DOUBLE-LAYER STRUCTURE AND ADSORPTION

In Chapter 1, we introduced some elementary ideas about the double layer, including notions about its capacitance and structure. In the remainder of the text to this point, we have made repeated references to its influence on electrode processes and electrochemical measurements. It is time now to delve into this aspect of electrochemical science in more detail. Here our goal is to examine the kinds of experimental measurements that can illuminate the structure of the double layer, as well as the important structural models and their implications for electrode kinetics. 

A great deal of our knowledge about the double layer comes from measurements of macroscopic, equilibrium properties, such as interfacial capacitance and surface tension. In general, we are interested in the way in which these properties change with potential and with the activities of various species in the electrolyte. The next section will deal with experimental aspects in some detail. For the moment, we will concentrate on the theory that we use to suggest and interpret experiments. Since our concern now is with macroscopic, equilibrium properties, we can expect a thermodynamic treatment to describe the system rigorously without a postulated model. This is an important aspect, because it implies that we can obtain data that any successful structural model must rationalize. 

We begin by developing the Gibbs adsorption isotherm, which describes interfaces in general, and from that we obtain the electrocapillary equation, which describes the properties of electrochemical interfaces more particularly. 

The reason for defining the reference system is that the properties of the interface are governed by excesses and deficiencies in the concentrations of components that is, we are concerned with differences between the quantities of various species in the actual interfacial region, with respect to the quantities we would expect if the existence of the interface did not perturb the pure phases, a and j8. These differences are called surface excess quantities. For example, the surface excess in the number of moles of any species, such as potassium ions or electrons, would be 

One of those variables is the electrochemical free energy, which can be considered profitably in a general way (1-4). For the reference system, the electrochemical free energy depends on the usual variables temperature, pressure, and the molar quantities of all components. That is, = G (T, P, nf). The surface area has no impact on because the interface does not perturb phases a and j8. There is, therefore, no energy of interaction. On the other hand, we know from experience that real systems have a tendencyjo minimize or maximize the interfocial ea hence the free energy of the actual system, G, must depend on the area. Thus, G = G (T, P, A, nf). 

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In situ Fourier transform infrared and in situ infrared reflection spectroscopies have been used to study the electrical double layer structure and adsorption of various species at low-index single-crystal faces of Au, Pt, and other electrodes.206"210 It has been shown that if the ions in the solution have vibrational bands, it is possible to relate their excess density to the experimentally observed surface. 

The double-layer structure and adsorption of various surface-active substances may be examined by this method, which replaces cumbersome balancing bridge measurements. 

A comprehensive treatment of the information about double layer structure and adsorption of organic sufactants that can be obtained for both solid and liquid metals by measurements of double layer capacitance has been published by Damaskin, Petrii, Batrakov [11] (see also Chapter 5). 

Zhou, W., Inoue, S., Iwahashi, T., Kanai, K., Seki, K., Miyamae, T., Kim, D., Katayama, Y. and Ouchi, Y., Double layer structure and adsorption/desorption hysteresis of neat ionic liquid on Pt electrode surface - An in-situ IR-visible sum-frequency generation spectroscopic study, Electwchem. Commun. 12, 672-675 (2010). 

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