Some Thermodynamic Properties of Methane Adsorbed on Rutile Between 80 and 140°K

Heat capacities of methane adsorbed on rutile (titanium dioxide) have been determined from 80 to 140°K for concentrations corresponding to the fractions 0.28, 0.56, 0.84, and 1.11 of the monolayer capacity. Integral heats of adsorption, determined at 110, 120, 130, and 140°K, show correlation with the heat capacities within the experimental error. The thermodynamic functions, ΔH, ΔS, and ΔF of the differential adsorption process, have been calculated from the data. The heat capacities indicate an adsorbate having properties differing from bulk methane and possessing some degree of mobility over the surface. No first-order phase changes were in evidence for the concentrations studied. At low surface coverages, the shape of the differential heat of adsorption curve points to the presence of a heterogeneous adsorbent surface, while at coverages approaching the monolayer, dispersion forces are probably in effect. All the differential thermodynamic quantities approach the values for liquid methane at coverages greater than the monolayer in the temperature range from 110 to 140°K.