Simulations of the structure and properties of amorphous silica surfaces

The structure and transport properties of solid surfaces have been described using models of varying complexity and rigor without systematic comparisons among available methods. Here, we describe the surface of amorphous silica using four techniques: (1) an ordered surface created by cutting the structure of a known silica polymorph (alpha -cristobalite); (2) an unrelaxed amorphous surface obtained by cutting bulk amorphous silica structures created by molecular dynamics methods; (3) a relaxed amorphous surface created by relaxing the amorphous surface; and (4) a random surface created by Monte Carlo sphere packing methods. Calculations of the adsorption potential surface and simulation of the surface diffusion of weakly bound adsorbates (N-2, Ar, CH4) interacting via Lennard-Jones potentials with these surfaces were used to compare surface models and to judge their fidelity by comparisons with available experimental values. Similar heats of adsorption were obtained on the relaxed, unrelaxed, and random surfaces (+/-0.5 kJ/mol), but the relaxed surface showed greater heterogeneity with a wider distribution of adsorption energies. Surface diffusion on the relaxed surface was slower than on the other surfaces, with slightly higher activation energies (0.5-1.0 kJ/mol). Rigorous comparisons between simulated and experimental surface diffusivities are not possible, because of scarce surface diffusion data on well-characterized surfaces. The values obtained from simulations on silica were similar to experimental surface diffusivities reported on borosilicate glasses. (C) 2001 Published by Elsevier Science Ltd.