Understanding the linear correlation between diffusion coefficient and molecular weight. A model to estimate diffusion coefficients in acetonitrile solutions

In this work, diffusion coefficients (D) for a family of quinones, nitroaromatics, ferrocenes and aromatic hydrocarbon compounds, in acetonitrile, were obtained by single potential step chronoamperometry. A plot of the diffusion coefficient against the molecular weight of each compound shows a linear correlation, which is unexpected from the point of view of the Stokes-Einstein equation. The rearrangement of this equation as a function of the molecular weight (M_w), reveals in fact, a non-linear functional dependence D = (M_w^{- 1/3}), however, the linear experimental behaviour was consistent with a model derived from linearization of this function. The resulting model involves variables easily obtained, and can be applied to predict diffusion coefficients of compounds in acetonitrile that cannot be easily measured, either by physicochemical or electrochemical methods. In this first approach, the model seems to be general and open the possibility to generalize the correlation considering other solvents and different kind of molecules.