Bockris, Devanathan, and Muller model

More recent models of the double layer have taken into account the physical nature of the interfacial region. In dipolar solvents, such as water, it is clear that an interaction between the electrode and the dipoles must exist. That this is important is reinforced by the fact that solvent concentration is always much higher than solute concentration. For example, pure water has a concentration of 55.5 mol dm-3. 

The Bockris, Devanathan, and Muller model16 recognizes this situation and shows the predominance of solvent molecules near the interface (Fig. 3.9). The solvent dipoles are oriented according to the electrode charge where they form a layer together with the specifically adsorbed ions. 

Regarding the electrode as a giant ion, the solvent molecules form its first solvation layer the IHP is the plane that passes through the centre of these dipoles and specifically adsorbed ions. In a similar fashion, OHP refers to adsorption of solvated ions that could be identified with a second solvation layer. Outside this comes the diffuse layer. Note that the actual profile of electrostatic potential variation with distance (Fig. 3.9b) is the same in qualitative terms as in the Grahame model (Fig. 3.8b). 

These authors also defined a shear plane, not necessarily coincident with the outer Helmholtz plane, which is extremely important in electrokinetic effects (Section 3.7). The shear plane limits the zone where the rigid holding of ions owing to the electrode charge ceases to operate. The potential of this plane is called the zeta or electrokinetic potential, f. 

The concept of double layer structure is far from being well established and evaluated. The models presented above give emphasis to electrostatic considerations. Chemical models have been developed that consider the electronic distribution of the atoms in the electrode, which is related to their work function. This was only possible after experimental 

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