Molecular interaction in mixed monolayers of fatty acids

Abstract

AbstractForce‐area isotherms have been obtained for mixed monolayers of arachidic acid and α‐ethyl‐stearic acid on conductivity water and M/400‐CaCl2 solution at pH 7.5. There is evidence of molecular interaction on both substrates as the mixed films are always more condensed than an ideal mixture of the two constituents. On M/400‐CaCl2 solution, mixed surface films containing more than 75% of α‐ethylstearic acid are fluid at all surface pressures; this indicates that the condensation in the mixed film is due to the van der Waals' forces between the hydrocarbon chains and not to calcium ions.Partial molar areas for the constituents of the mixed films on M/400‐CaCl2 solution have been calculated. The small values of partial molar areas obtained for arachidic acid at low mole fractions and low surface pressures are explained by strong van der Waals' forces.