Electron-transfer reactions across interfaces

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. 

In Chapter 7 general kinetics of electrode reactions is presented with kinetic parameters such as stoichiometric number, reaction order, and activation energy. In most cases the affinity of reactions is distributed in multiple steps rather than in a single particular rate step. Chapter 8 discusses the kinetics of electron transfer reactions across the electrode interfaces. Electron transfer proceeds through a quantum mechanical tunneling from an occupied electron level to a vacant electron level. Complexation and adsorption of redox particles influence the rate of electron transfer by shifting the electron level of redox particles. Chapter 9 discusses the kinetics of ion transfer reactions which are based upon activation processes of Boltzmann particles. 

Studies in nonaqueous dipolar aprotic solvents allowed the elucidation of the complicated role of the solvent nature in determining the - double layer structure and kinetics of electrochemical reactions. Special attention was paid to the phenomenon of ion - solvation and its effect on -> standard electrode potentials. Experimental studies of the various electrochemical systems in nonaqueous media greatly contributed to the advancement of the theory of elemental electron-transfer reactions across charged interfaces via the so-called energy of solvent reorganization. 

These examples should serve to underscore the difficulty in predicting the effects that interfacial potentials, membrane structure and microphase organization will have on electron-transfer reactions across the membrane interface and within the bilayer itself. The principles involved are common to micelles and vesicles, but the more anisotropic and highly ordered vesicles provide a more complex reaction environment for solubilized or adsorbed reactants. 

Schmickler, W. (1997). Electron-transfer reactions across Uquid/Uquid interfaces. J. Electroanal. Chem. 428, 123-127. 

Brown AR, Yellowlees LJ, Girault HH (1993) Photoinitiated electron-transfer reactions across the interface between two immiscible electrolyte-solutions. J Chem Soc Faraday Trans 89 207... 

Two heterogeneous electron transfer reactions across the RuO /water interface are involved in the overall reaction. A decrease in the particle potential results in an increase of the rate of the cathodic electron transfer process, while that of the anodic reaction is retarded. Under steady state conditions, the RUO2 will assume a potential Ep which is given by the intersection of the two current-voltage c-urves. At this potential a current i will flow which defines the overall reaction rate. It appears that in the case of oxidizing agents such as Fe(bipy) 2 Ru(bipy) Ce, or ZnTMPyP a driving force of ca. I50 mV for the water oxidation... 

Zhang, Z., Yuan, Y, Sun, R, Su, B., Guo, J., Shao, Y. Study of electron-transfer reactions across an externally polarized water/l,2-dichloroethane interface by scamilng electrochemical microscopy. J. Phys. Chem. B 2002,106, 6713-6717. 

Among the dynamical properties the ones most frequently studied are the lateral diffusion coefficient for water motion parallel to the interface, re-orientational motion near the interface, and the residence time of water molecules near the interface. Occasionally the single particle dynamics is further analyzed on the basis of the spectral densities of motion. Benjamin studied the dynamics of ion transfer across liquid/liquid interfaces and calculated the parameters of a kinetic model for these processes [10]. Reaction rate constants for electron transfer reactions were also derived for electron transfer reactions [11-19]. More recently, systematic studies were performed concerning water and ion transport through cylindrical pores [20-24] and water mobility in disordered polymers [25,26]. 

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 are the simplest class of electrochemical reactions. They play a special role in that every electrochemical reaction involves at least one electron-transfer step. This is even true if the current across the electrochemical interface is carried by ions since, depending on the direction of the current, the ions must either be generated or discharged by an exchange of electrons with the surroundings. 

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. 8-14. Electron state density for a redox electron transfer reaction and profile of electrostatic inner potential, across an electrode interface = potential...

During reduction, electrons travel/rom the power pack, through the electrode, transfer across the electrode-solution interface and enter into the electroactive species in solution. Conversely, during oxidation, electrons move in the opposite direction, and are conducted away from the electroactive material in solution and across the electrode-solution interface as soon as the electron-transfer reaction occurs. (Incidentally, these different directions of electron movement explains why an oxidative current has the opposite sign to a reductive current, cf. Section 1.2.)... 

In the above expression, ci k is the concentration of species i in phase k, and si kj is the stoichiometric coefficient of species i in phase k participating in heterogeneous reaction 1 (see eq 8). is the specific surface area (surface area per unit total volume) of the interface between phases k and p. ih.k- is the normal interfacial current transferred per unit interfacial area across the interface between the electronically conducting phase and phase k due to electron-transfer reaction h, and it is positive in the anodic direction. In the above expression, Faraday s law... 

This expression, known as Sand s equation, gives the variation of the interfacial concentration of M"+ with time after application of a constant current density. But one seeks also to know the time variation of the potential difference across the interface at which the electronation reaction M"+ + ne — M is occurring. To obtain this information, one recalls that the charge-transfer reaction across the interface is assumed in the present treatment to be virtually in equilibrium and therefore the Nenist equation (7.177) can be used to relate the potential difference to the concentration at the interface. That is, by substituting (7.181) in (7.177),... 

Among electrode processes with at least one charge transfer step, several different types of reaction can be found. The simplest interfacial electrochemical reactions are the exchange of electrons across the electrochemical interface by flipping oxidation states of transition metal ions in the electrolyte adjacent to the electrode surface. The electrode in this case is merely the source or sink of electrons, uptaking electrons from the reduced species and releasing them to the oxidized redox species in solution. Examples of simple electron transfer reactions 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. 

In the dark, the junction between an extrinsic (doped) semiconductor and a redox electrolyte behaves as a diode because only one type of charge carrier (electrons for n-type and holes for p-type) is available to take part in electron transfer reactions. The potential distribution across the semiconductor/electrolyte interface differs substantially from that across... 

Many photoelectrochemical reactions of interest involve multistep electron transfer reactions. Examples that have been studied are hydrogen evolution on InP [29] and Si [30], oxygen evolution on Ti02 [31] and photodecomposition of CdS [32]. These reactions involve the formation of charged surface bound intermediates, and the accumulation of surface charge modified the potential distribution across the interface. For example, the photodecomposition of n-CdS is believed to occur by the route... 

By using cyclic voltammetry, Schiffrin and coworkers [26, 186, 187, 189] studied electron transfer across the water-1,2-dichloroethane interface between the redox couple FefCNls /Fe(CN)6 in water, and lutetium(III) [186] and tin(IV) [26, 187] diphthalocyanines and bis(pyridine)-me50-tetraphenylporphyrinato-iron(II) or ru-thenium(III) [189] in the organic solvent. An essential advantage of these systems is that none of the reactants or products can cross the interface and interfere with the electron transfer reaction, which could be clearly demonstrated. Owing to a much higher concentration of the aqueous redox couple, the pseudo-first order electron transfer reactions could be analyzed with the help of the Nicholson-Shain theory. However, though they have all appeared to be quasireversible, kinetic analysis was restricted to an evaluation of the apparent standard rate constant o. which was found to be of the order of 10 cm s [186, 189]. Marcus [199] has derived a relationship between the pseudo-first-order rate constant for the reaction (8) and the rate... 

In membranes, the motional anisotropies in the lateral plane of the membrane are sufficiently different from diffusion in the transverse plane that the two are separately measured and reported [4b, 20d,e]. Membrane ffip-ffop and transmembrane diffusion of molecules and ions across the bilayer were considered in a previous section. The lateral motion of surfactants and additives inserted into the lipid bilayer can be characterized by the two-dimensional diffusion coefficient (/)/). Lateral diffusion of molecules in the bilayer membrane is often an obligatory step in membrane electron-transfer reactions, e.g., when both reactants are adsorbed at the interface, that can be rate-limiting [41]. Values of D/ have been determined for surfactant monomers and probe molecules dissolved in the membrane bilayer typical values are given in Table 2. In general, lateral diffusion coefficients of molecules in vesicle... 

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