Diffusion of Dioxygen in Alkanes and Cycloalkanes

Abstract

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.

Diffusion of aromatic compounds in nonaqueous solvents: A study of solute, solvent, and temperature dependences

“…Except for macromolecules, however, numerous experimental studies 3,[57][58][59][60][61][62][63][64][65][66] have shown that tracer diffusivity is inversely proportional to a fractional power of the viscosity of the solvent instead. This fractional power dependence of viscosity on diffusion is often referred to in the literature as the fractional Stokes-Einstein (FSE) relation, (D 12 /T) ∝ η −t , where t is known to be normally less than unity and generally around 2/3.…”

Photophysics and Photochemistry of a Water-Soluble C60 Dendrimer: Fluorescence Quenching by Halides and Photoinduced Oxidation of I-

“…The effect of the uncertainty on the parameter R is not critical for the conclusions. If a value as low as R ) 0.6, appropriate for the diffusion of the much smaller molecular oxygen molecule (r ) 1.8 Å) in alkanes, 35 is used instead of 1, a somewhat higher charge of z F ) -2.4 ( 0.3 is obtained. One may now remark that the rate constant for the quenching of the fullerene trisadduct in water by iodide (Table 1) is indeed very close to that of the dendrimer, as expected for a similar effective charge.…”

Size-dependent diffusion in cycloalkanes

“…This is not always the case for molecular solutions; deviations from SE behaviour have been found when the size of the solute is smaller than or comparable to that of the solvent. In several studies using the n-alkanes as solvents [5][6][7][8][9][10][11], values of D/T for a given solute were not linear in À1 ; the value of r obtained from equation (3) decreased as the solvent chain length and viscosity increased. For a non-spherical solute, r is a length determined by its shape and dimensions.…”

“…The application of equation (4) to the experimental results for a large number of solutes in the n-alkanes gave values of p<1; examples [6][7][8][9] range from 0.553 AE 0.009 for O 2 to 0.718 AE 0.004 for biphenyl (BPH) to 0.943 AE 0.014 for rubrene (RUB), indicating that the SE limit is approached as the solute size increases. Another proposal, due to Zwanzig and Harrison [3], noted reasons for retaining the viscosity dependence of equations (2) and (3) and characterized r as a solventdependent effective radius that measures the strength of solute-solvent interactions.…”