Kinetics of Solubilizates and Counterions

One of the most important features of micellar solutions from a chemical point of view is their ability to solubilize otherwise water insoluble molecules. The liquid-like apolar micellar interior acts as a solvent for apolar substances. The solubilized molecules are of course also in dynamic equilibrium with the aqueous environment and other micelles. The kinetics of the solubilizate exchange has been studied by ESR methods using nitroxide radicals with a significant water solubility278. These studies indicated that the exchange process is rapid, but a detailed picture did not emerge. 

The rate constant for the reentry is of the magnitude expected for a diffusion-controlled reaction as in Eq. (5.6). This means that the exit rate is determined by the partition coefficient of the solubilizate in its triplet state between the micelle and the aqueous solution. Table 5.2 shows the exit rate constants k for several systems. The water solubilities of the probes are also given to show the correlation between kt and the solubility in water. These studies give further support to the view that the micelle has a very dynamic structure, which makes it easy for the solubilizate to enter and leave the aggregate. 

The exchange between bound and free counterions is usually very rapid which probably explains why there exist so few experimental determinations of this rate constant. For example, in NMR studies of counterions one invariably observes a rapid exchange between free and bound ions. However, in solutions containing cetyl- 

For counterions, Eq. (5.7) predicts a larger rate constant for the association process than Eq. (5.6) since the ions are attracted by the micelle. An estimate using Eq. (5.7) and a solution of the Poisson-Boltzmann equation (cf. Sect. 6) gives k2 - lO M s-1 for the counterion association. 

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