Diffusion of water and ethanol in ion-exchange membranes: limits of the geometric approach

Diffusion coefficients of water as a solvent and ethanol as a tracer in sul phonated polyethylene (SPE) were measured by the pulsed-gradient spin-echo NMR method. Comparison with the data available for a number of ionic and no n-ionic polymers and solvents shows that, except for Nafion, all the materi als exhibit a fairly universal dependence of diffusivity on the volume frac tion of the solvent. This result is explained on the basis of the geometric obstruction approach and general conditions are formulated for the observe d universal dependence. Explanations are proposed for the peculiar behaviou r of Nafion and for the low diffusivity of ethanol within the same framewor k. (C) 1999 Elsevier Science B.V. All rights reserved.