Freezing-points of solutions of typical colloidal electrolytes; soaps, sulphonates, sulphates and bile salt

Abstract

On the basis of experience with different families of colloidal electrolytes, each of which exhibits a whole range of typical behaviour different from each other, and in agreement with the experience of Hartley and his collaborators (1937), it is emphasized that conductivity alone is an untrustworthy guide in many cases as to whether or not a particular solution is that of a colloidal electrolyte. In all such cases it is essential to adduce other physical chemical evidence. Most conclusive is the direct comparison of thermodynamic with electrical properties. Careful freezing-point studies have been made with a number of colloidal electrolytes. It is shown that the behaviour of different groups in solution falls into several different types, although all have in common the replacement of ions by colloid particles with increase in concentration. For example, in the family of bile salts, as in those of certain wetting agents, the conductivity almost approaches the behaviour of an ordinary electrolyte, whereas the lowering of freezing-point falls off strongly and rather abruptly. It is pointed out that the term ' critical concentration ’ of micelles, as often used, is either an over-simplification or a misconception. Micelles are actually formed over a fairly wide range of concentration, amounting to at least 10-fold. It is found that, in accordance with previous measurements of vapour pressure by McBain & O’Connor (1940) that loading potassium oleate solutions with solubilized iso -octane scarcely affects the lowering of freezing-point, showing that it is not solubilized as independent molecules, but that it is taken up in or upon existing colloidal particles.