Ionic reorientational motions in aqueous solutions: a depolarized light scattering study of NO3-and SCN-

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Abstract
Depolarized Rayleigh scattering has been measured in the frequency range 0·2 cm-1 to 200 cm-1 using a triple monochromator and Fabry-Perot interferometry for solutions of ammonium, sodium and lithium nitrates, for nitric acid and ammonium and potassium thiocyanate over a range of concentrations and temperatures. From comparisons with scattering from water and alkali halide solutions it is concluded that the narrow central component of the scattering arises predominantly from anion reorientational motions. From intensity measurements there is no evidence for significant anion-anion correlations. Infinite dilution relaxation times are discussed in relation to the motions of water molecules in the hydration spheres. Except for very dilute solutions, a simple hydrodynamic model accounts for the temperature and concentration dependence of SCN- reorientation. For NO3 - it is necessary to assume that the boundary conditions are concentration dependent. In very concentrated nitrate solutions the lineshape is non-lorentzian due, it is argued, to specific cation-anion interactions.