Phase diagrams of multilayer adsorbed methane

We propose multilayer phase diagrams based on recent observations of methane adsorbed on graphite and gold (111). Methane wets graphite at all temperatures observed, but it fully wets gold only above the bulk triple point. Possible reasons for this difference in behavior are discussed. The methane-graphite phase diagram also includes layer by layer condensation with critical points leading to a predicted bulk roughening temperature, and an extension of the bulk melting curve into the multilayer film region where it closes at a triple point with a compressed incommensurate first layer solid phase. Experimental evidence for these features is presented and discussed.