Micelle 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". ... 

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. 

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. 

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... 

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. 

An additional question is that of water penetration. Menger [35] attempted to construct a micelle using space-filling models, and found that if the chains are extended there will be extensive voids into which water molecules, or solutes, can penetrate. However, these voids will be reduced in size if some eclipsing is introduced into the hydrophobic alkyl groups. This porous-cluster model is supported by physical and chemical evidence of extensive water penetration into the micelle. 

Therefore, provided that there is an equilibrium between the micellar and aqueous pseudophases, the problem resolves itself into estimation of the distribution of reactants between aqueous and micellar pseudophase and calculation of the rate constants of reaction in each pseudophase. Menger and Portnoy [70] developed an equation which successfully accounted for micellar inhibited saponification of 4-nitrophenyl alkanoates. This model was also applied to spontaneous, unimolecular, hydrolyses of dinitrophenyl sulfate monoanions [66] and phosphate dianions [68] which are speeded by cationic micelles in water. 

So the Menger and Doll statement is irrefutable but also misleading. The model amphiphile chains have segregated themselves away from the water and their own headgroups almost as much as is physically possible. 94% of the chain volume resides inside the dry core and the hydrocarbon volume fraction falls off rapidly outside the core (fig. 8). There is contact of the chains with water, simply b ause of the large area per amphiphile at the micelle surface (more than 56 A for the micelle of figure 8). 

Although the porous cluster micellar model may not be considered completely different from the two-state miceUar model of Hartley in view of the fact that the so-called Stem layer and micellar core of Hartley micelles have never been clearly and precisely defined, a constractive scientific debate has started among the proponent and opponent scientific groups of porous cluster model of micelle. Proponents - and opponents of porous cluster micellar stmcture have published a series of papers over more than a decade since Menger proposed this model in 1979, but, as usual, the findings described in these papers always have almost equally convincing alternative explanations and thus, the real issue on water penetration in micelles remains unclear. 

Experimentally determined rate constants for various micellar-mediated reactions show either a monotonic decrease (i.e., micellar rate inhibition) or increase (i.e., micellar rate acceleration) with increase in [Suifl CMC, where [Surf]T represents total micelle-forming surfactant concentration (Figure 3.1). Menger and Portnoy obtained rate constants — [SurfJx plots — for hydrolysis of a few esters in the presence of anionic and cationic surfactants, which are almost similar to those plots shown in Figure 3.1. These authors explained their observations in terms of a proposed reaction mechanism as shown in Scheme 3.1 which is now called Menger s phase-separation model, enzyme-kinetic-type model, or preequilibrium kinetic (PEK) model for micellar-mediated reactions. In Scheme 3.1, Kj is the equilibrium... 

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