ROTATIONAL DIFFUSION IN ELECTROLYTE-SOLUTIONS

Rotational diffusion of the dye molecule resorufin in both aqueous and DMSO solutions of LiNO3 is used to assess the importance of ion atmosphere friction and ion pairing on the solute/solvent frictional coupling. Rotational diffusion times for resorufin in aqueous LiNO3 follow the solution viscosity and dielectric parameters as LiNO3 concentration increases, while rotation times for resorufin in DMSO solutions of LiNO3 show a much larger increase than can be accounted for by either of these factors. Evidence for ion pairing is presented, and its effect on the hydrodynamic component of the friction is evaluated. The experimental rotational diffusion times are compared with diffusion times calculated by incorporating terms for hydrodynamic, dielectric, and ion atmosphere friction. The term resulting from the ion pairing is the largest, and its inclusion is able to reproduce the experimentally measured relaxation times to within 15%.