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/6πηr). 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.

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