On the stability of unimolecular films. Part II.— Dissolution in alkaline solutions

Abstract

In the previous communications (Parts I and Parts II) it was shown that the conditions of equilibrium of a unimolecular film palmitic acid were influenced both by the temperature and the attraction of the polar head-group for the underlying solution, which in the case of the carboxyl group could be altered by changing the acidity or alkalinity of the surface layers of the solution. On alkaline surfaces this attraction increases to such an extent that not only does an expanded film tend to contract to form a condensed film, but actual dissolution may occur. Adam noticed that condensed films of palmitic acid showed a small slow decrease in area on a solution of P H 8∙5, while on a solution of P H 10∙0 this decrease in area look place so rapidly that he could not obtain force/area curves. An attempt has been made to examine the mechanism of the-dissolution process and to investigate the relationship between the rate of solution and the alkalinity of the underlying solution. The experiments were carried out on an apparatus designed so that the pressure of the film was maintained constant as the film dissolved. As the molecules left the film for the underlying solution the gaps in the film are closed by the pressure, and the area of the film gradually decreases. The factors influencing this rate of decrease in area at constant pressure have been studied.