Prewetting at a fluid-solid interface via Monte Carlo simulation

Abstract

An isobaric-isothermal Monte Carlo simulation method has been used to investigate the approach to wetting in a 12-6/9-3 potential model of fluid Ar and solid ${\mathrm{CO}}_2$. We have studied adsorption from the subcritical vapor at pressures up to saturation along isotherms at several temperatures. At each state the density profile and the adsorption excess were determined. At the lowest temperatures studied, the adsorption isotherms and density profiles exhibit behavior consistent with partial wetting at coexistence, in that the adsorption remains finite all the way up to coexistence. Over an intermediate range of temperatures we have found behavior consistent with the existence of a prewetting line, a locus of first-order phase transitions from thin- to thick-film adsorption. The adsorption excess increases discontinuously at pressures below saturation as thick liquidlike films form. These liquid films grow in thickness until the saturation pressure is reached and the adsorption excess diverges. At a still higher range of temperatures, behavior consistent with complete wetting at saturation is observed but with a continuous growth in the film thickness as the pressure is increased toward saturation. In contrast with earlier work on the same model system using the grand ensemble Monte Carlo method, our observations are in qualitative agreement with the predictions of density-functional and related theories of wetting phenomena. We discuss possible reasons for the failure to observe such phenomena in previous simulation work.