COAGULATION

From Big Medical Encyclopedia

COAGULATION (Latin coagulatio coagulation, a condensation) — adhesion of particles in liquid or gaseous disperse systems (colloidal solutions, suspensions, emulsions, aerosols) with formation of more or less large units. To. is of interest in connection with questions of aggregate stability of many biol, disperse systems, napr, blood and a lymph, suspensions of viruses, phages, bacterial suspensions, suspensions of medicinal substances, and also in connection with broad use of the techniques based on To., in medical, vt. and biol, researches (see. Coagulative tests , Veltmanna coagulative test , Koagulogramma ). To. widely apply in various tekhnol. processes in the food industry, at purification of drinking water and air for the purpose of release from suspended particles and bacteria. To. plays an important role in many biol, the phenomena (e.g., a denaturation of complete proteins), in geol., meteorol, and soil processes. To. also can lead to emergence continuous though, and loose, spatial structure — coagulative structure of gel (koagelya). From To. it is necessary to distinguish coacervation (see) — division of a disperse system with formation of a new phase (e.g., in the form of drops). Adhesion of particles of emulsions and vials of foam quite often turns into their merge (see. Coalescence ). To. represents manifestation of thermodynamic instability of disperse systems.

Distinguish two stages To.: 1) loss of aggregate stability and adhesion of particles — hidden To.; 2) loss of the formed units of particles in a deposit (coagulate) often in the form of flakes — explicit To. (see. Flocculation ). About hidden To. in system judge by increase opalescences (see), to decolourization, increase viscosity (see) and to other signs. The first stage To. optional passes into the second. To. can spontaneously occur (autokoagulyation) under the influence of chemical and physical. the processes proceeding in system in time (so-called aging), but, as a rule, To. is caused by external influences: addition to a disperse system of various substances — coagulants (electrolytes or nonelectrolytes), long dialysis (see), change of temperature, effect of light and other high-frequency electromagnetic oscillations, and also action of ultrasound, stirring and hashing.

Hydrosols can coagulate at agitation with unpolar liquids, particles at the same time gather on limit of the section of liquids. In aerosols (smokes and fogs) of a particle bear very small electric charges on the surface or are not loaded at all because of what in aerosols there is almost always spontaneous K. V industrial conditions To. aerosols make by filtering them through porous materials or by means of electric fields with a high gradient of potential.

To. electric charge of particles and solvation of their surface, and sometimes adsorption of large molecules of surfactants, napr, proteins interfere (see. Colloid protection ). Decrease in size of electric charge or electrokinetic potential of colloid particles (see. Electrokinetic phenomena ) and them solvation (see) which usually occur at the same time, call K.

Skorost K. it is measured by number of the particles which are sticking together for a unit of time in unit volume. It depends on temperature and by nature disperse system. Size, the return speeds To., serves as a measure of stability of a disperse system. Speed To. define, including particles under an ultramicroscope, measuring light scattering, and also other methods. Speed To. it is regulated by a ratio of forces of an attraction and the pushing away operating between particles in short distances. Resistance to thinning and deformation of a diffusion layer of a solvation shell is explained with emergence of the so-called wedging pressure, a cut causes pushing away of the approaching particles. In unequigranular systems K. occurs quicker, than in monodisperse. The particle shape also influences speed K.

Naiboley is studied To., caused by electrolytes. Coagulating effect of electrolytes is connected with compression of a diffusion part of a double electric layer on a surface of particles (i.e. with reduction of electrokinetic potential) and simultaneous decrease in their lyophilic property, a surface (poverkhnostnoaktivny nonelectrolytes sometimes promote To., removing lyophilic property of particles). The smallest concentration of electrolyte causing To., for certain (usually short) time term, is called threshold of coagulation (at). For various electrolytes size at can strongly vary even if it is found for the same koaguliruyemy system. By theoretical calculations of B. V. Deryagin and JI, D. of Landau it was shown that the relations at one - two - and three-charging ions are with other things being equal inversely proportional the sixth degree of size of a charge. This consistent pattern determined earlier to E. Schultze and L. Ph. A. Hardy is observed, however, not always since for ions of identical valency size at is defined by their situation in lyotropic series (see). At To. mix of electrolytes their influence can develop (additivity), be weakened (antagonism) or amplify (synergism). Three - and tetravalent coagulating ions are capable to change a sign of nuclear charge of colloid particles in this connection at change of concentration of these ions in colloid solution two or more zones of coagulation and respectively two or more thresholds are observed To. (alternation of zones K.). Determination of size at is complicated by the phenomenon of accustoming, a cut consists that at addition of electrolyte in the small portions a threshold To. above, than at introduction of all electrolyte at once. Also the return phenomenon when a threshold To is observed. at gradual administration of electrolyte below, than at its bystry addition (negative accustoming).

To., caused by dialysis or electrodialysis, it is connected with removal of the ions giving stability to colloid particles. During the mixing of colloid solutions (especially if they contain opposite charged particles) the decrease in their resistance (astabilization) bringing to their mutual To is often observed.

Sols of metal hydroxides easily coagulate at an elevated temperature. The current of sols, and also their hashing can sometimes accelerate To., but, on the other hand, too vigorous mechanical impact on system can lead to disintegration of units. Radoactive radiations cause To. sols of ferrous hydroxides, aluminum, etc., containing positively charged colloid particles. Action of gamma radiation, X-ray and visible light on To. it is connected with chemical reactions which can happen under their influence in sols, in particular with oxidation reactions — recovery. Ultrasound can cause To. or, on the contrary, to disperse droplets in emulsions.

In some cases between the aggregated particles in a coagulate there are thinnest layers of a dispersion medium. It determines the small durability of coagulative coupling and a possibility of division of units (fresh rainfall in which there was no recrystallization yet) on primary particles, i.e. the return transition of a coagulate to a condition of sol (see. Peptization ) under the influence of mechanical influence (e.g., hashing) or as a result of formation of the adsorption layers. At long storage of coagulates owing to aging their ability to pass back into a condition of sol decreases. If particles in a coagulate are in close contact with each other, then over time they grow together also To. it is irreversible. The pure coagulate turns out at long dialysis or electrodialysis. At irreversible To. ions of coagulating electrolyte force out ions from an outside (diffusion) part of a double electric layer of colloid particles and are absorbed by a coagulate. See also Aggregation .



Bibliography: Voyutsky S. S. Course of colloid chemistry, M., 1975; Sontag G. and Shtrenge K. Coagulation and stability of disperse systems, the lane with it., L., 1973, bibliogr.


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