Concentration dependence of Fickian diffusivity in solutions and sorption systems

Abstract

Using transition-state theory a general relation for the concentration dependence of Fickian diffusivity is derived. This relation is simplified by means of assumptions which are expected to reflect the behaviour of real diffusion systems. In this way one obtains two special forms of the general relation which render it possible to calculate the concentration dependence of Fick's diffusivity from experimental data. Diffusion systems are discussed for which the assumptions leading to the special equations can hold. Experimental examples are given by using diffusivity data from the literature. The concentration dependence D(c)=D(0) d ln a/d ln c determined by other authors for different diffusion systems is shown to be one of the special equations derived in this paper.