Free-Volume Theory Applied to Diffusion in Liquids: A Critical Analysis

Abstract

The aim of this paper is to evaluate free-volume theory when applied to diffusion in liquids. A classical strategy for computing diffusive mass transfer consists of using the Vignes equation, but this empirical interpolation is restricted to binary systems. Thus, particular emphasis has been placed on the development of a multicomponent model based on Vrentas and Duda approach, which is widely accepted for polymeric systems. For weakly interacting species, it is shown that robust mass-transfer computations can be achieved if local tracer diffusivities, which can be theoretically computed by free-volume theory, are known. A case study based on the benzene−cyclohexane system has been performed and shows that the simplest free-volume formalism does not achieve a good prediction of experimental tracer diffusion data. Nevertheless, accurate interpolations are obtained when the excess volume is taken into account. Finally, the empirical Vignes relationship is shown to be consistent with basic free-volume arguments.