Microphase Separation in Mixed Langmuir Monolayers

Abstract

The surface pressure-area (II-A) isotherm curves of pure and mixed monolayers which have been reported by Mori and Imae [Langmuir 11, 4799 (1955)] were analyzed to get the molecular insight into mixed monolayers. The II-A isotherm curves of the pure octadecyldimethylamine oxide {CH₃ (CH₂) ₁₇NO (CH₃) ₂, C₁₈DAO}, the pure arachidic acid {CH₃ (CH₂) ₁₈COOH, AA} monolayers and the mixed monolayers were analyzed to assume the mixed situation between C₁₈DAO and AA at the air-water interface. As a result, it was assumed that part of C₁₈DAO molecules forms mixed clusters with AA molecules and the residual C₁₈DAO molecules exit individually without any formation of mixed clusters with AA in the mixed monolayer. Not only the ratio of the clustered C₁₈DAO molecules to the total C₁₈DAO molecules was estimated, but also the mixed cluster's composition in the mixed monolayers was deduced to be 2 : 1 for AA and C₁₈DAO. The resultant H-A curves well fitted with the observation.
It is concluded that the microphase separation occurs with the mixed clusters as well as free C₁₈DAO molecules in the mixed monolayer.