Colloid Aging

COLLOID AGING, a change occurring with time in a freshly prepared colloidal solution. This change consists primarily in a decrease in the degree of dispersion of the colloid. This includes, for example, the well-known phenomenon of the "ripening" of hematoxylin and certain other colloidal dyes. Only after a more or less prolonged time after the preparation of the solution do their particles acquire the degree of dispersion necessary for staining. The gradual decrease in the degree of dispersion of a colloidal solution may end in its complete coagulation. Simultaneously, the hydrophilicity of the colloid, its degree of hydration, may decrease. In the case of a gel, this leads to a decrease in swelling and to the release of part of the water bound by the colloid (see Syneresis). If a certain substance has been adsorbed by a freshly precipitated colloidal sediment, the decrease in the adsorbing surface that occurs as a result of C. k. leads to the gradual release back into the solution of part of the initially adsorbed substance. In the case of an adsorbed dye, such "desorption" is easily noticeable by the secondary (weaker) staining of the decolorized (as a result of primary adsorption) solution. All these changes indicate the extreme slowness of the establishment of true equilibrium in colloidal systems. Colloids in a metastable state approach equilibrium only very slowly, often meaning the complete destruction of the colloidal state and a transition to a more coarsely heterogeneous system. Age changes of colloids in the organism are closely connected with similar phenomena of C. k., which therefore acquire essential significance for the general problem of the aging of the organism. In rough outline, such changes are shown, for example, by the figures of the water content in the human body. In the beginning of intrauterine life water constitutes up to 95% of the total body weight, then, gradually decreasing, reaches in a newborn 70–75%, and in an adult slowly decreases to approximately 59–00%. When comparing age changes in water content not in the aggregate over the whole body, but in the same tissues and organs, the differences may be even more significant. A vivid indicator of such progressive drying of tissue colloids with age can serve the changes in their physical properties, first of all a decrease in their elasticity. For the skin, measurements made with an elastometer by Bonniger give a quantitative representation of the successive age changes in elasticity. Similarly, the elasticity (and the brittleness increases) of other tissues—muscles, vessel walls, muscle tissue—decreases. Another characteristic indicator can serve the decrease in the stability of colloids, expressed, for example, in a decrease in the alcoholic number of serum and other tissue colloids. Ružička and his colleagues collected extensive experimental material illustrating the gradual decrease in the alcoholic number and the corresponding lowering of other indicators of the stability of cellular colloids. Unfortunately, this material is not sufficiently reliable; in some cases it apparently is based directly on methodological errors and therefore requires a very cautious and critical attitude toward it. Ružička himself widely uses it for theoretical conclusions, for constructing the colloidal theory of aging [wherein the very phenomenon of C. k. in the organism he incorrectly calls "hysteresis of protoplasm" (see)]. One of the vivid examples of the decrease in the stability of tissue colloids can serve the often occurring turbidity of the refracting media of the eye in old age. Simultaneously with the decrease in the stability of colloids, their protective action also falls (see). Blood is normally supersaturated with calcium salts and other poorly soluble substances (e.g., urates), which are held in solution by its protective colloids. The insufficiency of the decreasing with age protective action of the colloids of blood must play a significant role in senile (and in pathological) salt deposition. The presented, far from complete list of age changes of colloids in the organism shows the enormous importance of these changes in the general picture of the aging of the organism. Any general theory of aging will have to take this aspect of the physiological process of aging into account. It would be incorrect, however, to fully identify (as is often done) age changes of colloids in the organism with the phenomenon of C. k. in the form in which it is studied by colloid chemistry. In the organism, time, during which a given colloidal system is maintained, plays no less a role than the influences to which it is subjected from the side of metabolic products. One must also remember that the very colloids in the organism are products of metabolism and may depend in their qualitative composition and in their properties on the latter. For example, a decrease in the amount of albumin and an increased content of globulins in the blood leads to a lowering of its stability, i.e., creates a picture of "aging" of the blood, although in its nature this phenomenon is fundamentally different from the aging of hematoxylin or another homogeneous colloidal solution.

D. Pyoikhantiih.