Diffusion of Dioxygen in Alkanes and Cycloalkanes

The translational diffusion constant, D, of dioxygen, O 2, has been measured in the odd n -alkanes n -C 7 H 16 to n -C 15 H 32, two branched alkanes (isooctane and squalane), and several cycloalkanes (cyclohexane, methylcyclohexane, n -butylcyclohexane, dicyclohexyl, cis -decalin, and trans -decalin). The D values were determined using Taylor−Aris dispersion theory in solutions drawn through a microcapillary by reduced pressure. The initial analysis of the data was in terms of the Stokes−Einstein relation ( D = k B T /6πη r ). In both the n -alkanes and cycloalkanes, the values of the hydrodynamic radius r for O 2 are smaller than its known dimensions and decrease as the viscosity η increases, i.e., O 2 is diffusing faster than predicted by a constant solute “size.” The data can be fitted to D / T = A /η p with p < 1 ( p = 1 for the Stokes−Einstein relation). When the data for the odd n -alkanes are combined with our earlier results for O 2 in the even n -alkanes ( n -C 6 H 14 to n -C 16 H 34, Kowert, B. A.; Dang, N. C. J. Phys. Chem . 1999, 103, 779), we find p = 0.553 ± 0.009. For O 2 in the cycloalkanes the fit gives p = 0.632 ± 0.017. The data for isooctane and squalane are in approximate agreement with the n -alkane fit. The D values are also discussed in terms of computer simulations for small penetrants in hydrocarbons, the molar volumes of the solvents, and free volume approaches. A correlation between the p values and results of the free volume analyses is noted and discussed.