Wetting and nonwetting of molecular films at zero temperature

Abstract

The wetting characteristics of classical rare gases adsorbed on smooth solid substrates at zero temperature are studied as a function of the molecule-substrate interaction. Comparisons of the relative stabilities of monolayers, bilayers, and trilayers with respect to each other and to the bulk solid phase show a succession of growth regimes with increasing substrate interaction strength relative to molecular pair interactions: nonwetting, incomplete wetting with maximum thicknesses of one layer, incomplete wetting with a mutually commensurate bilayer, and complete wetting. Their order of appearance and the values of substrate strength at crossover boundaries depend sensitively on substrate potential shape, substrate screening of interactions between adsorbate atoms, and the magnitude of the bulk solid energy. Close correspondence with experiment is obtained from Cole-Klein potentials with enhanced first-layer well depths and with substrate screening. This model has a narrow complete wetting region with boundaries at relative substrate strengths near those observed in films on graphite. The complete wetting regime is bounded on the high-strength side by incomplete wetting films consisting of mutually commensurate close-packed bilayers, and on the low-strength side by close-packed monolayers. At still weaker substrate potentials the adsorption is nonwetting. The layer densities of the incompletely wet films are found to be several percent greater than surface planes of the bulk solid, supporting the conjecture that the transition from complete to incomplete, with increasing substrate strength, is driven by structural mismatch between the film and the bulk solid. These features are consistent with observations of light molecular gases on graphite and other surfaces.