Guoy-Stern double layer
A diffuse double layer (DDL) occurs near surfaces of negatively charged layer silicate minerals (colloids) in soils. An aqueous solution is assumed to be in contact with mineral surfaces. Cations in solution are attracted to the surface; anions are repelled, setting up a DDL of cations and anions adjacent to the surface.
The Guoy-Chapman model assumes cations are point charges, colloid surfaces are planar and essentially infinite, and surface charge is distributed of the entire surface. These assumptions did not match with observations, however.
Stern improved the model by assuming that some cations were held tightly next to colloid surfaces in a layer of specifically adsorbed or Stern-layer cations. [Cations in this probably most closely approximate cation exchange capacity in soils.] The DDL exists outside the Stern layer.
The above information was obtained from H.L. Bohn et al. 1985. Soil Chemistry, 2nd Edition. John Wiley & Sons, New York, Chichester. This or a number of other texts on soil chemistry or colloidal chemistry should provide a more detailed explanation.
