Examining the Contributions of Image-Charge Forces to Charge Reversal: Discrete Versus Continuum Modeling of Surface Charges

The effects of both repulsive and attractive image-charge forces on the structure of electric double layers are addressed by Monte Carlo determination, based on a primitive model of electrolytes in contact with two types of identically charged surfaces: one with a homogeneously smeared-out charge density and the other with discrete interfacial groups. It is shown that the behavior of ions is closely related to surface charge distributions. Moreover, charge reversal in the absence of image charges witnesses an initial enhancement and then follows a fast suppression with increasing valence of the interfacial groups. The situation is quite similar to what are observed in the presence of repulsive image charges, which can significantly facilitate counterion condensation by overcoming the electrostatic barrier presented by the low dielectric substrate. With transition to attractive image-charge interactions, however, charge reversal remains widely unaffected in different surface charge representations, which even becomes much weaker when compared to the corresponding cases of both no images and repulsive images, provided that the interfacial groups have adequate valences. The overall scenario is found to be independent of the surface charge density values under study. These findings clearly illustrate the enormous improvement in our quantitative understanding of the electric double layer structure and the associated charge reversal phenomenon at the interface of various substrates.