Thermodynamic and magnetic resonance studies on the hydration of polymers: I. Interactions of water in a synthetic cation-exchange resin |
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Authors: | M. Busch E. v. Goldammer |
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Affiliation: | (1) Institut für Biophysik, Ruhr-Universität Bochum, Postfach 10 21 48, D-4630 Bochum 1, West Germany |
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Abstract: | Measurements of NMR spin-lattice relaxation times T1 were performed for sorbed H2O and D2O in a sulfonated ion-exchange resin at varying degrees of hydration with alkaline cations as counter ions. From the sorption isotherms at three different temperatures the partial molar enthalpies![Delta](/content/h47k213765j5q710/xxlarge916.gif) and entropies![Delta](/content/h47k213765j5q710/xxlarge916.gif) of sorption show a minimum for all alkaline cations at water concentrations of n 0.8, i.e., there are 0.8 water molecules per –SO3– group. The first water molecules sorbed in the ion-exchange resin matrix are characterized by anisotrtopic rotational diffusion processes with correlation times of the order of 1 50 ns and 2 30 ps, respectively. This indicates that they are located in the electrostatic field between the corresponding ion pair. Although these two correlation times are very similar at a given temperature for all alkaline cations studied in the present investigation, the existence of a second spin-lattice relaxation time for sorbed H2O at n=0.8 indicates that for Cs about 50% of the sorbed water diffuses between locations in the resin. This fraction decreases with ionic radii and with falling temperatures. For Li the amount is less than 20%. |
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Keywords: | NMR-relaxation hydration ion-exchange resin enthalpy entropy |
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