ADSORPTION

From Big Medical Encyclopedia

ADSORPTION (adsorptio; lat. ad — on, at and sorbeo — I absorb) — absorption of gases, vapors or solutes a surface of a solid or liquid. The body, on a surface to-rogo occurs And., call adsorbent, the adsorbed substance — adsorbtivy or an adsorbate. And. gases solids (wood charcoal) it was opened by the Swedish chemist Sheele (Page W. Scheele) in 1773 and irrespective of it Italian F. Foiitana in 1777; adsorption of substances from solutions wood charcoal — the Russian chemist academician T. E. Lovits in 1785. Opportunity And. on the surfaces of liquid and communication of this phenomenon with surface intention for the first time were theoretically proved by the American mathematician and the physicist J. W. Gibbs in 1876. Afterwards it was shown that And. treats extremely widespread phenomena and it is found almost everywhere where gases, vapors and solutes are in contact with the surface of liquid or a solid.

Distinguish physical and chemical A. V the first case the adsorbed molecules keep the identity; at chemical And., the chemisorption called also, the molecule of the adsorbed substance enters a chemical bond with the surface of adsorbent. Physical And., to-ruyu usually call simply And., represents the reversible process which is coming to an end with establishment of the adsorption equilibrium at Krom speed And. it is equal to the speed of reverse — a desorption. Chemisorption is, as a rule, irreversible.

And. often is followed absorption (see), chemisorption and steam condensation of an adsorbtiv in a time of adsorbent. In all these cases complex process of absorption is called sorption (see).

Size A. on the surface of the liquid adjoining on gas or with other liquid, directly do not measure, and calculate by means of Gibbs's equation:

where — amount of adsorbate in moths on 1 cm 2 the surfaces of liquid, With — molar absorbate concentration in liquid, R — a gas constant from Klapeyron's equation — Mendeleyeva, T — absolute temperature, Δσ — the change of surface intention corresponding to change of absorbate concentration on ΔС.

Size A. gas or solute on solid adsorbent measure directly and express in moths of adsorbate on 1 g of adsorbent. The amount of adsorbed gas is usually determined by an additional weight of the adsorbent which absorbed gas; size A. solute — on reduction of its concentration in solution.

Dependence of size A. at a constant temperature from equilibrium concentration of an adsorbtiv, expressed graphically or as the equation, call an isotherm And. In fig. the typical experimental isotherm is presented And. At very small concentration size A. it is proportional to concentration that finds the expression in the rectilinear course of an isotherm And. in the field of small concentration. With the further growth of concentration growth of size A. it is slowed down. At very big concentration size A. reaches limiting value that is explained by saturation of a surface of adsorbent the molecules of an adsorbtiv forming the monomolecular adsorption layer. When the adsorption layer polimolekulyaren or And. it is complicated by capillary condensation (see. Sorption ), the isotherm of absorption has more difficult appearance.

In practice for analytical expression of an isotherm And. at average values of concentration usually use Friendlich's equation:

= kcn,

where and — size A. in moths on 1 g of adsorbent, with — concentration in moths on 1 l, k and n — constants. The constant k is in number equal to size A. at the concentration equal 1; size defines it height of a curve over abscissa axis and changes over a wide range depending on the nature of adsorbent and an adsorbtiv. Constant P defines curvature of an isotherm; its numerical size changes with the nature of adsorbent and an adsorbtiv a little and fluctuates ranging from 0,1 to 0,6. Friendlich's equation graphically represents a parabolic curve (fig).

Fig. Adsorption isotherm: a — the size of adsorption; with — equilibrium concentration; asub ?/sub — a yield value of adsorption; 0 — 1 — area of small concentration; 1 — 2 — area of average concentration; 2 — 3 — area of big concentration

To wider area of concentration apply Langmyur's equation more often:

where and and with have the same values, as in Friendlich's equation, and ∞ and α — constants which numerical values depend by nature adsorbent and an adsorbtiv. The constant ah is equal to a yield value And., the surface of an absorber corresponding to full saturation by molecules of an adsorbtiv. The constant α is equal to the relation of a kinetic constant of a desorption to a kinetic constant And.

In addition to absorbate concentration, size A. depends by nature adsorbent and an adsorbtiv; And. increases with increase in a specific surface of an absorber. For organic matters of the same homologous series size A. increases approximately by 3 times with increase in a hydrocarbon chain at one SN group 2 (Dyuklo's rule — Traube). Size A., as a rule, decreases with temperature increase. Size A. from solutions depends by nature solvent: the better substance is dissolved in this solvent, the worse it from it is adsorbed.

And. from solutions of strong electrolytes subdivide into the following types: equivalent And., at a cut cations and anions of electrolyte are adsorbed in equivalent quantities; selective And. — in this case either cations, or anions of electrolyte are selectively adsorbed; the surface of adsorbent at the same time gets respectively positive or negative electric charge. Ion exchange adsorption represents exchange of cations or anions between adsorbent and solution (see. Ionites ).

In the saturated adsorption layer of a molecule of an adsorbtiv are oriented, napr, on a surface of aqueous solutions polar groups of the adsorbed molecules are shipped in solution. Idea of orientation of molecules played a big role during the studying of structure of biological membranes in the saturated adsorption layer.

Practical value of the adsorptive phenomena is very big. And. is the cornerstone of dyeing of fabrics, a tanning of skin etc. And. is an initial stage of process of absorption of any substances cells and fabrics. S A. also processes of interaction of enzymes are closely connected with substrates, antibodies with antigens, polarization and depolarization of biological membranes.

And. plays the main role at course of many catalytic reactions (see. Catalysis ) and in chemistry of colloidal solutions.

A number of adsorbents (coal, a kaolin, ferric hydroxide, ion-exchange resins and others) apply as antidotes (see) for binding of the poisons and toxins which got in went. - kish. a path, and also for And. intestinal gases at meteorism (see). Activated charcoal is used as adsorbent in military and industrial gas masks. In the equipment A. apply to purification of substances, including and medicinal, regenerations of valuable volatile solvents, to division and the analysis of complex mixtures (see. Chromatography ).

See also Surface intention .



Bibliography: Adam N. K. Fizika and chemistry of surfaces, the lane with English, M. — L., 1947; Bladergryon V. Physical chemistry in medicine and biology, the lane with it is mute., M., 1951; Brunauer S. Adsorption of gases and vapors, the lane with English, t. 1, M., 1948, bibliogr.; Mac Ben D. V. Sorption of gases and vapors solids, the lane with English, M. — L., 1934.

V. P. Mishin.

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