Monte Carlo determination of mixed electrolytes next to a planar dielectric interface with different surface charge distributions

摘要

Employing canonical ensemble Monte Carlo simulations, we report a calculation of the distribution of small ions next to a planar negatively charged surface in the presence of mixed electrolytes of monovalent and trivalent salt ions within the framework of the primitive model under more realistic hydrated ion size conditions. The effects of surface charge discreteness and dielectric breakdown on charge inversion are discussed based on increasing concentration of both monovalent and trivalent salt. Moreover, a comparison of the simulation results for different discretization models is made along with the case of uniformly distributed charge in terms of the ionic density profiles as well as the integrated charge distribution function. For finite size charged groups located inside the lower dielectric region, a complete equivalence with the case of uniform distribution is observed if the quantities of interest are exclusively analyzed as a function of the distance to the charged interface. With protruding head groups into the aqueous solution, the excluded volume dominates over the correlation effect, therefore the ions are less accumulated in the vicinity of the charged surface, inducing that the onset position of charge inversion experiences an evident shift toward the aqueous environment. Overall, the effect of repulsive image forces on the diffuse double layer structure can be significant at low surface charge density irrespectively of surface charge distributions.