Electric double-layer

The electric double-layer is a model describing the variation of the electric Potentiel with the accesses of a surface. It intervenes mainly at the time of the study of the behavior of the Colloïde S and surfaces in contact with solution S.

History of the models of the electric double-layer

The initial model of the electric double-layer is allotted to Helmholtz (1879). Mathematically, it simply compared the double-layer to a condensing , while being based on a physical model in which a layer of Ion S is adsorbed on the surface.

Later, Gouy and Chapman resulted in significant improvements to this model, by developing the theory of the double-layer separately diffuses, related to thermal agitation. They published it in three years of interval, respectively in 1910 and 1913. In this extended model, the potential of surface decrease in way Exponentielle because of a layer of against-ions of the solutions, adsorbed on the surface.

The model used today is the model of Gouy-Chapman- Stern, which combines the two models while utilizing the dense layer of Helmholtz (called then layer of Stern) and the diffuse layer of Gouy-Chapman. The diffuse layer starts then at a finite distance from the wall.

The notable approximations of this model are the following ones:

the Ion S are regarded as specific load S.
Seules the interactions Coulomb S are significant.
the electric Permittivité is constant inside the double-layer.
the solvent is uniform on an atomic scale.

Origin of the loads of surface

Often, when a Liquide is in contact with a surface, this one takes care, because of the dissociation of the molecules of surface. Thus, for a surface of Silica, containing groupings silanols SiOH, Proton S H⁺ can be released, leaving of SiO^- on surface. This dissociation depends on the species present in the solution, sound pH, etc Typiquement, for water on silica, the Ionization of surface approximately gives rise to a negative charge all the 16nm².

The potential $\ zeta$ (zeta) is the difference of potential between the dense layer of Stern and the liquid. It thus characterizes the electric burden-sharing on surface.

Application to the Colloid S

----

Random links: