adsorption and absorption. Adsorption isotherms. Definition of sorption and its types CRS on the topic of adsorption

Sorption (from Latin sorbeo - I absorb, draw in) is any process of absorption of one substance (sorbtiva) by another (sorbent), regardless of the mechanism of absorption. Depending on the mechanism of sorption, adsorption, absorption, chemisorption and capillary condensation are distinguished.

Adsorption called the change in the concentration of a substance at the interface. Adsorption occurs on any interfacial surfaces, and any substances can be adsorbed. Adsorption equilibrium, i.e. the equilibrium distribution of the substance between the boundary layer and the adjacent phases is a dynamic equilibrium and is quickly established. Adsorption decreases with increasing temperature.

In some cases, the absorption of one substance by another is not limited to the surface layer, but occurs throughout the entire volume of the sorbent. This absorption is called absorption. An example of an absorption process is the dissolution of gases in liquids. The absorption of one substance by another, accompanied by chemical reactions, is called chemisorption. Thus, the absorption of ammonia or hydrogen chloride by water, the absorption of moisture and oxygen by metals with the formation of oxides and hydroxides, the absorption of carbon dioxide by calcium oxide are examples of chemisorption processes. capillary condensation consists in liquefying vapors in microporous sorbents. It occurs due to the fact that the vapor pressure over the concave meniscus of the liquid in the narrow capillaries wetted by it is less than the saturated vapor pressure over the flat surface of the liquid at the same temperature.

Thus, sorption processes are different in their mechanism. However, any sorption process begins with adsorption at the boundary of contiguous phases, which can be liquid, gaseous or solid.

As indicated in § 106, all spontaneous processes at the phase boundaries occur in the direction of decreasing free surface energy. Consequently, positive adsorption, leading to an increase in the concentration of a substance in the boundary layer, is possible only if, in this case, the surface tension decreases.

Let us consider the relationship between the surface tension of solutions and adsorption at the liquid | gas. The surface tension of solutions depends on the nature of the solvent and the solute, on the concentration of the latter, and on the temperature. The dependence of the surface tension of solutions at constant temperature on the concentration of the solute is called surface tension isotherm. Solutes or lower the surface tension of the solvent, in which case they are called surface-active substances (surfactants), or increase the surface tension (surface-inactive substances), or do not affect the surface tension of the solvent (Fig. 95). In aqueous solutions, polar organic compounds (alcohols, acids, amines, phenols) are surface-active. Most strong electrolytes are surface-inactive.

Surfactants are divided into two large subgroups: 1) truly water-soluble and 2) micellar colloids.

Surfactants of the first subgroup are amphiphilic molecules with short hydrocarbon radicals, and surfactants of the second subgroup are amphiphilic molecules with long hydrocarbon radicals, slightly soluble in water.

The difference between the concentrations of a solute in the surface layer and in the same layer inside the volume of the solution is called the surface excess of this substance and is denoted by the Greek letter G (“gam-

Rice.

Rice.

Surface layer structure: but- pure solvent; b- unsaturated monomolecular surfactant layer; in- saturated monomolecular surfactant layer

(a - surface tension, C- solution concentration): 1,2 - solutions of surface-active substances (surfactants) with greater (/) and less (2) surface activity; 3 - solution of surfactant ma"). Surfactants are positively adsorbed in the surface layer, and, consequently, for them T > 0, since this leads to a decrease in surface tension. On the contrary, surfactants are adsorbed negatively; their concentration in the surface layer is less than in the volume of the solution (G

An example of an adsorption isotherm for a surfactant is shown in fig. 96. As you can see, with an increase in the concentration of the solution, G reaches the limit value (T 00), when the entire surface layer is occupied by surfactant molecules that have displaced the solvent molecules. In such saturated monomolecular surface layers, surfactant molecules are correctly oriented - with their polar group towards the polar phase (for example, water), and with their non-polar hydrocarbon radical - towards the non-polar phase (for example, air), forming a kind of palisade.

The boundary tension changes similarly and the third component is adsorbed at the boundary of two immiscible liquids.

The adsorption of gases and vapors on the surface of solids also occurs as a result of a decrease in the free surface energy. In view of the difficulty in measuring the surface tension of solids, adsorption on them is judged by directly determining the amount of the adsorbed substance. The latter is the greater, the larger the surface of the adsorbent. Therefore, for the implementation of adsorption processes, it is very important to create highly porous adsorbents with a developed inner surface, which is characterized by a specific surface area, i.e. surface per 1 g of sorbent. The most important porous sorbents are active carbon and silica gel. The absorbing ability of coal was noticed as early as the 18th century. However, only in 1915 N.D. Zelinsky developed a method for obtaining active carbons, proposing them as universal absorbers of toxic substances, and together with E.L. Kumantom designed a coal gas mask with a rubber mask. One of the first ways to activate charcoal was to treat it with superheated steam to remove resinous substances formed during the dry distillation of wood and filling the pores in ordinary coal.

Modern methods for obtaining and studying activated carbons in our country were developed by M.M. Dubinin. The specific surface of activated carbons reaches 1000 m 2 per gram. Activated carbon is a hydrophobic adsorbent, absorbs water vapor poorly and hydrocarbons very well.

To absorb water vapor, a hydrophilic adsorbent is widely used, which is an airgel of dehydrated silicic acid and is called silica gel. The industry produces a number of grades of silica gel with different pore sizes and distributions.

Unlike the surface of liquids, not all points on the surfaces of solids are equal in terms of their adsorption capacity. At low gas concentrations, adsorption occurs monomolecularly over the most active sites of the adsorbent - its "active centers", which are individual atoms or groups of surface atoms, the force field of which is the least saturated. When gases are adsorbed at temperatures below their critical temperature, monomolecular adsorption can transform into polymolecular adsorption with increasing pressure.

An increase in temperature and a decrease in pressure lead to the desorption of gases and vapors. As a result, sorption-desorption methods are widely used in industry to extract various substances from the air, as well as to separate gases and vapors.

When solutes are adsorbed from solutions on solid adsorbents, solvents are always adsorbed to some extent. Therefore, adsorption from solutions is competitive between the absorption of solutes and the solvent. Both dissolved non-electrolytes and electrolytes can be adsorbed. In this regard, a distinction is made between molecular and ionic adsorption from solutions.

In order to reduce the adsorption of the solvent during molecular sorption from aqueous solutions, a hydrophobic adsorbent is usually used - activated carbon, and in sorption from non-polar solvents (hydrocarbons) a hydrophilic adsorbent - silica gel. Adsorption proceeds along the active centers of the adsorbent, often monomolecularly and highly selectively. Isotherms of molecular adsorption from solutions, as well as gases and vapors, have the form of the curve shown in Fig. 96. Desorption carried out with the help of liquids is usually called elution and liquids or solutions used for these purposes, eluents.

Sorption can occur under static or dynamic conditions. Sorption is called static, when the absorbed substance (sorbent), which is in the gaseous or liquid phase, is brought into contact with the immobile sorbent or mixed with it. The static activity of the sorbent is characterized by the amount of absorbed substance per unit mass of the sorbent under certain conditions.

dynamic sorption is called when the absorbed substance is in a mobile liquid or gaseous phase, which is filtered through a layer of sorbent. The dynamic activity of the adsorbent is characterized by the time from the beginning of the passage of the adsorbent to its breakthrough, i.e. before it appears behind the adsorbent layer (N.A. Shilov, 1917). In industry, sorption-desorption processes, as a rule, are carried out under dynamic conditions, since this ensures the continuity of technological processes and the possibility of their automation.

• Nikolai Dmitrievich Zelinsky (1861 - 1953) - academician, founder of a large school of organic chemists. He owns classical works in the field of organic catalysis, as well as on the chemistry of oil and the production of many valuable products from it.
• Mikhail Mikhailovich Dubinin (1901-1993) - academician, laureate of State Prizes, head of a large scientific school in the field of sorption. He made a great contribution to the development of modern ideas about the mechanism of sorption of gases and vapors, as well as methods for obtaining and studying sorbents.

ADSORPTION(from Latin ad-na, at and sorbeo-absorb), a change (usually an increase) in the concentration of matter near the surface of the phase separation ("absorption on the surface"). In the general case, the cause of adsorption is the uncompensated intermol. forces near this surface, i.e. the presence of adsorption force field. The body that creates such a field is called. adsorbent, in-in, molecules to-rogo can be adsorbed, and d sorb t and in om, already adsorbed. in-in-adsorbate. The reverse process of adsorption, called. desorption.

The nature of the adsorption forces m. very different. If these are van der Waals forces, then adsorption is called. physical, if valence (i.e., adsorption is accompanied by the formation of surface chemical compounds), - chemical, or chemisorption. Distinguish. features of chemisorption - irreversibility, high thermal effects (hundreds of kJ / mol), activated character. Between physical and chem. adsorption, there are many intermediates. cases (eg, adsorption due to the formation of hydrogen bonds). Also possible diff. types of physical adsorption max. universal manifestation of dispersion intermol. forces of attraction, since they are approximately constant for adsorbents with a surface of any chemical. nature (the so-called non-specific adsorption). Phys. adsorption can be caused by electrostatic. forces (mutually between ions, dipoles or quadrupoles); while adsorption is determined by chem. the nature of the adsorptive molecules (the so-called specific adsorption). Means. role in adsorption the geometry of the surface of the section also plays: in the case of a flat surface, they speak of adsorption on an open surface, in the case of a slightly or strongly curved surface, adsorption in the pores of the adsorbent.

In the theory of adsorption, a distinction is made between statics (the adsorbent-adsorbate system is in thermodynamic equilibrium) and kinetics (there is no equilibrium).

Adsorption statics

Because the system is in equilibrium, then the potentials of the adsorbate and adsorbate are the same; the entropy of the adsorbate due to the decrease in the mobility of molecules during adsorption is less than the entropy of the adsorbate. Therefore, with an inert adsorbent, the enthalpy is always negative, i.e. adsorption is exothermic. Taking into account the change in the entropy of the adsorbent can change this conclusion. For example, during sorption by polymers in-in, in which the polymer swells, the entropy of the latter (due to an increase in the mobility of macromolecules) can increase so much that adsorption becomes endothermic. In the future, the article considers only exothermic. adsorption .

There are integral, differential, isosteric. and average heat of adsorption. The integral heat Q is equal to the loss of enthalpy (at V \u003d const - internal energy) when adsorption changes from a 1 to a 2 (in a particular case, maybe a 1 \u003d 0): Q \u003d - (H 2 - H 1) This the value is usually referred to the mass of the adsorbent and is expressed in J/kg.

There is another mechanism leading to additional adsorption of adsorbents below their critical. t-ry on porous adsorbents at relatively high values ​​of p/p s . This is capillary condensation. If a concave adsorbate meniscus is formed in a pore, condensation begins in it at p/p s<1. Согласно ур-нию Кельвина:

where is the surface tension of the adsorbate, V is its molar volume, r is the radius of curvature of the meniscus. Capillary condensation leads to a sharp rise in the adsorption isotherm. In this case, the so-called is often (but not always) observed. adsorption hysteresis, i.e. adsorption mismatch. and desorbts. branches of the isotherm. As a rule, this is due to the fact that the shapes of the menisci do not coincide during adsorption and desorption.

Using the potential theory, M.M. Dubinin proposed and developed the theory of volumetric filling of micro-pores (TOZM). It has been postulated that this theory only applies to microporous adsorbents. A feature of such adsorbents, in which the linear dimensions of the pores are r1 nm, is that the entire volume of their pores is "filled" with adsorbents. field. Therefore, during adsorption, they are filled not in layers, but volumetrically. The value in the case under consideration is not adsorption. potential, and up to the sign of the chemical. adsorbate potential, measured from the level of chemical. potential of a normal liquid at the same t-re. The entire set of adsorbent pores is divided into three classes: micropores (r0.6 nm), mesopores (0.6 nm-20 nm) and macropores (r20 nm). Adsorption in micropores occurs according to the TOZM scheme, i.e. volumetrically, in mesopores - by the mechanism of layer-by-layer filling, completed by capillary condensation. Macropores during adsorption. equilibrium play no role.

Introducing the concept of f-tsii distribution of pore volumes on the values ​​of chemical. adsorbate potential in them, M.M. Dubinin and L. V. Radushkevich received the equation for the TOZM adsorption isotherm, which is usually written down as a trace. form:

where p, E and a 0 are parameters (a 0 \u003d a for p \u003d p s). Temperature dependence a 0:

where = -(da 0 /dT); a 0 0 \u003d a 0 at T \u003d T 0. The parameters n and E are practically independent of t-ry. In most cases, n \u003d 2. Only for cases where the initial heats of adsorption are very large, n\u003e 2. To recalculate adsorption isotherms from one adsorbent to another, it is approximately assumed that E 1 /E 2 P 1 /P \u003d and that a 01 / a 02 V 1 /V 2 , where P i is a parachor, V i is the molar volume of the adsorbent.

Using the notion that in a real adsorbent there are pores of different sizes, and introducing the distribution of E values ​​with a dispersion equal to F. Stekli, he proposed a generalization of equation (23), called the Dubinin-Stöckli equation:

Adsorption kinetics

Adsorption, like any real process, occurs in time. Therefore, a complete theory of adsorption should contain a section on the kinetics of adsorption. The elementary act of adsorption is carried out almost instantly (the exception is chemisorption). Therefore, the time dependences of adsorption are determined in the main. diffusion mechanism, i.e., the supply of an adsorbent to the place of adsorption. If adsorption on an open surface is not instantaneous, such a process occurs in an external diffusion region; in this case, the laws of diffusion are not specific to adsorption. In the case of porous adsorbents, in addition to ext. diffusion, an important role begins to play vnutr. diffusion, i.e. transfer of the adsorbent in the pores of the adsorbent in the presence of a concentration gradient in them. The mechanism of such transfer may depend on the adsorbate concentration and pore sizes.

There are molecular, Knudsen and surface (Volmer) diffusion. Molecular diffusion is carried out if the length is free. the range of molecules in the pores is less than the pore size, the Knudsen length is if this length exceeds the pore size. During surface diffusion, molecules move along the surface of the adsorbent without transition to the bulk phase. However, the values ​​of the coefficient diffusions are not the same for different diffusion mechanisms. In many cases, it is not possible to establish experimentally exactly how diffusion occurs, and therefore the so-called. effective coefficient. diffusion describing the process as a whole.

Main experimental material on the kinetics of adsorption is the so-called. kinetic curve, i.e. f-tion \u003d a / a equals \u003d f (t) where is the relative adsorption equal to the ratio of the current value of adsorption a to a equal to its value at time t. To interpret the kinetic curve in the simplest case, it is assumed that the adsorbent grain has a porous structure completely uniform in volume (this model is called quasi-homogeneous). means. improvement of the quasi-homogeneous model - the notion that each grain contains regions with larger and finer pores. Diffusion in such a grain is described by two dec. coefficients.

In the case of an open surface, taking the Langmuir model, it is easy to obtain a kinetic. adsorption level. The rate of approach to equilibrium is the difference between the rates of adsorption and desorption. Assuming, as usual in kinetics, that the rates of processes are proportional to the concentrations of reacting substances, we have:

where k ads and k dec are the rate constants respectively. adsorption and desorption. The pressure in the gas phase is assumed to be constant. When integrating this equation from t = 0 to any value of t, we get:

Hence, for f we have:= equal. So we finally have:

where k = k ads + k dec.

The effect of t-ry on the rate of adsorption is expressed by an equation similar to the Arrhenius equation. With increasing t-ry k ads exponentially increases. Because diffusion in the pores of the adsorbent is associated with overcoming activation. barriers, the temperature dependences of k ads and k des are not the same.

Knowledge of diffusion rates is important not only for the theory of adsorption, but also for calculating prom. adsorption processes. In this case, they usually deal not with individual grains of the adsorbent, but with their layers. The kinetics of the process in the layer is expressed by very complex dependencies. At each point of the layer at a given point in time, the amount of adsorption is determined not only by the type of equation of the adsorption isotherm and the laws of the kinetics of the process, but also by aero- or hydrodynamic. conditions for the flow of gas or liquid around grains. The kinetics of the process in the adsorbent layer, in contrast to the kinetics in a single grain, is called. adsorption dynamics, the general scheme for solving problems is as follows: a system of differentials is compiled. ur-tions in partial derivatives, taking into account the characteristics of the layer, the adsorption isotherm, diffusion characteristics (diffusion coefficient, types of mass transfer along the layer and inside the grains), aero- and hydrodynamic. flow features. Initial and boundary conditions are set. The solution of this system of equations, in principle, leads to the values ​​of the adsorption values ​​at a given point in time at a given point in the layer. As a rule, analytical the solution can be obtained only for the simplest cases; therefore, such a problem is solved numerically with the help of a computer.

In the experimental study of the dynamics of adsorption, a gas or liquid stream with specified characteristics is passed through the adsorbent layer and the composition of the outgoing stream is examined as a function of time. The appearance of the absorbed in-va for a layer called. breakthrough, and the time to breakthrough - the time of the protective action. The dependence of the concentration of this component behind the layer on the time called. output curve. These curves serve as the main experimental material that makes it possible to judge the patterns of adsorption dynamics.

Hardware design of adsorption processes

There are many technologies. adsorption techniques. processes. Widespread cyclic. (periodic) installations with a fixed adsorbent bed, osn. node to-rykh - one or several. adsorbers made in the form of hollow columns filled with granular adsorbent. The gas (or liquid) stream containing the adsorbed components is passed through the adsorbent bed until breakthrough. After that, the adsorbent in the adsorber is regenerated, and the gas flow is sent to another adsorber. Adsorbent regeneration includes a number of stages, of which the main one is desorption, i.e. release of previously absorbed matter from the adsorbent. Desorption is carried out by heating, depressurizing in the gas phase, displacement (for example, with sharp water

If absorption is a physical process not accompanied by other physical or chemical processes, it usually follows the Nernst distribution law:

"at equilibrium, the ratio of the concentrations of the third component in two liquid states is a constant.";

The volume of the constant K N depends on the temperature and is called the distribution coefficient. This equality is true provided that the concentrations are not too high and if the "x" molecules do not change their shape in any other of the two states. If such a molecule undergoes association or dissociation, then this equality still describes the equilibrium between "x" in both states, but only for the same form - the concentrations of all remaining forms must be calculated taking into account all other equilibria.

In many technologically important processes, chemical absorption is used instead of a physical process, such as the absorption of carbon dioxide by sodium hydroxide - such processes do not follow the Nernst distribution law.

For some examples of this effect, extraction can be considered, in which a component can be extracted from one liquid phase of a solution and transferred to another without a chemical reaction. Examples of such solutions are noble gases and osmium oxide.

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Sorption- - the general name of the phenomenon and mass transfer processes in which a solid or liquid (sorbent) absorbs a substance (sorbent) from the environment. [Usherov Marshak A. V. Concrete science: lexicon. Moscow: RIF Building Materials. 2009 ... Encyclopedia of terms, definitions and explanations of building materials

- (from Latin sorbeo I absorb) absorption by a solid or liquid of any substance from the environment. The main types of sorption are adsorption, absorption, and chemisorption. The absorbing body is called a sorbent, absorbed by a sorbate (sorbate). ... ... Big Encyclopedic Dictionary

- (from Latin sorbeo I absorb), TV absorption. body or liquid (sorbent) liquid in va or gas (sorbate) from the environment. Absorption in VA from the gas phase by the entire volume of the liquid sorbent called. absorption, absorption in the surface layer ... ... Physical Encyclopedia

Adsorption, chemisorption, absorption, sorption, absorption, chemical absorption Dictionary of Russian synonyms. sorption noun, number of synonyms: 7 absorption (5) ... Synonym dictionary

sorption- - the ability of one substance to absorb (concentrate) another. General chemistry: textbook / A. V. Zholnin Sorption is the general name for the phenomena and processes of mass transfer in which a solid or liquid absorbs a substance from ... ... Chemical terms

The process of absorption by the whole mass (absorption) or surface (adsorption) of a solid or liquid of substances from the environment. To intercellular interactions (virus cell, macrophage lymphocyte, etc.), the term is applicable in the case of adding ... ... Dictionary of microbiology

The process by which a body absorbs gases, vapors, or solutes from its environment. Includes absorption and adsorption, which may also be accompanied by chem. the interaction of the absorbed substance with the absorber ... ... Geological Encyclopedia

SORPTION- physical. chem. processes of absorption of gases, vapors and dissolved substances by solids or liquids, called (see). Distinguish the following types of S.: (see); (see), (see); capillary (see), as well as ion-exchange S., when selective ... ... Great Polytechnic Encyclopedia

AND; well. [from lat. sorbere absorb] Phys., chem. Absorption by a solid or liquid of some kind. substances from the environment. ◁ Sorption, oh, oh. C th processes. C. pump. * * * sorption (from Latin sorbeo I absorb), absorption by a solid body or ... ... encyclopedic Dictionary

sorption- ▲ absorption in direction, condensed state, out, medium sorption selective absorption by a solid or liquid to l. substances from the environment. sorbent. absorption volumetric sorption. absorbent. absorb. adsorption ... ... Ideographic Dictionary of the Russian Language

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• Ion-exchange sorption of biologically active substances, Demin A., Chernova I., Shataeva L. The monograph is devoted to the latest achievements in the field of synthesis of highly permeable polyelectrolyte networks with fractal morphology and high hydrophilic pore surface. Comparison…

The particles located on the surface of each phase form a special surface phase, the properties of which differ significantly from the properties of the internal regions of the phase. Particles located on the surface interact both with homogeneous particles and with particles of a different kind (Fig. 6.1).

Rice. 6.1. The surface layer of a substance in a condensed state

The consequence of this phenomenon is that the average energy gs of a particle located on the phase interface differs from the average energy of the same particle in the volume of the phase gv .. An important characteristic of the surface phase is the surface energy G s - the difference in the average energy of a particle located on the surface, and the particle in the volume of the phase, multiplied by the number of particles on the surface N:

G s \u003d N (g s -g v)
Under surface energy refers to the Gibbs energy (Gs) - surface formation. It is equal to the product of the specific surface energy σ and the area of ​​the interface S:

Specific surface energy (J / m 2) is equal to the work required to form a unit area of ​​the free surface. The specific surface energy is determined by the nature of a particular substance. The higher the interaction energy between particles of a substance, the higher the specific surface energy. As the temperature increases, the specific surface energy decreases. Near the critical temperature, the surface tension is zero.

The term surface energy is applied to the gas-solid interface. For the interface between condensed phases (liquid - liquid, liquid - solid), the term interfacial energy is used. For the phase boundary liquid - gas (steam) usually use the term specific surface energy, called surface tension.

Surface tension σ- the most important thermodynamic characteristic of the interface, defined as the work of reversible isothermal formation of a unit area of ​​this surface. In the case of a liquid interface, the surface tension can also be legitimately considered as the work required to increase the unit length of the free surface contour:

where ΔL is the increase in the length of the free surface contour, m;

σ – surface tension, N/m.

Due to surface tension, in the absence of external force influences, the liquid takes the form of a ball corresponding to the minimum surface area and, consequently, the smallest value of the free surface energy. One of the ways to lower the free surface energy is sorption.

Sorption(from lat. sorbeo - I absorb) - absorption by a solid or liquid of a substance from the environment. The absorbing body is called a sorbent, the substance absorbed by it is called a sorbate (or sorbtiv). Distinguish between the absorption of a substance by the entire mass of a liquid sorbent - it's absorption; surface layer of a solid or liquid sorbent is adsorption.

Absorption is the absorption of substances from a gas mixture by liquids. In engineering, absorption is usually used to extract a component from a gas mixture. Absorption improves with increasing pressure and decreasing temperature.

Adsorption is the process of concentration of a substance from the volume of phases at their interface.

An adsorbent is a substance that can adsorb another substance. An adsorbent is a substance that can be adsorbed. An adsorbate is an adsorbed substance.

Adsorption.

Sorption

Sorption(from the Latin sorbeo - I absorb, draw in) they call any process of absorption of one substance ( sorbtiva) others ( sorbent), regardless of the absorption mechanism.

Depending on the sorption mechanism, adsorption, absorption, chemisorption and capillary condensation are distinguished.

Adsorption

Adsorption is a process that occurs at the interface. It affects only the surface layers of the interacting phases, and does not extend to the deep layers of these phases.

Adsorption is the phenomenon of accumulation of one substance on the surface of another. In the general case, adsorption is a change in the concentration of a substance at the interface.

Absorption

Absorption, in contrast to adsorption, this process captures not only the interface, but spreads for the entire volume of the sorbent.

An example of an absorption process is the dissolution of gases in a liquid.

Chemisorption

Chemisorption called the absorption of one substance by another, accompanied by their chemical interaction.

capillary condensation

capillary condensation- liquefaction of steam in capillaries, cracks or pores in solids.

The phenomenon of condensation is different from physical adsorption.

Thus, sorption processes are different in their mechanism. However, any sorption process begins with adsorption at the boundary of contiguous phases, which can be liquid, gaseous or solid.

Adsorption

Recall that adsorption called the accumulation of one substance on the surface of another. In general, adsorption called the change in the concentration of a substance at the interface.

Adsorption occurs on any interfacial surfaces and any substances can be adsorbed.

Adsorption equilibrium, i.e. the equilibrium distribution of matter between the boundary layer and the adjacent phases is a dynamic equilibrium and is quickly established.

Adsorption decreases with decreasing temperature.

The absorbed substance, which is still in the volume of the phase, is called adsorbent, absorbed - adsorbate. The substance on whose surface adsorption occurs adsorbent.

Adsorption is a reversible process. The reverse process of adsorption is called desorption.

The removal of adsorbed substances from adsorbents using solvents is called elution.

Distinguish molecular And ion adsorption. This distinction occurs depending on what is adsorbed - molecules or ions of the substance.

Adsorption on the surface of liquids

Particles of substances dissolved in liquids can be adsorbed on the surface of liquids. Adsorption accompanies the process of dissolution, affecting the distribution of particles of the dissolved substance between the surface layer of the solvent and its internal volume.

In accordance with the second law of thermodynamics, the surface energy of liquids tends to a minimum. In pure solvents, this energy is reduced by reducing the surface.

In solutions, the surface energy can decrease or increase due to a change in the concentration of particles in the surface layer of the liquid.

Gibbs it was found that the distribution of the substance dissolved in the liquid occurs in such a way that the maximum reduction in surface tension is achieved.

He also proposed an equation that determines the amount of adsorption G, i.e., an excess of a substance that accumulates in 1 cm 2 of the surface layer, having a thickness of about one molecule, compared with the content of this substance in the same volume inside the liquid.

Where Δσ - change in surface tension corresponding to a change in concentration ∆C.

Value Δσ/ΔС called surface activity.

Consequently, adsorption G depends on surface activity values And concentration of substance C.

If the surface tension decreases, then adsorption G It has positive value.

positive adsorption. Surfactants.

The more the substance lowers the surface tension, the more it will accumulate in the surface layer.

The concentration of the dissolved substance in the surface layer will become much higher than in the rest of the volume of the liquid. The resulting concentration difference will inevitably cause diffusion, which will be directed from the surface layer into the liquid and will be an obstacle to the complete transition of all dissolved particles to the surface layer. A mobile adsorption equilibrium will be established between the dissolved substance in the surface layer and the rest of the volume of the liquid.

Adsorption, accompanied by the accumulation of a substance in the surface layer, is called positive. Its limit is the complete saturation of the surface layer with the adsorbed substance.

Positively adsorbing substances are also called surfactants (surfactant). In aqueous solutions, the role surfactant ov will play substances with a fatty and diphilic nature (fats, most fatty acids, ketones, alcohols, cholesterol, etc.).

negative adsorption. Surface-inactive substances.

If the solute increases the surface tension, then it will be pushed out of the surface layer into the adsorbent. This adsorption is called negative.

The limit of negative adsorption is the complete displacement of the adsorbent from the surface layer inside the adsorbent (solvent).

As a result of the concentration difference diffusion will occur, which will be directed to the surface layer. Therefore, there will always be some amount of adsorbate in the surface layer.

Substances that sharply increase surface tension are almost absent in the surface layer of dilute solutions. Only a significant increase in the concentration of such solutions leads to the movement of noticeable amounts of the dissolved substance into the surface layer, which is accompanied by an increase in surface tension.

Negatively adsorbed substances are called surface-inactive.

Adsorption and surface tension of biological fluids

Negative and positive adsorption of various substances in the blood and protoplasm of cells is of great importance for metabolism in living organisms.

The surface tension of biological fluids is significantly below than water. Therefore, hydrophobic substances, such as fatty acids, steroids, will accumulate at the walls of blood vessels, cell membranes which facilitates their penetration through these membranes.

For adsorption from aqueous solutions, the presence of polar molecules ( hydrophilic) and non-polar ( hydrophobic) groups.

So, in the molecule of butyric acid there is a polar group UNSD and a hydrophobic hydrocarbon chain:

Molecules that have both types of groups at the same time are called diphilic.

In an amphiphilic molecule with short hydrophobic chain dominated hydrophilic properties, therefore, such molecules dissolve well in water, being adsorbed negatively.

With the lengthening of the hydrocarbon chain, the hydrophobic properties of the molecules are enhanced and their solubility in water decreases.

Consequently, surfactants include amphiphilic structure substances that have a surface tension lower than the solvent, and the dissolution of which leads to positive adsorption, causing a decrease in surface tension.

Surface-inactive substances have the opposite properties.

Simultaneously with an increase in the hydrophobic properties of molecules, their surface activity increases. So chain elongation in the homologous series of fatty acids, alcohols, amines, etc. per radical –CH2– increases their ability to positive adsorption in dilute solutions in 3.2 times(Traube-Duclos rule).

Molecules of substances with a predominance of hydrophobic properties (fatty acids with a high molecular weight, etc.) are located mainly on the surface of the water, forming surface films.

With a small amount of such molecules, a surface film is not formed. If there are many molecules, then they are arranged in an orderly manner, one next to the other, and their hydrophobic parts protrude above the water surface, forming the so-called Langmuir fence.

1 - random arrangement of amphiphilic molecules;
2 - Langmuir palisade;
3 - excess of molecules;
4 - hydrophilic part of the molecules;
5 - hydrophobic part of the molecules;

Surface film It is formed by a monomolecular layer of molecules, each of which occupies a certain area on the surface of the water. The layer thickness and the area occupied by each molecule can be calculated.

Thus, fatty acid molecules having one polar group each (butyric, valeric, capric acids, etc.) occupy an area on the water surface
21 10 -16 cm 2, regardless of the length of the hydrocarbon chain.

Fatty acids with two polar groups (for example, oleic acid) occupy an area twice as large, and molecules with three polar groups (for example, tristearin) occupy three times as much area, and so on.

With an excess of a substance with predominantly hydrophobic properties, its molecules are located above the molecular film.

decompression sickness

The formation of surface films often complicates the filtration process.

At the air–water interface, a surfactant can be adsorbed in air bubbles in solution. A film of this substance forms, as it were, a shell around the bubble. Such a bubble, when pressed through narrow pores in the filter, is not capable of sharply deforming and, therefore, can clog larger holes in the filter than a bubble without a film.

Divers working at great depths sometimes experience the so-called decompression sickness. Air is supplied to their suits under pressure and, consequently, an increased amount of gases dissolves in the blood of divers.

When you ascend too quickly to the surface, the pressure in the suits drops sharply, and a significant part of the blood gases is released in the form of bubbles, on which a surface film is formed from surfactants contained in the blood.

Gas bubbles clog small blood vessels in various tissues and organs, which leads to a serious illness or even death of a person.

A similar pathology can also occur as a result of a sharp drop in atmospheric pressure during depressurization of pilots' spacesuits and aircraft cabins during high-altitude flights.

For the treatment of decompression sickness, the patient is placed in a pressure chamber, where a lot of pressure is created. Bubbles of gases again dissolve in the blood. Over several days, the pressure in the pressure chamber is slowly reduced. During this time, excess gas from the blood is just as slowly removed through the lungs, without creating blockages.

Adsorption by solids

Solids can adsorb gases and vapors, as well as molecules and ions of dissolved substances.

The nature of the forces causing adsorption

Adsorption on solids can be explained by the presence of attractive force fields that arise due to unbalanced bonds in the crystal lattice.

On the protruding sections of the solid adsorbent (on active centers), adsorption proceeds especially strongly. So ledges on a piece of coal 4.5 times absorb more oxygen, than indentations on its surface.

The adsorption forces are made up of valence interaction forces(chemical) and weaker van der Waals(physical). The role of both in different cases of adsorption is different. So, at the very beginning of the adsorption of most gases, when their pressure is low, chemical adsorption is observed. With increasing pressure, it gives way to physical, which mainly determines the adsorption of gases.

Adsorption forces can be quite large. Thus, in order to completely remove adsorbed water molecules from glass, it must be strongly heated in a vacuum.

Adsorbents, which have powerful force fields, turn out to be completely covered with adsorbed particles. At insignificant adsorption forces, only the more active centers are covered with adsorbed particles.

Adsorption is influenced not only by the nature of the adsorbent, but also by the adsorbate. So, on solid adsorbents, those gases that are easier to liquefy are adsorbed more strongly, i.e. whose critical temperature is higher.

Reversibility of adsorption

Adsorption represents reversible process. Adsorbed particles do not remain immobile. They are retained on the adsorbent for only hundredths and thousandths of a second and, being desorbed, are replaced by new particles. In addition, they are not strictly fixed on the adsorbent, but can move over its surface. As a result, set dynamic adsorption equilibrium between free and adsorbed particles.

Adsorption rate

The rate of adsorption is of great importance for the practical use of various adsorbents.

For example, in a gas mask, the air passing through the box must be very quickly cleared of impurities of toxic substances, which is possible only at high rates of adsorption processes.

It should be pointed out that activated carbon in a gas mask plays the role of not only an adsorbent for a number of toxic substances, but also a catalyst for the decomposition reactions of some of them.

In particular, activated carbon catalyzes the hydrolysis of phosgene:

COCl2 + H2O = HCl + CO2.

Temperature rise lowers physical adsorption - adsorption, since the movement of molecules in the adsorption layer increases, the orientation of adsorbed molecules is disturbed, i.e. desorption increases.

On the other hand, an increase in temperature increases the energy of adsorbed particles, which enhances chemical adsorption.

Therefore, in some cases, an increase in temperature enhances desorption, in others it increases adsorption.

Thus, for most gases, an increase in temperature reduces adsorption. At the same time, an increase in temperature from –185 to +20°С increases oxygen adsorption by platinum by a factor of 10, since chemical adsorption increases in this case.

Increasing pressure gases and vapors increases adsorption.

capillary condensation

When adsorbing vapors, the so-called capillary condensation flowing on coal and other porous adsorbents.

The liquid condensed in the capillaries forms concave meniscus over which the vapor is saturated at a lower pressure than over a flat surface. This increases the vapor condensation in the adsorbent capillaries.

Capillary condensation is especially pronounced in easily liquefied gases.

Chemisorption

During chemisorption, the substance enters into a chemical reaction with the adsorbent, for example:

O2 + 2Cu = 2CuO.

If newly formed chemisorption molecules diffuse into the depth of the adsorbent substance, then the achievement of sorption equilibrium occurs more slowly, since it depends on the diffusion rate.

If at chemisorption non-diffusing molecules appear on the surface of the sorbent; If a film is formed, it slows down and eventually stops the chemisorption process.

So, an aluminum plate, sorbing oxygen, is covered with a film of aluminum oxide, which quickly stops the chemisorption process:

4Al + 3O2 = 2Al2O3 .

Chemisorption, like any chemical reaction, can be exo- or endothermic. Therefore, an increase in temperature enhances some chemisorption processes and weakens others.

It is impossible to completely distinguish between adsorption and chemisorption. Usually these two processes proceed together.