Reactive counterion micelles

The previous section discussed reactions in which a reagent, e.g., a nucleophilic anion, is added to a cationic micelle of a surfactant whose counteranion is chemically inert, so that the two anions compete for the micelle. However one has a conceptually simpler situation when the only micellar counterion is also the reactant. There is then no interionic competition, and if the micelle is saturated with reactive counterions, i.e., if /8 is constant, the observed first order rate constants should increase as the substrate binds to the micelle, and will be constant once the substrate is fully micellar bound. 

This treatment accounts for a large amount of data for aromatic nucleophilic substitution, dephosphorylation and deacylation in reactive counterion micelles involving OH, F and RC02 [94,96] (Table 5). Examples are shown in Table 5 together with values of It is important to note that if > 500/M, as it is for Br or CN , Eqn. 11 predicts that will vary little over a wide range of... 

In cetyltrimethylammonium micelles with a reactive counterion. Relative to the second order rate constant in water. 

The distribution of a reactive counterion is described by ion exchange constant because substantial experimental work has demonstrated that micellar and other aqueous ionic interfaces act as selective ion exchangers. For example, reactive counterions, for example, H+ in anionic micelles exchange with other cations such as Li" or Cs" " and reactive anions, for example, OH or CN in cationic micelles are displaced by nonreactive counterions. Their distributions are generally described by ion exchange constants, (14) ... 

Reactive counterion surfactants have only one counterion and it is also the reactant, for example, OH in cetyltrimethylammonium hydroxide (CTAOH) micelles. The stoichiometric concentration of micellized N, [Nm] increases incrementally with added surfactant, but the interfacial molarity, Nm, is constant because, in the original development of pseudophase models, the assumption was made that [Nm]/[Sm] = P and that is constant and independent of surfactant concentration and the concentration of any added salt. Equation (15) reduces to (17). Equation (17) has the same mathematical form as (11) for first-order reactions. 

This make sense because, if the interfacial reactive counterion concentration is constant, (13), then it was reasonable to assume that the ratio of bound counterions to micellized surfactant, P, and Vm are constant. This prediction has been observed many times when kobs increases with added [CTANt] until aU the substrate. A, is micellar bound and then obs becomes constant. However, this prediction has also failed when a significant amount of counterion is added as salt, above 0.2 M to more than 1 The experimental results at high concentrations of added reactive counterion are qualitatively consistent with the assumption that the total interfacial counterion concentration is given by... 

Cuenca, A. Alkaline hydrolysis of 2-phenoxyquinoxaline in reactive counterion micelles effects of head group size. Int. J. Chem. Kinet. 1998, 50(11), 777-783. 

The results in Figs. 4 and 5 show clearly that the values of X (and also H20m) are not independent of surfactant and salt concentrations as required by Eq. (2) if both and Vm are constant, as generally assumed in applications of the PIE model to micellar catalyzed reactions [20-22]. The increase in Xm with added salt is consistent with several sets of kinetic results in micelles in which the observed rate constants increase steadily with added reactive counterions and never reach a plateau. The linear regions of the results in Figs. 7 and 8 are qualitatively consistent with Eq. (4) ... 

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). 

It is more difficult to interpret micellar effects upon reactions of azide ion. The behavior is normal , in the sense that k /kw 1, for deacylation, an Sn2 reaction, and addition to a carbocation (Table 4) (Cuenca, 1985). But the micellar reaction is much faster for nucleophilic aromatic substitution. Values of k /kw depend upon the substrate and are slightly larger when both N 3 and an inert counterion are present, but the trends are the same. We have no explanation for these results, although there seems to be a relation between the anomalous behavior of the azide ion in micellar reactions of aromatic substrates and its nucleophilicity in water and similar polar, hydroxylic solvents. Azide is a very powerful nucleophile towards carboca-tions, based on Ritchie s N+ scale, but in water it is much less reactive towards 2,4-dinitrohalobenzenes than predicted, whereas the reactivity of other nucleophiles fits the N+ scale (Ritchie and Sawada, 1977). Therefore the large values of k /kw may reflect the fact that azide ion is unusually unreactive in aromatic nucleophilic substitution in water, rather than that it is abnormally reactive in micelles. 

For a surface active betaine ester the rate of alkaline hydrolysis shows significant concentration dependence. Due to a locally elevated concentration of hydroxyl ions at the cationic micellar surface, i.e., a locally increased pH in the micellar pseudophase, the reaction rate can be substantially higher when the substance is present at a concentration above the critical micelle concentration compared to the rate observed for a unimeric surfactant or a non-surface active betaine ester under the same conditions. This behavior, which is illustrated in Fig. 10, is an example of micellar catalysis. The decrease in reaction rate observed at higher concentrations for the C12-C18 1 compounds is a consequence of competition between the reactive hydroxyl ions and the inert surfactant counterions at the micellar surface. This effect is in line with the essential features of the pseudophase ion-exchange model of micellar catalysis [29,31]. 

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). 

Quite large enhancements in the rate of photochemical reaction have been observed in heterogeneous environments such as those that occur in aqueous micelle solutions or surface semiconductors (Cooper W.J. and Herr. F.L., I 987). The ways that micelles may influence solute chemical reactivity have been sumimarized below. These influences include cage, localization-compartmentalization, micro viscosity, polarity, pre-orientation, counterion and local electric field effects. 

The fact that the gaseoues reactants react very quickly means that, in practice and according to model B, the reaction takes place at the phase boundary or in an interfacial layer with a relatively small thickness [30, 32], The latter has been proven which - via process modeling on the basis of appropriate kinetic models -made possible a more optimal reactor and mixing design [43], Additionally, there is much (industrially initiated) work underway to check the addition of counterions or surface active ligands (Sections 3.2.4 and 3.2.6) or to test measures which increase the widths of the interfacial layers or the consequences of micelle/vesicle-forming devices (Section 4.5) [45]. The dependence of the reactivity of aqueous systems on the solubility of the reactants in the aqueous catalyst solutions is of appreciable importance for the problem of universal applicability (cf., e.g., Sections 4.1, 4.2, 6.1.3.2, and Chapter 7). 

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