Micellar catalysis - kinetic models

A micelle-bound substrate will experience a reaction environment different from bulk water, leading to a kinetic medium effect. Hence, micelles are able to catalyse or inhibit organic reactions. Research on micellar catalysis has focused on the kinetics of the organic reactions involved. An overview of the multitude of transformations that have been studied in micellar media is beyond the scope of this chapter. Instead, the reader is referred to an extensive set of review articles and monographs  

The kinetic data are essentially always treated using the pseudophase model, regarding the micellar solution as consisting of two separate phases. The simplest case of micellar catalysis applies to unimolecTilar reactions where the catalytic effect depends on the efficiency of bindirg of the reactant to the micelle (quantified by the partition coefficient, P) and the rate constant of the reaction in the micellar pseudophase (k ) and in the aqueous phase (k ). Menger and Portnoy have developed a model, treating micelles as enzyme-like particles, that allows the evaluation of all three parameters from the dependence of the observed rate constant on the concentration of surfactant .  

Partition coefficients are usually determined using ultrafiltration or NMR or UV-vis  

Herein k js is the observed pseudo-first-order rate constant. In the presence of micelles, analogous treatment of the experimental data will only provide an apparent second-order rate constant, which is a weighed average of the second-order rate constants in the micellar pseudophase and in the aqueous phase (Equation 5.2). 

Berezin and co-workers have analysed in detail the kinetics of bimolecular micelle-catalysed reactions . They have derived the following equation, relating the apparent rate constant for the reaction of A with B to the concentration of surfactant  

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General Mechanisms of Micellar Catalysis Kinetic Models for M i ce 11 e-Catalyzed Reactions... 

A kinetic study of the basic hydrolysis in a water/AOT/decane system has shown a change in the reactivity of p-nitrophenyl ethyl chloromethyl phosphonate above the percolation threshold. The applicability of the pseudophase model of micellar catalysis, below and above the percolation threshold, was also shown [285],... 

Micellar catalysis in organic reactions kinetic and mechanistic implications, 8, 271 Micelles, aqueous, and similar assemblies, organic reactivity in, 22, 213 Micelles, membranes and other aqueous aggregates, catalysis by, as models of enzyme action, 17, 435... 

Piskiewicz [119] has developed a kinetic model of micellar catalysis, based on the Hill equation of enzyme kinetics, which assumes a cooperative interaction between reactants and surfactant to form reactive substrate-micelle complexes. This model is probably not applicable to systems in which the surfactant is in large excess over substrate, as in most micellar mediated reactions, but it gives a very reasonable explanation of the rate effects of very dilute surfactants. 

Alkaline hydrolysis of ethyl caprylate (itself insoluble in water) yields sodium caprylate, initially at a very slow rate bnt as soon as sufficient caprylate was formed for aggregation into micelles to take place, the authors observed an exponential increase in reaction rate owing to micellar catalysis. These self-assembling surfactant strucmres may consequently provide a model system for studies of pre-biotic chemistry. The possible relevance of this process to prebiotic chemistry was emphasized by their observation that the micelles can be converted into more robust vesicles by a pH change induced by dissolved CO2, and latter on, Luisi extended this approach to vesicular systems (see Section 3.3). Kinetic models for this kind of autocatalytic dynamic systems were also developed in the literature." ... 

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. ... 

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. 

Rezende, M.C., Nome, R Kinetic behavior of cetyltrimethylammonium hydroxide the dehydroclorination of l,l,l-trichloro-2,2-bis(p-chlorophenyl)ethane and some of its derivatives. J. Phys. Chem. 1984, SS(9), 1892-1896. (c) Neves, M. de RS., Zanette, D., Quina, R, Moretti, M.T., Nome, R Origin of the apparent breakdown of the pseudophase ion-exchange-model for micellar catalysis with reactive counterion surfactants. J. Phys. Chem. 1989, 93(4), 1502-1505. 

The rate of cleavage of 1 has been studied in the absence and presence of metallomicelles of surfactant with tridentate ligand head groups (10 and 11) at a constant pH, but the observed kinetic data are not snfficient to provide a quantitative explanation of these data in terms of a micellar kinetic model. However, these kinetic data may provide a qualitative assessment of catalytic factor (Y) due to aqueous metal ion catalysis (Y" ), metal ion-tridentate Ugand complex catalysis (Y ), and metal ion-tridentate ligand surfactant complex catalysis (Y ls), xhns, Y = (k b kg)/k, = (k m - - ko)/ko, and Y ls =... 

Micelles are prevalent in naturally occurring and biological catalytic reactions. However, it is only in recent decades that scientists have developed kinetic models clarifying how micelle-mediated catalysis works at a molecular level. Written by a leading expert in the field, Micellar Catalysis is an in-depth examination of how micelles affect reaction mechanisms and reaction rates in organic and inorganic reactions. 

Catalysis, enzymatic, physical organic model systems and the problem of, 11,1 Catalysis, general base and nucleophilic, of ester hydrolysis and related reactions, 5,237 Catalysis, micellar, in organic reactions kinetic and mechanistic implications, 8,271 Catalysis, phase-transfer by quaternary ammonium salts, 15,267 Catalytic antibodies, 31,249... 

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