Layer growth of methane on MgO: An adsorption isotherm study

The layering properties of methane on MgO (100) were measured between 70 K and 96 K using high-resolution adsorption isotherms. The results demonstrate that at least five layers can form. Phase transitions for the different layers were identified by monitoring the changes in the isothermal compressibility of a particular layer as a function of chemical potential. A phase transition (melting) for the monolayer is observed at about 80 K, whereas layers 2 through 4 show a similar transition at about 85 K. The isosteric heat of adsorption was calculated as a function of coverage to gain insight into the growth process of the CH₄/MgO system. The thermodynamic quantities determined here are compared with published experimental values. Our data provide key information for defining current methane-MgO interaction potentials.