Hydration Structure of Na+ in Concentrated Aqueous Solutions

X-Ray diffraction and Raman spectroscopic measurements have been carried out for aqueous 10 mol% NaX (X = Cl, Br, ClO4, and NO3) solutions in order to determine the hydration structure of Na+ in concentrated aqueous solutions. Isotropic Raman spectra for NaX–H2O solutions exhibit a polarized peak centered at 183—187 cm−1. The corresponding peak in the isotropic spectra for NaX–D2O solutions is shifted to the lower frequency side by ca. 10 cm−1, indicating that the peak can be assigned to the totally symmetric vibrational mode of hydrated Na+, Na+(H2O)n. The least-squares fitting analysis was applied to the X-ray interference term observed for these solutions in order to discuss the hydration number of Na+ in concentrated aqueous solutions.