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Systematic variations of Ion hydration in aqueous alkali metal fluoride solutions

Buchner, R., Wachter, W. and Hefter, G. (2019) Systematic variations of Ion hydration in aqueous alkali metal fluoride solutions. The Journal of Physical Chemistry B, 123 (50). pp. 10868-10876.

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Aqueous solutions of three alkali metal fluorides—NaF, KF, and CsF—have been studied by dielectric relaxation spectroscopy (DRS) over the frequency range 0.2 ≲ ν/GHz ≤ 89 at 25 °C and at concentrations 0.05 ≲ c/mol L–1 ≲ 1. The combination of these measurements with analogous literature data for RbF(aq) and M′Cl(aq) (M′ = Li, Na, K, and Cs) made possible a systematic analysis of the hydration of F– and the alkali metal cations. Unlike the other halide ions, F– was found to have a well-defined hydration shell which contains ∼7 water molecules, consistent with the appearance of a “slow”-water mode at ∼10 GHz in the spectra. Limiting total effective hydration numbers (Zt0) for M′F(aq), obtained from the solvent-related modes, did not follow a simple sequence, varying in the order CsF < NaF < RbF < KF. However, it is shown that this anomalous sequence results from subtle variations in the strength of the M′+–OH2 bonding. Thus, it was established that Zib0(M′+) values, corresponding to the numbers of strongly (“irrotationally”) bound (ib) water molecules around the cations, do vary with charge density in the order Li+ ≫ Na+ > K+ > Rb+ > Cs+. It was also found that Zs0(M′+), the number of moderately bound (“slow”) water molecules, varied in the same order: K+ > Rb+ > Cs+. However, the presence of ib water molecules in the hydration shells of Li+ and Na+ attenuates their further interaction with surrounding water molecules such that Zs0(M′+) ≈ 0 for both ions.

Item Type: Journal Article
Murdoch Affiliation(s): Chemistry and Physics
Publisher: American Chemical Society
Copyright: © 2019 American Chemical Society
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