Helmholtz double layer, inner

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... 

The simplest model for the electrical double layer is the Helmholtz condenser. A distribution of counterions in the bulk phase described by a Boltzmann distribution agree with the Gouy-Chapman theory. On the basis of a Langmuir isotherm Stem (1924) derived a generalisation of the double layer models given by Helmholtz and Gouy. Grahame (1955) extended this model with the possibility of adsorption of hydrated and dehydrated ions. This leads to a built-up of an inner and an outer Helmholtz double layer. Fig. 2.14. shows schematically the model of specific adsorption of ions and dipoles. 

Helmholtz Double Layer A simplistic description of the electric double layer as a condenser (the Helmholtz condenser) in which the condenser plate separation distance is the Debye length. The Helmholtz layer is divided into an inner Helmholtz plane (IHP) of adsorbed, dehydrated ions immediately next to a surface, and an outer Helmholtz plane (OHP) at the center of a next layer of hydrated, adsorbed ions just inside the imaginary boundary where the diffuse double layer begins. That is, both Helmholtz planes are within the Stern layer. 

IHP Inner Helmholtz plane, see Helmholtz Double Layer. 

Inner Helmholtz Plane (IHP) See Helmholtz Double Layer. 

The double layer can be divided into two regions the compact double layer that includes the area between the electrode and the plane of closest approach, and the diffuse double layer extending from the plane of closest approach to the bulk of the solution. The compact double layer is also referred to as the Helmholtz double layer or inner double layer [1]. In an outer or diffuse layer the force holding the ions in the interphase is the non-specific Coulombic interaction between the charge on the electrode q (together with that on the inner layer and the charge on the ions [4]. In the inner layer there may or may not be adsorbed ions held partly by Coulombic and partly by specific forces. Potential 02 at the plane of closest approach (separating the inner and outer layers) depends on the electrode potential and concentration of the electrolyte [1]. Potential 0 in the diffuse double layer decreases almost exponentially with distance x from the plane of closest approach, and it can be written [1] ... 

The water molecules sometimes contain the specifically adsorbed anions. The water molecules form the inner Helmholtz layer. The line drawn through the center of these molecules is called the inner Helmholtz plane. The outer Helmholtz plane pz (OHP) represents the locus of the electrical centers of the positive charges. This plane resides at a fixed distance from the metal due to the water molecules that are between the surface of the metal and ions. The outer Helmholtz plane (OHP) is significantly affected by hydrated cations (hydrated). In the simple model of the Helmholtz double layer developed earlier, the adsorption of dipoles was not considered. When a metal surface has a slight excess charge, the dipoles are adsorbed. This process contributes significantly to the potential difference across the double layer. Two factors seriously affect the potential difference across the double layen... 

Pig. 3. Representation of the electrical double layer at a metal electrode—solution interface for the case where anions occupy the inner Helmholtz plane... 

Fig. 1. Schematic representation of the electrochemical or diffuse double layer showing the inner (IHP) and outer (OHP) Helmholtz planes and the...

Fig. 1. The structure of the electrical double layer where Q represents the solvent CD, specifically adsorbed anions 0, anions and (D, cations. The inner Helmholtz plane (IHP) is the center of specifically adsorbed ions. The outer Helmholtz plane (OHP) is the closest point of approach for solvated cations or molecules. O, the corresponding electric potential across the double layer, is also shown.

FIGURE 1-11 Schematic representation of the electrical double layer. IHP = inner Helmholtz plane OHP = outer Helmoltz plane. 

By means of the thermodynamic theory of the double layer and the theory of the diffuse layer it is possible to determine the charge density ox corresponding to the adsorbed ions, i.e. ions in the inner Helmholtz plane, and the potential of the outer Helmholtz plane 2 in the presence of specific adsorption. 

Anions may exhibit a tendency toward specific adsorption at the O/S interface. This may be related in some way to the complexing affinity. This effect, occurring at the inner Helmholtz plane of the electrochemical double layer, may significantly change the charge transfer situation [cf. Section III(5(iii))]. 

For a long time, the electric double layer was compared to a capacitor with two plates, one of which was the charged metal and the other, the ions in the solution. In the absence of specific adsorption, the two plates were viewed as separated only by a layer of solvent. This model was later modified by Stem, who took into account the existence of the diffuse layer. He combined both concepts, postulating that the double layer consists of a rigid part called the inner—or Helmholtz—layer, and a diffuse layer of ions extending from the outer Helmholtz plane into the bulk of the solution. Accordingly, the potential drop between the metal and the bulk consists of two parts ... 

Fig. 5-8. An interfadal double layer model (triple-layer model) SS = solid surface OHP = outer Helmholtz plane inner potential tt z excess charge <2h = distance from the solid surface to the closest approach of hydrated ions (Helmluritz layer thickness) C = electric capacity.

Chemisorption of anions at the electrode interface involves dehydration of hydrated anions followed by adsorption of dehydrated anions which, then, penetrate into the compact double layer to contact the interface directly, this result is called the contact adsorption or specific adsorption. The plane of the contact adsorption of dehydrated anions is occasionally called the inner Helmholtz plane... 

In the course of ionic contact adsorption on the interface of metal electrode, hydrated ions are first dehydrated and then adsorbed at the inner Helmholtz plane in the compact layer as shown in Fig. 5-27 and as described in Sec. 5.6.1. In the interfacial double layer containing adsorbed ions, the combined charge of motal and adsorbed ions = z eF on the metal side is balanced with the... 

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. 

Fig. 6-99. An interfacial electric double layer on semiconductor electrodes a = charge of surface states 0.1 = interfadal charge of adsorbed ions IHP = inner Helmholtz plane.

Figure 5.4 Schematic representation of the double-layer around an electrode, showing the positions of the inner and outer Helmholtz planes, and the way that ionic charges are separated. The circles represent solvated ions.

It is noted that the molecular interaction parameter described by Eq. 52 of the improved model is a function of the surfactant concentration. Surprisingly, the dependence is rather significant (Eig. 9) and has been neglected in the conventional theories that use as a fitting parameter independent of the surfactant concentration. Obviously, the resultant force acting in the inner Helmholtz plane of the double layer is attractive and strongly influences the adsorption of the surfactants and binding of the counterions. Note that surface potential f s is the contribution due to the adsorption only, while the experimentally measured surface potential also includes the surface potential of the solvent (water). The effect of the electrical potential of the solvent on adsorption is included in the adsorption constants Ki and K2. 

Not all ceramic materials behave the same at a given pH, however. As the material begins to dissolve, ions form at the snrface, water molecnles orient themselves accordingly, and an electrical double layer is established, as shown in Eigure 3.18. The first layer of charged ions and oriented water molecules is called the inner Helmholtz plane (IHP), and the second layer of oppositely charged particles is called the outer Helmholtz... 

Figure 3.18 Formation of the electrical double layer of a surface in solution, showing the inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP). Reprinted, by permission, from B. D. Craig, Fundamental Aspects of Corrosion Films in Corrosion Science, p. 4. Copyright 1991 by Plenum Press.

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