Hydrophilic anion reactions, micellar

Micellar effects upon reactions of hydrophilic anions... 

It seems possible that a very hydrophilic anion such as OH- might not in fact penetrate the micellar surface (Scheme 1) so that its interaction with a cationic micelle would be non-specific, and it would exist in the diffuse, Gouy-Chapman layer adjacent to the micelle. In other words, OH" would not be bound in the Stem layer, although other less hydrophilic anions such as Br, CN or N 3 probably would bind specifically in this layer. In fact the distinction between micellar and aqueous pseudophases is partially lost for reactions of very hydrophilic anions. The distinction is, however, appropriate for micellar reactions of less hydrophilic ions. 

Photochemical electron transfer reactions have been examined in micellar systems as probes for the diffusion and location of quenchers, and as environments for solar energy storage 2 3>90 95 96>. The relative rates of quenching will depend on the location of the donor and acceptor (Scheme XXXII). For example, the rate of quenching of a hydrophobic donor located inside the micelle by Cu2+ is much faster in anionic micelles compared to cationic micelles. Similarly a hydrophobic excited state is quenched faster by a hydrophobic donor or acceptor than by a hydrophilic one in micellar systems. 

Table II. Micellar Effects on Reactions of Hydrophilic Anions...

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... 

Specific-ion electrodes are expensive, temperamental and seem to have a depressingly short life when exposed to aqueous surfactants. They are also not sensitive to some mechanistically interesting ions. Other methods do not have these shortcomings, but they too are not applicable to all ions. Most workers have followed the approach developed by Romsted who noted that counterions bind specifically to ionic micelles, and that qualitatively the binding parallels that to ion exchange resins (Romsted 1977, 1984). In considering the development of Romsted s ideas it will be useful to note that many micellar reactions involving hydrophilic ions are carried out in solutions which contain a mixture of anions for example, there will be the chemically inert counterion of the surfactant plus the added reactive ion. Competition between these ions for the micelle is of key importance and merits detailed consideration. In some cases the solution also contains buffers and the effect of buffer ions has to be considered (Quina et al., 1980). 

The situation is different for reactions of very hydrophilic ions, e.g. hydroxide and fluoride, because here overall rate constants increase with increasing concentration of the reactive anion even though the substrate is fully micellar bound (Bunton et al., 1979, 1980b, 1981a). The behavior is similar for equilibria involving OH" (Cipiciani et al., 1983a, 1985 Gan, 1985). In these systems the micellar surface does not appear to be saturated with counterions. The kinetic data can be treated on the assumption that the distribution between water and micelles of reactive anion, e.g. Y, follows a mass-action equation (9) (Bunton et al., 1981a). 

Little is known about the structures of these kinetically effective complexes, or even about the aggregates of the amphiphile. Both hydrophobic and coulombic interactions are important because these aggregates are much less effective than micelles at assisting reactions of hydrophilic nucleophilic anions. These observations are consistent with the view that the aggregates are much smaller than micelles. It is probable that the structures and aggregation numbers of these aggregates depend on the nature of the solutes which bind to them and Piszkiewicz (1977) has suggested that such interactions play a role in micellar kinetics. 

The second order rate constants, A and k, for reactions in the micellar and aqueous pseudophases have the same dimensions, and can now be compared directly, and within all the uncertainties of the treatment it seems that A and k are of similar magnitudes for most reactions, and in some systems A > A . This generalization is strongly supported by evidence for reactions of relatively hydro-phobic nucleophilic anions such as oximate, imidazolide, thiolate and aryloxide, typically with carboxylate or phosphate esters [61,82-85]. These similarities of second-order rate constants in the aqueous and micellar pseudophases are consistent with both reactants being located near the micellar surface in a water-rich region. Therefore the micellar rate enhancements of bimolecular reactions are due largely to concentration of the reactants in the small volume of the micelles. Some examples are in Table 3 for reactions of or hydrophilic nucleophilic anions and in Table 4 for reactions of more hydrophobic nucleophiles. 

Overall, the use of surfactants in water for the studied BV reactions implies the partition of all reaction partners (substrate, oxidant and catalyst) between the micelle, bulk water and the interphase between the two. As a consequence, the lipophilicity of all species is crucial to rationalize their positioning in the micellar system, and as a general observation, more hydrophilic substrates worked well in neutral surfactants while more lipophilic ones worked well in anionic micelles. 

These effects are noted especially in region D of the plot in Fig. 11.12a the results for three samples of NaDS are shown in Fig. 11.12b. In this work the nickel concentration at the micellar surface was estimated by titration with murexide, a chromophoric anion. This interaction has been studied in more detail the reaction between Ni " and murexide (below) results in a pronounced colour change making the reaction easy to follow. Murexide being hydrophilic is not solubilized by the micellar species and thus can be used as an indicator of the appearance of micellar surface . 

---