Methods for obtaining dispersed systems. Condensation methods

Methods for producing colloidal solutions can also be divided into two groups: condensation and dispersion methods (a separate group is the peptization method, which will be discussed later). Another necessary condition for obtaining sols, in addition to bringing the particle sizes to colloidal ones, is the presence in the system of stabilizers - substances that prevent the process of spontaneous enlargement of colloidal particles.

Rice. Classification of methods for producing dispersed systems (the type of system is indicated in brackets)

Dispersion methods

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

Dispersion can be spontaneous or non-spontaneous. Spontaneous dispersion is characteristic of lyophilic systems and is associated with an increase in the disorder of the system (when many small particles are formed from one large piece). When dispersing at a constant temperature, the increase in entropy must exceed the change in enthalpy.

ΔH > TΔS; ΔG > 0.

The dispersion process in this case is typically non-spontaneous and is carried out due to external energy.

Dispersion is characterized by the degree of dispersion. It is determined by the ratio of the sizes of the initial product and the particles of the dispersed phase of the resulting system. The degree of dispersion can be expressed as follows:

α 1 = d n / d k; α 2 = B n / B k; α 3 = V n / V k,

where d n; d to; Bn; B to; V n; Vк - respectively diameter, surface area, volume of particles before and after dispersion.

Thus, the degree of dispersion can be expressed in terms of size (α 1), surface area (α 2) or volume (α 3) of dispersed phase particles, i.e. can be linear, superficial or volumetric.

The work W required to disperse a solid or liquid is spent on deforming the body W d and on the formation of a new phase interface W a, which is measured by the work of adhesion. Deformation is a necessary prerequisite for the destruction of a body. According to P.A. Rebinder, the work of dispersion is determined by the formula

W = W a + W d = σ*ΔB + kV,

where σ* is a value proportional to or equal to the surface tension at the interface between the dispersed phase and the dispersion medium; ΔB—increase in the phase interface as a result of dispersion; V is the volume of the original body before dispersion; k is a coefficient equivalent to the work of deformation per unit volume of a body.

Condensation methods

Condensation methods for producing dispersed systems include condensation, desublimation and crystallization. They are based on the formation of a new phase under conditions of a supersaturated state of a substance in a gas or liquid medium. In this case, the system goes from homogeneous to heterogeneous. Condensation and desublimation are characteristic of a gas medium, and crystallization is characteristic of a liquid medium.

A necessary condition for condensation and crystallization is supersaturation and uneven distribution of the substance in the dispersion medium (concentration fluctuation), as well as the formation of condensation centers or nuclei.

The degree of supersaturation β for solution and steam can be expressed as follows:

β f = s/s s , β P = r/p s ,

where p, c are the supersaturated vapor pressure and the concentration of the substance in the supersaturated solution; р s is the equilibrium pressure of saturated vapor over a flat surface; c s is the equilibrium concentration corresponding to the formation of a new phase.

To carry out crystallization, the solution or gas mixture is cooled.

The condensation methods for obtaining dispersed systems are based on the processes of crystallization, desublimation and condensation, which are caused by a decrease in the Gibbs energy (ΔG< 0) и протекают самопроизвольно.

During the nucleation and formation of particles from a supersaturated solution or gaseous medium, the chemical potential µ changes and an interface appears, which becomes the carrier of excess free surface energy.

The work spent on the formation of particles is determined by the surface tension σ and is equal to:

W 1 = 4πr 2 σ,

where 4πr 2 is the surface of spherical particles with radius r.

The chemical potential changes as follows:

Δμ = μ i // - μ i /< 0; μ i // >μ i / ,

where μ i / and μ i // are the chemical potentials of homo and heterogeneous systems (during the transition from small drops to large ones).

The change in chemical potential characterizes the transfer of a certain number of moles of a substance from one phase to another; this number n moles is equal to the volume of the particle 4πr 3 /3 divided by the molar volume Vm:

The work of formation of a new surface during the condensation process W k is equal to:

where W 1 and W 2 are, respectively, the work spent on the formation of the particle surface and the work on the transfer of matter from a homogeneous medium to a heterogeneous one.

The formation of dispersed systems can occur as a result of physical and chemical condensation, as well as when replacing the solvent.

Physical condensation occurs when the temperature of a gas medium containing vapors of various substances decreases. When the necessary conditions are met, particles or drops of the dispersed phase are formed. A similar process takes place not only in the volume of gas, but also on a cooled solid surface, which is placed in a warmer gas environment.

Condensation is determined by the difference in chemical potentials (μ i // - μ i /)< 0, которая изменяется в результате замены растворителя. В отличие от обычной физической конденсации при solvent replacement the composition and properties of the dispersion medium do not remain constant. If alcohol or acetone solutions of sulfur, phosphorus, rosin and some other organic substances are poured into water, the solution becomes supersaturated, condensation occurs and dispersed phase particles are formed. The solvent replacement method is one of the few by which sols can be obtained.

At chemical condensation the formation of a substance occurs with its simultaneous supersaturation and condensation.

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 watering. Osmotic pressure causes water to rise in the plant from the roots to the top. Thus, leaf cells, losing water, osmotically absorb it from stem cells, and the latter take it from 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 making calculations, take into account the standard EMF values:

ε o (Zn 2+ /Zn 0) = – 0.74 V and ε o (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 brief description. Which method of obtaining dispersed systems is most beneficial from a thermodynamic point of view?

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

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

Condensation methods are based on the process of 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 a colloidal system should be obtained. 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 dispersed substance is less soluble.

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

Solvent replacement results in the formation of a sol when another liquid is added to the original solution, which mixes well with the original solvent but is a poor solvent for the solute.

Chemical condensation methods are based on performing 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 reactions, but also on redox reactions, hydrolysis, etc.

Dispersed systems can also be obtained by peptization, which consists of converting sediments, the particles of which already have colloidal sizes, into a colloidal “solution”. The following types of peptization are distinguished: peptization by washing the sediment; peptization with surfactants; chemical peptization.

For example, a freshly prepared and quickly washed precipitate of iron hydroxide turns into a red-brown colloidal solution by adding a small amount of FeCl 3 solution (adsorption peptization) or HCl (dissolution).

The mechanism of formation of colloidal particles using the peptization method has been studied quite fully: chemical interaction of particles on the surface occurs according to the following scheme:

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

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

where N А is Avogadro’s number, 6 10 23 molecules/mol;

h – 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 – absolute temperature, K;

number 3.14.

2) Root mean square displacement:

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

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

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

At low concentrations, surfactants form true solutions, i.e. particles are dispersed and they are reduced to individual molecules (or ions). As the concentration increases, micelles appear. in aqueous solutions, the organic parts of the molecules in micelles are combined into a liquid hydrocarbon core, and the polar hydrated groups are in water, while the total area of ​​​​contact of the hydrophobic parts of the 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 reduced to values ​​that ensure the thermodynamic stability of such aggregates compared to a molecular solution and the surfactant macrophase.

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

Surface activity is associated with chemical composition substances. As a rule, it increases with decreasing 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 the water, and the hydrocarbon non-polar radical is pushed out. the resulting decrease in Gibbs energy limits the size of the surface layer to one molecule thick. in this case, a so-called monomolecular layer is formed.

Depending on the structure, surfactant molecules are divided into nonionic, built on the basis of esters, including 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 cationic (salts of primary, secondary and tertiary amines).

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

The peculiarity of surfactant molecules is that they have high surface activity towards 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 concentration.

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

When studying homologous series, a clear dependence of activity on the length of the hydrocarbon radical was discovered.

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

As the concentration increases, adsorption on the surface of the liquid 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 ​​\u200b\u200bthe orientation of molecules in the surface layer. According to Langmuir, during adsorption, a polar group, which has a high affinity for the polar phase - water, is drawn into the water, and the hydrocarbon non-polar radical is pushed out. The resulting decrease in Gibbs energy limits the size of the surface layer to one molecule thick. In this case, a so-called monomolecular layer is formed.

Methods for obtaining dispersed systems

Lecture 20. Electrokinetic phenomena

Self-test questions

1. What is the difference between adsorption on a solid surface and adsorption on a liquid surface?

2. What is physical and chemical adsorption, what is their essence?

4. On what principles is Langmuir’s theory of monomolecular adsorption based?

5. Give the equation for the Langmuir adsorption isotherm. What is limiting adsorption?

6. Consider the Freundlich equation. Under what conditions and for what systems is it applicable?

7. Explain the principle of graphically determining adsorption constants using the Freundlich equation?

20.1 Methods for obtaining dispersed systems

20.2 Electrophoresis, electroosmosis, sedimentation and percolation potentials

20.3 Electrokinetic potential and its definition

A chemical substance can be obtained in a colloidal state under the following conditions:

1) the particle size of a given substance must be brought to colloidal sizes (10−5–10−7 cm), which can be done by two methods: a) crushing the particles of the substance to the size of a colloidal degree of dispersion (dispersion methods); b) enlargement of molecules, atoms, ions to particles of colloidal size (condensation methods);

2) the presence of a stabilizer, for example, electrolyte ions, which form an ionic hydrate shell on the surface of colloidal particles and create a charge that prevents particles from sticking together when they collide in a solution;

3) colloidal particles (dispersed phase) must have poor solubility in a dispersion medium, at least at the time of their preparation.

If these conditions are met, colloidal particles acquire an electrical charge and a hydration shell, which prevents them from precipitating.

Dispersion methods for producing colloidal systems are based on grinding relatively large particles of the dispersed phase substance to colloidal sizes by mechanical, electrical, chemical, and ultrasonic dispersion. Chemical methods of dispersion also include the so-called. spontaneous dispersion method. For example, by dissolving in water, colloidal solutions of starch, gelatin, agar-agar, etc. can be obtained. Spontaneous dispersion occurs without external mechanical influences. This method is widely used to obtain solutions of high molecular weight substances from solid polymers.

Condensation methods are based on the transition of molecular or ionic solutions into colloidal solutions due to the enlargement of particles of the dispersed phase substance. Condensation methods include the solvent replacement method, chemical methods for producing colloidal solutions using reactions of oxidation, reduction, exchange decomposition, hydrolysis, etc., as well as the peptization method. As a result of all chemical reactions, molecular or ionic solutions become colloidal by converting dissolved substances into an insoluble state. Condensation methods, in addition to chemical processes, can also be based on physical processes, mainly the phenomenon of vapor condensation. In chemical methods for producing dispersed systems, one of the starting substances acts as a stabilizer, and is taken in excess.

Oxidation method. It is based on oxidation reactions, as a result of which one of the substances can be obtained in a colloidal state. For example, by oxidizing hydrogen sulfide with atmospheric oxygen or sulfur dioxide, a sulfur sol can be obtained:

2H 2 S + O 2 → 2H 2 O + 2S

2H 2 S + SO 2 → 2H 2 O + 3S

Recovery method. As an example, we give the reaction of producing a gold sol by reducing its salt with hydrogen peroxide or formaldehyde:

2HAuCI 4 + 3H 2 O 2 → 2Au + 8HCI + 3O 2

2HAuCI 4 + 3HCHO + 11KOH → 2Au + 3HCOOK + 8KCI + 8H 2 O

The reduction reaction produced many metals in a colloidal state, for example, Au, Ag, Pt, Pd, Os, Hg, etc.

Exchange decomposition method. An example is the reaction for producing barium sulfate sol:

BaCI 2 + K 2 SO 4 → BaSO 4 + 2KCI

or silver chloride

AgNO 3 + KCI → AgCI + KNO 3.

Hydrolysis method. Slightly soluble Fe(III) hydroxide is formed during the hydrolysis of iron(III) chloride:

FeCI 3 + 3HOH → Fe(OH) 3 + 3HCI,

Fe(OH) 3 + HCI → FeOCI + 2H 2 O

The ferric oxychloride formed as a result of these reactions dissociates partially into ions:

FeOCI ↔ FeO + + CI −

These ions provide an ionic layer around the Fe(OH) 3 particles, keeping them suspended.

Peptization method. Peptization is the transition of sediments formed during coagulation into a colloidal solution. It can occur when washing sediments under the influence of peptizing agents, which use electrolytes. There is no change in the degree of dispersion of sediment particles, but only their disconnection.

For this reason, the peptization method, condensation in the initial stages, and dispersion in the final stages, occupies an intermediate position between condensation and dispersion. An example of a sol obtained by peptization is the synthesis of Prussian blue sol.

Methods for obtaining dispersed systems, their classification and brief characteristics. Which method of obtaining dispersed systems is most beneficial from a thermodynamic point of view?

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

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

Condensation methods are based on the process of 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 a colloidal system should be obtained. 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 dispersed substance is less soluble.

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

Solvent replacement results in the formation of a sol when another liquid is added to the original solution, which mixes well with the original solvent but is a poor solvent for the solute.

Chemical condensation methods are based on performing 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 reactions, but also on redox reactions, hydrolysis, etc.

Dispersed systems can also be obtained by peptization, which consists of converting sediments, the particles of which already have colloidal sizes, into a colloidal “solution”. The following types of peptization are distinguished: peptization by washing the sediment; peptization with surfactants; chemical peptization.

For example, a freshly prepared and quickly washed precipitate of iron hydroxide turns into a red-brown colloidal solution by adding a small amount of FeCl 3 solution (adsorption peptization) or HCl (dissolution).

The mechanism of formation of colloidal particles using the peptization method has been studied quite fully: chemical interaction of particles on the surface occurs according to the following scheme:

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

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;

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 - absolute temperature, K;

Number 3.14.

2) Root mean square displacement:

Where? ?? ???mean square displacement (averaged shift value) of a disperse particle, m 2 ;?

Time during which the particle is displaced (diffusion duration), s;??

D?? diffusion coefficient, m2. s -1 .

? ? ????·D·?=2*12.24*10 -10 *5=12.24*10 -9 m 2

Answer: ? ? ?? 12.24*10 -9 m2.

1.2. Methods for obtaining dispersed systems

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

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

Dispersion method

This method combines, first of all, mechanical methods in which overcoming intermolecular forces and accumulating free surface energy during the dispersion process 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. Ball mills are the most common. These are hollow rotating cylinders into which the crushed material and steel or ceramic balls are loaded. As the cylinder rotates, the balls roll, abrading the material being crushed. Shredding can also occur as a result of ball impacts. Ball mills produce systems whose particle sizes are within a fairly wide range: from 2-3 to 50-70 microns. A hollow cylinder with balls can be set into a circular oscillatory motion, which promotes intensive crushing of the loaded material under the influence of the complex movement of the crushed bodies. This device is called a vibration mill.

Finer dispersion is achieved in colloidal mills of various designs, the operating principle of which is based on the development of breaking forces in a suspension or emulsion under the influence of centrifugal force in a narrow gap between the rotor rotating at high speed and the stationary part of the device - the stator. The suspended large particles experience a significant breaking force and are thus dispersed.

High dispersion can be achieved ultrasonic dispersion. The dispersive effect of ultrasound is associated with cavitation - the formation and slamming of a cavity in a liquid. The collapse of cavities is accompanied by the appearance of cavitation shock waves, which destroy the material. It has been experimentally established that dispersion is directly dependent on the frequency of ultrasonic vibrations. Ultrasonic dispersion is especially effective if the material is previously finely ground. Emulsions obtained by ultrasonic method are characterized 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 progresses, the strength of the particles increases, which is usually used to create stronger materials. At the same time, an increase in the strength of materials as they are crushed leads to a large consumption of energy for further dispersion. The destruction of materials can be facilitated by using the Rebinder effect - adsorption reduction in the strength of solids. This effect is to reduce the surface energy with the help of surfactants, resulting in easier deformation and destruction of the solid. 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 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, since, by covering the surface of the particles, they thereby prevent them from sticking back together. This also helps to achieve a highly dispersed state.

It is usually not possible to achieve high dispersity using the dispersion method. 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, foam masses.

Condensation method

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

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

During physical condensation, a new phase is formed in a gas or liquid medium under conditions of a supersaturated state of the substance. Condensation involves the formation of a new phase on existing surfaces (walls of a vessel, 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 a substance in the system. In the first case, 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, so the reactivity of the condensed substance is greater than the macrophases in accordance with the Kelvin equation of capillary condensation. Therefore, in order for the condensed substance not to return to the original phase and condensation to continue, there must be supersaturation in the system.

1.3. Classification of disperse systems

Dispersed systems are classified according to the following criteria:

degree of dispersion;

state of aggregation of the dispersed phase and dispersion medium;

structural and mechanical properties;

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

Classification by degree of dispersion

Depending on the particle size, highly dispersed, medium dispersed and coarsely dispersed systems are distinguished (Table 1.1).

Table 1.1

The specific surface area of ​​particles of the dispersed phase is maximum in highly dispersed systems; when moving 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.

Based on the particle size of the dispersed phase, one and the same product can belong to different disperse systems. For example, the particles of premium wheat flour have a size of (1-30)10 –6 m, i.e. flour of this grade simultaneously belongs to the medium-disperse and coarse systems.

Classification by state of aggregation

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

Each disperse system has its own designation and name: the numerator indicates the aggregate state of the dispersed phase, and the denominator indicates the dispersion medium. Eight options for 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 is ​​added to the word sol to characterize the dispersion medium. If the dispersion medium is solid – xerosols, liquid – lyosols(hydrosols), gas – aerosols.

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

For example, dough after kneading is a complex disperse system consisting of solid, liquid and gaseous phases. It can be represented as a system of type T, G, F/T. Starch grains, particles of grain shells and swollen insoluble proteins make up the solid phase. Mineral and organic substances (water-soluble proteins, dextrins, sugars, salts, etc.) are dissolved in unbound water. Some of the proteins that swell indefinitely form colloidal solutions. The fat present in the dough is in the form of droplets. The gaseous environment is formed due to the capture of air bubbles during kneading and during the fermentation process.

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

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

Table 1.2

Classification according to structural and mechanical properties

Distinguish freely dispersed And cohesively dispersed systems.

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

In cohesively 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.). Cohesively dispersed food masses can be in the form of intermediate products (dough, minced meat) or prepared food products (cottage cheese, butter, halva, marmalade, processed cheese, etc.).

Classification by nature of interaction

dispersed phase and dispersion medium

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

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

Questions and tasks to reinforce the material

Name the characteristic features of disperse 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 disperse systems? How does the specific surface area change when the dispersed phase is crushed?

Calculate the specific surface area (in m2/m3) of cubic sugar crystals with an edge length of 210 -3 m.

The diameter of oil droplets in sauces depends on the method of their preparation. With manual shaking it is 210 -5 m, and with machine mixing - 410 -6 m. Determine the dispersion and specific surface area (m 2 /m 3) of oil droplets for each case. Draw a conclusion about the effect of particle size on the specific surface area.

Determine the specific surface area of ​​fat globules and their quantity in 1 kg of milk with a fat content of 3.2%. The diameter of fat globules is 8.510 -7 m, the density of milk fat
900 kg/m3.

What is the cause of excess surface energy?

What is surface tension? In what units is it measured? Name the factors influencing surface tension.

What are the known methods for producing disperse systems?

By what criteria are disperse systems classified? Give a classification of disperse 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 have? Give examples.

Chapter2 . LYOPHILIZED DISPERSE SYSTEMS

The most common and widely used lyophilic systems in the food industry are solutions of colloidal surfactants and high-molecular compounds.

2.1. Solutions of colloidal surfactants

Colloidal are 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 form micelles.

Due to the high degree of association of molecules between the micelle and the dispersion medium, an interface appears,
i.e. micellar surfactant solutions are heterogeneous systems. But, despite the heterogeneity and large interfacial surface, they are thermodynamically stable. This is due to the fact that surfactant molecules in micelles are oriented by polar groups towards the polar medium, which causes 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 by the International Organization for Standardization (ISO) in 1960, colloidal surfactants are divided into anionic, cationic, nonionic and amphoteric. Sometimes high molecular weight (polymer), perfluorinated and organosilicon surfactants are also isolated, however, based on the chemical nature of the molecules, these surfactants can be classified into 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 groups of surfactants in removing dirt from contact surfaces, which determines their use in a variety of detergents.

Polar groups in anionic surfactants are carboxyl, sulfate, sulfonate, and 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 a number of carbon atoms of 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 - alkylsulfonates, alkylbenzenesulfonates, etc.

Anionic substances make up the majority of the world's surfactant production. Main reason The popularity of these surfactants is their simplicity and low production costs.

Cationic are surfactants whose molecules dissociate in an aqueous solution to form a surfactant cation with a long hydrophobic chain and an anion - usually a halide, sometimes an anion of sulfuric or phosphoric acid. These include amines of varying 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 ones, but have a good ability to adsorb on negatively charged surfaces - metals, minerals, plastics, fibers, cell membranes, which determined their use as anti-corrosion and antistatic agents, dispersants, conditioners, bactericidal and additives that reduce caking of fertilizers.

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 the polyethylene glycol chain. This is the most promising and rapidly developing class of surfactants.

Nonionic surfactants, compared to anionic and cationic ones, are less sensitive to salts that cause water hardness. This type of surfactant makes the detergent soft, safe, and environmentally friendly (the biodegradability of nonionic surfactants is 100%). Nonionic surfactants exist only in liquid or paste form, and therefore cannot be contained in solid detergents (soaps, powders).

Amphoteric (ampholytic) surfactants contain both types of groups in the molecule: acidic (most often carboxyl) and basic (usually an amino group of different degrees of substitution). Depending on the pH of the environment, they exhibit properties as cationic surfactants (at pH< 4), так и анионактивных (при рН 9-12). При
pH 4-9 they can behave as nonionic 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, and therefore are often used in high-quality shampoos and cosmetics.

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

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

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

Most experimental data indicate that near the CMC in aqueous solutions, micelles are spherical formations both in the case of cationic and anionic active and nonionic 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 the micelle) ranges from 30 to 2000 molecules. The attractive forces of the hydrocarbon parts of surfactant molecules in water can be identified with hydrophobic interactions; repulsion of polar groups limits the growth of micelles. In non-polar solvents, the orientation of the surfactant molecules is opposite, i.e. the hydrocarbon radical faces the non-polar liquid (Fig. 2.1, b).

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

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

KKM value – most important characteristic A surfactant that depends 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 in the property-composition curve. Therefore, most methods for determining CMC are based on measuring any physicochemical property - surface tension, electrical conductivity, refractive index, osmotic pressure, etc. - and establishing the concentration at which a sharp change in this property is observed.

Thus, surface tension isotherms  solutions of colloidal surfactants, instead of the usual smooth motion described by the Shishkovsky equation, a kink is detected in CMC (Fig. 2.3). With a further increase in concentration above the CMC, the surface tension values ​​remain practically unchanged.

Curve of specific electrical conductivity æ versus concentration With ionic colloidal surfactants with CMC has a sharp break (Fig. 2.4).

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

Solubilization is an important factor in the detergent action of surfactants. Typically, pollutant particles are hydrophobic and are not wetted by water. Therefore, even at high temperatures, the cleaning effect of water is very small and colloidal surfactants are added to increase it. When a detergent comes into contact with a contaminated surface, surfactant molecules form an adsorption layer on the dirt particles and the surface being cleaned. Surfactant molecules gradually penetrate between the dirt particles and the surface, promoting the detachment of dirt particles (Fig. 2.6). The contaminant enters the micelle and can no longer settle on the surface to be washed.