Formation and Collapse of Mixed Monolayers of Trilaurin with Cholesterol and with Tristearin

Abstract

Abstract The surface pressure-area curves of mixed monolayers and the equilibrium spreading pressure of solid mixtures were measured at 25 °C for cholesterol and trilaurin as well as tristearin and trilaurin. The phase diagrams of these mixtures were obtained by the measurement of the melting point. From two curves, one representing the relation between the collapse pressure (if the mixed monolayer was not overcompressed) and the composition of mixed monolayer, and the other representing the relation between the equilibrium spreading pressure and the composition of solid mixture, it was shown that the compositions of the mixed monolayer and the solid mixture coexisting in equilibrium were obtained. It was found that when the mixed monolayer of cholesterol and trilaurin was compressed, pure trilaurin began to collapse and deposit, and that the mixed monolayer of tristearin and trilaurin was apt to be overcompressed. It was concluded on the basis of phase rule and thermodynamics for plane interfaces that the equilibrium spreading pressure is constant regardless of the composition of solid mixture if the solid mixture consists of two phases of mixed crystals, and that cholesterol and trilaurin form a regular solution in mixed monolayer.