Probing water dynamics with OH−

Isotropic Raman spectra of aqueous solutions of LiOH, NaOH and KOH at concentrations ranging from high dilution to saturation have been measured and the frequency and width of the OH− stretching band have been analyzed. The dependence of the bandwidth on solute concentration suggests that the OH− vibration undergoes a transition from fast to slow modulation regimes as the solvent concentration decreases below the value of ∼20 water molecules per solute molecule. A correlation between this finding and structural modifications of the H-bond network of the solvent at similar concentrations is envisaged.