Solute-solvent frictional coupling in electrolyte solutions. Role of ion pairs

Rotational diffusion of the organic anion resorufin in different electrolyte solutions is studied. The rotational relaxation time tau increases as the concentration of electrolyte increases for all electrolytes studied. Although tau follows the viscosity in NaNO3 aqueous solution, the tau in Mg(NO3)(2) solution is too large to be explained only by the increase of the viscosity. This effect is more significant in DMSO electrolyte solutions. The increase of the rotational relaxation time is well correlated with the charge-to-size ratio of the cation. The analysis of the experimental data using theoretical models that incorporate the different mechanisms of friction in electrolyte solutions strongly indicates the existence of ion pairs in DMSO electrolyte solution.