Monte Carlo investigations of hexadecane films on a metal substrate

The structural properties of liquid n-hexadecane films, ranging in thickness from 1 to 4 nm and adsorbed on a flat metal (Au) surface, have been studied by the configurational-bias Monte Carlo method. Over the temperature range 350 K to 650 K, the substrate was found to induce a well-defined layering of the molecules for the portion of the film closest to the surface. Even at the highest temperature, this layering extends beyond a monolayer. In the surface layer, molecules are predominantly oriented with their long axes and their backbone zig-zag planes parallel to the substrate. Molecules close to the surface are also characterized by fewer gauche defects and are more densely packed than those present in the bulk region of the film. At a given temperature, the density and the characteristics of the molecules in the first adsorbed layer are substantially independent of the total thickness of the film. With increasing temperature, the layering diminishes and molecules in the first layer exhibit properties closer to those of the bulk liquid at a lower temperature. Comparisons are made with experiments and previous simulation studies on related systems.