Complex Dynamical Aspects of Organic Electrolyte Solutions

The molecular dynamics of acetone alkali halide (LiBr and LiI) solutions at different compositions, from the pure solvent up to the solubility limit and at different temperatures, from 5.5 to 44.6 °C, was studied using depolarized Rayleigh scattering (DRS) and low-frequency Raman spectroscopy, which probe dynamical fluctuations of the polarizability anisotropy of the system. With the increase of salt concentration an overall slowing down of the picosecond relaxation was observed because of the constraining imposed by the cation on the orientational mobility of the solvent. In particular, two distinct relaxation processes were detected: a slower process (tens of picosecond) due to acetone molecules within the Li+ solvation shell and a fast one (units of picosecond) due to bulk-like acetone. The retardation effect on the global solvent relaxation is larger in LiI then LiBr solutions owing to the larger degree of ion-ion association (ion pairs) in the latter. The modulation of cation-solvent (ion–dipole) interactions induced by cation-anion (ion–ion) ones affects the solvent mobility and explains, on a molecular basis, viscosity trends. Overall, a correlation between fast solvation dynamics and ion association propensity was found. Furthermore, analysis of the spectral susceptibility ”(̃) evidences that the enhancement of Li+…O=C contacts, largely responsible for electrostriction phenomena in these ionic solutions, also modifies the ultrafast librational motion of acetone molecules localized in the ionic solvation shell.