Micellar catalysis aqueous systems

A particularly interesting type of micellar catalysis is the autocatalytic self-replication of micelles [58]. Various examples have been described, but a particularly interesting case is the biphasic self-reproduction of aqueous caprylate micelles [59]. In this system ethyl caprylate undergoes hydroxyl catalysed hydrolysis to produce the free carboxylate anion, caprylate. Caprylate micelles then fonn. As these micelles fonn, they solubilize ethylcaprylate and catalyse further production of caprylate anion and caprylate micelles. 

Let us recall the micellar aqueous system, as this procedure is actually the basic one. The chemistry is based on fatty acids, that build micelles in higher pH ranges and vesicles at pH c. 8.0-8.5 (Hargreaves and Deamer, 1978a). The interest in fatty acids lies also in the fact that they are considered possible candidates for the first prebiotic membranes, as will be seen later on. The experimental apparatus is particularly simple, also a reminder of a possible prebiotic situation the water-insoluble ethyl caprylate is overlaid on an aqueous alkaline solution, so that at the macroscopic interphase there is an hydrolysis reaction that produces caprylate ions. The reaction is very slow, as shown in Figure 7.15, but eventually the critical micelle concentration (cmc) is reached in solution, and thus the first caprylate micelles are formed. Aqueous micelles can actually be seen as lipophylic spherical surfaces, to which the lipophylic ethyl caprylate (EC) avidly binds. The efficient molecular dispersion of EC on the micellar surface speeds up its hydrolysis, (a kind of physical micellar catalysis) and caprylate ions are rapidly formed. This results in the formation of more micelles. However, more micelles determine more binding of the water-insoluble EC, with the formation of more and more micelles a typical autocatalytic behavior. The increase in micelle population was directly monitored by fluorescence quenching techniques, as already used in the case of the... 

S. Kobayashi, T. Wakabayashi, S. Nagayama, H. Oyamada, Tetrahedron Lett. 1997,38,4559-4562. Judging from the amount of the surfactant used in the present case, the aldol reaction would not proceed only in micelles, (a) J. H. Fendler, E. J. Fendler, Catalysis in Micellar and Macromolecular Systems, Academic, London, 1975. (b) Mixed Surfactant Systems (Eds. P. M. Holland, D. N. Rubingh), ACS, Washington, DC, 19W. (c) Structure and Reactivity in Aqueous Solution (Eds. C. J. Cramer, D. G. Truhlar), ACS, Washington, DC, 1994. (d) Surfactant-Enhanced Subsurface Remediation (Eds. D. A. Sabatini, R. C. Knox, J. H. Harwell), ACS, Washington, DC, 1994. 

Work in the area of micellar catalysis in both aqueous and nonaqueous solvent systems is certain to continue to grow in importance as a tool for better understanding the chemistry and mechanics of enzymatic catalysis, as a probe for studying the mechanistic aspects of many reactions, and as a route to improved yields in reactions of academic interest. Of more practical significance, however, may be the expanding use of micellar catalysis in industrial applications as a method for obtaining maximum production with minimum input of time, energy, and materials. 

Foidler, J. H. Fendl, E. J. Principles of Micellar Catalysis in Aqueous Solutions. Catalysis in Micellar and Macromokcular Systems-, Academic Press New York, 1975 pp 86-103. 

There are two other examples from Buchmeiser [107] and Mingotaud [108] that describe catalyst modification at the X-type Hgand for use in aqueous media, although both of these are based on a micellar catalysis approach (see section MiceUar Catalysis Approaches ). Buchmeiser s system involved the use of an amphiphilic, poly(2-oxazoline)-based block copolymer that was functionalized and subsequently attached to the catalyst through a fluorinated silver carboxylate. 

In an early stage of developing organic reactions in aqueous systems, rare earth triflates were used for aldol reactions in THF-water or ethanol-water, giving successful results. On the other hand, when the reactions were carried out in pure water, the corresponding aldol products were obtained only in low yields. " This was probably because solubility of organic substrates was low and decomposition of silyl enol ethers occurred faster than the desired aldol reactions in water. To address this issue, micellar catalysis in water was... 

Sangwan and Schneider have studied the effect of cyclodextrins on a number of aqueous Diels-Alder reactions between acrylate, fumarate and maleate derivatives of varying hydrophobicities and (mainly) cyclopenta-diene [26]. No simple correlation between substrate hydrophobicity and cyclodextrin-catalyzed rate enhancement was found. However, those systems that did respond to p-cyclodextrin catalysis exhibited enzyme-like saturation kinetics. This led these workers to conclude that the hydrophobic effect can, in fact, be counterproductive to the Diels-Alder reaction if it leads to unproductive orientation of the reactants. The same can be said about the effect of amphiphiles (detergents capable of micellization) on aqueous Diels-Alder reactions since sodium dodecylsulfate (SDS) decelerated the reaction between cyclopentadiene and methyl acrylate. Those cases in the literature claiming micellar catalysis of the aqueous Diels-Alder reaction may simply be benefiting from the solubilizing effect of the amphiphilic additives rather than any bona fide preorganization of the reactants within a micelle [27,28]. 

The thermodynamic equilibria of amphiphilic molecules in solution involve four fundamental processes (1) dissolution of amphiphiles into solution (2) aggregation of dissolved amphiphiles (3) adsorption of dissolved amphiphiles at an interface and (4) spreading of amphiphiles from their bulk phase directly to the interface (Fig. 1.1). All but the last of these processes are presented and discussed throughout this book from the thermodynamic standpoint (especially from that of Gibbs s phase rule), and the type of thermodynamic treatment that should be adopted for each is clarified. These discussions are conducted from a theoretical point of view centered on dilute aqueous solutions the solutions dealt with are mostly those of the ionic surfactants with which the author s studies have been concerned. The theoretical treatment of ionic surfactants can easily be adapted to nonionic surfactants. The author has also concentrated on recent applications of micelles, such as solubilization into micelles, mixed micelle formation, micellar catalysis, the protochemical mechanisms of the micellar systems, and the interaction between amphiphiles and polymers. Fortunately, almost all of these subjects have been his primary research interests, and therefore this book covers, in many respects, the fundamental treatment of colloidal systems. 

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