Wang Zhi-Yong, Zhang Pengli, Ma Zengwei
School of Science, Chongqing University of Technology, Chongqing 400054, China.
Phys Chem Chem Phys. 2018 Feb 7;20(6):4118-4128. doi: 10.1039/c7cp08117k.
The conventional paradigm for characterizing surface overcharging and charge reversal is based on the so-called Stern layer, in which surface dissociation reaction and specific chemical adsorption are assumed to take place. In this article, a series of Monte Carlo simulations have been applied to obtain useful insights into the underlying physics responsible for these two kinds of anomalous phenomena at the interface of two dielectrics, with special emphasis on the case of divalent counterions that are more relevant in natural and biological environments. At a weakly charged surface, it is found that independent of the type of surface charge distribution and the dielectric response of the solution, the overcharging event is universally driven by the ion size-asymmetric effect. Exceptionally, the overcharging still persists when the surface is highly charged but is only restricted to the case of discrete surface charge in a relatively low dielectric medium. As compared to the adsorption onto the homogeneously smeared charge surface that has the same average affinity for counterions, on the other hand, charge reversal under the action of a dielectric response can be substantially enhanced in the discrete surface charge representation due to strong association of counterions with interfacial groups, and the degree of enhancement depends in a nontrivial way on the reduction of the medium dielectric constant and the steric effects of finite ion size. Rather interestingly, the charge reversal is of high relevance to the overcharging of interfaces because the overwhelming interfacial association forces the coions closer to the surface due to their smaller size than the counterions. Upon the addition of a monovalent salt to the solution, the interfacial association with divalent counterions makes surface overcharging and charge reversal widely unaffected, in contrast to the prevailing notion that screening of surface charge of a homogeneous nature is determined by the competitive effects between size-exclusion effects and energetic contributions. Overall, the present work highlights that the complex interplay between the electrostatic and steric interactions should be coupled to the realistic character of surface charge to establish a faithful description of the overcharging and charge reversal at heterophase interfaces.
表征表面过充电和电荷反转的传统范式基于所谓的斯特恩层，其中假定发生表面离解反应和特定化学吸附。在本文中，已应用一系列蒙特卡罗模拟来深入了解在两种电介质界面处导致这两种异常现象的潜在物理机制，特别强调了在自然和生物环境中更相关的二价抗衡离子的情况。在弱带电表面，发现无论表面电荷分布类型和溶液的介电响应如何，过充电事件普遍由离子尺寸不对称效应驱动。例外的是，当表面高度带电时过充电仍然存在，但仅限于相对低介电介质中离散表面电荷的情况。另一方面，与对抗衡离子具有相同平均亲和力的均匀涂抹电荷表面上的吸附相比，由于抗衡离子与界面基团的强烈缔合，在离散表面电荷表示中，介电响应作用下的电荷反转可得到显著增强，增强程度以一种复杂的方式取决于介质介电常数的降低和有限离子尺寸的空间效应。相当有趣的是，电荷反转与界面的过充电高度相关，因为压倒性的界面缔合由于共离子尺寸小于抗衡离子而迫使它们更靠近表面。与普遍观点认为均匀性质的表面电荷屏蔽由尺寸排阻效应和能量贡献之间的竞争效应决定相反，向溶液中添加单价盐时，与二价抗衡离子的界面缔合使表面过充电和电荷反转基本不受影响。总体而言，本工作强调静电和空间相互作用之间的复杂相互作用应与表面电荷的实际特性相结合，以建立对异相界面上过充电和电荷反转的忠实描述。
