Surfactant energy, electron transfer

An attractive way to overcome this problem is to use microheterogeneous photocatalytic systems based on lipid vesicles, i.e. microscopic spherical particles formed by closed lipid or surfactant bilayer membranes (Fig. 1) across which it is possible to perform vectorial photocatalytic electron transfer (PET). This leads to generation of energy-rich one-electron reductant A" and oxidant D, separated by the membrane and, thus, unable to recombine. As a result of such PET reactions, the energy of photons is converted to the chemical energy of spatially separated one electron reductant tmd oxidant. 

In the artificial system Figure 4b, a polymerized surfactant vesicle is substituted for the thylakoid membrane. Energy is harvested by semiconductors, rather than by PSI and PSII. Electron transfer is rather simple. Water (rather than C02) is reduced in the reduction half cycle to hydrogen, at the expense of benzyl alcohol. In spite of these differences, the basic principles in plant and mimetic photosyntheses are similar. Components of both are compartmentalized. The sequence of events is identical in both systems energy harvesting, vectorial charge separation, and reduction. 

Exploitation of Potentials for Energy and Electron Transfer and Charge Separation In Surfactant Vesicles... 

The surface energy of a substrate may be increased by oxygen plasma treatment to introduce polar functionaUties. Alternatively, the surface energy of the ink may be decreased by the addition of surfactants. Hov rever, surfactants are a nuisance in many electrochemical systems. They aggregate in solution, adsorb at interfaces, and inhibit some electron transfer reactions. Indeed, the presence of surfactants in many commercial inks is a serious problem that is often overlooked by academic researchers. For biomedical appUcations, it should also be noted that some surfactants are endocrine disruptors, and therefore are not approved by the U.S. 

Fig, 1. Comparison of the initial stage of Photosynthesis (a) vn th polymer-bound(b) and surfactant-bound(c) electron transfer sensitizer. (The depiction(a) is too simplified and presented here only to show the difference between energy transfer and energy migration.)... 

The exit rate constants of the excited anions after the photoprotolytic dissociation of l,4-dichloro-2-naphthol within decylsulfate, dedecylsulfate, and cetylsulfate micelles were measured with a fluorescence quencher hardly penetrating the micelles, - the nitrate ion [121]. The addition of nitrate into the solution quenched the fluorescence of those anions which escape from the micelles within the lifetime of the excited state only. The exit rate constant of the naphtholate anion increases with increasing length of the hydrocarbon radical in the micelle-forming surfactant. The exit rate is thus controlled by the lowering of the micelle polarity (i.e. by the free energy of the exit process) rather than by the micelle size or the distance that the anion must diffuse. Perhaps one can establish a kind of correlation between the rate constant of this process and its free energy as was done for photochemical electron transfer [126] and proton transfer [156,157]. 

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. 

Electron transfer rates between adrenaline and related benzene diols and complexes of iron(III) with some substituted 1,10-phenanthrolines have been reported [67] in surfactant systems. In cationic systems the reactions take place in the aqueous phase and reaction rates are lower than they are in simple aqueous systems, but in anionic surfactant systems the reaction rates are enhanced, reactions probably taking place at the micellar interface. The rates of exit and entrance of aromatic compounds from and into micelles have recently been studied using phosphorescence decay measurements [68] exit rate constants of aromatic hydrocarbons are of the order of 10 to 10 s " S whereas values of 10 to 10 (moll ) s have been reported for intramicellar energy transfer processes. Release of aromatic phosphorescence probes from micelles followed by their deactivation in the aqueous phase is hence expected to be an important mode of deactivation of the triplet state [69]. Kinetic schemes for triplets that are partitioned between aqueous and micellar phases are considered for the cases of single occupancy and double occupancy of the micellar units. 

Electron transport systems perform important functions concerning respiration and energy metabolism in eucaryotes [22, 23], The electron transport reactions occur at the mitochondria inner membrane formed by electron transport proteins [24] and the lipid bilayer built up by the self-assembly of phospholipids as vital smfactants [25, 26]. The electron transport proteins include redox catalysts such as nicotinamide, iron [27, 28], and quinones [29]. The electrons produced by these redox reactions transfer through the lipid bilayer. While the relationship between the electron transport mechanisms and the molecular self-assembly in vivo has been clarified, control of the self-assembly by electron transport has been applied for an artificial polymeric surfactant. 

Luminescence and electronic energy transfer have been used frequently tq determine aggregation numbers of hydrocarbon type surfactants [3]. In contrast, a few luminescence studies have been made for fluorinated surfactants [16-19]. 

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