Application of the hypernetted chain approximation to the electrical double layer. Comparison with Monte Carlo results for 2:1 and 1:2 salts

The hypernetted chain approximation (HNC/MSA version) is applied to the electrical double layer problem. We consider an asymmetric binary salt consisting of a divalent and a monovalent ion. The ions, solvent, and electrode are modeled as charged hard spheres, a dielectric continuum, and a uniformly charged hard wall, respectively. The HNC/MSA results are in good agreement with recent computer simulations. Specifically we find that the double layer is generally narrower than that indicated by the Poisson-Boltzmann theory of Gouy and Chapman and that the density and potential profiles can be nonmonotonic which contrasts with the monotonic nature of these profiles in the Gouy-Chapman theory. This nonmonotonic behavior is more pronounced when the counterions are divalent and when the concentration is increased and results from the existence of layers of charge alternating sign. This also causes a nonmonotonic dependence of the diffuse layer potential and ionic adsorption isotherms on the electrode charge density. © 1983 American Chemical Society.