Multilayers of methane adsorbed on graphite

Abstract

Adsorption isotherms and heats of adsorption have been determined for the methane–graphite system over wide ranges of temperature and surface coverage. Some data are reported for the region below monolayer coverage where liquid and vapour surface phases coexist but the majority pertain to multilayer adsorption. The fluid–incommensurate-solid transition has been examined in detail and an analysis of the results yields a tricritical temperature of 105 K and an order parameter exponent of 1/3. An explanation is given for an unexpected difference in the coverage dependence of the isosteric heat of adsorption of multilayers at low and high temperatures (Phys. Rev. B, 1984, 30, 3486). Hysteresis is observed in adsorption isotherms and heats of adsorption for surface coverages greater than the equivalent of a monolayer. Its magnitude depends upon the amount of methane adsorbed before desorption takes place which suggests that it may arise from an irreversible compression of the adlayer as adsorption proceeds. An attempt is made to correlate all of the results with existing information about the methane–graphite phase diagram.