Comparative study on the properties of hydration water of Na- and K-halide ions by Raman OH/OD-stretching spectroscopy and dielectric relaxation data

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" T_str and mole fraction of the high T_str water region, giving the hydration number N_hyd, for each electrolyte solution. The determined T_str 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 N_hyd were observed between NaX and KX and among even halide ion species within the experimental errors. Measured N_hyd values of 25-27 were much greater than the reported numbers by NMR chemical shift and ¹⁷O 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 T_str 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 H₂O/D ₂O mixed solvent of 50 mol %, and we found that both OH-stretching and OD-stretching bands have almost equivalent T_str ≈ 330 K and mole fractions with each other.