Diffusivity of cyclohexane

Single crystals, suitable for X-ray diffraction were obtained from slow diffusion of cyclohexane into a concentrated solution of 13e in dichloromethane. As proposed, the... 

These thermodynamic approaches to hydrophobic effects are complemented by spectroscopic studies. Tanabe (1993) has studied the Raman spectra manifested during the rotational diffusion of cyclohexane in water. The values of the diffusion coefficients are approximately half those expected from data for other solvents of the same viscosity, and the interpretations made are in terms of hindered rotation arising from the icebergs presumably formed (c/. Frank and Evans) around the cyclohexane. 

Fig. 4 Coefficients of long-range diffusion of cyclohexane in zeolite Na-X at a sorbate concentration of 1.9 molecules per cavity in the pure adsorbent-adsorbate system (o), and under the influence of an argon atmosphere of < 0.06 MPa (A), 0.13 MPa ( ), and 0.2 MPa >). From [85] with permission...

A similar result can be concluded from the hydrophilicity of hydrophilic-hydrophobic copolymers. For example, it is well- known that at least 10 % hydrophilic segments is needed to disperse a statistical hydrophilic-hydrophobic copolymer in water. Whereas, in our work with vinyl acetate-neutralized acrylic acid block copolymers, stable dispersion in water was achieved even at 4 wt % acrylic acid content (Caneba and Dar, 2005). Finally, a comparative study of permeabilities has been made between block and random copolymers for sorption and diffusion of cyclohexane in styrene-butadiene copolymers (Caneba et al., 1983-1984). Since the permeability is proportional to the product of the diffusivity and solubility, numerical results for 10 wt % S contents indicate an increase in permeability for the block copolymer membrane compared to the random copolymer membrane. 

Fig. 2 Effective diffusivities of cyclohexane in PSi with an average pore diameter of 10 nm as a function of the relative gas pressure z at room temperature (Valiullin et al. 2005b)...

The conclusion of all these thermodynamic studies is the existence of thiazole-solvent and thiazole-thiazole associations. The most probable mode of association is of the n-rr type from the lone pair of the nitrogen of one molecule to the various other atoms of the other. These associations are confirmed by the results of viscosimetnc studies on thiazole and binary mixtures of thiazole and CCU or QHij. In the case of CCU, there is association of two thiazole molecules with one solvent molecule, whereas cyclohexane seems to destroy some thiazole self-associations (aggregates) existing in the pure liquid (312-314). The same conclusions are drawn from the study of the self-diffusion of thiazole (labeled with C) in thiazole-cyclohexane solutions (114). 

Fig. 3.1.7 The surface diffusion coefficient / surface of cyclohexane (squares) and acetone (circles) in porous silicon with 3.6-nm mean...

No single model can exactly describe molecular reorientation in plastic crystals. Models which include features of the different models described above have been considered. For example, diffusion motion interrupted by orientation jumps has been considered to be responsible for molecular reorientation. This model has been somewhat successful in the case of cyclohexane and neopentane (Lechner, 1972 De Graaf Sciesinski, 1970). What is not completely clear is whether the reorientational motion is cooperative. There appears to be some evidence for coupling between the reorientational motion and the motions of neighbouring molecules. Comparative experimental studies employing complementary techniques which are sensitive to autocorrelation and monomolecular correlation would be of interest. 

The conversion of cyclohexanes to aromatics is a highly endothermic reaction (AH 50 kcal./mole) and occurs very readily over platinum-alumina catalyst at temperatures above about 350°C. At temperatures in the range 450-500°C., common in catalytic reforming, it is extremely difficult to avoid diffusional limitations and to maintain isothermal conditions. The importance of pore diffusion effects in the dehydrogenation of cyclohexane to benzene at temperatures above about 372°C. has been shown by Barnett et al. (B2). However, at temperatures below 372°C. these investigators concluded that pore diffusion did not limit the rate when using in, catalyst pellets. 

The self-diffusion of benzene in PIB [36], cyclohexane in BR [37] and toluene in PIB [38-40] has been investigated by PFG NMR. In addition more recently Schlick and co-workers [41] have measured the self-diffusion of benzene and cyclohexane mixtures in polyisoprene. In the first reported study of this kind, Boss and co-workers [36] measured the self-diffusion coefficients of benzene in polyisoprene at 70.4 °C. The increase in Dself with increasing solvent volume fraction could be described by the Fujita-Doolittle theory which states that the rate of self-diffusion scales with the free volume which in turn increases linearly with temperature. At higher solvent volume fractions the rate of selfdiffusion deviates from the Fujita-Doolittle theory, as the entanglement density decreased below the critical value. 

Figure 13.2 Self-diffusion coefficients of cyclohexane in BR solutions as a function of temperature and solvent volume fraction (Vj = 0.05 (O), 0.19 ( ), 0.29 ( ), 0.375 ( ) and 0.45 (A) [37]. The solid and dashed lines are best fits to the WLF equation [37]...

One-electron Fe redox catalysts may also be immobilized by incorporation into aluminophosphate frameworks. Dugal el al. (143) reported the oxidation of cyclohexane to give adipic acid with air as the oxidant in the presence of Fe-AlPO-31. This molecular sieve has narrow pores, with a 0.54-nm diameter. Cyclohexane is easily adsorbed in the micropores, but desorption of initial products such as cyclohexyl hydroperoxide or cyclohexanone is slow. Consequently, subsequent radical reactions occur until the cyclohexyl ring is broken to form linear products that are sufficiently mobile to diffuse out of the molecular sieve ... 

The liquid phase oxidation of cyclohexane involves both physical and chemical mechanisms. Upon entering the liquid, oxygen diffuses towards the bulk of the liquid. Reaction takes place simultaneously with diffusion. It is possible that the relative speed of reaction and diffusion can affect the course of reaction and the final produce distribution. It is obvious, however, that the reaction must be fast before such an effect is appreciable. 

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