Effect of Substrate Salts on Stearic Acid Films on Water

Abstract

converted largely into calcium stearate which gives a condensed solid film at moderate pressures. The fresh distilled water may contain a minute concentration of metallic ions but not enough to give a condensed film immediately. In the case of the hydrochloric acid the film may be expected to be only stearic acid. Langmuir and Schaefer (5) have determined that the monolayer on a subphase of 10-4 molal calcium carbonate is the nearly neutral soap at pH 11, at pH 5.1 it is one half neutral soap, while at pH 3 it consists entirely of the free fatty acid. Adams'3 and Lyons and Rideal(7 have observed that surface area pressure curves varied with pH of the substrate which was regulated by the use of buffer solutions. As the underlying solution was made more alkaline, the low pressure and large area form of the film gradually disappears with decreasing limiting area until, on relatively alkaline solutions, only a single straight line is obtained with the limiting area of 20.6 A(2). In most of the cases reported, no attempt was made to assure that only a single species of positive ion was present in the substrate. In most cases, pH was regulated by the use of buffer solutions which, of course, could serve as a source of foreign cations. To the authors' knowledge it has not been suggested, except for the formation of calcium soaps5), that this change in surface area may be due to a simple exchange of the hydrogen ions of the surface film acid for cations of the substrate.