The Electrosorption Valency

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. 

When a species Sz is adsorbed at an electrode surface, it may form a purely physical bond (physisorption) or a much stronger chemical bond (chemisorption). In the latter case the species may transfer a positive or negative charge to the electrode, according to the formal adsorption reaction  

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The definition of the electrosorption valence involves the total surface excess, not only the amount that is specifically adsorbed. It is common to correct the surface excess F, for any amount that may be in the diffuse double layer in order to obtain the amount that is specifically adsorbed. This can be done by calculating the excess in the... 

Usually the electrosorption valence is denoted by 7, which we use for the surface tension. The symbol l was used earlier by Lorenz and Salie [2]. 

The interpretation of the electrosorption valence is difficult. The following, somewhat naive argument shows that it involves both the distribution of the potential and the amount of charge transferred during the adsorption process. Suppose that an ion Sz is adsorbed and takes up A electrons in the process. A need not be an integer since there can be partial charge transfer (cf. Chapter 4). We can then write the adsorption reaction formally as ... 

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 ... 

Another parameter, the electrosorption valence. Y introduced by Vetter and Schulze (1973), is also widely used in electrochemistry. It is a macroscopic measure defined as y——(1/ (dq /dT). For most conditions, X = —y. 

B. E. Conway and H. Angerstein-Kozlowska, Interaction Effects in Electrodeposited Monolayers and the Role of the Electrosorption Valency Factor, J- Electroanal. Chem. 113 63 (1980). 

Partial charge transfer during adsorption is difficult to evaluate because separation of the charge transferred to the electrode and that part of the charge transferred across the double layer to give specifically adsorbed ions cannot be done through measurements of the total charge in the external circuit. Vetter and Schultze [102] defined the electrosorption valence as... 

The electrosorption valency usually increases as the underpotential decreases to approach the ionic charge (total discharge of the cation) close to the Nernst potential, for instance in the case of lead and thallium upd on silver [114]. However, the co-adsorption of anions may contribute to the observed apparent electrosorption valence, as rotating ring disc electrode (RDE) experiments have shown [113]. 

Fig. 12.20. Electrosorption of Br and I" on polycrystalline gold. Plot of charge passed vs. coverage determined by EQCM. Slope is the electrosorption valency (from Ref. 60 with permission).

Another useful thermodynamic relationship that allows the potential dependence of l to be determined from the differential capacity Co, of the interphase at constant rs is readily obtained from the very definition of l. Choosing Ez as the reference potential and denoting by lz the electrosorption valency at Ez, the l value at any other applied potential E is given by ... 

Lorenz refers to f as the macroscopic pet coefficient and regards it as experimentally accessible. C M can be measured by the extrapolated value of the differential capacity C at infinite frequency, when the surface concentration rs is frozen, i.e., when it cannot keep up with the ac signal. Since, however, the partial charge associated with chemisorption does follow the ac signal, this is only true if A can be regarded as potential independent. It can be readily seen that l in Eq. (31) is the opposite of the electrosorption valency l. In fact, replacing from Eq. (9) into Eq. (2) yields ... 

This kinetic determination of the electrosorption valency l is based on the relatively mild assumption of a potential independent X. 

In conclusion, kinetic procedures based on Eqs. (42), (45) and (46) for the estimate of the electrosorption valency l are often expected to yield inaccurate results, especially if carried out at high surface... 

Even though this equation is based on an overly simple model, it illustrates well the double-layer properties that govern the electrosorption valency in the absence of pet. In particular, it shows that a fractional value of / need not necessarily indicate pet. We shall return to the hard-sphere electrolyte model when we discuss dipole moments of adsorbates. 

This is due to the fact that the electrosorption valency, l, obtained from diffusion-controlled adsorption relies on measurements corresponding to the same small, initial amount of adsorbed material at all potentials. Consequently, this procedure does not suffer from the limitations involved in the use of Eq. (2) at high surface coverages. 

The electrosorption valency of several neutral aliphatic molecules on mercury was examined by Koppitz at al.54 As usual, the experimental l values were referred to the potential of zero charge in the absence of specific adsorption, Ez, and to low surface coverages. With the remarkable exception of thiourea, all these molecules do not undergo pet (A = 0). Since they are also neutral (z = 0), their electrosorption valency is exclusively determined by the two dipole terms ... 

Figure 9. Dependence of the electrosorption valency / on the dipole terms Ks. The inset shows the expectations from Eq. (61). Systems with expected charge transfer are marked by vertical arrows. (Reprinted from Ref.54 with permission from Elsevier Science)...

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. 

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 ... 

Table 1 summarizes a few values of the dipole moment fx and of the electrosorption valency l of halide and alkali metal ions adsorbed on mercury. The experimental values of l are relative to low coverages near the potential of zero charge and are taken from Schultze and Koppitz,50 while the corresponding // values were calculated from Eq. (75). The theoretical values in the last column are from a hard sphere electrolyte model. Further data can be found in the article by Schmickler49 Note that, in the electrochemical environment, the dipole moments are much smaller than in vacuo, where they can reach values of the order of 7 D for the alkali metal ions. No doubt, this difference is caused by the screening of the adsorbate dipole by the solvent molecules. 

After making an approximate estimate of the absolute potential difference y/ across the mercury/aqueous solution interphase, let us detennine the electrosorption valency of three mercury-supported thiol monolayers for which uM was measured by potential-step chronocoulometry. As already reported in Sec. IV.1, n-octadecanthiol... 

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