Equilibrium Spreading Pressure of the Mixed System of Normal Long Chain Fatty Acids

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Abstract
Equilibrium spreading pressure were measured at various compositions and temperatures for mixed systems of tridecanoic acid-myristic acid, myristic acid-pentadecanoic acid, and pentadecanoic acid-palmitic acid, and thermodynamical treatment of these data with respect to the equilibrium between crystal and monolayer was performed. By evaluating the equilibrium composition of monolayer phase from the measured curve of equilibrium spreading pressure vs. composition of solid phase, the phase diagram was constructed. It was found that the equilibrium curve has a maximum point where the solid and monolayer phases have the same composition and a point where the solid phase is in equilibrium with two monolayer phases. The partial molar enthalpy change accompanied by the phase transition from solid to monolayer were found to be dependent on the composition.