A Molecular Dynamics Simulation Study of the Effect of External Electric Field on the Water Surface Potential

doi:10.6023/A19060205

Abstract

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

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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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Fig. & Tab. 6

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

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

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

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

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

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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