Liquid-vapor interface of the Stockmayer fluid in a uniform external field

Abstract

The effect of a uniform (non-spatially varying) external field on the liquid-vapor interface of the Stockmayer fluid (Lennard-Jones particles embedded with a point-dipole) has been investigated by molecular dynamics simulations. The long-ranged parts of both the dipole and Lennard-Jones interactions are treated using an Ewald summation, which removes the effects of the cutoff. The direction of the field shifts the critical point and interfacial properties in different directions. For an external field parallel to the interface, the critical temperature increases, while for a field applied perpendicular to the interface, it decreases. The effects of the field on surface tension and interfacial width are also investigated. For zero field, dipoles near the liquid-vapor interface show a weak orientation parallel to the interface. For fields parallel to the interface, ordering in the liquid phase is greater than the vapor, while for fields perpendicular to the interface, the opposite is true. * stamoor@sandia.gov

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

“…We found that, compared with their 0 V bulk value, the density decreases by 0.5% when electrodes voltage difference increases to 9 V. Our finding disagree with the argument in a recent MD study, in which bulk water density was reported unchanged under foreign uniform E‐field up to 1.2 V/nm . Meanwhile, the decreasing trend of the liquid water is consistent with that of the dipolar Stockmayer liquid‐ vapor coexistence system under applied uniform external E‐field …”

“…With this scaling relationship and the data demonstrated in Figure b, considering constant interfacial dimension and constant temperature, we could estimate a decreasing trend in the surface tension when external E‐field is applied, and the trend should appear differently depending on the type of the electrode which faces the water surface. This decreasing trend in surface tension agrees with both Stockmayer fluid surfaces and the TIP4P/2005 water surfaces in the presence of a perpendicular uniform E‐field. There appears no molecular‐level theory that can quantitatively interpret the mechanism for the change in the surface tension due to E‐field.…”

“…also reported two surface tension decreasing rates under opposite field directions. However, no asymmetric behavior has been reported in Stockmayer system, indicating that the field direction does not affect the dependence of γ on field strength for Stockmayer fluid surfaces . The disparities between the molecular water system and point dipolar system could arise from the geometric nature of the model particles, and the degrees of the surface polarization induced by different directional E‐fields.…”

“…In a series of previous atomistic simulation studies, Grest and coworkers have validated the capillary wave boarding effect for different liquid‐vapor interfaces, by examining the scaling dependence of interfacial width on interfacial dimensions, Δ 2 =Δ 0 2 + k B T /2π γ ln( L / B 0 ), in which L is the interfacial dimension, ∆ 0 and B 0 are characteristic lengths for specific materials. With this scaling relationship and the data demonstrated in Figure b, considering constant interfacial dimension and constant temperature, we could estimate a decreasing trend in the surface tension when external E‐field is applied, and the trend should appear differently depending on the type of the electrode which faces the water surface.…”

“…A few molecular simulation studies on the simple dipolar liquid‐vapor interface properties exposed to a spatially uniform external field have been carried out recently . It is reasonable to employ uniform electric fields in the studies of the structural and thermodynamic behaviors of bulk water or ices with the periodic boundary conditions applied in the simulation .…”