Proton Magnetic Resonance Studies of Ionic Solvation in Ion-exchange Resins. Part III. Multivalent Counterions

The p.m.r. spectra of Dowex AG 50W, X8, resins containing multivalent counterions have been measured. The ionic molal shifts in the exchanger phase are compared with those reported in homogeneous aqueous solutions. The spectral results are consistent with the concentrated electrolyte model of ion-exchange resins. Concentration and temperature dependence of proton chemical shifts in multivalent ionic forms of the resin suggests that ion association at high crosslinkages is more pronounced for counterions with high charge densities. The shapes of the shift-molality plots are explained by postulating that, at high crosslinkages, Mg2+ ion and the fixed —SO3− group form "contact-ion pair" while the formation of "solvent-separated ion pair" is important in Be2+ and Al3+ forms of the resin. The anomalous trend in the hydration number values for magnesium and aluminum resinates can be understood as due to the long-range ordering of water structure in the presence of structure-forming ions. The p.m.r. spectra of resin–methanol system have been recorded at room temperature and at −65 °C. Failure to observe the resonance signal due to the cationic solvation shell at low temperature is attributed to the high degree of ion association under the conditions prevailing in the resin medium.

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.

Proton magnetic resonance studies of ionic solvation in ion-exchange resins.: Part I. Sulfonated cation-exchange resins

The proton magnetic resonance spectra of cation-exchange resins of crosslinkage X1 to X16 in the H+, Li+, Na+, K +, Rb+, Cs+, and NH4+ forms have been recorded and the values of the molal chemical shift, δM+0, are found to be −0.321, −0.005, 0.041, 0.031, 0.028, 0.015, and 0.00 (assumed) p.p.m./mole, respectively, for the cations. These values are very nearly equal to those observed in corresponding aqueous solutions, indicating that ion–solvent interactions are similar in both the resin matrix and the aqueous solution. The temperature dependence of the chemical shift shows the same trend. The effective hydration numbers calculated from the temperature dependence are 2.0 (4.81 m) for the H+ form, 4.8 (3.25 m) and 3.6 (5.26 m) for the Na+ form, 4.4 (3.9 m) for the K+ form, and 2.4 (6.18 m) for the Cs+ form. The ions in the resin phase behave as solutions of strong electrolyte, which is consistent with the polyelectrolyte gel model for the ion-exchange resin.