The Mechanism of Ion Transfer

In the model presented here, ion transfer is assumed to occur in many small steps. Considering that the ion is initially hydrated and it is squeezed through a layer of water molecules adsorbed at the metal surface, the component of its movement in the direction perpendicular to the interface is expected to cause some distortion in the hydration shell around it. In this sense, each step involves some degree of solvent rearrangement. This is similar to outer-sphere charge transfer, but there are several important differences  

1) Ion transfer under the influence of the externally applied electrostatic field involves only a singe time scale - that of the movement of the hydrated ion. This is of the order of 20 ps, which is similar to the relaxation time of molecules in bulk water. Hence, there is enough time for the water molecules in the hydration shell to rearrange around the ion, as it moves in the direction perpendicular to the electrode surface. 

Curve 1 applies to the system at equilibrium. A value of = 0.5 eV was chosen here for the electrochemical Gibbs energy of activation, and the position of the activated complex was taken to be closer to the initial state of the solvated ion (which is 

The Gibbs energy of activation is modified by the applied overpotential so that 

Where n is the number of unit charges transferred. Thus, the value of the cathodic transfer coefficient for n = 2 is given by 

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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... 

The primary purpose of this review is to summarize comprehensively advances in the study of this kinetic aspect of charge transfer across ITIES since 1981, when Koryta and Vanysek gave a timely review at that early stage of the development of electrochemistry at ITIES. Reviews [5-14] and monographs [15, 16] are available of other aspects of the electrochemistry at ITIES, e.g., ion transfer facilitated by ionophores, applications to analytical purposes or to liquid extraction, and instrumentation. In a recent review on charge transfer across ITIES, Girault [14] addressed key issues regarding the mechanism of ion transfer the dependence of the rate constant of ion transfer on the applied potential, the presence of an activation barrier, the double layer correction, the effect of solvent viscosity, theoretical treatments, etc. Since the author s [14] opinions differ in several respects from ours, we have tried to review this subject as systematically and critically as possible. 

Electron transfer between a redox couple 01 /R1 in the W phase and a redox couple 02/R2 in the O phase, Eq. (8), represents the second basic type of charge transfer across a liquid-liquid interface. Although this is a second-order process, its mechanism is better understood than the mechanism of ion transfer. 

Bockris, J. O M. The Mechanism of Charge Transfer from Metal Electrodes to Ions in Solution 6... 

Various types of research are carried out on ITIESs nowadays. These studies are modeled on electrochemical techniques, theories, and systems. Studies of ion transfer across ITIESs are especially interesting and important because these are the only studies on ITIESs. Many complex ion transfers assisted by some chemical reactions have been studied, to say nothing of single ion transfers. In the world of nature, many types of ion transfer play important roles such as selective ion transfer through biological membranes. Therefore, there are quite a few studies that get ideas from those systems, while many interests from analytical applications motivate those too. Since the ion transfer at an ITIES is closely related with the fields of solvent extraction and ion-selective electrodes, these studies mainly deal with facilitated ion transfer by various kinds of ionophores. Since crown ethers as ionophores show interesting selectivity, a lot of derivatives are synthesized and their selectivities are evaluated in solvent extraction, ion-selective systems, etc. Of course electrochemical studies on ITIESs are also suitable for the systems of ion transfer facilitated by crown ethers and have thrown new light on the mechanisms of selectivity exhibited by crown ethers. 

In the case of non-HBD solvents, such as DMSO, the measured pK values are absolute (that is, free from ion pairing) and can be directly compared with gas-phase acidities6 in addition, knowledge of the heats of ionization in DMSO7 allows the evaluation of a possible entropy effect when the two phases are compared. The mechanism of proton transfer between oxygen and nitrogen acids and bases in aqueous solution has been reviewed8. 

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. 

Extraction of Ions from an Aqueous Solution. Its Implications on the Mechanism of Phase Transfer Catalyzed Polvetherifications... 

At this point it is necessary to consider the mechanism of electron-transfer luminescence in solutions which cannot involve ion-radical annihilation because both cation and anion of the fluorescer are not formed. Such emission can be achieved by treating anion radicals with chemical oxidants or electrochemically under conditions where the corresponding cation cannot be produced, and it may also be achieved by electrochemical reduction of cations without producing the corresponding anion. In addition to triplets, three types of processes and pathways have been proposed to help explain why such emission occurs. These may be described as (7) impurities, (2) ion-radical aggregates, and (5) heterogeneous electron transfer. It is evident63 that impurities,... 

Harry Gray Two points in Prof. Taube s paper quoted as experiments in progress haven t been mentioned. Both are concerned with the mechanism of electron transfer, because the transmission in the ligand, wherever the attack is, is through the 7r-system, and in cobalt(III) in the detectable radical ion intermediate, because of the improbability of resonance transfer from tt to electron resonance experiment in which one tries the reduction by chromous and looks for the ESR signal of the radical ion. 

The radial dependence of the fluorescence-quenching interaction between terbium, holmium, and neodymium in aqueous chloride solution was examined by Holloway and Kestigian (108a). From the concentration dependencies of the fluorescence lifetimes, they concluded that the probability for quenching interaction falls off as 1/r6, where r is the average spacing between the ions. If the mechanism of resonance transfer is assumed, the observed radial dependence implies a dipole-dipole interaction. 

It might be assumed that there would be little to study in the mechanism of electron transfer—that the reducing agent and the oxidizing agent would simply bump into each other and electron transfer would take place. Reactions in solution are complicated, however, by the fact that the oxidized and reduced species are often metal ions surrounded by shields of ligands and solvating molecules. Electron transfer reactions... 

Enhancement of Eu3+ fluorescence has also been observed [612] when incorporating Tb3+ ion in Eu3+ doped tungstate host lattice. The degree of enhancement is proportional to the Tb3+ ion concentration. The mechanism of energy transfer in this case may probably involve interaction with Tb3+ affecting the cohesive force operating on Eu3+. 

Electron transfer from the excited states of Fe(II) to the H30 f cation in aqueous solutions of H2S04 which results in the formation of Fe(III) and of H atoms has been studied by Korolev and Bazhin [36, 37]. The quantum yield of the formation of Fe(III) in 5.5 M H2S04 at 77 K has been found to be only two times smaller than at room temperature. Photo-oxidation of Fe(II) is also observed at 4.2 K. The actual very weak dependence of the efficiency of Fe(II) photo-oxidation on temperature points to the tunneling mechanism of this process [36, 37]. Bazhin and Korolev [38], have made a detailed theoretical analysis in terms of the theory of radiationless transitions of the mechanism of electron transfer from the excited ions Fe(II) to H30 1 in solutions. In this work a simple way is suggested for an a priori estimation of the maximum possible distance, RmSiX, of tunneling between a donor and an acceptor in solid matrices. This method is based on taking into account the dependence... 

In this obvious case, there is no ambiguity in writing partial chemical reactions. It can only be noted that the copper and tin ions, not atoms, are most likely to diffuse across the growing Cu3Sn and Cu6Sn5 intermetallic layers, whereas electrons simply accompany them, so that the final result is such, as if the atoms were the diffusing species. For formal kinetics, such details of the mechanism of atomic transfer are clearly of no importance. 

In recent years, there has been a great deal of interest in the mechanisms of electron transfer processes.52-60 It is now recognized that oxidation-reduction reactions involving metal ions and their complexes are mainly of two types inner-sphere (ligand transfer) and outer-sphere (electron transfer) reactions. Prototypes of these two processes are represented by the following reactions. 

Oxide dissolution has been reviewed by Diggle [49]. According to the type of ion transfer rate-determining step, the different mechanisms can be classified into a chemical... 

The aqua ion has been extensively used as a reductant in studies on the mechanism of electron transfer reactions, best exemplified by the classical example of the reaction of Cr2+(aq) with [Com(NH3)5X]2+. This reaction proceeds via an inner-sphere (ligand-bridged) mechanism, as in the general reaction sequence... 

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