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

Abstract

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.