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Methods for obtaining dispersed systems include. Obtaining, stabilization and purification of dispersed systems. Membranes and membrane processes

1.2. Methods of obtaining dispersed systems

There are two known methods for obtaining dispersed systems. In one of them, solid and liquid substances are finely ground (dispersed) in an appropriate dispersion medium, in the other they cause the formation of particles of a dispersed phase from individual molecules or ions.

Methods for obtaining dispersed systems by grinding larger particles are called dispersive... Methods based on the formation of particles by crystallization or condensation are called condensation.

Dispersion method

This method combines, first of all, mechanical methods, in which the overcoming of intermolecular forces and the accumulation of free surface energy in the process of dispersion occurs due to external mechanical work on the system. As a result, solids are crushed, abraded, crushed or split.

In laboratory and industrial conditions, the processes under consideration are carried out in crushers, millstones and mills of various designs. The most common are ball mills. These are hollow rotating cylinders, which are loaded with the material to be ground and steel or ceramic balls. As the cylinder rotates, the balls roll, abrading the crushed material. Grinding can also occur as a result of impacts of balls. In ball mills, systems are obtained, the particle sizes of which are in a fairly wide range: from 2-3 to 50-70 microns. A hollow cylinder with balls can be set in a circular oscillatory motion, which contributes to intensive crushing of the loaded material under the action of the complex movement of the crushed bodies. Such a device is called a vibration mill.

A finer dispersion is achieved in colloidal mills of various designs, the principle of which is based on the development of breaking forces in a suspension or emulsion under the action of centrifugal force in a narrow gap between the rotor rotating at high speed and the stationary part of the device - the stator. Suspended coarse particles are subjected to a significant breaking force and are thus dispersed.

High dispersion can be achieved ultrasonic dispersion. The dispersing effect of ultrasound is associated with cavitation - the formation and collapse of a cavity in a liquid. The collapse of the cavities is accompanied by the appearance of cavitation shock waves, which destroy the material. It has been experimentally established that dispersion is in direct proportion to the frequency of ultrasonic vibrations. Ultrasonic dispersion is especially effective if the material has been previously finely ground. Emulsions obtained by the ultrasonic method are distinguished by a uniform particle size of the dispersed phase.

When crushing and grinding, materials are destroyed, first of all, in places of strength defects (macro- and microcracks). Therefore, as grinding increases, the strength of the particles increases, which is usually used to create stronger materials. At the same time, an increase in the strength of materials as they grind leads to a large energy consumption for further dispersion. The destruction of materials can be facilitated by using the Rebinder effect - adsorption reduction in strength solids... This effect consists in the reduction of surface energy with the help of surfactants, as a result of which the deformation and destruction of the solid is facilitated. For hardness reducers, small amounts are characteristic, causing the Rebinder effect and specificity of action. Additives, wetting the material, help the medium to penetrate into the places of defects and, using 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, since, covering the surface of the particles, they thereby prevent their reverse adhesion. This also contributes to the achievement of a highly dispersed state.

The dispersion method usually fails to achieve high dispersion. Disperse systems obtained by dispersion methods are flour, bran, dough, powdered sugar, cocoa (nibs, powder), chocolate, praline, marzipan masses, fruit and berry purees, suspensions, emulsions, foamy masses.

Condensation method

The condensation method is based on the processes of the formation of a heterogeneous phase from a homogeneous system by combining molecules, ions or atoms. Distinguish between chemical and physical condensation.

Chemical condensation based on the release of a poorly soluble substance as a result of a chemical reaction. To obtain a new phase with a colloidal degree of dispersion, an excess of one of the reagents, the use of dilute solutions, and the presence of a stabilizer in the system are required.

During physical condensation, a new phase is formed in a gaseous or liquid medium under conditions of a supersaturated state of matter. Condensation presupposes the formation of a new phase on already existing surfaces (vessel walls, particles of foreign substances - condensation nuclei) or on the surface of nuclei that arise spontaneously as a result of fluctuations in the density and concentration of matter in the system. In the first case, the condensation is called heterogeneous, in the second - homogeneous. As a rule, condensation occurs on the surface of condensation nuclei or nuclei of very small sizes; therefore, the reactivity of the condensed substance is greater than that of the macrophase in accordance with the Kelvin capillary condensation equation. Therefore, in order for the condensed substance not to return to the initial phase and the condensation to continue, the presence of supersaturation in the system is necessary.

1.3. Classification of dispersed systems

Dispersed systems are classified according to the following criteria:

degree of dispersion;

aggregate state of the dispersed phase and dispersion medium;

structural and mechanical properties;

the nature of the interaction of the dispersed phase and the dispersion medium.

Classification by degree of dispersion

Depending on the size of the particles, there are highly dispersed, medium-dispersed and coarse-dispersed systems (Table 1.1).

Table 1.1

	particles, m	Dispersion
Highly dispersed (colloidal systems)			Hydrosols, aerosols
Medium dispersed			Instant coffee, powdered sugar
Coarsely dispersed	More than 10 -5
True solutions	Less than 10 -9

The specific surface area of the dispersed phase particles is maximum in highly dispersed systems, while passing to medium- and coarsely dispersed systems, the specific surface area decreases (Fig. 1.3). When the particle size is less than 10 -9 m, the interface between the particle and the medium disappears, and molecular or ionic solutions (true solutions) are formed.

According to the particle size of the dispersed phase, one and the same product can belong to different dispersed systems. For example, particles wheat flour of the highest grade have a size of (1-30) 10 -6 m, that is, flour of this grade simultaneously belongs to medium-dispersed and coarse-dispersed systems.

Physical state classification

The dispersed phase and the dispersed medium can be in any of three states of aggregation: solid (T), liquid (L) and gaseous (G).

Each dispersed system has its own designation and name: the numerator indicates the state of aggregation of the dispersed phase, in the denominator - the dispersion medium. Eight variants of dispersed systems are possible (Table 1.2), since the H / H system cannot be heterogeneous.

In general, all highly dispersed colloidal systems are called sols... A prefix characterizing the dispersion medium is added to the word sol. If the dispersion medium is solid - xerosols, liquid - lyosols(hydrosols), gas - aerosols.

In addition to simple dispersed systems, there are also complex dispersed systems that consist of three or more phases.

For example, the dough after kneading is a complex dispersed system consisting of solid, liquid and gaseous phases. It can be represented as a system of type T, G, Zh / T. Starch grains, particles of grain shells and swollen insoluble proteins make up the solid phase. In unbound water, mineral and organic substances are dissolved (water-soluble proteins, dextrins, sugars, salts, etc.). Some of the infinitely swelling proteins form colloidal solutions. The fat present in the dough is in the form of drops. A gaseous medium is formed by trapping air bubbles during mixing and during fermentation.

The dispersion medium of the chocolate mass is cocoa butter, and the dispersed phase consists of powdered sugar and cocoa liquor particles, that is, the chocolate mass without filler is a complex dispersed system T, T / W.

Complex dispersed systems include industrial aerosols (smog), consisting of solid and liquid phases, distributed in a gaseous medium.

Table 1.2

Dispersion	Dispersed	Dispersed	System name,
			Colloidal state is impossible
			Liquid aerosols: mist, deodorant
			Solid aerosols, powders: dust, smoke, powdered sugar, cocoa powder, milk powder
			Foams, gas emulsions: carbonated water, beer, foam (beer, soap)
			Emulsions: milk, mayonnaise
			Sols, suspensions: metal sols, natural reservoirs, cocoa mass, mustard
			Hard foams: pumice, styrofoam, cheese, bread, aerated chocolate, marshmallows
			Capillary systems: oil, fruit fillings
			Metal alloys, precious stones

Classification by structural and mechanical properties

Distinguish free dispersed and coherently dispersed systems.

In free-dispersed systems, the particles of the dispersed phase are not bound to each other and freely move throughout the volume of the system (lyosols, dilute suspensions and emulsions, aerosols, almost all free-flowing powders, etc.).

In coherently dispersed systems, particles of the dispersed phase contact each other, forming a framework that imparts structural and mechanical properties to these systems - strength, elasticity, plasticity (gels, jellies, solid foams, concentrated emulsions, etc.). Coherently dispersed food masses can be in the form of intermediates (dough, chopped meat) or ready-made food (cottage cheese, butter, halva, marmalade, processed cheese etc.).

Classification by the nature of the interaction

dispersed phase and dispersion medium

All dispersed systems form two large groups- lyophilic and lyophobic:

Lyophilic (hydrophilic) dispersed systems are characterized by a significant predominance of the forces of surface interaction of the dispersed and dispersive phases over cohesive forces. In other words, these systems are characterized by high affinity of the dispersed phase and the dispersion medium and, consequently, low values of the surface energy G pov. They form spontaneously and are thermodynamically stable. The properties of lyophilic dispersed systems can be manifested by solutions of colloidal surfactants (soaps), solutions of high-molecular compounds (proteins, polysaccharides), critical emulsions, microemulsions, and some sols.

Lyophobic (hydrophobic) - systems in which the intermolecular particle - medium interaction is small. Such systems are considered thermodynamically unstable. For their formation, certain conditions and external influence are necessary. To increase stability, stabilizers are introduced into them. Most dispersed food systems are lyophobic.

Questions and tasks to consolidate the material

What are the characteristic features of dispersed systems? What is the dispersed phase and dispersion medium in the following systems: milk, bread, mayonnaise, butter, dough?

What parameters characterize the degree of fragmentation of dispersed systems? How does the specific surface area change upon crushing of the dispersed phase?

Calculate the specific surface area (in m 2 / m 3) of cubic sugar crystals with an edge length of 210 -3 m.

The diameter of the oil droplets in sauces depends on how they are prepared. With manual shaking, it is 210 -5 m, and with machine stirring - 410 -6 m. Determine the dispersion and specific surface area (m 2 / m 3) of oil drops for each case. Make a conclusion about the effect of particle size on the specific surface area.

Determine the specific surface area of fat globules and their number in 1 kg of milk with a fat content of 3.2%. The diameter of the fat globules is 8.510 -7 m, the density of milk fat
900 kg / m 3.

What is the reason for the occurrence of excess surface energy?

What is surface tension? In what units is it measured? What are the factors that affect surface tension?

Bring known methods obtaining dispersed systems?

What are the characteristics of dispersed systems classified? Give the classification of dispersed systems according to the degree of dispersion and the state of aggregation of the phases.

On what basis are dispersed systems divided into lyophobic and lyophilic? What properties do these systems possess? Give examples.

Chapter2 ... LYOPHILIC DISPERSION SYSTEMS

The most common and widely used in Food Industry lyophilic systems are solutions of colloidal surfactants and high molecular weight compounds.

2.1. Colloidal surfactant solutions

Colloidal are called surfactants capable of forming micelles in solutions (from Latin mica - tiny) - associates consisting of a large number of molecules (from 20 to 100). Surfactants with a long hydrocarbon chain containing 10-20 carbon atoms have the ability to micelle formation.

Due to the high degree of association of molecules between the micelle and the dispersion medium, an interface arises,
that is, micellar surfactant solutions are heterogeneous systems. But, despite the heterogeneity and large interfacial area, they are thermodynamically stable. This is due to the fact that surfactant molecules in micelles are oriented by polar groups to a polar medium, which leads to a low interfacial tension. Therefore, the surface energy of such systems is low; these are typical lyophilic systems.

2.1.1. Classification of colloidal surfactants

by polar groups

According to the classification adopted at the III International Congress on surfactants and recommended An international organization according to standardization (ISO) in 1960, colloidal surfactants are classified into anionic, cationic, nonionic and amphoteric. Sometimes high molecular weight (polymeric), perfluorinated and organosilicon surfactants are also isolated, however, according to the chemical nature of the molecules, these surfactants can be assigned to one of the above classes.

Anionic surfactants contain one or more polar groups in the molecule and dissociate in an aqueous solution to form long-chain anions, which determine their surface activity. They are better than all other surfactant groups to remove dirt from contact surfaces, which determines their use in a variety of detergents.

Polar groups in anionic surfactants are carboxyl, sulfate, sulfonate, phosphate.

A large group of anionic surfactants are derivatives of carboxylic acids (soaps). The most important are alkali metal salts of saturated and unsaturated fatty acids with the number of carbon atoms 12-18, obtained from animal fats or vegetable oils... When used under optimal conditions, soaps are ideal surfactants. Their main drawback is sensitivity to hard water, which determined the need to create synthetic anionic surfactants - alkyl sulfonates, alkyl benzene sulfonates, etc.

Anionic substances make up a large part of the world production of surfactants. The main reason for the popularity of these surfactants is their simplicity and low production cost.

Cationic are surfactants whose molecules dissociate in aqueous solution to form a surface-active cation with a long hydrophobic chain and an anion — usually a halide, sometimes an anion of sulfuric or phosphoric acid. These include amines of various degrees of substitution, quaternary ammonium bases and other nitrogen-containing bases, quaternary phosphonium and tertiary sulfonium bases. Cationic surfactants do not reduce surface tension as much as anionic surfactants, but they have a good ability to adsorb on negatively charged surfaces - metals, minerals, plastics, fibers, cell membranes, which determined their use as anticorrosive and antistatic agents, dispersants, conditioners, bactericidal and reducing the caking of fertilizers additives.

Nonionic surfactants do not dissociate into ions in water. Their solubility is due to the presence in the molecules of hydrophilic ether and hydroxyl groups, most often a polyethylene glycol chain. This is the most promising and rapidly developing class of surfactants.

Compared to anionic and cationic surfactants, nonionic surfactants are less sensitive to salts that cause water hardness. This type of surfactant brings softness, safety, environmental friendliness to the detergent (biodegradability of non-ionic surfactants is 100%). Non-ionic surfactants exist only in liquid or paste form, therefore they cannot be contained in solid detergents (soap, powders).

Amphoteric (ampholytic) surfactants contain both types of groups in the molecule: acid (most often carboxyl) and basic (usually amino group of different degrees of substitution). Depending on the pH of the medium, they exhibit properties as cationic surfactants (at pH< 4), так и анионактивных (при рН 9-12). При
pH 4-9 they can behave like non-ionic compounds.

This type of surfactant includes many natural substances, including amino acids and proteins.

Amphoteric surfactants are characterized by very good dermatological properties, soften the effect of anionic cleansing ingredients, therefore they are often used in high-quality shampoos and cosmetics.

More information on the classification of surfactants and the main representatives of each class can be found in.

2.1.2. Critical concentration of micelle formation.
Structure and properties of surfactant micelles. Solubilization

The surfactant concentration at which micelles appear in the solution is called critical micelle concentration(KKM). The structure and properties of surfactant micelles are due to intermolecular interactions between the components of the system.

Most of the experimental data indicate that near CMC in aqueous solutions, micelles are spherical formations both in the case of cation- and anion-active and non-ionic surfactants. When micelles are formed in a polar solvent, for example, water, the hydrocarbon chains of surfactant molecules are combined into a compact core, and the hydrated polar groups facing the aqueous phase form a hydrophilic shell (Fig.2.1, a). The diameter of such a micelle is equal to twice the length of the surfactant molecule, and the aggregation number (the number of molecules in a micelle) ranges from 30 to 2000 molecules. The forces of attraction of the hydrocarbon parts of surfactant molecules in water can be identified with hydrophobic interactions; repulsion of polar groups leads to restriction of micelle growth. In non-polar solvents, the orientation of surfactant molecules is opposite, i.e., the hydrocarbon radical is facing a non-polar liquid (Fig.2.1, b).

There is a dynamic equilibrium between surfactant molecules in the adsorption layer and in solution, as well as between surfactant molecules included in micelles (Fig. 2.2).

The shape of micelles and their sizes do not change over a fairly wide range of concentrations. However, with an increase in the surfactant content in the solution, the interaction between micelles begins to manifest itself and at concentrations exceeding the CMC by a factor of 10 or more, they enlarge, first forming cylindrical micelles, and then, at higher concentrations, rod-shaped, disc-shaped, and plate-like micelles with a pronounced anisometry. ... At even higher values of the surfactant concentration in solutions, spatial networks appear, and the system becomes structured.

The CMC value is the most important characteristic of a surfactant, depending on many factors: the length and degree of branching of the hydrocarbon radical, the presence of impurities, the pH of the solution, the ratio between the hydrophilic and hydrophobic properties of the surfactant. The longer the hydrocarbon radical and the weaker the polar group, the lower the CMC value. When the surfactant concentration is higher than the critical one, corresponding to the CMC, the physicochemical properties change sharply, and a kink appears on the property-composition curve. Therefore, most of the methods for determining the CMC are based on the measurement of some physicochemical properties- surface tension, electrical conductivity, refractive index, osmotic pressure, etc. - and the establishment of the concentration at which there is a sharp change in this property.

So, isotherms of surface tension  solutions of colloidal surfactants, instead of the usual smooth run described by the Shishkovsky equation, a break is found at CMC (Fig. 2.3). With a further increase in concentration above the CMC, the surface tension values remain practically unchanged.

The curve of the dependence of the specific electrical conductivity æ on the concentration With ionogenic colloidal surfactants at CCM has a sharp break (Fig. 2.4).

One of characteristic properties solutions of colloidal surfactants associated with their micellar structure is solubilization- dissolution in solutions of colloidal surfactants, which are usually insoluble in a given liquid. The solubilization mechanism consists in the penetration of non-polar molecules of substances added to the surfactant solution into the non-polar micelle core (Fig. 2.5), or vice versa. In this case, the hydrocarbon chains p move, and the volume of the micelle increases. As a result of solubilization in aqueous solutions of surfactants, hydrocarbon liquids dissolve: gasoline, kerosene, as well as fats that are insoluble in water. The salts of bile acids - sodium cholate and deoxycholate - have an exceptionally high solubilizing activity, which solubilize and emulsify fats in the intestine.

Solubilization is an important factor in the detergent action of surfactants. Typically, the contaminant particles are hydrophobic and not wetted with water. Therefore, even with high temperature the washing effect of water is very small and colloidal surfactants are added to increase it. On contact detergent with a contaminated surface, surfactant molecules form an adsorption layer on dirt particles and the surface to be cleaned. Surfactant molecules gradually penetrate between the dirt particles and the surface, helping to tear off the dirt particles (Fig. 2.6). The contaminant gets inside the micelle and can no longer settle on the washed surface.

Osmotic pressure ensures the movement of water in plants due to the difference in osmotic pressure between the cell sap of plant roots (5-20 bar) and the soil solution, which is additionally diluted during irrigation. Osmotic pressure causes water to rise in the plant from the roots to the top. Thus, the leaf cells, losing water, osmotically absorb it from the stem cells, and the latter take it from the root cells.

49. Calculate the EMF of a copper-zinc galvanic cell, in which the concentration of C ions u 2 + is equal to 0.001 mol / l, and ions Zn 2+ 0.1 mol / L. When calculating, take into account the standard EMF values:

ε о (Zn 2+ / Zn 0) = - 0.74 V and ε о (Cu 2 + / Cu 0) = + 0.34 V.

To calculate the EMF value, the Nernst equation is used

54. Methods for obtaining dispersed systems, their classification and a brief description of... What is the method of obtaining dispersed systems with thermodynamic point view is most beneficial?

Dispersion method. It consists in mechanical crushing of solids to a given fineness; dispersion by ultrasonic vibrations; electrical dispersion under the action of variable and direct current... To obtain dispersed systems by dispersion, mechanical devices are widely used: crushers, mills, mortars, rollers, paint grinders, and shakers. Liquids are sprayed and sprayed using nozzles, tops, rotating discs, centrifuges. Dispersion of gases is carried out mainly by bubbling them through a liquid. In foam polymers, foam concrete, foam gypsum, gases are obtained using substances that emit gas at elevated temperatures or in chemical reactions.

Despite the widespread use of dispersion methods, they cannot be applied to obtain disperse systems with a particle size of -100 nm. Such systems are obtained by condensation methods.

Condensation methods are based on the formation of a dispersed phase from substances in a molecular or ionic state. Necessary requirement with this method - the creation of a supersaturated solution, from which should be obtained colloidal system... This can be achieved under certain physical or chemical conditions.

Physical methods of condensation:

1) cooling of vapors of liquids or solids during adiabatic expansion or mixing them with a large volume of air;

2) gradual removal (evaporation) of the solvent from the solution or replacing it with another solvent, in which the dispersible substance dissolves worse.

Thus, physical condensation refers to the condensation of water vapor on the surface of solid or liquid particles, ions or charged molecules in the air (fog, smog).

Solvent substitution leads to the formation of a sol in cases where another liquid is added to the original solution, which mixes well with the original solvent, but is a poor solvent for the solute.

Chemical methods of condensation are based on the implementation of various reactions, as a result of which an undissolved substance is precipitated from a supersaturated solution.

Chemical condensation can be based not only on exchange, but also on redox reactions, hydrolysis, etc.

Disperse systems can also be obtained by the peptization method, which consists in transferring sediments into a colloidal "solution", the particles of which already have colloidal sizes. There are the following types of peptization: peptization by washing the precipitate; peptization with surfactants; chemical peptization.

For example, a freshly prepared and quickly washed precipitate of iron hydroxide transforms into a colloidal solution of red-brown color from the addition of a small amount of FeCl 3 (adsorption peptization) or HCl (dissolution) solution.

The mechanism of the formation of colloidal particles by the peptization method has been studied quite fully: there is a chemical interaction of particles on the surface according to the scheme:

adsorbs ions Fe +3 or FeO +, the subsequent ones are formed as a result of hydrolysis of FeCl 3 and the micelle core receives a positive charge. The micelle formula can be written as:

From a thermodynamic point of view, the most advantageous is the dispersion method.

1) The diffusion coefficient for a spherical particle is calculated using the Einstein equation:

where N A is Avogadro's number, 6 10 23 molecules / mol;

h is the viscosity of the dispersion medium, N · s / m 2 (Pa · s);

r — particle radius, m;

R - universal gas constant, 8.314 J / mol · K;

T is the absolute temperature, K;

Number 3.14.

2) RMS bias:

   D    mean square displacement (average shear value) of a dispersed particle, m2; 

time for which the particle is displaced (diffusion duration), s; 

D  diffusion coefficient, m 2. with -1.

   D  = 2 * 12.24 * 10 -10 * 5 = 12.24 * 10 -9 m 2    12.24 * 10 -9 m 2.

74. Surfactants. Describe the reasons and mechanism for the manifestation of their surface activity.

At low concentrations, surfactants form true solutions, i.e. particles are dispersed in them to individual molecules (or ions). as the concentration increases, micelles appear. in aqueous solutions, organic parts of molecules in micelles combine into a liquid hydrocarbon core, and polar hydrated groups are in water, while the total contact area of hydrophobic parts of molecules with water is sharply reduced. Due to the hydrophilicity of the polar groups surrounding the micelle, the surface (interfacial) tension at the core-water interface is lowered to values that ensure the thermodynamic stability of such aggregates in comparison with the molecular solution and the surfactant macrophase.

At low micellar concentrations, spherical micelles (Gartley micelles) with a liquid apolar core are formed.

Surface activity is associated with chemical composition substances. As a rule, it increases with a decrease in the polarity of the surfactant (for aqueous solutions).

According to Langmuir, during adsorption, the polar group, which has a high affinity for the polar phase, is drawn into water, while a non-polar hydrocarbon radical is pushed out. the resulting decrease in the Gibbs energy limits the size of the surface layer to a thickness of one molecule. in this case, a so-called monomolecular layer is formed.

Depending on the structure, surfactant molecules are subdivided into nonionic, based on ethers containing ethoxy groups, and ionic, based on organic acids and bases.

Ionic surfactants dissociate in solution to form surface-active ions, for example:

If surface-active anions are formed during dissociation, surfactants are called anionic (salts of fatty acids, soaps). If surface-active cations are formed during dissociation, surfactants are called cation-active (salts of primary, secondary and tertiary amines).

There are surfactants that, depending on the pH of the solution, can be both cationic and aninoactive (proteins, amino acids).

The peculiarity of surfactant molecules is that they have a high surface activity with respect to water, which reflects the strong dependence of the surface tension of an aqueous surfactant solution on its concentration.

At low surfactant concentrations, adsorption is proportional to the concentration.

Surface activity is related to the chemical composition of a substance. As a rule, it increases with decreasing surfactant polarity (for aqueous solutions). For example, the activity is higher for carboxylic acids than for their salts.

In the study of homologous series, a clear dependence of the activity on the length of the hydrocarbon radical was found.

On the basis of a large amount of experimental material at the end of the 19th century, Duclos and Traube formulated a rule: the surface activity in a series of homologues increases by 3-3.5 times with an increase in the hydrocarbon chain by one CH 2 group.

As the concentration increases, adsorption on the liquid surface first increases sharply and then approaches a certain limit, called the limiting adsorption.

Based on this fact and a large number of studies, Langmuir put forward the idea of the orientation of molecules in the surface layer. According to Langmuir, during adsorption, the polar group, which has a high affinity for the polar phase - water, is drawn into water, and a non-polar hydrocarbon radical is pushed out. The resulting decrease in the Gibbs energy limits the size of the surface layer to a thickness of one molecule. This forms a so-called monomolecular layer.

INTRODUCTION

The proposed tutorial contains a description of 7 laboratory works for the main sections of the course colloidal chemistry.

Each work consists of theoretical and practical parts. The first part sets out the basics of the corresponding section of the colloidal chemistry course, which will allow students to consciously and successfully complete laboratory work. This is followed by a practical part, which describes the purpose of the work, the necessary reagents and equipment, the procedure for its implementation and processing of experimental results, the requirements for the report and questions for self-control.

The main goals of laboratory work in colloidal chemistry are to instill in students the skills of independent experimental work and to help them master the basic theoretical material discussed in the lectures.

LABORATORY WORK No. 1

OBTAINING ASHES BY THE METHOD OF REPLACING THE SOLVENT.

STUDYING THE PHENOMENA OF WRONG SERIES.

THEORETICAL PART

Colloidal chemistry subject

The science of surface phenomena and dispersed systems is called colloidal chemistry.

Surface phenomena include processes occurring at the interface, in the interfacial surface layer and resulting from the interaction of conjugated phases. Each body is limited by a surface; therefore, bodies of any size can be objects of colloidal chemistry. However, surface phenomena are most pronounced in bodies with a highly developed surface, which gives them new important properties.

Disperse systems considered in colloidal chemistry, consist of at least two phases. One of them is solid and is called dispersion medium. The other phase is fragmented and distributed in the first, it is called dispersed phase.

Classification of dispersed systems

The most general classification of dispersed systems is based on the difference in the state of aggregation of the dispersed phase and the dispersed medium. Three aggregate states (solid, liquid and gaseous) make it possible to distinguish nine types of dispersed systems (Table 1.1). For brevity, they are conventionally designated by a fraction, the numerator of which indicates the state of aggregation of the dispersed phase, and the denominator, the dispersion medium. For example, the fraction T / W denotes systems with a solid dispersed phase and a liquid dispersion medium (solid in liquid). One of the nine combinations of G / G under normal conditions cannot form a colloidal system, since gases give true solutions at any ratios. However, gases can also exhibit some properties of colloidal systems due to continuous fluctuations of density and concentration, which cause inhomogeneities in the system.

It can be seen from the presented classification that all dispersed systems can be divided into two classes according to the kinetic properties of the dispersed phase: free dispersed systems in which the dispersed phase is mobile, and coherently dispersed systems - systems with a solid dispersion medium in which the particles of the dispersed phase cannot move freely. Freely dispersed systems include sols, cohesive dispersed systems - gels.

Disperse systems are classified according to dispersion. For free-dispersed and coherent-dispersed systems, classifications by dispersion have significant differences.

Table 1.1

Classification of dispersed systems by the state of aggregation of phases

Symbol systems	System name and examples
T / T	Solid heterogeneous systems: minerals, alloys, concrete, composite materials
W / T	Capillary systems: liquid in porous bodies, adsorbents in solutions, soils, soils
G / T	Porous bodies: adsorbents and catalysts in gases
T / F	Suspensions and sols: industrial suspensions, pulps, suspensions, pastes, sludge
F / F	Emulsions: natural oils, creams, milk
G / F	Gas emulsions and foams: flotation, fire fighting, soap foams
T / G	Aerosols (dust, fumes), powders
W / G	Aerosols, fogs, including industrial ones, clouds
Y / Y	Colloidal systems are absent

Freely dispersed systems are subdivided into ultramicroheterogeneous, the particle size of which ranges from 10 -7 to 10 -5 cm (from 1 to 100 nm), microheterogeneous with a particle size from 10 -5 to 10 -3 cm (from 0.1 to 10 microns) and coarsely dispersed with particles larger than 10 ‑3 cm.

Ultramicroheterogeneous systems are often called truly colloidal or simply colloidal, since previously only such systems were considered an object of colloidal chemistry. Now the term “colloidal” has come to be used in a broad sense, equivalent to the term “heterogeneous-dispersed”, and the name “sol” remained for ultramicroheterogeneous systems.

Connected-dispersed systems, more precisely, porous bodies, are classified into microporous - with pore sizes up to 2 nm, transitional-porous - from 2 to 200 nm, and macroporous - above 200 nm. T / T systems are often more conveniently subdivided according to their dispersion, in the same way as free-dispersed systems.

According to thermodynamic stability, dispersed systems are classified into lyophilic (thermodynamically stable) and lyophobic (thermodynamically unstable).

Methods for obtaining dispersed systems

Lyophobic dispersed systems (thermodynamically nonequilibrium) can be obtained in two ways: condensation molecules and crushing larger particles to the desired degree of dispersion.

The condensation path of the formation of dispersed systems is associated with the release of a new phase from a homogeneous system in a metastable state, for example, crystallization from a supersaturated solution, condensation of a supersaturated vapor, etc. This process occurs if the chemical potential of a substance in a new (stable) phase is less than in the old one (). However, this ultimately beneficial process goes through a stage that requires the expenditure of energy - the stage of the formation of nuclei of a new phase. Conditions for the emergence of nuclei of a new phase arise in a metastable system in places where local supersaturation is formed - fluctuations of density (concentration) of sufficient magnitude. The radius of the equilibrium nucleus of a new phase is related to the degree of supersaturation by the known dependence (for a liquid droplet formed in a supersaturated vapor):

(1.1)

where s and - surface tension and molar volume of a liquid droplet; p and p - the elasticity of the supersaturated and saturated vapor, respectively.

It can be seen from the equation that the formation of nuclei of a new phase requires supersaturation p / p> 1. The greater the degree of supersaturation, the smaller the equilibrium nucleus size, the easier it is formed.

The sizes of the resulting particles depend on the conditions of the condensation process, in principle, on the ratio between the rates of simultaneously occurring processes: the formation of nuclei and their growth. To obtain small particles (i.e., particles of a dispersed phase in a future dispersed system), a significant prevalence of the rate of the first process over the rate of the second is necessary. In practice, such conditions are created either in very dilute solutions of reacting substances, or, conversely, in sufficiently concentrated solutions, when many nuclei are formed at once during the crystallization process, which did not have time to grow to large sizes... In the first case, a sol (colloidal system) is formed, in the second, a fine-crystalline precipitate is obtained, which can be certain conditions transfer to a colloidal solution.

Chemical condensation

If a sparingly soluble compound is formed during a chemical reaction, then under certain conditions it can be obtained in the form of a colloidal solution. For this it is necessary, first, to carry out a reaction diluted in a solution so that the growth rate of crystalline particles is low, then the particles are small (10 –7 ¼ 10 –9 m) and sedimentation stability will be provided to the system; secondly, one of the reactants should be taken in excess so that an electric double layer, the main factor of aggregate stability, could form on the surface of the crystal.

Physical condensation

The method is based on the condensation of molecules of one substance - the future dispersed phase, in another substance - the future dispersion medium. In practice, this can be done in various ways, for example, by passing one substance into another.

One example of physical condensation is the solvent exchange method: a solution of a substance is gradually, with stirring, added to a liquid in which this substance is insoluble. In this case, the condensation of molecules and the formation of colloidal particles occurs.

In this way, hydrosols of sulfur, phosphorus, rosin, anthracene and other substances can be obtained by pouring their alcohol solutions into water. The structure of the electric double layer in these systems is not well known.

Splitting up

Mechanical crushing is carried out in various types of mills (disperse mills are used to obtain colloidal dispersion), using ultrasound, in a volt arc (to obtain metal sols), etc.

The fragmentation of small particles requires a large expenditure of work, since the interface between the phases in such systems must be very large. The particles formed during crushing have a tendency of spontaneous adhesion (coagulation), therefore crushing should be carried out in a dispersed medium in the presence of stabilizers - ions or surfactants.

Crushing in the presence of a surfactant (surfactant) requires less work. The effect of a significant decrease in the resistance of solids to destruction as a result of surfactant adsorption was discovered by P.A. Rebinder. and received the name of the adsorptive strength reduction.

Objective: get acquainted with various methods of obtaining dispersed systems.

Brief theoretical introduction.

Methods for preparing dispersed systems can be divided into two groups: dispersion methods and condensation methods.

Dispersion methods are based on crushing large pieces of material to the required degree of dispersion. These methods are more often used to obtain suspensions and emulsions. Systems with particle sizes of 10 -6 - 10 -7 cm are obtained by condensation methods. Condensation methods are the combination of molecules or ions to the size of colloidal particles, resulting in the formation of a phase boundary.

To obtain dispersed systems using any of these methods, the following conditions must be met:

a) insolubility or limited solubility of the dispersed phase in the dispersion medium;

b) the presence of a stabilizer in the system, which should ensure the stability of suspended particles and stop their growth.

Dispersion methods.

Expending work against the molecular forces of adhesion, it is possible to achieve the desired degree of dispersion in various ways.

1. Mechanical dispersion.

The method consists in vigorous and prolonged grinding, grinding or spraying the substance of the dispersed phase and mixing it with a liquid that serves as a dispersion medium. Large particles are crushed using mortars, colloidal mills, paint grinders. Pharmaceuticals, lubricants, food products are obtained by mechanical dispersion.

2. Dispersion by ultrasound.

The method is based on the use of ultrasonic vibrations (more than 20,000 vibrations per second). Dispersion using ultrasound is effective only for substances with low strength: sulfur, graphite, paints, starch, rubber, gelatin. Emulsions are very easily obtained by this method, for example, cocoa emulsions, high-quality creams, etc.

Condensation methods.

Condensation methods are based on the processes of formation of dispersed phase particles from substances in a molecular or ionic state. These processes can be both physical and chemical in nature.

Physical condensation.

1. Method of replacing the solvent.

The essence of the method lies in the fact that the solvent in which the substance dissolves, forming a true solution, is replaced by a solvent in which this substance is insoluble. For example, if an alcoholic solution of sulfur, phosphorus or rosin is poured into water, then the solution becomes saturated, condensation occurs, and particles of the dispersed phase are formed. This is because these substances are poorly soluble in a water-alcohol mixture.

2. Condensation during cooling of steam.

The most obvious example of vapor condensation is the formation of fog or smoke. Another example of the formation of colloidal particles as a result of vapor condensation is the Wilson chamber used in nuclear physics.

Chemical condensation.

The production of dispersed systems by chemical condensation methods is reduced to the formation of molecules of insoluble substances as a result of a chemical reaction, followed by their enlargement to the size of colloidal particles. Chemical condensation methods are classified according to the type of chemical reaction underlying the preparation of the sol. Among the reactions, as a result of which, under appropriate conditions, substances in a colloidal state can be formed, include the reactions of oxidation, reduction, exchange, hydrolysis.

1. Oxidation reactions.

An example of an oxidative reaction is the oxidation of hydrogen sulfide in aquatic environment:

H 2 S + O 2 = 2S + 2H 2 O

2. Reactions of exchange.

An example of such a reaction is the formation of an arsenic (III) sulfide sol:

As 2 O 3 + 3H 2 S = As 2 S 3 + 3H 2 O

3. Hydrolysis reactions.

Hydrolysis is most often used to obtain metal hydroxide sols:

FeCl 3 + 3H 2 O = Fe (OH) 3 + 3HCl

Peptization method.

Peptization is the process of transition into a colloidal solution of sediments formed during coagulation. Peptization can be caused by washing the coagulum with a solvent, as well as by exposure to peptizing agents (electrolytes, non-electrolytes, surfactants, high molecular weight compounds). Only freshly obtained sediments in which crystallization phenomena have not passed and the particles have not lost their individuality can be peptized.

Experimental part.

I. Physical condensation methods.

Test 1... Sulfur sol production by solvent replacement.

Pour 10 ml of distilled water into a test tube, add 5 drops of a solution of sulfur in ethanol and vigorously mix the contents of the test tube. A transparent opalescent sol is formed. Sulfur is soluble in alcohol but insoluble in water. When alcohol is replaced with water, the molecules of the solute are combined into aggregates of colloidal sizes.

To observe the Faraday-Tyndall effect, a test tube with colloidal solution is placed in the path of the light beam of the projection lamp. Consider the tube at an angle of 90 ° to the direction of the incident beam.

Methods for obtaining dispersed systems, their classification and brief description. What method of obtaining dispersed systems is the most advantageous from the thermodynamic point of view?

Dispersion method. It consists in mechanical crushing of solids to a given fineness; dispersion by ultrasonic vibrations; electrical dispersion under the action of alternating and direct current. To obtain dispersed systems by dispersion, mechanical devices are widely used: crushers, mills, mortars, rollers, paint grinders, and shakers. Liquids are sprayed and sprayed using nozzles, tops, rotating discs, centrifuges. Dispersion of gases is carried out mainly by bubbling them through a liquid. In foam polymers, foam concrete, foam gypsum, gases are obtained using substances that emit gas at elevated temperatures or in chemical reactions.

Despite the widespread use of dispersion methods, they cannot be applied to obtain disperse systems with a particle size of -100 nm. Such systems are obtained by condensation methods.

Condensation methods are based on the formation of a dispersed phase from substances in a molecular or ionic state. A necessary requirement for this method is the creation of a supersaturated solution, from which the colloidal system should be obtained. This can be achieved under certain physical or chemical conditions.