Diffusion of Aromatic Hydrocarbons in n-Alkanes and Cyclohexanes

The translational diffusion constants, D, of biphenyl, diphenylacetylene, diphenylbutadiyne, anthracene, pyrene, rubrene, perylene, and coronene have been determined in the n -alkanes using capillary flow techniques. Pyrene and rubrene were also studied in cyclohexane and several of its derivatives. The solutes showed varying degrees of deviation from the Stokes−Einstein (SE) relation ( D = k B T /6πη r ); the values of the hydrodynamic radius r decrease as the viscosity η increases. The data can be fitted to D / T = A /η p with p < 1 ( p = 1 for the SE relation). The values of p increase as the solute size increases. In the n -alkanes, they range from p = 0.718 ± 0.004 for biphenyl to p = 0.943 ± 0.014 for rubrene; the largest value ( p = 0.982 ± 0.019) is found for rubrene in the cyclohexanes. The D values have been compared with reorientational results and molecular dynamics calculations; they have also been fitted to the Doolittle−Cohen−Turnbull free volume equation. A correlation between the p values and the results of the free volume analyses is discussed.