Impact of Head Group Charges, Ionic Sizes, and Dielectric Images on Charge Inversion: A Monte Carlo Simulation Study

The ionic density profiles and corresponding integrated charge distribution close to an aqueous interface carrying charged functional groups of different valences exposed to mixed-salt solutions of 1:1 and 3:1 electrolytes were computed by use of extensive Monte Carlo simulations within the framework of the primitive model. The influence of varying ion sizes on charge inversion was analyzed, along with the crucial role of electrostatic images arising from dielectric discontinuities. For small sizes of charged species, it was found that charge inversion is enhanced by the presence of multivalent interfacial groups. Moreover, dielectric images further intensify the degree of charge inversion despite strong depletion effects. In particular, the onset of charge inversion shifts closer to the nonaqueous phase if the surface charge density is sufficiently low. These observations are opposite to the general belief of image repulsion. When large-sized ions were considered, the degree of charge inversion was found to be independent of the structure of interfacial groups and to remain widely unaffected in the absence and presence of dielectric images. Overall, our results show that interionic excluded-volume interactions are an overwhelming factor in determining counterion condensation. In addition, the inclusion of ionic size asymmetry at weakly charged interface results in surface charge amplification, which becomes more pronounced with increasing valence of the interfacial groups. Our work emphasizes the fact that surface equilibria mainly result from a subtle balance between steric and charge correlations.