Divalent/Monovalent Selectivities and Secondary Interactions of Multibasic Acids on Anion Exchange Resins

The ion exchange equilibria of Br^– on three strongly basic anion exchange resins of different exchange capacities and different cross-linking degrees by five alkanoates (MC_m), a phosphate, and eight alkanedioates (DC_n; m,n = number of carbon atoms) were analyzed, and the selectivity coefficients for their monoanion and dianion, K₁₁ and K₂₁ were evaluated as a function of percent exchange (%E). The decrease in the K₁₁ values of MC_m with an increase in %E was attributed to the interference with hydration of MC_m in the resin phase, while the increase in the K₁₁ values of DC_n to the intermolecular hydrogen bonding between exchanged acidic anions. The log K₂₁ values of DC_n at %E = 0 plotted against n showed the minimum rather than the maximum as described previously. The reason has been discussed in terms of the hydrophobic interaction of DC_n with the polymer matrix, intramolecular hydrogen bonding, and the charge density.