Obtaining disperse systems by dispersion methods. Preparation, stabilization and purification of disperse systems

Condensation methods are based on the processes of the emergence of a new phase by combining molecules, ions or atoms in a homogeneous medium. These methods can be divided into physical and chemical.

physical condensation. The most important physical methods for obtaining dispersed systems- vapor condensation and solvent replacement. Most good example condensation from vapor is the formation of fog. When the parameters of the system change, in particular, when the temperature decreases, the vapor pressure can become higher than the equilibrium vapor pressure over the liquid (or over the solid) and a new liquid (solid) phase appears in the gas phase. As a result, the system becomes heterogeneous - fog (smoke) begins to form. In this way, for example, camouflage aerosols are obtained, which are formed by cooling the vapors of P2O5, ZnO and other substances. Lyosols are obtained in the process of joint condensation of vapors of substances that form a dispersed phase and a dispersion medium on a cooled surface.

The method of replacing the solvent is widely used, based, like the previous one, on such a change in the parameters of the system, in which the chemical potential of the component in the dispersion medium becomes higher than the equilibrium one and the tendency to transition to an equilibrium state leads to the formation of a new phase. In contrast to the vapor condensation method (temperature change), in the solvent replacement method, the composition of the medium is changed. So, if a saturated molecular solution of sulfur in ethyl alcohol is poured into a large volume of water, then the resulting solution in an alcohol-water mixture is already supersaturated. Supersaturation will lead to the aggregation of sulfur molecules with the formation of particles of a new phase - dispersed.

The solvent replacement method is used to obtain sols of sulfur, phosphorus, arsenic, rosin, cellulose acetate and many organic matter by pouring alcohol or acetone solutions of these substances into water.

chemical condensation. These methods are also based on the condensation separation of a new phase from a supersaturated solution. However, unlike physical methods, the substance that forms the dispersed phase, appears as a result of a chemical reaction. Thus, any chemical reaction proceeding with the formation of a new phase can be a source of obtaining a colloidal system. The following chemical processes are given as examples.

• 1. Recovery. A classic example of this method is the preparation of a gold sol by the reduction of chloroauric acid. Hydrogen peroxide can be used as a reducing agent (Zigmondy method):
• 2HauCl2+3H2O22Au+8HCl+3O2

Other reducing agents are also known: phosphorus (M. Faraday), tannin (W. Oswald), formaldehyde (R. Zhigmondy). For example,

• 2KauO2+3HCHO+K2CO3=2Au+3HCOOK+KHCO3+H2O
• 2. Oxidation. Oxidative reactions are widespread in nature. This is due to the fact that during the rise of magmatic melts and gases separated from them, fluid phases and groundwater, all mobile phases pass from the zone recovery processes at great depths to zones of oxidative reactions near the surface. An illustration of such processes is the formation of a sulfur sol in hydrothermal waters, with oxidizing agents (sulfur dioxide or oxygen):
• 2H2S+O2=2S+2H2O

Another example is the process of oxidation and hydrolysis of iron bicarbonate:

4Fe(HCO3)2+O2+2H2O4Fe(OH)3+8CO2

The resulting iron hydroxide sol imparts a red-brown color to natural waters and is the source of rusty-brown sediment zones in the lower soil layers.

• 3. Hydrolysis. Widespread in nature and importance in technology has the formation of hydrosols in the processes of hydrolysis of salts. Salt hydrolysis processes are used for wastewater treatment (aluminum hydroxide obtained by hydrolysis of aluminum sulfate). The high specific surface of the colloidal hydroxides formed during hydrolysis makes it possible to effectively adsorb impurities - surfactant molecules and heavy metal ions.
• 4. Exchange reactions. This method is the most common in practice. For example, obtaining an arsenic sulfide sol:
• 2H3AsO3+3H2SAs2S3+6H2O,

obtaining silver iodide sol:

AgNO3+KIAgI+KNO3

Interestingly, exchange reactions make it possible to obtain sols in organic solvents. In particular, the reaction

Hg(CN)2+H2SHgS+2HCN

It is carried out by dissolving Hg (CN) 2 in methyl, ethyl or propyl alcohol and passing hydrogen sulfide through the solution.

Well-known reactions in analytical chemistry, such as the production of precipitates of barium sulfate or silver chloride

Na2SO4 + BaCl2 BaSO4 + 2NaCl

AgNO3 + NaCl AgCl + NaNO3

under certain conditions lead to the production of almost transparent, slightly cloudy sols, from which precipitation may subsequently occur.

Thus, for the condensation preparation of sols, it is necessary that the concentration of the substance in the solution exceed the solubility, i.e. the solution must be supersaturated. These conditions are common both for the formation of a fine sol and for a normal solid phase deposit. However, in the first case, compliance special conditions, which, according to the theory developed by Weimarn, consists in the simultaneous appearance of a huge number of nuclei of the dispersed phase. The nucleus should be understood as the minimum accumulation of a new phase, which is in equilibrium with environment. To obtain a highly dispersed system, it is necessary that the rate of nucleation be much higher than the rate of crystal growth. In practice, this is achieved by pouring a concentrated solution of one component into a very dilute solution of the other with vigorous stirring.

Sols are formed more easily if special compounds, called protective substances, or stabilizers, are introduced into solutions in the process of their preparation. Soaps, proteins and other compounds are used as protective substances in the preparation of hydrosols. Stabilizers are also used in the preparation of organosols.

Sol is a dispersed system with a solid-particle dispersed phase. An aerosol corresponds to a gaseous dispersion medium, and a lyosol (hydrosol) corresponds to a liquid dispersion medium.

The dispersion of liquids is usually called atomization when it occurs in the gas phase, and emulsification when it is carried out in another liquid that is immiscible with the first.

Dispersion- fine grinding of solids or liquids, resulting in powders, suspensions, emulsions ( emulsification, or emulsification). When solids are dispersed, their mechanical destruction occurs.

Dispersion methods

– mechanical dispersion- carried out under the influence of external mechanical work. Methods: abrasion, crushing, splitting, spraying, bubbling (passing a jet of air through a liquid), shaking, explosion, the action of sound and ultrasonic waves. This method produces flour, powdered sugar, cocoa powder, spices, ground coffee and others. Particle size obtained by this method, k.p. quite large, at least 100 nm. Equipment: mortars, mills, crushers of various types, millstones.

To improve efficiency, mechanical dispersion is carried out in a liquid medium. Liquids (solutions of surfactants, electrolytes) that wet a solid are adsorbed on it and reduce strength during machining. This is called adsorption strength reduction of solids or rebinder effect(justified in 1982 by P.A. Rebinder).

– electrical dispersion– is based on the formation of a voltaic arc between the electrodes of the sprayed metal, placed in a cooled DC. Metals evaporate at the temperature of a voltaic arc, and then condense in a cold DC. Metal hydrosols (the dispersion medium is water), for example, silver, gold and platinum, are mainly obtained by this method.

– ultrasonic dispersion- based on exposure to ultrasonic vibrations with a frequency above 20 thousand per 1 s, which are not caught by the human ear, it is effective only for substances with low strength. These include sulfur, graphite, starch, rubber, gelatin, etc.

to the physicochemicaldispersion applies method peptization. It consists in the transfer of freshly prepared loose sediments into a colloidal solution under the action of special stabilizing additives (peptizers - electrolytes, surfactant solutions). The action of the peptizer is that the sediment particles are separated from each other and transferred to a suspended state, forming a sol. This method can be used to obtain, for example, hydrosol of iron hydroxide (III). The peptization method can only be used for freshly prepared sediments, since recrystallization and aging processes occur during storage, leading to particles coalescing with each other. Particle sizes obtained by this method are about 1 nm.

DISPERSION

Dispersion can be spontaneous and non-spontaneous. Spontaneous dispersion is characteristic of lyophilic systems. With regard to lyophobic systems, spontaneous dispersion is excluded, dispersion in them is possible by spending a certain amount of work.

Dispersion is characterized degree of dispersion ( a ) . It is determined by the ratio of the sizes of the initial product and the particles of the dispersed phase of the resulting system.

a = d n/ d k,(7.1)

d n , d k is the particle diameter before and after grinding.

Job W, necessary for the dispersion of a solid or liquid, is spent on deforming the body W e and on the formation of a new interface W a, which is measured by the work of adhesion. Deformation is a necessary prerequisite for the destruction of the body. The work of dispersion is determined by the formula:

W = W A + W d = * DB + kV (7.2)

* - a value proportional to or equal to the surface tension at the interface,

DB- increase in the phase interface as a result of dispersion,

V- the volume of the original body before dispersion,

k- coefficient equivalent to the work of deformation of a unit volume of the body.

With the help of colloid chemistry methods, it is possible to reduce the energy required for dispersion. Adsorption strength reduction is one of such methods. As a result of the adsorption of surfactants on the outer and inner surfaces of the solid body, the interfacial surface tension decreases, and the deformation of the solid body is facilitated.

Reducing the energy of dispersion can be achieved by the following methods: carrying out the process in a liquid medium, grinding with simultaneous vibration, the use of an ultrasonic method.

OBTAINING DISPERSIVE SYSTEMS DUE TO CONDENSATION PROCESSES

Condensation methods : condensation, desublimation, crystallization. They are based on the formation of a new phase under conditions of a supersaturated state of matter in a gaseous or liquid medium. In this case, the system changes from homogeneous to heterogeneous. Condensation and desublimation are characteristic of a gas medium, and crystallization is characteristic of a liquid medium.

Necessary condition for condensation and crystallization - supersaturation and uneven distribution of matter in the dispersion medium and the formation of condensation centers (embryos).

Degree of supersaturation b for solution and vapor can be expressed as follows:

b f = s/s s, b n = r/r s (7.3)

p, c - supersaturated vapor pressure and concentration of a substance in a supersaturated solution, p s- equilibrium pressure of saturated vapor over a flat surface, s s- equilibrium solubility corresponding to the formation of a new phase.

Condensation is facilitated by the smallest particles. For example, combustion products of aircraft fuel, soil particles, etc. can serve as condensation nuclei for water vapor.

When there are no condensation nuclei, droplets can exist in a supercooled state. When vapors condense under these conditions, not drops, but crystals will form. The process by which a gaseous substance changes to a solid state without going through the liquid state is called desublimation.

Sublimation -the transition of a solid to a gas without going through a liquid.

The condensation methods are based on spontaneous processes, which are accompanied by a decrease in the Gibbs energy.

During the nucleation and formation of particles from a supersaturated solution or gas phase, the chemical potential changesm, an interface appears, which becomes the carrier of excess free surface energy.

Condensation can be physical or chemical.

physical condensation - is carried out with a decrease in the temperature of the gaseous medium containing vapors of various substances.

Isothermal distillation : reduction in the size of small particles until their complete disappearance and the growth of large particles.

MEMBRANES AND MEMBRANE PROCESSES

membranes- semi-permeable partitions, with the help of which osmosis is carried out. Osmosis- spontaneous process of transfer of a solvent (dispersion medium) through a membrane from a solvent (less concentrated or colloidal solution) into a solution (or into a more concentrated solution).

Membranes are thin porous films; they are two-dimensional highly dispersed systems.

Most often, membranes are used to purify liquids from impurities using reverse osmosis (the movement of dissolved impurities through the membrane under the action of external pressure)

Fig. 7.2. Scheme of osmosis (a), reverse osmosis (b), ultrafiltration and dialysis (c)

1.dispersion medium (pure liquid) L, 2.colloidal S/L or true solution, 3.membrane, 4.pure liquid (solvent) flow, 5.impurity flow.

If pressure P is applied from the side of the dispersed system 2, then the liquid flow from region 2 will move to region 1. Only solvent molecules pass through membrane 3 (due to their greater mobility). The contents of area 1 will be enriched with pure liquid, and impurities will concentrate in area 2.

The direction of movement of the liquid in reverse osmosis is opposite to its movement in the case of osmosis.

The work required to implement reverse osmosis is expended on forcing liquid through the pores:

W oc = D R V (7.4)

Dp is the pressure drop on both sides of the membrane,

Vis the volume of fluid that has passed through the membranes.

D p = P - p (7.5)

P - excess pressure over the solution,

p- osmotic pressure.

Equation 7.5 implies that Р> p. This condition determines the overpressure required to carry out reverse osmosis.

With help dialysis(7.2, c) the disperse system is cleaned from impurities in the form of ions or molecules. The dispersed system is placed in the right part 2 of the vessel, separated from the left part 1 by a membrane 3. The membrane is permeable to molecules and ions, but retains particles of the dispersed phase. Impurities as a result of diffusion from the region of higher concentration 2 will spontaneously move to the region of lower concentration 1.

The purification of a colloidal solution by means of dialysis can be intensified by applying an external pressure p (7.2, c). In this case, the process is called ultrafiltration.

Reverse osmosis, dialysis, ultrafiltration are used for various purposes, but they have much in common, similar equipment is used.

The basic principle of membrane operation is selective permeability, which is determined by the pore size, the properties of the systems to be cleaned, and the external pressure.

In addition to cleaning solutions, membranes contribute to the equilibrium of electrolytes in the presence of particles or ions, the size of which does not allow them to penetrate through the pores, the so-called membrane equilibrium, which is of practical importance for IUD solutions, in the processes of swelling of substances and in various physiological processes.

Membrane technology is much more efficient than other similar technologies and requires less energy costs.

CAUSE OF MOLECULAR-KINETIC PROPERTIES

All molecular-kinetic properties are caused by the chaotic thermal motion of the molecules of the dispersion medium, which consists of the translational, rotational and vibrational motions of the molecules.

Molecules have different kinetic energies. However, at a given temperature, the average value of the kinetic energy of molecules remains constant. The fluctuation of the values ​​of the kinetic energy of the molecules of the dispersion medium is the cause of the molecular kinetic properties.

Molecular-kinetic properties are manifested in liquid and gaseous dispersion medium.

BROWNIAN MOTION

The smallest particles of insignificant mass experience unequal impacts from the molecules of the dispersion medium, the figure shows the resulting force F, which makes the particles move.

Fig.7.3. Impact of the molecules of the dispersion medium on the particle of the dispersed phase.

The direction and momentum of this force is constantly changing, so the particles move randomly.

Einstein and Smoluchowski managed to determine the direction of the resulting force and connect it with the molecular-kinetic properties of the medium in 1907 independently of each other.

Their calculations were based not on the true path of particles, but on the shift of particles (Fig. 7.4).

The path of the particle is determined by the polyline, and the shift X characterizes the change in the particle coordinate over a certain period of time. The mean shift will determine the rms shift of the particle:

(7.6)

x 1, x 2, x i- particle shifts for a certain time.

The theory of Brownian motion comes from the concept of the interaction of a random force f( t) , which characterizes the impacts of molecules and, and forces Ft, depending on time and friction force during the motion of particles of the dispersed phase in the dispersion medium at a speed v. Brownian motion equation(Langevin equation) has the form:

m(dv/ dt) + hv = Ft + f( t) (7.7)

Where m is the mass of the particle,his the coefficient of friction during particle motion.

For large intervals of time, the inertia of the particles, that is, the term m(dv/ dt) can be neglected. After integration 7.7. provided that the average product of impulses of a random force is equal to zero, find the average shift:

(7.8)

Where t- time, h- viscosity of the dispersion medium, r is the radius of the particles of the dispersed phase.

Brownian motion is most pronounced in highly dispersed systems. Understanding the causes and developing the theory of Brownian motion is a brilliant proof of the molecular nature of matter.

DIFFUSION

Diffusion- the process of spontaneous spread of a substance from an area with a higher concentration to an area with a lower concentration.

Types of diffusion:

1. molecular;

2. ionic;

3. diffusion of colloidal particles.

Ionic diffusion is associated with the spontaneous movement of ions. The formation of a diffuse layer of counterions on the surface of particles of the dispersed phase occurs according to the mechanism of ion diffusion.

Diffusion of highly dispersive colloidal particles is shown in fig. 7.5.n 1 > n 2 . That is, diffusion goes from bottom to top. Diffusion is characterized by a certain rate of movement of a substance through the cross section B, which is equal to dm/ dt.

On distance Dx the concentration difference will ben 2 - n 1 , is a negative value.

dn/ dx is the concentration gradient.

The speed of movement of matter:

dm = – D·( dn/ dx) · Bdt (7.9)

D- diffusion coefficient.

Equation 7.9 - basic diffusion equation V differential form. It is valid for all types of diffusion. In integral form, it is applicable to two processes: stationary and non-stationary.

For a stationary process, the concentration gradient is constant. Integrating 7.9., we get:

m = – D(dn/ dx) Bt- Fick's first law (7.10)

The physical meaning of the diffusion coefficient : If- dn/ dx= 1, B = 1, t= 1, then m = D, that is, the diffusion coefficient is numerically equal to the mass of the diffusing substance, when the concentration gradient, the cross-sectional area of ​​the diffusion flow, time are equal to one.

Colloidal particles are characterized by a minimum diffusion coefficient.

Diffusion is quantified diffusion coefficient, which is related to the mean shift:

x -,2 = 2 Dr, r= x -,2 /(2 Dt) (7.11)

D= kT/ (6 phr) (7.12)

k= R/ N A .

It can be seen from this formula that the diffusion coefficient also depends on the shape of the particles; thus, knowing the diffusion coefficient, one can determine the particle size of the dispersed phase.

OSMOSIS

When two solutions of different concentrations are separated by a semi-permeable partition, a solvent flow occurs from a lower concentration to a higher one. This process is called osmosis.

1 - a vessel with a solution, 2 - a container with a pure liquid, 3 - a semi-permeable partition (membrane).

Thermodynamic explanation of osmosis:

Chemical potential of a pure liquidm 2 exceeds the chemical potential of the same liquid in solutionm 1 .The process goes spontaneously towards a lower chemical potential until the chemical potentials are equalized.

As a result of the movement of liquid in the tank 1, excess pressure is createdpcalled osmotic. The solvent penetrating into region 1 raises the liquid level to a height H, which compensates for the pressure of the pure solvent.

Osmotic pressure - excess pressure over the solution, which is necessary to prevent the transfer of the solvent through the membrane.

The osmotic pressure is equal to the pressure that the dispersed phase would produce if it, in the form of a gas at the same temperature, occupied the same volume as the colloidal system (solution). Osmotic pressure arises spontaneously as a consequence of the molecular-kinetic properties of the dispersion medium.

Osmotic pressure for ideal solutions of non-electrolytes:

pV = RTln(1 – x) (7.13)

Vis the molar volume of the solvent, x is the mole fraction of the solute.

In the case of dilute solutions of non-electrolytes:

pV = nRT (7.14)

Where n is the number of moles of the solute.

If the mass of the solute = q, mass \u003d M, then n = q/M, then:

p = n(RT/V) = (q/V)(RT/V)(7.15)

M= mN A, m = 4/3 pr 3 r (7.16)

r- particle density, m- molecular weight of particles of the dispersed phase, r is the radius of the particles of the dispersed phase.

Then:

(7.17)

It follows from this formula that the osmotic pressure is directly proportional to the concentration of the dispersed phase and inversely proportional to the size of these particles.

The osmotic pressure of colloidal solutions is insignificant.

SEDIMENTATION

Sedimentation- settling of particles of the dispersed phase, reverse sedimentation - floating of particles.

Each particle in the system is affected by gravity and the lifting force of Archimedes:

F g = mg= vgr And F A = vgr 0 (7.18)

Where r, r 0 - density of particles of the dispersed phase and dispersion medium, m is the mass of the particle, v is the volume of the particle, g- acceleration of gravity.

These forces are constant and directed in different directions. The resultant force causing sedimentation is:

F sed = F g -F A = v( r - r 0 ) g (7.19)

If r> r 0 , then the particle settles, if vice versa, then it floats.

During the laminar motion of a particle, resistance arises - the friction force:

F tr = B u (7.20)

B - coefficient of friction, u is the speed of the particle.

Force acting on a particle during motion:

F = F sed - F tr = vg(r - r 0 ) – B u (7.21)

With an increase in speed at a sufficiently large coefficient of friction, there comes a moment when the friction force reaches the force that causes sedimentation and the driving force will be equal to zero. After that, the speed of the particle becomes constant:

u = vg(r (7.23)

Knowing the quantities included in the equation, one can easily find the radius of the particles of the dispersed phase.

The ability to sediment is expressed through sedimentation constant:

S sed = u/g (7.24)

The phenomenon of sedimentation is widely used in various industries, including often used to analyze disperse systems.

Two methods for obtaining dispersed systems - dispersion and condensation

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

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

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

Condensation occurs during the formation of fog, during crystallization.

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

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

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

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

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

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

Purification of disperse systems.

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

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

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

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

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

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

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

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

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

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

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

Colloidal systems, in terms of the degree of dispersion, occupy an intermediate position between true solutions (molecular or ion-dispersed systems) and coarsely dispersed systems. Therefore, there are two groups of methods for obtaining disperse systems: group 1 - dispersion, i.e. pulverization of particles of the dispersed phase of coarse systems, group 2 is based on aggregation (condensation) processes, in which molecules under the action of cohesive forces unite and first give rise to a new phase embryo, and then real particles of a new phase

Another necessary condition for obtaining sols, in addition to bringing the particle size to colloidal, is the presence in the system of stabilizers - substances that prevent the process of spontaneous enlargement of colloidal particles.

Dispersion methods

Dispersion methods are based on the crushing of solids to particles of colloidal size and thus the formation of colloidal solutions. The dispersion process is carried out various methods: mechanical grinding of the substance in colloid mills, electric arc spraying of metals, crushing of the substance with the help of ultrasound.

Condensation methods

A substance in a molecularly dispersed state can be converted into a colloidal state by replacing one solvent with another - those. solvent replacement method. An example is the preparation of a rosin sol, which is insoluble in water, but highly soluble in ethanol. With the gradual addition of an alcohol solution of rosin to water, a sharp decrease in the solubility of rosin occurs, resulting in the formation of a colloidal solution of rosin in water. Sulfur hydrosol can be obtained in a similar manner.

Colloidal solutions can also be obtained by the method chemical condensation, based on carrying out chemical reactions accompanied by the formation of insoluble or poorly soluble substances. For this purpose, various types of reactions are used - decomposition, hydrolysis, redox, etc. So, a red gold sol is obtained by reducing the sodium salt of gold acid with formaldehyde:

NaAuO 2 + HCOH + Na 2 CO 3 ––> Au + HCOONa + H 2 O

End of work -

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For the convenience of comparing thermal effects, as well as other thermodynamic functions, the concept of the standard state of matter is introduced. For solids and liquids as standard with

The first corollary of Hess' law
This consequence is associated with the heats of formation of compounds. The heat (enthalpy) of formation of a compound is the amount of heat released or absorbed during the formation of 1 mol

The second corollary of Hess' law
In some cases, it is more convenient to calculate the thermal effect of a reaction from the heats (enthalpies) of combustion of the substances involved in the reaction. The heat (enthalpy) of combustion of a compound is called those

Kirchhoff equation. The dependence of the thermal effect of the reaction on temperature
Differentiating with respect to temperature (at constant pressure) the equality DH = H2 − H1 we obtain ¶(

The concept of entropy. Statistical thermodynamics and the physical meaning of entropy
All processes occurring in nature can be divided into spontaneous and non-spontaneous. Spontaneous processes proceed without the expenditure of energy from the outside; for pro

Entropy Change as a Criterion for the Spontaneous Flow of a Process in an Isolated System
Spontaneous processes proceed without the expenditure of energy from the outside. The spontaneous course of the process is associated with irreversibility. irreversible in thermodynes

Planck's postulate. (Third law of thermodynamics)
Unlike internal energy and enthalpy, entropy can be given absolute values. This possibility appears when using Planck's postulate, which

Thermodynamic potentials
The mathematical apparatus of thermodynamics is based on the combined equation of the first and second laws of thermodynamics for reversible processes: dU = T d

Gibbs energy change in chemical reactions
DG calculation for chemical processes can be done in two ways. In the first method, relation (27) is used: DG = D

Chemical potential
Consider systems in which the quantities of substances change. These changes can occur as a result of chemical reactions or phase transitions. At the same time, they change

Gibbs phase rule
Component - a chemically homogeneous substance contained in the system that can be isolated from the system and can exist in an isolated form for a long time.

One-component systems
At kn = 1, the equation of the phase rule will take the form: С = 3 - Ф, If there is 1 phase in equilibrium, then С = 2,

Phase diagram of water
The phase diagram of water in the coordinates p - T is shown in Fig.8. It is composed of 3 phase fields - areas of different (p, T)-values, for which

Sulfur phase diagram
Crystalline sulfur exists in the form of two modifications - rhombic (Sp) and monoclinic (Sm). Therefore, it is possible that there

Clausius–Clapeyron equation
Movement along the lines of two-phase equilibrium on the phase diagram (C=1) means a coordinated change in pressure and temperature, i.e. p = f(T). General form such a function for one-component

Entropy of evaporation
The molar entropy of evaporation DSsp = DHisp/Tboil is equal to the difference Svapor - Sliquid. Since Sp

Chemical equilibrium
Thermodynamic equilibrium is such a state of the system, the characteristics of which (temperature, pressure, volume, concentration) do not change in time at constant

Mass action law. Equilibrium constants
The quantitative characteristic of chemical equilibrium is the equilibrium constant, which can be expressed in terms of the equilibrium concentrations of Ci,

Isobar and isochore of a chemical reaction
To obtain the dependence of the equilibrium constant Kp on temperature, we use the Gibbs-Helmholtz equation:

Thermodynamics of solutions
The existence of absolutely pure substances is impossible - any substance necessarily contains impurities, or, in other words, any homogeneous system is multicomponent. The solution is a homogeneous system

Formation of solutions. Solubility
The concentration of a component in a solution can vary from zero to some maximum value, called the solubility of the component. Solubility is the concentration of a component in a saturated

Solubility of gases in liquids
The solubility of gases in liquids depends on a number of factors: the nature of the gas and liquid, pressure, temperature, concentration of substances dissolved in the liquid (especially

Mutual solubility of liquids
Depending on their nature, liquids can be mixed in any ratio (in this case they speak of unlimited mutual solubility), they can be practically indefinite.

Solubility of solids in liquids
The solubility of solids in liquids is determined by the nature of the substances and, as a rule, depends significantly on temperature; information about the solubility of target solids

Relationship between the composition of liquid solution and vapor. Laws of Konovalov
The relative content of the components in the vapor, as a rule, differs from their content in the solution - the vapor is relatively richer in the component whose boiling point is lower. This fact

Saturated vapor pressure of dilute solutions. Raoult's law
Imagine that a certain substance B is introduced into the equilibrium system liquid A - vapor A. When a solution is formed, the mole fraction of the solvent XA becomes

Deviations from Raoult's law
If both components of a binary (consisting of two components) solution are volatile, then the vapor above the solution will contain both components. Consider a binary solution, cos

Crystallization temperature of dilute solutions
A solution, unlike a pure liquid, does not completely solidify at a constant temperature. At a certain temperature, called the temperature of the onset of crystallization

Boiling point of dilute solutions
The boiling point of solutions of a non-volatile substance is always higher than the boiling point of a pure solvent at the same pressure. Let us consider a p-T diagram with

The concept of solute activity
If the concentration of the solute does not exceed 0.1 mol/l, then the non-electrolyte solution is usually considered diluted. In such solutions, the interaction between molecules

Colligative properties of solutions
Some properties of solutions depend only on the concentration of dissolved particles and do not depend on their nature. Such properties of the solution are called colligative. At the same time, even

Theory of electrolytic dissociation. Degree of dissociation
Electrolytes are substances whose melts or solutions conduct electric current due to dissociation into ions. To explain the features of the properties of electrolyte solutions, S. Arrhenius proposed

Weak electrolytes. Dissociation constant
The process of dissociation of weak electrolytes is reversible. A dynamic equilibrium is established in the system, which can be quantified by the constant pa

Strong electrolytes
Strong electrolytes in solutions of any concentration completely dissociate into ions and, therefore, the laws obtained for weak electrolytes cannot be applied to strong electrolytes.

Electrical conductivity of electrolyte solutions
An electric current is an ordered movement of charged particles. Electrolyte solutions have ionic conductivity due to the movement of ions in an electric

Electric potentials at phase boundaries
When a metal electrode (conductor with electronic conductivity) comes into contact with a polar solvent (water) or an electrolyte solution at the electrode-liquid interface, a double

Galvanic cell. EMF of a galvanic cell
Consider the simplest Daniel-Jacobi galvanic cell, consisting of two half-cells - zinc and copper plates, placed in solutions of zinc and copper sulfates, respectively, which are connected

Electrode potential. Nernst equation
EMF of a galvanic cell E is conveniently represented as a difference of some quantities characterizing each of the electrodes - electrode potentials; O

Reference electrodes
To determine the electrode potential, it is necessary to measure the EMF of a galvanic cell, composed of the electrode under test and an electrode with a precisely known potential

Indicator electrodes
Electrodes that are reversible with respect to the hydrogen ion are used in practice to determine the activity of these ions in solution (and hence the pH of the solution) with a potentiome

Redox electrodes
In contrast to the described electrode processes, in the case of redox electrodes, the processes of receiving and giving off electrons by atoms or ions occur

The rate of a chemical reaction
The basic concept of chemical kinetics is the rate of a chemical reaction. The rate of a chemical reaction is the change in the concentration of reactants per unit time. Mathematic

Basic postulate of chemical kinetics
(the law of mass action in chemical kinetics) Chemical kinetics is based on the basic postulate of chemical kinetics: The rate of a chemical reaction is directly proportional to

Zero order reactions
We substitute expression (71) into equation (74), taking into account the fact that the calculation is based on the initial substance A (which determines the choice of the minus sign):

First order reactions
We substitute expression (71) into equation (75): Integration

Second order reactions
Let us consider the simplest case, when the kinetic equation has the form (76). In this case, taking into account (71), we can write:

CH3COOC2H5 + H2O -–> CH3COOH + C2H5OH
If this reaction is carried out at close concentrations of ethyl acetate and water, then the overall reaction order is two and the kinetic equation has the following form:

Methods for determining the order of the reaction
To determine the partial orders of the reaction, the method of excess concentrations is used. It lies in the fact that the reaction is carried out under conditions when the concentration of one of the reagents is much

Parallel Reactions
The starting materials can simultaneously form various reaction products, for example, two or more isomers:

chain reactions
These reactions consist of a series of interrelated steps, where the particles produced by each step generate subsequent steps. As a rule, chain reactions proceed with the participation of free

Van't Hoff and Arrhenius equations
The reaction rate constant k in equation (72) is a function of temperature; an increase in temperature generally increases the rate constant. The first attempt to take into account the effect of temperature was

Photochemical reactions
Overcoming the activation barrier during the interaction of molecules can be carried out by supplying energy to the system in the form of light quanta. Reactions in which particle activation

Catalysis
The rate of a chemical reaction at a given temperature is determined by the rate of formation of the activated complex, which, in turn, depends on the energy

Michaelis equation
Enzymatic catalysis - catalytic reactions occurring with the participation of enzymes - biological catalysts of protein nature. Enzymatic catalysis has two characteristic features

Molecular and kinetic properties of dispersed systems
Broken particles are characterized by Brownian motion. It is the more intense, the smaller the particle diameter and the lower the viscosity of the medium. With a particle diameter of 3–4 µm, Brownian motion is

Optical properties of colloidal systems
Colloidal systems are characterized by a dull (usually bluish) glow, which can be observed against a dark background when a beam of light is passed through them. This glow on

Adsorption. Gibbs equation
Adsorption is the phenomenon of spontaneous thickening in the surface layer of a mass of a substance that lowers the surface tension by its presence. Adsorption value (G, mol/m

Adsorption at the solid-gas interface
When gases are adsorbed on solids, the description of the interaction between the molecules of the adsorbate (substance that is adsorbed) and the adsorbent (substance that adsorbs) is very complex.

Adsorption from solutions
Surfactants (surfactants) Surfactants (surfactants) reduce surface tension. Surfactant molecules adsorbed at the water p

Micellization
Like adsorption, the phenomenon of micelle formation is associated with molecular interactions of its polar molecules (parts of molecules) and hydrophobic linkages of the hydrocarbon chain. Higher

Double electric layer and electrokinetic phenomena
When considering the structure of micelles, it was shown that a double electric layer (EDL) is formed on the surface of colloidal particles. The first theory of the DEL structure was developed by Helmholtz and Perret