NMR study of thick films of methane on graphite

We report the results of an NMR survey of the system methane adsorbed on graphite near the triple point temperature, in the thick-film regime (10–50 layers) where the methane behavior is substantially bulk-like. We study the interaction of the film with the substrate and find that T_1 is affected by a dipolar coupling with paramagnetic spin centers in the substrate. Since the systematics of these effects are now understood, they can be used as diagnostics in their own right. We are able to conclude that solid methane wets the graphite basal plane substrate to a thickness of at least 50 layers in the temperature range 70 K < T < 105 K. On the Grafoil substrate we employ, such thick films ought to be unstable with respect to capillary condensation, but capillary condensation is evidently not nucleated in our experiments