Diffusion of Benzene and Alkylbenzenes in <i>n</i>‑Alkanes

The translational diffusion constants, <i>D</i>, of benzene and a series of alkylbenzenes have been determined in four <i>n</i>-alkanes at room temperature using capillary flow techniques. The alkylbenzenes are toluene, ethylbenzene, 1-phenylpropane, 1-phenylpentane, 1-phenyloctane, 1-phenylundecane, 1-phenyltetradecane, and 1-phenylheptadecane. The <i>n</i>-alkanes are <i>n</i>-nonane, <i>n</i>-decane, <i>n</i>-dodecane, and <i>n</i>-pentadecane. Ratios of the solutes’ <i>D</i> values are independent of solvent and in general agreement with the predictions of diffusion models for cylinders and lollipops. For the latter, an alkylbenzene’s phenyl ring is the lollipop’s candy; the alkyl chain is its handle. A model that considers the solutes to be spheres with volumes determined by the van der Waals increments of their constituent atoms is not in agreement with experiment. The diffusion constants of 1-alkene and <i>n</i>-alkane solutes in <i>n</i>-alkane solvents also are compared with the cylinder model; reasonably good agreement is found. The <i>n</i>-alkanes are relatively extended, and this appears to be the case for the alkyl chains of the 1-alkenes and alkylbenzenes as well

Diffusion of Squalene in <i>n</i>‑Alkanes and Squalane

Squalene, an intermediate in the biosynthesis of cholesterol, has a 24-carbon backbone with six methyl groups and six isolated double bonds. Capillary flow techniques have been used to determine its translational diffusion constant, <i>D</i>, at room temperature in squalane, <i>n</i>-C₁₆, and three <i>n</i>-C₈–squalane mixtures. The <i>D</i> values have a weaker dependence on viscosity, η, than predicted by the Stokes–Einstein relation, <i>D</i> = <i>k</i>_B<i>T</i>/(6πη<i>r</i>). A fit to the modified relation, <i>D</i>/<i>T</i> = <i>A</i>_SE/η^<i>p</i>, gives <i>p</i> = 0.820 ± 0.028; <i>p</i> = 1 for the Stokes–Einstein limit. The translational motion of squalene appears to be much like that of <i>n</i>-alkane solutes with comparable chain lengths; their <i>D</i> values show similar deviations from the Stokes–Einstein model. The <i>n</i>-alkane with the same carbon chain length as squalene, <i>n</i>-C₂₄, has a near-equal <i>p</i> value of 0.844 ± 0.018 in <i>n</i>-alkane solvents. The values of the hydrodynamic radius, <i>r</i>, for <i>n</i>-C₂₄, squalene, and other <i>n</i>-alkane solutes decrease as the viscosity increases and have a common dependence on the van der Waals volumes of the solute and solvent. The possibility of studying squalene in lipid droplets and membranes is discussed