslides during the construction of dams, to lower the groundwater level, to repair railway tracks and to dry buildings.

IN earth's crust groundwater flows through soils and rocks, and they are accompanied by so-called flow potentials, which are used by geophysicists to search for minerals, map groundwater, and find ways for water to seep through dams. Flow potentials arise during the transportation of liquid fuel, when filling tanks, cisterns, oil tankers, gasoline tanks of aircraft. When fuel flows through the pipes, sufficiently high potential differences arise at the ends of the pipelines, due to which tremendous fires occurred on oil tankers. There are also potentials of settling (this is also a flow, i.e. movement) of water droplets in clouds - the cause of lightning discharges in the atmosphere.

Electrochemical methods are widely used in medicine. When blood flows through the capillaries of the circulatory system, flow potentials arise, which are one of the sources of biopotentials. It has been established, for example, that one of the peaks electrocardiograms due to the occurrence of blood flow potentials in the coronary vessels of the heart. These potentials are measured in cardiac clinics and laboratories.

Electrophoresis is used as a method for determining and separating proteins.(and other electrically charged particles)) in solution by passing an electric current through this solution. The speed of movement of colloidal particles in an electric field depends on their charge and mass, so they gradually separate, moving to different poles of the electrode. Electrophoresis can be used to obtain drugs and biologically active substances.

Electrophoresis can also be used to analyze the composition of colloidal systems.Electrophoresis, like chromatography, can be performed on paper. Electropherograms of blood plasma proteins for all healthy people are almost the same. In pathology, they acquire a characteristic, and specific for each disease, appearance. Electrophoresis is widely used for research chemical composition body tissues. For example, to analyze various proteins and lipoproteins in blood serum, analyze the composition of proteins in urine, etc.

Electrophoresis is very often used for therapeutic purposes. For example: for the introduction of drugs through the skin (drugs are colloidal solutions); acceleration of the migration of leukocytes to the focus of inflammation (during inflammation, the destruction of cellular structures occurs with the formation of acidic products, in which case the surface of the tissues acquires a positive charge); or acceleration of the movement of erythrocytes to tissues suffering from hypoxia (the potential of human erythrocytes is a stable value and is equal to -16.3 mV).

Electrophoresis has become more widespread in the clinic of therapeutic dentistry as one of the methods of pain relief. For this purpose, 5 - 10% solutions of novocaine, dicaine, trimecaine, nicotinic acid are used.

The problem of CS stability is one of the main problems in colloid chemistry. IUD solutions and some lyophilic colloids (clays, soaps) are thermodynamically stable, they form spontaneously. During the formation of lyophobic CS, dispersion (grinding) occurs due to mechanical or other work, for these processes G > 0, i.e. thermodynamically unstable systems are formed. But, nevertheless, such systems can exist for quite a long time.

Distinguish between kinetic and aggregative stability of colloidal systems . Under kinetic stability understand the ability of the dispersed phase to be in suspension and not precipitate. Highly dispersed systems are kinetically more stable; the smaller the particle, the faster it moves, and the less the force of gravity acts on it. Therefore, sols are kinetically more stable than classical emulsions and suspensions. The kinetic stability is also affected by the density and viscosity of the medium. In viscous liquids, the settling of even large particles occurs slowly. In a gaseous medium, the density and viscosity are very low; therefore, systems with only very small particles, aerosols, can exist in gaseous media.

Aggregative stability is the ability of the system to maintain a certain degree of dispersion, i.e. do not coalesce into larger particles.

What contributes to the aggregative stability of the CS? Or: What prevents particles from sticking together?

The presence of a charge on the particles. The charge appears on the particles as a result of selective ion adsorption. (see the structure of colloidal particles, electric double layer). This usually occurs in aqueous electrolyte solutions.

Adsorption on surfactant particles. This process leads to a decrease in surface tension and by reducing the overall energy of the system, making it more stable. But this also happens mainly in solutions.

Hydration of colloidal particles. This phenomenon is observed in aqueous solutions, but only in lyophilic colloids, for example, in protein solutions.

Violation of aggregative stability, which occurs due to the adhesion of particles into large aggregates and their precipitation, is called coagulation.

Often in received dispersed systems, except for micelles, stabilizer and solvent contains low molecular weight substances (impurities). They reduce the stability of DS (they can neutralize the charge of colloidal particles, which leads to coagulation and destruction of colloidal systems). To purify colloidal systems from low molecular weight impurities, dialysis, electrodialysis, and ultrafiltration are used.

Dialysis(proposed and named by T. Graham) is based on passing a colloidal solution through a semipermeable membrane. The simplest dialyzer (Fig. 5) is a bag made of a semi-permeable material, into which a colloidal solution is poured, and the bag is lowered into a vessel with water (solvent). Due to the small size of the holes, semi-permeable membranes retain colloidal particles, while low-molecular ones pass through the membrane into the solvent. As a result, low molecular weight substances are removed from the colloidal solution. Previously, the walls of the urinary or gallbladder, intestines, and parchment were used as a semipermeable membrane. At present, membranes made of collodion (a solution of cellulose nitrate) are cellophane. They are very convenient, because. membranes can be made with any hole size.

Rice. 5. Dialyzers T. Graham.

It should be noted that long-term dialysis, in addition to removing impurities from the solution, can lead to coagulation of the system as a result of the removal of the stabilizer.

Electrodialysis. Since low molecular weight impurities in sols are electrolytes, dialysis can be accelerated by applying an electric current. To do this, a colloidal solution is placed between two membranes, outside of which

Dialysis is used in biotechnology and pharmaceuticals to purify proteins, IUDs from salt impurities, to obtain valuable drugs - globulin, flocculants, etc. Dialysis is used in the clinic as a method of treatment ("hemodialysis") for patients with diseases of the liver, kidneys, long-term pressure syndrome, in acute poisoning. In this case, the patient's blood is passed through the "artificial kidney" apparatus. It is a system with a membrane, one side of which is washed with a saline (physiological) solution having the same composition as the blood plasma, and the other with the patient's blood. During hemodialysis, low molecular weight metabolic products leave the blood through the membrane, while proteins remain in the blood (due to their large size). The salts necessary for the body are also preserved, because. there is no concentration gradient between blood and saline.

Ultrafiltration is used in the same way as dialysis and electrodialysis, in particular, to purify the culture fluid from the bodies of bacteria that produce antibiotics, separate proteins and sterilize their solutions. In this case, bacteria, viruses remain on the filter, and the necessary medicinal substances (serums, vaccines) are isolated from the filtrate.

Lecture number 5. Theories of the electric double layer

General ideas about disperse systems

Chemical interaction in homogeneous reactions occurs with effective collisions of active particles, and in heterogeneous reactions, at the interface between the phases when the reactants come into contact, and the rate and mechanism of the reaction depend on the surface area, which is the greater, the more developed the surface. From this point of view, dispersed systems with a high specific surface area are of particular interest.

A disperse system is a mixture consisting of at least two substances that do not chemically react with each other and have almost complete mutual insolubility. Disperse system - This is a system in which very fine particles of one substance are evenly distributed in the volume of another.

Considering disperse systems, two concepts are distinguished: the dispersed phase and the dispersion medium (Fig. 10.1).

Dispersed phase - This is a collection of particles of a substance dispersed to small sizes, evenly distributed in the volume of another substance. Signs of the dispersed phase are fragmentation and discontinuity.

Dispersion mediumis a substance in which the particles of the dispersed phase are evenly distributed. A sign of a dispersion medium is its continuity.

The dispersed phase can be separated from the dispersion medium by a physical method (centrifugation, separation, settling, etc.).

Figure 10.1 - Dispersed system: particles of the dispersed phase s (in the form of small solid particles, crystals, liquid drops, gas bubbles, associates of molecules or ions), having an adsorption layer d, are distributed in a homogeneous continuous dispersion medium f.

Dispersed systems are classified according to different hallmarks: dispersion, aggregate state of the dispersed phase and the dispersion medium, the intensity of interaction between them, the absence or formation of structures in disperse systems.

Classification according to the degree of dispersion

Depending on the particle size of the dispersed phase, all disperse systems are conditionally divided into three groups (Fig. 10.2).

Figure 10.2 - Classification of dispersed systems by particle size (for comparison, particle sizes in true solutions are given)

1. Coarsely dispersed systems , in which the particle size is more than 1 µm (10 –5 m). This group of dispersed systems is characterized by the following features: particles of the dispersed phase settle (or float) in the field of gravitational forces, do not pass through paper filters; they can be viewed with a conventional microscope. Coarse systems include suspensions, emulsions, dust, foam, aerosols, etc.

Suspension - is a dispersed system in which the dispersedphase is solid, and the dispersion medium is a liquid.

An example of a suspension can be a system formed by shaking clay or chalk in water, paint, paste.

Emulsion - this is a dispersed system in which the liquid dispersed phase is uniformly distributed in the volume of the liquid dispersion medium, i.e. An emulsion consists of two mutually insoluble liquids.

Examples of emulsions include milk (drops of liquid fat act as the dispersed phase, and water is the dispersion medium), cream, mayonnaise, margarine, ice cream.

When settling, suspensions and emulsions are separated (separated) into their constituent parts: the dispersed phase and the dispersion medium. So, if benzene is vigorously shaken with water, an emulsion is formed, which after some time is divided into two layers: the upper benzene and the lower water. To prevent separation of emulsions, they are added emulsifiers- substances that impart aggregate stability to emulsions.

Foam - a cellular coarse-dispersed system in which the dispersed phase is a set of gas (or vapor) bubbles, and the dispersion medium is a liquid.

In foams, the total volume of the gas in the bubbles can be hundreds of times greater than the volume of the liquid dispersion medium contained in the interlayers between the gas bubbles.

2. Microheterogeneous (orfinely dispersed ) intermediate systems in which the particle size ranges from 10 – 5 –10 –7 m. These include fine suspensions, fumes, porous solids.

3. Ultramicroheterogeneous (orcolloid-dispersed ) systems in which particles with a size of 1–100 nm (10–9 –10 –7 m) consist of 10 3_ 10 9 atoms and are separated from the solvent by an interface. Colloidal solutions are characterized by a limiting highly dispersed state, they are usually called ash, or often lyosolsto emphasize that the dispersion medium is a liquid. If water is taken as the dispersion medium, then such sols are calledhydrosols, and if the organic liquid -organosols.

For most finely dispersed systems, certain features are inherent:

low diffusion rate;

particles of the dispersed phase (i.e. colloidal particles) can only be examined using an ultramicroscope or an electron microscope;

scattering of light by colloidal particles, as a result of which they take the form of light spots in an ultramicroscope - the Tyndall effect (Fig. 10.3);

Figure 10.3 - Ultramicroheterogeneous (finely dispersed) system: a) colloidal solution; b) scheme of deflection of a narrow beam of light when passing through a colloidal solution; c) scattering of light by a colloidal solution (Tyndall effect)

• on the phase interface in the presence of stabilizers (electrolyte ions), an ionic layer or a solvation shell is formed, which contributes to the existence of particles in a suspended form;
• the dispersed phase is either completely insoluble or slightly soluble in the dispersion medium.

Examples of colloidal particles include starch, proteins, polymers, rubber, soaps, Aluminum and Ferum (III) hydroxides.

Classification of dispersed systems according to the ratio of aggregate states of the dispersed phase and the dispersion medium

This classification was proposed by Ostavld (Table 10.1). When schematically recording the aggregate state of dispersed systems, the aggregate state of the dispersed phase is first indicated by the letters G (gas), F (liquid) or T (solid), and then a dash (or a fraction sign) is written and the aggregate state of the dispersion medium is written.

Table 10.1 - Classification of dispersed systems

Classification of dispersed systems according to the intensity of molecular interaction

This classification was proposed by G. Freindlich and is used exclusively for systems with a liquid dispersion medium.

1. Lyophilic systems , in which the dispersed phase interacts with the dispersion medium and, under certain conditions, is able to dissolve in it - these are solutions of colloidal surfactants (surfactants), solutions of macromolecular compounds (HMCs). Among the various lyophilic systems, the most important in practical terms are surfactants, which can be both in a molecularly dissolved state and in the form of aggregates (micelles) consisting of tens, hundreds or more molecules.
2. Lyophobic systems , in which the dispersed phase is not able to interact with the dispersion medium and dissolve in it. In lyophobic systems, the interaction between molecules of different phases is much weaker than in the case of lyophilic systems; interfacial surface tension is high, as a result of which the system tends to spontaneous coarsening of the particles of the dispersed phase.

Classification of dispersed systems by physical state

The author of the classification is P. Rebinder. According to this classification, a disperse system is denoted by a fraction, in which the dispersed phase is placed in the numerator, and the dispersion medium is in the denominator. For example: T 1 /W 2 denotes a dispersed system with a solid phase (index 1) and a liquid dispersion medium (index 2). The Rehbinder classification divides disperse systems into two classes:

1. Freely dispersed systems – sols in which the dispersed phase does not form continuous rigid structures (grids, trusses or frames), has fluidity, and the particles of the dispersed phase do not contact each other, participating in random thermal motion and moving freely under the action of gravity. These include aerosols, lyosols, diluted suspensions and emulsions.

Examples of free-dispersed systems:

• Dispersed systems in gases with colloidal dispersity (T 1 /G 2 - dust in the upper layers of the atmosphere, aerosols), with coarse dispersion (T 1 /G 2 - smoke and Zh 1 /G 2 - fogs);
• Dispersed systems in liquids with colloidal dispersity (T 1 /L 2 - lyosols, dispersed dyes in water, latexes of synthetic polymers), with coarse dispersion (T 1 /L 2 - suspensions; L 1 /L 2 - liquid emulsions; G 1 / Zh 2 - gas emulsions);
• Dispersed systems in solids: T 1 /T 2 - solid sols, for example, yellow metal sol in glass, pigmented fibers, filled polymers.

2. Cohesive-dispersed (or continuous) systems . In continuous (coherently dispersed) systems, particles of the dispersed phase form rigid spatial structures. Such systems resist shear deformation. Cohesive-dispersed systems are solid; they arise when the particles of the dispersed phase come into contact, leading to the formation of a structure in the form of a skeleton or network, which limits the fluidity of the dispersed system and gives it the ability to keep its shape. Such structured colloidal systems are called gels.

Examples of connected disperse systems:

• Dispersed systems with a liquid interface (G 1 / Zh 2 - foam; Zh1 / Zh 2 - foamy emulsions);
• Dispersed systems with a solid phase interface (G 1 /T 2 - porous bodies, natural fibers, pumice, sponge, charcoal; W 1 /T 2 - moisture in granite; T 1 /T 2 - interpenetrating networks of polymers).

Preparation and purification of colloidal solutions

Obtaining colloidal solutions

Colloidal solutions can be obtaineddispersive or to condensation methods.

1. Dispersion methods- these are methods for obtaining lyophobic sols by crushing large pieces to aggregates of colloidal sizes.

mechanical crushing of coarse-dispersed systems is carried out by: crushing, impact, abrasion, splitting. The grinding of particles to sizes of several tens of microns is carried out using ball mills.Very fine crushing (up to 0.1-1 micron) is achieved on specialcolloid millswith a narrow gap between a rapidly rotating rotor (10-20 thousand rpm) and a fixed body, and the particles are torn or abraded in the gap.The works of P. A. Rebinder established the phenomenon of a decrease in the resistance of solids to elastic and plastic deformations, as well as mechanical destruction under the influence of adsorption of surfactants. Surfactants facilitate dispersion and contribute to a significant increase in the degree of dispersion.

2. Condensation methods- these are methods for obtaining colloidal solutions by combining (condensing) molecules and ions into aggregates of colloidal sizes. The system transforms from homogeneous into heterogeneous, i.e., a new phase appears (dispersed phase). The prerequisite is supersaturation original system.

Condensation methods are classified according to the nature of the forces causing condensation into physical condensation and chemical condensation.

physical condensation can be carried out from vapors or by changing the solvent.

vapor condensation. The starting material is in pairs. As the temperature decreases, the vapor becomes supersaturated and partially condenses, forming a dispersed phase. Hydrosols of mercury and some other metals are obtained in this way.

Solvent replacement method. The method is based on changing the composition and properties of the dispersion medium. For example, an alcohol solution of sulfur, phosphorus or rosin is poured into water, due to a decrease in the solubility of the substance in the new solvent, the solution becomes supersaturated and part of the substance condenses, forming particles of the dispersed phase.

Chemical condensation consists in the fact that the substance that forms the dispersed phase is obtained as a result of a chemical reaction. In order for the reaction to form a colloidal solution, and not a true solution or precipitate, at least three conditions must be met:

1. the substance of the dispersed phase is insoluble in the dispersion medium;
2. the rate of formation of nuclei of crystals of the dispersed phase is much greater than the rate of crystal growth; this condition is usually met when a concentrated solution of one component is poured into a highly dilute solution of another component with vigorous stirring;
3. one of the starting materials is taken in excess, it is it that is the stabilizer.

Methods for purification of colloidal solutions.

Colloidal solutions obtained in one way or another are usually purified from low-molecular impurities (molecules and ions). Removal of these impurities is carried out by methods of dialysis, (electrodialysis), ultrafiltration.

Dialysis– cleaning method using a semi-permeable membrane that separates the colloidal solution from a pure dispersion medium. As a semi-permeable (i.e., permeable to molecules and ions, but impermeable to particles of the dispersed phase) membranes, parchment, cellophane, collodion, ceramic filters and other finely porous materials are used. As a result of diffusion, low-molecular impurities pass into the external solution.

Ultrafiltration called dialysis, carried out under pressure in the inner chamber. Essentially, ultrafiltration is not a method for purifying sols, but only a method for concentrating them.

Optical properties of colloidal solutions

When light falls on a disperse system, the following phenomena can be observed:

• the passage of light through the system;
• refraction of light by particles of the dispersed phase (if these particles are transparent);
• reflection of light by particles of the dispersed phase (if the particles are opaque);
• scattering of light;
• absorption ( absorption) of light by the dispersed phase.

light scattering observed for systems in which the particles of the dispersed phase are smaller or commensurate with the wavelength of the incident light. Recall that the particle size of the dispersed phase in colloidal solutions is 10 -7 -10 -9 m. Consequently, light scattering is a characteristic phenomenon for the colloidal systems we study.

Rayleigh created the theory of light scattering. He derived an equation that relates the scattered light intensity I to the incident light intensity I 0 . fair, provided that:

• the particles are spherical;
• particles do not conduct electricity (i.e. are non-metallic);
• the particles do not absorb light, that is, they are colorless;
• the colloidal solution is diluted to such an extent that the distance between the particles is greater than the wavelength of the incident light.

Rayleigh equation:

• Where V is the volume of one particle,
• λ - wavelength;
• n 1 is the refractive index of the particle;
• n o is the refractive index of the medium.

The following conclusions follow from the Rayleigh equation:

1. The intensity of the scattered light is the greater, the more the refractive indices of the particle and the medium differ (n 1 - P 0 ).
2. If the refractive indices P 1 And n 0 are the same, then light scattering will be absent in an inhomogeneous medium.
3. The intensity of the scattered light is the greater, the greater the partial concentration v. Mass concentration c, g / dm 3, which is usually used in the preparation of solutions, is associated with a partial concentration by the expression:

where ρ is the particle density.

It should be noted that this dependence is preserved only in the region of small particle sizes. For the visible part of the spectrum, this condition corresponds to the values ​​2 10 -6 cm< r < 4 10 -6 см. С увеличением r рост I slows down, and for r > λ, scattering is replaced by reflection. The intensity of the scattered light is directly proportional to the concentration.

4. The intensity of scattered light is inversely proportional to the wavelength to the fourth power.

This means that when a white light beam passes through a colloidal solution, short waves - the blue and violet parts of the spectrum - are predominantly scattered. Therefore, a colorless sol has a bluish color in scattered light, and a reddish color in transmitted light. The blue color of the sky is also due to the scattering of light. tiny droplets water in the atmosphere. The orange or red color of the sky at sunrise or sunset is due to the fact that in the morning or evening there is mainly light that has passed through the atmosphere.

light absorption. The Rayleigh equation was derived for uncolored sols, i.e., not absorbing light. However, many colloidal solutions have a certain color, i.e. absorb light in the corresponding region of the spectrum - the sol is always colored in a color complementary to that absorbed. So, absorbing the blue part of the spectrum (435-480 nm), the sol turns yellow; upon absorption of the bluish-green part (490-500 nm), it takes on a red color.If the rays of the entire visible spectrum pass through a transparent body or are reflected from an opaque body, then the transparent body appears colorless, and the opaque body appears white. If a body absorbs radiation from the entire visible spectrum, it appears black.The optical properties of colloidal solutions capable of absorbing light can be characterized by the change in light intensity as it passes through the system. To do this, use the Bouguer-Lambert-Beer law:

where I 0 is the intensity of the incident light ; I etc is the intensity of the light transmitted through the sol; k - absorption coefficient; l- thickness of the sol layer; With- sol concentration.

If we take the logarithm of the expression, we get:

The value is called optical density solution . When working with monochromatic light, they always indicate at what wavelength the optical density was determined, denoting it D λ .

Micellar theory of the structure of colloidal systems

Let us consider the structure of a hydrophobic colloidal particle using the example of the formation of an AgI sol by the exchange reaction

AgNO 3 + KI → AgI + KNO 3.

If the substances are taken in equivalent quantities, then a crystalline precipitate of AgI precipitates. But, if one of the initial substances is in excess, for example, KI, the AgI crystallization process leads to the formation of a colloidal solution - AgI micelles.

The structure diagram of AgI hydrosol micelles is shown in Fig. 10.4.

Figure 10.4 - Scheme of an AgI hydrosol micelle formed with an excess of KI

An aggregate of molecules [ mAgI ] of 100-1000 (microcrystals) - the core, is the embryo of a new phase, on the surface of which the adsorption of electrolyte ions in the dispersion medium takes place. According to the Panet-Fajans rule, ions are better adsorbed, the same as the ions that enter the crystal lattice of the nucleus and complete this lattice. Ions that adsorb directly to the nucleus are called potential-determining, since they determine the magnitude of the potential and the sign of the surface charge, as well as the sign of the charge of the entire particle. Potential-determining ions in this system are I - ions, which are in excess, are part of crystal lattice AgI nuclei act as stabilizers and form the inner shell in the hard part of the electric double layer (EDL) of the micelle. The aggregate with adsorbed ions I - forms the core of a micelle.

To the negatively charged surface of AgI particles at a distance close to the radius of a hydrated ion, ions of the opposite sign (counterions) - positively charged K + ions - are attracted from the solution. The layer of counterions - the outer shell of the double electric layer (EDL), is held both by electrostatic forces and by the forces of adsorption attraction. An aggregate of molecules together with a solid double layer is called a colloidal particle - a granule.

Part of the counterions due to thermal motion is located diffusely around the granule, and are associated with it only due to electrostatic forces. Colloidal particles, together with the diffuse layer surrounding it, are called micelles. A micelle is electrically neutral, since the charge of the nucleus is equal to the charge of all counterions, and the granule usually has a charge, which is called electrokinetic or ξ - zeta - potential. In an abbreviated form, the micelle structure scheme for this example can be written as follows:

One of the main provisions of the theory of the structure of colloidal particles is the concept of the structure of a double electric layer (EDL). According to modern ideas, double electrical layer DELconsists of adsorption and diffusion layers. The adsorption layer consists of:

• the charged surface of the micelle core as a result of the adsorption of potential-determining ions on it, which determine the magnitude of the surface potential and its sign;
• a layer of ions of the opposite sign - counterions, which are attracted from the solution to the charged surface. Adsorption layer of counterions located at a distance of a molecular radius from the charged surface. Both electrostatic and adsorption forces exist between this surface and the counterions of the adsorption layer, and therefore these counterions are bonded especially strongly to the core. The adsorption layer is very dense, its thickness is constant and does not depend on changes in external conditions (electrolyte concentration, temperature).

Due to thermal motion, part of the counterions penetrate deep into the dispersion medium, and their attraction to the charged surface of the granule is carried out only due to electrostatic forces. These counterions make up the diffuse layer, which is less strongly bonded to the surface. The diffuse layer has a variable thickness, which depends on the concentration of electrolytes in the dispersion medium.

When the solid and liquid phases move relative to each other, a DEL break occurs in the diffuse part and a potential jump occurs at the phase interface, which is called electrokinetic ξ - potential(zeta - potential). Its value is determined by the difference between the total number of charges (φ) of potential-determining ions and the number of counterion charges (ε) contained in the adsorption layer, i.e. ξ = φ - ε. The drop in the interfacial potential with distance from the solid phase deep into the solution is shown in Fig. 10.5.

Figure 10.5 DPP structure

The presence of a potential difference around the particles of a hydrophobic sol prevents them from sticking together during a collision, that is, they are a factor in the aggregate stability of the sol. If the number of diffuse ions decreases or tends to zero, then the granule becomes electrically neutral (isoelectric state) and has the least stability.

Thus, the magnitude of the electrokinetic potential determines the repulsive forces, and hence the aggregate stability of the colloidal solution. Sufficient stability of the colloidal solution is ensured at the value of the electrokinetic potential ξ = 0.07V, at values ​​less than ξ = 0.03V, the repulsive forces are too weak to resist aggregation, and therefore coagulation occurs, which inevitably ends with sedimentation.

The magnitude of the electrokinetic potential can be determined using an electrophoresis device using the formula (10.5):

where η is the viscosity; ϑ - speed of movement of particles; l is the distance between the electrodes along the solution; E - electromotive force, D - dielectric constant.

Factors affecting ξ - potential:

1. The presence in the solution of an indifferent electrolyte - an electrolyte that does not contain a potential-determining ion.

• The indifferent electrolyte contains a counterion. In this case, the diffusion layer is compressed and ξ falls and, as a consequence, coagulation occurs.
• An indifferent electrolyte contains an ion of the same sign as a counterion, but not the counterion itself. In this case, it happens ion exchange: the counterion is replaced by ions of an indifferent electrolyte. A drop in ξ is observed, but the degree of drop will depend on the nature of the substituent ion, its valence, and the degree of hydration. Lyotropic series of cations and anions - series in which ions are arranged according to their ability to compress the diffuse layer and cause a drop in the ξ potential.

Li + - Na + - NH 4 + - K + - Rb + - Cs + - Mg 2+ - Ca 2+ - Ba 2+ ...

CH 3 COO - - F - - NO 3 - - Cl - - I - - Br - - SCN - - OH - - SO 4 2 -

2. Adding solution stabilizer electrolyte- an electrolyte containing a potential-determined ion causes an increase in ξ - potential, and therefore contributes to the stability of the colloidal system, but up to a certain limit.

Stability and coagulation of colloidal systems

The modern theory of stability and coagulation of colloidal systems was created by several well-known scientists: Deryagina, Landau, Verwey, Overbeck, and therefore it is abbreviated as DLVO theory . According to this theory, the stability of a disperse system is determined by the balance of attractive and repulsive forces that arise between particles as they approach each other as a result of Brownian motion. There are kinetic and aggregate stability of colloidal systems.

1. Kinetic (sedimentation) stability- the ability of dispersed particles to be in suspension and not settle (not sediment). In dispersed systems, as in natural solutions, there is Brownian motion. Brownian motion depends on particle size, dispersion medium viscosity, temperature, etc. Finely dispersed systems (sols), whose particles practically do not settle under the action of gravity, are kinetically (sedimentation) stable. They also include hydrophilic sols - solutions of polymers, proteins, etc. Hydrophobic sols, coarse systems (suspensions, emulsions) are kinetically unstable. In them, the separation of the phase and the medium takes place quite quickly.
2. Aggregate stability- the ability of the particles of the dispersed phase to keep a certain degree of dispersion unchanged. In aggregate-stable systems, particles of the dispersed phase do not stick together during collisions and do not form aggregates. But if the aggregate stability is violated, the colloidal particles form large aggregates, followed by the precipitation of the dispersed phase. Such a process is called coagulation, and it proceeds spontaneously, since the free energy of the system decreases (Δ G<0) .

Factors that affect the stability of colloidal systems include:

1. The presence of an electric charge of dispersed particles. Dispersed particles of lyophobic sols have the same charge, and therefore, upon collision, they will repel each other the stronger, the higher the zeta potential. However, the electrical factor is not always decisive.
2. The ability to solvate (hydrate) stabilizing ions. The more hydrated (solvated) counterions in the diffuse layer, the larger the total hydrated (solvate) shell around the granules and the more stable the dispersed system.

According to the theory, during Brownian motion, colloidal particles freely approach each other at a distance of up to 10 -5 see. The nature of the change in the van der Waals forces of attraction (1) and electrostatic repulsive forces (2) between colloidal particles is shown in fig. 10.6. The resulting curve (3) was obtained by geometric addition of the corresponding ordinates. At minimum and large distances, the attraction energy prevails between the particles (I and II energy minima). In the second energy minimum, the particle cohesion energy is insufficient to keep them in an aggregated state. At medium distances corresponding to the thickness of the electrical double layer, the repulsive energy prevails with a potential barrier AB preventing particles from sticking together. Practice shows that at a zeta potential ξ = 70 mV, colloidal systems are characterized by a high potential barrier and high aggregative stability. To destabilize the colloidal system, i.e. implementation of the coagulation process, it is necessary to reduce -potential up to values ​​0 - 3 mV.

Figure 10.6. Potential curves of interaction of colloidal particles

Coagulation of dispersed systems

Coagulation - the process of adhesion of colloidal particles. This process proceeds relatively easily under the influence of a variety of factors: the introduction of electrolytes, non-electrolytes, freezing, boiling, mixing, exposure to sunlight, etc.. In the process electrolytic coagulation (under the influence of electrolytes) ion-exchange adsorption is often observed: coagulant ions with a higher valency or a higher adsorption potential displace the counterions first of the diffuse layer and then of the adsorption layer. The exchange takes place in an equivalent amount, but the replacement of counterions leads to the fact that, at a sufficient concentration of electrolytes in a dispersed medium, the particles lose their stability and stick together upon collision.

A number of experimental general rules have been established for electrolytic coagulation:

1. Coagulation of lyophobic sols is caused by any electrolytes, but it is observed at a noticeable rate when a certain electrolyte concentration is reached. Coagulation threshold(C to) is the minimum electrolyte concentration required to start coagulation of the sol. In this case, external changes are observed, such as cloudiness of the solution, a change in its color, etc.

• where Sal is the molar concentration of the electrolyte, mmol/l;
• Vel - volume of electrolyte solution, l;
• Vz is the volume of the sol, l.

The reciprocal of the coagulation threshold is called the coagulating ability () of the electrolyte:

where Ck is the coagulation threshold.

2. Schultz–Hurdy rule:

• the coagulating effect is exhibited by that ion, the charge of which is opposite in sign to the charge of the surface of colloidal particles (the charge of the granule), and this effect increases with increasing valency of the ion;
• the coagulating effect of ions increases many times with an increase in the valency of the ions. For one - two and trivalent ions, the coagulating effect is roughly related as 1: 50: 500.

This is explained by the fact that multivalent highly charged ions of coagulants are much stronger attracted by the charged surface of a colloidal particle than monovalent ones, and much easier to displace counterions from the diffuse and even adsorption layer.

3. The coagulating effect of organic ions is much higher than that of inorganic ones. This is due to their high adsorption capacity, the ability to be adsorbed in a superequivalent amount, and also to cause recharging of the surface of colloidal particles.

4. In a number of inorganic ions with the same charges, the coagulating ability depends on the radius of the ion - coagulant: the larger the radius, the greater the coagulating ability (see. lyotropic series). This is explained by the fact that the degree of ion hydration decreases, for example, from L + to Cs + , and this facilitates its incorporation into the double ionic layer.

5. Electrically neutral particles of lyophobic colloidal sols coagulate with the highest speed.

6. The phenomenon of sol addiction. If a coagulant is quickly added to the sol, then coagulation occurs, if slowly, there is no coagulation. This can be explained by the fact that a reaction occurs between the electrolyte and the sol, as a result of which peptizers are formed that stabilize the disperse system:

Fe (OH) 3 + HCl → FeOCl + 2H 2 O,

FeOCl → FeO + + Cl - ,

where FeO + is a peptizer for the Fe (OH) 3 sol.

The coagulating effect of a mixture of electrolytes manifests itself differently depending on the nature of the ion - coagulator. In a mixture of electrolytes, the action can be added to the coagulating action of each electrolyte. This phenomenon is called additivity ions (NaCl, KCl). If the coagulating effect of electrolyte ions decreases with the introduction of ions of another electrolyte, antagonism of ions (LiCl, MgCl 2 ). In the case when the coagulating effect of electrolyte ions increases with the introduction of ions of another electrolyte, this phenomenon is called synergy ions.

The introduction, for example, of 10 ml of a 10% NaCl solution in 10 ml of Fe (OH) 3 sol leads to coagulation of this sol. But this can be avoided if one of the protective substances is added to the sol solution: 5 ml of gelatin, 15 ml of egg albumin, 20 ml of dextrin.

Protection of colloidal particles

Colloidal protection- increasing the aggregate stability of the sol by introducing a macromolecular compound (HMC) into it. For hydrophobic sols, proteins, carbohydrates, pectins are usually used as IUDs; for non-aqueous sols - rubbers.

The protective effect of the IUD is associated with the formation of a certain adsorption layer on the surface of colloidal particles (Figure 10.7). The reverse of coagulation is called peptization.

Figure 10.7 Mechanism of peptization

To characterize the protective effect of various naval forces, Zsigmondy proposed using the golden number.golden numberis the number of milligrams of IUD to be added to 10 cm 3 0.0006% red gold sol to prevent it from turning blue (coagulation) when 1 cm is added to it 3 10% NaCl solution. Sometimes, instead of gold sol, colloidal solutions of silver (silver number), iron hydroxide (iron number), etc. are used to characterize the protective effect of the IUD.Table 10.2 shows the meaning of these numbers for some IUDs.

Table 10.2 Protective action of the IUD