Effect of temperature on hydrogen isotope effects in dehydration of ion-exchange resins: Alkali metal ionic (Li<sup >+</sup>, K<sup >+</sup> and Cs<sup >+</sup>) forms of Dowex-50W resins

Abstract

Hydrogen isotope effects in dehydration of alkali metal ionic (Li⁺, K⁺ and Cs⁺) forms of Dowex-50W resins (with 2, 4, 8 and 12% divinylbenzene content), having n_w, (number of moles of water per equivalent of resin) values of ~2 to ~27, have been determined at 303, 318 and 333K using a Rayleigh distillation type technique. For very low n_w, values, the observed single stage separation factor, α_obs" for all the systems is independent of temperature and has a constant value of ~1.025 ± 0.004, which is much lower than α_w<i>, </i> the single stage separation factor for pure water distillation at that temperature. In the case of Li<sup >+</sup> resins at all temperatures and K<sup >+</sup> and Cs<sup >+</sup> resins at higher temperatures only, the <i>α</i><sub >obs</sub> varies with n<sub >w</sub> as follows: as n<sub >w</sub>, -value increases, <i>α</i><sub >obs</sub> increases gradually, reaches a maximum value, α<sub >max</sub> (at n<sub >max</sub> which is greater than <i>a_w. </i> At still higher n<sub >w</sub>, -values the α<sub >obs</sub> decreases gradually and finally approaches the α<sub >w</sub>. In these systems where a maximum is observed in the ε<sub >obs</sub>[= α<sub >obs</sub> - 1] versus n<sub >w</sub>, plots, the data have been analysed in terms of the three-water sub-system-model for cation hydration and the ε<sub >1</sub>, ε<sub >2</sub> and ε<sub >3</sub> representing the separation factors for the water molecules in the primary shell, the secondary shell and the bulk water, respectively have been obtained. Analysis of the results for different temperatures shows that ε<i>₁</i>, and ε<i>₂</i> originating from electrostatic interaction, are temperature independent. The fact that ε<sub >3</sub>,= ε<i>_w </i> (within the experimental error), shows that the effect of cationic charge does not extend beyond the second layer of water molecules. At lower temperature (303K), ε<sub >obs</sub> versus n<sub >w</sub>; plots for K<sup >+ </sup>and Cs<sup >+</sup> resins do not exhibit a maximum and ε<sub >obs</sub> approaches ε<sub >w</sub> at high n<sub >w</sub> , values. Further, ε<sub >2</sub> (353 or 303K) < ε<sub >w</sub>(303K) for these systems. These data show that water structure is preferentially formed at high n<sub >w</sub>., values at the expense of hydration shells, i.e., the hydration shells are destroyed by the more stable water structure at these temperatures. For any cationic form of the resin, the sharpness in the maximum of ε<sub >obs</sub> versus n<sub >w</sub>, curve increases with increase in temperature, implying that with increasing temperature the bulk water structure is destroyed more rapidly than the hydration shell water structure. These observations are in consonance with the views reported in literature (see ref. 13) on electrolyte solutions.