Diffusion of fluids in confined geometry.

Pulsed field gradient NMR measurements of restricted self-diffusion of three nonspecific liquids are reported in well characterised samples with average pore diameters ranging between 35 A and 500 A. Structural information has been obtained in the long-times and small "wave-vectors" regime, using an exact analytic solution for the related problem of "thin" semi-permeable barriers. Experimental results suggest that the permeability of a porous medium depends on the porosity (connectivity) of the pore space and the molecular size of the self-diffusing molecules.