Electrified Interface

In recent years, advances in experimental capabilities have fueled a great deal of activity in the study of the electrified solid-liquid interface. This has been the subject of a recent workshop and review article [145] discussing structural characterization, interfacial dynamics and electrode materials. The field of surface chemistry has also received significant attention due to many surface-sensitive means to interrogate the molecular processes occurring at the electrode surface. Reviews by Hubbard [146, 147] and others [148] detail the progress. In this and the following section, we present only a brief summary of selected aspects of this field. 

Electrochemistry is concerned with the study of the interface between an electronic and an ionic conductor and, traditionally, has concentrated on (i) the nature of the ionic conductor, which is usually an aqueous or (more rarely) a non-aqueous solution, polymer or superionic solid containing mobile ions (ii) the structure of the electrified interface that fonns on inunersion of an electronic conductor into an ionic conductor and (iii) the electron-transfer processes that can take place at this interface and the limitations on the rates of such processes. 

The classical model of the electrified interface is shown in figure A2.4.7 [15], and the following features are apparent. 

One of the most important advances in electrochemistry in the last decade was tlie application of STM and AFM to structural problems at the electrified solid/liquid interface [108. 109]. Sonnenfield and Hansma [110] were the first to use STM to study a surface innnersed in a liquid, thus extending STM beyond the gas/solid interfaces without a significant loss in resolution. In situ local-probe investigations at solid/liquid interfaces can be perfomied under electrochemical conditions if both phases are electronic and ionic conducting and this... 

The singlet multidensity Ornstein-Zernike approach for the density profile described in this section has also been applied to study the role of association effects in the ionic liquid at an electrified interface [22]. 

K. Heinzinger. Molecular dynamics of water at interfaces. In J. Lipkowski, P. N. Ross, eds. Structure of Electrified Interfaces, Erontiers of Electrochemistry. New York VCH 1993, Chap 7, p. 239. 

II. SHORT OVERVIEW OF THE STATE OF THE ART A. An Example of Electrified Interface... 

In this section we introduce the basic ingredients of a field-theoretic approach to electrified interfaces and compare it with both the standard method of statistical mechanics and the density functional theory. 

J. Stafiej, Z. Borkowska, J. P. Badiah. A description of electrified interfaces based on methods of statistical field theory. J Electroanal Chem 395 1-14, 1995. J. Stafiej, J. P. Badiah. On a new theoretical approach of electrified interfaces. J Electroanal Chem 409 12-19>, 1996. 

J. Stafiej, M. Dymitrowska, J. P. Badiali. Field theoretic approach of electrified interfaces. Eleetroehim Aeta 47 2107-2113, 1996. 

The electrified interface between the metal and the electrolyte solution (the metal surface may be film-free or partially or completely covered with films or corrosion products). 

The present Section, which provides an outline of selected relevant topics in electrochemistry, is intended primarily as an introduction to aqueous corrosion for those readers whose basic training has not involved a study of electrochemistry. The scope of electrochemistry is enormous and cannot be treated adequately here, but there are now a number of excellent books on the subject, and it is hoped that this outline will serve to stimulate further study. The topics selected are as follows a) the nature of the electrified interface between the metal and the solution, (b) adsorption, (c) transfer of charge across the interface under equilibrium and non-equilibrium conditions, d) overpotential and the rate of an electrode reaction and (e) the hydrogen evolution reaction and hydrogen absorption by ferrous alloys. For reasons of space a number of important topics, such as the electrochemistry of electrolyte solutions, have been omitted. 

The adsorption of ions due to an electrified interface can be evaluated by considering a series of laminae of the solution at various distances from the metal surface and assessing the number of ions present as compared with those that would have been present if the electrified interface had been... 

The majority of work on the electrified interface has been carried out using a mercury electrode, which has the advantage that it has a well-defined and reproducible surface and a highly polarisable interface when immersed in a solution. In the case of solid metals the concepts outlined are equally applicable, but modifications are necessary to allow for the following ... 

An ideal reversible cell is characterised by an e.m.f. that remains constant irrespective of the rate of reaction in either direction, i.e. each interface constituting the cell must be so completely non-polarisable that it resists any attempt to change its potential. Although this is impossible to achieve in practice, a number of interfaces approximate to ideality providing the rate of reaction is maintained at a very low value. These reversible electrodes (or half-cells) are used as reference electrodes for determining the potential of a single electrified interface. 

It is evident that it is only possible to measure the e.,m.f. of a cell, and that in order to determine the potential at a single electrified interface it is necessary to assign an arbitrary potential to a specified electrified interface, which is then used as a reference for all others. The equilibrium at... 

The potential of the electrified interface of a metal immersed in an aqueous solution is of fundamental importance in studying the mechanism of corrosion reactions and in corrosion testing and monitoring, and it is, therefore, of some importance to consider the factors that determine the potential of a metal in a practical environment. The determination of the potential can be achieved without difficulty, but the significance of the potential is far more complex and some of the factors that affect the potential are as follows ... 

Carnie and Chan and Blum and Henderson have calculated the capacitance for an idealized model of an electrified interface using the mean spherical approximation (MSA). The interface is considered to consist of a solution of charged hard spheres in a solvent of hard spheres with embedded point dipoles, while the electrode is considered to be a uniformly charged hard wall whose dielectric constant is equal to that of the electrolyte (so that image forces need not be considered). 

Figure 22. Correlations between the interfacial term, AX, derived from Fig. 14, and the enthalpy of formation of the oxide MO, corrected for the work to break metal-metal bonds. I, II, in mean first, second, and third periods of the periodic table of elements. From Ref. 26, updated. (From R. Guidelli, ed, Electrified Interfaces in Physics, Chemistry, and Biology, p. 252, Fig. 3. Copyright 1992 Kluwer Academic Publishers. Reproduced with permission.)...

S. Trasatti, in Electrified Interfaces in Physics, Chemistry and Biology, R. Guidelli, ed., Kluwer, Dordrecht, 1992, p. 229. 

U. Palm, T. Silk, and T. Raud, Chemistry and Physics of Electrified Interfaces Solid/Elec-trolyte and Biological Systems. Ext. Abstr. Int. Confi, 1988, p. 103. 

The Volta potential is defined as the difference between the electrostatic outer potentials of two condensed phases in equilibrium. The measurement of this and related quantities is performed using a system of voltaic cells. This technique, which in some applications is called the surface potential method, is one of the oldest but still frequently used experimental methods for studying phenomena at electrified solid and hquid surfaces and interfaces. The difficulty with the method, which in fact is common to most electrochemical methods, is lack of molecular specificity. However, combined with modem surface-sensitive methods such as spectroscopy, it can provide important physicochemical information. Even without such complementary molecular information, the voltaic cell method is still the source of much basic electrochemical data. 

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