Coupling of acoustic phonons in LiCl aqueous solutions to a relaxation mode of the ionic hydration shell and observation of central peaks in inelastic light scattering

We have measured Brillouin spectra of LiCl aqueous solutions as a function of concentration. A classical model of phonons damped by a viscous loss process does not describe the spectra adequately. They are better described by a coupled mode system consisting of a phonon and a relaxation mode of water molecules in the ionic hydration shell. The hydration shell relaxation is well-described by a single relaxation time which, for example, is 24 ps at 36 mol % LiCl. The relaxation time and the coupling strength depend on the concentration, which indicates that the dynamic properties of the water molecules in the ionic hydration shell are affected by interionic interactions.