Coupling of ion and water fluxes in synthetic membranes

When the salt fluxes across a fixed charge membrane predicted by the Teorell-Meyer-Sievers (TMS) theory are compared with experiment it is found that in low-resistance ion-exchange membranes the coupling of ion and water fluxes should be taken into account. An examination of equivalent conductance and electro-osmotic data show that coupling occurs to different extents for counterions and co-ions. A satisfactory practical correction is achieved when all sorbed electrolyte, co-ions and counterions, is treated as fully coupled with the osmotic flux while those counterions that exactly balance the fixed charges are regarded as unaffected by the osmotic flux. This procedure can be shown to have a phenomenological justification in terms of the behaviour of pair-wise frictional coefficients.