Molecular dynamics simulations of the structures of alkane/hydroxylated α-Al₂O₃(0001) interfaces

Molecular dynamics simulations were carried out on several linear alkane systems of different chain lengths, confined between hydroxylated a-aluminum oxide surfaces at experimental densities, to study structural and dynamical features of the film/surface interfaces. The simulations show that there is extensive multilayer adsorption on the hydroxylated α-aluminum oxide surface. The adsorption layers are separated by the diameter of the linear alkanes, independent of chain length. Alkanes near the surface show a high percentage of the trans conguration as well as greater densities, indicating efficient packing of chains in the inner adsorption layer. The simulations indicate that this "solidification" increases as the alkane chain becomes longer. Radial distribution functions also show a more-ordered multilayer packing structure for alkane chains physisorbed on the surface as compared to the bulk. The dynamics of chains in the first adsorption layer are strongly affected by their interactions with the solid surface. Diffusion within this innermost layer is efficient, although there is negligible diffusion out of this layer on a 500-ps time scale. The surface also affects the diffusion dynamics of the second layer, but for the smaller alkanes the diffusion dynamics of the third and other, further-removed layers are the same as those for the bulk.