Estimation of diffusion coefficients by using a linear correlation between the diffusion coefficient and molecular weight

Diffusion coefficients (D) for a family of quinones, nitroaromatics, ferrocenes and aromatic hydrocarbon compounds, in acetonitrile, dimethylsulfoxide and dimethylformamide, were determined by single potential step chronoamperometry. The behaviour of the diffusion coefficient against the molecular weight of each compound was linear, which is unexpected from the point of view of the Stokes-Einstein equation, whose rearrangement as a function of the molecular weight (M_w), predict a non-linear functional dependence D = f(M_w^-1/3). However, the linear experimental behaviour was consistent with a model resulting from expansion of this function around a pivot mass whose value depends on the solvent-electrolyte system. The linear behaviour is due to the fact that the molecular density of the analysed compounds is in the same order of magnitude. Three empirical linear models to estimate diffusion coefficients in acetonitrile, dimethylsulfoxide and dimethylformamide were established while a semi-empirical model gave good prediction capability in other solvent systems. These models involve variables easily available such as the temperature, viscosity and the molecular weight of the interest compound.