Dynamics of Sodium and Lithium Counter-Ions and Water Molecules in Cation-Exchange Resins as Shown by NMR Spectroscopy

Abstract

The dynamics of water molecules and cations (lithium and sodium counter-ions) in cation-exchange resins were investigated by measuring relaxation time and self-diffusion coefficients of 1H, 7Li, and 23Na NMR. Five gel-type cation-exchange resins with different by the crosslinked poly(styrene sulfonic acid) copolymer swollen in water and salt aqueous solution were studied. A macroporous-type resin was studied for comparison. Activation energy for the lithium and sodium ions obtained from the Arrhenius plots of spin-lattice relaxation times T1 correlated well with the cross-linking of resins. Since T1 of the cations in every resin are shorter than in corresponding aqueous solution, the correlation time of the rotational motion of cations is longer in the resins. With higher cross-linking of the resin, the rotational motion of the cations becomes slower and more restricted. Self-diffusion coefficients of water molecules and lithium and sodium counter-ions in the gel-type resin were directly measured by pulsed field-gradient NMR methods. Translational diffusion of these species was closely related to the cross-linking of resin similar to T1. The diffusion of water molecules and cations becomes slower in higher cross-linked resin. Apparent diffusion coefficients of Li+ ions and water molecules were dependent on diffusion time. This may be explained by restricted diffusion in a three dimensional network structure of gel-heteroporous type ion-exchange resins.