Comicelles

In practice it is usually necessary to take into account partial deprotonation of the potentially nucleophilic group, or partial micellar binding of the substrate, and if comicelles are used allowance has to be made for dilution of the nucleophile in the micellar pseudophase. 

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. 

Based on a molar volume of the Stern layer of 0.14 M 1 litre unless specified b Comicelle with inert surfactant c Micellar molar volume of 0.36 M -1 litre... 

Comicelles of hydroxy-, phenol-, thiophenoxide-, thiol- and imidazole-functionalized surfactants. Rate comparisons... 

Comicelles of histidine functionalized surfactants. Rate comparisons... 

Deacylation or hydrolysis of chiral carbamates, carbonates and alkanoates Micelles and comicelles of N-hexadecyl-N-methylephedrinium bromide or N -myristoyl-histidine with CTABr. Rate effects and enantioselectivities examined Fomasier and Tonellato, 1984... 

Fig. 3.6. Partial (10%) neutralization of colloidal CaC03with dithiodiglycolic acid leading to comicellized calcium carbonate and calcium dithiodiglycolic carboxylate...

The Armstrong model (1 7) can describe the retention of apolar, polar and even ionic solutes, provided they were binding solutes. The highest IL-, values corresponded to electrostatic interactions (1600 for CTAB with SDS micelles and 2600 for benzoic acid with CTAB with SDS micelles and 2600 for benzoic acid with CTAB micelles) or to comicellization (190 for SOBS with SDS micelles and 3000 for CPC with CTAB micelles). 

Low charge density, hydrophobically modified polybetaines were shown to interact and comicellize with nonionic, anionic, cationic, and amphoteric surfactants [181-183] and many ionic organic dyes [264,265]. The association mechanism of hydrophobically modified polymers and surfactants in dependence on the concentration of interacting components can be modeled by two pathways (Scheme 21) [183]. 

The first pathway is the formation of mixed micelles or hemimicelles, composed of polymer-bound hydrophobes comicellized with surfactant molecules. Intermolecular physical cross-links often enhance the viscosity of the micellar solutions. The second pathway is intramolecular comicellization so that the hydrodynamic size of the associates contracts. 

Interpolymer, polymer-surfactant, and coordination complexes of polybetaines are less developed. The cascade -type complexation observed for the polybetaine-polyelectrolyte system is similar to the layer-by-layer deposition found for oppositely charged polyelectrolytes. The behavior of the polybetaine-surfactant system differs from that of polyelectrolyte-surfactant and polyampholyte-surfactant complexes, leading to inter- or intramolecular comicellization or converting the whole macromolecule to either a polycation or polyanion. 

Deacylation or hydrolysis of Micelles and comicelles of N-hexadecyl- Fomasier and Tonellato,... 

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. 

In the more general case, in which a functional group is only partially deproto-nated to the reactive nucleophile, a correction can easily be made for partial deprotonation. If a comicelle of functional and non-functional surfactant is used it is necessary to correct Eqn. 12 for the mole fraction of functional groups in the micellar pseudophase [107,108]. 

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. 

These results are illustrated in Table 6 where the second-order rate constants in the micelle of comicelle are given relative to those for reaction of a chemically similar monomeric nucleophile in water. (Different volume elements of reaction have been used by the various workers in this area, as footnoted in Table 6.)... 

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

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