Wetting and melting in a methane/graphite system

Abstract

We report a new thermodynamic analysis of melting in thick adsorbed films and experimental results of an NMR survey of melting in methane films adsorbed on graphite. We find that solid-liquid stratification, related to surface melting in the bulk, may occur in a system that also exhibits triple point dewetting. Each phenomenon has distinctive asymptotic thermodynamic behavior in the thick film limit. We predict the existence of surface tension induced surface freezing, symmetric with surface melting, and new first-order phase transitions at coexistence between stratified and homogeneous films. The NMR data are used to show that solid methane wets graphite to at least 50 layers and that capillary condensation is not nucleated in our system (methane on Grafoil). We confirm that solid-liquid stratification does not occur in this system. Instead, melting is a first-order phase transition between homogeneous phases of solid and liquid, which we trace from a triple point dewetting transition at bulk coexistence down to 1.8 layers, where the latent heat of melting is known to have vanished. We conclude that melting in this system has a tricritical point at about 4 layers. Finally, we observe an increase in molecular mobility associated with the surface layers of the roughened solid phase.