Equilibrium position of carboxylic acid molecules in monolayers on aqueous subphases

Abstract

Langmuir films of the carboxylic acids C22, C20, and C18 disclosed an unusual pressure π i at large molecular areas before compression when spread on subphases containing alkali hydroxydes. The pressure appeared only when the acids were fully ionized and increased with increasing electrolyte concentration in the subphase. π i was ion dependent and varied in the same manner as the sequence Li + < Na + < K + and it decreased with increasing chain length. We have concluded that π i must be a consequence of carbon chains situated in the plane of the interface. This position is not in equilibrium since π i disappeared with time. We have attributed this to an evolution of the monolayer configuration into a more or less upright chain position. Further spontaneous rearrangements took place in the monolayers. These rearrangements were favored through successive compressions.