The determinants of divalent/monovalent selectivity in anion exchangers

Abstract

This research demonstrates that it is the distance of fixed-charge separation in the resin which is the primary determinant of divalent/monovalent selectivity. Anion resins, particularly acrylics and epoxies, with closely spaced N-containing (amine) functional groups are inherently divalent-ion selective. This is because the uptake of a divalent anion, e.g. sulfate, requires the presence of two closely-spaced positive charges.Results from this study of 30 commercially-available, strong- and weak-base, anion resins indicate that in order to bring positive charges into close proximity within a resin one can: (1) incorporate the amine functional groups into the polymer chains as opposed to having them pendant on the chains; (2) minimize the size and number of organic ("R") groups attached to the N atom, i.e., minimize the size of the amine; and (3) maximize the resin flexibility, i.e., its ability to reorient to satisfy divalent counterions, by minimizing the degree of crosslinking.The distance-of-charge-separation theory is not restricted to divalent anion exchangers but also applies to cation exchangers and to polyvalent ions in general.