Electrokinetic, electrochemical, and electrostatic surface potentials of the pristine water liquid–vapor interface

Abstract

Although conceptually simple, the air-water interface displays rich behavior and is subject to intense experimental and theoretical investigations. Different definitions of the electrostatic surface potential as well as different calculation methods, each relevant for distinct experimental scenarios, lead to widely varying potential magnitudes and sometimes even different signs. Based on quantum-chemical density-functional-theory molecular dynamics simulations, different surface potentials are evaluated and compared to force-field MD simulations. As well explained in literature, the laterally averaged electrostatic surface potential, accessible to electron holography, is dominated by the trace of the water molecular quadrupole moment and using DFT-MD amounts to +4.35V inside the water phase, very different from results obtained with FF water models which yield negative values of the order of -0.4V to -0.6V. Thus, when predicting potentials within water molecules, as relevant for photoelectron spectroscopy and non-linear interface-specific spectroscopy, DFT simulations should be used. The electrochemical surface potential, relevant for ion transfer reactions and ion surface adsorption, is much smaller, less than 200mV in magnitude, and depends specifically on the atom radius. Charge transfer between interfacial water molecules leads to a sizable surface potential as well. However, when probing electrokinetics by explicitly applying a lateral electric field in DFT-MD simulations, the electrokinetic zeta-potential turns out to be negligible, in agreement with predictions using continuous hydrodynamic models. Thus, interfacial polarization charges from intermolecular charge transfer do not lead to significant electrokinetic mobility at the pristine vapor-liquid water interface, even assuming these transfer charges are mobile in an external electric field.

Quantifying the Molecular Polarization Response of Liquid Water Interfaces at Heterogeneously Charged Surfaces

“…Systems without a distinct liquid interface, such as intrinsically disordered proteins or protein condensates, pose a challenge in allowing for the unambiguous definition of κ⃗. Finally, as we have noted previously, the physical relevance of this molecular polarization (e.g., arising from nuclear reorientation) omits the important influence of electronic polarization and thus provides incomplete insight into properties such as interfacial dielectric and surface potential . For the future work, it will be useful to investigate how the inferred polarization results depend on the water model by testing other rigid point-charge models , as well as flexible/polarizable models. − …”

“…For these properties, molecular multipoles (which are not well reproduced by classical force fields) play a crucial role. 44,45 For example, accounting for the quadrupole and octupole contributions in water's polarization is crucial to accurately describe the anisotropy in interfacial dielectric profiles. 46,47 These multipoles arise through electronic polarization and can thus be accounted for with electronic structure methods.…”

“…However, the information contained in P ( κ⃗ | σ s ) is incomplete for quantifying certain physical properties, such as surface potential and dielectric response. For these properties, molecular multipoles (which are not well reproduced by classical force fields) play a crucial role. , For example, accounting for the quadrupole and octupole contributions in water’s polarization is crucial to accurately describe the anisotropy in interfacial dielectric profiles. , These multipoles arise through electronic polarization and can thus be accounted for with electronic structure methods. ,− They are not included in the SMIR model. The importance of collective solvent fluctuations is also highlighted in ref in the context of water’s local polarization response to a charged surface, which we revisit along with a detailed analysis using our method in the later section entitled Role of Water–Water Interactions in the Nonideal Response of Interfacial Water.…”

Multiscale Modeling of Aqueous Electric Double Layers

“…Data are shown for water densities above 0.1% of the bulk density. The DFT standard deviations have been obtained from block averaging in the time domain . (C) ⟨cos θ⟩ at the silica–water interface from FF MD simulations at surface charge density σ = 0 and at σ = −0.26 e /nm 2 and 0.15 M NaCl. , (D) ⟨cos θ⟩ at the hexane/water interface from DFT-based MD simulations in the presence of 5 M HCl (symbols), as well as from FF MD simulations at both the pristine hexane/water interface (orange line) and in the presence of 2 M HCl, modeled as H 3 O + and Cl – ions (red broken line) .…”

The Three-Phase Contact Potential Difference Modulates the Water Surface Charge

“…The surface charging mechanism has important implications for atmospheric science, biological transport, nanofluidics, − static electrification, , electrochemical power systems, − and the understanding of the fundamental properties of water . However, there is a puzzle: the information about the magnitude and the sign of the charge at the water interface is contradictory. − A molecular-level mechanism that drives the charging of water in each particular case is incompletely known. − The situation hinders the development of modern electrochemical and energy-harvesting systems and limits our understanding of the mechanisms of natural electrification on scales from microscopic to global. This unsolved interdisciplinary problem has attracted the attention of scientists from different fields.…”