Metal-solution interphase charging

The simplest model of the structure of the metal-solution interphase is the Helmholtz compact double-layer model (1879). According to this model, all the excess charge... 

Equality of i and i on an atomic scale means that a constant exchange of charge carriers (electrons or ions) takes place process the metal-solution interphase. Figure 6.3... 

The simplest model of the structure of the metal-solution interphase is the Helmholtz compact double-layer model (1879). According to this model, all the excess charge on the solution side of the interphase, qs. is lined up in the same plane at a fixed distance away from the electrode, the Helmholtz plane (Fig. 4.4). This fixed distance xH is determined by the hydration sphere of the ions. It is defined as the plane of the centers of the hydrated ions. All excess charge on the metal, qM, is located at the metal surface. 

Figure 1. Schematic picture of the metal/solution interphase in the case of nonspecific (a) and specific (b) anionic adsorption, x = 0, x = P and x = d are die electrode surface plane, the plane of closest approach for the specifically adsorbed anions, and that for the nonspecifically adsorbed ions. Curve 1 represents the potential-distance profile. In (b), curve 1 results from the combination of curve 2, expressing die contribution from the charge density as of the specifically adsorbed anions, and curve 3, expressing die contribution from die charge density Om on the metal. The potential difference, ft1 — d> across die inner layer is the same in (a) and (b). (Reprinted from Ref.7 with permission from the Am. Chem. Soc.)...

One such properly is the capacitance, which is observed whenever a metal-solution interphase is formed. This capacitance, called the double layer capacitance, is a result of the charge separation in the interphase. Since the interphase does not extend more than about 10 nm in a direction perpendicular to the surface (and in concentrated solutions it is limited to 1.0 nm or less), the observed capacitance depends on the structure of this very thin region, called the double layer. If the surface is rough, the double layer will follow its curvature down to atomic dimensions, and the capacitance measured under suitably chosen conditions is proportional to the real surface area of the electrode. 

When the current does not flow through battery the measurable diflerence in electric potential between the terminals of the two electrodes is the result of all the equilibrium potential differences at the interphase between the conducting phases in contact. In the example of the Daniell cell, with both electrodes having copper terminals, there are three interfacial potential differences (apart from the small liquid junction potential difference at the contact between the two electrolyte phases) one potential difference at the contact between the zinc rod and the copper terminal (Zn/Cu) and two potential differences at the metal-solution interphases (Zn/Zn + and Cu/Cu +), which are mainly due to the charge transfer processes. 

When an electrode is at equilibrium, the equilibrium partial current densities i and i are equal and they are designated by one symbol, i0. This equality on an atomic scale means that a constant exchange of charge carriers (electrons or ions) takes place across the metal-solution interphase (Fig. 1). When the interphase is not in equilibrium, a net current density i flows through the electrode (the double layer). It is given by the difference between the anodic partial current density i (a positive quantity) and the cathodic partial current density i (a negative quantity) ... 

The rigoroiis analysis of the effect of temperature variations on interfacial properties is a key tool to provide new and valuable information on the structure and reactivity of the metal solution interphase. The entropy of the components that form the interphase is a unique probe of their stmctural properties. Therefore, this experimental data is particularly useful for the validation of molecular models of electrified interphases. In addition, the use of fast temperature perturbations is especially suitable for the selective characterization of different inter-facial components, based on their different response time towards the temperature change. In this way, the entropic properties of doublelayer phenomena and charge-transfer adsorption processes can be evaluated separately. It will be shown in this chapter that the combina-... 

The Chapter by N. Garcia, V. Climent and J. Feliu provides a lucid and authoritative overview of the use of laser-pulsed induced temperature variations at the platinum single-ciystal/aqueous solution interphases and of the rigorous analysis of these experiments via Gibbs thermodynamics to extract new and very valuable information on the stracture and reactivity of the metal/solution interphase. The authors show how some key interfacial properties can be evaluated directly via this elegant analysis, such as the entropy of charge-transfer adsoibed species, the entropy of formation of the interfacial water network and the potential of water reorientation. 

Apart from the most electropositive metals, most other metals extracted through molten salt routes are recovered as solids these include many important refractory and other transition metals, the lanthanides, and some actinides. Particularly interesting problems arise in the electrowinning of the refractory metals. Attempts to deposit these metals in a coherent, massive form of theoretical density usually meet with a number of difficulties. Deposits may be dendritic, for example, if electrodeposition proceeds under mass transfer control, or they may be powdery and nonadherent if secondary reactions, such as alkali metal deposition, followed by backreaction with the solute, occurs. Moreover, powdery deposits may also arise if low oxidation states, formed as intermediates during the reduction process, disproportionate in the metal-melt interphase. Charge-transfer-controlled electrodeposition or coupled chemical steps appear to be a prerequisite for obtaining dense, coherent, and adherent deposits. Such deposits have been obtained... 

Charging of Interphase. Let us consider a case where a metal M is immersed in the aqueous solution of its salt, MA. Both phases, metal and the ionic solution, contain ions, as discussed earlier. At the metal-solution interface (physical boundary) there will be an exchange of metal ions M+ between the two phases (Fig. 4.2). 

In the case of a metal/solution interface, the charge on the metal is one of the signals that can be picked up. This electrode charge is mirrored on the solution side by an equal and opposite net charge constituted of separate contributions of the positive and negative charges, i.e., the relative concentrations of cations and anions in the interphase. However, are these ions on the metal or near the metal ... 

In the mechanisms to be described in this section, one of the idealizations of electrochemistry is being portrayed. Thus, in perfectly polarizable metal electrodes, it is accepted that no charge passes when the potential is changed. However, in reality, a small current does pass across a perfectly polarizable electrode/solution interphase. In the same way, here the statement free from surface states (which has been assumed in the account given above) means in reality that the concentration of surface states in certain semiconductors is relatively small, say, less than 10 states cm. So when one refers to the low surface state case, as here, one means that the surface of the semiconductor, particularly in respect to sites energetically in the energy gap, is covered with less than the stated number per unit area. A surface absolutely free of electronic states in the surface is an idealization. (If 1012 sounds like a large number, it is in fact only about one surface site in a thousand.) A consequence of this is the location of the potential difference at the interphase of a semiconductor with a solution. As shown in Fig. 10.1(a), the potential difference is inside the semiconductor, and outside in the solution there is almost no potential difference at all. 

At the metal/liquid interphase, the conversion from electronic to ionic conduction occurs. The electrode metal is the source or sink of electrons, and electron transfer is the key process whereby the electrode exchanges charges with the arriving ions, or ionizes neutral substances (a second mechanism of charge transfer is by oxidation of the electrode metal the metal leaves the surface as charged cations and enters the solution). Without electron transfer, there is no chemical electrode reaction, no DC electrode current, and no faradaic current. In the solution at the electrode surface, the electric double layer is formed as soon as the metal is wetted. Electron transfer takes place somewhere in the double layer. 

The surface tension of an electrode in contact with an electrolyte depends on the metal-solution potential difference, A ( ). The equation describing this dependence is called the electrocapiilary equation. It follows by simple logic from the Gibbs adsorption isotherm. Thus, the sum X Tjdp in Eq. (9.6) should represent the surface excess (or deficiency, i.e. negative surface excess) of all the species in the interphase. On the solution side there are terms of the type (Fcr dpd-) and (Frh dp,RH) charged and neutral species, respectively, where the subscript "RH stands for an unspecified organic molecule). On the metal side, the surface excess is... 

At any interface between two different phases there will be a redistribution of charge in each phase at the interface with a consequent loss of its electroneutrality, although the interface as a whole remains electrically neutral. (Bockris considers an interface to be sharp and definite to within an atomic layer, whereas an interphase is less sharply defined and may extend from at least two molecular diameters to tens of thousands of nanometres the interphase may be regarded as the region between the two phases in which the properties have not yet reached those of the bulk of either phase .) In the simplest case the interface between a metal and a solution could be visualised as a line of excess electrons at the surface of the metal and an equal number of positive charges in the solution that are in contact with the metal (Fig. 20.2). Thus although each phase has an excess charge the interface as a whole is electrically neutral. 

The presence of electrical charge affects the interfacial tension in the interphase. If one of the phases considered is a metal and the other is an electrolyte solution, then the phenomena accompanying a change in the interfacial tension are included under the term of electrocapillarity. 

In Chapters 2 and 3 we have described basic structural properties of the components of an interphase. In Chapter 2 we have shown that water molecules form clusters and that ions in a water solution are hydrated. Each ion in an ionic solution is surrounded predominantly by ions of opposite charge. In Chapter 3 we have shown that a metal is composed of positive ions distributed on crystal lattice points and surrounded by a free-electron gas which extends outside the ionic lattice to form a surface dipole layer. 

The next question concerns how these excess charges are distributed on the metal and solution sides of the interphase. We discuss these topics in the next four sections. Four models of charge distribution in the solution side of the interphase are discussed the Helmholtz, Gouy-Chapman, Stern, and Grahame models. 

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