Micelle electron tunneling

These reactions are regarded as electron tunnelling processes, and the results are interpreted in terms of the estimated distributions of occupied and unoccupied electronic levels in the redox systems aq-e, D-D+, and A-A- involved. If the overlap of the occupied levels of the donor with the unoccupied levels of the acceptor is not good, transfer is slow, irrespective of how thermodynamically favourable it may be. These levels are shifted, relative to one another, by the charge on the micellar head-group, which determines the direction of the electrical double layer at the interface. Reaction rates are therefore influenced both by the charge on the micelle and the concentration of electrolyte in the aqueous phase.47 48 The lifetime of normally unstable intermediates may be enormously enhanced by working in a micellar system.49... 

The photoionization of phenothiazine and its derivatives in SDS micelles, which is not observed in homogeneous solutions and the premicellar region was studied by Gratzel et al. [106-109]. They suppose that the anionic micelles decrease the ionization potential of the phenothiazine derivatives and the photoionization includes an electron tunnelling from the micelle into the solution through the Stem layer. However, there are no direct data for these systems to give a value of the photoionization threshold. A drastic increase of the photoionization quantum yield at the transition to anionic micelles could also be caused by a decrease of the role of the geminate recombination. 

Consider a hydrophobic sensitizer such as tetramethylbenzidine145) or phenothiazine146 incorporated into an anionic micelle. After excitation these molecules will eject electrons via a tunnelling mechanism147,148) from the lipid into the aqueous phase where formation of hydrated electrons (e q) occurs. The sensitizer cation will remain within the micellar aggregate. 

The ionization by light at 347.1 nm (3.57 eV energy) of phenothiazine incorporated in NaLS micelles in water has been attributed to the rapid tunnelling of an electron from excited phenothiazine through the double layer into unoccupied electronic redox levels of the system aq/cj [57]. This photoionization is promoted by co-solubilization of duroquinone which prevents ejection of electrons from the phenothiazine into the water phase. It is suggested [57] that the phenothiazine/water/quinone/micelle system offers a simple model for electron transfer in photosynthetic systems and for the heterogeneous catalysis of the photodecomposition of water via the freed electrons. A schematic representation of the processes when the surfactant is anionic [58] is shown in Fig. 11.10. 

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