Micelle aqueous

The thermodynamics of micelle formation has been studied extensively. There is for example a mass action model (Wennestrdm and Lindman, 1979) that assumes that micelles can be described by an aggregate Mm with a single aggregation number m, so that the only descriptive equation is mMi Mm. A more complex form assumes the multiple equilibrium model, allowing aggregates of different sizes to be in equilibrium with each other (Tanford, 1978 Wennestrdm and Lindman, 1979 Israelachvili, 1992). 

Hydrophobic substances added to an aqueous micellar solution tend to be entrapped into the oily interior, and if the substance contains a fiuorofor, its fluorescence properties may change drastically upon entrapment, due to the change 

Micellar catalysis is a broad field (Fendler and Fendler, 1975 Rathman, 1996 Rispens and Engberts, 2001), and caution is needed when using this term. In fact, whereas the broad term catalysis is justihed when referring to an increase of the velocity of reachon, this does not always mean that the velocity constant is increased (namely that there is a decrease of the specific activation energy). Rather, the velocity effect can be due to a concentration effect operated by the surface of the micelles. This is also the case for the autocatalytic self-reproduction of micelles discussed in the previous chapter, where the lipophilic precursor of the surfactant is concentrated on the hydrophobic surface of the fatty acid micelles (Bachmann et al., 1992), a feature that has given rise to some controversy (Mavelli and Luisi, 1996 Buhse etal, 1991 1998 Mavelli, 2004). 

micelles are equilibrium systems. What does this mean It means that the final state, for example the average micellar size and all corresponding physical properties, are reached regardless of the pathway used to form them. The final state is thus independent of the mixing order of the components, and does not depend on the previous history of the sample. It is possible to make larger or smaller micelles by changing the environmental parameters (e.g., salt concentration) - and if two 

We have seen that aqueous micelles are important in all cases in which hydrophobic, lipophilic substances have to be solubilized - and this, as already mentioned, is the basis of the large teclmical importance of micelles in laundry, oil refining, cosmetics, and chemical reactivity at the oil/water interphase. Reverse micelles, as the term implies, are important in the reverse case, when a hydrophilic substance needs to be solubilized in an oily environment. Typical solvents in this case are hydrocarbons, chloroform, or CCI4. 

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Because the core of an aqueous micelle is extremely hydrophobic, it has the abiHty to solubiHze oil within it, as weU as to stabilize a dispersion. These solubilization and suspension properties of surfactants are the basis for the cleansing abiHty of soaps and other surfactants. Furthermore, the abiHty of surfactants to stabilize interfacial regions, particularly the air—water interface, is the basis for lathering, foaming, and sudsing. 

Mazumdar S, Mitra S (1993) Biomimetic Chemistry of Hemes Inside Aqueous Micelles. 81 115-145... 

With the development of new instrumental techniques, much new information on the size and shape of aqueous micelles has become available. The inceptive description of the micelle as a spherical agglomerate of 20-100 monomers, 12-30 in radius (JJ, with a liquid hydrocarbon interior, has been considerably refined in recent years by spectroscopic (e.g. nmr, fluorescence decay, quasielastic light-scattering), hydrodynamic (e.g. viscometry, centrifugation) and classical light-scattering and osmometry studies. From these investigations have developed plausible descriptions of the thermodynamic and kinetic states of micellar micro-environments, as well as an appreciation of the plurality of micelle size and shape. 

The typical cream, a soft, emulsified mass of solidified particles in an aqueous, micelle-rich medium, does not form a water-impermeable (occlusive) film on the skin. Nevertheless, creams contain lipids and other moisturizers that replace substances lost from the skin in the course of everyday living. Creams thus make good emollients because, by replenishing lipids and in some instances also polar, hygroscopic substances,... 

Figure 1 Typical cross-sectional schematic representing the classical view of an aqueous micelle. Counterions are not shown. (From Ref. 2 with permission.)...

Organic Reactivity in Aqueous Micelles and Similar Assemblies... 

The general principles which govern the effects of normal, aqueous, micelles upon reaction rates and equilibria are considered first, and then we discuss some specific reactions and the relation of micellar effects to mechanism. We also briefly consider some non-micellar species generated by amphiphiles which can also mediate reactivity. 

Studies of chemical reactivity in self-assembling colloids were initially based on reactions in aqueous micelles, but recently reactivity has been examined in other colloidal systems such as microemulsions and synthetic vesicles (Mackay, 1981 Fendler, 1982 O Connor et al., 1982, 1984 Cuccovia et al. 1982b). Some hydrophobic trialkylammonium salts, which are phase-... 

Provided that equilibrium is maintained between the aqueous and micellar pseudophases (designated by subscripts W and M) the overall reaction rate will be the sum of rates in water and the micelles and will therefore depend upon the distribution of reactants between each pseudophase and the appropriate rate constants in the two pseudophases. Early studies of reactivity in aqueous micelles showed the importance of substrate hydropho-bicity in determining the extent of substrate binding to micelles for example, reactions of a very hydrophilic substrate could be essentially unaffected by added surfactant, whereas large effects were observed with chemically similar, but hydrophobic substrates (Menger and Portnoy, 1967 Cordes and Gitler, 1973 Fendler and Fendler, 1975). 

This limited amount of kinetic evidence suggests that the kinetic models developed for reactivity in aqueous micelles are directly applicable to reactions in vesicles, and that the rate enchancements have similar origins. There is uncertainty as to the appropriate volume element of reaction, especially if the vesicular wall is sufficiently permeable for reaction to occur on both the inner and outer surfaces, because these surfaces will have different radii of curvature and one will be concave and the other convex. Thus binding, exchange and rate constants may be different at the two surfaces. 

This section gives tabulated examples of recent work on micellar effects upon chemical and photochemical reactions. In general the examples given in this section do not duplicate material covered elsewhere in the chapter for example micellar effects on some photochemical reactions and reactivity in reversed micelles are listed here although they are neglected in the body of the text. For many ionic reactions in aqueous micelles only overall rate effects have been reported, in many cases because the evidence did not permit estimation of the parameters which describe distribution of reactants between aqueous and micellar pseudophases. These reactions are, nevertheless, of considerable chemical importance, and they are briefly described here. 

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