Double layer: counter ions description

Conversely, according to the description of the electrical double layer based on the Stern-Gouy-Chapman (S-G-C) version of the theory [24], counter ions cannot get closer to the surface than a certain distance (plane of closest approach of counter ions). Chemically adsorbed ions are located at the inner Helmholtz plane (IHP), while non-chemically adsorbed ions are located in the outer Helmholtz plane (OHP) at a distance x from the surface. The potential difference between this plane and the bulk solution is 1 ohp- In this version of the theory, Pqhp replaces P in all equations. Two regions are discernible in the double layer the compact area between the charged surface and the OHP in which the potential decays linearly and the diffuse layer in which the potential decay is almost exponential due to screening effects. 

Electrostatic stabilization is of importance in solution synthesis as another way to stabilize dispersions.1 1 Colloidal particles almost always have charged surfaces that tend to repel each other. One of the most common charging processes is the adsorption of charged species on the surface of the particle. To maintain electroneutrality, a diffuse cloud of counter ions forms in the fluid around the suspended particle. This phenomenon is described by the diffuse double-layer theory. When the diffuse ion clouds of particles interpenetrate, the particles tend to repel each other electrostatically. The electrostatic repulsive forces are opposed by attractive van der Waals forces that are always present between particles in suspension. The description of the potentials created by these two opposing forces is known as the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The DLVO theory predicts... 

From this work it would appear that the greatest limitation in the understanding of colloidal stability is not the theory of stability, but rather the associated theories relating to the electric double layer. To be complete, double layer theory must include precise descriptions of ion pairing, adsorbed counter ions, and ionic strengths of solutions. 

The description of the double layer properties by the Stem-Gouy model is a very crude one. A veiy weak point is the assumption that the dielectric contact suddenly changes from that of the solution to that of the Helmholtz double layer. The main information comes, therefore, from the minimum which indicates the potential of zero excess charge on the metal. This is, however, only correct in the absence of specific adsorption of ions. If ions are adsorbed, the counter charge for the diffuse double layer is the sum of the surface charge in the metal and of the adsorbed ions. Since the concentration of adsorbed ions also varies with the applied potential, this effect increases the apparent capacity of the Helmholtz double layer. 

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