Functional micelles and comicelles

Several workers have attempted to draw analogies between reactions in functional micelles and in enzymes, but in general this seems to be unjustified because there is generally little substrate- or stereo-specificity in the micellar reactions and usually limited turnover rates. 

The functional micellized surfactant contained both imidazole and hydroxyethyl groups and the final products of reaction with p-nitrophenyl alkanoates are acyl derivatives of the hydroxyethyl group. However, these O-acylated products were formed from an acylimidazole intermediate which 

Several reasons can be adduced for concerted reactions apparently being relatively unimportant in functional micelles. 

micelles have very loose, mobile, structures and there are considerable entropy costs in a concerted reaction. These costs are much less serious in enzymic systems where conformation at the active site is tailor-made to fit the transition state. Secondly, the sites of micellar reactions are very wet and omnipresent water molecules are available to transfer protons. 

The problem of slow turnover has been noted. Initial reaction of a nucleophilic group, e.g. imidazole or oximate, in a functional micelle, with a carboxylic or phosphoric ester, for example, gives an acylated or phosphory-lated imidazole or oxime, and these derivatives hydrolyze slowly to regenerate the nucleophile. Kunitake and Shinkai (1980) discuss a number of reactions in micelles which contain both nucleophilic and basic groups which are potentially capable of acting as bifunctional reagents (Tonellato, 1979, Kunitake and Shinkai, 1980 Bunton, 1984) 

---

For a number of reactions in functional micelles and comicelles second-order rate constants are similar in micelles and in water. Except for aromatic nucleophilic substitution they are slightly smaller in the micelles than in water, and the pattern of behavior is exactly that found for reactions of organic nucleophilic anions in non-functional micelles. Some examples of these comparisons are in Table 9. 

Relative rate constants of reactions in functional micelles and comicelles ... 

Micelles and comicelles functionalized with histidinamide or alaninamide moieties. Rate comparisons... 

Comicelles of hydroxy-, phenol-, thiophenoxide-, thiol- and imidazole-functionalized surfactants. Rate comparisons CTABr, SDS and non-functional and hydroxamate-functionalized micelles. Rate comparisons... 

However, as for reactions in non-functional micelles, sec , cannot be compared directly with second-order rate constants in water, whose dimensions are, conventionally, M /sec. But this comparison can be made provided that one specifies the volume element of reaction, which can be taken to be the molar volume of the micelles, or the assumed molar volume of the micellar Stern layer. This choice is an arbitrary one, but the volumes differ by factors of ca. 2 [107,108], so it does not materially affect the conclusions. The rate constants in the micelle for dephosphorylation, deacylation and nucleophilic substitution by a functional hydroxyethyl surfactant are similar to those in water [99], and similar results have been observed for dephosphorylation using functional surfactants with imidazole and oximate [106,107]. Similar results have been obtained by Fornasier and Tonalleto [108] for deacylation of carboxylic esters by a variety of functional comicelles. 

In all the bimolecular reactions considered thus far the surfactant has been chemically inert, but a functionalized surfactant will generate a micelle in which reactant is covalently bonded (Scheme 3). The functional groups are basic or nucleophilic, and include amino, imidazole, oximate, hydroxamate, thiolate or hydroxyl [3-6,97-108]. In some cases comicelles of a functional and an inert surfactant have also been used. The reactions studied include deacylation, dephosphorylation, nucleophilic aromatic substitution, and nucleophilic addition to preformed carbocations, and some examples are shown in Scheme 7. 

When micelles are formed from surfactants with functional head groups that contain an anionic nucleophilic moiety such as an amino, hydroxyl, thionate, iodosobenzoate, imidazole, or gem-diolate group, they are often referred to as reactive micelles. Such surfactants can either be micellized by themselves or comicellized with common inert surfactants (Fendler and Fendler, 1975 Sudholter et al., 1979 Fendler, 1982 Romsted, 1984). It can generally be assumed that aqueous-phase reaction is negligible (i.e., all of the substrate is micellar-bound), and the micellar head groups are completely neutralized by counterions (Bunton, 1991). 

Comicellization of NF and STrS is shown in Fig. 7.15 as a function of the overall mole fraction of STrS. No micelles exist in region I. In region II, micelles... 

Fig. 7.19 Comicellization shown as a function of total concentration and overall mole fraction of STS in 0.05M sodium chloride solution at 30°C region I, no micelles region II, NF-rich micelles region III, two kinds of mixed micelles region IV, STS-rich micelles. The dashed lines represent the asymptotes at infinity total concentration. (From Ref. 111. Reproduced by permission of the American Chemical Society.)...

---