Reverse micelles reaction kinetics

Reactivities of pentacyanoferrates(II) in micelles and reversed micelles have been studied. The hexadecyltrimethylammonium cation causes a modest increase in rate constant for the anion-anion reaction [Fe(CN)5(4-CNpy)] + CN. This can equally well be interpreted according to the pseudophase model developed from the Olson-Simonson treatment of kinetics in micellar systems or by the classical Bronsted equation. 

A logical development of this study will be to consider the microscopic point of view i.e the elementary reactions to build the infinite reverse micelles of the network. Detailed structural information is necessary to undertake a more microscopic approach of the kinetics. The aggregation mechanisif involves hydrogen bonding as clearly demonstrated by an IR spectrocopy study (3 ) and has to be understood. 

In principle, silica growth kinetics may be controlled by (1) slow release of monomer via alkoxide hydrolysis in the particle-free reverse micelles, (2) slow surface reaction of monomer addition to the growing particle, and (3) slow transport processes as determined by the dynamics of intermicellar mass transfer. There is strong experimental evidence to support the view that the rate of silica growth in the microemulsion environment is controlled by the rate of hydrolysis of TEOS (23,24,29). Silica growth kinetics can be analyzed in terms of the overall hydrolysis and condensation reactions ... 

The experimental data for oxidation of benzyl alcohol,1 aliphatic primary and secondary alcohols,2 and cholesterol3 with cetyltrimethylammonium (CTA) dichromate indicated that the reactions occur in a reverse micelle system produced by the oxidant. Michaelis-Menten-type kinetics were observed with respect to the reductants. The product of the oxidation of cholesterol depends on the solvent. In dichloromethane, the product is 7-dehydrocholesterol, whereas with dichloromethane containing acetic acid the product is 5-cholesten-3-one. A low kinetic isotope effect, k /ku = 2.81, was observed in the oxidation of methanol- this, combined with the rate data and the reverse solvent isotope effect [ (H20)/fc(D20) = 0.76], suggests that these reactions... 

The kinetics of the piperidino-defluorination reaction of l-fluoro-2,4-dinitrobenzene have been studied in non-aqueous reverse micelles consisting of ethylene glycol-AOT-/ -heptane or DMF-AOT-n-heptane. The reaction, which is not base catalysed, is accelerated when DMF, a non-hydrogen bond donor solvent, is used in the micelle core.22 Catalysis by human glutathione 5-transferase M la-la of the reaction of glutathione with 1-chloro- and l-fluoro-2,4-dinitrobenzenes has been investigated. Much stronger enzymatic catalysis was observed in the case of the dechlorination reaction than for the defluorination and a transition-state model was proposed 23... 

The rapid development of biotechnology during the 1980s provided new opportunities for the application of reaction engineering principles. In biochemical systems, reactions are catalyzed by enzymes. These biocatalysts may be dispersed in an aqueous phase or in a reverse micelle, supported on a polymeric carrier, or contained within whole cells. The reactors used are most often stirred tanks, bubble columns, or hollow fibers. If the kinetics for the enzymatic process is known, then the effects of reaction conditions and mass transfer phenomena can be analyzed quite successfully using classical reactor models. Where living cells are present, the growth of the cell mass as well as the kinetics of the desired reaction must be modeled [16, 17]. 

Inhibition of peroxidation of unsaturated lipid chains in biomembranes is of particular significance and interest, because uncontrolled oxidation disrupts the protective layer around cells provided by the membranes. Furthermore, radical chain transfer reactions can also initiate damage of associated proteins, enzymes and DNA. The volume of literature is immense and expanding in the field of antioxidants. We will select certain milestones of advances where micelles and lipid bilayers, as mimics of biomembranes, provided media for quantitative studies on the activities of phenolic antioxidants. One of us, L. R. C. Barclay, was fortunate to be able to spend a sabbatical in Dr. Keith Ingold s laboratory in 1979-1980 when we carried out the first controlled initiation of peroxidation in lipid bilayers of egg lecithin and its inhibition by the natural antioxidant a-Toc . A typical example of the early results is shown in Figure 4. The oxidizability of the bilayer membrane was determined in these studies, but we were not aware that phosphatidyl cholines aggregate into reverse micelles in non-protic solvents like chlorobenzene, so this determination was not correct in solution. This was later corrected by detailed kinetic and P NMR studies, which concluded that the oxidizability of a lipid chain in a bilayer is very similar to that in homogeneous solution . ... 

When considering enzyme kinetics in a reversed micellar system it is critical to recognize that there are two volumes upon which to base concentration dependent constants and variables, the total or observed reaction volume and the micellar or aqueous water pool volume (111. Additionally, the partitioning of the substrate between the hulk organic solvent mixture and the reversed micelles... 

If the substrate is preferentially soluble in the reversed micelles (Ps l), a comparison of the kinetic parameters observed for the overall system and the kinetic parameters intrinsic to the reversed micellar reaction medium gives... 

The rate of a-chymotrypsin-catalyzed hydrolysis as a function of overall GPANA concentration in CTAB reversed micelles and in aqueous solution are shown in Figure 5. It is apparent that the reaction rate in the reversed micellar solution is on the order of 50 times more rapid than in the aqueous system. Furthermore, in the reversed micellar system there is no indication of enzyme saturation as the reaction is first order in substrate concentration. As enzyme saturation kinetics are not observed, it is impossible to differentiate between the parameters kcat and Kg. Instead a second order bimolecular rate constant for both the micelle interior ( micelle) and for what is experimentally observed ( observed) is defined. 

A second synthetic substrate, BTPNA, was investigated to clarify if this rate enhancement in CTAB reversed micelles is observed for all substrates (Figure 7). Two major differences are observed first, that there is no reaction rate enhancement in reversed micelles versus an aqueous based system, and second, that saturation kinetics are observed. 

Observed reaction kinetics for some enzyme substrate systems solubilized within a reversed micellar solution are enhanced relative to those observed In aqueous solution. This enhancement can be due to simple concentration of the reactants within the micelles, but can also be Influenced by the localization and orientation of the substrates being used. 

The use of microemulsions or reverse micelles as media for chemical and enzymatic reactions has been reviewed in recent years [20,37,38]. Microemulsions, including those based on organogels, are also useful media for enzyme-catalyzed synthetic reactions [37,39-43] and for preparation of nanoparticles [44]. In a very different direction, Vanag and Hanazaki [45] showed that the ferroin-catalyzed Belousov -Zhabitinskii oscillatory reaction exhibits frequency-multiplying bifurcations in reverse AOT microemulsions in octane, A clear understanding of reactivity in microemulsions and insight into how to optimize the experimental conditions requires kinetic models with predictive power. We focus attention primarily on this problem. 

Marcia-Rio et al. used this W/O AOT microemulsion as the medium for nitroso transfer to secondary amines from A-methyl-/V-nitroso-/ -toluenesulfonamide (8). Their quantitative treatment, which includes consideration of reactant solubilities, shows that reaction occurs at the microemulsion interface, where it is slower than in water. This rate difference is understandable on the very reasonable assumption that the polarity of the microemulsion interface is lower than that of water [99-101]. These kinetic data indicate that the interfacial regions of the water pool microdroplets in O/W microemulsions and reverse micelles can be regarded as reaction media corresponding to descriptions applied to normal aqueous association colloids. This concept has also been applied to acid-base equilibria, especially by El Seoud and his group [112,116,117]. 

Hirai et al. [97] used the AOT/isooctane/water system and titanium tetrabutoxide (TTBO). Particle diameters on the order of 3 nm were obtained by dynamic light scattering, a dimension that is smaller than the diameters of the reverse micelles (9-19.3 nm). Particle formation was strongly influenced by the water/surfactant molar ratio (R) and by the alkoxide concentration. The reaction kinetics was followed with UV-Vis absorption spectrophotometry. The absorbance of the reaction system increased monotonically with time for R = 9, whereas it went through a peak (after 400 min) for R = 30. These results were rationalized in terms of differences in availability of reactant water molecules. For... 

Several groups of investigators have looked at the effect of reverse micelles on reactions involving small molecules or ions a rationalization of the kinetic data obtained requires some knowledge about a) the solubilization sites for the small molecules within the micelles b) the nature of the water pool and c) the dynamics of solubilizate exchange among micelles. From a practical point of view, carrying out... 

Fig. 7 b. Normalized reaction rate plotted against substrate concentration for a reaction obeying Michaelis-Menten kinetics in reverse micelles. The abscissa shows the two scales of concentration, overall and local (or water pool) with 1% water, v v, and the two corresponding K -values... 

The enzymatic reaction in PFPE/SCCO2 reverse micelles was found to follow the classical Michaelis-Menten kinetics, whereas the activity was a strong function of the molar ratio of H2O-PEPE. The optimum ChOx activity occurred when the molar ratio of H2O-PFPE exceeded 12. The values of the apparent first-order rate constant (A cat,app) were comparable with that obtained in AOT/ isooctane system. In contrast, the authors observed that the best case for the apparent Michaelis constant (A mapp) was -twofold better than that reported for reverse micelles in liquid isooctane. There was no significant change in A cat,app or /fm,app when the CO2 pressure was changed between 100 and 260 bar. 

Main-group metal ions of and electronic configurations form kinetically labile complexes with halo ligands both in homogeneous aqueous solutions and in water droplets inside reverse micelles. Complexes of the 5 metal ions are exclusively featured by LMCT reactions as with Pb(IV), Sb(V), and Hg(II) [8, 22, 23, 24]. The Hg(II) cation possesses a filled d subshell thus, its chemical features are rather similar to main-group metal ions. In the case of the configuration, however, electron ejection (with oxidation of the metal center) can also occur, demonstrated by the example of hydroxo complexes of Sn(II), T1(I), and Pb(II) [25, 26]. 

The kinetics of enzymatic reactions in microemulsions obey, as a rule, the classic Michaelis-Menten equation [6,26,35], but difhculties arise in interpreting the results because of the distribution of reactants, products, and enzyme molecules among the microphases of the microemulsion [8,36-38], In addition, there are some enzymes in reverse micelles that exhibit enhanced activity as compared to that expressed in water this has given rise to the concept of superactivity [6,26,39], The superactivity has been explained in terms of the state of water in reverse micelles, the increased rigidity of the enzymes caused by the surfactant layer, and the enhanced substrate concentration at the enzyme microenvironment [36,40],... 

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