The Influence of Electrolytes on the Solution and Precipitation of Casein and Gelatin. by Jacques Loeb and Robert F. Loeb

The modem concepts of colloid chemistry originated largely f rom a study of the precipitation of colloids by electrolytes, and this accounts, perhaps, for the fact that the tendency to form aggregates was considered the chief characteristic of colloids in solution. As a consequence it is assumed in the text-books of colloid chemistry that the ultimate units of a colloid in solution are not isolated molecules or ions but larger aggregates of molecules or ions, the so called micella • (small crumbs) of Naegeli, which are supposed to be kept in a stable solution or suspension through forces of repulsion due to the fact that they are electrically charged. The precipitation of the particles by electrolytes is ascribed to a diminution of the charge of the micella through the adsorption of the ions of the electrolyte. I t was noticed by Hardy that the active or precipitating ion of the electrolyte was always that ion which had the opposite sign of charge as the colloidal particle, and Picton and Linder had noticed that the precipitating efficiency of ions increased rapidly with their valency. ~ The experiment s published by one of us have led to results which are compatible with some but not all of the assumptions just enumerated. In the first place, experiments on the osmoti~ pressure and the viscosity of protein solutions have made it probable that the ultimate units in certain protein solutions, such as crystalline egg albumin, are essentially isolated protein ions or molecules, though such solutions may, in a secondary way, also contain aggregates of ions or molecules. 2