Coadsorption phase diagram for CH_4/CCl_4 on graphite

Abstract

We present an extensive thermodynamic study of methane coadsorbed on graphite precoated with a saturated monolayer of carbon tetrachloride. A combination of heat capacity and volumetric equation of state data permit construction of the multilayer coadsorption phase diagram between 70 and 115 K. Displacement of the preadsorbed CCl_4 by CH_4 occurs by a continuous process across the temperature range studied. While the continuous nature of the transition implies a single phase mixed film during displacement, the observation of a commensurability transition characteristic of CH_4 on bare graphite suggests the emergence of order in the coadsorbed CH_4 in the early stages of displacement. At low temperatures and multilayer coverages, the coadsorption phase diagram shows small but measurable differences from the pure CH_4 data, indicating that the multilayer film formed after displacement is not identical to that of CH_4 in the absence of CCl_4. At higher temperatures, a new first order phase transition is observed to correspond with the completion of displacement. One interpretation is that the phase boundary represents the melting of a nearly pure CH_4 monolayer solid into a mixed liquid film phase, with the size of the melting point depression indicating a concentration of nearly 1% CCl_4 in the liquid phase.