A molecular simulation study on the role of ion sizes and dielectric images in near-surface ion distribution far from the strong coupling limit

Abstract

A series of Monte Carlo simulations of the planar electric double layers are carried out in the primitive model for two electrolyte mixtures next to a smooth and uniformly charged hard wall representing an ideal biological interface with low and moderate surface charge densities. The structural information of the double layers is applied to reveal charge inversion and overcharging through the addition of multivalent electrolyte at a certain physiological concentration. Various values for the radius of the ions are taken into account to capture the impact of short-range correlations. Meanwhile, the influence of image charges on ion distribution is analyzed, which stems from dielectric discontinuity between the interior and exterior of the membrane matrix. It is clearly shown that depending on the amount of foreign salt, the large size of charged species regardless of its polarity plays a positive role in promoting charge inversion. Moreover, our findings indicate that charge inversion do not signify the reversal of the electrophoretic mobility, in consistent with the recent theoretical predictions by Horno and co-workers [J. Colloid Interface Sci. 356, 325 (2011)]. In addition, the depletion effect triggered by repulsive image forces which are intertwined with the excluded volume correlations gives rise to an anomalous overcharging for low surface charged surface in the high concentrations of trivalent salt. Overall, the ion distribution in a double layer is exclusively governed by entropic and electrostatic contributions but with preferentially leading status for different magnitudes of surface charge.