Definitions and Experimental Observations

Let us first consider diffusion in an idealized single cylindrical pore. Pick s law for a binary system with equimolar counter diffusing occurring is  

Equation 3.5.a-2 was derived assuming totally random, or diffusive, collisions of the gas molecules with the wall, which is reasonable when the pore size is still large with respect to molecular dimensions (but much smaller than the mean free path). A further extension of this reasoning is to the case where the pore size is, in fact, of the same order of magnitude as the molecules themselves. Weisz [32] terms this region configurational diffusion, and Fig. 3.5.a-l presents his estimate of the order of magnitude of the observed diffusivities. 

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