Interactions, metal-electrolyte

These primary electrochemical steps may take place at values of potential below the eqnilibrinm potential of the basic reaction. Thns, in a solntion not yet satnrated with dissolved hydrogen, hydrogen molecnles can form even at potentials more positive than the eqnilibrinm potential of the hydrogen electrode at 1 atm of hydrogen pressnre. Becanse of their energy of chemical interaction with the snbstrate, metal adatoms can be prodnced cathodically even at potentials more positive than the eqnilibrinm potential of a given metal-electrolyte system. This process is called the underpotential deposition of metals. 

Models for the compact layer of the metal-electrolyte interface have become more and more elaborate, providing better and better representations of observed electrocapillary data for different metals, solvents, and temperatures, but almost always leaving the metal itself out of consideration, except for consideration of image interactions of the solvent dipoles. For reviews of these models, see Parsons,13 Reeves,14 Fawcett et a/.,15 and Guidelli,16 who gives detailed discussion of the mathematical as well as the physical assumptions used. 

The experimental data bearing on the question of the effect of different metals and different crystal orientations on the properties of the metal-electrolyte interface have been discussed by Hamelin et al.27 The results of capacitance measurements for seven sp metals (Ag, Au, Cu, Zn, Pb, Sn, and Bi) in aqueous electrolytes are reviewed. The potential of zero charge is derived from the maximum of the capacitance. Subtracting the diffuse-layer capacitance, one derives the inner-layer capacitance, which, when plotted against surface charge, shows a maximum close to qM = 0. This maximum, which is almost independent of crystal orientation, is explained in terms of the reorientation of water molecules adjacent to the metal surface. Interaction of different faces of metal with water, ions, and organic molecules inside the outer Helmholtz plane are discussed, as well as adsorption. 

For the metal in the electrochemical interface, one requires a model for the interaction between metal and electrolyte species. Most important in such a model are the terms which are responsible for establishing the metal-electrolyte distance, so that this distance can be calculated as a function of surface charge density. The most important such term is the repulsive pseudopotential interaction of metal electrons with the cores of solvent species, which affects the distribution of these electrons and how this distribution reacts to charging, as well as the metal-electrolyte distance. Although most calculations have used parameterized simple functional forms for this term, it can now be calculated correctly ab initio. 

What one requires is a self-consistent picture of the interface, including both metal and electrolyte, so that, for a given surface charge, one has distributions of all species of metal and electrolyte phases. Unified theories have proved too difficult but, happily, it seems that some decoupling of the two phases is possible, because the details of the metal-electrolyte interaction are not so important. Thus, one can calculate the structure of each part of the interface in the field of the other, so that the distributions of metal species are appropriate to the field of the electrolyte species, and vice versa. 

Nancollas, G. H. "Interactions in Electrolyte Solutions (Metal Complex and Ion Pair Formation in Solution)" Elsevier Publishing Co., Amsterdam, 1966. 

Thus, both cations and anions are forced away from the insulator/electrolyte interface over a distance r. As a result, their concentration profile is inverted, relative to the one created at the metal/electrolyte interface, as shown in Fig. 5.3. There the ions are attracted to and repelled from the metal surface by Coulombic interactions... 

Figure 28 Metal electrode on O2 -conducting (left) and Na+-conducting (right) solid electrolyte. The figure depicts the metal-electrolyte double layer at the metal-gas interface due to electric potential-controlled ion migration, as well as its interaction with adsorbed reactants during CO oxidation (from Vayenas and Koutsodontis, 2008 reprinted with permission. Copyright 2008, American Institute of Physics).

Computer simulations of metal/electrolyte interfaces are a great challenge. Explicit water molecules bring new degrees of freedom for atoms and electrons, and accurate and physically realistic simnlations require solvent dynamics. Taylor and Neurock reviewed recent work on metal/water interfaces focusing on water stracture and other interactions with electrode surfaces. ... 

It was discussed in Section III.C that salts, which are inert at low concentrations and close to PZC can induce effects characteristic for specific adsorption at sufficiently high concentrations. In organic solvents (water free or containing small amounts of water) the specific interactions of electrolytes that are inert in water are commonplace, even at concentrations below 10 mol dm" (but the activities of alkali metal cations in such solutions can be higher that in 1 molar aqueous solution). Reversal of sign potential of inorganic materials from negative... 

Spohr describes in detail the use of computer simulations in modeling the metal/ electrolyte interface, which is currently one of the main routes towards a microscopic understanding of the properties of aqueous solutions near a charged surface. After an extensive discussion of the relevant interaction potentials, results for the metal/water interface and for electrolytes containing non-specifically and specifically adsorbing ions, are presented. Ion density profiles and hydration numbers as a function of distance from the electrode surface reveal amazing details about the double layer structure. In turn, the influence of these phenomena on electrode kinetics is briefly addressed for simple interfacial reactions. 

A similar analysis was carried out for the case of an electrochemical system in which the ion is adsorbed at the charged metal-electrolyte solution interface. The derived analytical expressions for the interaction potential become a bit more complicated [89], but all the above-formulated quahtative conclusions remain... 

The quantum mechanical treatment of the metallic surface has been developed to a large extent for metal-vacuum interfaces. Similar techniques should be applicable to the metal electrolyte interface. The main difference is that the fluid contributes not only to the electric fields in the metal surface, but also to specific chemical bonding interactions. 

Study of the inhibition mechanism of molecules requires systematic investigation of the adsorption/desorption processes of inhibitor molecules on the metal/electrolyte interface. The adsorption phenomena of inhibitors on corroding metals are fairly complex and dependent on the surface feature, such as the composition and structure of the oxide-hydroxide layer, the local pH gradient near the interface, and interaction between the inhibitor molecules and components of the oxide layer. The study of adsorption/desorption processes of corrosion inhibitors on a noble metal surface is of great importance for fundamental aspects. Therefore knowledge of the adsorption properties of inhibitor molecules on a well-defined surface structure might be beneficial, and may contribute to a better understanding of the kinetics and mechanisms of inhibition processes on constructional materials of industrial importance. 

In further developments, with Schmickler et al. (1984), two models of the solvent layer at the metal interface were considered. These self-consistent calculations of charge-induced electron relaxation predict in one form or another the well known hump in the compact-layer capacitance and introduce a dependence of the capacitance behavior on the properties of the metal electron system, that is, of course, not indicated in previous, purely molecular treatments of the metal/electrolyte interface. In general, metal-specific behavior (apart from that associated with specific orientation of solvent dipoles due to donor-acceptor interaction with the metal) is related to the free electron density of the metal. For further details, readers are referred to the review mentioned earlier (Feldman et al., 1986). 

Metalliding. MetaUiding, a General Electric Company process (9), is a high temperature electrolytic technique in which an anode and a cathode are suspended in a molten fluoride salt bath. As a direct current is passed from the anode to the cathode, the anode material diffuses into the surface of the cathode, which produces a uniform, pore-free alloy rather than the typical plate usually associated with electrolytic processes. The process is called metalliding because it encompasses the interaction, mostly in the soHd state, of many metals and metalloids ranging from beryUium to uranium. It is operated at 500—1200°C in an inert atmosphere and a metal vessel the coulombic yields are usually quantitative, and processing times are short controUed... 

LPR probes measure the electrochemical corrosion mechanism involved in the interaction of the metal with the electrolyte. To measure hnear polarization resistance R, l/cm", the following assumptions must be made ... 

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