Coadsorption phase diagram for CH4/CCl4 on graphite

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 CCl4 by CH4 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 CH4 on bare graphite suggests the emergence of order in the coadsorbed CH4 in the early stages of displacement. At low temperatures and multilayer coverages, the coadsorption phase diagram shows small but measurable differences from the pure CH4 data, indicating that the multilayer film formed after displacement is not identical to that of CH4 in the absence of CCl4. 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 CH4 monolayer solid into a mixed liquid film phase, with the size of the melting point depression indicating a concentration of nearly 1% CCl4 in the liquid phase.