Oleic Acid Monolayers on Concentrated Salt Solutions

Abstract

Equilibrium spreading pressures (E.S.P.) and force-area (π-A) curves have been measured for films of oleic acid spread on solutions of a range of salts and non-ionic compounds at various concentrations. There is a linear relationship between E.S.P. and activity of salt in the subphase up to an activity of about 5. The films are still of the liquid expanded type on salt solutions but the π-d curves are shifted to larger areas. Ions follow the order of the lyotropic series in the magnitude of their effects, anions having a greater effect than cations. Results are explained by visualizing both a hydration effect and a charge effect causing an increase in attraction between oleic acid and the substrate, together with an increase in the repulsive terms of the system of forces between the film molecules.