Electrode-solution interface, structural

Interfacial water molecules play important roles in many physical, chemical and biological processes. A molecular-level understanding of the structural arrangement of water molecules at electrode/electrolyte solution interfaces is one of the most important issues in electrochemistry. The presence of oriented water molecules, induced by interactions between water dipoles and electrode and by the strong electric field within the double layer has been proposed [39-41]. It has also been proposed that water molecules are present at electrode surfaces in the form of clusters [42, 43]. Despite the numerous studies on the structure of water at metal electrode surfaces using various techniques such as surface enhanced Raman spectroscopy [44, 45], surface infrared spectroscopy [46, 47[, surface enhanced infrared spectroscopy [7, 8] and X-ray diffraction [48, 49[, the exact nature of the structure of water at an electrode/solution interface is still not fully understood. 

The question arises of the extent to which the build-up of an electrode potential may significantly alter the original concentration of the solution in which the electrode is placed. Let us take the example of a silver electrode. Once the electrode has been immersed in an Ag+ solution, part of the Ag+ ions will be discharged by precipitation of the corresponding amount of Ag and to an extent such that the Nemst potential has been reached. In fact, a double layer at the electrode/solution interface has been formed whose structure cannot be as precisely described as has appeared from the model proposed by... 

All of these techniques, although powerful, do not reveal the structure and geometric arrangement of atomic species at the interface. Thus, in spite of its importance, our knowledge of the structure of the electrode/solution interface at the atomic level is still very rudimentary. 

As mentioned previously, this can be attributed in part to the lack of structure-sensitive techniques that can operate in the presence of a condensed phase. Ultrahigh-vacuum (UHV) surface spectroscopic techniques such as low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and others have been applied to the study of electrochemical interfaces, and a wealth of information has emerged from these ex situ studies on well-defined electrode surfaces.15"17 However, the fact that these techniques require the use of UHV precludes their use for in situ studies of the electrode/solution interface. In addition, transfer of the electrode from the electrolytic medium into UHV introduces the very serious question of whether the nature of the surface examined ex situ has the same structure as the surface in contact with the electrolyte and under potential control. Furthermore, any information on the solution side of the interface is, of necessity, lost. 

Very often, the electrode-solution interface can be represented by an equivalent circuit, as shown in Fig. 5.10, where Rs denotes the ohmic resistance of the electrolyte solution, Cdl, the double layer capacitance, Rct the charge (or electron) transfer resistance that exists if a redox probe is present in the electrolyte solution, and Zw the Warburg impedance arising from the diffusion of redox probe ions from the bulk electrolyte to the electrode interface. Note that both Rs and Zw represent bulk properties and are not expected to be affected by an immunocomplex structure on an electrode surface. On the other hand, Cdl and Rct depend on the dielectric and insulating properties of the electrode-electrolyte solution interface. For example, for an electrode surface immobilized with an immunocomplex, the double layer capacitance would consist of a constant capacitance of the bare electrode (Cbare) and a variable capacitance arising from the immunocomplex structure (Cimmun), expressed as in Eq. (4). 

The central issue which has to be addressed in any comprehensive study of electrode-surface phenomena is the determination of an unambiguous correlation between interfacial composition, interfacial structure, and interfacial reactivity. This principal concern is of course identical to the goal of fundamental studies in heterogeneous catalysis at gas-solid interfaces. However, electrochemical systems are far more complicated since a full treatment of the electrode-solution interface must incorporate not only the compact (inner) layer but also the boundary (outer) layer of the electrical double-layer. The effect of the outer layer on electrode reactions has been neglected in most surface electrochemical studies but in certain situations, such as in conducting polymers and... 

Valuable information can be obtained from thermal desorption spectra (TDS) spectra, despite the fact that electrochemists are somewhat cautious about the relevance of ultrahigh vacuum data to the solution situation, and the solid/liquid interface in particular. Their objections arise from the fact that properties of the double layer depend on the interaction of the electrode with ions in the solution. Experiments in which the electrode, after having been in contact with the solution, is evacuated and further investigated under high vacuum conditions, can hardly reflect the real situation at the metal/solution interface. However, the TDS spectra can provide valuable information about the energy of water adsorption on metals and its dependence on the surface structure. At low temperatures of 100 to 200 K, frozen molecules of water are fixed at the metal. This case is quite different from the adsorption at the electrode/solution interface, which usually involves a dynamic equilibrium with molecules in the bulk. 

Adsorption of a condensed 1-hydroxy-adamantane layer at the Hg elec-trode/(Na2S04 or NaF) solution interface has been studied as a function of temperature by Stenina et al. [174]. Later, Stenina etal. [175] have determined adsorption parameters and their temperature dependence for a two-dimensional condensation of adamantanol-1 at a mercury electrode in Na2S04 solutions. They have also studied coadsorption of halide (F , Cl , Br ) anions and 1-adamantanol molecules on Hg electrode [176]. More recently, Stenina etal. [177] have described a new type of an adsorption layer comprising organic molecules of a cage structure condensed at the electrode/solution interface. This phenomenon was discovered for adsorption of cubane derivatives at mercury electrode. 

Electrodic reactions that underlie the processes of metal deposition, etc., cannot be understood without knowing the potential difference at the electrode/solution interface and how it varies with distance from the electrode. The ions from the solution must be electrically energized to cross the interphase region and deposit on the metal. This electrical energy must be picked up from the field at the interface, which itself depends upon the double-layer structure. Thus, control over metal deposition processes can be improved by an increased understanding of double layers at metal/solutioii interfaces. 

Adsorption phenomena significantly influence the rate of electrode reactions. The heterogeneous nature of electrode reactions determines that energetics and local activities of reacting species in the vicinity of the electrode may be different from those in the bulk solution, even when mass transport limitations can be regarded as negligible. The structure and properties of the electrode—solution interface then play a key role in the adsorption of electroactive as well as electroinactive surface active substances (SAS) at electrodes. 

The electrochemical behavior of a modified electrode ultimately depends on structural details at the molecular level. For example, the molecular-level interaction between the redox site in the film and the solvent from the contacting solution phase might play an important role in the electrochemical response. Molecular-level details are often difficult to infer from electrochemical methods alone, but do lend themselves to spectroscopic analyses. In recent years there has been an explosion of new spectroscopic techniques for characterizing modified electrodes and the electrode-solution interface in general [44,45]. In this section, we review some of these spectroelectrochemical methods. 

Electrolytes The above issue of double layer structure is important to the mechanism of nucleation and growth in ionic liquids, it may therefore be possible to control the structure at the electrode/solution interface by addition of an inert electrolyte. In this respect most Group 1 metals are soluble in most ionic liquids, although it is only generally lithium salts that exhibit high solubility. In ionic liquids with discrete anions the presence of Group 1 metal ions can be detrimental to the deposition of reactive metals such as A1 and Ta where they have been shown to be co-deposited despite their presence in trace concentrations. 

The only models that exist for double layer structure in ionic liquids suggest a Helmholz layer at the electrode/solution interface [103, 104], If the reduction potential is below the point of zero charge (pzc) then this would result in a layer of cations approximately 5 A thick across which most of the potential would be dropped, making it difficult to reduce an anionic metal complex. Hence, the double layer models must be incorrect. 

S. Trasatti. The Electrode Potential, in Comprehensive Treatise of Electrochemistry, Vol. 1, J. O M. Bockris, B.E. Conway and E. Veager. Eds. Plenum (1980), chapter 2 B.E. Conway, The State of Water and Hydrated Ions at Interfaces, Adv. Colloid Interface Sci. 8 (1977) 91 W.R. Fawcett, Molecular Models for Solvent Structure at Polarizable Interfaces. Isr. J. Chem. 18 (1979) 3 M.A. Habib, Solvent Dipoles at the Electrode-Solution Interface. in Modem Aspects of Electrochemistry, Vol. 12, J. O M. Bockris and B.E. Conway. Eds. Plenum (1977) 131 S. Trasatti, Solvent Adsorption and Double Layer Potential Drop at Electrodes, in Modem Aspects of Electrochemistry, B.E. Conway and J. O M. Bockris, Eds. Vol. 13 Plenum (1979) chapter 2 J. O M. Bockris. K-T. Jeng, Water Structure at Interfaces The Present Situation. Adv. Colloid Interface Set 33 (1990) 1. 

Overpotential depends on the structure of a solution near an electrode, which differs from that in the bulk solution. Near an electrode, electrons are transferred across an electrode-solution interface that depends on the solution and the condition of the electrode. 

Structural changes of the electrode-solution interface, such as absorption and desorption of species that change the potential of the electrode and solution composition without charge transfer. 

In the present chapter, the relationship between the electrode potential and the activity of the solution components in the cell is examined in detail. The connection between the Galvani potential difference at the electrode solution interface and the electrode potential on the standard redox scale is discussed. This leads to an examination of the extrathermodynamic assumption which allows one to define an absolute electrode potential. Ion transfer processes at the membrane solution interface are then examined. Diffusion potentials within the membrane and the Donnan potentials at the interface are illustrated for both liquid and solid state membranes. Specific ion electrodes are described, and their various modes of sensing ion activities in an analyte solution discussed. The structure and type of membrane used are considered with respect to its selectivity to a particular ion over other ions. At the end of the chapter, emphasis is placed on the definition of pH and its measurement using the glass electrode. 

On the electrode side of the double layer the excess charges are concentrated in the plane of the surface of the electronic conductor. On the electrolyte side of the double layer the charge distribution is quite complex. The potential drop occurs over several atomic dimensions and depends on the specific reactivity and atomic structure of the electrode surface and the electrolyte composition. The electrical double layer strongly influences the rate and pathway of electrode reactions. The reader is referred to several excellent discussions of the electrical double layer at the electrode—solution interface (26-28). 

Figure 3 schematically depicts the structure of the electrode—solution interface. The inner Helmholtz plane (IHP) refers to the distance of closest approach of specifically adsorbed ions, generally anions to the electrode surface. In aqueous systems, water molecules adsorb onto the electrode surface. 

The combination of infrared (IR) spectroscopy and electrochemistry, IR spectro-electrochemistry, is a powerful tool for investigation of the electrode/solution interface. It is extremely useful for studies of the structure and bonding of species adsorbed on electrode surfaces. In situ IR-spectroelectrochemistry reveals important details of the potential-dependent surface chemistry of adsorbing species. 

Under some conditions an electrode may be in a potential region where charge-transfer reactions do not occur because they are either thermodynamically or kinet-ically unfavorable. However, such processes as adsorption can occur, and the structure of the electrode-solution interface can change, causing transitory changes in current and/or potential. These processes are called nonfaradaic processes. 

Another very important property of a given electrode/solution interface with regard to its structure and functioning is its potential of zero charge (pzc). This is the unique value of electrode potential at which the excess charge on the electrode is zero. Acquisition of pzc data for solid electrodes is dependent on the chemical nature of the electrode surface, and wide variations in pzc values for electrodes of the same metal in identical solutions have been reported. 

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