Thermodynamics of multicomponent monolayers: III. Equilibrium spreading pressure of two-component mixtures

Abstract

The thermodynamic equations representing the equilibrium of a two-component bulk phase with monolayer phases formed by the spontaneous spreading have been derived from the equations in Part I of this series. They have been applied to the experimental results on the system of pentadecanoic and myristic acids which are miscible in the crystalline state. By plotting the equilibrium spreading pressure against the composition in the solid phase and then comparing the resultant curve with the equations, the phase diagram can be constructed which represents the equilibrium compositions of solid and monolayer phases. Interesting features have been observed; this system behaves in such a way that a system forming the positive azeotrope behaves and the equilibrium of the solid phase with two monolayer phases, that is, condensed and expanded ones, is realized at higher temperatures while not at lower temperatures. It has also been shown that the partial molar en thalpy change associated with the spreading process can be evaluated by observing the temperature dependence of equilibrium spreading pressure.