On the effect of image charges and ion-wall dispersion forces on electric double layer interactions

Two effects of interactions between polarizable ions and polarizable walls in electric double layers are investigated: ionic image charge forces and ion-wall dispersion forces. The first must be included for a consistent treatment of the wall-wall van der Waals (vdW) interaction, since it contains the effect of screening of the static part of the vdW interaction. The second has been suggested to give rise to ion specificity in double layer interactions. The strength of the ion-wall dispersion forces are estimated from quantum mechanical calculations of ionic polarizability and from experimental data for the dielectric functions of the media. The ion density profiles and the anisotropic ion-ion distribution functions in the double layer are calculated in the highly accurate anisotropic hypernetted chain approximation, which allows the correct treatment of the image charge forces. The double layer interactions are evaluated from these distribution functions. It is found that it is important to include both kinds of ion-wall forces. Quantitative and sometimes even qualitative differences occur in the double layer interactions depending on the ionic species of the electrolyte due to different strengths of the ion-wall dispersion interactions.