Multilayer adsorption of ethylene on graphite: Layering, prewetting, and wetting

We report the results of a comprehensive x-ray-scattering study of multilayer ethylene physisorbed on graphite. At temperatures well below the triple point of bulk ethylene (103.8 K), adsorption only occurs up to a finite number of layers; any further addition of material results in the nucleation of bulk crystallites. The number of layers that coexist with bulk increases at successively increasing transition temperatures. Reversible, first-order transitions between the total number of layers absorbed and the coexisting bulk are observed at $T=75\mathrm{}$ K (oneto two layers) and at $T=98\mathrm{}$ K (two to three layers). Bulk particle-size information is obtained as a function of both temperature and the amount of material in the cell. We present evidence that suggests that ethylene undergoes a thin-film to thick-film transition immediately below the triple point and that the triple point is the wetting temperature of ethylene on graphite. Pronounced hysteresis effects are observed both at the layering transitions and in the neighborhood of the triple point.