Micellar charge effects

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). 

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... 

All the evidence to date fits the hypothesis that micellar charge effects are related to mechanism, but the results are not so easy to explain. In a reaction dominated by bond breaking, positive charge developing at the reaction center should interact favorably with an anionic head group and the... 

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. 

Spontaneous hydrolyses of carboxylic anhydrides, diaryl carbonates and aryl chloroformates are faster in cationic than in anionic micelles, regardless of the nature of the counteranion in the cetyltrimethylammonium micelle (Al-Lohedan et al., 1982b Bunton et al., 1984). This charge effect does not seem to be related to substrate hydrophobicity, although the extent of micellar inhibition (relative to reaction in water) is clearly dependent upon substrate hydrophobicity for anhydride hydrolyses. 

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. 

It is of interest to note the resemblance of our data to that reported by Friberg et al. (24). They have investigated the rate of hydrolysis of p-nitrophenol laurate in the microemulsion system consisting of ceryltrimethyl ammonium bromide, butanol and water. Two pronounced and broad peaks of the reaction rate were observed. The enhancements have been ascribed to the conventional micellar catalysis effect in which the micellar surface charge density plays a dominant role. However, this seems unlikely to be the reasons for the enhancements observed in our studies in view of the sharpness of the peak at 0.855 as compared to that reported by Friberg et al. (24). 

Much less is known about micellar charge and counterion binding in the case of bile salts. Based on the result of ionic self-diffusion measurements [20,163,173], conductance studies [17,18,187], Na, and Ca activity coefficients [16,19,144,188,189] and NMR studies with Na, Rb and Cs [190], a number of generalities can be made. Below the operational CMC, all bile salts behave as fully dissociated 1 1 electrolytes, yet interionic effects between cations and bile salt anions decrease the equivalent conductance of very dilute solutions [17,18,187]. With the onset of micelle formation, counterions become bound to a small degree values at this concentration are about < 0.07-0.13 and are not greatly influenced by the species of monovalent alkali cations [163,190]. At concentrations above the CMC, values remain relatively constant to 100 mM in the case of C and this... 

In addition to a generalized medium effect, micelles also have a charge effect that seems to be related to the reaction mechanism. Most of the experiments were made by using CTAX as the cationic and SDS as the anionic surfactant, and the rate constants for reaction of fully micellar bound substrates are designated k+ and k that is, k+ and k are values of k M in cationic and anionic micelles, respectively (54, 55). 

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