Selective ion transport through hydrated cation and anion exchange membranes I. The effect of specific interactions

Ion partitioning between membranes with fixed ionic groups and aqueous solutions containing different simple salts and acids (LiCl, CsCl, HCl) is studied experimentally for dilute and concentrated solutions and cation and anion exchange membranes including Nafion®, Aquivion®, sulfonated polystyrene, sulfonated polyether ketone, sulfonated poly(phenylene sulfone), poly(acrylic acid) and a poly(phenylene oxide) functionalized with quaternized ammonium groups. For the given membranes and ions, ion partitioning is fully described by the degree of ion pairing which is controlled by specific interactions between counter- and fixed-ions (site-binding), ion hydration and membrane swelling. In the case of cation exchange membranes, specific interaction is highest for the most acidic fixed ionic groups and cations with the highest polarizability and lowest electronegativity (here Cs+). Data on ion partitioning combined with data on ion diffusion (obtained by 7Li, 133Cs NMR) and direct electrochemical transference number measurements show that selective ion uptake does not significantly change counter-ion transference numbers (compared to transference numbers of the corresponding aqueous solution), but it does reduce the total conductivity. Especially for membranes in highly concentrated salt solutions, where Donnan-exclusion does not apply, all ionic transference numbers approach these of the corresponding aqueous solutions. Under these conditions, the transport properties of cation and anion exchange membranes are similar and close to the transport characteristics of simple diaphragms. With appropriate implementation of counter-ion association/dissociation equilibria and ion hydration numbers, the Donnan-equation describes co- and counter-ion partitioning in a quantitative way. Apart from counter-ion/co-ion discrimination in dilute solution, no selective transport of specific ions significantly beyond selectivity characteristics of homogeneous aqueous solutions is expected. For larger ions or other dissolved species within ionomers with narrow aqueous ionic domains, however, there are additional steric effects (ion-sieving) which will be discussed in the second part of this series while the third part will present results of atomistic modelling.