Gouy-Chapman shell

The two limiting cases are again specific binding and atmospheric condensation. Specific binding, as exemplified by the divalent cations Ca and Mg ", Implies entrapment within a Stern layer and attachment to a specific group, such as carboxylate or phosphate. Atmospheric condensation refers to the presence of the fully hydrated Ion In a diffuse double layer or In a Gouy-Chapman shell. 

Figure 2.13 illustrates what is currently a widely accepted model of the electrode-solution interphase. This model has evolved from simpler models, which first considered the interphase as a simple capacitor (Helmholtz), then as a Boltzmann distribution of ions (Gouy-Chapman). The electrode is covered by a sheath of oriented solvent molecules (water molecules are illustrated). Adsorbed anions or molecules, A, contact the electrode directly and are not fully solvated. The plane that passes through the center of these molecules is called the inner Helmholtz plane (IHP). Such molecules or ions are said to be specifically adsorbed or contact adsorbed. The molecules in the next layer carry their primary (hydration) shell and are separated from the electrode by the monolayer of oriented solvent (water) molecules adsorbed on the electrode. The plane passing through the center of these solvated molecules or ions is referred to as the outer Helmholtz plane (OHP). Beyond the compact layer defined by the OHP is a Boltzmann distribution of ions determined by electrostatic interaction between the ions and the potential at the OHP and the random jostling of ions and... 

Stern layers can be introduced at different levels of sophistication. In the simplest case we only consider the finite size effect of the counterions (Fig. 4.5). Due to their size, which in water might include their hydration shell, they cannot get infinitely close to the surface but always remain at a certain distance. This distance <5 between the surface and the centers of these counterions marks the so-called outer Helmholtz plane. It separates the Stern from the Gouy-Chapman layer. For a positively charged surface this is indicated in Fig. 4.5. 

However, this is actually more complex. The ions that are not adsorbed (generally cations) are kept at a certain distance from the electrode by their solvation shell and by a layer of solvent molecules adsorbed on the electrode. Conversely, those that are specifically adsorbed (anions) are directly in contact with the electrode surface. Therefore, there are two Helmhotz planes. On the other hand, the ions accumulated close to the electrode are under the influence of the thermal movement. They constitute the diffuse layer (or Gouy-Chapman layer). 

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