Capillary condensation transitions in a slab geometry

A simple model, previously used to explore wetting transitions, is evaluated for the case of a slab geometry in which adsorption occurs between two semi-infinite solids, with parallel faces separated by a distance L. The model yields a universal description of possible wetting and capillary condensation (CC) transitions. The system's thermodynamic behavior is predicted from the values of two dimensionless parameters: D* (the reduced gas-surface interaction strength, a function of temperature) and L* (the reduced separation). If D*<2.9, negligible adsorption occurs at all pressures below saturated vapor pressure (SVP). For somewhat larger values of D*, CC occurs for sufficiently small L* close to SVP. For very large values of D*, an additional prewetting transition (formation of a film) is predicted for large L*; this is accompanied by a CC transition close to SVP. The model is generally consistent with limited results of density-functional calculations for the He liquids at zero temperature.