Ion Association Effects of Lipophilic Quaternary Ammonium Salts in Ion-Exchange and Potentiometric Selectivity

Abstract

Strong effects of ion association on the anion-exchange selectivity in systems water -solutions of high quaternary ammonium salts (QAS) in organic solvents as well as on the potentiometric selectivity of plasticized polyvinylchloride (PVC) membranes containing QAS as ion exchangers have been established experimen¬tally and substantiated theoretically. Based on the linear Gibbs energy relations (LGER) and ion association theory by Eigen-Denison-Ramsey-Fuoss, an approach to separate estimation of the parts played by the solvation and ion association fac¬tors in ion-exchange and potentiometric selectivity has been proposed. The results obtained serve as the basis for revision of the formed notions about QAS as of «nonselective» ion exchangers and enable the development of methods for the con¬trol of ion-exchange and potentiometric selectivity using the factor of ion associa¬tion. Specifically, it has been demonstrated that variation of steric accessibility of the QAS exchange center is a powerful means to control the selectivity. It has been found experimentally that due to varying steric accessibility of the QAS exchange center the selectivity values have changed by 3 orders of magnitu¬de in case of single- charged anions exchanged for single-charged ones and by more than 7 orders - in case of double-charged anions exchanged for the sing¬le-charged ones. The above-mentioned effects revealed also in the potentiometric selectivity of QAS-based PVC membranes and to some extent - in the potentiometric selectivity of the membranes based on neutral anion carriers, doped with QAS for provision of anion permselectivity, are of great practical importance for the development of ISE with improved selectivity.