Helmholtz, Gouy-Chapman, Stern, and Grahame

The first models Helmholtz, Gouy-Chapman, Stern and Grahame Helmholtz Model (1879) 

The two principal defects of this model are first that it neglects interactions that occur further from the electrode than the first layer of adsorbed species, and secondly that it does not take into account any dependence on electrolyte concentration. 

At the beginning of this century Gouy13 and Chapman13 independently developed a double layer model in which they considered that the applied potential and electrolyte concentration both influenced the value of the double layer capacity. Thus, the double layer would not be compact as in Helmholtz s description but of variable thickness, the ions being free to move (Fig. 3.6a). This is called the diffuse double layer. 

In this model, the distribution of species with distance from the electrode obeys Boltzmann s law 

The Poisson equation relates the potential with the charge distribution 

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The next question concerns how these excess charges are distributed on the metal and solution sides of the interphase. We discuss these topics in the next four sections. Four models of charge distribution in the solution side of the interphase are discussed the Helmholtz, Gouy-Chapman, Stern, and Grahame models. 

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