Helmholtz and later Stern divided the solution side of a diffuse double layer into an inner part (Stern or Helmholtz layer) and an outer part (Gouy or diffuse layer). In a more simple model, the ions with a finite size “d” adsorb on a surface and form a so-called outer Helmholtz plane. In the so-called triple layer model, the Helmholtz plane is subdivided into an inner Helmholtz plane (iHp, where specifically adsorbed ions such as protons or K on high-charged clay minerals reside) and an outer Helmholtz plane (oHp, where non-specifically adsorbed ions reside). Note that for all models the following equation holds:
Sigma_zero + Sigma _iHp + Sigma_oHp + Sigma_diffuse = zero.
where: Sigma_zero denotes the charge density (in Coulomb per square meter) of the solid, Sigma_iHp denotes the charge density of species adsorbed in the inner Helmholtz plane (usually specifically adsorbed ions), Sigma_iHp denotes the charge density of species adsorbed in the outer Helmholtz plane (usually nonspecifically adsorbed species), and Sigma_Diffuse is the charge density of the diffuse double layer formed in the solution.
See diffuse double layer