Raman vibrational dynamics of hydrated ions in the low-frequency spectral region

The hydration structure of ions in aqueous environments can have a significant influence on their chemical and biological properties. Due to its inherent dynamical character, determination of the hydration shell around dissolved ions has proved challenging, mainly so for cations such as sodium and potassium which form diffuse and dynamic hydrating structures. The low frequency polarized Raman spectrum, as retrieved by time resolved isotropic optical Kerr effect measurements, is sensitive to structural fluctuations and can reveal information about ion-water interactions through their Raman active vibrational modes. Here we study a series of mixtures of sodium, potassium and lithium hydroxide solutions by changing cation concentration pairwise (namely, sodium/potassium or sodium/lithium) while keeping constant the hydroxide concentration. The hydroxide-water hydrogen bond vibration, which produces a well-defined isotropic Raman mode, appears at higher frequencies from the cation-water Raman active vibrations. In addition to previously reported lithium-water low frequency vibrations, clear spectral features could be resolved from the concentration studies and assigned to sodium-water hydration shell vibrations. However, potassium related low frequency spectral features remain elusive. The same method was applied to mixtures of the same cations with a halide anion (chloride) in order to rule out any specific features related to the dissolved hydroxide anion. Comparison between halide and hydroxide measurements confirmed the presence of the cation modes and further revealed a low frequency spectral feature related to hydroxide induced changes in water polarizability.