Micellar charge

With these assumptions, ion exchange between a reactive anion, Y , and an inert anion, X , for example, was written in terms of (7).2 It then was relatively straightforward to write the concentration of reactive ion in the micelle in terms of an assumed constancy of fractional micellar charge, a, and the ion exchange parameter, K, and to analyse rates in terms of these parameters, the binding constants of the substrate, Ks, and the second-order rate constants, kw and A M (Romsted, 1977, 1984 Quina and Chaimovich, 1979 Bunton and Romsted, 1979). 

In the discussions of micellar effects thus far there has been essentially no discussion of the possible effect of micellar charge upon reactivity in the micellar pseudophase. This is an interesting point because in most of the original discussions of micellar rate effects it was assumed that rate constants in micelles were affected by the presence of polar or ionic head groups. It is impracticable to seek an answer to this question for spontaneous reactions of anionic substrates because they bind weakly if at all to anionic micelles (p. 245). The problem can be examined for spontaneous unimolecular and water-catalysed reactions of non-ionic substrates in cationic and anionic micelles, and there appears to be a significant relation between reaction mechanism and the effect of micellar charge upon the rate of the spontaneous hydrolysis of micellar-bound substrates. 

The foregoing discussion of micellar charge effects has implicitly assumed that differences in water activity or substrate location in cationic and anionic micelles are not of major importance. If such differences were all important it would be difficult to explain the differences in k+/k for carbonyl addition and SN reactions, because increase of water content in an aqueous-organic solvent speeds all these reactions (Johnson, 1967 Ingold, 1969). As to substrate location, there is very extensive evidence that polar organic molecules bind close to the micelle-water interface in both anionic and cationic micelles, although the more hydrophobic the solute the more time it will spend in the less polar part of the micelle. Substrate hydrophobicity has a marked effect on the overall rate effects in both cationic and anionic micelles, but less so on values of k+/k. It seems impossible to explain all these charge effects in terms of differences in the location of substrates in cationic and anionic micelles. 

If micellar charge effects can be related to charge distribution in the transition state, as we suggest, they should be applicable to the elucidation of mechanisms of spontaneous hydrolyses, and we illustrate this approach by... 

These micellar charge effects also seem to be present for reactions at heteroatoms. For example, in the spontaneous hydrolysis of a series of benzenesulfonyl chlorides (12), values of k+/k for reactions in CTAC1 and SDS increase from 0.85 for X = OMe to 22 for X = NOz, suggesting that there is a substituent effect upon the relative extents of bond-making and breaking (Bunton et al., 1985). 

The effect of micelles on these spontaneous hydrolyses is difficult to explain in terms of kinetic solvent effects on these reactions. Mukerjee and his coworkers have refined earlier methods for estimating apparent dielectric constants or effective polarities at micellar surfaces. For cationic and zwitterionic betaine sulfonate micelles Def is lower by ca 15 from the value in anionic dodecyl sulfate micelles (Ramachandran et al., 1982). We do not know whether there is a direct connection between these differences in effective dielectric constant and the relation between reaction rates and micellar charge, but the possibility is intriguing. 

In much the same way it should be possible to discriminate between attack by anionic and non-ionic nucleophiles. Micelles, regardless of charge, generally speed attack by non-ionic nucleophiles, but the enhancements are typically small, whereas large inhibition or enhancement is observed for attack of nucleophilic anions, depending upon micellar charge. 

The electrical conductance shows a weaker concentration dependence above than below the CMC corresponding to a decrease in the equivalent conductance (Fig. 2.10). The transport number of the surfactant ion rises sharply at the CMC while that of the counterion may become negative. This as well as electrophoretic mobilities may yield information on micellar charge. At high concentrations, conductance anisotropies have been observed for flowing systems. This, as well as flow birefringence, is useful for the demonstration of nonspherical micelle shape. 

The second boundary condition is obtained from the micellar charge Qm. It follows from Gauss theorem that... 

Studies of the spontaneous hydrolysis of a series of substituted benzoyl chlorides and of 4-X-benzenesulfonyl chlorides at 25 °C in cationic, anionic, and sulfobetaine micelles have allowed an assessment to be made of micellar charge effects on hydrolysis mechanisms.72... 

FIGURE 31.9 Correlation of surfactant micellar zeta potential and micelle charge density with zein dissolution showing that protein denaturation potential scales linearly with the micellar charge/potential. 

The micellar charge was also corroborated by the Guoy—Chapman diffused, double-layer model. At equilibrium, the surface charge of the micelle alters the ionic composition of the interface with respect to the bulk concentration. The difference between the actual proton concentration on the interface and the one measured at the bulk by pH electrode is observed as a pK shift of the indicator and is related with the Gouy-Chapman potential (Goldstein, 1972). 

Figure 34. Determination of micellar charge from equilibrium and kinetic measurements. The decrement of micellar charge as a function of sodium dodecyl sulfate added to Brij 58 micelles was calculated from the pK shift according to Gouy-Chapman equation (/ = lOm/W) (A) or from the second-order rate constant of protonation using Debye s equation (Eigen etal., 1964) for rates measured in the presence of ionic screening (O) at/ = 10mM,or from rates extrapolated to / = 0 ( ) (Gutman et al., 1981a).

Figure 35. The correlation between the second-order diffusion-controlled rate constant of protonation of adsorbed Neutral Red and the micellar charge. The micellar charge was calculated as described in the text. The continuous line was calculated according to Debye s equation (Eigen et al., 1964).

Bromo Cresol Green adsorbed on Brij 58 micelles shifts its pK from pK = 4.96 (/ — 0) in water to pKcbs = 6.5 0.05. The distribution ratio of the acidic (a) and alkaline (P) states are a = 30,000 1000 and (3 = 1100 100. Thus, protonation increases the lipophilicity of the indicator by 30-fold. The pK of the adsorbed indicator is independent of the ionic strength (Table VI), suggesting that the micellar charge is close to zero. 

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