A Model for the Surface Diffusion of Molecules on a Heterogeneous Surface

Abstract

A new model to explain the effects of the variation of temperature and surface coverage on the surface diffusion of molecules on heterogeneous surfaces has been presented. In this model, the emphasis is placed on understanding the influence of surface structure and geometry on the surface diffusion process. The model is a multifractal version of the so-called “homotattic patch” model that is often used in studies of gas adsorption on heterogeneous surfaces. Some previous experimental results, reported in the literature, on the surface diffusion of benzene on a fumed silica, have been reanalyzed in the light of the new model. This reanalysis of past data has suggested that, at a given value of overall average surface coverage, the surface diffusion flux in this system is dominated by only a small fraction of the total number of molecules on the surface. In addition, the small fraction of molecules that are the predominant contributors to the surface diffusion flux are generally adsorbed within particular regions of the surface. The particular regions contained within the different surface patches in the model were found to have differing degrees of surface roughness, which could be characterized by different surface fractal dimensions. This work has also shown that it may be possible to use surface diffusion methods to determine the characteristic size and spatial distribution of the different surface patches with different adsorption energies in homotattic patch models of heterogeneous adsorbent surfaces.