Micelle cage effect

Calculations of micelle sizes and aggregation numbers have been made from static and time-resolved quenching experiments in which deviations from the behavior in homogeneous solutions due to compartmentalization of the reactants in micelles (cage effect) were quantitatively exploited (cf. Section IV). It is assumed that the number of quenchers per micelle follows Poisson statistics, and it must be guaranteed that the time which donors and acceptors spend in a micelle is long compared to fluorescence lifetimes of donors and that quenching by the acceptors is the main deactivation process of the donors. 

In micelles of the surfactant cetyltrimethylammonium chloride (CTAC), the ratio of AA AB BB is <1 >98 <1 %. The CTAC micelles provide a cage effect, which greatly enhances the joining of the A and B radicals produced by the photolysis (Scheme 12.3). 

Figure 12 Cage effect during the photolysis of dibenzyl ketone. Top GC traces of products in micelles and solution. Bottom Cage effect with respect to the detergent concentration. Note the sudden change in the cage effect at the cmc.

Since the cage effect is less than 100 % in most cases (Table 1), some of the radical pairs exit the micelle and become water solubilized free radicals. At some later time, the free radicals recombine to form DPE. This has been verified by Cu2+ quenching experiments (Fig. 6)13,19). The disappearance of DBK is not dependent upon the concentration of Cu2+. However, the yield of DPE drops very rapidly with increasing copper concentration and then levels off. The leveling off region is directly related to the amount of cage reaction. The products formed by reaction with copper are benzyl chloride (4), and benzyl alcohol (5) (Scheme V)20). 

Because of their different hydrophilicities, the two free radicals formed at the same time can separate from each other quickly which can eliminate the cage effect. In a micelle, the local BA concentration may be quite high. Once a micelle is initiated, a number of BA molecules may be added quickly. As a result, some short BA blocks would be incorporated into a poly(MAETAC) chain to form something like multi-block copoly(MAETAC-BA), as shown in Fig. 19 [170]. Surfactant should stabilize the BA blocks so that the block copolymer remains in the aqueous phase. 

Due to the cage effect in micelles, unsymmetrically substituted dibenzyl ketones such as 13 yield predominantly the unsymmetrical diphenyl-ethanes on photodecurbonylation, whereas in homogeneous solution all three possible products are formed in the statistical ratio 1 2 1 (Turro and Kraeutler, 1978). 

However, in the smaller micelles of cetyltrimethylanunoniumbromide (CTAB) and of SDS signiflcant amounts of the combination product biphenyl were found, increasing with decreasing micellar volume. These experiments also show the difference from ordinary cage effects, which increase with solvent viscosity the microviscosity of SDS micelles is smaller than that of CTAB micelles (measured by several spectroscopic probes see Section VI), so that opposite results (if any) should be excepted from ordinary cage effects. 

Organic Photoreactions in Micelles.—The reactivity of excited states may be strongly perturbed by micelles, for a variety of reasons. Enforced approximation may make intermolecular reactions more efficient in some cases. In others, the cage effect of the micelle can entrap transients, and especially inhibit radical-pair diffusion processes. Micelle-induced conformational changes may affect the product distribution in intramolecular hydrogen-atom abstraction, and the reactivity of external reagents towards excited states may be enhanced by ionic micelles. 

Photochemistry of dibenzyl ketones has been examined in a number of organized media. Photolysis of 3-(4-methylphenyl)-l-phenylacetone (MeDBK) results in a 1 2 1 mixture of three products shown in Scheme 31. Photolysis of the same molecule solubilized in a micelle (hexadecyltrimethylammonium chloride) gives a single product (AB). This remarkable change in product distribution is due to the cage effect brought forth by the micellar structure. The change in product distribution occurs at or above the critical micelle concentration. 

The photolyses of 1,2-dipheny1-2-methyl-1-propa-none and its 2h and derivatives in micellar solution are now described and further demonstrate the enhanced cage and magnetic isotope effects of mlcelllzatlon. We report also the observation of CIDP during the photolyses of micellar solutions of several ketones, and demonstrate the validity of the radical pair model to these systems. Analyses of the CIDNP spectra in the presence and absence of aqueous free radical scavengers (e.g., Cu2+) allow us to differentiate between radical pairs which react exclusively within the micelle and those that are formed after diffusion into the bulk aqueous phase. In some cases this allows us to estimate a lifetime associated with the exit of free radicals from the micelles. 

If we are to use the radical pair theory to explain the effects of micellization on the cage reaction probability as well as the magnetic field effect, it is mandatory that we be able to observe CIDNP in these systems. In addition, since CIDNP is sensitive to events on the time scale of the radical pair lifetime, detailed analysis of the CIDNP can often lead to mechanistic insight to the dynamics of the radical pair. Below we describe one such result. 

---