Water liquid-vapor interface subjected to various electric fields: A molecular dynamics study

Simple rose model of water in constant electric field

A simple two-dimensional statistical mechanical water model, called the rose model, was used in this work. We studied how a homogeneous constant electric field affects the properties of water. The rose model is a very simple model that helps explain the anomalous properties of water. Rose water molecules are represented as two-dimensional Lennard-Jones disks with potentials for orientation-dependent pairwise interactions mimicking formations of hydrogen bonds. The original model is modified by addition of charges for interaction with the electric field. We studied what kind of influence the electric field strength has on the model's properties. To determine the structure and thermodynamics of the rose model under the influence of the electric field we used Monte Carlo simulations. Under the influence of a weak electric field the anomalous properties and phase transitions of the water do not change. On the other hand, the strong fields shift the phase transition points as well as the position of the density maximum.

Insights into the mechanism of electrostatic field promoting ozone mass transfer in water: A molecular dynamics perspective

Ozone is the main role of ozone-based advanced oxidation process for organic wastewater treatment, which is usually added in water by aeration. However, the low solubility of ozone in water seriously affects the degradation efficiency. In this article, the external electrostatic field (EF) was proposed to improve the ozone solubility in water. The mass transfer characteristics of ozone in a gas-liquid two-phase system under EF were studied by molecular dynamics (MD) simulation. The microscopic mechanism of ozone mass transfer in water promoted by external EF was revealed by analyzing Gibbs dividing surface (GDS) interface structure, interfacial water molecular orientation, surface tension, liquid phase viscosity, hydrogen bond network and ozone self-diffusion coefficient. Our findings reveal that EF can enhance the thickness of GDS region (from 0.4648 nm to 0.4996 nm when EF is 0.2 V/nm) as well as its ozone content. The dipole moment orientation of water molecules also tends to point in the EF direction due to the influence of EF, making the difference in dipole moment orientation of water molecules in the first and second layers of GDS region gradually disappear. In addition, compared with the absence of EF, the existence of external EF can decrease the surface tension (from 77.6162 mN/m to 73.3480 mN/m when EF is 0.2 V/nm) at the gas-liquid interface and the viscosity of liquid phase (from 0.293 mP_a·s to 0.162 mP_a·s when EF is 0.2 V/nm), break the network of hydrogen bond in liquid phase, and increase the mobility of ozone (self-diffusion coefficient of ozone changes from 1.3232 × 10^-6 cm²·s^-1 to 1.8812 × 10^-6 cm²·s^-1 when EF is 0.2 V/nm). All these properties changes indicate that the presence of external EF enhances the ability of ozone to penetrate the interface of the two-phase system, and then improves the ozone mass transfer efficiency in water.

Wettability of cellulose surfaces under the influence of an external electric field

HYPOTHESIS

Interfacial tensions play an important role in dewatering of hydrophilic materials like nanofibrillated cellulose, and are affected by the molecular organization of water at the interface. Application of an electric field influences the orientation of water molecules along the field direction. Hence, it should be possible to alter the interfacial free energies to tune the wettability of cellulose surface through application of an external electric field thus, aiding the dewatering process.

SIMULATIONS

Molecular dynamics simulations of cellulose surface in contact with water under the influence of an external electric field have been conducted with GLYCAM-06 forcefield. The effect of variation in electric field intensity and directions on the spreading coefficient has been addressed via orientational preference of water molecules and interfacial free energy analyses.

FINDINGS

The application of electric field influences the interfacial free energy difference at the cellulose-water interface. The spreading coefficient increases with the electric field directed parallel to the cellulose-water interface while it decreases in the perpendicular electric field. Variation in interfacial free energies seems to explain the change in contact angle adequately in presence of an electric field. The wettability of cellulose surface can be tuned by the application of an external electric field.

Water electrification: Principles and applications

Deep engineering of liquid water by charge and impurity injection, charged support, current flow, hydrophobic confinement, or applying a directional field has becoming increasingly important to the mankind toward overcoming energy and environment crisis. One can mediate the processes or temperatures of molecular evaporation for clean water harvesting, HO bond dissociation for H₂ fuel generation, solidification for living-organism cryopreservation, structure stiffening for bioengineering, etc., with mechanisms being still puzzling. We show that the framework of "hydrogen bonding and electronic dynamics" has substantiated the progress in the fundamental issues and the aimed engineering. The segmental disparity of the coupled hydrogen bond (O:HO or HB with ":" being lone pair of oxygen) resolves their specific-heat curves and turns out a quasisolid phase (QS, bound at -15 and 4 °C). Electrification shows dual functionality that not only aligns, orders, polarizes water molecules but also stretches the O:HO bond. The O:HO segmental cooperative relaxation and polarization shift the QS boundary through Einstein's relation, ΔΘ_Dx ∝ Δω_x, resulting in a gel-like, viscoelastic, and stable supersolid phase with raised melting point T_m and lowered temperatures for vaporization T_V and ice nucleation T_N. The supersolidity and electro structure ordering provide additional forces to reinforce Armstrong's water bridge. QS dispersion and the secondary effect of electrification such as compression define the T_N for Dufour's electro-freezing. The T_V depression, surface stress disruption, and electrostatic attraction raise Asakawa's molecular evaporability. Composition of opposite, compatible fields eases the HO dissociation and soil wetting. Progress evidences not only the essentiality of the coupled O:HO bond theory but also the feasibility of engineering water and solutions by programmed electrification.

Electrochemical local etching of silicon in etchant vapor

Direct machining and imprinting of Si are beneficial for simplifying the fabrication of microelectromechanical systems, nanoelectromechanical systems, optical devices, and fin field-effect transistors, and for reducing process costs. Electrochemical micromachining has been introduced for highly doped Si, but complex structures cannot be imprinted directly. With chemical imprinting, complex nano/micropatterns can be imprinted even on low-doped Si, but the physical contact can damage the templates. In this study, we demonstrated an electrochemical local etching (ELE) method for fabricating nano/micrometer structures on semiconductors in a noncontact manner. Polygon tips were prepared as templates on highly doped n-type Si via etching in KOH. A constant space is maintained between the template and the target Si using a gap layer to prevent damage and contamination. In the etchant vapor, the voltage bias between the template and the target Si leads to condensation of the etchant. Because the etching region is localized by the condensation of the etchant, even low-doped semiconductors can be imprinted in submicrometer patterns in a single step. When the etchant condensation is suppressed, the etching area is reduced and the resolution is increased, allowing direct imprinting of the polygonal submicrometer pattern. ELE has the potential to produce complex nano/micrometer structures in a single step without photoresists and physical contact.

Electromechanics of the liquid water vapour interface

Two collective properties distinguishing the thin liquid water vapour interface from the bulk liquid are the anisotropy of the pressure tensor giving rise to surface tension and the orientational alignment of the molecules leading to a finite dipolar surface potential. Both properties can be regarded as capillary phenomena and are likely to be coupled. We have investigated this coupling by determining the response of the tangential component of the surface tension to the application of an electric field normal to the surface using finite field molecular dynamics simulations. We find an upside down parabola with a maximum shifted away from zero field. Comparing the molecular dynamics results to a phenomenological electromechanical model we relate the zero field derivative of the tangential part of the surface tension to the electrostatic potential generated by the spontaneous interface polarization. When interpreted with this model our simulations also indicate that Kelvin forces due to electric field gradients at a polarized interface play an important role in the effective dielectric response.

Formation and migration of H3O+ and OH− ions at the water/silica and water/vapor interfaces under the influence of a static electric field: a molecular dynamics study

Water ‘layers’ 1 and 2 in pink; ‘layer’ 3 in blue and green over portion of glass surface (grey). +90° field causes water migration and clustering.

Enhanced conductivity of water at the electrified air–water interface: a DFT-MD characterization

DFT-based molecular dynamics simulations of the electrified air–liquid water interface are presented, where a homogeneous field is applied parallel to the surface plane (i.e. parallel to the 2D-HBonded-Network/2DN).

Orientational and Folding Thermodynamics via Electric Dipole Moment Restraining

The projection of molecular processes onto a small set of relevant descriptors, the so-called reaction coordinates or collective variables (CVs), is a technique nowadays routinely employed by the biomolecular simulation community. In this work, we implemented two CVs to manipulate the orientation (i.e., angle) (μ⃗_a) and magnitude (|μ⃗|) of the electric dipole moment. In doing so, we studied the thermodynamics of water orientation under the application of external voltages and the folding of two polypeptides at zero-field conditions. The projection of the free-energy [potential of mean force (PMF)] along water orientation defined an upper limit of around 0.3 V for irrelevant thermodynamic effects. On the other hand, sufficiently strong μ⃗_a restraints applied on 12-alanine (Ala₁₂) triggered structural effects because of the alignment of local dipoles; for lower restraints, a full-body rotation is achieved. The manipulation of |μ⃗| produced strong perturbations on the secondary structure of Ala₁₂, promoting an enhanced sampling to its configurational space. Rigorous free-energy calculations in the form of 2-D PMFs for deca-alanine showed the utility of |μ⃗| as a reaction coordinate to study folding in small α helices. As a whole, we propose that the manipulation of both components of the dipole moment, μ⃗_a and |μ⃗|, provides thermodynamics insights into the structural conformation and stability of biomolecules. These new CVs are implemented in the Colvars module, available for NAMD and LAMMPS.

Effect of an external electric field on capillary filling of water in hydrophilic silica nanochannels

Development of functional nanofluidic devices requires understanding the fundamentals of capillary driven flow in nanochannels.