Some problems of gibbs' two dividing surfaces: Thermodynamics of multicomponent foam films

Abstract

The system of a multicomponent foam film in equilibrium with a bulk liquid (meniscus) and a gas phase is considered. A rigorous thermodynamic scheme of describing this kind of systems is presented. The idealized system is introduced by making use of two dividing surfaces separating two bulk phases — the gas phase and the liquid phase of the meniscus. It is shown that the problems of the surface thermodynamics of small phases have been first developed by Gibbs. The conditions of the mechanical equilibrium of the system considered are examined in order to join the experimental method of liquid film investigations with the thermodynamic theory. Thermodynamical and riechanical definitions of the film disjoining pressure, tension, film surface tension, contact angle are presented. The problem of the independent thermodynamic variables, important for the comparison between the experimental data and the thermodynamic theory, is also discussed. Some fundamental Gibbs—Duhem equations are obtained. They correspond to suitable independent, experimental variables. A number of experimentally determinable quantities, that follow from those equations, are examined in detail and an emphasis is laid both on the possibilities for their physical interpretation and their comparison with similar quantities, obtained by model calculations.