Surface diffusion of organic vapor mixtures through porous glass

Abstract

This study reports the experimental results of surface diffusion on porous glass for four vapor mixtures, i.e. benzene/hexane, benzene/cyclohexane, benzene/acetone and benzene/methanol, at 308 K. For all components, the surface diffusion flux was changed by the presence of the accompanying component. This suggests that interaction among permeants plays a significant role in the surface diffusion of the mixtures. To discuss the interaction quantitatively, the adsorption equilibrium and surface diffusion coefficients, which determine surface diffusion rates, were obtained experimentally. For the adsorption equilibrium, an extended BET equation was derived by introducing interaction terms among different components; the equation expressed the experimental results of the adsorption equilibrium well. For the surface diffusion coefficients, the hopping model was extended to a binary system by considering the changes in the potential barrier of hopping due to the interaction between a diffusing molecule and an adsorbed molecule; the extended model explained the dependence of the surface diffusion coefficients on the amounts adsorbed up to monolayer capacities. These results show that interaction among components affects both the adsorption equilibrium and the surface diffusion coefficients. Furthermore, correlations were found between the interaction term in the expression of surface diffusion coefficients and the corresponding interaction term in the expressions of the adsorption equilibrium.