Effects of ion association of lipophilic quaternary ammonium salts in ion-exchange and potentiometric selectivity

It has been found experimentally and substantiated theoretically that the anion-exchange selectivity in water—lipophilic quaternary ammonium salts (QAS) toluene solutions systems as well as the potentiometric selectivity of plasticized PVC membranes containing QAS, are strongly influenced by ion association. In particular, it has been demonstrated that varying the steric accessibility of QAS exchange center is a powerful tool for the selectivity control. The experimental values of the selectivity change caused by variations in the QAS exchange center steric accessibility were about 3 orders of magnitude when the single-charged ions were exchanged for the single-charged ones and more than seven orders when double-charged ions were exchanged for the single-charged. The above effects have also been observed for the potentiometric selectivity of QAS-based PVC membranes and, to some extent, for the potentiometric selectivity of the neutral anion carrier-based membranes doped by QAS to provide anion permselectivity. This fact is of immediate practical interest for the development of ISE with improved selectivity. The obtained results allow to revise the generally accepted idea of QAS as “nonselective” ion-exchangers and to suggest specific ways for controlling the ion-exchange and potentiometric selectivity using the ion association as a tool.