A Constant Potential Molecular Dynamics Simulation Study of the Atomic-Scale Structure of Water Surfaces Near Electrodes

Novel and technologically important processes and phenomena arise at water surfaces in the presence of electric fields. However, experimental measurements on water surfaces are challenging, and the results are scarce and inconclusive. In this work, the constant potential molecular dynamics method, in which the electrode charges are allowed to fluctuate to keep the electric potential fixed, was implemented in the study of a near-electrode water surface systems. This simulation system was set up with a vapor/liquid-water/vapor slab and two electrodes under different sets of applied electrostatic potential, yielding i) a detailed characterization of the external E-field dependent electrostatic potential/density/dipole moment density profiles, and ii) the relationship between the water surface width and the applied electrode voltage differences which has been rarely reported. The adjustments in the number density profiles in the vicinity of water surfaces due to external E-fields were observed, while the capillary interfacial widths for the surfaces near both cathode and anode were found with different increment rates under increasing E-fields. By examining dipole density profiles across the water surfaces, we found that external E-field induced polarization occurs in both bulk and surface regimes, yet the surface polarization densities vary asymmetrically with respect to the increasing E-fields. Detailed discussions were carried out to explain the correlation between water surface tension and surface widths, as well as the interplay between the surface polarization densities and the hydrogen bond network structure. We conclude that the mechanical and structural properties of the water surfaces could be tuned by both magnitude and direction of the strong external E-fields. We also recognize that more surface properties with application value, such as dielectric permittivity tensor or surface potential, could also be regulated by the external E-fields.