HEAT-CAPACITY STUDY OF THE MELTING OF SUBMONOLAYER BUTANE FILMS ABSORBED ON GRAPHITE

We have used ac calorimetry to study the melting transition of submonolayer butane films adsorbed on graphite. We investigated eight surface coverages ranging from n = 0.56 to n = 0.89 for temperatures between 98 and 126 K. We find one single, asymmetric, heat-capacity peak at melting. The melting-peak height is typically 15 in units of C/Nk(B) (where C is the total heat capacity of the film and N the number of butane molecules), and the peak temperature is 112.6+/-0.3 K. We identify the transition as first order. Our heat-capacity results are in good agreement with the elastic neutron-scattering results as well as with very recent computer simulations for this system. We also compare our results with predictions by Janke and Kleinert that the monolayer melting transition changes from first order to continuous as the adsorbed molecules studied go from spherical to elongated.