Unusual behavior of Stokes–Einstein relation in liquid mixtures

The Stokes–Einstein (S–E) relation is well studied for pure liquids. Here, we report the applicability of the S–E relation in liquid mixtures. The breakdown of the S–E relation in organic and aqueous liquid mixtures might be attributed to the self-diffusion coefficient, shear viscosity, or the hydrodynamics radius of the solute–solvent species, which are strongly correlated. The hydrodynamic radius must be adjusted with an effective radius that accounts for the size of the diffusing solute wetted by the solvent molecules. Furthermore, the mutual diffusivity could be the correct option instead of self-diffusivity for the prediction of hydrodynamic radius using the S–E relation, as the mobility of the solute is strongly coupled with the mobility of the solvent molecules. The theoretical hydrodynamic radius obtained from the S–E relation is predicted to be well correlated with the distance between the solute and solvent center of mass calculated from molecular dynamics simulations. The acceptance of this effective hydrodynamic radius is ensured by testing its applicability for a large collection of liquid mixtures.