Sorption-diffusion in heterogeneous systems. Part 1.—General sorption behaviour and criteria

Abstract

In diffusion-controlled sorptive processes, Fick's second equation can be solved analytically only for special functional forms of an immobilization isotherm. The dissimilarity in the formal appearance of attainable solutions hides common features of physical behaviour. The rate processes can, however, be broadly characterized by adopting a modified characteristic time constant containing constants which characterize geometrical and equilibrium properties of the systems. In the absence of information concerning the shape of the immobilization isotherm, diffusion coefficients of the mobile species can then be derived from observed “apparent diffusivities” within ±30 % of uncertainty in magnitude.