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Menger model

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". ... [Pg.129]

The model proposed by Menger et al. (Fig. 2) shows two extreme conformations, one in which the hydrocarbon chains are fully extended and another in which they are folded [18, 19], The surface of Menger s micelle is less defined than in the classical model and the surfactants that form the micelle are randomly orientated. The water can penetrate and enter in contact with the hydrophobic part of the surfactants. This model, apart from being more acceptable from an esteric point of view, gives a better explanation than the classical model of a series of experimental results such as viscosity, polarity, or kinetics. [Pg.291]

The other two, more recent, micellar models consist of a core that has barely any contact with the water (as in the classical model), and part of the hydrocarbon chains of the surfactants, as well as the heads of these, are exposed toward the surface, with which they are in contact with the water (as in Menger s model). [Pg.292]

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 ... [Pg.162]

Recently, Menger (1979) proposed an alternative model of the micellar structure. According to his reef model , micelles possess rugged, dynamic surfaces and water molecules penetrate close to the micelle core. [Pg.437]

The rate constants for micelle-catalyzed reactions, when plotted against surfactant concentration, yield approximately sigmoid-shaped curves. The kinetic model commonly used quantitatively to describe the relationship of rate constant to surfactant, D, concentration assumes that micelles, D , form a noncovalent complex (4a) with substrate, S, before catalysis may take place (Menger and Portnoy, 1967 Cordes and Dunlap, 1969). An alternative model... [Pg.448]

J. H. Barker, F. Hammersen, I. Bondar, T. J. Galla, M. D. Menger, W. Gross, and K. Messmer. Direct monitoring of nutritive blood flow in a failing skin flap The hairless mouse ear skin-flap model. Plast. Reconstr. Surg. 84 303-313 (1989). [Pg.27]

The most straightforward of the various models describing micellar kinetics is the Menger-Portnoy model for (pseudo) unimolecular reactions.The Menger-Portnoy model assumes rapid equilibration of the reactant of interest over bulk water and the micellar pseudophase with equilibrium constant K. The reaction then proceeds in both pseudophases with rate constants and in bulk water and the micellar pseudophase, respectively (Scheme 4). [Pg.11]

The Menger-Portnoy model is closely related to the Berezin model employing partition coefficients instead of equilibrium constants.For the case where only two pseudophases (bulk water and micelle) are considered, the partitioning of the reactant is given by the partition coefficient P. This leads to Equation (4) describing observed rate constants as a function of surfactant concentration. [Pg.12]

Both the Menger-Portnoy model and the model by Berezin were effectively derived on the assumption that micellar solutions contain two pseudophases, namely the micellar pseudophase and bulk water. However, both models can be expanded to take more than one micellar pseudophase into account. For example, this could be done when the micellar pseudophase is seen to consist of two separate pseudophases (zones) itself, namely a pseudophase corresponding to the hydrophobic core and a pseudophase corresponding to the micellar Stern region. " If one then assumes a reaction to occur with a rate constant k in the Stern region while the reaction does not occur in the micellar core, the expression for k includes the distribution of the reactant over different zones [Equation (6)]. " ... [Pg.13]

For the first case, one can use the so-called pseudophase ion exchange (PIE) model.The PIE model is based on the Menger-Portnoy model but additionally allows for ion exchange to occur in the micellar Stern region where a reactive counterion competes with nonreactive counterions (Scheme 5). [Pg.13]

The oxidation of D-fructose with cerium(IV) in sulfuric acid medium is inhibited by an increase in the acidity. A cationic surfactant, CTAB, catalyses the reaction, whereas SDS has no effect. The catalytic role of CTAB has been explained using the pseudophase model of Menger and Portnoy. A mechanism involving the formation of an intermediate complex between /3-D-fructopyranose and Ce(S04)32- has been proposed.61 The oxidation of cycloalkanones with cerium(IV) in sulfuric acid medium showed a negligible effect of acidity. Formation of an intermediate complex, which decomposes in the rate-determining step, has been suggested.62... [Pg.98]

The question is often posed of whether biocatalysts operate by different scientific principles from organic catalysts. Careful analysis reveals that they do not (Knowles, 1991 Menger, 1993). Enzymes are not different, just better (Knowles, 1991). The multitude of enzyme models including oligopeptidic or polypeptidic catalysts (Chapter 18) proves that all enzyme action can be explained by rational chemical and physical principles. However, enzymes can create unusual and superior reaction conditions, such as extremely low pKa values for a lysine residue (Westheimer, 1995) or a high positive potential for a redox metal ion (Wittung-Stafshede, 1998). [Pg.14]


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See also in sourсe #XX -- [ Pg.23 ]




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