Proton Magnetic Resonance Studies of Ionic Solvation in Ion-Exchange Resins. Part II

The proton magnetic resonance spectra of anion-exchange resin Dowex AG1 of various crosslinkages ranging from X1 to X10 in the F−, Cl−, Br−, I−, NO3−, and ClO4− forms have been recorded and the values of the molal chemical shift, δ0x−, are found to be −0.035, 0.026 (assumed), 0.035, 0.045, 0.037, and 0.058 p.p.m./mol, respectively, for the anions. These values are generally lower compared to those observed in corresponding aqueous solutions, indicating that ion-solvent interactions are similar in both the resin matrix and the aqueous solution but there is probably increased ion association in the resin phase. The effective hydration numbers have been deduced from the temperature dependence measurements of proton chemical shifts for the alkali-metal ions and for a few anions in various crosslinkages of Dowex AG 50W and Dowex AG1 resins. The values of the hydration number decrease as the crosslinkage increases for the alkali-metal ions. The decrease in the hydration number at high crosslinkage is attributed to ion association in the resin phase. The reversal of selectivity in K+/Cs+ system below X16 in the sulfonated resin is explained on the basis of decrease in the hydration of K+ form of resin compared to that of Cs+ form, consistent with the experimental observations. The electrostatic interactions between the counterion and fixed group and ions and solvent in the resin phase have been discussed in the light of the proposed models.