Thermodynamic study of monolayer methane on graphite

High-resolution heat-capacity and vapor-pressure isotherm measurements have been made to study the phases and phase transitions of monolayer methane on graphite. In the submonolayer region, heat-capacity results indicate a continuous transition between the 3 × 3 commensurate solid (CS) phase and an expanded incommensurate solid (ES) phase and triple-point melting from the ES phase. Within the resolution of our data no heat-capacity evidence of an orientational ordering transition is found. The liquid-vapor coexistence boundary is studied very carefully and the results show that the critical point of methane on graphite belongs to the two-dimensional Ising universality class. In the pure-solid-phase near-monolayer-completion region, the transition from the compressed or dense incommensurate solid (DS) phase to the ES phase and melting from the ES phase are studied. Both heat-capacity and vapor-pressure isotherm data show evidence of the existence of a thin sliver of (3 × 3) phase sandwiched between the DS and ES phases. This thin sliver of CS phase terminates near a coverage of ne=1 and T=88 K. Beyond this coverage and temperature, the transition between the DS and ES phases becomes continuous. The melting transition from the ES phase is found to be first order in all the coverages studied in this experiment. The isotopic effect is studied for the commensurate-incommensurate, melting, and liquid-vapor critical transitions.