Critical Potential in the Coagulation of Colloids by Electrolytes

Abstract

THE accepted view of the coagulation of so-called hydrophobe colloids by electrolytes is that it is the result of a diminution of the potential of the electrical double layer. The Potential is measured from the cataphoretic speed, u, by means of Helmholtz's equation, u = DV/4π, where D is the dielectric constant, V the potential of the double layer, and ŋ the viscosity coefficient. In 1926, S. G. Choudhury made observations in this laboratory which threw doubt on the above explanation of coagulation by electrolytes. Kruyt and Willigen (Zeits. Phys. Chem., 130, 170; 1927) have recently observed that in the coagulation of arsenious sulphide sol by potassium ferrocyanide, the cataphoretic speed is greater than that of the pure colloid up to the concentrations studied by them; their measurements, however, do not go up to the coagulating concentration. They attempt to get over the difficulty by assuming that the dielectric constant of the medium increases with the concentration of the electrolyte, so that though the cataphoretic speed is higher, the potential of the double layer is smaller than that of the pure sol. A number of observations show, however, that an impossibly large value of the dielectric constant must be assumed in order to reconcile the data with the above explanation. It also appears that the nature and even the manner of prepartion of the colloid have a great influence on the cataphoretic speed at the coagulating concentration of the electrolyte. We reproduce our data in the accompanying curves (Fig. 1).