Properties of three capillary fluids in the critical region

Nitrogen, xenon, and sulfur hexafluoride when adsorbed on a mesoporous controlled pore glass all exhibit capillary condensation at temperatures less than T-c, the bulk critical temperature, and all exhibit a capillary critical temperature, T-cc, where adsorption-desorption hysteresis vanishes. For all temperatures less than T-cc, the density and surface tension of these capillary liquids are essentially the same as bulk liquid. Within the intercritical region, T-cc < T < T-c, these capillary fluids at, or near, saturation pressure are also liquidlike. We conclude that fluid behavior at the capillary critical point is not analogous to fluid behavior at the bulk critical point. The capillary critical point is identified as the point at which homogeneous nucleation of the vapor phase within capillary liquid replaces Kelvin failure as the mechanism responsible for abrupt capillary evaporation on desorption. We also discuss the phenomenon of critical depletion reported by Findenegg and colleagues.