Adsorption and wetting transitions at a model of the interface between a solid and a binary fluid mixture

摘要

We have developed a statistical-mechanical model for the adsorption of binary fluid mixtures (liquids or gases) at solid substrates. This model, which is a generalization to mixtures of that due to Sullivan, allows us to relate the relative adsorption of the two species and the solid-fluid interfacial tensions to parameters which characterize the solid-fluid and fluid-fluid potential functions. By making simplifying assumptions for the form of the potential functions and imposing certain mixing rules the problem of calculating the interfacial structure of the binary mixture can be reduced to an effective one density problem. The wetting characteristics of the solid-binary fluid system are then determined by Sullivan's procedure. For a binary mixture which exhibits liquid-liquid phase separation and an upper critical end point the wetting ‘phase diagram’., which determines the boundaries between different classes of wetting behaviour, differs from that for a one component fluid for temperatures below the critical end point. This has important repercussions for the behaviour of the relative adsorption, the total coverage and the interfacial tensions as a function of temperature.