Reactions electron transfer, at interfaces

Monolayer and multilayer thin films are technologically important materials that potentially provide well-defined molecular architectures for the detailed study of interfacial electron transfer. Perhaps the most important attribute of these heterogeneous systems is the ease with which their molecular architecture can be synthetically varied to tailor the properties of the ensemble. Assemblies incorporating specifically designed structures can, in principle, meet the needs of a variety of technological applications and be used as models for understanding fundamental interfacial reaction mechanisms. In fact, molecular assemblies are nearly ideal laboratories for the fundamental study of electron-transfer reactions at interfaces. In this chapter, the use of monolayer and multilayer assemblies to probe fundamental questions regarding electron transfer in surface-confined molecular assemblies will be addressed. 

Electron transfer reactions at interfaces between a semiconducting oxide and an electrolyte can take place in two ways ... 

VIII. Kinetics of Electron Transfer Reactions at Interfaces... 

Lewis NS (1998) Progress in understanding electron-transfer reactions at semiconduc-tor/hquid interfaces. JPhys Chem B 102 4843-4855... 

Michael Faraday first studied electron transfer reactions at oil-water interfaces to prepare colloidal metals by reducing metal salts at the ether-water or carbon disulfide-water interfaces. As the field progressed after Faraday s pioneering observations, it... 

Electron-transfer reactions at liquid-liquid interfaces have the form ... 

A. Electron-Transfer Reactions at Externally Polarized interfaces... 

As mentioned above, the distribution of the various species in the two adjacent phases changes during a potential sweep which induces the transfer of an ion I across the interface when the potential approaches its standard transfer potential. This flux of charges across the interface leads to a measurable current which is recorded as a function of the applied potential. Such curves are called voltammograms and a typical example for the transfer of pilocarpine [229] is shown in Fig. 6, illustrating that cyclic voltammograms produced by reversible ion transfer reactions are similar to those obtained for electron transfer reactions at a metal-electrolyte solution interface. 

Electron-transfer reactions at liquid-liquid interfaces involve redox couples on each side of the interface. The basic scheme is (see Fig. 12.5) ... 

Electron-transfer reactions at ITIES resemble electron-transfer reactions across biological membranes, which adds a special interest. Also, in contrast to homogeneous electron-transfer reactions, they allow a separation of the reaction products. So it is disappointing to report that only very few experimental investigations of electron-transfer reactions at ITIES have been performed. This is mainly due to the fact that it is difficult to find systems where the reactants do not cross the interface after the reaction in addition, side reactions with the supporting electrolyte can be a problem. 

Most theoretical studies of outer-sphere (nonbond-breaking) electron transfer reactions at the metal-solution interface involve major simplifying assumptions regarding the molecular and electronic structure of the solvents and the metal. Although the importance of molecular structure and the dynamics of the solvent has been recognized, most of the theoretical work in this area has been based on a highly simplified continuum model. ... 

Despite the fact that electron transfer reactions at the electrode/electrolyte interface are of fundamental importance to many chemical processes, a quantitative understanding of the factors that influence the rate of these reactions is still lacking. Although the general theoretical framework was established many years ago by Marcus, Levich, Dogonadze, and oth-... 

The theoretical modeling of electron transfer reactions at the solution/metal interface is challenging because, in addition to the difficulties associated with the quantitative treatment of the water/metal surface and of the electric double layer discussed earlier, one now needs to consider the interactions of the electron with the metal surface and the solvated ions. Most theoretical treatments have focused on electron-metal coupling, while representing the solvent using the continuum dielectric media. In keeping with the scope of this review, we limit our discussion to subjects that have been adi essed in recent years using molecular dynamics computer simulations. 

Following the early studies on the pure interface, chemical and electrochemical processes at the interface between two immiscible liquids have been studied using the molecular dynamics method. The most important processes for electrochemical research involve charge transfer reactions. Molecular dynamics computer simulations have been used to study the rate and the mechanism of ion transfer across the water/1,2-dichloroethane interface and of ion transfer across a simple model of a liquid/liquid interface, where a direct comparison of the rate with the prediction of simple diffusion models has been made. ° ° Charge transfer of several types has also been studied, including the calculations of free energy curves for electron transfer reactions at a model liquid/liquid... 

Fig. 8-8. Energy levels for redox electron transfer reaction at a metal electrode (a) in equilibrium, (b) in anodic polarization with reao tion rate determined by interfadal electron transfer, (c) anodic polarization with reaction rate determined by both interfadal electron transfer and diffusion of hydrated partides. EF0)Eooxj.a= Fenni level of redox electrons at an interface.

Gao YQ, Gerogievskii Y, Marcus RA (2000) On the theory of electron transfer reactions at semiconductor/liquid interfaces. J Chem Phys 112 3358-3369... 

In discussing this modification, it is helpful to consider a concrete example. Suppose that the electron-transfer reaction at the interface under study is... 

A cell is said to act reversibly if the net cell reaction is reversed when the current through the cell is made to flow in the opposite direction. When no current is being drawn, such a cell is in a true equilibrium state. Note that the absence of net current flow does not necessarily signify that a cell is in equilibrium. If an iron wire is placed in a solution of low pH, the most likely electron transfer reactions at the metal/solution interface are... 

The kinetics of electron transfer reactions at electrodes can be explained either by surmounting an activation barrier due to the chemical reorganization of the reactants or by tunnelling through the potential barrier across the electrode—solution interface. 

The electron transfer reactions at the semiconductor/electrolyte interface occur either via the conduction band or the valence band. The total current is therefore given by the sum of four partial currents, denoted as represent electron transfer via the conduction anc valence bands, respectively, and the superscripts, a and c, indicate anodic anc cathodic processes, respectively. Let us assume nereafter that the electron transfer occurs only via the conduction band. In a simple case where the concentration of the electrolyte is sufficiently high and only the overvoltages at the Helmholtz layer (tjh) and in the space charge layer (rjsc) are important, the ica and cc can be given as follows4)... 

Activation volume — As in case of homogeneous chemical reactions, also the rate of heterogeneous electron transfer reactions at electrode interfaces can depend on pressure. The activation volume AVZ involved in electrochemical reactions can be determined by studying the pressure dependence of the heterogeneous -> standard rate constant ks AVa = -RT j (p is the molar - gas constant, T absolute temperature, and P the pressure inside the electrochemical cell). If AI4 is smaller than zero, i.e., when the volume of the activated complex is smaller than the volume of the reactant molecule, an increase of pressure will enhance the reaction rate and the opposite holds true when A14 is larger than zero. Refs. [i] Swaddle TW, Tregloan PA (1999) Coord Chem Rev 187 255 [ii] Dolidze TD, Khoshtariya DE, Waldeck DH, Macyk J, van Eldik R (2003) JPhys Chem B 107 7172... 

Refs. [i] Samec Z (1979) J Electroanal Chem 99 197 [ii] Samec Z, Marecek V, Weber J (1979) J Electroanal Chem 96 245 [iii] Chen QZ, Iwamoto K, Seno M (1991) Electrochim Acta 36 291 [iv] Wei C, Bard Aj, Mirkin MV (1995) J Phys Chem 99 16033 [v] Shi C, Anson FC (1998) I Phys Chem B 102 9850 [vi] Geblewicz G, Schiffrin Df (1988) ] Electroanal Chem 244 27 [vii] Marcus RA (1990) J Phys Chem 94 1050 [viii] Barker AL, Unwin PR, Amemiya S, Zhou JF, Bard AJ (1999) JPhys Chem B 103 7260 [ix] Fermln DJ, Lahtinen R (2001) Dynamic aspects of heterogeneous electron-transfer reactions at liquid-liquid interfaces. In Volkov AG (ed) Liquid interfaces in chemical, biological, and pharmaceutical applications. Marcel Dekker, New York, pp 179-227 [x] Cheng Y, Schiffrin DJ (1996) J Chem Soc Faraday Trans 92 3865 [xi] Fermin DJ, Doung H, Ding Z, Brevet PF, Girault HH (1999) Electrochem Com-mun 1 29... 

The question of the free-energy dependence of heterogeneous electron-transfer reactions at liquid-liquid interfaces was addressed by Bard and coworkers. They ex-... 

Although the lifetime of the reactive electron-hole pair is not known, the reasonable estimate of 10 -10 s leads to an electron transfer rate constant between 10 and 10 M s . In general, electron transfer reactions at the semiconductor-liquid interface are very fast [see Ref [33] and N. Serpone, E. Pellizetti (Eds.), Homogeneous and Heterogeneous Photocatalysis, Reidel, Dordrecht, 1986, p. 51]. 

Figure 8 shows the free energy curves for a model electron transfer reaction at the water/1,2-dichloroethane (DCE) interface. The model[72] represents the... 

A detailed study[81] of the solvent non-equilibrium response to electron transfer reactions at the interface between a model diatomic non-polar solvent and a diatomic polar solvent has shown that solvent relaxation at the liquid/liquid interface can be significantly slower than in the bulk of each liquid. In this model, the solvent response to the charge separation reaction A + D —> A + D+ is slow because large structural rearrangements of surface dipoles are needed to bring the products to their new equilibrium state. 

I. Benjamin, Molecular dynamics study of the free energy functions for electron transfer reactions at the liquid-liquid interface, J. Phys. Chem. 95 (1991) 6675. 

I. Benjamin and Y. I. Kharkats, Reorganization free energy for electron transfer reactions at liquid/liquid interfaces, Electrochimica Acta, in press (1998). 

Katz, E., WiHner, B., and Willner, I. Light-controlled electron transfer reactions at photo isomerizable monolayer electrodes by means of electrostatic interactions Active interfaces for the amperometric transduction of recorded optical signals. Biosens. Bioelectron. 1997, 22, 703-719. 

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