**SURFACE INTENTION** — force operating per acre interfaces of two phases which are in balanced state, and in number equal to the work spent for formation of unit of area of such surface. Any biol, system, including. cell (see), is a heterogeneous system and differs in a large number of interfaces, generally in a look biol, membranes (see. Membranes biological ), therefore studying of patterns of change of P. of N on the interphase boundary in biol, systems is normal also at funkts, disturbances or under the influence of pharmaceuticals and toxic connections finds application in biol, and medical researches.

The concept «surface tension» appeared in scientific literature at the beginning of 19 century thanks to Jung's works (T. Young); P. of N is detailed Van der J. D. van der Waals and representatives of his school studied. The essential contribution to studying of the superficial phenomena was made also by J. Gibbs, J. Du-claux, E. Guggenheim, S. Kondo, P.A. Rehbinder, B. V. Deryagin, etc.

Pct usually designate grech, a letter σ, units of P. of N in the SI system — J/m^{ 2 } or N/m, in the SGS system — an erg/cm^{ 2 } or dynes / see.

Under the influence of P. of N. liquid (see) aims to reduce the surface and if influence of gravity is not enough (e.g., at reduction of the size of drops or at immersion of the studied liquid in other liquid of similar density), then the studied liquid takes the form of a sphere, i.e. the body possessing the minimum surface at this volume.

The item of N on border of two liquids, called by interfacial tension, decreases with increase of mutual solubility of these liquids. In case of two beyond all bounds the mutually dissolved liquids P. of N it is equal to zero.

Unlike a pure liquid the structure of a surface layer in solution differs from composition of solution in the depth of volume: in a surface layer that from components of solution which causes P.'s reduction N is adsorbed. It is counteracted by process diffusions (see), promoting alignment of concentration in all parts of volume of solution. A result of these multidirectional processes is establishment of balance (see. chemical equilibrium ). In colloid solutions and solutions of high-molecular compounds equilibrium time 10^{ 4 } — 10^{ 5 } sec., whereas in pure liquids apprx. 10^{ - 4 } sec.

At dissolution of any substance in its P.'s liquid the N changes. Solute can raise usually insignificant P. of N of liquid; the substances having this property call superficial and inactive. On otnoshensho to water such substances are inorganic salts, sugar, polyatomic alcohol, alpha amino acids. Those substances which lower Pct of liquid-solvent call surfactants (the standard reduction — surfactant). In relation to water such substances are fat to - you, monoatomic alcohol, ethers, glycosides, proteins, etc. Superficial activity is usually characterized by the size of the relation of change of P. of N to change of concentration of the substance which caused P.'s change N of pure solvent.

Superficial activity of organic matters increases in a homologous series approximately three times at accession to a hydrocarbon chain of the next SNZ-group (Dyuklo's rule — Traube). Fiziol, activity of substances of the same homologous series quite often changes in proportion to change of their superficial activity.

Researches P. of N tselnokhr blood, plasmas and blood sera revealed essential distinctions between P. of N static (equilibrium) and dynamic, i.e. the Pct measured in conditions when again formed surface layer did not reach an equilibrium state. Dynamic P. of N of blood is almost equal to P. to N of water (72,8 erg/cm^{ 2 } ). Static the Pct of blood, in to-ruyu for prevention of coagulation is added oxalate, makes 56 ±1 erg/cm^{ 2 } (for men) and 61 ±2 erg/cm^{ 2 } (for women). Dynamic P. of N of blood considerably changes at pregnancy and nek-ry patol, states that is caused by change of concentration in blood of various substances which are surface-active. The sharpest reduction of P. of N is observed at an acute anaphylaxis.

The item of N of blood and blood serum very rezistentno to effect of surfactants and quickly enough (approximately for 5*10^{ 2 } sec.) it is recovered at addition, e.g., of 0,1% of sodium oleate, it is strong and irreversible the N of water reducing P. (see. Detergents ).

Interfacial tension on surfaces of cellular membranes very little also does not exceed 5 erg/cm^{ 2 } . However change of this tension plays an essential role in processes of the amoeboid movement, cell fission, permeability of cellular membranes.

There are several methods of measurement of P. of N. The most exact method of measurement of size of static N. of N is the method of a capillary ascent. It consists in measurement of height of raising of liquid (h) in a glass capillary which radius r. In this case σ = (rhρg^{ 2 } )/2, where ρ — fluid density, g — acceleration of gravity. The method is applicable for pure liquids and solutions of the low-molecular substances which are well moistening a surface of a capillary.

The method of maximal pressure, the Crimea also determine the size of Pct, consists in determination of pressure (p) necessary for breakdown of air traps through a capillary, a tip to-rogo concerns the surface of the studied liquid. In this case P. of N is determined by the equation: σ = (σ_{ 0 } * p)/p^{ 0 } , where σ_{ 0 } and ^{ 0 } - surface intention and pressure, about the Crimea air traps are pressed through through the capillary concerning (usually pure) standard liquid. The method is offered by P. A. Rehbinder and is suitable both for pure liquids, and for solutions of various substances.

The method of the account of drops is based on comparison of number of drops of n and n_{ 0 } the studied liquid and the liquids of comparison respectively which are contained in a certain volume. The item of N is calculated on a formula: σ = (h_{ 0 } ρ/nρ_{ 0 } ) *σ_{ 0 } , where ρ — density of the studied liquid, and ρ_{ 0 } and σ_{ 0 } — density and liquid tension of comparison (usually waters) respectively. This method can be used for measurement of interfacial tension or at an effluence of liquid in other liquid.

The method of a separation of a ring of a pla of a frame from the surface of liquid is based on measurement of the maximum force of a separation (F), proportional P. in N, length of a frame or length of a ring. However this method is insufficiently exact and is a little used in practice.

**Bibliography:** Adamson A. U. Physical chemistry of surfaces, the lane with English, M., 1979; Vasilyev Yu. M. and Malenkov A. G. Cellular surface and reaction of a k.yaetka, L., 1968; Malenkov A. G. also Feel the p. A. Intercellular contacts and reactions of fabric, M., 1979; It is S. and To about N d about Page. The molecular theory of surface intention in liquids, the lane with English, M., 1963; Progress STAKE. YAOIDNOY chemistry, under the editorship of P. A. Rya-binder, M., 1973; Proceedings of symposium on surface chemistry of biological systems, N. Y., 1970.

*B. B. Akopyan.*