Potential distribution, double layer interface

Fig. 5.38. A. (a) An electrical double layer and (b) an electrical triple layer. B. Potential distribution at the interface. OHP = Outer Helmholtz Plane, IHP = Inner HP, <1> = Galvani potential.

The double layer acts to neutralize the surface charge creating an electrokinetic potential distribution between the interface and the solution. This voltage difference is on the order of millivolts and is referred to as the surface... 

The formation of a double layer has considerable consequences for the chaise and potential distribution across the interface. In the case of a metal electrode the counter charges are located just below the surface. Since, however, the carrier density in a semiconductor is usually much smaller than in a metal electrode, the counter charges can be distributed over a considerable distance below the interface, that is, a space charge layer is formed, similar to that in pure solid devices (compare with Chapter 2). The potential and charge distribution across the Helmholtz layer, Gouy layer, and space charge region will be treated separately in... 

Fig. 20.38 Illustration of the electric potential distribution (< ) across the interface between a metal electrode (Me) coated with a neutral polymer (poly) film and an electrolyte solution containing ions that are partitioned into the polymer. El, E2, E3 are three applied electrode potentials at which the polymer is neither oxidized nor reduced. The electrical double layers (1) and (2) correspond to two different bulk concentrations of ions in the polymer, d.p. is the distribution potential.

When two conducting phases come into contact with each other, a redistribution of charge occurs as a result of any electron energy level difference between the phases. If the two phases are metals, electrons flow from one metal to the other until the electron levels equiUbrate. When an electrode, ie, electronic conductor, is immersed in an electrolyte, ie, ionic conductor, an electrical double layer forms at the electrode—solution interface resulting from the unequal tendency for distribution of electrical charges in the two phases. Because overall electrical neutrality must be maintained, this separation of charge between the electrode and solution gives rise to a potential difference between the two phases, equal to that needed to ensure equiUbrium. 

On the electrode side of the double layer the excess charges are concentrated in the plane of the surface of the electronic conductor. On the electrolyte side of the double layer the charge distribution is quite complex. The potential drop occurs over several atomic dimensions and depends on the specific reactivity and atomic stmcture of the electrode surface and the electrolyte composition. The electrical double layer strongly influences the rate and pathway of electrode reactions. The reader is referred to several excellent discussions of the electrical double layer at the electrode—solution interface (26-28). 

Previous considerations have shown that the interface between two conducting phases is characterised by an unequal distribution of electrical charge which gives rise to an electrical double layer and to an electrical potential diflFerence. This can be illustrated by considering the transport of charge (metal ions or electrons) that occurs immediately an isolated metal is immersed in a solution of its cations ... 

Fig. 4.1 Structure of the electric double layer and electric potential distribution at (A) a metal-electrolyte solution interface, (B) a semiconductor-electrolyte solution interface and (C) an interface of two immiscible electrolyte solutions (ITIES) in the absence of specific adsorption. The region between the electrode and the outer Helmholtz plane (OHP, at the distance jc2 from the electrode) contains a layer of oriented solvent molecules while in the Verwey and Niessen model of ITIES (C) this layer is absent...

The distribution of potential in TC is practically the same as that near the flat surface if the electrolyte concentration is about 1 mol/1 [2], So the discharge of TC may be considered as that of a double electric layer formed at the flat electrode surface/electrolyte solution interface, and hence, an equivalent circuit for the TC discharge may be presented as an RC circuit, where C is the double layer capacitance and R is the electrolyte resistance. 

The argument for the formation of the double layer has proceeded simply. The existence of a boundary for the electrolyte necessarily implies a basic anisotropy in the forces operating on the particles in the intcrphase region. Owing to this anisotropy, there occurs a redistribution of the mobile charges and orientable dipoles (compared with their distribution in the bulk of the phases). This redistribution is the structural basis of the potential difference across the interface. 

There is a functional relationship between the charge on each phase (or the potential difference across the interface) and the structure of the interphase region. The fundamental problem of double-layer studies is to unravel this functional relationship. One has understood a particular electrified interface if, on the basis of a model (i.e., an assumed type of arrangement of the particles in the interphase), one can predict the distribution of charge (or variation of potential) across the interphase. 

The previous section discussed the structure at the junction of two phases, the one a solid electron conductor, the other an ionic solution. Why is this important Knowledge of the structure of the interface, the distribution of particles in this region, and the variation of the electric potential in the double layer, permits one to control reactions occurring in this region. Control of these reactions is important because they are the foundation stones of important mechanisms linked to the understanding of industrial processes and problems, such as deposition and dissolution of metals, corrosion, electrocatalysis, film formation, and electro-organic synthesis. 

Fig. 3. The structure of electrical double layer at a semiconductor-electrolyte interface (a) and the distribution of the potential (b) and charge (c) at the interface. The electrode is charged negatively. is the space-charge region thickness, La is the Helmholtz layer thickness, Qlc and Qtl are the charge of the semiconductor and ionic plates of the double layer, respectively (for further notations see the text).

---