Simulations of hydrocarbon adsorption and subsequent water penetration on an aluminum oxide surface

Abstract

Static and dynamic equilibrium properties of butane octane, and dodecane films adsorbed on α- Al 2 O 3 (0001) at a variety of coverages and temperatures, and the subsequent penetration of such films by 30 molecule water clusters are examined using classical molecular dynamics. Model potential functions are constructed from existing alkane united atom and “simple point charge” model water parameters, experimental alkane desorption energies and other available theoreticalinformation. The adsorbed films exhibit a distinct layering parallel to the surface, and a pronounced densification, reduction in gauche defects and orientational ordering within the innermost layer. Strong surface corrugation 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 butane molecules show any tendency to tilt their long axes out of the first layer toward the second. ( H 2 O ) 30 clusters are attracted toward the alumina surface and easily penetrate most of the adsorbed alkane films, either by displacing alkane molecules to more distant layers or causing 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 alkane coverages, lower temperatures, and longer chains.