Adsorption from a binary fluid mixture

Abstract

The nature of adsorption at the interface between a model binary fluid mixture and a solid substrate is investigated using a two-density free-energy functional. For model fluids which exhibit a liquid-liquid upper critical end point we show that the solid-vapour interface can be wet by a ‘double’ wetting film which is a composite of both phase-separated liquids. The structure and growth of such composite films is described and their possible observation discussed briefly. Results of calculations of the density profiles, relative adsorption and surface tension of the solid-vapour interface are presented. For certain choices of the substrate-fluid potential the binary fluid can exhibit re-entrant partial wetting as the temperature is increased, i.e. the solid-vapour interface undergoes transitions from partial wetting, to complete wetting by one liquid phase, to partial wetting and, finally, to wetting by the common liquid phase that occurs above the critical end point.