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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.
