Overcharging and Free Energy Barriers for Equally Charged Surfaces Immersed in Salt Solutions

The stability of dispersions containing charged particles may obviously be regulated by salt. In some systems, the effective charge, as measured by the potential some small distance away from the particles, can have a sign opposite to the bare surface charge. If charge reversal takes place, there is typically a salt concentration regime within which colloidal stability increases with added salt. These experimental findings on dispersions have been corroborated by atomic force microscopy investigations, where an attraction is found at short separations. This attraction is stronger than expected from standard DLVO theory, and there has been considerable debate concerning its origin. In this work, we use simple coarse-grained models of these systems, where the bare surfaces carry a uniform charge density, and ion-specific adsorption is absent. Our hypothesis is that these experimental observations can be explained by such a simplistic pure Coulomb based model. Our approach entails grand canonical Metropolis Monte Carlo (MC) simulations as well as correlation-corrected Poisson-Boltzmann (cPB) calculations. In the former case, all ions have a common size, while the cPB utilizes a point-like model. We devote significant attention on apparent surface charge densities and interactions between large flat model surfaces immersed in either a 2:1 salt or a 3:1 salt. In contrast to most of the previous theoretical efforts in this area, we mainly focus on the weak long-ranged repulsion and its connection to an effective surface charge. We find a charge reversal and a concomitant development of a free energy barrier for both salts. The experimentally observed nonmonotonic dependence of colloidal stability on the salt concentration is reproduced using MC simulations as well as cPB calculations. A strong attraction is observed at short range for all investigated cases. We argue that in our model, all non-DLVO aspects can be traced to ion–ion correlations.含带电粒子的分散体系的稳定性，显然可通过电解质盐进行调控。在部分体系中，通过颗粒表面某一微小距离处的电势测得的有效电荷，其符号可能与本征表面电荷相反。当发生电荷反转时，通常存在一段盐浓度区间，在此区间内胶体稳定性随添加的盐浓度升高而增强。上述关于分散体系的实验发现已得到原子力显微镜(Atomic Force Microscopy, AFM)研究的佐证，该研究在短程间距下观测到了吸引作用。这种吸引作用强于标准DLVO理论的预测值，其起源问题此前已引发广泛讨论。 在本研究中，我们采用了这类体系的简单粗粒化模型，模型中的裸露表面带有均匀电荷密度，且不存在离子特异性吸附现象。我们的假设是，上述实验观测结果可通过这类基于纯库仑相互作用的简化模型加以解释。本研究采用的方法包括巨正则系综梅特罗波利斯蒙特卡洛（MC）模拟，以及关联修正泊松-玻尔兹曼（cPB）计算。在前者的模拟中，所有离子具有统一的尺寸；而cPB理论则采用了点电荷模型。我们重点关注了表观表面电荷密度，以及浸没于2-1价电解质盐或3-1价电解质盐中的大型平面模型表面之间的相互作用。与该领域此前的多数理论研究不同，我们主要聚焦于弱长程斥力及其与有效表面电荷的关联。 我们在两种电解质盐体系中均观测到了电荷反转现象，以及伴随出现的自由能势垒。通过MC模拟与cPB计算，我们复现了实验中观测到的胶体稳定性随盐浓度变化的非单调依赖关系。在所有研究工况下，短程范围内均观测到了强烈的吸引作用。我们认为，在本模型中，所有非DLVO理论的效应均可归因于离子-离子关联相互作用。