Single electrode/solution interface

In the case of a redox reaction in a single electrode-solution interface... 

The entropy of formation of the interface was calculated from the temperature coefficient of the interfacial tension.304 The entropy of formation has been found to increase with the nature of the electrolyte in the same sequence as the single cation entropy in DMSO.108, 09,329 The entropy of formation showed a maximum at negative charges. The difference in AS between the maximum and the value at ff=ocan be taken as a measure of the specific ordering of the solvent at the electrode/solution interface. Data 108,109304314 have shown that A(AS) decreases in the sequence NMF > DMSO > DMF > H90 > PC > MeOH. 

But before dying to understand the behavior of electrochemical systems, or cells, it was considered useful to disassemble, or analyze, them conceptually into two isolated electrode/electrolyte interfaces and then to study single interfaces. This has been done. The whole treatment so far has concerned itself with a single electrode/ solution interface98 and with the current-potential laws that govern its behavior. The Butler-Volmer equation is the key equation for a single interface. The behavior of an electrochemical system, or cell, must be conceptually synthesized from the behavior of the individual interfaces that combine to form a cell... 

X-ray surface diffraction has been applied in situ to study several processes at the electrode solution interface [13, 14]. An important phenomenon in electrochemistry at Au is surface reconstruction in which a monolayer of atoms on the surface of a single crystal acquires a different arrangement from that of the... 

Fig. 10.23 Models of the solvent monolayer at the electrode solution interface according to (a) the two-state model with spherical solvent dipoles in either the up (f) or down orientations ( ) (b) the three-state model in which a third state with solvent dipoles parallel ( ) to the interface has been added (c) the cluster model with clusters in the up (/ / ) and down ( / ) orientations, and single molecules up (t) and down (f,).

Up to this point, we have considered potentials associated with a single metal/solution interface (i.e., (])m>s> and( ) ). It is, of course, not possible to measure directly either the absolute potentials or differences between them. Potential is only experimentally measurable or controllable relative to that of another electrode of defined, invariant potential (i.e., a nonpolarizable reference electrode). Apart from defining the applied potential and enabling it to be measured, a reference electrode is required in order to complete the circuit and maintain electrical neutrality with zero current flow throughout the potentialmeasuring circuit of the cell. 

Ellipsometry at noble metal electrode/solution interfaces has been used to test theoretically predicted microscopic parameters of the interface [937]. Investigated systems include numerous oxide layer systems [934-943], metal deposition processes [934], adsorption processes [934, 944] and polymer films on electrodes [945-947]. Submonolayer sensitivity has been claimed. Expansion and contraction of polyaniline films was monitored with ellipsometry by Kim et al. [948]. Film thickness as a function of the state of oxidation of redox active polyelectrolyte layers has been measured with ellipsometry [949]. The deposition and electroreduction of Mn02 films has been studied [950] below a thickness of 150 nm, the anodically formed film behaved like an isotropic single layer with optical constants independent of thickness. Beyond this limit, anisotropic film properties had to be assumed. Reduction was accompanied by an increase in thickness, which started at the ox-ide/solution interface. 

Anions and neutral organic compounds are among other species whose adsorption on single-crystal electrodes has been probed with sensitivity by infrared spectroscopy. Because of their importance as common electrolytes, there has been long-standing interest in the behavior of simple oxoanions such as perchlorate, sulfates, and phosphates, at the electrode/solution interface (cf. Refs. 

Structural control of the electrode/solution interface is a complex problem of fundamental importance in electrochemical sciences (7). To achieve some elements of such control, it would be desirable to impart molecular character onto the otherwise "naked" electrode surface so that, as a result, it might acquire desired catalytic properties, gain some elements of molecular selectivity, or exhibit other desirable molecular characteristics. To accomplish this, electrochemists have explored numerous possibilities of coating the electrode surface with thin films (from a single monolayer to micrometers in thickness) of a wide variety of materials (2). This area of electrochemistry, often referred to as the chemical mo fication of electrodes, is the subject of a number of recent reviews (7-5). 

In the electrochemical shorthand the contact zone of two different electrolyte solutions (also, for example, for the same electrolyte at different concentrations) is represented by a double line. This is in contrast to the electrode/solution interface which is symbolized by a single line. Thus for a cell with liquid junction made up of an ion-selective measuring electrode M in contact with a solution containing the corresponding ions complemented with a standard hydrogen electrode we would write ... 

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