Compact layer, of ions

The expressions for the rates of the electrochemical reactions given in Section II. A have not taken into account the detailed structure of the interfacial region. In general, the solution adjacent to the electrode will consist of at least two regions. Immediately adjacent to the metal there will be a compact layer of ions and solvent molecules which behaves as a capacitor. A potential difference will be established between... 

Stern15 combined the Helmholtz model for values of potential far from Ez with the Gouy-Chapman model for values close to Ez (Fig. 3.7ayb). He considered that the double layer was formed by a compact layer of ions... 

Historically, the first elue for the existenee of such double layer emerged from Helmholtz s studies, in 1879. His first theory assumed the presenee of a compact layer of ions in direct contact with the eharged electrode surfaee. This first proposal was followed by the eoneepts of Gouy and Chapman (1913), who adapted the previously described model to... 

One can write acid-base equilibrium constants for the species in the inner compact layer and ion pair association constants for the outer compact layer. In these constants, the concentration or activity of an ion is related to that in the bulk by a term e p(-erp/kT), where yp is the potential appropriate to the layer [25]. The charge density in both layers is given by the algebraic sum of the ions present per unit area, which is related to the number of ions removed from solution by, for example, a pH titration. If the capacity of the layers can be estimated, one has a relationship between the charge density and potential and thence to the experimentally measurable zeta potential [26]. 

The inner layer (closest to the electrode), known as the inner Helmholtz plane (IHP), contains solvent molecules and specifically adsorbed ions (which are not hilly solvated). It is defined by the locus of points for the specifically adsorbed ions. The next layer, the outer Helmholtz plane (OHP), reflects the imaginary plane passing through the center of solvated ions at then closest approach to the surface. The solvated ions are nonspecifically adsorbed and are attracted to the surface by long-range coulombic forces. Both Helmholtz layers represent the compact layer. Such a compact layer of charges is strongly held by the electrode and can survive even when the electrode is pulled out of the solution. The Helmholtz model does not take into account the thermal motion of ions, which loosens them from the compact layer. 

Specific adsorption occurs, i.e. ions enter the compact layer, in a considerable majority of cases. The most obvious result of specific adsorption is a decrease and shift in the maximum of the electrocapillary curve to negative values because of adsorption of anions (see Fig. 4.2) and to positive values for the adsorption of cations. A layer of ions is formed at the interface only when specific adsorption occurs. 

An adsorbed layer of water molecules at the interface separates hydrated ions from the solid surface. The interfacial electric double layer can be represented by a condenser model comprising three distinct layers a diffuse charge layer in the ionic solution, a compact layer of adsorbed water molecules, and a diffuse charge layer in the solid as shown in Fig. 5-8. The interfacial excess charge on the... 

In the absence of specific adsorption of anions, the GCSG model regards the electrical double layer as two plate capacitors in series that correspond respectively, to two regions of the electrolyte adjacent to the electrode, (a) An inner compact layer of solvent molecules (one or two layers) and immobile ions attracted by Coulombic forces (Helmholtz inner plane in Fig. 2). Specific adsorption of anions at the electrode surface may occur in this region by electronic orbital coupling with the metal, (b) An outer diffuse region of coulombically attracted ions in thermal motion that complete the countercharge of the electrode. 

The modeling approaches used to describe the surface reactions of metal ions differ in their definition of surface structure and the charge/potential relationships within the compact layer of the EDL ( 2, 5,. In our previous calculations ( 2) we... 

We cannot be certain whether Equation 15 or 16 should be compared with the acidity of the adsorbed Cu(II) ion. Equation 14. The surface bond will undoubtedly affect hydrolysis of an adsorbed Cu(II) ion, and perhaps release of the second proton from the hydration sheath of Cu (aq) may be more appropriate for comparison. Such comparisons are of limited value, however, since uncertainty in the available thermodynamic data for hydrolysis of metal ions is rather large (30). Nonetheless, with the present data it would appear that metal ions are more easily hydrolyzed within the compact layer of the EDL. 

Refinement of the model awaits further experimental work on the physico-chemical nature of surface bonding of ions within the compact layer of the EDL. At present our conclusions concerning the speciation of adsorbed ions are supported by 1) enthalpy/ entropy arguments for analogous reactions in solution, and 2) a limited knowledge of the solvent medium of the compact layer of the EDL. 

When the electrolyte concentration, c, is low, while the concentration of colloidal particles, n, and their effective charge are high, i.e. when c q nleNA, the value of x is close to the initial electrolyte concentration. In the other words, under these conditions essentially all of the electrolyte should transfer into pure dispersion medium. This means that in the case of highly developed diffuse layers of ions and rather compact arrangement of particles, when the ionic atmospheres come into contact, the co-ions (Na+ in the present... 

The passivation of zinc depends on the pH of the pore solution. In contact with alkaline solutions, as long as the pH remains below 13.3, zinc can passivate due to formation of a layer of calcium hydroxyzincate. Figure 15.6 shows the typical corrosion rate of zinc as a function of pH however, even at pH values higher than 12, in the presence of calcium ions, such as in concrete pore solution, zinc can be passive and has a very low corrosion rate. In saturated calcium hydroxide solutions it was found that for pH values up to about 12.8 a compact layer of zinc-corrosion products forms, which will protect the steel even if the pH changes in a subsequent phase. For pH values between 12.8 and 13.3, larger crystals form that can still passivate the bar. Finally, for values above 13.3, coarse corrosion products form that cannot prevent corrosion. 

In a certain sense, interfaces between two liquids are simpler than those between a metal and a liquid, since they do not involve solid-state properties. However, for a long time the structure of liquid-liquid interfaces has remained a matter of controversy. Essentially, there are two different views one holds that the interface is sharp and contains a compact layer of solvent molecules into which the ions cannot penetrate. The other view posits the... 

When a metal electrode is placed in an electrolyte solution, an equilibrium difference usually becomes established between the metal and solution. Equilibrium is reached when the electrons left in the metal contribute to the formation of a layer of ions whose charge is equal and opposite to that of the cations in solution at the interface. The positive charges of cations in the solution and the negative charges of electrons in the metal electrode form the electrical double layer [4]. The solution side of the double layer is made up of several layers as shown in Fig. 2.7. The inner layer, which is closest to the electrode, consists of solvent and other ions, which are called specifically adsorbed ions. This inner layer is called the compact Helmholtz layer, and the locus of the electrical centers of this inner layer is called the inner Helmholtz plane, which is at a distance of di from the metal electrode surface. The solvated ion can approach the electrode only to a distance d2. The locus of the centers of the nearest solvated ion is called the outer Helmholtz plane. The interaction of the solvated ion with metal electrode only involves electrostatic force and is independent of the chemical properties of the ions. These ions are called non-specifically adsorbed ions. These ions are distributed in the 3D region called diffusion layer whose thickness depends on the ionic concentration in the electrolyte. The structure of the double layer affects the rate of electrode reactions. 

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