Anthracene, viscosity effect

Effect of Viscosity on the Rate of Photosensitization of Diaryliodonium Salts by Anthracene... 

Photosensitization of diaryliodonium salts by anthracene occurs by a photoredox reaction in which an electron is transferred from an excited singlet or triplet state of the anthracene to the diaryliodonium initiator.13"15,17 The lifetimes of the anthracene singlet and triplet states are on the order of nanoseconds and microseconds respectively, and the bimolecular electron transfer reactions between the anthracene and the initiator are limited by the rate of diffusion of reactants, which in turn depends upon the system viscosity. In this contribution, we have studied the effects of viscosity on the rate of the photosensitization reaction of diaryliodonium salts by anthracene. Using steady-state fluorescence spectroscopy, we have characterized the photosensitization rate in propanol/glycerol solutions of varying viscosities. The results were analyzed using numerical solutions of the photophysical kinetic equations in conjunction with the mathematical relationships provided by the Smoluchowski16 theory for the rate constants of the diffusion-controlled bimolecular reactions. 

The experimental and simulation results presented here indicate that the system viscosity has an important effect on the overall rate of the photosensitization of diary liodonium salts by anthracene. These studies reveal that as the viscosity of the solvent is increased from 1 to 1000 cP, the overall rate of the photosensitization reaction decreases by an order of magnitude. This decrease in reaction rate is qualitatively explained using the Smoluchowski-Stokes-Einstein model for the rate constants of the bimolecular, diffusion-controlled elementary reactions in the numerical solution of the kinetic photophysical equations. A more quantitative fit between the experimental data and the simulation results was obtained by scaling the bimolecular rate constants by rj"07 rather than the rf1 as suggested by the Smoluchowski-Stokes-Einstein analysis. These simulation results provide a semi-empirical correlation which may be used to estimate the effective photosensitization rate constant for viscosities ranging from 1 to 1000 cP. 

The conditions that most favored mass transfer of anthracene (250 and 300 rpm and presence of Triton X-100) were evaluated in terms of enzyme inactivation as well as all viscosities of silicone oil. Inactivation coefficients, kd, were calculated according to first-order kinetics (Fig. 10.14). The increase of the agitation rate to 300 rpm did not have a remarkable effect on the inactivation in presence of Triton, whereas in aqueous medium inactivation coefficients slightly increased. The viscosity of solvent does not seem to affect inactivation, except for 10 cSt, which led to the highest values. 

Once mass transfer and enzyme inactivation was studied, the degradation of anthracene was evaluated in a TPPB. In vitro degradation experiments were carried out at 250 rpm, since 300 rpm did not give any improvement in mass transfer and enzyme stability. Furthermore, the overall effect of the different viscosities was evaluated for the maximization of anthracene removal by VP. 

A summary of the outcomes in terms of the average anthracene degradation rate (in 38 h) and efficiency is presented in Fig. 10.15. The increase of viscosity led to higher degradation rates and in parallel, higher efficiencies. A similar effect was observed for experiments with Triton X-100. 

On the other hand, the presence of Triton increased the removal rate. The highest degradation rate was obtained with silicone oil 50 cSt in presence of 0.25 CMC Triton X-100 and at 250 rpm. Mass transfer experiments demonstrated that lower viscosities favored increased mass transfer coefficients. However, it seems that there were no mass transfer limitations in the degradation experiments, and other effects such as enzyme protection were more important to increase anthracene removal. As the interfacial area decreases for high solvent viscosity, the interfacial interaction with the enzyme also decreases, which is the main mechanism for the inactivation of biocatalysts by organic solvents [54]. 

Figure 8, is an interesting SEM scanning electron micrograph of units of mesophase formed by co-carbonization of anthracene and phen-anthrene (3 7) to 823 K at 300 MPa pressure (19). The effect of enhanced pressure is to increase the viscosity of the mesophase and... 

Accelerated solvent extraction is a closed system of extraction which utilises higher temperature and pressure. Closed systems are designed to minimise the loss of volatiles, improve the efficiency and increase the throughput. The elevated temperature improves analyte solubility. For example, anthracene, a PAH, is 15 times more soluble in methylene chloride at 150°C than at 50°C. High temperature also helps to overcome sample extraction matrix effects and gives faster desorption kinetics. The lower solvent viscosity allows diffusion of the solvent into the matrix to occur more quickly than other extraction techniques. Increased pressure also elevates the boiling point of the solvent. 

Experimental work is consistent with the view that solvent specific corrections to the intramolecular potential are not required in the absence of strong polymer/solvent interactions. Glowinkowski et al. used CNMR to study the local dynamics of polyisoprene in ten solvents as a function of temperature [31]. They measured correlation times due to the motion of differmt C-H vectors in the chain backbone. They found that these correlation times were determined by oidy the temperature and the solvent viscosity. Variables such as solvent shape, flexibility, and chemical functionality had no effect on the correlation times, except through the solvent viscosity. (Highly polar solvents were excluded from this study as they do not dissolve polyisoprene.) Similar conclusions have been reached in NMR studies of polybutadiene [52] and in time-resolved optical studies of polyisoprene [53] and polystyrene [54] with anthracene labels. NMR studies of PEO [55] have been interpreted as supporting this same conclusion [31] (except when the solvent was water [56]). 

For polymerization in the liquid state, the reaction kinetics is even more complicated due to the heat and the increase in viscosity associated with the reaction, the so-caUed Tromsdorff effects [47]. The reaction accelerates itself as the polymerization proceeds as a result of a positive feedback loop generated by the increase in heat as well as in viscosity and the diffusion-controlled termination of the polymerization process [48, 49]. An example is illustrated in Figure 6.5 for a mixture of a polystyrene doubly labeled with anthracene and fluorescein (PSAF) dissolved in MMA monomer [50]. A PSAF/MMA (5/95) mixture containing 2wt% of Lucirin TPO as a photoinitiator and 6 wt% of ethylene glycol dimethacrylate (EGDMA) as a... 

Bunker et al. studied the photodimerization reaction of anthracene in supercritical CO2 at 35°C (179). They found that the reaction quantum yields are up to an order of magnitude higher in supercritical CO2 (35 C, Pr = 1.9) than in liquid benzene at the same anthracene concentrations however, for the fluid density dependence, the yields obtained at different densities agree well with the yields calculated on the basis of experimentally determined viscosities (Figure 24). Since the results provided no evidence of solute-solute clustering effects, the higher photodimerization yields in the supercritical fluid were atttibuted to more efficient anthracene diffusion associated with the lower viscosity. (See Scheme 6.)... 

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