Modification of the Stokes–Einstein Equation with a Semiempirical Microfriction Factor for Correlation of Tracer Diffusivities in Organic Solvents

Characterized by conceptual simplicity, a semiempirical formula based on the Stokes–Einstein equation and microfriction theory was constructed for correlating tracer diffusivities and, alternatively, for evaluating solute aggregation in organic solvents. For nonassociated systems, van der Waals radii determined by Bondi’s method were adopted to establish a base equation for treating associated solvents and solute–solvent systems. Values of molecular association numbers for hydrogen-bonded solvents are in good agreement with those characterized by X-ray diffraction and near-infrared spectroscopy. The generalized equation incorporating independently evaluated solutes’ solvation numbers and solvents’ association numbers as needed is capable of predicting tracer diffusivities to within ±8% for nonassociated and associated solute–solvent systems without resorting to adjustable parameters. Overall, this new approach performs better than the widely used Wilke–Chang equation with a comparable level of simplicity, including delineating temperature dependencies from 298 to 473 K of tracer diffusivities of solute–solvent systems consisting of relatively compact molecules.