Quenching the Aggregation

As the reaction consvunes more and more of the reactants, the particles grow by atomistic and collisional growth. When the reaction is complete, atomistic growth stops but collisional growth continues as long as the particles are still at a temperature in which they are molten or sticky. To control the size of the particles produced by a gas phase reactor it is necessary to suddenly cool the particles to well below their melting point at the point at which they are the desired size. This step 

FIGURE 7SI Quenching (a) Heat transfer quenching by double pipe heat exchanger in an aerosol reactor, (b) Dilution quenching by the addition of a cold gas. 

Chapter 7 Pouxler Synthesis with Gas Phase Eeaetants 

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The introduction of bulky side chains that contain adamantyl groups to poly(p-phenylenevinylene) (PPV), a semiconducting conjugated polymer, decreases the number of interchain interactions. This action will reduce the aggregation quenching and polymer photoluminescence properties would be improved [93]. 

Burke M, Edge R, Land EJ, McGarvey DJ, and Truscott TG. 2001. One-electron reduction potentials of dietary carotenoid radical cations in aqueous micellar environments. FEBS Letters 500(3) 132-136. Bystritskaya EV and Karpukhin ON. 1975. Effect of the aggregate state of a medium on the quenching of singlet oxygen. Doklady Akademii Nauk SSSR 221 1100-1103. 

Mullineaux, C.W., Pascal, A.A., Horton, P. and Holzwarth, A.R. 1992. Excitation energy quenching in aggregates of the LHCII chlorophyll-protein complex A time-resolved fluorescence study. Biochim. Biophys. Acta 1141 23-28. 

As noted earlier, environments such as water/methanol mixtures are useful models of membrane environments. These mixed solvents lead to a reduced efficiency of 02 quenching and the quenching becomes negligible at high water concentrations. Figure 14.2 shows an example of this behavior for zeaxanthin (ZEA), as the aggregation of ZEA is increased. 

The aggregation and the orientation of a carotenoid in the lipid bilayer may be major factors in determining the efficiency of 02 quenching, for example, ZEA may span the membrane and aggregate while P-CAR, p-CRYP, and LYC are more randomly ordered. 

Anderson and coworkers [183-185] reported water-soluble polyrotaxanes 149 and 150, containing sulfonated PPV chains surrounded by mechanically bound a- and (3-cyclodextrin macrocycles (Chart 2.32). The cyclodextrin rings play the role of a wire insulator, preventing aggregation and interchain quenching. The effect was demonstrated by atomic force... 

Fluorescence quenching studies in micellar systems provide quantitative information not only on the aggregation number but also on counterion binding and on the effect of additives on the micellization process. The solubilizing process (partition coefficients between the aqueous phase and the micellar pseudo-phase, entry and exit rates of solutes) can also be characterized by fluorescence quenching. 

The results presented above show that the structure of the aggregates formed by the quenching of PVC solutions depends on the starting concentration. If the starting concentration is low enough (Cq = 0.5 10 g the particles are characterized by an average radius... 

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