The thermodynamic properties of n-octane, nnonane, and n-decane adsorbed on the MgO(100) surface were investigated using high-resolution, volumetric adsorption isotherms in the temperature ranges of 225−295, 245−280, and 250−310 K, respectively. Two distinct molecular layers were observed in all isotherms. The heat, differential enthalpy, differential entropy, and isosteric heat of adsorption were determined. The temperature dependence of the two-dimensional compressibility was used to identify that two-phase transitions occur for the first and second layers, respectively. The average area per molecule of 137 ± 5,128 ± 5, and 202 ± 5 Å 2 for octane, nonane, and decane, respectively, suggests that the carbon backbone is preferentially oriented parallel to the MgO(100) surface. A substantial set of molecular dynamics (MD) temperature and coverage runs for all three molecules using the COMPASS force field were used to calculate both the minimum energy configurations for individual molecules and near-completed layers on the MgO(100) surface. These calculations support the thermodynamic evidence of the carbon backbone oriented parallel to the surface and additionally suggest a preferential alignment of the molecule along the ⟨11⟩ and ⟨10⟩ directions in the surface (100) plane. The MD simulations were used to evaluate the distribution of the molecules perpendicular to the MgO(100) surface as a function of temperature and nominal surface coverage. Evidence is also recorded that suggests that the interface broadens, the orientational order decreases, and liquid-like layers appear as the temperature is increased. As observed previously with the behavior of heptane on MgO(100), an entropic contribution to the free energy is apparent as a result of the odd number of carbon atoms in the nonane backbone. The near surface layer of all three molecules appears to remain orientationally ordered (i.e., noticeably more planar) and stabilized (i.e., more solid-like) at substantially higher temperatures when the second and third layers are observed to be liquid-like. This observation is consistent with earlier experimental observation of rare gases and alkanes on graphite that indicate that the near-surface layer melts at temperatures above the bulk melting temperature.

Thermodynamic properties of n-heptane adsorbed on the MgO(100) surface were investigated using high-resolution, volumetric adsorption isotherms in the temperature range of 205–275 K. Two distinct molecular layers were observed in all isotherms. The heat, differential enthalpy, differential entropy, and isosteric heat of adsorption were determined. Using the temperature dependence of the two-dimensional compressibility, two phase transitions were observed at 246.0 ± 2.0 K and 249.3 ± 1.2 K for the first and second layers, respectively. The average area per n-heptane molecule adsorbed on MgO was estimated to be 99 ± 10 Å2, suggesting that the carbon backbone is preferentially oriented parallel to the surface. The COMPASS force field was used to calculate the minimum energy configurations of an n-heptane molecule on the MgO(100) surface. The calculations support experimental evidence of the carbon backbone oriented parallel to the surface and additionally suggest a preferential alignment of the molecule along the ⟨11⟩ and ⟨10⟩ directions in the surface (100) plane.

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