Heat-capacity study of butane on graphite

Heat-capacity measurements were performed on butane films adsorbed on graphite for coverages between n = 0.50 and the equivalent to n = 6.0 layers. The evolution of the melting of the first layer was studied as a function of coverage. Below n = 0.83 melting occurs at a triple line at 112.6 K. This transition moves to higher temperature as the coverage is increased; above n = 1.20 the melting of the first layer produces a sharp heat-capacity peak near 134 K. Beginning at n = 2.34 the height of this sharp peak starts to decrease with increasing coverage; the peak disappears near n = 3.0. The decrease and disappearance of the sharp first-layer melting peak coincide with a compression of the first layer into full commensuration observed in a recent neutron-scattering study. A broad heat-capacity peak at 127 K appears in the data starting near n = 2.65. This broad peak is identified as resulting from two phase transitions occurring at close-by temperatures: melting of the fully commensurate first layer, and a transition involving the bulk. The heat-capacity peak from the melting of the first layer remains broad for all coverages above n = 2.65; this melting transition is continuous. The results for butane and those for other systems are compared to the Janke and Kleinert theory of two-dimensional melting. We suggest that the extended monolayer regime is the region where this theory is applicable. Evidence of the presence of a second solid layer is presented. Second-layer melting occurs at 114 K. No heat-capacity peaks corresponding to any additional layers beyond the second are found, indicating that solid butane incompletely wets graphite. The heat-capacity results are extensively compared, and found to be in very good agreement with results from a recent neutron-scattering study.