Dispersed method. Preparation and purification of dispersed systems (DS). Preparation of sols by solvent replacement method

A dispersed system is a system in which one substance is distributed in the medium of another, and there is a phase boundary between the particles and the dispersion medium. Dispersed systems consist of a dispersed phase and a dispersion medium.

The dispersed phase is particles distributed in the medium. Its signs: dispersion and intermittency.

Dispersion medium is the material medium in which the dispersed phase is located. Its sign is continuity.

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.

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

Cleaning methods:

1. Dialysis - purification of sols from impurities using semi-permeable membranes washed with a pure solvent.

2. Electrodialysis – dialysis accelerated by an electric field.

3. Ultrafiltration – purification by pressing a dispersion medium along with low-molecular impurities through a semi-permeable membrane (ultrafilter).

Molecular-kinetic and optical properties of dispersed systems: Brownian motion, osmotic pressure, diffusion, sedimentation equilibrium, sedimentation analysis, optical properties of dispersed systems.

All molecular kinetic properties are caused by the spontaneous movement of molecules and are manifested in Brownian motion, diffusion, osmosis, and sedimentation equilibrium.

Brownian motion is the continuous, chaotic, equally probable in all directions movement of small particles suspended in liquids or gases due to the influence of molecules of a dispersion medium. The theory of Brownian motion is based on the idea of ​​the interaction of a random force, which characterizes the impacts of molecules, a time-dependent force, and a frictional force when particles of a dispersed phase move in a dispersive medium at a certain speed.

In addition to translational motion, rotational motion is also possible, which is typical for two-dimensional particles of irregular shape (threads, fibers, flakes). Brownian motion is most pronounced in highly dispersed systems, and its intensity depends on the dispersion.

Diffusion is the spontaneous spread of a substance from an area of ​​higher concentration to an area of ​​lower concentration. The following types are distinguished:

1.)molecular

3) colloidal particles.

The rate of diffusion in gases is the highest, and in solids it is the least.

Osmotic pressure is the excess pressure above a solution that is necessary to prevent the transfer of solvent through the membrane. OD occurs when a pure solvent moves towards a solution or from a more dilute solution towards a more concentrated one, and is therefore related to the concentration of the solute and solvent. Osmotic pressure is equal to the pressure that the dispersed phase (solute) would produce if it, in the form of a gas, at the same temperature, occupied the same volume as the colloidal system (solution).

Sedimentation is the separation of dispersed systems under the influence of gravity with the separation of the dispersed phase in the form of sediment. The ability of dispersed systems to sediment is an indicator of their sedimentation stability. Separation processes are used when it is necessary to isolate one or another component from some component from some natural or artificially prepared product, which is a heterogeneous liquid system. In some cases, a valuable component is removed from the system, in others, unwanted impurities are removed. In public catering, the processes of separation of dispersed systems are necessary when it is necessary to obtain clear drinks, clarify the broth, and free it from meat particles.

The behavior of a light beam encountering particles of the dispersed phase on its path depends on the ratio of the wavelength of light and the size of the particles. If the particle size is greater than the wavelength of light, then the light is reflected from the surface of the particles at a certain angle. This phenomenon is observed in suspensions. If the particle size is smaller than the wavelength of light, then the light is scattered.

Src="http://present5.com/presentation/3/40492240_88526628.pdf-img/40492240_88526628.pdf-1.jpg" alt=">Methods for obtaining dispersed systems">!}

Src="http://present5.com/presentation/3/40492240_88526628.pdf-img/40492240_88526628.pdf-2.jpg" alt="> Dispersed systems are obtained with the necessary set of physical and chemical properties (composition, state of aggregation,"> Дисперсные системы получают с необходимым набором физических и химических свойств (состав, агрегатное состояние, размер, форма, структура, поверхностные свойства). При получении дисперсных систем решают две важные задачи: получение дисперсных частиц нужного размера и формы; стабилизация дисперсных систем, т. е. сохранение размеров дисперсных частиц в течение достаточно длительного времени (особенно актуальна для наночастиц). Методы получения дисперсных систем делятся на: диспергационные, конденсационные и метод пептизации. 2!}

Src="http://present5.com/presentation/3/40492240_88526628.pdf-img/40492240_88526628.pdf-3.jpg" alt="> Dispersion methods Methods involve grinding large (macroscopic) samples of a given"> Диспергационные методы Методы заключаются в измельчении крупных (макроскопических) образцов данного вещества до частиц дисперсных размеров. При диспергировании химический состав и агрегатное состояние вещества обычно не меняются, меняется размер частиц и их форма. Диспергирование происходит, как правило, не самопроизвольно, а с затратой внешней работы, расходуемой на преодоление межмолекулярных сил при дроблении вещества. Диспергационные методы используют в основном для получения грубодисперсных частиц – от 1 мкм и выше - производство цемента (1 млрд. т в год), измельчении руд полезных ископаемых, получение пищевых продуктов и лекарств и т. д. 3!}

Src="http://present5.com/presentation/3/40492240_88526628.pdf-img/40492240_88526628.pdf-4.jpg" alt="> The mechanism for reducing hardness is that the added substance (hardness reducer ) is adsorbed in"> Механизм уменьшения твердости заключается в том, что добавляемое вещество (понизитель твердости) адсорбируется в местах дефектов кристаллической решетки твердого тела, что приводит к экранированию сил сцепления, действующими между противоположными поверхностями щели (при адсорбции электролитов возникают силы электростатического отталкивания между одноименно заряженными ионами, ПАВы понижают поверхностное натяжение на границе раздела твердое тело – газ, что облегчает деформирование твердого тела). Добавки помогают не только разрушить материал, но и стабилизируют систему в дисперсном состоянии, т. к. , адсорбируясь на поверхности частиц, мешают их обратному слипанию. 4!}

Src="http://present5.com/presentation/3/40492240_88526628.pdf-img/40492240_88526628.pdf-5.jpg" alt="> Condensation methods are based on the association of molecules into aggregates from true solutions"> Конденсационные методы основаны на ассоциации молекул в агрегаты из истинных растворов (гомогенных сред). Путем конденсации в зависимости от условий могут быть получены системы любой дисперсности, с частицами любого размера. Эти методы в основном используют для получения дисперсных систем с размерами частиц 10 -8 – 10 -9 м (высокодисперсные и ультрадисперсные), поэтому эти методы широко используют в нанотехнологиях. Конденсационные методы не требуют затраты внешней работы. Появление новой фазы происходит при пересыщении среды, т. е. создании концентраций, превышающих равновесные. 5!}

Src="http://present5.com/presentation/3/40492240_88526628.pdf-img/40492240_88526628.pdf-6.jpg" alt="> The condensation mechanism includes stages: 1. Nucleation stage - the appearance of nuclei ( centers"> Механизм конденсации включает стадии: 1. Стадия зародышеобразования - возникновение зародышей (центров кристаллизации) в пересыщенном растворе; зародыши образуются тем легче, чем больше в растворе центров зародышеобразования (чужеродных частиц). 2. Рост зародышей. 3. Формирование слоя стабилизатора (слоя противоионов), определяющего устойчивость полученной дисперсной системы (для дисперсных систем с жидкой дисперсионной средой). 6!}

Src="http://present5.com/presentation/3/40492240_88526628.pdf-img/40492240_88526628.pdf-7.jpg" alt="> Rules for obtaining dispersed systems by condensation methods 1. The greater the degree"> Правила получения дисперсных систем конденсационными методами 1. Чем больше степень пересыщения, тем меньше радиус зародыша, тем легче он образуется. 2. Для получения мелких частиц необходимо, чтобы скорость образования зародышей была больше скорости их роста. Пересыщение можно вызвать физическим процессом или проведением химической реакции. Различают физические и химические конденсационные методы. 7!}

Src="http://present5.com/presentation/3/40492240_88526628.pdf-img/40492240_88526628.pdf-8.jpg" alt="> Chemical condensation methods Methods are based on the formation of a new phase (m.p. ."> Химические конденсационные методы Методы основаны на образовании новой фазы (м. р. с.) в результате протекания химических реакций. Для получения высокодисперсных золей концентрированный раствор одного компонента добавляют к разбавленному раствору другого компонента при постоянном перемешивании. 8!}

Src="http://present5.com/presentation/3/40492240_88526628.pdf-img/40492240_88526628.pdf-9.jpg" alt="> Examples of chemical reactions used to form colloidal systems: 1. Reactions"> Примеры химических реакций, используемых для образования коллоидных систем: 1. Реакции восстановления (получение золей Au, Ag, Pt и др. металлов). Восстановление аурата калия формальдегидом. 2 Na. Au. O 2 + 3 HCOH + Na 2 CO 3 = 2 Au + 3 HCOONa +Na. HCO 3 + H 2 O В результате получается золь золота, стабилизированный ауратом калия. Строение мицеллы этого золя можно представить: 2. Реакции обмена (метод, наиболее часто встречающийся на практике). Получение золя иодида серебра. Ag. NO 3 + KJ(изб.) = Ag. J↓ + KNO 3 Строение мицеллы: 9!}

Src="http://present5.com/presentation/3/40492240_88526628.pdf-img/40492240_88526628.pdf-10.jpg" alt="> Peptization method Peptization is a method based on translation into"> Метод пептизации Пептизация – метод, основанный на переводе в коллоидный раствор осадков, первичные размеры которых уже имеют размеры высокодисперсных систем. Суть метода: свежевыпавший рыхлый осадок переводят в золь путем обработки пептизаторами (растворами электролитов, ПАВов, растворителем). 10!}

Src="http://present5.com/presentation/3/40492240_88526628.pdf-img/40492240_88526628.pdf-11.jpg" alt="> Methods for purifying dispersed systems The resulting sols often contain low molecular weight impurities (foreign"> Методы очистки дисперсных систем Полученные золи часто содержат низкомолекулярные примеси (чужеродные электролиты), способные разрушать коллоидные системы. Полученные золи во многих случаях приходится очищать. Очищают также и дисперсные системы природного происхождения (латексы, нефть, вакцины, сыворотки и т. д.). Для очистки от примесей используют: диализ, электродиализ, ультрафильтрацию. Диализ – извлечение из золей низкомолекулярных веществ чистым растворителем с помощью полупроницаемой перегородки (мембраны), через которую не проходят коллоидные частицы. Электродиализ – диализ, ускоренный применением внешнего электрического поля. Ультрафильтрация – электродиализ под давлением (гемодиализ). 11!}

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

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

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

By replacing the solvent, sols of sulfur, phosphorus, arsenic, rosin, cellulose acetate and many organic substances are obtained by pouring alcohol or acetone solutions of these substances into water.

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

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

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

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

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

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

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

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

Preparation of silver iodide sol:

AgNO3+KIAgI+KNO3

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

Hg(CN)2+H2SHgS+2HCN

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

Reactions well known in analytical chemistry, such as the production of precipitates of barium sulfate or silver chloride

Na2SO4 + BaCl2 BaSO4 + 2NaCl

AgNO3 + NaCl AgCl + NaNO3

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

Thus, for the condensation production of sols, it is necessary that the concentration of the substance in the solution exceeds the solubility, i.e. the solution must be supersaturated. These conditions are common both for the formation of a highly dispersed sol and an ordinary solid phase sediment. However, in the first case, special conditions must be met, which, according to the theory developed by Weymarn, consists in the simultaneous appearance of a huge number of dispersed phase nuclei. The embryo should be understood as a minimal accumulation of a new phase that is in equilibrium with the environment. To obtain a highly dispersed system, it is necessary that the rate of nucleation formation be much greater than the rate of crystal growth. In practice, this is achieved by pouring a concentrated solution of one component into a very dilute solution of another with vigorous stirring.

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

There are two general approaches to obtaining disp. systems – dispersion and condensation. The dispersion method is based on grinding macroscopic particles to nanosizes (1-100 nm).

Mechanical grinding is not widely used due to its high energy consumption. In laboratory practice, ultrasonic grinding is used. During grinding, two processes compete: dispersion and aggregation of the resulting particles. The ratio of the rates of these processes depends on the duration of grinding, temperature, the nature of the liquid phase, and the presence of stabilizers (most often surfactants). By selecting optimal conditions, it is possible to obtain particles of the required size, but the particle size distribution can be quite wide.

The most interesting is the spontaneous dispersion of solid bodies in the liquid phase. A similar process can be observed for substances with a layered structure. In such structures, there is a strong interaction between the atoms inside the layer and a weak v-d-v interaction between the layers. For example, molybdenum and tungsten sulfides, which have a layered structure, spontaneously disperse in acetonitrile to form nanometer-sized bilayer particles. In this case, the liquid phase penetrates between the layers, increases the interlayer distance, and the interaction between the layers weakens. Under the influence of thermal vibrations, nanoparticles are detached from the surface of the TV phase.

Condensation methods are divided into physical and chemical. The formation of nanoparticles occurs through a series of transition states during the formation of intermediate ensembles, leading to the emergence of a nucleus of a new phase, its spontaneous growth and the appearance of a physical phase interface. It is important to ensure a high rate of embryo formation and a low rate of growth.

Physical methods are widely used to obtain ultradisperse metal particles. These methods are essentially dispersion-condensation methods. In the first stage, the metal is dispersed to atoms by evaporation. Then, due to supersaturation of the vapor, condensation occurs.

Molecular beam method used to produce coatings with a thickness of about 10 nm. The feed material in a chamber with a diaphragm is heated to high temperatures in a vacuum. The evaporated particles, passing through the diaphragm, form a molecular beam. The intensity of the beam and the rate of condensation of particles on the substrate can be changed by varying the temperature and vapor pressure above the source material.

Aerosol method consists in the evaporation of metal in a rarefied atmosphere of an inert gas at a low temperature, followed by condensation of the vapor. Using this method, nanoparticles of Au, Fe, Co, Ni, Ag, Al were obtained; their oxides, nitrides, sulfides.

Cryochemical synthesis is based on the condensation of metal atoms (or metal compounds) at low temperature in an inert matrix.

Chemical condensation. A colloidal solution of gold (red) with particle size was obtained in 1857 by Faraday. This sol is on display in the British Museum. Its stability is explained by the formation of EDL at the solid phase-solution interface and the appearance of an electrostatic component of disjoining pressure.

Often, the synthesis of nanoparticles is carried out in solution during chemical reactions. Reduction reactions are used to produce metal particles. Aluminum and borohydrides, hypophosphites, etc. are used as reducing agents. For example, a gold sol with a particle size of 7 nm is obtained by reducing gold chloride with sodium borohydride.

Nanoparticles of metal salts or oxides are obtained through exchange or hydrolysis reactions.

Natural and synthetic surfactants are used as stabilizers.

Nanoparticles of mixed composition were synthesized. For example, Cd/ZnS, ZnS/CdSe, TiO 2 /SiO 2. Such nanoparticles are obtained by deposition of molecules of one type (shell) on a previously synthesized nanoparticle of another type (core).

The main disadvantage of all methods is the wide size distribution of nanoparticles. One of the methods for regulating the size of nanoparticles is associated with the production of nanoparticles in reverse microemulsions. In reverse microemulsions, the dis phase is water and the dis medium is oil. The size of droplets of water (or other polar liquid) can vary widely depending on the production conditions and the nature of the stabilizer. A drop of water plays the role of a reactor in which a new phase is formed. The size of the resulting particle is limited by the size of the drop; the shape of this particle follows the shape of the drop.

Sol-gel method contains the following stages: 1. preparation of a starting solution, usually containing metal alkoxides M(OR) n, where M is silicon, titanium, zinc, aluminum, tin, cerium, etc., R is alkal or aryl; 2. gel formation due to polymerization reactions; 3. drying; 4. heat treatment. Hydrolysis is carried out in organic solvents

M(OR) 4 +4H 2 OM(OH) 4 +4ROH.

Then polymerization and gel formation occurs

mM(OH) n (MO) 2 +2mH 2 O.

Peptization method. There are peptization when washing the sediment, peptization of the sediment with an electrolyte; peptization with surfactants; chemical peptization.

Peptization when washing the sediment is reduced to removing the electrolyte from the sediment that causes coagulation. In this case, the thickness of the EDL increases, the forces of ionic-electrostatic repulsion prevail over the forces of intermolecular attraction.

Peptization of the sediment with an electrolyte is associated with the ability of one of the electrolyte ions to be adsorbed on particles, which promotes the formation of DES on the particles.

Peptization with surfactants. Surfactant macromolecules, adsorbed on particles, either give them a charge (ionogenic surfactants) or form an adsorption-solvation barrier that prevents particles from sticking together in the sediment.

Chemical peptization occurs when a substance added to the system reacts with a substance in the precipitate. In this case, an electrolyte is formed, which forms an DES on the surface of the particles.

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 collapse of a cavity in a liquid. The slamming 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

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

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

Dispersive	Dispersed	Dispersed	System name,
			Colloidal state is impossible
			Liquid aerosols: fog, 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
			Solid foams: pumice, polystyrene foam, cheese, bread, aerated chocolate, marshmallows
			Capillary systems: oil, fruit fillings
			Metal alloys, precious stones

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 finished 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. Their formation requires 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 disadvantage 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. The main reason for 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.

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 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 CCM (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 surfactant solutions: 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.