Inverse micelles reactions

It was found the pK s involved in the inverse micelle reaction could not readily be measured spectrophotometrically... 

The subject of micellar catalysis and inhibition of reactions can be divided into the types of reaction occurring, e.g. base-catalysed and acid-catalysed hydrolyses, oxidation, etc., or in terms of mechanisms, e.g. juxtaposition of reactive groups in micelles, attraction of counterions to an oppositely charged micellar surface, protection by solubilization within non-ionic micelles, etc. It is not possible to adhere rigidly to either scheme but we will attempt here to consider, in turn, hydrolysis, oxidation in aqueous micelles, reactions in inverse micelles, reactions involving drugs and miscellaneous reactions of interest. Bunton s summary of the topic in his recent review of the subject is worth repeating here [12] ... 

Surfactants have also been of interest for their ability to support reactions in normally inhospitable environments. Reactions such as hydrolysis, aminolysis, solvolysis, and, in inorganic chemistry, of aquation of complex ions, may be retarded, accelerated, or differently sensitive to catalysts relative to the behavior in ordinary solutions (see Refs. 205 and 206 for reviews). The acid-base chemistry in micellar solutions has been investigated by Drummond and co-workers [207]. A useful model has been the pseudophase model [206-209] in which reactants are either in solution or solubilized in micelles and partition between the two as though two distinct phases were involved. In inverse micelles in nonpolar media, water is concentrated in the micellar core and reactions in the micelle may be greatly accelerated [206, 210]. The confining environment of a solubilized reactant may lead to stereochemical consequences as in photodimerization reactions in micelles [211] or vesicles [212] or in the generation of radical pairs [213]. 

The rate constants and k represent rate constants for a surface reaction and have units m mol s and s respectively. The accelerative effects are about 10 -10 fold. They indicate that both reactants are bound at the surface layer of the micelle (surfactant-water interface) and the enhanced rates are caused by enhanced reactant concentration here and there are no other significant effects. Similar behavior is observed in an inverse micelle, where the water phase is now dispersed as micro-droplets in the organic phase. With this arrangement, it is possible to study anion interchange in the tetrahedral complexes C0CI4 or CoCl2(SCN)2 by temperature-jump. A dissociative mechanism is favored, but the interpretation is complicated by uncertainty in the nature of the species present in the water-surfactant boundary, a general problem in this medium. 

To use templates or envelopes as a controlled reaction space was developed in the early 1980s, such as the use of inverse micelle technique (4). Another fundamental idea is to use the atomic periodicity of surfactant molecules by using them as surface ligands for sequential addition of anions and cations under the concept of semiconductive compounds like CdSe as a living polymer (3). 

The parameter r2 is independent of the initiator type for the emulsion, however, and is slightly higher than that obtained in benzene (r2=1.23) (Table 3). This behavior results from good compatibility of the macromonomer with poly-BzMA. Therefore the reactivity of the macromonomer does not depend so much on the reaction medium type. In contrast, reversed apparent reactivity was observed in heptane in which the clear solution of monomer turned into a polymer suspension upon polymerization. Since BzMA is soluble in the medium, it has been suggested that the polymerization occurs preferentially on the (inverse) micelle surface which is enriched by the macromonomers. 

The structure and dynamics of inverse (water in oil) micellar solutions and microemulsions are of interest because of the unique properties of the water core, the view that such micelles may serve as models of enzyme active sites, and the potential use of inverse micelles as hosts for enzymatic reactions (80-82). 

The inhomogeneous structure of a micelle (or inverse micelle) can influence the course of a photoinduced electron transfer. Such a micelle is biphasic, containing a hydrocarbon-like core and a water-like surface. If the photoinduced electron transfer produces a product which has lower solubility in the aqueous phase (a situation which might obtain if a cationic acceptor is reduced to a neutral product), this product will be directed by solubility considerations to move toward the hydrophobic center of the micelle, i.e., remote from the site of the forward electron transfer. This spatial separation, shown conceptually in Scheme 4, in turn will retard the rate of the back reaction compared with that of the forward reaction. 

Nevertheless, for inverse miniemulsions the surfactant is used in a very efficient way, at least as compared to inverse micro emulsions [47,48] or inverse suspensions [49] which are used for subsequent polymerization processes. Again, the surface coverage of the inverse miniemulsion droplets with surfactant is incomplete and empty inverse micelles are absent. Again this is important for the interpretation of the reaction mechanism. 

It should be emphasized that these structural changes within a one-phase region may change the kinetics of a chemical reaction in a pronounced manner. As an example may be mentioned the catalytic effect of Inverse micelles on ester hydrolysis. Fig. 5 is from the first publication — on this subject. It clearly shows the lack of catalytic effect by the premicellar aggregates and the sudden increase of hydrolysis rate in the concentration range where the Inverse micelles begin being formed. 

Figure 11 The preparation of nanoparticles by the inverse micelle process in which a chemical reaction between microemulsion or inverse micelles after collision and perhaps fusion converts the soluble salt into an insoluble metal or metal oxide as shown. Source From Ref. 75.

The ultrasonification process is connected with the rapidly increased oil-water interfacial area as well as the significant re-organization of the droplet clusters or droplet surface layer. This may lead to the formation of additional water-oil interface (inverse micelles) and, thereby, decrease the amount of free emulsifier in the reaction medium. This is supposed to be more pronounced in the systems with non-ionic emulsifier. Furthermore, the high-oil solubility of non-ionic emulsifier and the continuous release of non-micellar emulsifier during polymerization influence the particle nucleation and polymerization kinetics by a complex way. For example, the hairy particles stabilized by non-ionic emulsifier (electrosteric or steric stabilization) enhance the barrier for entering radicals and differ from the polymer particles stabilized by ionic emulsifier. The hydro-phobic non-ionic emulsifier (at high temperature) can act as hydrophobe. 

In Chapter 29, Bunton presents a brief review of micellar effects on nucleophilicity, and he describes recent work of his own in this area. A major contribution of Bunton s has been his development of a quantitative model for calculating nucleophile concentration in the pseudophase of the micelle thus, calculation of rate constants in the micelle is possible. Using this model, Bunton finds that the reaction rates in micelles are very similar to those in water. Thus, micellar accelerations result from reactant concentration. Bunton notes that this conclusion also applies to microemulsions, vesicles, and inverse micelles. A second important contribution of this chapter is a summary of the large amount of experimental work on the contrasting effects of cationic and anionic micelles on reactions of anionic and neutral nucleophiles and on hydrolyses. 

The overall conclusion is that cationic micelles do not increase the nucleophilicity of anions (Tables I and II), except as noted for some reactions of azide ion. Nonionic nucleophilicity is slightly lower in micelles than in water simply because of their lower polarity. The overall rate enhancements are generally due to increased concentration of reagents in the micelles, and similar conclusions can be drawn for reactions in microemulsions (46) vesicles (47, 48), and inverse micelles in apolar solvents (49). 

Isopar-M SMO AIBN 1 1 6 30-50 47 Baade and Reichert [49] Inverse micelles not detected. Polymerization in monomer droplets. Kinetic latex stability. Solution-like kinetics, with interfacial reactions Inverse- Suspension... 

Unlike the aqueous micells just described (often referred to simply as micelles), there can also be micelles in which the apolar groups are in contact with the solvent at the exterior and the polar or ionic groups are associated with water in the interior. These are referred to as reverse or inverse micelles. Reverse micelles are particularly noted for their accelerating or retarding effect on biochemical reactions. 

Hawker and co-workers reported the synthesis of unimolecular inverse micelles based on benzyl ether dendrimers with hydroxyl groups in its interior (142). They showed that this dendrimer was able to catalyze the El elimination reaction of 2-iodo-2-methyIheptane. The tridendron of G4 seems to be particularly effective in catalyzing this reaction. The conversions to products were found to be greater than 90%. The regioselectivity of the elimination was moderate. Turnover... 

---