Raman and nuclear magnetic resonance spectroscopic investigations of water structure in aqueous univalent electrolyte solutions.

摘要

Polarized and unpolarized Raman spectroscopy of the water O--H stretching vibration and nuclear magnetic resonance (nmr) chemical shifts of aqueous electrolyte solutions were used to examine the changes in the structure of pure water clusters that occur with changes in univalent electrolyte concentration.; Unpolarized Raman spectroscopic observations were used to provide support for the view that the environment of the electrolyte in the liquid is that of a liquid crystalline hydrate. A two state model of solution structure, beta(alphaI water + (1-alpha)Iconcentrated electrolyte ), was applied to Raman spectra of nine univalent electrolytes. Analysis of the resulting fits suggests that the presence of the hydrated electrolyte clusters perturbs the structure of the bulk water clusters in the solution. The analysis also reveals the approximate hydration value for the ion pair.; Depolarization ratios calculated from polarized Raman spectra revealed changes in water structure symmetry. The changes are correlated to changes in the apparent density of bulk water.; Nmr chemical shifts plotted as a function of electrolyte concentration were used to demonstrate further changes in water structure. Changes in the hydrogen bond strength of water occur as a result of changes in electrolyte concentration. An equilibrium shift in water structure occurs subsequent to changes in the hydrogen bond strength. A hypothesis that the minimum in the specific heat capacity of electrolyte solution is caused by a weak continuous transition in the water structure is supported by correlation of the changes in the slope of the nmr chemical shifts to the minimum in the specific heat capacity.