Coupling of ion and water fluxes in synthetic membranes*

The behaviour of ion-exchange membranes has, in most circumstances, been interpreted in terms of the fixed charge theory of Teorell, Meyer and Sievers. Generally it has been recognised that some allowance should be made in the theory for coupling between ion and water fluxes. In making approximate corrections for this, it has usually been assumed that close coupling between all ion fluxes and the osmotic or electro-osmotic water flux occurs. In this paper the specific nature of ion/water flux coupling is explored and it is shown that close coupling is a good approximation for co-ion and water fluxes but that the coupling of the counter-ion and water fluxes is less strong. p]The interactions between ions, water and membrane matrix in a cation exchange membrane are revealed by determining the values of the pairwise molecular friction coefficients. These findings support long standing qualitative ideas about the interplay of electrostatic, hydration and hydrophobic interactions in membranes. p]The picture that emerges shows that an exact extension of the usual equations of the fixed charge theory to include flux coupling would be too complex to be useful. An approximate procedure is to divide conceptually the counter-ions into two groups, those balancing the fixed charges and those accompanying the sorbed co-ions, i.e. constituting sorbed electrolyte. If coupling of the former group of counter-ions with the water flux is ignored whilst close coupling is assumed between the sorbed electrolyte and the water flux, a very good correction is achieved for the overall effects of coupling.