The layering properties of ethane on MgO(100) were measured between 91 and 144 K using high-resolution adsorption isotherms. In contrast to previous studies, the results demonstrate that only three layers are formed. The thermodynamic functions derived from the data (isosteric heat, differential enthalpy, and entropy of adsorption) compare well with literature values and show a steady trend toward the bulk properties as the number of layers increased. Phase transitions for two of the layers were identified by monitoring the changes in the two-dimensional isothermal compressibility as a function of chemical potential. Both of these phase transitions occur at approximately 127 K and within 1 K of each other. Through the use of neutron diffraction, it is possible to identify at least one solid surface phase that melts at approximately 75 K. The transition at 127 K is therefore a transition between a liquidlike phase and a hyper-critical fluid. A comparison is made between the present data and our recent study of methane on MgO.

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Thermodynamic measurements using high-resolution volumetric adsorption isotherms were performed on n-pentane films physisorbed on MgO(100) surfaces between 181 K and 244 K. The isotherms show two distinct adsorption steps before the saturated vapor pressure is reached. The heat of adsorption is found to be 33.7 ± 0.3 kJ mol −1 for the first layer and 32.9 ± 0.3 kJ mol −1 for the second layer. Evolution of the two-dimensional compressibility, as a function of temperature, suggests that a phase transition occurs at 185.5 ± 1 K in the second layer. Neutron diffraction is used to establish that the melting of the pentane monolayer takes place between 101 K and 105 K. Computer modeling studies indicate that the pentane molecules adsorb with the molecular axis parallel to the substrate plane. These results suggest that the monolayer forms a solid with a rectangular unit cell, consistent with the neutron diffraction measurements.

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