Micropores Concentration Dependence and Predicting Mixed Diffusivities

Diffusion in Micropores Concentration Dependence and Predicting Mixed Diffusivities 

For diffusion in gases and colloidal systems, concentration is the origin (or driving force ) for diffusion. Einstein first showed in 1905 that from the concentration gradient, the diffusivity is (Kauzmann, 1966) 

For diffusion in liquid mixtures, it has been argued that chemical potential is the driving force. (Haase and Siry, 1968). 

The mechanisms of diffusion in these two systems (gas and liquid) are different and unrelated diffusion in gases is the result of the collision process, whereas that in liquids is an activated process (Bird et al., 1960). Diffusion in microp-orous materials is neither gaseous nor liquid diffusion. The closest case for such diffusion is surface diffusion, where molecules hop within the surface force field (see review by Kapoor et al., 1989b). Pick s law is used for both application (in modeling of adsorption processes) and experimental measurement of diffusion. Extensive reviews are available on diffusion in microporous materials and zeolites (Karger and Ruthven, 1992 Do, 1998). A lucid discussion on the nonlinear, and in some cases peculiar, phenomena in zeolite diffusion was given 

The Fickian diffusivities for diffusion in zeolites and microporous materials are generally concentration dependent. Although a variety of concentration dependence has been reported (Yang, 1987), an increase in the diffusivity with concentration is generally the rule. The observed concentration dependence is similar to that seen for surface diffusion, that is. 

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