From Big Medical Encyclopedia

FOAMS — the concentrated disperse systems formed by a set of the vials of gas (usually air) divided by pellicles of liquid (most often aqueous solution of frother).

Items find application in medicine. Pharmaceuticals in the form of P. use at treatment of burns, diseases of skin, etc. The industry the large number of hygienic and cosmetics of different function, action is produced to-rykh it is based on their property to form P. (soaps, detergents, shampoos, creams, etc.). Items use for the prevention of release of toxicants in air from plating baths, etc. P. are widely applied to fight against dust in the equipment, napr, at benefication by method of flotation, and also in fire-prevention business at suppression of the fires by means of foam fire extinguishers.

The vials of gas called by P.'s cells represent a dispersed phase, and pellicles of liquid dividing cells and forming a framework of foam — a dispersion medium (see. Disperse systems ).

The volume of a gaseous dispersed phase of V considerably exceeds the volume of a fluid dispersion medium of V . Relation of b = (V + V ) / V call frequency rate of the Item. If the size b no more than 10 — 20, then P.'s cells have spherical shape and thickness of liquid films is rather big. If size E exceeds several tens and even hundreds, then P.'s cells have the form of polyhedrons (polyhedrons) and are divided very much by pellicles, the minimum thickness to-rykh makes from 4,0*10 - 9 To 1,0*10-8 m. The sizes of cells depending on conditions of receiving P. vary over a wide range — from 10 - 8 to 10 - 3 m.

Pure liquids do not form the Item. The substances added to liquid for P.'s education are called frothers. Low-molecular surfactants concern to them (alcohols, carboxylic to - you, amines, sulfonic acids, etc.). Especially high foaming ability hemicolloids, napr, soaps and surface-active high-molecular compounds have (proteins, pectins, etc.).

The item — rather unstable disperse systems: over time the liquid films separating P.'s cells utonchatsya owing to running off of liquid, films are broken off and P. collapse. P.'s stability, first of all, depends by nature and concentration of frother. Low-molecular frothers, napr, alcohols, give P., life time to-rykh is measured by several tens seconds. With increase in concentration of these frothers P.'s stability increases in the beginning, and then decreases. Otherwise high-molecular frothers (proteins, pectins) behave and colloids (see). They give P., life time to-rykh is rather big. At the same time P.'s stability with increase in concentration of frother only increases. In addition to the nature and concentration of frother P.'s stability is influenced by temperature and viscosity of a dispersion medium. Temperature increase, as a rule, lowers stability of the Item. Increase in viscosity of liquid films increases stability of the Item.

The films formed by high-molecular frothers sometimes harden the chemical processes resulting in them, and the liquid framework of P. passes into firm. Such firm disperse systems — polyfoams are characterized by big stability. They find broad application in the national economy and in scientific research.

Items, as well as other disperse systems, receive dispersion (stirring, intensive hashing of the corresponding gas with solution of frother, breakdown of vials of gas in solution of frother via porous membranes, etc.) or method of condensation (gas evolution from peresyshchenny gas of solution of frother or at boiling, etc.).

When P.'s education is undesirable, they are destroyed by means of the substances called pe-but quenchers or antivspenivate-lyam. These substances, napr, the amyl and octyl alcohols having high superficial activity and very small lathering power are forced out by frothers from a surface of cells of P. and make impossible P. Izvestna's education and other ways of destruction of P., napr, heating or mechanical influence.

Bibliography: Voyutsky S. S. Course of colloid chemistry, page 24, etc., M., 1975; Tikhomirov V. K. Foams — теория^ and practice of their receiving and destruction * M., 1975, bibliogr.

V. P. Mishin