The Electrical Double Layer. A Deeper Examination

The charge on the metallic electrode, qM, (which, depending on the potential applied, can be negative or positive compared to the charge of the solution, qs) is considered to be concentrated in a layer of about 0.1 A close to the electrode surface. 

By contrast, the charge of the solution, qs, is distributed in a number of layers. The layer in contact with the electrode, called the internal layer, is largely composed of solvent molecules and in a small part by molecules or anions of other species, that are said to be specifically adsorbed on the electrode. As a consequence of the particular bonds that these molecules or anions form with the metal surface, they are able to resist the repulsive forces that develop between charges of the same sign. This most internal layer is also defined as the compact layer. The distance, xj, between the nucleus of the specifically adsorbed species and the metallic electrode is called the internal Helmholtz plane (IHP). The ions of opposite charge to that of the electrode, that are obviously solvated, can approach the electrode up to a distance of x2, defined as the outer Helmholtz plane (OHP). 

As the electrostatic interaction between the solvated ions and the metal is indirect, it is virtually independent of the chemical nature of the ions these latter are said to be non-specifically adsorbed. 

The region of solution between the OHP and the bulk of the solution is called the diffusion layer. The majority of the charge present in solution resulting from the applied electrode potential resides in this layer. 

The potential difference (4 M - J S), known as the interface potential, cannot be measured, in that it would need the insertion of another electrode, i.e. another interface. What one can measure is the electrochemical potential of the electrode, E, with respect to a reference electrode obviously every change of ( J M — s) is reflected in a variation of E, or vice versa. 

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