Single-file diffusion process

Since the fraction of empty sites in a zeolite channel determines the correlation factor (Section 5.2.2), as is well known from single-file diffusion in the pores of a membrane, the strong dependence of the diffusion coefficients on concentration can be understood. This is why a simple Nernst-Planck type coupling of the diffusive fluxes (see, for example, [H, Schmalzried (1981)]) is also not adequate. Therefore, we should not expect that sorption and desorption are symmetric processes having identical kinetics. Surveys on zeolite kinetics can be found in [A. Dyer (1988) J. Karger, D.M. Ruthven (1992)]. 

When investigating single-file diffusion in zeolites, one must face not only the general experimental problems of diffusion studies with zeolites, but also additional aggravations. These are due to possible transitions between the regimes of single-file and normal diffusion and the influence of the real structure on these transitions. There is also substantial need for a satisfactory and handy theoretical description of the expected processes. Sects. 2 and 3 of this chapter are devoted to this problem, while Sect. 4 provides a survey of the available experimental data in this rather new held of research. 

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