Solvation Structure of Bromide Ion in Anion-Exchange Resins

Abstract

The local structures of Br- in ion-exchange resins are studied with XAFS at the Br−K edge. Br--form anion-exchange resins having different ion-exchange groups, i.e., tertiary ammonium (R-3) and quaternary ammonium ion (R-4), are examined after being soaked in water, methanol (MeOH), and some polar aprotic solvents. The resins soaked in aprotic solvents give almost the same spectral features as dry resins, indicating that Br- is tightly bound by the ion-exchange sites. Br- is entrapped inside the tripod composed of three equivalent methyl groups of the ion-exchange group in the R-4 resin, while it strongly interacts with the ammonium hydrogen of the ion-exchange group of the R-3 resin. In the latter case, the scattering from methyl and methylene carbon atoms in the ion-exchange group also contributes to XAFS spectra. In contrast, characteristic features are confirmed when the resins are soaked in water and MeOH. Detailed analyses of XAFS spectra imply that two species, i.e., solvated Br- and that bound on ...