Phase transitions of ethylene on graphite

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.