SIMULATIONS OF HYDROCARBON ADSORPTION AND SUBSEQUENT WATER PENETRATION ON AN ALUMINUM-OXIDE SURFACE

Static and dynamic equilibrium properties of butane octane, and dodeca ne films' adsorbed on alpha-Al2O3(0001) at a variety of coverages and temperatures, and the subsequent penetration of such films by 30 molec ule water clusters are examined using classical molecular dynamics. Mo del potential functions are constructed from existing alkane united at om and ''simple point charge'' model water parameters, experimental al kane desorption energies and other available theoretical information. The adsorbed films exhibit a distinct layering parallel to the surface , and a pronounced densification, reduction in gauche defects and orie ntational ordering within the innermost layer. Strong surface corrugat ion allows molecules to rotate relatively freely about their long axes at intermediate temperatures and assists them in orienting their zig- zag planes perpendicular to the surface at lower temperatures. Only bu tane molecules show any tendency to tilt their long axes out of the fi rst layer toward the second. (H2O)(30) clusters are attracted toward t he alumina surface and easily penetrate most of the adsorbed alkane fi lms, either by displacing alkane molecules to more distant layers or c ausing them to pack more closely within existing layers. The molecules in the clusters tend to remain connected during penetration. Kinetic barriers to penetration become increasingly significant for higher alk ane coverages, lower temperatures, and longer chains. (C) 1997 America n Institute of Physics.