Fig. 1. Electrical double layer defined by electrostatic planes in 3-plane surface complexation models. Charge separation due to adsorbed and electrolyte ions located in the electrostatic planes defines two molecular capacitors which are termed the inner- and outer-Helmholz layers. These layers are collectively known as the Stern Layer. A layer of diffuse electrolyte ions separates the Stern layer and bulk solution. Electrical potential decreases from some positive value at the mineral surface to zero in the bulk solution.