Phase Transitions in CH_3Cl Monolayers Physically Adsorbed on the Surface of Graphite

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Heat capacity measurements have been made to study the phase transitions occur-ing in the monolayer of CH,Cl physically adsorbed on the surface of graphite. Itreveals the existence of two different two-dimensional solid phases depending on thecoverage: the low-density phase (co. 5 molecule/ nm') melts at 120 K; the high-densityphase (ca. 7 molecule / nm') melts at 146K. There is a coexistence region in between,where the melting point of the high-density phase becomes lower with a decrease inthe coverage. The high-density solid undergoes an order-disorder phase transition at128 K accompanied by the excess entropy of approximately R In 2, which suggests thepossibility that a random head-tail disordering occurs in the admolecules. An anom-aly in the fluid phase is also found near the full coverage of the monolayer, which mayindicate the presence of some ordered fluid just above the melting point.[heat capacity, phase diagram, phase transition, adsorbed monolayer, order- ll disorder, two-dimensionalll
社団法人日本物理学会の論文
1991-01-15