Diffusion of Squalene in n-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 the translational diffusion constant, $D$, of squalene at room temperature in several nonpolar solvents; they include squalane, $n$-hexadecane, and three $n$-octane-squalane mixtures. Values of $r$, squalene's hydrodynamic radius, are calculated from the Stokes-Einstein relation and decrease as the viscosity increases. These solvent-dependent $r$ values are a consequence of the relative sizes of squalene and the solvents. The Stokes-Einstein limit assumes the solute is much larger than the solvent; this is not the case for our solutions. A number of $n$-alkane solutes diffusing in $n$-alkane solvents also have $r$ values that decrease as the viscosity increases. The $r$ values for squalene and these $n$-alkanes have a common dependence on the ratio of the solvent's van der Waals volume to that of the solute probe. The translational motion of squalene appears to be similar to that of $n$-alkane solutes with similar chain lengths diffusing in $n$-alkane solvents; $n$-tetracosane is an example.