Thermodynamic consideration of the pressure dependence of interfacial tension

The pressure dependence of the interfacial tension of oil/water interface is related thermodynamically to the excess number of moles of oil per unit area defined with reference to Gibbs' dividing surface, the distance between the two dividing planes satisfying the Hansen convention, or the volume of interface formation. In order to know what the most useful thermodynamic concept is, the inter-facial tension has been measured as a function of pressure for the interfaces of four members of the homologous series of alkane (hexane, octane, decane, and dodecane), the substances of symmetrical ring structure (cyclohexane and benzene), and the substances of unsymmetrical linear structure (butyl bromide and hexyl bromide) against water. It has been clarified that the volume of interface formation is appropriate to elucidate the behavior of oil and water molecules in the interfacial region. Further, this conclusion has been substantiated by measuring the interfacial tension as a function of temperature and evaluating the entropy and energy of interface formation.