Bratko D, Mulpuri N
Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23221, USA.
J Chem Phys. 2024 Oct 21;161(15). doi: 10.1063/5.0233894.
Molecular and collective reorientations in interfacial water are by-and-large decelerated near surfaces subjected to outgoing electric fields (pointing from surface to liquid, i.e., when the surface carries positive charge). In incoming fields at negatively charged surfaces, these rates show a nonmonotonic dependence on field strength where fastest reorientations are observed when the field alignment barely offsets the polarizing effects due to interfacial hydrogen bonding. This extremum coincides with a peak of local static permittivity. We use molecular dynamics simulations to explore the impact of background static field on high frequency AC permittivity in hydration water under an electric field mimicking the conditions inside a capacitor where one of the confinement walls is subject to an outgoing field and the other one to an incoming field. At strong static fields, the absorption peak undergoes a monotonic blue shift upon increasing field strength in both hydration layers. At intermediate fields, however, the hydration region at the wall under an incoming field (the negative capacitor plate) features a red shift coinciding with maximal static-permittivity and reorientation-rate. The shift is mostly determined by the variation of the inverse static dielectric constant as proposed for mono-exponentially decaying polarization correlations. Conversely, hydration water at the opposite (positively charged) surface features a monotonic blue shift consistent with conventional saturation. The sensitivity of absorption peaks on the field suggests that surface charge densities could be deduced from sub-THz dielectric spectroscopy experiments in porous materials when interfaces accommodate a major fraction of water contained in the system.
在受到向外电场作用的表面（电场从表面指向液体，即表面带正电荷时）附近，界面水的分子重排和集体重排总体上会减速。在带负电荷表面的向内电场中，这些速率对场强呈现非单调依赖性，当场的取向几乎抵消了由于界面氢键引起的极化效应时，会观察到最快的重排。这个极值与局部静态介电常数的峰值重合。我们使用分子动力学模拟来研究背景静态场对模拟电容器内部条件下的水化水中高频交流介电常数的影响，其中一个限制壁受到向外电场，另一个受到向内电场。在强静态场下，两个水化层中吸收峰随着场强增加而单调蓝移。然而，在中间场中，在向内电场作用下的壁处的水化区域（负电容器极板）出现红移，与最大静态介电常数和重排速率一致。这种位移主要由单指数衰减极化相关性所提出的逆静态介电常数的变化决定。相反，在相对的（带正电荷的）表面处的水化水呈现与传统饱和一致的单调蓝移。吸收峰对场的敏感性表明，当界面容纳系统中大部分水时，表面电荷密度可以从多孔材料中的亚太赫兹介电光谱实验中推导出来。
