Diffusion of spherical solutes: A fractional molecular-hydrodynamic study of solvent dependence

Diffusion of Benzene and Alkylbenzenes in Nonpolar Solvents

The translational diffusion constants, D, of benzene and a series of alkylbenzenes have been determined in n-pentadecane, 2,6,10,14-tetramethylpentadecane (pristane), 2,2,4,4,6,8,8-heptamethylnonane (isocetane), and 2,6,10,15,19,23-hexamethyltetracosane (squalane) using capillary flow techniques. The solutes' D values are compared with the predictions of a cylinder diffusion model as are those for (a) benzene and alkylbenzenes in n-nonane, n-decane, n-dodecane, and supercritical CO₂ and (b) n-alkanes and 1-alkenes in n-hexane, n-heptane, n-octane, benzene, and toluene. The D values for benzene and the alkylbenzenes also are compared with the predictions of lollipop diffusion for which the phenyl ring is the candy and the alkyl chain is the handle. Both models give an average difference of less than 4% between experimental and calculated diffusion constants in solvents whose viscosities vary by a factor of more than 600 when benzene and toluene (as solutes) are omitted; the comparisons include 150 and 85 D values for the cylinder and lollipop models, respectively. The differences increase when benzene and toluene are included and are most likely because of their shapes and the shapes assumed by the models. The agreement with the models indicates that the chains of the alkylbenzenes and 1-alkenes, like those of the n-alkanes, are relatively extended. The D values for several of the solutes also are fitted to a modification of the Stokes-Einstein relation that varies their dependence on viscosity instead of chain dimensions.