Gupta Ankur, Govind Rajan Ananth, Carter Emily A, Stone Howard A
Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544, USA.
Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309, USA.
Phys Rev Lett. 2020 Oct 30;125(18):188004. doi: 10.1103/PhysRevLett.125.188004.
Electrical double layers (EDLs) play a significant role in a broad range of physical phenomena related to colloidal stability, diffuse-charge dynamics, electrokinetics, and energy storage applications. Recently, it has been suggested that for large ion sizes or multivalent electrolytes, ions can arrange in a layered structure inside the EDLs. However, the widely used Poisson-Boltzmann models for EDLs are unable to capture the details of ion concentration oscillations and the effect of electrolyte valence on such oscillations. Here, by treating a pair of ions as hard spheres below the distance of closest approach and as point charges otherwise, we are able to predict ionic layering without any additional parameters or boundary conditions while still being compatible with the Poisson-Boltzmann framework. Depending on the combination of ion valence, size, and concentration, our model reveals a structured EDL with spatially oscillating ion concentrations. We report the dependence of critical ion concentration, i.e., the ion concentration above which the oscillations are observed, on the counter-ion valence and the ion size. More importantly, our model displays quantitative agreement with the results of computationally intensive models of the EDL. Finally, we analyze the nonequilibrium problem of EDL charging and demonstrate that ionic layering increases the total charge storage capacity and the charging timescale.
双电层（EDLs）在与胶体稳定性、扩散电荷动力学、动电现象和能量存储应用相关的广泛物理现象中起着重要作用。最近，有人提出，对于大离子尺寸或多价电解质，离子可以在双电层内部排列成层状结构。然而，广泛使用的双电层泊松 - 玻尔兹曼模型无法捕捉离子浓度振荡的细节以及电解质价态对这种振荡的影响。在这里，通过将一对离子在最接近距离以下视为硬球，在其他情况下视为点电荷，我们能够在没有任何额外参数或边界条件的情况下预测离子分层，同时仍与泊松 - 玻尔兹曼框架兼容。根据离子价态、尺寸和浓度的组合，我们的模型揭示了一个具有空间振荡离子浓度的结构化双电层。我们报告了临界离子浓度（即观察到振荡的离子浓度）对反离子价态和离子尺寸的依赖性。更重要的是，我们的模型与双电层计算密集型模型的结果显示出定量一致性。最后，我们分析了双电层充电的非平衡问题，并证明离子分层增加了总电荷存储容量和充电时间尺度。
