Adsorption at the Electrode-Electrolyte Interface

In the previous section the distance of closest approach of ions to a planar electrode-electrolyte interface was discussed. In solid-electrolyte systems, this distance is assumed to be approximately the radius of the mobile ion. In the presence of a polar solvent the hydration sheath of the ion and the solvent layer adjacent to the metal are also important. The only forces acting on the interface have been assnmed to be electrostatic in origin. These forces orient the solvent dipoles and detennine the distribution of ions with distance from the interface. 

It is possible, however, that an ion can interact chemically with the electrode material. If this happens the ion may break through the solvent layers or, as in the case of the solid, become displaced from a normal lattice site. This possibility is known as specific adsorption. In aqueous electrochemistry the locus of the centers of the specifically adsorbed ions is known as the inner Helmholtz plane. Neutral molecules may also adsorb and hence affect the faradic current, for example by blockage of the reaction sites. Neutral molecule effects have not been studied in the case of solid systems and will therefore not be considered further. 

In order to include adsorption in a discussion of the electrical response, it is necessary to know the relationship between the surface concentration of the adsorbed species and the concentration in the elecbolyte just outside the double layer. This last concentration can then be related to the bulk or average concentration through appropriate diffusion equations. 

For a neutral molecule, potential dependence will still be expected, since at large potential differences the force acting on the dipole of a polar solvent will be sufficient to compete with all but the strongest adsorption bonds. 

A simple isotherm, due originally to Langmuir, assumes that the free energy of adsorption AG° is the same all over the surface and that interactions between adsorbed species are neglected. Under these conditions, the surface concentration E, is related to the surface concentration at full coverage Eq by the expression 

---