Mobility of solubilizate molecules

As with surfactant monomers, the solubilizate molecules are not rigidly fixed in the micelle, but have a freedom of motion that depends on the solubilization site. The Ufetime of a solubilizate in the micelle is very short, usually less than 1ms. These short relaxation times were obtained using NMR and ultrasonic techniques. 

Recent studies indicate that like the surfactant monomers themselves, the solubilized molecules are not rigidly fixed in the micelle but have a freedom of motion which is dependent to some extent on the solubilization site. In fact, as discussed in Chapter 3, solubilized probes are used as indicators of the fluidity of their micellar micro-environment. In particular, a decrease in the polarization of the fluorescent radiation from fluorescent probes located in the micellar core is indicative of the molecular motion of these probes. Similarly, tumbling of solubilized nitroxide probes proceeds at a more rapid rate than can be accounted for by simple rotation of the micelle itself indicating that the solubilized probe undergoes dynamic motion within the micelle. Such motion produces a characteristic hyperfine pattern on e.s.r. spectra. 

Not only does the solubilizate have freedom of movement, albeit restricted within the micelle but it is also in constant dynamic equilibrium with the bulk aqueous phase. Only a limited number of studies have been reported of the determination of the rate constants for entry and exit of solubilizate molecules. 

Ultrasonic relaxation studies of the kinetics of the exchange of butanol, pentanol and hexanol between the aqueous phase and micelles of CTAB have been reported [130, 131]. The ultrasonic relaxation associated with the alcohol exchange process was well separated from that associated with the exchange process involving the CTAB monomers and could be treated independently. A simple collision-type kinetic model was used to describe the alcohol-micelle exchange process in such a way that rate constants could be evaluated. Table 5.5 shows that the rate constant associated with the dissociation of alcohol monomers from the micelles, fc i, increases as the hydrocarbon chain length of 

A study by stopped-flow techniques of the rate of solubilization of positively charged acridine (and related) dyes by the micelles of several sodium n-alkyl sulphates was reported by Robinson and co-workers [132]. The solubilization process was relatively slow k = 47.6 s for acridine orange in NaDS) due largely to charge effects and the rate was very dependent on the geometrical shape 

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