Comparative Study on the Properties of Hydration Water of Na- and K‑Halide Ions by Raman OH/OD-stretching Spectroscopy and Dielectric Relaxation Data
- Publication date
- 2014
- Publisher
Abstract
Properties
of hypermobile water (HMW) were studied by Raman OH-stretching spectroscopy.
Hydration water properties measured by Raman OH-stretching spectra
of NaX/KX (X: Cl, Br, I) solutions (0.05–0.2 M) were comparatively
analyzed with the data by dielectric relaxation spectroscopy (DRS),
NMR, and statistical mechanical studies. The Raman OH-stretching spectra
were well-fitted with linear combinations of the spectra of pure water
both at the same and the higher temperatures. The fitting analysis
determined the “structure temperature” <i>T</i><sub>str</sub> and mole fraction of the high <i>T</i><sub>str</sub> water region, giving the hydration number <i>N</i><sub>hyd</sub>, for each electrolyte solution. The determined <i>T</i><sub>str</sub> was much higher than the solution temperature
of 293 K for each tested salt and was higher for larger halide ions,
consistent with commonly known “structure-breaking”
order Cl < Br < I. No significant differences in <i>N</i><sub>hyd</sub> were observed between NaX and KX and among even halide
ion species within the experimental errors. Measured <i>N</i><sub>hyd</sub> values of 25–27 were much greater than the
reported numbers by NMR chemical shift and <sup>17</sup>O NMR relaxation
studies and comparable to the numbers of hypermobile water reported
in the previous DRS studies. The results indicated that the hydration
region around NaX or KX measured by the present Raman study was nearly
overlapped with the region of HMW by DRS. It was also suggested that
differences in the ion size effects on <i>T</i><sub>str</sub> and the DR frequency resulted from the sensitivity difference to
long-range many-body interactions among water molecules. High structure–temperature
regions were also detected by the analysis of OH-stretching and OD-stretching
bands for 0.2 M NaI in H<sub>2</sub>O/D<sub>2</sub>O mixed solvent
of 50 mol %, and we found that both OH-stretching and OD-stretching
bands have almost equivalent <i>T</i><sub>str</sub> ≈
330 K and mole fractions with each other