Surface thermodynamics metal/solution interface

The liquid metal mercury-solution interface presents the advantage that it approaches closest to an ideal polarizable interface and, therefore, it adopts the potential difference applied between it and a non-polarizable interface. For this reason, the mercury-solution interface has been extensively selected to carry out measurements of the surface tension dependence on the applied potential. In the case of other metal-solution interfaces, the thermodynamic study is much more complex since the changes in the interfacial area are determined by the increase of the number of surface atoms (plastic deformation) or by the increase of the interatomic lattice spacing (elastic deformation) [2, 4]. 

Two aspects of Table 1 are important. The standard conditions are 298 K and all reactants and products are at unity activity. The second key is the selection of the hydrogen reaction as having a standard reversible potential of 0.0 V. The table allows the first use of thermodynamics in corrosion. For a metal in a 1 M solution of its salt, the table allows one to predict the electrochemical potential below (i.e., more negative) which net dissolution is impossible. For example, at +0.337 V(NHE), copper will not dissolve to cuprous ion if the solution is 1 M in Cu2+. In fact, at more negative potentials, there will be a tendency at the metal/solution interface to reduce the cuprous ions to copper metal on the surface. 

When a metal, M, is immersed in a solution containing its ions, M, several reactions may occur. The metal atoms may lose electrons (oxidation reaction) to become metaUic ions, or the metal ions in solution may gain electrons (reduction reaction) to become soHd metal atoms. The equihbrium conditions across the metal-solution interface controls which reaction, if any, will take place. When the metal is immersed in the electrolyte, electrons wiU be transferred across the interface until the electrochemical potentials or chemical potentials (Gibbs ffee-energies) on both sides of the interface are balanced, that is, Absolution electrode Until thermodynamic equihbrium is reached. The charge transfer rate at the electrode-electrolyte interface depends on the electric field across the interface and on the chemical potential gradient. At equihbrium, the net current is zero and the rates of the oxidation and reduction reactions become equal. The potential when the electrode is at equilibrium is known as the reversible half-ceU potential or equihbrium potential, Ceq. The net equivalent current that flows across the interface per unit surface area when there is no external current source is known as the exchange current density, f. 

Surface Thermodynamics of Metal/Solution Interface the Untapped Resources... 

The mercury/electrolyte interface played a major role in the early studies of the structure of metal/solution interfaces, and electrode kinetics in general. The surface of the liquid metal is highly reproducible and the low catalytic activity of Hg towards hydrogen evolution provided a rather wide range of potentials where the thermodynamic properties of the interface coidd be determined experimentally, allowing theories to be verified or discarded. However, mercury is of little industrial interest, and its use has been all but eliminated in recent decades because of its high toxicity and devastating influence on the environment. 

Anodic oxidation often involves the formation of films on the surface, i.e. of a solid phase formed of salts or complexes of the metals with solution components. They often appear in the potential region where the electrode, covered with the oxidation product, can function as an electrode of the second kind. Under these conditions the films are thermodynamically stable. On the other hand, films are sometimes formed which in view of their solubility product and the pH of the solution should not be stable. These films are stabilized by their structure or by the influence of surface forces at the interface. 

The electrochemical behavior of an Mg electrode in thionyl chloride/ Mg(AlCl4)2 solutions was investigated extensively by Meitav and Peled [426], The Mg electrode in this electrolyte system is covered by MgCl2, which forms a bilayered surface film a compact one close to the metal and a porous one at the film-solution interface. This surface film determines the electrochemical behavior of these systems and can only conduct Cl ions, and thus the mobility of Mg2+ through it is practically zero. Thus, Mg deposition does not occur in this system, and Mg dissolution at a reasonable rate occurs via a breakdown and repair mechanism. Since the active metal is thermodynamically unstable in thionyl chloride when a fresh metal is exposed to solution, it reacts readily with the solvent to form this film. 

Various chemical surface complexation models have been developed to describe potentiometric titration and metal adsorption data at the oxide—mineral solution interface. Surface complexation models provide molecular descriptions of metal adsorption using an equilibrium approach that defines surface species, chemical reactions, mass balances, and charge balances. Thermodynamic properties such as solid-phase activity coefficients and equilibrium constants are calculated mathematically. The major advancement of the chemical surface complexation models is consideration of charge on both the adsorbate metal ion and the adsorbent surface. In addition, these models can provide insight into the stoichiometry and reactivity of adsorbed species. Application of these models to reference oxide minerals has been extensive, but their use in describing ion adsorption by clay minerals, organic materials, and soils has been more limited. 

The precise arrangement and degree of ordering of the lipid molecules in the final structure is not known for certain. However, it seems highly probable that the bilayer nature of the assembly is a consequence of the thermodynamics of free-energy changes at the metal-lipid surface and at the lipid-aqueous solution interface [4,13]. Our measurements of the electrical properties of supported lipid bilayers described here are consistent with those of conventional BLMs and closely related systems. 

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