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.
Electron spin resonance (ESR) studies of the planar bis(maleonitriledithiolato)nickel anion radical, Ni(mnt) 2 -, have been carried out from the motional narrowing region to the glassy limit in ethanol (EtOH), tributyl phosphate (TBP), 4-allyl-2-methoxyphenol (eugenol), and a mixed solvent containing equal volumes of dimethylformamide and chloroform (DMF-CHCl 3 ). In all solvents, axially symmetric Brownian rotational diffusion produces agreement between the experimental and calculated ESR line widths of Ni(mnt) 2 -when the anisotropic Zeeman interaction makes the dominant contribution to the widths. The long in-plane axis of the radical is the diffusional symmetry (or parallel) axis; the rotational motion is fastest about this axis with D | /D ⊥ ) 3.0-4.0 (D | and D ⊥ are the diffusion constants for reorientation about the parallel and perpendicular axes, respectively). In EtOH and DMF-CHCl 3 , however, the calculated widths are not in agreement with experiment in the fast motional region near the minimum experimental width; possible reasons for this lack of agreement are discussed (spectra were taken in the fast motional region for the other solvents but not at temperatures high enough to observe the minimum experimental width). The calculated widths were determined as a function of the correlation time τ 2 (0) ) (6D ⊥ ) -1 . The temperature dependence of τ 2 (0) is discussed in terms of the modified Stokes-Einstein-Debye (SED) model and the Vogel-Tammann-Fulcher (VTF) equation. The results from the SED model indicate that Ni(mnt) 2 -has the strongest interaction with EtOH and the weakest interaction with TBP. The VTF analyses give the solvent-dependent parameters D (a measure of the strong or fragile nature of the liquid) and T 0 (a temperature below the glass transition temperature, T g ). The values of D and T 0 obtained from the Ni(mnt) 2 -line widths are (a) compared with the values from viscosities, dielectric relaxation, and light scattering and (b) used to calculate values of T g , which are compared with experiment. For both comparisons, the agreement is generally good, although some differences are found.
The translational diffusion constants, D, of trans-stilbene, 1,4-dipheny-1,3-butadiene, 1,6-dipheny-1,3,5-hexatriene, 1,1,4,4,-tetraphenyl-1,3-butadiene, tetraphenylethylene, 9,10-diphenylanthracene, p-terphenyl, bibenzyl, 1,1‘-binaphthyl, [2.2]paracyclophane, triptycene, and dodecahydrotriphenylene have been determined in the n-alkanes using capillary flow techniques. The solutes showed deviations from the Stokes−Einstein (SE) relation (D = k B T/(6πηr)); the values of the hydrodynamic radius, r, decrease as the viscosity, η, increases. The data can be fitted to D/T = A SE/η p with p < 1 (p = 1 for the SE relation). The values of p increase as the solute size increases; they range from p = 0.712 for p-terphenyl to p = 0.942 for 1,1,4,4,-tetraphenyl-1,3-butadiene. The deviations from SE behavior are discussed in terms of the ratio V s/V p, where V s and V p are the van der Waals volumes of a solvent and diffusing probe, respectively. The diffusion constants also are discussed in terms of the Wilke−Chang equation. The values of D -1 for several of the solutes are compared with their rotational correlation times, τθ, in the n-alkanes. The values of τθ, which showed deviations from the Stokes−Einstein−Debye expression (τθ = 4πr 3η/(3k B T)), have the same general dependence on viscosity as D -1.
The translational diffusion constants, D, of biphenyl, diphenylacetylene, diphenylbutadiyne, anthracene, pyrene, rubrene, perylene, and coronene have been determined in the n-alkanes using capillary flow techniques. Pyrene and rubrene were also studied in cyclohexane and several of its derivatives. The solutes showed varying degrees of deviation from the Stokes-Einstein (SE) relation (D ) k B T/6πηr); the values of the hydrodynamic radius r decrease as the viscosity η increases. The data can be fitted to D/T ) A/η p with p < 1 (p ) 1 for the SE relation). The values of p increase as the solute size increases. In the n-alkanes, they range from p ) 0.718 ( 0.004 for biphenyl to p ) 0.943 ( 0.014 for rubrene; the largest value (p ) 0.982 ( 0.019) is found for rubrene in the cyclohexanes. The D values have been compared with reorientational results and molecular dynamics calculations; they have also been fitted to the Doolittle-Cohen-Turnbull free volume equation. A correlation between the p values and the results of the free volume analyses is discussed.
The translational diffusion constant, D, of dioxygen, O 2 , has been measured in n-alkane solutions drawn through a microcapillary by reduced pressure. The passage of O 2 through the capillary has been monitored using its UV absorption at 190 nm. Data were taken in hexane, octane, decane, dodecane, tetradecane, and hexadecane. The diffusion constants decrease as the chain length increases and are not in agreement with the Stokes-Einstein relation. The data can be fitted to D/T ) A/η p with A ) 1.27 × 10 -8 and p ) 0.562 (p ) 1 for the Stokes-Einstein relation); T is the absolute temperature, and η is the viscosity in P.
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