Relation to the Electrosorption Valency

We consider a small amount of ions adsorbed at a potential close to 

Here Cac denotes the Gouy-Chapman capacity, Agm gives the first order deviation from the Gouy-Chapman theory in the absence of 

Setting as = 0 we obtain A = 1/Cn, where C is the Helmholtz or inner-layer capacity.72 By definition, this is the first-order correction to the Gouy-Chapman theory. Setting as = -aM with as = zeNs, from Eqs. (71) and (72) we obtain for the dipole moment  

As mentioned before, the electrosorption valency is usually corrected for double-layer effects. By disregarding the Gouy-Chapman term in Eq. (72), the following expression for the electrosorption valency is obtained  

Substituting B from Eq. (74) into Eq. (73), the following relationship between the electrosorption valency and the dipole moment is obtained  

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Experimental results are available for the diffraction rods and structure of an overlayer of Pb on Ag(l 11)[23, 24]. Since these measurements are very accurate it is also possible to measure the compression of the UPD overlayer of Pb as the potential is changed[25, 26]. The compressibility is certainly related to the electrosorption valency, discussed in another section of this book. 

There is no direct way of measuring the partial charge transfer A, the most direct experimental evidence comes from NEXAFS experiments. It is related to the electrosorption valency [160]. 

The electrosorption valence can be related to the dipole moment of an adsorbed species introduced in Chapter 4. For this purpose consider an electrode surface that is initially at the pzc and free of adsorbate. When a small excess charge density o is placed on the metal, its potential changes by an amount A given by ... 

In this review, we will consider the adsorption of a single species coadsorption phenomena will not be considered, since it is generally impossible to divide the flow of charge among several species. We will present the thermodynamics on which the concept of the electrosorption valency is based, discuss methods by which it can be measured, and explain its relation to the dipole moment and to partial charge transfer. The latter can be explained within an extension of the Anderson-Newns model for adsorption, which is useful for a semi-quantitative treatment of electrochemical adsorption. Our review of concepts and methods will be concluded by a survey of experimental data on thiol monolayers, which nowadays are adsorbates of particular interest. 

Electrochemistry shares many concepts with surface science, and for the last two decades there has been an exchange of methods and ideas between these two neighboring disciplines. However, the electrosorption valency has no equivalent in surface science, since experiments at the solid/gas or solid/vacuum interface cannot be performed at constant potential. However, for low coverages, and near the potential of zero charge, the electrosorption valency can be related to the dipole moment of the adsorbate, which can be measured both in surface science and, though with greater difficulty, also in electrochemistry. In the following, we point out the relation between these two quantities. 

Chapter 3, by Rolando Guidelli, deals with another aspect of major fundamental interest, the process of electrosorption at electrodes, a topic central to electrochemical surface science Electrosorption Valency and Partial Charge Transfer. Thermodynamic examination of electrochemical adsorption of anions and atomic species, e.g. as in underpotential deposition of H and metal adatoms at noble metals, enables details of the state of polarity of electrosorbed species at metal interfaces to be deduced. The bases and results of studies in this field are treated in depth in this chapter and important relations to surface -potential changes at metals, studied in the gas-phase under high-vacuum conditions, will be recognized. Results obtained in this field of research have significant relevance to behavior of species involved in electrocatalysis, e.g. in fuel-cells, as treated in chapter 4, and in electrodeposition of metals. 

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