电场作用下水表面电势的分子动力学研究

doi:10.6023/A19060205

摘要/Abstract

水表面电势在诸多电化学过程与反应中扮演关键角色, 然而实验上直接测量却极具挑战. 本论文提出一套基于平衡态恒定电势分子动力学的模拟-分析-计算方法, 可实现通过保持恒定电势且伴随电荷涨落的电极板将电场作用于附近的水表面, 并以平均探针电势计算方法精确测量空间电势分布. 凭借此套方法, 首次计算了不同电极电势下水表面区域的空间电势分布函数, 并测得了鲜有报道的水表面电势随外电场的变化关系. 发现了阴极附近水的表面电势随外电场增强而降低而阳极附近水的表面电势随外电场增强而增大的非对称性. 同时计算了平衡态水表面分子数密度和偶极矩极化密度分布函数, 展示出逐渐增强的外电场能够强烈改变水表面区域的极化行为也能够使液体水整体微弱的极化. 论文最后提出水表面电势随电场变化的非对称性源自水表面极化行为的非对称性以及液体区域的整体极化.

关键词: 水, 分子动力学, 恒定电势法, 液-汽界面, 平均探针电势计算法

The surface potential of the liquid-vapor interface of water plays a critical role in electrochemistry, interfacial reactivity, and solvation thermodynamics. However, direct experimental measurement of the surface potential of pure water is exceedingly challenging. Here we present a methodology to explore the effect of external electric field on the water surface potential. The methodology contains constant electrostatic potential molecular dynamics simulation[J. Chem. Phys., 126, 084704(2007)], in which, the electrode charges are allowed to fluctuate to keep the potential fixed, as well as a recently developed probe and average method[J. Phys.: Cond. Matter, 28, 464006(2016)] to accurately map out the electrostatic potential across the water surfaces. The methodology is applied to the coexistence of the vapor phase and the liquid phase of the room temperature pure water (described by a simple SPC/E water model) under different magnitudes of E-fields generated from the nearby electrodes, yielding a first-time calculation of the external E-field dependent water surface potential profiles, and the relationship between the water surface potential and the external E-field strength which has been rarely reported. We found an asymmetric effect of external E-field on the surface potential, i.e., the surface potential decreases with increasing the external E-field strength for the water surface close to the cathode, while the surface potential increases with increasing field strength for the surface close to the anode. The water surfaces are also characterized by calculating the number density and dipole polarization density profiles, which depict the presence of the external E-fields induced bulk polarization under high strength field. By comparing the dipole polarization density profiles and the potential profiles, we conclude that the asymmetric effect of external E-field on the surface potential is due to the asymmetric behavior in surface polarization under external E-field for the water surfaces near cathode or anode, and is also due to the polarization within bulk part of the liquid water. The methodology presented in the current study can be easily applied to more advanced water models such as polarizable water models which are beyond the SPC/E used in current work. The achievement of the fundamental data and the physics relationship between the surface potential of water and the applied external E-field could potentially facilitate the advancements in electrodynamics and thermodynamics of the liquid-vapor interfaces.

Key words: water, molecular dynamics, constant potential method, liquid-vapor interface, probe and average potential calculation method

引用此文

杨鹏里, 王振兴, 梁尊, 梁洪涛, 杨洋. 电场作用下水表面电势的分子动力学研究[J]. 化学学报, 2019, 77(10): 1045-1053.

Yang, Pengli, Wang, Zhenxing, Liang, Zun, Liang, Hongtao, Yang, Yang. A Molecular Dynamics Simulation Study of the Effect of External Electric Field on the Water Surface Potential[J]. Acta Chimica Sinica, 2019, 77(10): 1045-1053.

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图/表 6

图1. (a)恒定电势分子动力学模拟坐标快照, 两金属铂电极板电势2 V, 温度300 K. (b) 4组不同的施加电势组合下, 阴阳电极板表面原子所带电荷的概率密度. 纵坐标为对数标度

Figure 1. (a) Constant potential method MD simulation snapshot. The simulation box consists of a water slab between two electrodes maintained at constant potential difference 2V, temperature 300K. The color code on the FCC electrode Pt atoms indicates instantaneous charge, with central palette color-blue corresponding to negative charge and red corresponding to positive charge. In the liquid water region, the red spheres represent oxygen and the smaller white spheres represent hydrogen. (b) Log-linear plot of the probability distribution of electrode atom charges for the surface layers of electrodes at four different values of the applied electrode potential differences. Dashed lines correspond to the results at cathode and the solid lines correspond to the results at anode

图2. 阴阳两极附近水表面平均分子数密度(a,b)和偶极矩极化密度(c,d)随恒定电势电极电势差变化.两平行竖线标出了密度数值从液相密度的10%变化到90%的表面宽度范围

Figure 2. ean water molecules number density (a, b) and dipole moment (z component) density (c, d) profiles for water surfaces near cathode and near anode, in the systems with four different values of the applied electrode potential differences. Parallel vertical thin lines locate a distance (10~90 width) over which the density profile changes from 10% to 90% of the density in the bulk liquid relative to its value in the vapor as one traverses the surface from the vapor into liquid

Δ?/V	$w_{1090}^{C}\text{/nm}$	$w_{1090}^{A}\text{/nm}$	$\chi _{1090}^{C}\text{/V}$	$\chi _{1090}^{A}\text{/V}$	$P 1090 C /$ (Debye?nm^–3)	$P 9010 A$ / (Debye?nm^–3)	E₀/(V?nm^–1)	E_p/(V?nm^–1)
0	0.389(7)	0.388(6)	0.49(1)	0.49(1)	2.78(3)	2.77(3)	0.00	0.000
2	0.393(10)	0.385(6)	0.48(2)	0.51(2)	2.10(4)	3.46(5)	0.34	0.006
6	0.421(7)	0.398(7)	0.43(3)	0.56(1)	0.56(5)	4.58(3)	1.02	0.015
9	0.467(8)	0.433(12)	0.36(3)	0.62(2)	–0.60(3)	5.31(4)	1.55	0.023

Δ?/V	$w_{1090}^{C}\text{/nm}$	$w_{1090}^{A}\text{/nm}$	$\chi _{1090}^{C}\text{/V}$	$\chi _{1090}^{A}\text{/V}$	$P 1090 C /$ (Debye?nm^–3)	$P 9010 A$ / (Debye?nm^–3)	E₀/(V?nm^–1)	E_p/(V?nm^–1)
0	0.389(7)	0.388(6)	0.49(1)	0.49(1)	2.78(3)	2.77(3)	0.00	0.000
2	0.393(10)	0.385(6)	0.48(2)	0.51(2)	2.10(4)	3.46(5)	0.34	0.006
6	0.421(7)	0.398(7)	0.43(3)	0.56(1)	0.56(5)	4.58(3)	1.02	0.015
9	0.467(8)	0.433(12)	0.36(3)	0.62(2)	–0.60(3)	5.31(4)	1.55	0.023

表1. 从恒定电势分子动力学方法进行的外电场作用下水表面模拟中提取的表面热力学参量. C代表阴极、A代表阳极. Δ?是两电极板电势差, w1090是10~90表面宽度, χ1090和代表水表面10%和90%液相密度位置间电势落差(或表面电势)和偶极极化密度落差. E0是体系真空层区域匀强电场强度, Ep是水膜内部极化电场强度. 括号内数字代表95%置信度的统计误差

Table 1. Surface thermodynamic parameters extracted from the constant potential method MD simulation of the liquid-vapor water surfaces under different external E-field generated by nearby cathode (C) and anode (A). Δ? is the voltage difference between cathode and anode, w1090is the 10~90 width of the water surface,χ1090 andP1090stand for the water surface potential and surface dipolar polarization density difference, E0is the strength of E-field in the vapor/vacuum region between electrodes and water surfaces, Epis the strength of E-field within water film due to bulk polarization. Numbers in parentheses are 95% confidence errors for the last digits shown

图3. 应用平均探针法计算获得的全局电势分布函数随恒定电势电极电势差变化结果, 插图放大展示液膜内部的电势分布

Figure 3. Electric potential profile from 4 different voltage difference simulations, calculated using the probe and average method. The inset shows selected data portion in the bulk water region

图4. 水表面局域电势分布随电势差变化的差异. (a)阴极附近的水表面, (b)阳极附近的水表面

Figure 4. Selected surface portion of the electric potential profile from 4 different voltage difference simulations. (a) Water surface near cathode, (b) water surface near anode

图5. (a)水表面电势$\chi _{1090}^{C}$、$\chi _{1090}^{A}$和水表面偶极极化密度落差 P 1090 C 、 P 9010 A 随外电场强度的变化关系呈现非对称性. (b)水膜内部区域极化电场强度Ep随外电场强度E0的变化关系

Figure 5. (a) Water surface potential ($\chi _{1090}^{C}$,$\chi _{1090}^{A}$) andwater surface dipolar polarization density difference ( P 1090 C , P 9010 A ) as the function of E0. (b) Strength of E-field within water film due to bulk polarization,Ep, as the function of E0

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