Complex Dynamical Aspects of Organic Electrolyte Solutions

Molecular dynamics of acetone-alkali metal halide (LiBr, LiI) solutions were investigated using depolarized Rayleigh scattering (DRS) and low-frequency Raman spectroscopy in the frequency range from similar to 0.5 to 200 cm(-1) (similar to 20 GHz to 6 THz). These experiments probe fast dynamical fluctuations of the polarizability anisotropy at picosecond and sub-picosecond time scales that are mainly driven by acetone orientational dynamics. Two distinct contributions were revealed: a fast process (units of picosecond, ps) related to the essentially unperturbed bulk solvent and a slow one (tens of ps) assigned to acetone molecules forming Li+ solvation shells, decelerated by the motional constraint imposed by the cation. The increase of LiBr and LiI concentration significantly slows down the overall solvent relaxation as a consequence of the increased fraction of acetone molecules involved in the ion solvation shells. The global retardation is larger in Li! than LiBr solutions consistently with viscosity trends. This is explained in terms of ion association (at least ion pairing) more favorably promoted by Br- than I-, with reduced Li+-acetone interactions in LiBr than LiI solutions. Anion-induced modulation of the Li+center dot center dot center dot O=C contacts, largely responsible for electrostriction phenomena, also affects the reduced THz-Raman spectral density, ascribed to ultrafast librational motions of acetone molecules. Overall, these findings enlighten the interplay between ion-dipole and ion-ion interactions on the fast solvation dynamics in electrolyte solutions of a typical polar aprotic solvent.