Protective Action

PROTECTIVE ACTION, the ability of certain colloids to prevent the precipitation of other colloids present in a solution. The addition of an emulsion colloid to a typical suspensoid, for example to a sol of some metal, protects it from precipitation by salts, imparting to it in this respect the properties of an emulsion. The smaller the amount of a given colloid that must be added for this purpose, the stronger is obviously its protective action. It is most convenient to observe the latter by using a gold sol as the suspensoid, which, upon beginning to precipitate, sharply changes its color. This color reaction is usually used for the quantitative characterization of the protective action (see Gold number). The protective action of colloids was studied in particularly great detail by Zsigmondy. Gelatin, gum arabic, and certain albumoses are most frequently used as protective colloids. The addition of these substances in relatively minute quantities makes it possible to prepare completely stable colloidal solutions of the most labile suspensoids (e.g., metal sols), which is of great importance in the preparation of colloidal preparations finding therapeutic use. An example is collargol, a commercial preparation of colloidal silver; when dried, upon the addition of water, like a true emulsion colloid, it passes back into solution. In the organism, particularly in the blood, emulsion colloids are abundant, and their protective action apparently plays an essential role in many vital phenomena. Thus, the blood contains calcium salts (calcium carbonate and calcium phosphate) in amounts significantly exceeding their normal solubility in water. To elucidate the mechanism of ossification, it is essentially necessary to solve not the question of why calcium salts are deposited in certain places, but rather the reverse question: why the deposition of lime does not occur everywhere in the organism. The obstacle to the deposition of lime is primarily the protective colloids of the blood, which do not allow insoluble salt particles to combine into large aggregates and precipitate out of solution. The study of protective action therefore acquires great importance both for the theory of normal ossification and for the understanding of pathological salt deposition in arteriosclerosis, gout, the formation of kidney and gallstones, etc. The mechanism of protective action, according to Bechhold's theory, consists in the fact that the protective colloid is adsorbed by the particles of the suspensoid and envelops them in a thin layer. The sensitivity of a colloidal particle to salts is determined by the properties of its surface layer, with which it borders the solution. Therefore, particles coated, for example, with a finest layer of gelatin must possess the stability of the latter in relation to salts. This explanation, however, encounters serious difficulties in cases where a minute amount of the protective colloid appears insufficient to cover the entire surface of more highly dispersed particles. Zsigmondy assumes in this case the combination of a suspensoid particle with one or several particles of the protective colloid.

PROTECTIVE DEVICES IN INDUSTRY

assumes in this case the combination of a suspensoid particle with one or several particles of the protective colloid.