On the production of soap jellies,and the physical conditions under which jelly formation takes place. (Preliminary communication.)

Abstract

In the course of experiments regarding the influence exerted by various electrolytes on the equilibrium of emulsions, published in the year 1913, the writer noted that NaCl, when used at a concentration in excess of .4M, caused a precipitation of some constituent of the aqueous phase of an emulsion of oil dispersed in water, and that the emulsion subsequently broke down, the oil and water layers separating. This effect was believed to be attributable to the precipitation of the surface film of soap on which the stability of the emulsion depended. To test this question Na oleate was treated with salt at different concentrations, and it was found that at .4 to .45M NaCl complete precipitation of the soap took place. It was noted, however, that prior to precipitation a tendency to jelly formation was exhibited in the zone from .2M NaCl to .4 or .45M NaCl.

An attempt to repeat this experiment with a soap, which had been slightly acidified either by the addition of a minute quantity of oleic acid or of mineral acid, gave an entirely different result, an opalescence with increasing cloudiness and tendency to precipitation was noted between .2M and .4M NaCl, followed by complete precipitation at .45M NaCl. Further tests using varying proportions of soap, varying proportions of NaOH, and of NaCl and other salts of Na, brought out the remarkable fact that, as long as the soap employed was not too greatly diluted and was slightly alkaline, a jelly would be formed at all points between .2M Na and .45M Na regardless of whether the Na was derived from NaOH, from NaCl or other salts of Na.

In very concentrated soap solutions or in very strong alkali the jelly formation commences at a somewhat higher concentration and continues also somewhat above .45M. But it may be stated as a general principle that a zone of jelly formation obtains within these ranges, provided the original concentration of OH ions is in excess of the amount required to produce a strong pink coloration of the soap solution with phenolphthalein.