Heats of adsorption of methane multilayers on graphite

Isosteric heats of adsorption of ${\mathrm{CH}}_4$ on exfoliated graphite (Grafoil) have been measured calorimetrically at 69.5, 77, 84.5, and 92.5 K up to surface coverages of the equivalent of 3.5 layers. At the lowest two temperatures, $q_{\mathrm{st}}$ varies rather little with the amount adsorbed beyond a monolayer. In the same region of adsorption, $q_{\mathrm{st}}$ at the highest two temperatures changes by as much as 10% in an oscillatory manner. A qualitative analysis of the results suggests that they are consistent with a transition (roughening?) occurring in the methane multilayers in the vicinity of $T=80\mathrm{}$ K. From isotherm and heat-capacity measurements on the same system, Hamilton and Goodstein [Phys. Rev. B 28, 3838 (1983)] have deduced that critical points in the multilayers occur at approximately 78 K.