Electrical double layer

The electrical double layer describes the variation of electric potential near a surface, and has a large bearing on the behaviour of colloids and other surfaces in contact with solutions.

Historical development of double layer models

The earliest model of the electrical double layer is usually attributed to Helmholtz (1879). Helmholtz treated the double layer mathematically as a simple capacitor, based on a physical model in which a single layer of ions is adsorbed at the surface.

Later Gouy and Chapman (1910-1913) made significant improvements by introducing a diffuse model of the electrical double layer, in which the potential at a surface decreases exponentially due to adsorbed counter-ions from the solution.

The current classical electrical double layer is the Gouy-Chapman-Stern model, which combines the Helmholtz single adsorbed layer with the Gouy-Chapman diffuse layer. Important approximations used in this model are:

Ions are effectively point charges

The only significant interactions are coulombic

Electrical permittivity is constant throughout the double layer
The solvent is uniform at the atomic scale