Structural analysis of ion semi-permeable membranes by co-ion uptake and diffusion studies

Abstract

Three different strongly basic exchangers with similar co-ion uptake behavior have very different co-ion diffusion characteristics. Not only are there differences in the order of magnitude of the co-ion permeabilities P at the same external electrolyte concentration c, but also marked differences in the shape of the P-c curves, which can be interpreted in terms of the internal micro- structure of the (inhomogeneous)membrand. Other structural details can be elucidated from the fact that polyvalent co-ions are more strongly excluded from local regions of high counterion concentration than are monovalent co-ions, and so lead to different P-c relations. Furthermore there was observed, especially for polyvalent ions, a great difference between the mechanisms of steady-state through-diffusion and of transient diffusion out of a saturated membrane, which also permits interpretation in terms of internal structure. The details obtained by the mathematical analysis of the diffusion data agree with, and go considerably beyond, what can be ascertained by electron microscopy. (auth)