Molecular dynamics study of the adsorption behavior of normal alkanes on a relaxed α-Al2O3 (0001) surface

Adsorption behavior of two normal alkanes (C-11 and C-200) with explicit hydrogens on a relaxed alpha-Al2O3 (0001) surface at 150 degrees C was studied using classical molecular dynamics (MD) simulation along with the use of the COMPASS force field. Prior to the MD simulations, first principle density functional theory (DFT) calculations were carried out to relax the alumina (0001) surface that was created by cleaving the corresponding crystal structure. It was found that orientation of the adsorbed segments and the number of carbons adsorbed seem to be insensitive to the chain length. The computed adsorption energy per mole of adsorbed CH2 agrees well with those obtained from inverse gas chromatography measurements. Also, both simulation and experimental results showed that the adsorption energy decreased with increasing chain length. It was observed that molecular planes containing the skeletal carbons of the alkane segments that formed the first adsorption layer tend to orient parallel to the alumina surface, regardless of chain length. However, there existed a small amount of C-11 segments adopting the perpendicular orientation. For both C-11 and C200, the average distance between the neighboring adsorbed segments was determined to be 4.6 angstrom which is slightly lower than the distance between two adjacent normal alkane molecules in their crystalline form but is equal to the interatomic distance between the aluminum atoms in the surface layer of the alumina surface. The present work suggests that both the orientation and adsorption energy of normal alkanes on the alumina (0001) surface depends on whether the surface is relaxed or not.