Melting of Methane Adsorbed on Graphite Studied by Nuclear Magnetic Resonance
An NMR survey of the system of methane adsorbed on graphite, over a range of 70-105 K in temperature and.87 -51 layers in coverage, is presented. The data are analyzed in terms of current models of the phenomena occurring in adsorbed films, such as wetting, roughening, surface melting, and melting. The interaction between the substrate and adsorbate and its effect on T_1 is quantitatively analyzed in terms of a model of fixed paramagnetic spin centers in the substrate. Since the T_1 effect is very sensitive to the distribution of the adsorbate with respect to the surface, it is shown that it can be used as a powerful diagnostic tool for the study of wetting behavior in thick films where other techniques are insensitive. While T_2 is also affected by the substrate, we show that it can still be used as a probe of molecular mobility in thin films. Roughening is found to cause an enhancement of mobility in a region of about 4 layers on the surface of the film. A new, complete thermodynamic model of surface melting, applicable to adsorbed films, is presented, and possible new phase transitions are predicted. The data for methane on graphite are found to be inconsistent with the hypothesis of surface melting. Finally, the bulk melting transition is traced from thick films all the way down to 1.39 layers. The transition is observed to persist to the thinnest supermonolayer films studied, in a region where previous heat capacity studies have shown the latent heat to vanish.
- Pub Date:
- Chemistry: Physical; Physics: Condensed Matter