Micellar environment

In all surfactant solutions 5.2 can be expected to prefer the nonpolar micellar environment over the aqueous phase. Consequently, those surfactant/dienophile combinations where the dienophile resides primarily in the aqueous phase show inhibition. This is the case for 5.If and S.lg in C12E7 solution and for S.lg in CTAB solution. On the other hand, when diene, dienophile and copper ion simultaneously bind to the micelle, as is the case for Cu(DS)2 solutions with all three dienophiles, efficient micellar catalysis is observed. An intermediate situation exists for 5.1c in CTAB or C12E7 solutions and particularly for 5.If in CTAB solution. Now the dienophile binds to the micelle and is slid elded from the copper ions that apparently prefer the aqueous phase. Tliis results in an overall retardation, despite the possible locally increased concentration of 5.2 in the micelle. 

The NOBS system undergoes an additional reaction that forms a diacyl peroxide as a result of the nucleophilic attack of the peracid anion on the NOBS precursor as shown in equation 21. This undesirable side reaction can be minimized by the use of an excess molar quantity of hydrogen peroxide (91,96) or by the use of shorter dialkyl chain acid derivatives. However, the use of these acid derivatives also appears to result in less efficient bleaching. The dependence of the acid group on the side product formation is apparentiy the result of the proximity of the newly formed peracid to unreacted NOBS in the micellar environment (91). A variety of other peracid precursor stmctures can be found (97—118). 

As a result of the micellar environment, enzymes and proteins acquire novel conformational and/or dynamic properties, which has led to an interesting research perspective from both the biophysical and the biotechnological points of view [173-175], From the comparison of some properties of catalase and horseradish peroxidase solubilized in wa-ter/AOT/n-heptane microemulsions with those in an aqueous solution of AOT it was ascertained that the secondary structure of catalase significantly changes in the presence of an aqueous micellar solution of AOT, whereas in AOT/n-heptane reverse micelles it does not change. On the other hand, AOT has no effect on horseradish peroxidase in aqueous solution, whereas slight changes in the secondary structure of horseradish peroxidase in AOT/n-heptane reverse micelles occur [176],... 

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. 

There is evidence that carotenoid radical cations can persist for up to one second in some micellar environments (Burke et al. 2001a) so that, in the absence of a repair process, the radical may survive until it reacts with another biomolecule. 

The studies of this equilibrium as a function of pH enabled the estimation of the absolute one-electron reduction potentials of CAR + in an aqueous micellar environment (Edge et al. 2000, Burke et al. 2001b) (see Table 14.12 for typical results). As can be seen, the potentials of all the dietary carotenoid radical cations are very similar but LYC + has the lowest potential implying that it is the best carotenoid antioxidant against free radicals (of course, this is an oversimplification, see above). 

Photochemical reactions are usually run in homogeneous solutions notwithstanding it is also possible to irradiate solid compounds directly. Examples of such reactions on a preparative scale 705) as well as a discussion on crystal lattice control on photoreactions 706) are found in the literature. Finally, specific effects of a micellar environement is also being used in photochemical reactions of preparative purposes707). 

Several review articles have been published on the catalytic functions of micelles and related systems (Fendler and Fendler, 1970, 1975 Menger, 1977 Berezin et al., 1973 Cordes and Dunlap, 1969 Cordes and Gitler, 1973 Kunitake, 1977 Kunitake and Okahata, 1976 Bunton, 1979). The conventional catalytic functions of micelles are, in most cases, related to (i) the concentration of reactants and catalytic acid-base species in the micellar phase due to electrostatic and/or hydrophobic forces and (it) the stabilization of transition states and/or destabilization of initial states by the micellar environments. The situation is more complex when one of the reagents is hydrophilic (Bunton et al., 1979). However, the last few years have witnessed several novel advances in this field especially in relation to enzymatic catalysis. 

Flavin oxidation of carbanions has also been of much concern since active intermediates in some flavoenzyme-mediated reactions (amino acid oxidase, lactate oxidase, etc.) are carbanions (Kosman, 1977). Flavin oxidation of nitroethane carbanion (20), which had not been achieved in non-enzymatic systems, occurs with [56] bound to CTAB micelles (Shinkai etal., 1976b). This suggests that the nitroethane carbanion is also activated by the micellar environment. 

An investigation of the effects of micellar environment on the thermal and photochemical atropisomerization of two picket fence porphyrins (4,0 isomers), one with long chains (Cjg), H2PF,THA, and one with short chains (C3), H2PF,TPro, has been carried out. As... 

The effect of charge delocalization en route to the activated complex is the result of the relatively nonpolar micellar environment compared to bulk water, charges in the micellar pseudophase are less stabilized by interactions with their environment (cf. stabilization of developing charges by the electrostatically non-neutral environment for (pseudo) unimolecular reactions). This effect was found for the dehydro-bromination reaction of 2-(p-nitrophenyl) ethyl bromide and the dehydrochlorination of 1,1,1 -trichloro-2,2-bis(p-chlorophenyl)ethane. ... 

It has been suggested that aqueous micellar systems simulate the electrostatic and hydrophobic interactions of the heme cavity [15-23]. Pioneering studies by Simplicio et al. [15-17] have shown that the heme is monodispersed when encapsulated in aqueous micelles. They have studied binding of cyanide and other axial ligands to ferric hemes in micellar environments. These studies [15-23] indicated that a heme encapsulated in an aqueous detergent micelle finds itself inside a large macromolecular cavity whose interactions is primarily... 

To circumvent the problem of lower water solubility and fast hydrolysis by water, some authors have investigated peroxyoxalate emission in micellar environments ... 

As enzymes could be used to carry out synthetic reactions in organic solvents [244-246] only under certain specific conditions, the appHcation ofRMs as enzyme hosts to perform biotransformations has attracted a great deal of research attention in the recent past. The reverse micellar environment represents a medium where the aqueous/organic interface is very large ( 100 m ml ) [247]. 

We saw in the last section how the removal of a surfactant molecule from water into a micellar environment has a negative AG° value. Qualitatively, it is not surprising that other... 

Incorporation of large amounts of functionalized monomer in the growing polymer creates a polar micellar environment that discourages migration of gaseous ethylene to the micelles. 

The importance of surface charge is amply demonstrated by the reductive quenching of surfactant [Ru(bipy)3]2+ derivatives by pyrene-N,AT-dimethylaniline (DMA). Figure 13 shows the decay of photogenerated [Ru(bipy)3]+ in the millisecond time domain for various different micellar environments and for free MeCN solution. Apparently the crucial step is the repulsion of DMA1 by the micelle and hence CTAC provides considerably more efficient charge separation than does SDS.328... 

Photochemical reactions are usually run in homogeneous solutions but recent developments are found in the literature on photoreactions in solide state, on solid matrix, or in a micellar environment [113]. These new methodologies have not yet been applied to carbohydrates but should be of great interest in the near future. 

The mechanism of emulsion polymerisation is complex. The basic theory is that originally proposed by Harkins21. Monomer is distributed throughout the emulsion system (a) as stabilised emulsion droplets, (b) dissolved to a small extent in the aqueous phase and (c) solubilised in soap micelles (see page 89). The micellar environment appears to be the most favourable for the initiation of polymerisation. The emulsion droplets of monomer appear to act mainly as reservoirs to supply material to the polymerisation sites by diffusion through the aqueous phase. As the micelles grow, they adsorb free emulsifier from solution, and eventually from the surface of the emulsion droplets. The emulsifier thus serves to stabilise the polymer particles. This theory accounts for the observation that the rate of polymerisation and the number of polymer particles finally produced depend largely on the emulsifier concentration, and that the number of polymer particles may far exceed the number of monomer droplets initially present. 

The emulsion polymerization process involves the polymerization of liquid monomers that are dispersed in an aqueous surfactant micelle-containing solution. The monomers are solubilized in the surfactant micelles. A water-soluble initiator catalyst, such as sodium persulfate, is added to the aqueous phase. The free radicals generated cause the dispersed monomers to react to produce polymer molecules within the micellar environment. The surfactant plays an additional role in stabilizing dispersion of the produced polymer particles. Thus, the surfactants used both provide micelles to house the monomers and macroradicals, and also stabilize the produced polymer particles [193,790], Anionic surfactants, such as dodecylbenzene sulfonates, are commonly used to provide electrostatic stabilization [193], These tend to cause production of polymer particles having diameters of about 0.1-0.3 pm, whereas when steric stabilization is provided by, for example, graft copolymers, then diameters of about 0.1-10 pm tend to be produced [790,791]. 

The ability of micelles or related aggregates to alter reaction rates and selectivity has been an area of active research for the past several decades. Reactants are partitioned into the aggregates by coulombic and hydrophobic interactions the observed rate accelerations are largely a result of the increased localization of the reactants and also of the typical physicochemical properties of the micellar environment, which are significantly different from those of the bulk solvents. This unique ability of the aggregate systems has therefore prompted several scientists to employ micellar media for catalytically carrying out specific reactions. 

This had been proposed, namely for a Rose Bengal derivative because the yield and the rate of formation of the radicals are dependent on the light intensity. A more recent analysis of the process performed in a micellar environment seems to agree with an alternative path (steps 25 and 26) still explaining a biphotonic behaviour [174] a more reactive but unidentified intermediate would be formed by absorption of a photon by the triplet state. 

Homolytic oc-cleavages (Type I reactions to produce a radical pair) involving the n, re states of ketones, have been studied extensively in homogeneous solutions 9c>, but only recently have ketones been employed to investigate this important class of photoreactions in micellar environments. For ease of discussion, we can classify reactions of this type in terms of the resulting products ... 

We shall now show how the reaction dynamics and reaction products can be affected by micellar environments, how the knowledge of the reaction mechanism in homogeneous solution can assist in determining the reaction mechanism in micelles and how understanding of the mechanism in a micelle can be employed to enhance our understanding of the structure and dynamics of micelles. 

Since the quantum yield for disappearance of ketone (triplet ketone nor the primary radical pair PhCH2CO CH2Ph is scavenged. As stated above, the proposed mechanism for the photolysis of DBK in micellar solution is illustrated in Fig. 5 13,21). The micellar environment inhibits the diffusion of radicals to the bulk aqueous phase the radical pair s distance maximum separation is maintained to a few tens of angstroms or less. The amount of escape being reduced, the radicals can then undergo more efficient intersystem crossing and recombination. 

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