Give the definition of coagulation, coagulation threshold and coagulating ability of the electrolyte. The importance of coagulation processes for the life of the body
Violation of the aggregative stability of a colloidal system in the direction of particle enlargement due to their sticking together under the influence of molecular forces of attraction is called coagulation (from lat. thickening, clotting). Coagulation of colloids can be caused by electrolytes, temperature changes, mechanical influences, changes in the composition of the dispersion medium, electric current, etc.
electrolyte coagulation.
Basic rules of coagulation:
All electrolytes can cause coagulation. But only the ion that is opposite to the charge of the granule has a coagulating effect.
Coagulation causes only a certain concentration of electrolyte.
The minimum electrolyte concentration that causes rapid coagulation is called coagulation threshold (PC).
It is usually expressed in millimoles per liter of colloidal solution.
The coagulating ability of the electrolyte is related to the valency of the ions. The coagulating ion must have a charge opposite to that of the particles.
The PC value depends on the charge value of the coagulating ion. The higher its valency, the lower the electrolyte concentration corresponds to the coagulation threshold. This is the Schultz-Hurdy rule. Exists general pattern: with an increase in the valency of the ion, the concentration of the added coagulating electrolyte decreases, and the ratio of coagulation thresholds for one-, two-, and three-valent ions corresponds to the ratio of numbers: hundreds, tens and ones.
According to the Schulze-Harde rule, the coagulation threshold (γ s) is measured inversely with the 6-degree of ion valence (in the limiting case)
γ c \u003d k / Z 6, k is the coefficient.
That is, the values of coagulation thresholds for one-, two-, and trivalent ions are as 1: (1/2) 6: (1/3) 6 = 1: 1/64: 1/729 = 729: 11: 1.
The coagulation threshold depends on the nature of the electrolyte and on the valency of the coagulating ion. The coagulation threshold is calculated by the formula:
γ = СV(electrolyte)/(V(sol) + V(electrolyte)) (mmol/l),
where C is the electrolyte concentration, mmol/l; V is the minimum volume of the electrolyte solution, upon addition of which the coagulation of the sol began, ml.
The coagulating ability of ions of the same charge increases with increasing ion radius.
The coagulating effect of the electrolyte is not limited to the compression of the diffuse layer. At the same time, selective adsorption on the colloidal particle of those electrolyte ions that have a charge opposite to the granule proceeds.
The mechanism of coagulation by electrolytes
Withdrawal in progress electric charge, i.e. bringing the colloidal particle into an isoelectric state (ζ = 0) and reducing the hydration shell of the colloidal particle.
In this case, the diffuse layer is compressed by the transition of counterions from the diffuse layer to the adsorption layer. In this case, the zeta potential decreases; at ζ = 0, all colloidal particles sediment. In addition, there is selective adsorption on the colloidal particle of those electrolyte ions that have a charge opposite to the granule.
SUSPENSIONS- these are suspensions of powders in liquids (soils and soils, clay dough, cement and lime mortars, oil paints). Diluted suspensions are used for dyeing fabrics, concentrated suspensions are used in construction. The particle sizes of suspensions are larger than the number of particles. Suspensions simultaneously absorb and scatter light, are sedimentationally unstable, do not exhibit osmotic pressure and Brownian motion, particles are visible in a conventional microscope, and are not capable of diffusion. As a rule, a DES or a solvation shell is formed on the surface of the particles. The -potential of particles has the same value as for typical sols. Under the influence of electrolytes, suspensions coagulate.
Aggregatively stable suspensions are plastic. Aggregatively unstable systems are fragile. Aggregatively unstable systems can be made aggregatively stable by introducing surfactants (for example, Al 2 O 3 in benzene, soot in water).
Emulsions - these are dispersed systems in which both the dispersed phase and the dispersion medium are liquid. The degree of dispersion of ordinary emulsions is not very large: the radius of their particles is about 10 -3 - 10 -5 cm. Ordinary emulsions are microheterogeneous systems consisting of two immiscible liquids, one of which is dispersed in the other in the form of very small drops. Usually one of the phases of the emulsion is water. The other phase can be any organic liquid that is immiscible with water: oil, benzene, gasoline, kerosene, etc. This other liquid is called, regardless of its chemical nature, oil. In addition to water and oil, any stable emulsion necessarily contains a third component, which gives it aggregative stability, which is called emulsifier. Water and oil form two types of emulsions. The first type: water is a dispersion medium, oil is a dispersed phase, these are oil-in-water (O/W) emulsions - direct emulsions. The second type - water-in-oil emulsions (W / O) - reverse.
Distinguish emulsions diluted and concentrated. Diluted emulsions (concentration of the dispersed phase up to 0.1%) are stabilized by electrolytes that create a double electric layer on the surface of the emulsified droplets. They are stable without special emulsifiers. Concentrated emulsions (concentration of the dispersed phase > 1%) are stable only in the presence of special emulsifiers, which are substances that form a strong film on droplets of an emulsified liquid that does not tear during collisions. These are Naval Forces, gelatin, rubber, resins and semi-colloids - soaps.
The emulsifier is chosen according to the rule: O/W emulsions are stabilized with water-soluble HMCs, such as proteins, and water-soluble hydrophilic soaps, such as sodium oleate. W/O emulsions are stabilized by HMCs soluble in hydrocarbons, for example, polyisobutylene, oleophilic resins and soaps with polyvalent cations (calcium oleate), insoluble in water and soluble in hydrocarbons.
Emulsions with a dispersed phase concentration of more than 74% are called gelatinized. Their physical properties are different from the usual ones. Conventional emulsions are liquids, such as milk, cream; gelatinous - solid, for example, greases, butter, margarine, mayonnaise, thick creams.
One type of emulsion can be converted into another type of emulsion. This phenomenon is called phase reversal in emulsions, it is achieved by changing the nature of the emulsifier.
Emulsions are obtained by mechanical dispersion of the dispersed phase in a dispersion medium in the presence of an appropriate emulsifier. To disperse emulsifiable liquids, strong mixing, shaking, vibrations are used, which is carried out using special mixers, colloid mills, and ultrasound.
Spontaneous emulsification plays a significant role in the processes associated with the digestion and assimilation of food by the animal body. When fat enters, for example, the intestines, fat first self-disperses under the influence of surfactants (cholic acids) contained in bile, and then the resulting highly dispersed emulsion is absorbed through the intestinal wall into the body.
Natural emulsions include milk, egg yolk, latex - the milky juice of rubber plants, from which natural rubber is obtained.
In the food industry, emulsions include not only dairy products, but also margarine, mayonnaise, and sauces. In the pharmaceutical industry, many drugs are used in the form of emulsions. Emulsions of bitumen in water are widely used in the construction and repair of roads. AT agriculture many herbicides, insecticides and fungicides are applied in the form of emulsions.
Penami are dispersed systems in which the dispersed phase is a gas, and the dispersion medium is a liquid extended into thin films. In other words, foams are highly concentrated gas-in-liquid emulsions. Foams are many building and insulating materials (foam concrete, foam plastics, pumice), as well as food products (marshmallow, marshmallow, mousse, etc.). Foams are used in fire extinguishing, flotation processes.
To obtain a stable foam, stabilizers are needed - foaming agents, which can be used as surface-active IUDs, soaps, etc. Foaming agent molecules are adsorbed at the interface in such a way that their hydrophobic part (hydrocarbon radical) is directed towards the gas phase, and the hydrophilic part towards water. The hydrophilic part of the molecule is hydrated by water, forming hydrated layers of a certain thickness, which protect gas bubbles from coalescence.
Foam can be obtained by shaking the foaming agent solution in a cylinder or by passing air through a porous filter placed in the foaming agent solution. The third way is that a jet of solution from a certain height falls on the surface of the same solution in the cylinder.
Foam life time is determined by the time of existence of its certain volume from the moment of occurrence to complete destruction or by the time during which the height of the foam column decreases by half.
Foam ratio- this is the ratio of the initial volume of foam to the volume of the foam concentrate solution used to form this foam.
Soap or IUD stabilized liquid foam can be broken down by adding a surfactant to displace the foaming agent from the surface of the bubbles.
If the liquid films that separate the gas bubbles in the foam are able to harden, then a practically stable hard foam (foam concrete, microporous rubber, etc.) can be obtained.
AEROSOLS. Aerosols play important role in meteorology, in agriculture (sprinkling, pest control), in military affairs (signal and masking smoke). Most of the fuel is burned in pulverized form. It is more efficient to extinguish fires with the help of mist (water curtain).
Aerosols differ from lyosols in the low viscosity of the disperse and the absence of a stabilizing solvate shell or DEL on the surface of disparticles. Aerosols are unstable. Any aerosol breaks down over time. Coarsely dispersed aerosols sediment. High-dispersion aerosols are destroyed due to frequent collisions of particles with each other or with the walls of the vessel (for natural aerosols with obstacles: trees, buildings, etc.). Aerosol particles move not only under the action of mechanical forces, but also under the action of other gradients: electric potential (electrophoresis); temperature (thermophoresis).
The charge on aerosol particles is a random variable and is determined by the capture of gas ions from the atmosphere. The settling of charged particles leads to the appearance of a settling potential in the vertical direction (tens of kV/m in atm). The rate of sedimentation can be enhanced by wind and downdrafts. In this case, the field strength corresponding to the breakdown of air (dielectric) is achieved, i.e. lightning occurs.
Methods for destroying aerosols include filtering through porous materials, bubbling aerosols through a liquid, adsorbing particles with a counter flow of atomized liquid, and depositing artificially charged aerosol particles on electrostatic precipitators.
SEMI-COLLOIDS
IUD solutions are monomolecular lyophilic systems, thermodynamically stable and reversible. To destroy the system, it is necessary to reduce the lyophilicity by reducing the activity of the dismedium, which will lead to a decrease in the activity of the solvent in the solvate layers. This can be achieved by adding desolvating agents such as electrolytes. This phenomenon is called salting out, and there is a struggle for water between macromolecules and ions. As a result of salting out, fibers, flakes, curdled sediments are formed.
IUDs are always characterized by some average mass M. The main methods for determining M are osmometry, diffusion, light scattering, ultracentrifugation, and viscosity measurements. Many swollen or dissolved IUDs are dissociated into ions and are colloidal electrolytes.
Colloidal electrolytes can exist as individual polyions. For example, a protein molecule in solution carries a number of ionic groups -COO - or NH; in the form of a matrix bearing fixed ions of the same sign, balanced by mobile counterions in ion-exchange resins. Micelles formed in surfactant solutions are close to polyelectrolytes.
The ability of disperse systems to maintain a certain degree of dispersion is calledaggregate stability.
Particles of the dispersed phase resist sticking due to different mechanisms. This ability is due, firstly, to the formation of a double electric layer on the surface of the particles of the dispersed phase, which provides electrical stabilization of the disperse system. Secondly, the molecular-adsorption stabilization mechanism works, which consists in the formation of adsorption layers around the particles, consisting of molecules of the dispersed medium and substances dissolved in it . Thirdly, there is a kinetic factor of stability - a low frequency of collisions of dispersed particles.
Sols (colloidal solutions) differ from coarse and molecular systems by their aggregative instability, so they change both in time and with the addition of various substances.
The essence of the mechanism of water purification from suspended colloidal particles is to disturb the equilibrium state of the system - to eliminate the balance of forces that do not allow particles to settle.
To achieve this goal, it uses the process of coagulation of colloidal impurities (simplified - water coagulation).
Coagulation - the process of adhesion of colloids into larger aggregates, which occurs as a result of their collisions during Brownian motion, mixing or directional movement in an external force field, the addition of coagulants. In this case, precipitation occurs - coagulate.
Coagulants (usually soluble iron or aluminum salts) intensify the coagulation process. The introduction of these substances into water contributes to the formation of a new poorly soluble phase (as a result of hydrolysis - the interaction of a substance with water). Thus, the process of coagulation consists in the progressive enlargement of particles and a decrease in their number in the volume of the dispersion medium.
Coagulation is slow and fast. With slow coagulation, only an insignificant part of the collisions of colloidal particles leads them to stick together, and the coagulate does not fall out. With rapid coagulation, each impact is effective and causes the particles to stick together, and a precipitate gradually forms in the colloidal solution.
The minimum concentration of a dosed substance (electrolyte or non-electrolyte) that initiates the coagulation process in a system with a liquid dispersion medium is called the coagulation threshold. Under certain conditions, coagulation is reversible. The process of transition of the coagulate back to the sol is called peptization, and the substances provoking this process are called peptizers. Peptizers, being the stabilizers of dispersed systems, are adsorbed on the surface of the particles, weakening the interaction between them, resulting in the disintegration of aggregates. The return to the primary state is especially effective when surfactants are introduced into the medium, which reduce the surface interfacial energy and facilitate dispersion.
Coagulation with iron salts
Let us consider what processes occur when iron (III) sulfate is added to a colloidal solution. This coagulant in an aqueous solution dissociates into iron ions and sulfate ion:
Fe 2 (SO 4) 3 → 2 Fe 3+ + 3 SO 4 2-
Fe 3+ + H 2 O ↔ Fe (OH) 2+ + H +
Fe(OH) 2+ + H 2 O ↔ Fe(OH) 2 + + H +
Fe(OH) 2 + + H 2 O ↔ Fe(OH) 3 ↓ + H +
Fe 3+ + 3H 2 O ↔ Fe(OH) 3 ↓ + 3H +
Micelle - structural unit lyophobic (weakly interacting with liquid) colloids that do not have a specific composition. Schematically, its structure on the example of an iron (III) hydroxide micelle can be depicted by the diagram:
(mFe(OH) 3 2nFe(OH) 2+ (2n - x) SO 4 2- )2x+ xSO 4 2-
The microcrystal of iron hydroxide, which forms a colloidal particle (see figure), selectively adsorbs from environment ions identical to the ions of its crystal lattice. Depending on the chemical composition solution (excess sulfate - ions or excess of iron ions), the microcrystal acquires a negative or positive charge. Such a charged crystal is called the micelle core, and potential-determining ions inform it of this charge.
The electric field of the charged surface of the crystal attracts from the solution counterions - ions that carry the opposite charge. A double electric layer is formed at the phase boundary, the thickness of which is determined by the outer boundary of the counterion cloud.
The double electric layer consists of adsorption and diffuse parts. The adsorption layer includes potential-forming ions and part of the counterions adsorbed on the surface of the nucleus. The diffuse layer is completed by the remaining counterions in an amount that contributes to the electrical neutrality of the micelles.
The electrical double layer surrounding colloids is rearranged under the influence of coagulants (electrolytes): counterions begin to be displaced from the diffuse to the adsorption part, and the thickness of the entire electrical layer decreases with time to the thickness of the adsorption layer. Dispersed particles fall into the region of mutual attraction, and rapid coagulation occurs.
Coagulation using aluminum salts
Most often, for water purification by coagulation at domestic water treatment stations and in pools, 18-aqueous crystalline aluminum sulfate - Al 2 (SO 4) 3 is used. 18H2O.
The processes that occur when aluminum salts are introduced into water are similar to those described above when iron salts are added:
Al 3+ + H 2 O ↔ Al (OH) 2+ + H +
Al(OH) 2+ + H 2 O ↔ Al(OH) 2 + + H +
Al(OH) 2 + + H 2 O ↔ Al(OH) 3 ↓ + H +
Overall hydrolysis equation:
Al 3+ + 3H 2 O ↔ Al(OH) 3 ↓ + 3H +
The formation of a precipitate of aluminum hydroxide occurs at pH values in the range from 5 to 7.5. At pH< 5 осадок не образуется. При рН >8.5 is the dissolution of the formed aluminum hydroxide with the formation of aluminates.
Al 2 (SO 4) 3 + 6 NaOH \u003d 2 Al (OH) 3↓ + 3 Na 2 SO 4
Al (OH) 3 + NaOH \u003d Na or (NaAlO 2. 2H 2 O)
Modern coagulants
Increasingly common in water treatment and purification processes Wastewater receive coagulants based on aluminum polyoxychloride.
The advantages of these coagulants compared to aluminum sulfate:
Delivery in the form of solutions, which makes their use more convenient (no need to dissolve);
Higher percentage of active substance;
Obtaining purified water of higher quality;
Reducing the volume of secondary waste;
Low residual aluminum content (< 0,2 мг/л);
No pH adjustment required;
Wide operating temperature range.
Technical characteristics of such coagulants produced by AURAT OJSC:
Contact coagulation
One of the options for cleaning by coagulation is contact coagulation. Contact coagulation occurs on the grains of the load of pressure vertical filters of mechanical cleaning. In this case, the introduction of the coagulant is carried out directly before the mechanical filter. The grains of the load and the particles adsorbed on them serve as coagulation centers. The process of flocculation in this case is significantly accelerated.
The higher speed of the coagulation process and the absence of the need for settling tanks for the formation and settling of sludge flakes are the undoubted advantages of contact coagulation.
The disadvantages of contact coagulation include accelerated contamination of pressure filters and the need for frequent regeneration of the load, as well as the risk of reagent leakage if the coagulation / filtration mode is selected incorrectly.
To check whether contact coagulation is carried out or not, water after mechanical filters is checked for coagulant content.
Dear Sirs, if you have a need to implement water purification using coagulants to bring the water quality to certain standards, make a request to the company's specialists Waterman. We will develop for you the optimal technological scheme of water purification.
Coagulation- the process of adhesion of colloidal particles with the formation of larger aggregates due to the loss of aggregative stability by the colloidal solution.
Coagulation threshold- the minimum amount of electrolyte that must be added to the colloidal solution to cause obvious coagulation (visible to the eye) - clouding of the solution or a change in its color.
spk \u003d sel Vel / Vkr + Vel
where With email- initial concentration of electrolyte solution; V email- the volume of the electrolyte solution added to the colloidal solution; V kr is the volume of the colloidal solution.
Coagulating ability - the reciprocal of the coagulation threshold, obeys the Schulze-Hardy rule.
Colloidal protection, its role in life. Peptization, biological role
Colloidal protection- increasing the aggregative stability of lyophobic sols by adding IUDs to them.
The mechanism is that around the micelles of the colloidal solution, adsorption shells are formed from flexible HMS macromolecules, which are amphiphilic and their hydrophobic regions are facing the particles of the dispersed phase, while the hydrophilic fragments are facing water.
In this case, the system is lyophilized, the micelles acquire an additional factor of aggregative stability due to their own hydration shells from IUD macromolecules.
- · good solubility of IUDs in a dispersed medium of a colloidal solution and adsorption of molecules on colloidal particles;
- sufficiently high concentration.
Thus, blood proteins prevent the precipitation and release of poorly soluble cholesterol and calcium salts on the walls of blood vessels, and also prevent the formation of stones in the urinary and biliary tract.
Peptization- process, reverse coagulation, i.e. the transformation of the precipitate formed as a result of coagulation into a stable colloidal solution.
It is carried out in two ways:
- 1. washing the coagulate with a pure solvent (DS);
- 2. adding a special electrolyte peptizer.
Conditions for effective peptization:
- Only freshly obtained sediments are capable of peptization;
- It is necessary to add small amounts of electrolyte-peptizer;
- Peptization is facilitated by stirring and heating.
This process underlies the resorption of atherosclerotic plaques on the walls of blood vessels, kidney and liver stones or blood clots.
Lyophobic colloidal solutions, as thermodynamically unstable systems, can collapse spontaneously or under the influence of external influences. The destruction of colloidal solutions begins with their coagulation
Coagulation- the process of combining colloidal particles with the formation of larger aggregates due to the loss of aggregative stability by the colloidal solution.
As a result of coagulation, the enlarged particles of the dispersed phase easily sediment, and the system is stratified. Thus, the cause of coagulation is the loss of aggregative stability of the colloidal solution, and the consequence of coagulation is a decrease in its sedimentation stability. In practice, coagulation can be caused by various external influences:
Ø adding small amounts of electrolyte,
Ø concentration of the colloidal solution,
Ø temperature change,
Ø ultrasonic action, electromagnetic field and etc.
During coagulation, the aggregative stability of colloidal systems is violated in the direction of particle enlargement (Fig. 28), and the sol is separated into two independent phases (liquid and solid). The phenomenon of coagulation underlies many physiological and pathological processes occurring in living systems: hemostasis (blood clotting in case of damage), coagulation of tissue proteins in case of burns, etc. Coagulation of colloidal solutions of calcium phosphate cholesterol in the blood leads to the formation of sediments and their deposition on the walls of blood vessels (sclerotic changes in blood vessels). In biological systems, coagulation with the addition of small amounts of electrolyte is of greatest practical importance, since cell sols are in contact with electrolytes. However, each electrolyte requires its own minimum concentration, called coagulation threshold electrolyte sol ( With PC). Coagulation threshold is the minimum electrolyte concentration that must be added to 1 liter of the sol in order to cause obvious coagulation (noticeable to the eye) - turbidity of the solution or a change in its color. The coagulation threshold can be calculated using the formula: , where With el is the initial concentration of the electrolyte solution; V el is the volume of the electrolyte solution added to the sol; V sol is the volume of the initial sol. el sol el el pc V V V s s 8 9
The reciprocal of the coagulation threshold is called coagulating action and determined by the formula: The coagulating effect of electrolytes on colloidal solutions with an ionic stabilizer is subject to Schulze-Hurdy rule : & Coagulation of the sols is caused by any ions that have a charge sign opposite to that of the granules. The coagulating ability of ions is the stronger, the higher the charge of the coagulant ion. The coagulating effect of the coagulant ion is directly proportional to its charge to the sixth power: f(z6). If NaCl, CaCl2, AlCl3 solutions are added to a sol having the structure of colloidal particles: (mAgI nI– (n–x)K+)x– xK+, then the coagulating effect of cations will increase sharply: (Na+) : (Ca2+) : (Al3+) = 1: 64: 729. At present, deviations from the Schulze-Hardy rule have been established. In addition to the charge of the coagulant ion, the coagulation threshold is affected by the radius of this ion and the nature of the ion accompanying the coagulant ion. The effect of electrolytes on the coagulation of colloidal solutions must be taken into account when introducing salt solutions into living organisms. It is important to understand that it is not only the concentration of the electrolyte that matters, but also the charge of the ion. Thus, an isotonic 0.9% NaCl solution cannot be replaced by an isotonic MgCl2 solution, since doubly charged ions have a higher coagulating effect than singly charged ones. PC s1
It is necessary to inject electrolyte solutions intravenously or intramuscularly So slow, to avoid coagulation. With rapid administration, due to the slow diffusion rate, local accumulation of the electrolyte can occur, exceeding the threshold concentration, which will lead to coagulation of biosubstrates. With slow administration, the electrolyte is carried away with the blood flow and diffuses into neighboring tissues. The threshold concentration is not reached and coagulation does not occur. In living tissues, this phenomenon is called habituation. In practice, coagulation is often caused by the action mixtures of electrolytes . Additivity is the summation of the coagulating action of the ions that cause coagulation. The additive effect is manifested in cases where electrolytes containing coagulating ions do not interact chemically with each other and act independently of each other. This phenomenon is observed when the coagulant ions have the same charge and a similar degree of hydration. For example, a mixture of KCl and NaNO3 salts exhibits an additive effect with respect to colloidal solutions with both negatively and positively charged granules. In the first case, coagulation is caused by K+ and Na+ cations, in the second case, by Cl– and NO3– anions. Antagonism - this is a weakening of the coagulating effect of one electrolyte in the presence of another. It is observed if the electrolytes in the mixture interact with each other and the coagulating ions bind into insoluble compounds (precipitate) or form a strong complex that does not have a coagulating ability. For example, the coagulating effect of Pb2+ relative to negatively charged granules is weakened in the presence of NaCl, since a precipitate of lead chloride is formed: Pb2+ + 2Cl– PbCl2. Synergy - this is an increase in the coagulating effect of one electrolyte in the presence of another. This is possible if between the electrolytes in the mixture occurs chemical reaction, as a result of which a multiply charged ion is formed, which has a higher coagulating ability. For example, the coagulating effect of FeCl3 and KCNS with respect to positively charged granules of a colloidal solution is significantly enhanced due to the formation of a multiply charged complex anion with a high coagulating ability: Fe3+ + 6CNS– 3–. heterocoagulation - coagulation of colloidal solutions containing heterogeneous particles that differ in chemical nature, sign or magnitude of charge. A special case of heterocoagulation is mutual coagulation. If a sol with positively charged particles is added to a sol with negatively charged particles, then their mutual coagulation will occur. At many water treatment plants, positively charged aluminum or iron hydroxide sols are added to water containing negatively charged organic mixtures. After mutual coagulation, the formed flakes are easily filtered out on sand filters. Spontaneous coagulation of many sols is often slow. It can be accelerated by increasing the speed of the particles, which helps them overcome the disjoining pressure. Acceleration of particle motion can be achieved, for example, by raising the temperature of the solution. An increase in the concentration of the sol can also accelerate its coagulation, since with an increase in the concentration, the number of effective collisions between colloidal particles increases. The coagulation process is very sensitive to the addition of electrolytes. Small amounts of electrolytes can speed it up dramatically. Therefore, on the one hand, electrolytes are necessary for the stabilization of sols, and on the other hand, their excessive addition leads to coagulation of sols. The effect of different electrolytes on this process is not the same. As can be seen, the first portions of the electrolyte do not cause changes in the sol visible to the eye. At the same time, the formation of particles of lower (I, II, III) orders begins, which proceeds imperceptibly to the naked eye, and is therefore called latent coagulation .
A further increase in the electrolyte concentration leads to a progressive development of coagulation, an increase in its rate, and is accompanied by the appearance of particles of higher orders. The sol undergoes visible changes: it becomes cloudy or its color changes. In this case, the value of the ξ-potential of the particles decreases. This stage of the process is called overt coagulation . The transition from covert to overt coagulation is called coagulation threshold . Explicit coagulation, in turn, is divided into two periods: slow coagulation , at which any increase in electrolyte concentration accelerates coagulation and fast coagulation , when a further increase in the concentration of the electrolyte no longer affects its speed, i.e. coagulation proceeds with maximum speed. With slow coagulation, not all collisions of colloidal particles in the ash are effective, and end with the association of particles, and with fast coagulation, all collisions lead to their association. During coagulation, along with a decrease in the number of particles and their enlargement, a number of properties of solutions change: the rate of diffusion and filtration of particles decreases, the rate of sedimentation increases, the intensity of scattered light changes, and at the same time the color of solutions, etc. Mechanism of coagulation Electrolyte coagulation consists in a decrease in the disjoining pressure of a thin liquid layer, which can occur due to: a) a decrease in the charge of the surface of the solid phase and, as a result, a decrease in the interfacial and then electrokinetic potentials; b) a decrease in the thickness of the ionic atmospheres of diffuse layers, and, as a consequence, a decrease in the zeta potential. In this regard, according to the mechanism, two types of coagulation are distinguished:
Neutralization coagulation occurs under the action of an electrolyte, which chemically interacts with potential-determining ions, binding them into a strong compound (for example, into a precipitate) and thereby reducing the charge of the surface of the nucleus. For example, when K2S is added to a colloidal solution of AgI with positively charged granules (potential-determining Ag+ ions), a reaction occurs between the coagulant ions S2– and potential-determining Ag+ ions with the formation of an Ag2S precipitate, which leads to the destruction of the AgI micelle. As a result of binding the potential-determining Ag+ ions, the interfacial potential decreases φ mf and the number of counterions NO3-, necessary to compensate for the charge of the surface of the nucleus. Thus, the ionic atmosphere becomes thinner, the disjoining pressure decreases, which leads to particles sticking together. concentration coagulation occurs under the action of an electrolyte, which does not chemically interact with stabilizer ions and does not change the surface charge of the micelle core. The coagulating effect is exhibited by those electrolyte ions that are counterions for these micelles. Concentration coagulation occurs at a constant value of the interfacial potential φ mf, but is accompanied by a decrease in the electrokinetic zeta potential ( ξ) .
Coagulation processes often occur in nature, for example, at the confluence of rivers into the sea. River water always contains colloidal particles of silt, clay, sand or soil.
When river water is mixed with salty sea water (containing a larger amount of electrolytes), coagulation of these particles begins, and a decrease in the speed of the water flow contributes to their settling at the mouth of the rivers, resulting in the formation of shoals and islands.
Coagulation is widely used in the purification of water entering the water supply network. To do this, aluminum and iron(III) sulfates are added to it, which, being good coagulants, are also hydrolyzed to form metal hydroxide sols. The particles of these sols usually have a charge opposite to the charge of the granules present in water. As a result, mutual coagulation of the sols and their precipitation occurs.
Colloidal solutions are found in wastewater from many industries: for example, stable emulsions of petroleum products, various other organic liquids. They are destroyed by treatment of wastewater with salts of alkaline earth metals.
In the sugar industry, coagulation processes are used in the purification of sugar beet juice. Its composition, in addition to sucrose and water, includes non-sugar substances, often in a colloidal-dispersed state. To remove them, Ca (OH) 2 is added to the juice. His mass fraction it usually does not exceed 2.5%. Impurities in a colloidal state coagulate and settle. To remove excess Ca (OH) 2 from juice, pass through it carbon dioxide. As a result, a precipitate of CaCO 3 is formed, which entrains many soluble impurities from the solution.
coagulation processes play essential role in a living organism, because biological fluids contain colloidal-dispersed particles in contact with dissolved electrolytes. Normally, these systems are usually in a state of equilibrium and coagulation processes do not occur in them. However, this equilibrium can be easily disturbed by introducing an additional amount of electrolyte from outside. Moreover, when introducing it into the body, it is necessary to take into account not only its concentration in the biological fluid, but also the charge of ions. Thus, an isotonic solution of NaCl cannot be replaced by an isotonic solution of MgCl 2, since this salt, unlike NaCl, contains doubly charged Mg 2+ ions, which have a higher coagulating ability than Na + ions.
When introducing a mixture of salts into the bloodstream, you should first make sure that this mixture does not have a synergistic effect, in order to avoid coagulation harmful to the body.
Solving many problems in medicine: prosthetics of blood vessels, heart valves, etc. - depends on the processes of blood coagulation. They can be considered as coagulation of erythrocytes. In surgery, during operations, anticoagulants (heparin, modified dextran, polyglucin) are injected into the blood. After operations and in case of internal bleeding, on the contrary, electrolytes that contribute to the flow of coagulation: caproic acid, protamine sulfate.
For the diagnosis of many diseases in clinical laboratories, the erythrocyte sedimentation rate (ESR) is determined. With various pathologies, for a number of reasons, the coagulation of erythrocytes increases, and the rate of their sedimentation becomes greater than normal.
The formation of bile, urinary and other stones in the body is also associated with an increase in pathological conditions of coagulation of cholesterol, bilirubin, uric acid salts due to a weakening of the natural protective effect. The study of the mechanism of these processes is extremely important for the development of ways to treat these diseases.