Physical Theory of Fluid Interfaces Near the Critical Point

Abstract

A continuum theory of surfaces is developed for fluids near their critical points. The free energy of the fluid interface is considered to consist of two terms: the self free energy of inhomogeneity and an energy of interaction across the interface due to van der Waals forces. This second term is computed via the principles of Lifshitz theory and gives a physical basis to Widom's modification of the Cahn?Hilliard theory of surfaces. The scaling laws as derived by Widom are recalculated, and several differences from the original laws emerge. The theory permits calculation of absolute values of surface tensions and interface widths near the critical point from experimental dielectric and free energy data. Furthermore, the fluids considered are not necessarily simple fluids where only pairwise forces are important.