The Methane/Graphite Phase Diagram

Methane adsorbed on graphite has been one of the most intensely studied of all multilayer film systems. A rather detailed phase diagram for the system was proposed by Goodstein et al. in 1984, [1] elaborated and embellished by Wortis in 1984,[2] and modified by Pettersen et al. in 1986.[3] The purpose of proposing the phase diagram was to stimulate further research by presenting hypotheses to be tested and drawing attention to open questions. The authors of the phase diagram had to confront some of the most interesting and vexing issues in the field of adsorbed films today: Does the adsorbed solid wet the substrate, and if so, why? Is there evidence of roughening, capillary condensation, or surface melting? Most intriguing of all, can one observe a dimensional crossover, from 2D to 3D behavior in the melting transition? The authors proposed answers to all of those questions. The purpose of this paper is to reexamine those proposed answers in the light of subsequent experimental and theoretical work on methane/graphite and other related systems. As we shall see, although much has been learned, many of the questions remain open. This is very much a work in progress.