Physical micellar catalysis

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... 

Several factors have been invoked to explain the aqueous rate acceleration aggregation of the reactants leading to micellar catalysis, effects connected with the internal pressure of the solvent, polarity of the solvent, H-bonding interactions with the solvent, and hydrophobic interactions (A y < 0). The initial literature was rather controversial, and there was a strong need for a systematic study using physical-organic techniques. 

While the end-cleavage of PCL within worm micelles appears consistent with both the chemical and the nano-scale physical changes, it is also considerably faster than the slow hydrolysis reported for PCL homo/copolymer bulk, particle, or films, i.e. on the time scale of months-years under the same condition. (2-5) The distinction arises with the specific effect of OCL worm micelles on PCL hydrolysis. As speculated from studies on spherical micelles (27), the terminal -OH of the hydrophobic PCL block is not strictly sequestered in the dry , hydrophobic core but tends to be drawn into the hydrated corona. A micellar catalysis effect involving interfacial water (28) plus the likely... 

The rate constants for unimolecular and solvolytic reactions generally show a monotonic decrease (i.e., micellar inhibition)" - or a monotonic increase (i.e., micellar catalysis) - or insensitivity (i.e., micellar-independent rate)"- " to an increase in micellar concentration. There seems to be no exception to this generalization and, if there is one, it is owing to some specific chemical or physical reasons. For example, the nnimolecular decarboxylation of 6-nitrobenzisox-azole-3-carboxylate ion (1) in CTABr micelles is enhanced by the salts of hydrophilic anions and slowed by the salts of hydrophobic anions, whereas salts such as sodium tosylate increased reaction rate when in low concentration, and retarded it when in high concentration. The first theoretical model, known as the... 

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... 

The chemical catalysis (i.e. the decrease of the activation energy of the particular hydrolysis of the caprylic ester) may in fact be positive or negative, but it is probably negligible with respect to the physical catalysis (increase of the active surface where the hydrolysis takes place). Lack of understanding of this concept has led to confiision and to the search of complicated mechanism to explain micellar autocatalysis. The same type of argument holds of course for vesicles. 

Solubilization and catalysis in reversed micelles is the subject of a recent review by Kitahara [93] the literature to 1976 was covered by Fendler in his review [94] with emphasis on the extensive work from his own laboratories. Reactions in reversed micelles will not be simple reflections of reactions in normal micelles, but are bound to be influenced by the nature of the water in the interior of the micelles. The size of the pools of solubilized water will be determined by the ratio of surfactant to water and by the nature of the head groups of the surfactants which congregate together in the centre of these aggregates. The physical properties of the solubilized water has been found to be quite different from the properties of bulk water especially at low levels of hydration of the head groups [95]. At higher concentrations of water in the micelle interior the water behaves more like bulk water. Fluorescence probe analysis of the micellar core has indicated a very rigid interior state with a viscosity of over 40 cP [96-99]. 

Focused on kinetic, chemical, and physical aspects of micelle-mediated catalysis, this book offers clear insight into the complex mechanisms that occur in biological reactions. Micellar Catal sis is an essential source of reference for scientists involved in the re.search and development of micelles for industrial and biochemical applications. 

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