ABSTRACT
The electrical double layer structure is commonly described with the Gouy-Chapman (GC) or Gouy-Chapman-Stern (GCS) picture. Extensive testing has confirmed its suitability for the description of the electrostatic potential and ion density in the vicinity of membranes. The GCS theory makes two drastic approximations: it neglects the correlation between positions of individual ions and the size of the ions. The physical quantities that cannot always be simplified to the level of the GCS theory are the interaction energy or the force between charged surfaces. The reason for this failure is a strong attractive contribution arising under typical solution conditions from the correlation between divalent ions in the solution. Some distance away from a surface, the electrostatic potential is small and counterions are not crowded. Under such conditions mean field approximation applies and the behavior of all physical quantities follows exponential variations of the GC theory.
