A Thermodynamic Study of Methane Multilayers Adsorbed on Graphite

Thermodynamic measurements of the properties of methane adsorbed on graphite have been made in the range, roughly, of 1-6 molecular layers and 64-105 K. Interpreting the results in the context of current models, a tentative conclusion that layer by layer critical points occur at approximately constant temperature near 78 K is drawn. A line of melting transitions extending from the bulk triple point, Tt = 90.66 K. into the multilayer region at temperaturesnear 90 K is also observed, and while these transitions do not appear to be first order, there is an associated change in entropy. This change in entropy gradually diminishes, and vanishes entirely at about two layers. It is not clear how this line of transitions ends, but it is thermodynamically forbidden from ending in a critical point. Finally, the Landau potential has been constructed and tabulated as a function of its proper variables, namely temperature and chemical potential. This gives a complete thermodynamic description of the methane film in the range studied.