Micellar catalysis model for

From time to time various models have been developed to explain the micellar effect on the rate of reaction. Each model has its own strength and weakness. 

The extent of solubilization of the substrate to the micelle can be related with the association constant or the binding constant. Some of the important models developed to explain the micellar effect are described briefly as follows  

In Menger and Portnoy s model, the variation of the rate constant with surfactant concentration has been treated on the basis of assumption that substrate S is distributed between aqueous and micellar pseudo-phase given as follows  

This model leads to the following relationship for micellar catalysis  

The binding constants for the association of the solute with the micelle can be evaluated with the help of model developed by Sepulveda and coworkers. According to the model, solute binds to micelles with following equilibrium  

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A novel kinetic model for micellar catalysis has been developed based on the assumption that the Stem layer is always saturated with respect to counterions. This means that the ground state for ions is the ion bound to the micellar surface and not the free-ion in the bulk phase. An analogy between micellar reactions and reactions catalysed by regulatory enzymes has led to the application of the Hill model to the dependence rate constants of micellar catalysed reactions upon the detergent concentration. The decrease in rate at high concentrations of detergent is interpreted in terms of substrate inhibition. ... 

Neves, M. de F.S., Zanette, D., Quina, R, Moretti, M.T., Nome, F. Origin of the apparent breakdown of the pseudophase ion-exchange model for micellar catalysis with reactive counterion surfactants. J. Phys. Chem. 1989, 95(4), 1502-1505. 

Khan, M.N., Ismail, E. An apparent weakness of the pseudophase ion-exchange (PIE) model for micellar catalysis by cationic surfactants with nomeactive counterions. J. Chem. Soc., Perkin Trans. 2. 2001, 1346-1350. 

Scheme 6.2 Asymmetric hydrogenation of unsaturated amino acid derivatives as a model reaction for micellar catalysis using amphiphilic block copolymers.

For a surface active betaine ester the rate of alkaline hydrolysis shows significant concentration dependence. Due to a locally elevated concentration of hydroxyl ions at the cationic micellar surface, i.e., a locally increased pH in the micellar pseudophase, the reaction rate can be substantially higher when the substance is present at a concentration above the critical micelle concentration compared to the rate observed for a unimeric surfactant or a non-surface active betaine ester under the same conditions. This behavior, which is illustrated in Fig. 10, is an example of micellar catalysis. The decrease in reaction rate observed at higher concentrations for the C12-C18 1 compounds is a consequence of competition between the reactive hydroxyl ions and the inert surfactant counterions at the micellar surface. This effect is in line with the essential features of the pseudophase ion-exchange model of micellar catalysis [29,31]. 

The effects of macromolecules other than surfactants on the rates of organic reactions have been investigated extensively (Morawetz, 1965). In many cases, substrate specificity, bifunctional catalysis, competitive inhibition, and saturation (Michaelis-Menten) kinetics have been observed, and therefore these systems also serve as models for enzyme-catalyzed reactions and, in these and other respects, resemble micellar systems. Indeed, in some macromolecular systems micelle formation is very probable or is known to occur, and in others mixed micellar systems are likely. Recent books and reviews should be consulted for a more detailed description of macromolecular systems and for their applicability as models for enzymatic catalysis and other complex interactions (Morawetz, 1965 Bruice and Benkovic, 1966 Davydova et al., 1968 Winsor, 1968 Jencks, 1969 Overberger and Salamone, 1969). 

The fundamental principles controlling activity in nonaqueous systems are the same as those for aqueous solutions, except that the specificity of the micellar core for the solubilization of polar substrates is much greater than for the aqueous situation. The popularity of reversed micelles as models for enzyme catalysis stems from the fact that the micellar core is capable of binding substrates in concentrations and orientations that can be very specific to certain functionalities, much as an enzyme would do. As a result, reaction rate enhancements can be obtained comparable (with luck) to those of the natural systems, and far in excess of what can be explained on the basis of partitioning or availability of substrate. 

Broxton, T.J., Sango, D.B. Micellar catalysis of organic reactions. X. Further evidence for the partial failure of the pseudophase kinetic model of micellar catalysis for reactions of hydroxide ions. Aust. J. Chem. 1983, 36(4), 711-717. 

In retrospect, this study has demonstrated the limitations of two commonly accepted methods of analysing solubilisation and micellar catalysis, respectively. It has become clear that solubilisate ririg-current induced shifts need to be interpreted with due caution. These data indicate a proximity of solubilisate and parts of the surfactant and, strictly, do not specify the location within the micelle where the encounter takes place. Also the use of the pseudophase model for bimolecular reactions requires precaution. When distribution of the reactants over the micelle is not comparable, erroneous results are likely to be obtained... 

At the present time, "interest in reversed micelles is intense for several reasons. The rates of several types of reactions in apolar solvents are strongly enhanced by certain amphiphiles, and this "micellar catalysis" has been regarded as a model for enzyme activity (. Aside from such "biomimetic" features, rate enhancement by these surfactants may be important for applications in synthetic chemistry. Lastly, the aqueous "pools" solubilized within reversed micelles may be spectrally probed to provide structural information on the otherwise elusive state of water in small clusters. 

Although much work has already been devoted to the use of polysoaps in micellar cataylsis application, in particular as models for esterases [79] and systems for photochemical catalyzed reactions [80], only a few reports have appeared on the use of such polymer supports in transition metal catalysis. 

In recent years micellar emulsifiers have been found to affect the rate of many reactions (15,16). This phenomenon of micellar catalysis originally attracted attention as a model for enzymatically catalysed reactions although the analogy is... 

Recently, the investigations of nitrobenzisoxazoles mainly 6-nitrobenzisoxazole-3-carboxilate ions have received considerable interest due to their participation in reverse micellar systems [679-682], Reverse micelles are of considerable interest as reaction media because they are powerful models for biological compartmental-ization, enzymatic catalysis, and separation of biomolecules. Solutions of ionic surfactants in apolar media may contain reverse micelles, but they may also contain ion pairs or small clusters with water of hydration [679], Molecular design of nonlinear optical organic materials based on 6-nitrobenzoxazole chromophores has been developed [451],... 

The electrostatic model for the micellar effect on the hydrolysis of phosphate monoesters is also consistent with the results of inhibition studies (Bunton et al., 1968, 1970). The CTAB catalyzed hydrolysis of the dinitrophenyl phosphate dianions was found to be inhibited by low concentrations of a number of salts (Fig. 9). Simple electrolytes such as sodium chloride, sodium phosphate, and disodium tetraborate had little effect on the micellar catalysis, but salts with bulky organic anions such as sodium p-toluenesulfonate and sodium salts of aryl carboxylic and phosphoric acids dramatically inhibited the micelle catalysis by CTAB. From equation 14 and Fig. 10, the inhibitor constants, K, were calculated (Bunton et al., 1968) and are given in Table 9. The linearity of the plots in Fig. 10 justifies the assumption that the inhibition is competitive and that incorporation of an inhibitor molecule in a micelle prevents incorporation of the substrate (see Section III). Comparison of the value of for phenyl phosphate and the values of K for 2,4-and 2,6-dinitrophenyl phosphates suggests that nitro groups assist the... 

The majority of the work is focused on the use of polysoaps in micellar catalysis [482-484], in particular as models for esterases [57, 71, 136-140, 332, 361]. In addition, the catalytic activities in diverse other model reactions were investigated [141, 200, 211, 308, 317, 333, 361, 463, 464, 485-487]. Various photochemical systems for energy harvesting [183, 217, 259], or for charge separation [217,257-259,263] can be subsumed by applications of polysoaps in catalysis as well. Frequently, enhanced catalytic activity is observed compared to micelles of low molecular weight surfactants [113, 200, 258, 361, 485, 487]. 

The immediate success of the Berezin model in accounting almost quantitatively for the observed catalysis effect of micelles has an interesting implication. Is this truly a case of catalysis In many instances, the micelles bring about considerable shifts in equilibrium positions, which forced Berezin to admit that the term micellar catalysis was somewhat incorrect [2]. He justified its continued use on the basis that the surfactant is not consumed in the reaction and that for most surfactants the concentration required to bring about marked effects is usually very low. Some workers in the field have opted for less controversial terms such as micellar rate enhancement or rate promotion. The title of a recent review, Micellar Catalysis, a Useful Misnomer [3], sums up the prevalent attitude of researchers today. 

The emphasis placed on the last assumption is responsible for the name of the model. It is now well known that these assumptions, especially the first two, are reliable with impunity only over very narrow and dilute micellar concentration ranges. Nevertheless, the PIE model has provided invaluable insight over the past 25 years in elucidating micellar catalysis. Its failures [27-31] are usually attributable to clear-cut violations of its simple assumptions. Refinements or alternatives to these basic premises such as solving the nonlinear Poisson Boltzmann equation for the cell model have not proved to be particularly enlightening nor more helpful [32]. The extension of the PIE model to complicated micellar systems where anomalous rate behavior is more often than not the rule rather than the exception is probably unwarranted [33]. Sudhdlter et al. [34] have critically reviewed the Berezin model and its Romsted variation, the PIE model, as matters stood 20 years ago. In... 

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