PHASE-TRANSITIONS IN CH3CL MONOLAYERS PHYSICALLY ADSORBED ON THE SURFACE OF GRAPHITE

Heat capacity measurements have been made to study the phase transitions occurring in the monolayer of CH3Cl physically adsorbed on the surface of graphite. It reveals the existence of two different two-dimensional solid phases depending on the coverage: the low-density phase (ca. 5 molecule/nm2) melts at 120 K; the high-density phase (ca. 7 molecule/nm2) melts at 146 K. There is a coexistence region in between, where the melting point of the high-density phase becomes lower with a decrease in the coverage. The high-density solid undergoes an order-disorder phase transition at 128 K accompanied by the excess entropy of approximately R ln 2, which suggests the possibility that a random head-tail disordering occurs in the admolecules. An anomaly in the fluid phase is also found near the full coverage of the monolayer, which may indicate the presence of some ordered fluid just above the melting point.