Water Penetration into Micellar Environment

Water penetration into micellar pseudophase is a controversial topic. In an excellent account, Menger has summed up a bewildering array of opinions on 

The great majority of chains have six or more water-exposed carbons outside the nucleus. 

The nucleus is defined as the region of direct chain contact which contains virtually no water and possesses a dielectric constant similar to that of pure hydrocarbon. 

The nucleus comprises only a fraction of total micelle volume (perhaps 15 to 20% depending on where one locates the borders). 

With a few of the chains, water molecules could reach well beyond the first six carbons. 

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As in the case of the base-catalyzed hydrolysis, the observed rate constant for the reaction of ester 9c in the presence of amine 5a was found to decrease with increasing concentration of CTAB. Using equation (28), the second order rate constants for the aminolysis and those for the alkaline hydrolysis were calculated as a function of CTAB concentration and were found to be decreased by factors of 30-6 and 13-3, respectively, by micellar CTAB. These results can be interpreted by an explanation analogous to that for the alkaline hydrolysis in the absence of amines. However, the magnitude of the inhibition for the aminolysis (30-6) as compared to that for the hydroxide ion-catalyzed hydrolysis (13-3) is not readily explicable. It is conceivable, however, that deep penetration of ester into the micelle could result in an environment for the ester group in which the amino group is either not suitably oriented for nucleophilic attack or is excluded to a greater extent than water and hydroxide ion. 

Menger and Boyer [34] have recently presented optical rotary dispersion data which indicate appreciable penetration of water into the micellar core. A large change in the sign of the Cotton effect is induced on transferrance of (-h )trans-2-chloro-5-methylcyclohexanone from heptane to water. This is ascribed to a diaxial-diequatorial equilibrium which responds to the nature of its environment, lying further to the right in water than in heptane. 

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