ADSORPTION(from Latin ad-on, with and sorbeo-absorb), a change (usually an increase) in the substance near the phase interface (“absorption on the surface”). In the general case, the reason for adsorption is the lack of compensation between the molecular weights. forces near this surface, i.e. presence of adsorbents. force field. The body that creates such a field is called. , in which they can be adsorbed, and d so r b t and in about m, are already adsorbers. in-in-adsorbate. Process, reverse adsorption, called .
Nature of adsorb. strength m.b. 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. Will distinguish. features - irreversibility, high thermal effects (hundreds of kJ/), activated character. Between physical and chem. There are many gaps between adsorption. cases (for example, adsorption due to the formation). Various types are also possible. types of physical adsorption max. universal manifestation of dispersion intermolecular. forces of attraction, since they are approximately constant for any chemical surface. nature (so-called nonspecific adsorption). Phys. adsorption can be caused by electrostatic. forces (interaction between, dipoles or quadrupoles); in this case, adsorption is determined by chemical. the nature of the adsorbent (so-called specific adsorption). Means. role in adsorption The geometry of the interface also plays a role: in the case of a flat surface one speaks of adsorption on an open surface, in the case of a slightly or strongly curved surface one speaks of adsorption in pores.
In the theory of adsorption, a distinction is made between statics (the adsorbent-adsorbate system is thermodynamic) and kinetics (not).
Adsorption statics
Because the system is in equilibrium, then the chemical the potentials of the adsorbate and adsorptive are the same; adsorbate due to a decrease in mobility during adsorption of less adsorbent. Therefore, when inert, it is always negative, i.e. adsorption is exothermic. Accounting for change may change this conclusion. For example, when a substance swells, the latter (due to an increase in mobility) can increase so much that adsorption becomes endothermic. In what follows, the article considers only exothermic. adsorption .
There are integral, differential, isosteric. and average heat of adsorption. The integral heat Q is equal to the decrease (at V = const -internal energy) when adsorption changes from a 1 to a 2 (in the particular case, a 1 = 0): Q = -(H 2 - H 1) This value usually referred to as mass and expressed in J/kg.
There is another mechanism leading to complementarity. adsorption of adsorptives below their critical value. t-ry on porous at relatively high values of p/p s. This - . If a concave adsorbate has formed in a pore, then it begins 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. leads to a sharp rise in the adsorption isotherm. In this case, the so-called. adsorbts. hysteresis, i.e. adsorption mismatch and desorption. isotherm branches. As a rule, this is due to the fact that the forms do not coincide during adsorption.
Using potential theory, M.M. Dubinin proposed and developed the theory of volumetric filling of micro-pores (VFM). It has been postulated that this theory only applies to microporous ones. The peculiarity of such pores, in which the linear pore sizes are r1 nm, is that the entire volume of their pores is “filled” with adsorbents. field. Therefore, during adsorption they are filled not layer by layer, but volumetrically. The quantity in the case under consideration is not adsorb. potential, and up to the chemical sign. adsorbate potential, measured from the chemical level. at the same temperature. The entire set of 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 - according to the mechanism of layer-by-layer filling, completed. Macropores during adsorption. do not play any role.
Having introduced an idea of the distribution of pore volumes according to chemical values. adsorbate potential in them, M.M. Dubinin and L.V. Radushkevich obtained the equation of the adsorption isotherm of TOZM, which is usually written in the following. form: 
where p, E and a 0 are parameters (a 0 = a for p = p s). Temperature dependence a 0:
where= -(da 0 /dT); a 0 0 = a 0 at T= T 0. Parameters p and E are practically independent of t-ry. In most cases, n = 2. Only for cases where the initial heats of adsorption are very high, n > 2. To recalculate adsorption isotherms from one adsorbent to another, it is approximately assumed that E 1 /E 2 P 1 /P = and that a 01 / a 02 V 1 /V 2, where Pi is a parachor, Vi is the molar volume of the adsorbent.
Using the idea that in reality there are pores of different sizes, and introducing a distribution of E values with a dispersion equal to F. Steckley proposed a generalization of equation (23), called the Dubinin-Steckley equation: 
Adsorption kinetics
Adsorption, like any real process, occurs over time. Therefore, a complete theory of adsorption must contain a section on adsorption kinetics. Elementary adsorption occurs almost instantly (the exception is chemisorption). Therefore, the time dependences of adsorption are determined mainly. mechanism, i.e., supplying the adsorbent to the site of adsorption. If adsorption on an open surface is not instantaneous, such a process occurs in the external diffusion region; however, the laws are not specific to adsorption. In the case of porous ones, except for external , internal begins to play an important role. , i.e. transfer of adsorbent in pores in the presence of a gradient in them. The mechanism of such transfer may depend on the adsorbent and pore size.
There are molecular, Knudsen and surface (Volmer) methods. Molecular is carried out if the length is free. mileage in pores smaller size pore, Knudsen - if this length exceeds the pore size. During the superficial phase, they move across the surface without transitioning to the bulk phase. However, the coefficient values. not the same for different mechanisms. In plural In cases, it is not possible to establish experimentally how exactly this happens, and therefore the so-called effective coefficient , describing the process as a whole.
Basic let's experiment material on the kinetics of adsorption is the so-called. kinetic curve, i.e. function = a/a equals =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 grain has a porous structure that is completely homogeneous in volume (this model is called quasi-homogeneous). Means. An improvement on the quasi-homogeneous model is the idea that each grain contains regions with larger and finer pores. in such a grain is described by two different types. coefficients.
In the case of an open surface, adopting the Langmuir model, it is easy to obtain the kinetic. adsorption level. The rate of approach to is the difference between the adsorption rates and . Assuming, as usual in kinetics, that the rates of processes are proportional to the reacting substances, we have:
where k adc and k des - respectively. adsorption and in the gas phase is considered constant. When integrating this equation from t = 0 to any value of t, we obtain:
Hence, for f we have: = equal. Therefore, we finally have:
where k = k ads + k des.
The influence of temperature on the rate of adsorption is expressed by an equation similar to the Arrhenius equation. With increasing temperature k adc increases exponentially. Because in the pores is associated with overcoming activation. barriers, the temperature dependences of kads and kdes are not the same.
Knowing the rates is important not only for the theory of adsorption, but also for calculating industrial processes. adsorbts. processes. In this case, they usually deal not with individual grains, 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 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 of gas or liquid flow around grains. The kinetics of the process in a layer, in contrast to the kinetics in an individual grain, is called. adsorption dynamics, general scheme for solving problems, the solution is as follows: a system of differentials is compiled. equations in partial derivatives, taking into account the characteristics of the layer, adsorption isotherm, diffusion characteristics (coefficient, types of mass transfer throughout the layer and inside grains), aero- and hydrodynamic. flow features. Initial and boundary conditions are specified. The solution of this system of equations, in principle, leads to the values of adsorption values at a given time at a given point in the layer. As a rule, analytical the solution can be obtained only for the simplest cases, so this problem is solved numerically using a computer.
When experimentally studying the dynamics of adsorption, a gas or liquid flow with given characteristics is passed through the layer and the composition of the outgoing flow is studied as a function of time. The appearance of the absorbed substance behind the layer called. breakthrough, and the time before breakthrough is the time of protective action. The dependence of this component behind the layer on time is called. output curve. These curves serve as the basis. let's experiment material that allows one to judge the patterns of adsorption dynamics.
Hardware design of adsorption processes
There are many technologies. methods of adsorption. processes. Widespread cyclic (intermittent) installations with a fixed bed, basic. knot of which - one or several. , made in the form of hollow columns filled with granular. A gas (or liquid) stream containing adsorbed components is passed through the layer until it breakthroughs. After this, it is regenerated, and the gas flow is directed to another. includes a number of stages, of which the main one is desorption, i.e. isolating a previously absorbed substance from. carried out by heating, discharge in the gas phase, displacement (eg, acute water) or a combination of these methods. Since the adsorption times do not coincide, such a number of simultaneously working and regenerated ones is selected so that the overall process proceeds continuously.
According to technical and economical considerations are not followed through. Therefore the working capacity
Adsorption is a universal method that allows you to almost completely remove an impurity from a gas or liquid medium. In the chemical industry, in particular in TNV, the adsorption method is widely used for smooth cleaning and drying of process streams, improving the quality of raw materials and products, and is one of the methods for protecting the environment.
Adsorption is the concentration of substances on the surface or in the volume of a solid. At least two components are involved in the adsorption process. A solid substance on the surface or in the volume of which the absorbed substance is concentrated is called adsorbent. Absorbable substance located in gas or liquid phase called adsorptive, and after it has passed into the adsorbed state - adsorbate. Any solid substance has a surface, and therefore is potentially an adsorbent. However, in technology, solid adsorbents with a developed internal surface are used. The development of the internal surface in a solid is achieved by creating special conditions during its synthesis or as a result of additional processing.
From a thermodynamic point of view, adsorption manifests itself with a decrease in the Gibbs free energy (G). Like all processes accompanied by a decrease in the Gibbs energy, adsorption is a spontaneous process. The transition of a substance from the gas or liquid phase to the adsorbed state is associated with the loss of at least one degree of freedom (three-dimensional bulk gas or liquid phase two-dimensional surface phase), which leads to a decrease in the entropy of the system (S). Since enthalpy (H) is related to the Gibbs energy and entropy by the equation H = G + TS, it decreases during the adsorption process, and therefore, adsorption is an exothermic process.
Adsorption phenomena are divided into two main types: physical adsorption and chemisorption (sorption based on chemical interaction forces). Physical adsorption caused by molecular interaction forces: dispersion and electrostatic. Dispersion forces make the main contribution to the interaction energy of molecules. Thus, the molecules of any adsorbent have fluctuating dipoles and quadrupoles, causing instantaneous deviations of the electron density distribution from the average distribution. When the adsorbent molecules approach the atoms or molecules of the adsorbent, the movement of the fluctuating dipoles acquires a systematic and strictly ordered character, which leads to the emergence of attraction between them. In some cases, dispersion forces are enhanced by electrostatic forces - orientational and inductive. Orientation forces arise from the interaction of polar molecules with a surface containing electrostatic charges (ions, dipoles), and induction forces are caused by a change in the electronic structure of the adsorbent and adsorbent molecules under the influence of each other.
Unlike physical adsorption, when chemisorption The individuality of the adsorbent and the adsorbent is not preserved. When the adsorbent molecules approach the surface of the adsorbent, a redistribution of electrons of the interacting components occurs with the formation chemical bond. If physical adsorption can be compared with condensation, then chemisorption is considered as a chemical process occurring at the interface.
Physical adsorption and chemisorption can be distinguished based on the numerical value of the heat of adsorption. The heat of adsorption of the components of industrial gases is comparable to the heat of their condensation and does not exceed 85-125 kJ/mol. The heat of chemisorption of one mole of a substance reaches several hundred kJ. Chemisorption, as a rule, occurs at a low speed; this circumstance is often used to recognize it. In addition, chemisorption can occur at high temperatures, when physical adsorption is negligible. Chemosorption is characterized by a sharp, abrupt change in the absorption capacity of the extracted component during the transition from an adsorbent of one chemical nature to an adsorbent of another nature. During chemisorption, adsorbed molecules cannot move along the surface of the adsorbent, their position is fixed, and such adsorption is called localized. Physical adsorption can be either localized or non-localized. Typically, as the temperature increases, the molecules become mobile and the nature of the process changes: localized adsorption becomes nonlocalized.
Adsorption.
Sorption
Sorption(from the Latin sorbeo - absorb, draw in) is 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 This is a process that occurs at the interface. It affects only the surface layers of 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 general, adsorption is a change in the concentration of a substance at the interface.
Absorption
Absorption, unlike adsorption, this is a process that involves not only the phase interface, but also propagates for the entire volume of the sorbent.
An example of an absorption process is the dissolution of gases in a liquid.
Chemisorption
Chemisorption is 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 differ in their mechanism. However, any sorption process begins with adsorption at the boundary of contacting phases, which can be liquid, gaseous or solid.
Adsorption
Let us remind you that adsorption is the phenomenon of accumulation of one substance on the surface of another. In general, adsorption call the change in the concentration of a substance at the interface.
Adsorption occurs on any interphase 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 adsorptive, absorbed - adsorbate. Substance on the surface of which 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 ionic 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 dissolution process, affecting the distribution of solute particles 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 decreases by reducing the surface area.
In solutions, surface energy can decrease or increase due to changes in the concentration of particles in the surface layer of the liquid.
Gibbs it was found that the distribution of a substance dissolved in a liquid occurs in such a way that a maximum decrease in surface tension is achieved.
He also proposed an equation that determines the amount of adsorption G, that is, an excess of a substance accumulating in 1 cm 2 of a surface layer having a thickness of approximately one molecule, compared to the content of this substance in the same volume inside the liquid.
Where Δσ - change in surface tension corresponding to a change in concentration ΔС.
Magnitude Δσ/ΔС called surface activity.
Hence, 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 a substance lowers surface tension, the more it will accumulate in the surface layer.
The concentration of the dissolved substance in the surface layer will become significantly higher than in the rest of the liquid volume. The resulting difference in concentrations 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 liquid volume.
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 substances with a fatty and diphilic nature (fats, most fatty acids, ketones, alcohols, cholesterol, etc.) will play a role.
Negative adsorption. Surface-inactive substances.
If the solute increases surface tension, 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 into the adsorbent (solvent).
As a result of the difference in concentrations diffusion will occur, which will be directed to the surface layer. Therefore, there will always be some amount of adsorbent in the surface layer.
Substances that sharply increase surface tension are almost not contained 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 dissolved substance into the surface layer, which is accompanied by an increase in surface tension.
Negatively adsorbing substances are called surface-inactive.
Adsorption and surface tension of biological fluids
Negative and positive adsorption 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 and steroids, will accumulate at vessel walls, cell membranes , which facilitates their penetration through these membranes.
For adsorption from aqueous solutions great importance molecules have polar ( hydrophilic) and non-polar ( hydrophobic) groups.
So, in the butyric acid molecule there is a polar group UNS and hydrophobic hydrocarbon chain:
Molecules that simultaneously possess both types of groups are called diphilic.
In a diphilic molecule with short hydrophobic chain prevail hydrophilic properties, therefore, such molecules dissolve well in water, being adsorbed negatively.
As the hydrocarbon chain lengthens, the hydrophobic properties of the molecules increase and their solubility in water decreases.
Consequently, surfactants include substances with a diphilic structure, which have a lower surface tension than the solvent, and the dissolution of which leads to positive adsorption, causing a decrease in surface tension.
Surfactants have the opposite properties.
Simultaneously with the increase in the hydrophobic properties of molecules, their surface activity increases. Thus, chain elongation in the homologous series of fatty acids, alcohols, amines, etc. by radical –CH2– increases their ability for positive adsorption in dilute solutions in 3.2 times(Traube-Duclos rule).
Molecules of substances with predominant hydrophobic properties (fatty acids with high molecular weight, etc.) are located mainly on the surface of the water, forming surface films.
With a small number 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 palisade.
1 - random arrangement of amphiphilic molecules;
2 - Langmuir palisade;
3 - excess molecules;
4 - hydrophilic part of the molecules;
5 - hydrophobic part of the molecules;
Surface film is formed by a monomolecular layer of molecules, each of which occupies a certain area on the surface of the water. The thickness of the layer and the area occupied by each molecule can be calculated.
Thus, fatty acid molecules with 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 twice the area, and molecules with three polar groups (for example, tristearin) occupy three times the area, etc.
When there is an excess of a substance with predominantly hydrophobic properties, its molecules are located above the molecular film.
Caisson disease
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. The film of this substance forms a kind of shell around the bubble. Such a bubble, when pressed through narrow pores in the filter, is not capable of sharp deformation and therefore can clog larger holes in the filter than a bubble without film.
Divers working at great depths sometimes experience the so-called decompression sickness. Air is supplied to their spacesuits under pressure and, therefore, an increased amount of gases dissolves in the divers’ blood.
If you rise to the surface too quickly, the pressure in the spacesuits drops sharply, and a significant portion of blood gases are released in the form of bubbles, on which a surface film is formed from surfactants contained in the blood.
Gas bubbles clog small vessels in various tissues and organs, which leads to severe illness or even death of a person.
A similar pathology can occur as a result of a sharp fall atmospheric pressure during depressurization of pilots’ spacesuits and aircraft cabins during high-altitude flights.
To treat decompression sickness, the patient is placed in a pressure chamber where high pressure is created. Gas bubbles dissolve again in the blood. Over the course of 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 arising due to unbalanced bonds in the crystal lattice.
On protruding areas of a solid adsorbent (on active centers), adsorption is especially strong. So projections on a piece of coal 4.5 times adsorb oxygen more intensively, than recesses on its surface.
Adsorption forces are composed of valence interaction forces(chemical) and weaker van der Waals(physical). The role of both in different cases of adsorption is different. Thus, 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, to completely remove adsorbed water molecules from glass, it must be strongly heated in a vacuum.
Adsorbents, with powerful force fields, turn out to be completely covered with adsorbed particles. With insignificant adsorption forces, only the more active centers are covered by adsorbed particles.
Adsorption is influenced not only by the nature of the adsorbent, but also by the adsorbent. Thus, on solid adsorbents, those gases that liquefy more easily are more strongly adsorbed, i.e. whose critical temperature is higher.
Reversibility of adsorption
Adsorption represents reversible process. Adsorbed particles do not remain stationary. They are retained on the adsorbent for only hundredths and thousandths of a second and, when desorbed, are replaced by new particles. In addition, they are not strictly fixed on the adsorbent, but can move along its surface. As a result, it is established 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 only possible at high speeds of adsorption processes.
It is necessary to point 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 + H2 O = HCl + CO2.
Temperature increase reduces physical adsorption adsorption, since this increases the movement of molecules in adsorption layer, the orientation of the adsorbed molecules is disrupted, i.e. desorption increases.
On the other hand, an increase in temperature increases the energy of adsorbed particles, which enhances chemical adsorption.
Consequently, in some cases, an increase in temperature enhances desorption, in others it increases adsorption.
Thus, for most gases, increasing temperature reduces adsorption. At the same time, an increase in temperature from –185 to +20°C increases the adsorption of oxygen by platinum by 10 times, since chemical adsorption increases.
Increased pressure gases and vapors increases adsorption.
Capillary condensation
During vapor adsorption, the so-called capillary condensation flowing on coal and other porous adsorbents.
The liquid condensed in the capillaries forms concave meniscus, above which the vapor becomes saturated at a lower pressure than above a flat surface. This increases the condensation of vapors in the capillaries of the adsorbent.
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 during chemisorption Since molecules diffuse deep into the adsorbent substance, the achievement of sorption equilibrium occurs more slowly, since it depends on the rate of diffusion.
If at chemisorption Non-diffusing molecules appear on the surface of the sorbent, i.e. If a film is formed, it slows down and eventually stops the chemisorption process.
Thus, an aluminum plate, sorbing oxygen, is covered with a film of aluminum oxide, which quickly stops the chemisorption process:
4Al + 3O2 = 2Al2 O3.
Chemisorption, like any chemical reaction, May be exo- or endothermic. Consequently, 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 occur together.
Process spontaneous concentration of gases or dissolved substances at the interface is called adsorption. Depending on the nature of the contacting phases, adsorption at the boundaries is distinguished: gas - solid, gas - liquid, liquid - solid and liquid - liquid.
Back in 1785, the Russian scientist T.E. Lovitz discovered the ability of coal to absorb dissolved matter. Since then, many works have been devoted to the study of adsorption phenomena, among which the works of Russian scientists are of paramount importance: Academician N.D. Zelinsky, who proposed coal as a universal means of protection against gaseous toxic substances; M.S. Tsvet, who developed a chromatographic method for separating substances according to their adsorption ability; Academician K.K. Giedroyets, who created the theory of the absorption capacity of soils; Academician M.M. Dumansky, who developed a method for producing active adsorbents. Foreign scientists Gibbs, Langmuir, Freundlich, Polyani, Branauer and others did a lot to develop the theory and practice of adsorption.
Adsorption is a consequence of a decrease in the unsaturation of molecular, atomic or ionic forces at the interface and is caused by the accumulation of a substance that reduces the free surface energy. Adsorption is a spontaneous process, because As a result of the adsorption process, the free surface energy decreases, and according to the second law of thermodynamics, such processes are spontaneous.
Substances that are adsorbed are called adsorbates(sometimes - adsorbents), and substances that adsorb on their surface - adsorbents.
Depending on the nature of the forces acting between the particles (molecules, atoms, ions) of the adsorbate and adsorbent, they distinguish physical or van der Waals adsorption and chemical or chemisorption.
The nature of adsorption can be established by studying its kinetics and energy. Indeed, physical adsorption occurs under the influence of relatively weak intermolecular cohesion forces (van der Waals forces) and is similar in nature to the processes of condensation of adsorbate vapor, its heat is close to the heat of condensation and amounts to 10 - 50 kJ/mol. Therefore, as temperature increases, physical adsorption decreases.
Chemisorption is associated with the overlap of electronic orbitals of adsorbate and adsorbent particles, i.e. is caused by their chemical interaction, which does not, however, lead to the formation of a bulk phase. The heat of chemisorption is comparable to the heat of chemical reactions and is usually 60 - 600 kJ/mol. Chemical adsorption increases with increasing temperature.
Adsorption is a reversible process. The reverse process of adsorption is called desorption.
Distinguish molecular and ionic chemisorption depending on what is adsorbed - molecules or ions of the substance. In turn, ionic adsorption is divided into exchange and adsorption of potential-determining ions.
Exchange adsorption. Exchange adsorption occurs at the solid/electrolyte solution interface and consists in the fact that the adsorbent and the solution exchange cations or anions with each other in equivalent quantities, due to which the principle of electrical neutrality of the electrolyte solution and the adsorbent remains intact.
The main factors of exchange adsorption that determine its specificity are: the presence of a double electrical layer on the surface of the solid adsorbent, valence, radius and degree of hydration of the ions of the electrolyte solution.
Exchange adsorption proceeds somewhat slower than usual.
To understand the process of exchange adsorption, we can consider the process of formation of a double electric layer during the interaction of a silver chloride particle with a solution of potassium chloride. Chlorine ions, colliding with n particles, will combine with silver ions, forming a firmly held layer of C1 ions, thereby charging the surface of the particle. Such ions are called potential-determining, and because added C1 - ions increase their concentration, i.e. are adsorbed on the surface, then this type of adsorption is called adsorption potential-determining ions.
Adsorbed C1 - ions charge the particle negatively, and under the influence of electrostatic forces of attraction the number of K + ions adjacent to the surface of the particle will increase. In other words, adsorption of counterions will occur under the influence of electrostatic forces. Since K + ions can be replaced by other ions of the same sign, interacting with the particle only electrostatically, such ions are called exchangeable, their adsorption is exchange.
Thus, exchange adsorption occurs in the process of exchange of ions of the double electric layer of the adsorbent and solution ions. This can be represented schematically by the following equations:
Adsorbent - ½H + + Na + + Cl - à Adsorbent - ½Na + + H++Cl -
Adsorbent + ½OH - + Na + + Cl - à Adsorbent + ½Cl - + Na + + OH-
From the above diagram it can be seen that during the adsorption of ions, the pH of the medium can change (H + or OH - ions pass into the solution), the solution acquires an acidic or alkaline reaction, this type of adsorption is called hydrolytic.
Since exchange adsorption is chemical, the exchange of ions occurs in strictly equivalent ratios.
Exchange ions on a solid surface have a certain magnitude and sign of charge, therefore, in order not to disrupt the electrical double layer (EDL), exchange ions from the solution can only be ions of the same sign. In this case, the magnitude of the surface charge should not change. Thus, exchange adsorption can only be anion exchange or cation exchange.
The phenomena of exchange adsorption play an important role in the processes occurring in soils. The exchange complex of soils is the soil absorption complex (SAC), consisting of colloidal particles that are negatively charged. The exchangeable ions in soil are cations. The most important properties soil: water permeability, moisture capacity, swelling, structure, pH of the soil solution, etc. - are determined by the composition of adsorbed ions. For example, soils containing a significant amount of sodium ions in the composition of exchangeable cations acquire special, so-called “solonetz properties.” They are characterized by high dispersion, dense composition, high alkalinity, increased swelling and viscosity, and low water permeability. These soils are difficult to cultivate and, despite the large supply of nutrients, are not very fertile. If the exchangeable cations of the soil include predominantly calcium ions, then such soils have a good structure, low atomization, and good water and air permeability. These soils are among the most fertile. An example of soils with a high content of exchangeable calcium and excellent physicochemical properties are chernozem soils.
Ion exchange processes in soils can be represented by the following diagram:
[PPK] - 2Na + + Ca 2+ + SO 4 2- = [PPK] - Ca 2+ + Na 2 SO 4
B.P. Nikolsky and E.N. Gapon proposed an equation describing exchange adsorption:
Here g 1 and g 2 are the number of g-mol (g-eq) of adsorbed and desorbed ions per unit mass of the adsorbent, and 1 and a 2 are the activities of exchanging ions in solution at equilibrium; z 1 and z 2 are the charge of ions, K is the constant of this adsorption process.
The adsorption phenomenon is widely used in industry and agriculture. Thus, activated carbon is used for adsorption purification (refining) of sucrose syrup. It is the adsorption forces that hold the ions mineral fertilizers(K +, PO 4 -3, etc.) and molecules (urea) in the soil. Urea adsorption is physical; its molecules are weakly retained by the soil. Therefore, urea is usually added in the spring to prevent it from being carried away by spring floods. Potassium fertilizers can be applied to the soil in the fall, since the adsorption of K + ions is caused by chemical forces ( ionic bonds) and it is durable.
In general, adsorption is a function of pressure P (for gases) or concentration C (for liquid solutions) and temperature, i.e. is represented by a plane in coordinates Г = f(C,T). Typically, one of the parameters is kept constant and the adsorption is graphically depicted as curves.
The quantitative relationship established between the adsorbent and the adsorbent at a constant temperature in the form of an equation or curve is called adsorption isotherm.
There are several types of adsorption isotherms - the simplest equations to describe adsorption are the equation Freundlich and equation Langmuir.
Freundlich adsorption isotherm. The adsorption of a dissolved substance on a solid surface follows a certain pattern, according to which the concentration of the adsorbed substance does not increase proportionally to its concentration in the solution, but much more slowly, and is proportional to the nth root of the solution concentration. This dependence at constant temperature can be represented by the following equation:
X/m = K C 1/ n
Where X– amount (mol) of substance adsorbed m g of adsorbent: C - equilibrium concentration; K and 1/n are empirical constants characteristic of the adsorbent and adsorbate data; the value of 1/n ranges between 0.1 – 0.7. This equation is known as adsorption isotherms and has the shape of a parabola.
To graphically plot the Freundlich adsorption isotherm, the equilibrium concentration in mmol/l is plotted on the abscissa axis, and the adsorption value per unit surface X/m in mmol/gram is plotted on the ordinate axis. Figure 7 shows graphic image Freundlich equations.
Basic Concepts
The absorbed substance, which is still in the volume of the phase, is called adsorptive, absorbed - adsorbate. In a narrower sense, adsorption is often understood as the absorption of an impurity from a gas or liquid. solid(in the case of gas and liquid) or liquid (in the case of gas) - adsorbent. In this case, as in the general case of adsorption, the impurity is concentrated at the adsorbent-liquid or adsorbent-gas interface. The reverse process of adsorption, that is, the transfer of a substance from the interface to the volume of the phase, is called desorption. If the rates of adsorption and desorption are equal, then we speak of the establishment adsorption equilibrium. In a state of equilibrium, the number of adsorbed molecules remains constant indefinitely, if external conditions (pressure, temperature and composition of the system) remain unchanged.
Adsorption and chemisorption
At the interface between two phases, in addition to adsorption, which is mainly caused by physical interactions (mainly van der Waals forces), a chemical reaction can occur. This process is called chemisorption. A clear division into adsorption and chemisorption is not always possible. One of the main parameters by which these phenomena differ is the thermal effect: thus, the thermal effect of physical adsorption is usually close to the heat of liquefaction of the adsorbate, the thermal effect of chemisorption is much higher. In addition, unlike adsorption, chemisorption is usually irreversible and localized. An example of intermediate options that combine features of both adsorption and chemisorption is the interaction of oxygen on metals and hydrogen on nickel: at low temperatures they are adsorbed according to the laws of physical adsorption, but as the temperature rises, chemisorption begins to occur.
Similar phenomena
The previous section discussed the case of a heterogeneous reaction occurring on a surface—chemisorption. However, there are cases of heterogeneous reactions throughout the entire volume, and not just on the surface - this is a common heterogeneous reaction. Absorption throughout the entire volume can also occur under the influence of physical forces; this case is called absorption.
Physical adsorption
| Physical adsorption models | |
| Monolayer formation | Energy diagram |
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| Rice. 1: a) adsorbent, b) adsorbate, c) adsorbent (gas phase or solution) | Rice. 2: a) adsorbent, b) adsorbate, c) gas phase, d - distance, E - energy, E b - adsorption energy, (1) desorption, (2) adsorption |
| Polycondensation | Selective adsorption |
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| Rice. 3: a) adsorbent, b) adsorbate, c) condensate, d) adsorbent (gas phase or solution) | Rice. 4: a) adsorbent, b) adsorbate, c) adsorptive agents (gas phase or solution): preferential adsorption of blue particles is shown |
The cause of adsorption is nonspecific (that is, independent of the nature of the substance) van der Waals forces. Adsorption, complicated by chemical interaction between the adsorbent and the adsorbate, is special occasion. Phenomena of this kind are called chemisorption And chemical adsorption. “Ordinary” adsorption in the case when it is necessary to emphasize the nature of the interaction forces is called physical adsorption.
Physical adsorption is a reversible process; the equilibrium condition is determined by equal adsorption rates of adsorbent molecules P on vacant areas of the adsorbent surface S* and desorption - release of the adsorbate from the bound state S−P:
;The equilibrium equation in this case is:
, ,where is the fraction of the surface area of the adsorbent occupied by the adsorbate, is the Langmuir adsorption coefficient, and P is the concentration of the adsorbent.
Since and, accordingly, , the adsorption equilibrium equation can be written as follows:
The Langmuir equation is one form of the adsorption isotherm equation. The equation of adsorption isotherm (the abbreviated term adsorption isotherm is more often used) refers to the dependence of the equilibrium value of adsorption on the concentration of the adsorbent a = f (C) at a constant temperature ( T=const). The concentration of the adsorbent for the case of adsorption from a liquid is expressed, as a rule, in moles or mass fractions. Often, especially in the case of adsorption from solutions, a relative value is used: C/C s, where C is the concentration, C s is the limiting concentration (saturation concentration) of the adsorbent at a given temperature. In the case of adsorption from the gas phase, the concentration can be expressed in absolute pressure units, or, which is especially typical for vapor adsorption, in relative units: P/P s, where P is the vapor pressure, P s is the pressure saturated vapors of this substance. The adsorption value itself can also be expressed in units of concentration (the ratio of the number of adsorbate molecules to total number molecules at the interface). For adsorption on solid adsorbents, especially when considering practical problems, the ratio of the mass or amount of absorbed substance to the mass of the adsorbent, for example mg/g or mmol/g, is used.
Adsorption value
Adsorption is a general and ubiquitous phenomenon that occurs always and wherever there is an interface between phases. Greatest practical significance has the adsorption of surfactants and the adsorption of impurities from gas or liquid with special highly effective adsorbents. A variety of materials with a high specific surface area can act as adsorbents: porous carbon (the most common form is activated carbon), silica gels, zeolites, as well as some other groups of natural minerals and synthetic substances.
The installation for carrying out adsorption is called an adsorber.
see also
- Nitrogen adsorption plants
Notes
Literature
- Frolov Yu. G. Course of colloid chemistry. Surface phenomena and disperse systems. - M.: Chemistry, 1989. - 464 p.
- Keltsev N.V. Fundamentals of adsorption technology. - M.: Chemistry, 1984. - 592 p.
- Greg S., Singh K. Adsorption, specific surface area, porosity. - M.: Mir, 1984. - 310 p.*
- Adamson A. Physical chemistry of surfaces. – M.: Mir. 1979. – 568 p.
- Oura K., Lifshits V.G., Saranin A.A. et al. Introduction to surface physics / Ed. V. I. Sergienko. - M.: Nauka, 2006. - 490 p.
- Karnaukhov A.P. Adsorption. Texture of dispersed and porous materials. - Novosibirsk: Science. 1999. - 470 p.
- Chemical encyclopedia. T. 1. - M.: Soviet Encyclopedia, 1990. - 623 p.
- Poltorak O.M. Thermodynamics in physical chemistry. - M.: graduate School, 1991. - 319 p.
Links
- // Encyclopedic Dictionary of Brockhaus and Efron: In 86 volumes (82 volumes and 4 additional ones). - St. Petersburg. , 1890-1907.