Diffusion of 1-alkenes and cyclohexene in alkane solvents

The translational diffusion constant, D, has been measured for each of the 1-alkenes 1-C6H12, 1-C8H16, 1-C12H24, and 1-C14H28 in each of the even n-alkanes n-C6H14-n-C14H30; the D values have also been measured for 1-C10H20 in each of the even n-alkanes n-C8H18-n-C14H30. Cyclohexene has been studied in each of the even n-alkanes n-C8H18-n-C14H30 and cyclohexane. Deviations from the Stokes-Einstein (SE) relation (D = kBT/6πηr) were found. For a given solute, the hydrodynamic radius r decreased as the viscosity η increased. Analyses of literature data for n-alkane solutes in n-alkane solvents, including self-diffusion, also gave values of r that decreased as η increased. These solvent-dependent r values are discussed in terms of the relative sizes of the solutes and solvents. The data also were analyzed using D/T = A/ηp (p = 1 for the SE relation). The p values for the 1-alkenes and the n-alkane solutes with six or more carbon atoms were all <1 and were not a strong function of size; those for the 1-alkenes ranged from 0.637 ± 0.027 for 1-hexene to 0.725 ± 0.017 for 1-tetradecene. The p values for the analogous n-alkane solutes were roughly the same and indicated that the similar shapes and polarities of the two types of hydrocarbon play key roles in determining their diffusion. In the n-alkane solvents, the p value of the more globular cyclohexene is somewhat larger than those of both 1-hexene and n-hexane.