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 behaviour 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 differe n m in the shape of the P-c curves, which can be interpreted in terms of the internal micro-structure of the (inhomogeneous) membrane. Other structural details can be elucidated from the fact that polyvalent co-ions are more strongly excluded from local regions of high counter-ion 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 throughdiffusion 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. SYMBOLS A, B minimum and maximum values of M in exchanger ; C d D f ko I m M R t P vr U W Y z ' 8 U Y 9 I -. concentration, mole/l. ; thickness of membrane, cm ; diffusion coefficient, cmZ/sec-l; parameter = E/EM ; constant of site distribution function; integral of eqn. (9) ;concentration, mole/kg H20 ; local counterion concentration in the absence of external electrolyte, equiv./kg H20 ; fraction of uptake electrolyte released by diffusion ;time, sec; permeability, cm2 sec-1; volume fraction of purely organic matrix ; function of 4 ;water content g H@/g swollen membrane ; parameter = M/(vgzgm) ; exponent of " site " distribution function ; ionic charge counterion ; constant related to activity coefficients ; activity coefficients ; volume fraction (from 0 to 1) ; number of ions (subscripts c and g indicate co-ions and counter ions respectively) ;indicates observable mean in exchanger phase ; indicates data of region of maximum continuity.bars on M , c, y indicate exchanger phase ; -*In the uptake of monovalent co-ions on strong base anion and on strong acid cation permeable membranes,l the Donnan equilibrium in its usual form is inapplicable to the observed overall uptake values for a concentration range of many 2639

Particle Interaction and Soil Swelling

“…The behaviour of clay water systems is so complex viewed from both the point of view of condensed structures with varying pore sizes and also from the relationship of ions with the charged surfaces, that it is clear that diffuse double layer theory is not applicable for predicting film thickness, or ion partition between the solid and liquid phases in such a system. By using co-ion exclusion and diffusion measurements Crabtree and Glueckauf [42] have been able to describe the structure of an ion exchange resin with variable pore sizes. Such an approach appears more germane for the examination of soil solution composition and particle interaction.…”