Layer critical points of multilayer ethane adsorbed on graphite

Abstract

The physisorption system ethane on graphite was investigated by an ellipsometric technique. We report layer condensation critical temperatures for layers 2--9. ${T}_{c}$(2) is 120.8\ifmmode\pm\else\textpm\fi{}0.3 K, somewhat lower than previously reported. ${T}_{c}$(n) decreases with increasing layer number n until for n=8 and 9 the extrapolated critical temperatures are below the bulk melting point (90.3 K), where these layers have been preempted by bulk solid. The coefficient of the compressibility above ${T}_{c}$ increases strongly with n. This phenomenon is not predicted in a theory based on the two-dimensional Ising model, and probably reflects the importance of a second interaction parameter related to the substrate holding potential.