Thermodynamic and Modeling Study of Thin n-Heptane Films Adsorbed on Magnesium Oxide (100) Surfaces

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 angstrom(2), 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.