Krucker-Velasquez Emily, Swan James W
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Chem Phys. 2021 Oct 7;155(13):134903. doi: 10.1063/5.0061230.
The electrostatic screening length predicted by Debye-Hückel theory decreases with increasing ionic strength, but recent experiments have found that the screening length can instead increase in concentrated electrolytes. This phenomenon, referred to as underscreening, is believed to result from ion-ion correlations and short-range forces such as excluded volume interactions among ions. We use Brownian Dynamics to simulate a version of the Restrictive Primitive Model for electrolytes over a wide range of ion concentrations, ionic strengths, and ion excluded volume radii for binary electrolytes. We measure the decay of the charge-charge correlation among ions in the bulk and compare it against scaling trends found experimentally and determined in certain weak coupling theories of ion-ion correlation. Moreover, we find that additional large scale ion structures emerge at high concentrations. In this regime, the frequency of oscillations computed from the charge-charge correlation function is not dominated by electrostatic interactions but rather by excluded volume interactions and with oscillation periods on the order of the ion diameter. We also find the nearest neighbor correlation of ions sharing the same charge transitions from negative at small concentrations to positive at high concentrations, representing the formation of small, like-charge ion clusters. We conclude that the increase in local charge density due to the formation of these clusters and the topological constraints of macroscopic charged surfaces can help explain the degree of underscreening observed experimentally.
德拜 - 休克尔理论预测的静电屏蔽长度会随着离子强度的增加而减小，但最近的实验发现，在浓电解质中屏蔽长度反而会增加。这种被称为欠屏蔽的现象，被认为是由离子 - 离子相关性和短程力（如离子间的排除体积相互作用）导致的。我们使用布朗动力学在广泛的离子浓度、离子强度和二元电解质的离子排除体积半径范围内，模拟电解质的限制性原始模型。我们测量本体中离子间电荷 - 电荷相关性的衰减，并将其与实验发现的以及在某些离子 - 离子相关性的弱耦合理论中确定的标度趋势进行比较。此外，我们发现在高浓度下会出现额外的大规模离子结构。在这种情况下，由电荷 - 电荷相关函数计算出的振荡频率并非由静电相互作用主导，而是由排除体积相互作用主导，且振荡周期约为离子直径的量级。我们还发现，具有相同电荷的离子的最近邻相关性从小浓度时的负值转变为高浓度时的正值，这代表了小的、同电荷离子簇的形成。我们得出结论，由于这些簇的形成导致的局部电荷密度增加以及宏观带电表面的拓扑约束，有助于解释实验中观察到的欠屏蔽程度。
