Electric double layer
Keyword chosen by Kelly Miller
An electric double layer is a structure that forms on the surface of a charged object in an ionic solution.
When a surface is placed in a liquid, it can become charged in one of two ways:
1) the ionization or dissociation of surface groups
2)adsorption of ions from solution onto a previously uncharged surface
Regardless of how the surface becomes charged, when it is placed in an electrolytic solution, an electric double layer forms such that, the final surface charge is balanced by an equal but oppositely charged region of counterions. These counterions arrange into 2 parallel layers of charge at the surface. The first layer (also known as the Stern Layer) consists of ions that are strongly bound to the surface. The second layer is made up of ions attracted to the first layer electrostatically. The second layer electrically screens the first layer from the liquid bulk. The ions in the second layer, are more loosely associated with the object, they are subject to electric and thermal forces and are not firmly anchored like the ions in the Stern Layer are. The second layer is also known as the diffuse layer.
The first model of the electric double layer is attributed to Helmholtz who modeled the system as a simple capacitor.
Louis Georges Gouy and David Chapman introduced the diffuse model of the electric double layer, in which the electric potential decreases exponentially as the distance from the surface is increased. The Gouy-Chapman model does not hold for highly charged double layers, however. This model was later improved upon by Stern who resolved the Gouy-Chapman limitation by suggesting a combination of the Helmholtz and Gouy-Chapman models, which gave rise to the internal Stern layer (similar to what Helmholtz proposed with the capacitor model) and an outer diffuse layer (Gouy-Chapman layer). This is the model most commonly used.