Reorientational relaxation of water molecules in LiCl solution studied by depolarised Rayleigh scattering

Brillouin scattering and depolarised Rayleigh scattering have been used to study the reorientational motion of the water molecules in the LiCl primary hydration shell. The relaxation time is found to be dependent on the LiCl concentration, varying from 0.7*10^-11 s at 9 mol.% to 1.8*10^-11 s at 36 mol.%. The relaxation times at 26 mol.% and 36 mol.% follow a simple Arrhenius relation with activation energies of 5.5 kcal mol^-1 and 6.3 kcal mol^-1 respectively. The results are consistent with NMR studies, but not with dielectric measurements. Both the measured refractive index and the scattering intensity indicate that the electronic polarisability of the hydration shell also is dependent on ion concentration. The concentration dependence of the adiabatic compressibility indicates a possible structural transition at about 20 mol.%.