Initial Stabilization of the Charge Separation Products

The photoreaction in micellar solutions results in a change of a molecule s location within a micelle. The photoionization of a model substance - tetramethylbenzidine (TMB) - in micellar solutions was studied in detail. Bales and Kevan [131] used electron acceptors with a different position of the acceptor site in order to determine the location of TMB within SDS micelles. The acceptors were j-doxylstearic acids with the general formula  

The illumination of TMB in the presence of electron acceptors with different j-values yields TMB radical cations with different quantum yields and consumption of the spin label. The highest quantum yield was obtained with j = 10 and especially with j = 16. This was explained in terms of TMB location in the hydrophobic region of the micelle asymmetrically to the electron acceptor molecule. However, another interpretation is possible as the stearic acid molecule does not coincide in the number of carbon atoms with the micelle-forming surfactant, one can assume that the carbon atom C-13 is located near the end of the methyl groups of the surfactant, i.e. near the centre of the micelle. In this case the results of Bales and Kevan show that TMB molecules are located inside the micelle symmetrically with the photoionization probabihty maximum near the third carbon atom from the micelle centre. Anyway, TMB molecules are located in hydrophobic nuclei of the micelles. The important 

The location of TMB within the micelles was studied by Kevan et al. [132] with the spin-echo technique. In the deuterated SDS solution in the spin-echo spectrum of TMB one can observe both the proton modulation from the hydrogen atoms of SDS and the deuterium atoms of water. The conclusion was that TMB is located at the micelle-water interface. As discussed above, parent TMB is located in hydrophobic micelle nuclei. So the location of TMB changes within its lifetime and it transfers from the hydrophobic region of the micelle to the interface. The evolution of TMB in CTAB and SDS micelles was studied by Beck and Brus with pulse Raman spectroscopy [132]. They found that TMB is formed in CTAB micelles with a delay and yields TMB and TMB in the millisecond range. TMB formed in SDS micelles is stable for several days [134]. The anionic micelles seem to stabilize TMB and the cationic ones to destabilize it. 

The stabilization of the radical cation in SDS micelles is consistent with the formation of the surface complex already mentioned in Sect. 2.1.1. There are a number of examples of the stabilization of radical ions in micelles of the opposite sign. The electrochemical reduction of nitrobenzene in homogeneous solutions is two-electronic, but in micellar CTAB solutions it is resolved into two separate reactions [135]. So the presence of cationic micelles inhibits the disproportionation of nitrobenzene radical anions. It may be caused by the reduced mobility of the radical ions in the surface layer of the micelles due to complex formation with the end groups of the micelles. It can account for the inhibition of the disproportionation because the latter needs a coUision of two radical ions. The salt formation of the radical cation of N-methylphenothiazine with the end groups of SDS micelles was proposed in the study of electrochemical oxidation of N-methylphenothiazine in micellar solution [136]. Tetracyanoquinodi-methane is solubilized in dodecylpyridinium-inverted micelles in the form of a radical anion with the oxidation of the iodide ion [137]. 

Similar examples can be given for acid-base reactions. For example, SDS micelles stabilize acridinium cation, leading to a shift of the acid-base equilibrium [8]. From the thermodynamic point of view, such a stabilization is the lowering of the energy levels of the charged forms with respect of their conjugated uncharged forms. 

---