Surface potentials, Voltaic cells

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. 

This review is arranged as follows. After a short review of the basic definitions and significance of the various potentials which are assumed to exist at free surfaces and interfaces, the nature and most important features of voltaic cells, including their measurement techniques, are... 

The surface potential of a liquid solvent s, %, is defined as the difference in electrical potentials across the interface between this solvent and the gas phase, with the assumption that the outer potential of the solvent is zero. The potential arises from a preferred orientation of the solvent dipoles in the free surface zone. At the surface of the solution, the electric field responsible for the surface potential may arise from a preferred orientation of the solvent and solute dipoles, and from the ionic double layer. The potential as the difference in electrical potential across the interface between the phase and gas, is not measurable. However, the relative changes caused by the change in the solution s composition can be determined using the proper voltaic cells (see Sections XII-XV). 

The non situ experiment pioneered by Sass uses a preparation of an electrode in an ultrahigh vacuum through cryogenic coadsorption of known quantities of electrolyte species (i.e., solvent, ions, and neutral molecules) on a metal surface. " Such experiments serve as a simulation, or better, as a synthetic model of electrodes. The use of surface spectroscopic techniques makes it possible to determine the coverage and structure of a synthesized electrolyte. The interfacial potential (i.e., the electrode work function) is measured using the voltaic cell technique. Of course, there are reasonable objections to the UHV technique, such as too little water, too low a temperature, too small interfacial potentials, and lack of control of ionic activities. ... 

The difference between the surface potentials of two solvents (e.g., an organic solvent and water) may also be measured by means of the following voltaic cells " (Fig. 12) ... 

This review has been restricted mainly to clarification ofthe fundamentals and to presenting a coherent view ofthe actual state of research on voltaic cells, as well as their applications. Voltaic cells are, or may be, used in various branches of electrochemistry and surface chemistry, both in basic and applied research. They particularly enable interpretations of the potentials of various interphase and electrode boundaries, including those that are employed in galvanic and electroanalytical cells. 

Every liquid interface is usually electrified by ion separation, dipole orientation, or both (Section II). It is convenient to distinguish two groups of immiscible liquid-liquid interfaces water-polar solvent, such as nitrobenzene and 1,2-dichloroethane, and water-nonpolar solvent, e.g., octane or decane interfaces. For the second group it is impossible to investigate the interphase electrochemical equilibria and the Galvani potentials, whereas it is normal practice for the first group (Section III). On the other hand, these systems are very important as parts of the voltaic cells. They make it possible to measure the surface potential differences and the adsorption potentials (Section IV). 

Thus, the Volta potential may be operationally defined as the compensating voltage of the cell of Scheme 16. However, it should be stressed that the compensating voltage of a voltaic cell is not always the direct measure of the Volta potential. The appropriate mutual arrangement of phases, as well as application of reversible electrodes or salt bridges in the systems, allows measurement of not only the Volta potential but also the surface and the Galvani potentials. These possibilities are schematically illustrated by [15]... 

Thus the conductor functions as both the electrodes and the wire in a natural voltaic cell, connecting the cathodic part of the conductor near surface with the anodic part in the deeper environment. The reactants are solid-phase and dissolved constituents in the low-Eh and high-Eh environments that respectively surround the anode and cathode. The difference in oxidation potential of the reactants arises from the ubiquitous redox... 

Farrell J. R. and McTigue P. (1982), Precise compensating potential difference measurements with a voltaic cell—the surface potential of water , J. Electroanal. Chem. 139, 37-56. 

Recent progress and main problems of the study of electrochemical equilibrium properties are reviewed for interfaces between two immiscible liquid electrolyte solutions. The discussed properties are mainly described in terms of the Galvani, Volta, zero charge, and surface (dipolar) potentials at the liquid-liquid interfaces and free liquid surfaces. Different galvanic and voltaic cells with liquid-liquid, mainly water-nitrobenzene interfaces, are described. These interfaces may be polarizable or reversible with respect to one or several ions simultaneously. 

It should be noted that in the present review the results of the investigations performed with the use of voltaic cells of the type water-nonpolar solvent, e.g. water-decane [1, 2,18] or water-octane [19-23] will be intentionally omitted. In the case of such interesting and important systems the organic phase does not constitute a solution of a dissociated electrolyte hence here, as justified in detail by Davis and Rideal [1,2], only changes of surface potentials could be examined in practice. 

A gas gap is a necessary phase in voltaic cells. This phase separate two condensed phases, e.g. metals, liquid electrolyte solutions or solid electrolytes. Investigations of these cells allow the determination of Volta potentials, i.e. differences of the outer potentials of the phases, separated by the gas gap. Volta potentials are directly related to such quantities as real potentials of charged species in investigated phases and surface potential changes. 

Volta s greatest contribution was, however, the discovery, in 1796. of the voltaic pile, which consisted of a series of units, each made from sheets of dissimilar metals such as zinc and silver separated by wet doth. Volta showed that metals could be arranged in au "electromotive series so that each became positive when placed in contact with the one next below it in the series. Although, as has already been mentioned, Volta considered that the source of the electric energy was at the surface of contact of, the metals, this theory was thrown in doubt when it was discovered that chemical action accompanied the operation of the pile. It is of interest that the question of the seat of the potential of the galvanic cell is not, even today, finally settled. Many improvements of the voltaic pile were made. It is, of course, the precursor of the modern galvanic cell. 

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