Gouy-Chapman diffuse charge, metal-solution

Some emphasis is given in the first two chapters to show that complex formation equilibria permit to predict quantitatively the extent of adsorption of H+, OH , of metal ions and ligands as a function of pH, solution variables and of surface characteristics. Although the surface chemistry of hydrous oxides is somewhat similar to that of reversible electrodes the charge development and sorption mechanism for oxides and other mineral surfaces are different. Charge development on hydrous oxides often results from coordinative interactions at the oxide surface. The surface coordinative model describes quantitatively how surface charge develops, and permits to incorporate the central features of the Electric Double Layer theory, above all the Gouy-Chapman diffuse double layer model. 

Fig. 6.64. The Gouy-Chapman model, (a) The excess charge density on the OHP is smaller in magnitude than the charge on the metal. The remaining charge is distributed in the solution. The diffuse charge region, (b), can be simulated by a sheath of charge gd placed at a distance k 1 from the x = 0 plane, as depicted in (c).

Gouy-Chapman (GC), that is, to consider the whole interphase as consisting of two layers, a compact (or inner or Helmholtz ) and a diffuse one. The former corresponds to the aforementioned hypothesis of an ion-free layer of the solvent at the metal surface. The diffuse layer is located between the compact one and the bulk solution and the whole counterion charge is distributed inside this region in accordance with the GC theory. 

The chemical system at the surface will be quite complex. There will be strongly bound species at various sites on the metal surface. Where these are ionic, the electric field established at the surface will tend to attract ions of opposite charge from the solution. The first layer has been termed the electrode double layer, while the gegenion distribution in the solution is called the diffuse double layer. A theoretical analysis of the double layer has been made by Gouy and Chapman and adapted to kinetic analysis by Stern. For references, and discussion see paper by D. C. Grahame, J. Chem, Phys, 21, 1054 (1953). 

Gouy and Chapman realised that in an electrolyte solution, the charges are free to move and are subject to thermal motion. They retained the concepts of the electrostatic theory to describe the coulombic metal-counter ion interaction but, in addition, they allowed for the random motion of the ions. The result is a diffuse layer of charge in which the concentration of counter ions is greatest next to the electrode surface and decreases progressively towards a homogeneous distribution of ions within the bulk electrolyte (Fig. 5.1b). 

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