Inner-layer Helmholtz

To a first approximation, the ions in both Helmholtz layers can be considered point charges. They induce an equal and opposite image charge inside the conductive electrode. When the electrode is negative to the point of zero charge, cations populate the inner Helmholtz layer. 

The model is most vulnerable in the way it accounts for the number of particles that collide with the electrode [50, 115], In the model, the mass transfer of particles to the cathode is considered to be proportional to the mass transfer of ions. This greatly oversimplifies the behavior of particles in the vicinity of an interface. Another difficulty with the model stems from the reduction of the surface-bound ions. Since charge transfer cannot take place across the non-conducting particle-electrolyte interface, reduction is only possible if the ion resides in the inner Helmholtz layer [116]. Therefore, the assumption that a certain fraction of the adsorbed ions has to be reduced, implies that metal has grown around the particle to cover an identical fraction of the surface. Especially for large particles, it is difficult to see how such a particle, embedded over a substantial fraction of its diameter, could return to the plating bath. Moreover, the parameter itr, that determines the position of the codeposition maximum, is an artificial concept. This does not imply that the bend in the polarisation curve that marks the position of itr is illusionary. As will be seen later on, in the case of copper, the bend coincides with the point of zero-charge of the electrode. 

IHP). The layer between the electrode interface and the IHP is called the inner Helmholtz layer, and the layer between the IHP and the OHP is called the outer Helmholtz layer. In general, the interfacial chemisorption of dehydrated ions takes place at the IHP, and the physisorption of hydrated ions takes place at the OHP. 

Ionic contact adsorption on metallic electrodes alters the potential profile across the compact layer at constant electrode potential. If anions are adsorbed on the metal electrode at positive potentials, the adsorption-induced dipole generates a potential across the inner Helmholtz layer (IHL) as illustrated in Fig. 5-29. The electric field in the outer part (OHL) of the compact layer, as a result, becomes dififerent fi om and frequently opposite to that in the inner part (IHL) of the compact layer. 

In general, the contact adsorption of anions creates an electric field of intensity Sto in the inner Helmholtz layer, which may be greater than the average field intensity of. B,v = ( in +. Eout)/2, where aat is the field intensity in the outer Helmholtz layer. The rate of field increase rj may be derived from electrostatics as shown in Eqn. 5-50 [Liu, 1983] ... 

As the field intensity in the inner Helmholtz layer becomes extremely high, the field intensity E in the outer Helmholtz layer is reversed as shown in Fig. 5-29. Figure 5-30 illustrates the potential profile across the interfacial double layer of a mercury electrode in an aqueous chloride solution this result was obtained by calculations at various electrode potentials ranging fi om negative (cathodic) to positive (anodic) potentials. 

Since the electron transfer of the interfacial redox reaction, + cm = H.a> on electrodes takes place between the iimer Helmholtz plane (adsorption plane at distance d ) and the electrode metal, the ratio of adsorption coverages 0h,j/ in electron transfer equilibrium (hence, the charge transfer coefficient, 6z) is given in Eqn. 5-58 as a function of the potential vid /diOMn across the inner Helmholtz layer ... 

Ions with a weak solvation shell, anions in general, lose a part of or the complete solvation shell in the double layer and form a chemical bond to the metal surface. The adsorption is termed specific since the interaction occurs only for certain ions or molecules and is not related to the charge on the ion. The plane where the center of these ions are located is called the inner Helmholtz layer. In the specific adsorption, ions are chemically bound to the surface and the interaction has a covalent nature. In the case of non-specific adsorption, in which an electrostatic force binds ions to the surface, the coverage of ions is below 0.1 -0.2 ML due to electrostatic repulsion between the ions. In contrast, the coverage of specifically adsorbed ions exceeds this value, and a close-packed layer of specifically adsorbed ions is often observed. Specifically adsorbed ions are easily observed by STM [22], indicating that the junction between the electrode surface and the inner Helmholtz layer is highly... 

The term A GE represents the electrical work done in moving an ion of charge ze0 and water molecules with dipole moments fi between the outer Helmholtz layer and the inner Helmholtz layer in the electric field, X, arising from the charge of the metal (Section 6.8.2.1). Thus, it can split into AGE - AG i + AG w. If some transfer of charge (Section 6.8.2.1) occurs during the adsorption process, Eq. (6.210) can be written as... 

If we also consider the role of adsorption on the distribution of products, then we should note that surface concentrations of substrate and intermediate(s) must be taken into account, i.e. their concentrations in the inner Helmholtz layer (cf. for example, Wendt, 1973). One effect of this would possibly be to... 

As already noted (p. 30) effects of surface concentrations are really effects of adsorption, since it is the adsorption properties of the components of the electrolyte solution that influence the structure of the inner Helmholtz layer. 

These terms are based on a simple geometric model of the interface. One distinguishes between an inner and an outer Helmholtz layer. The inner Helmholtz layer comprises all species that are specifically adsorbed on the electrode surface. If only one type of molecule or ion is adsorbed, and they all sit in equivalent positions, then their centers define the inner Helmholtz plane. The outer Helmholtz layer comprises the ions that are closest to the electrode surface, but are not specifically adsorbed. They have kept their -> solvation spheres intact, and are bound only by electrostatic forces. If all these ions are equivalent, their centers define the outer Helmholtz plane. 

Inner Helmholtz Layer Outer Helmholtz Gouy-Chapman (diffuse) Layer... 

The solvent molecules form an oriented parallel, producing an electric potential that is added to the surface potential. This layer of solvent molecules can be protruded by the specifically adsorbed ions, or inner-sphere complexed ions. In this model, the solvent molecules together with the specifically adsorbed, inner-sphere complexed ions form the inner Helmholtz layer. Some authors divide the inner Helmholtz layer into two additional layers. For example, Grahame (1950) and Conway et al. (1951) assume that the relative permittivity of water varies along the double layer. In addition, the Stern variable surface charge-variable surface potential model (Bowden et al. 1977, 1980 Barrow et al. 1980, 1981) states that hydrogen and hydroxide ions, specifically adsorbed and inner-sphere... 

A more sophisticated model is the triple-layer model, allowing the surface reaction of the background electrolyte (Hayes et al. 1991). The potential-determining ions (hydrogen and hydroxide) are directly on the surface (inner Helmholtz layer), the other ions are at a certain distance from the surface (outer Helmholtz layer), and there is a diffuse layer, also. 

M+and Lr adsorbed as inner-sphere complexes, entering the inner Helmholtz layer... 

For an organic ion the same reasoning can be applied in fact this can be done in any model. For specifically adsorbed organic ions v (x) becomes V where it is noted that if Inorganic Ions also adsorb specifically each type may have its own inner Helmholtz layer potential. Calling this contribution "coulombic". 

When the flow lines can pass through the particle, a similar equation can be given for the boundary between the particle (p) and the inner Helmholtz layer... 

Apportioning the potential distributed across the oxide film, the inner Helmholtz layer and the outer Helmholtz layer, and assuming AV p to be constant with current based on dV/dq plots, b values of 120 mV were rationalized. A dK/d pH of -2.3RT/Fwasattributed topHdependenceof APo p, which results in R + = 1/2. (OHP = outer Helmholtz plane.)... 

Electrode Surface The electrode surface is a complicated heterogeneous system as Fig. 4 shows. There is a net negative charge nearest the surface within a metal acting as a cathode. To maintain electroneutrality, there are adsorbed ions and water molecules next to the surface in the electrolyte side. Toward the bulk electrolyte there are cations surrounded by water molecules. This is the Helmholtz double layer. The inner Helmholtz layer contains water molecules and specifically adsorbed anions. The outer... 

All electrochemical phenomena occur in these thin layers whose composition and properties differ from those of the bulk electrolyte. The inner Helmholtz layer is... 

The potential drop over the inner Helmholtz layer and the outer Helmholtz layer can be described by two capacitances the inner and the outer Helmholtz layer capacitance. 

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