Abstract
The results of an extensive thermodynamic study of the phase transitions of monolayer and multilayer ethylene on graphite are presented. In the submonolayer and near-monolayer completion regions, the melting transition is found to be continuous. Heat-capacity anomalies indicative of orientational and structural transitions of monolayer solid phases are found. Our result indicates the existence of a third monolayer solid phase in addition to the low- and high-density phases defined according to the orientation of the carbon-carbon axis of the molecule with respect to the substrate surface. In the multilayer region, our result is consistent with the interpretation that the wetting transition is pinned at ${T}_{t}$, the bulk triple-point temperature, and it is approached via a sequence of layering transitions. Liquidlike ethylene film is found to be layered by the graphite substrate well above ${T}_{t}$. The layer critical point ${T}_{c}$(m), the high-temperature limit where a film of m-1 layers coexists with that of m layers, is found to decrease towards ${T}_{t}$ with increasing m. The monolayer-bilayer layer-critical-point transition is found to be two-dimensional Ising-like. The isotopic effects in two-dimensional orientational, melting, and liquid-vapor transitions were also studied in this experiment.
Published Version
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