Rotational Relaxation of Ionic Molecules in Electrolyte Solutions. Anisotropy Relaxation and Molecular Dynamics Study

Rotational diffusion of organic ions in electrolyte solution is studied via optically heterodyned polarization spectroscopy and molecular dynamics (MD) simulations. Significant differences between the behavior of organic cation and anion species were observed in the experiments. While the rotational relaxation time of the anion normalized by the viscosity of the solution increases with the electrolyte concentration, the normalized relaxation time of the cation decreases with increasing electrolyte concentration. The experimental data are analyzed by using a continuum theory approach and MD simulations. It is demonstrated that one must include ion pairs to describe the dynamics of the anion, but the analysis of the relaxation of the cation does not require ion pairing. MD simulations show that the difference in the dynamics of the anion and cation in electrolyte solution is caused by the different ability of free anion, free cation, and ion paired species to associate with the solvent (DMSO).