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.     

Transient phoretic migration of a permselective colloidal particle

We quantify the phoretic migration of a spherical cation-permselective colloidal particle immersed in a binary electrolyte under a time-dependent electric field. We invoke the thin-Debye-layer approximation, where the size of ionic Debye layer enveloping the particle is much smaller than the particle radius. The imposed electric field generates ion concentration gradients, or concentration polarization, in the bulk (electroneutral) electrolyte outside the Debye layer. The bulk ion concentration polarization--and consequently the particle's phoretic velocity--evolves on the time scale for ion diffusion around the particle, which can be on the order of milliseconds for typical colloidal dimensions. Notably, concentration polarization arises here solely due to the permselectivity of the particle; it does not require non-uniform ionic transport in the Debye layer (i.e., surface conduction). Thus, the phoretic transport of a permselective particle is significantly different to that of a inert, dielectric particle, since surface conduction is necessary to achieve bulk concentration polarization in the (more commonly studied) latter case. Calculations are presented for a permselective particle under oscillatory (ac) and suddenly applied electric fields. In the former case, the particle velocity possesses frequency-dependent components in phase and out of phase with the driving field; in the latter case, the particle approaches its terminal velocity with a long-time (algebraic) tail.     

The Structure of Water Bonded to Phosphate Groups at the Electrified Zwitterionic Phospholipid Membranes/Aqueous Interface

Gaining insight into the water structure at the electrified phospholipid membranes/aqueous interface is vital and essential for elucidating the mechanism of many biochemical reactions, but still remains a formidable challenge. Herein, based on the superiority of surface enhanced infrared absorption (SEIRA) spectroscopy combined with electrochemistry in interfacial analysis, the evolution of local water structure at the zwitterionic phospholipid membranes/aqueous interface with an external electric field is revealed by means of ion perturbation. The strongly hydrogen‐bonded water directly bonded to the phosphate groups (PO₂^?) has a strong mechanical strength to resist potential perturbations, and that portion of water greatly affects the electrostatic properties of the phospholipid membranes. This study innovates the basic understanding of electric double layer (EDL) at the membranes/aqueous interface.     

Switching on stray electric fields in ferroelectric liquid crystal cells

In liquid crystal dot-matrix displays light may leak through the display area between the pixels. To obtain sufficient contrast this non-pixel area has to be made non-transmissive. For ferroelectric liquid crystal (FLC) displays this may be done by switching the material in the gaps between the picture elements to a non-transmissive state by the stray electric fields that occur during application of voltages to the pixel electrodes. This is experimentally studied for test cells with an electrically modified smectic layer structure. The gap region considered is an asymmetric environment of the FLC material, as the transparent conductive coating has been removed on one substrate, whereas on the other substrate a conductor covers the glass. The FLC molecules in the non-pixel area prefer to direct their dipoles towards the covered substrate. To switch the FLC material with the stray electric fields, it is a prerequisite to outweigh this preference. We made spatially resolved observations for various gap widths and various applied voltages on 2 μm thick FLC layers. With bipolar voltage pulses of 64 μs width each, amplitudes of about 25 V are needed to switch the FLC in 3·2 or 4·0 μm wide gaps. It was found to be more difficult to switch gaps that, are 7 μm wide than was anticipated on the basis of the results for 4 μm gaps. This is attributed to the surface polarization charge due to the FLC permanent dipoles built up at the FLC-glass interface. Experimental results supporting this explanation are presented.     

Electrokinetic actuation of an uncharged polarizable dielectric droplet in charged hydrogel medium

Electrokinetic transport of an uncharged nonconducting microsized liquid droplet in a charged hydrogel medium is studied. Dielectric polarization of the liquid drop under the action of an externally imposed electric field induces a non-homogeneous charge density at the droplet surface. The interactions of the induced surface charge of the droplet with the immobile charges of the hydrogel medium generates an electric force to the droplet, which actuates the drop through the charged hydrogel medium. A numerical study based on the first principle of electrokinetics is adopted. Dependence of the droplet velocity on its dielectric permittivity, bulk ionic concentration, and immobile charge density of the gel is analyzed. The surface conduction is significant in presence of charged gel, which creates a concentration polarization. The impact of the counterion saturation in the Debye layer due to the dielectric decrement of the medium is addressed. The modified Nernst–Planck equation for ion transport and the Poisson equation for the electric field is considered to take into account the dielectric polarization. A quadrupolar vortex around the uncharged droplet is observed when the gel medium is considered to be uncharged, which is similar to the induced charge electroosmosis around an uncharged dielectric colloid in free-solution. We find that the induced charge electrokinetic mechanism creates a strong recirculation of liquid within the droplet and the translational velocity of the droplet strongly depends on its size for the dielectric droplet embedded in a charged gel medium.     

Temperature Dependence of the Dielectric Permittivity of Acetic Acid,Propionic Acid and Their Methyl Esters: A Molecular Dynamics Simulation Study

For most liquids, the static relative dielectric permittivity is a decreasing function of temperature, because enhanced thermal motion reduces the ability of the molecular dipoles to orient under the effect of an external electric field. Monocarboxylic fatty acids ranging from acetic to octanoic acid represent an exception to this general rule. Close to room temperature, their dielectric permittivity increases slightly with increasing temperature. Herein, the causes for this anomaly are investigated based on molecular dynamics simulations of acetic and propionic acids at different temperatures in the interval 283–363 K, using the GROMOS 53A6_OXY force field. The corresponding methyl esters are also considered for comparison. The dielectric permittivity is calculated using either the box‐dipole fluctuation (BDF) or the external electric field (EEF) methods. The normal and anomalous temperature dependences of the permittivity for the esters and acids, respectively, are reproduced. Furthermore, in the EEF approach, the response of the acids to an applied field of increasing strength is found to present two successive linear regimes before reaching saturation. The low‐field permittivity ε, comparable to that obtained using the BDF approach, increases with increasing temperature. The higher‐field permittivity ε′ is slightly larger, and decreases with increasing temperature. Further analyses of the simulations in terms of radial distribution functions, hydrogen‐bonded structures, and diffusion properties suggest that increasing the temperature or the applied field strength both promote a relative population shift from cyclic (mainly dimeric) to extended (chain‐like) hydrogen‐bonded structures. The lower effective dipole moment associated with the former structures compared to the latter ones provides an explanation for the peculiar dielectric properties of the two acids compared to their methyl esters.     

Spontaneous polarization and electro-optic behaviour of a ferroelectric liquid crystal cell fabricated with polyimide Langmuir-Blodgett films

The spontaneous polarization and electro-optic response of ferroelectric liquid crystals (FLCs) were investigated in a cell fabricated with a polyimide alignment layer coated by the Langmuir-Blodgett method. The surface properties of the cured polyimide layers were monitored by contact angle measurement, and by FTIR spectroscopy and AFM for the orientation and surface roughness, respectively. The apparent spontaneous polarization of an FLC determined in a practical sandwich-cell depended on various conditions such as cell thickness, cooling rate from the smectic A to chiral smectic C phase, and deposition pressure. Electro-optic response and decay times of FLCs were also measured. Furthermore, the ions in the FLC mixture reduced the magnitude of the effective electric field, but had no effect at high frequency.     

Polarized states of hybrid aligned nematic layers

Electric polarization arising in hybrid aligned nematic liquid crystal layers with rigid boundary conditions is studied numerically by solving the torques equation and Poisson equation. Three phenomena that give rise to the polarization are taken into account: flexoelectricity, surface polarization and adsorption of ions. The director orientation within the layer, as well as the distribution of electric potential and space charge density are calculated for layers deformed by an external magnetic field. The role of the ionic space charge is investigated. For a particular set of parameters of a model substance, the voltage arising between the layer surfaces varies from 10^-1 V (in an extremely pure nematic) to 10^-3 V (in material with a typical ion concentration). The surface polarization yields an additional voltage (of the order 10^-2 V) nearly independent of the ion concentration. The effect of simultaneous flexoelectric polarization and ion adsorption is evidently different from a linear superposition of their separate contributions. The flexoelectric polarization leads to partial separation of ions of opposite signs. In the case of positive flexoelectric coefficients, a thin sublayer of positive charge arises at the planar-orienting boundary plate. The negative charge is displaced towards the homeotropically aligning plate. The magnitude of this effect increases with the magnetic field. The surface phenomena introduce additional subsurface charges.     

A polarizable electrostatic model of the N‐methylacetamide dimer

Our previously developed polarizable electrostatic model is applied to isolated N‐methylacetamide (NMA) and to three hydrogen‐bonded configurations of the NMA dimer. Two versions of the model are studied. In the first one (POL1), polarizability along the valence bonds is described by induced bond charge increments, and polarizability perpendicular to the bonds is described by cylindrically isotropic induced atomic dipoles. In the other version (POL2), the induced bond charge increments are replaced by induced atomic dipoles along the bonds. The parameterization is done by fitting to ab initio MP2/6‐31++G(d,p) electric potentials. The polarizability parameters are determined by subjecting the NMA molecule to various external electric fields. POL1 turns out to be easier to optimize than POL2. Both models reproduce well the ab initio electric potentials, molecular dipole moments, and molecular polarizability tensors of the monomer and the dimers. Nonpolarizable models are also investigated. The results show that polarization is very important for reproducing the electric potentials of the studied dimers, indicating that this is also the case in hydrogen bonding between peptide groups in proteins. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1933–1943, 2001     

Analysis of self-electrophoretic motion of a spherical particle in a nanotube: effect of nonuniform surface charge density

Autonomous motions of a spherical nanoparticle in a nanotube filled with an electrolyte solution were investigated using a continuum theory, which consisted of the Nernst-Planck equations for the ionic concentrations, the Poisson equation for the electric potential in the solution, and the Stokes equation for the hydrodynamic field. Contrary to the usual electrophoresis, in which an external electric field is imposed to direct the motion of charged particles, the autonomous motion originates from the self-generated electric field due to the ionic concentration polarization of the liquid medium surrounding an asymmetrically charged particle. In addition to the particle motion, the interaction between the electric field generated and the free charges of the polarized solution induces electroosmotic flows. These autonomous motions of the fluid as well as the particle were examined with focus on the effects of the surface-charge distribution of the particle, the size of the nanotube, and the thickness of the electric double layer, which affected the direction and the speed of the particle significantly.     

Electro‐osmotic flow in nanochannels with voltage‐controlled polyelectrolyte brushes: Dependence on grafting density and normal electric field

Molecular dynamics simulations were performed for electro‐osmotic flow (EOF) confined in a polyelectrolyte‐grafted nanochannel under variable grafting density and normal electric field. With decreasing the value of the normal electric field, the brush undergoes a collapse transition, and the ion distribution is changed significantly. The brush thickness increases on increasing the grafting density at positive and weak negative electric fields, whereas a reduced brush thickness is observed at strong negative electric field. Our results further reveal that the flow velocity is not only dependent on conformational transition of the brush but also related to the cation and anion distributions. At low grafting density, the EOF is almost completely quenched at high electric field strength due to strong surface friction between ions and walls. For the case of very dense grafting, the flow velocity is influenced weakly within the brush when varying the grafting density. Additionally, a bidirectional flow occurs at an intermediate electric field. The investigation on fluid flux indicates that the fluid flux is insensitive to the grafting density, when the normal electric field is removed. For nonzero normal electric fields, a significant change in the fluid flux is observed at low grafting densities. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Enhanced pearl-chain formation by electrokinetic interaction with the bottom surface of vessel

Counterions in an electric double layer (EDL) around a colloidal particle accumulate on one side of the EDL and are deficient on the other side under an electric field, resulting in an imbalance of ionic concentration in the EDL, that is to say, the ionic polarization of EDL. It is well known that the ionic polarization of EDL induces electric dipole moments whereby the alignments of colloidal particles (e.g., pearl chains) are formed under alternating electric fields. In this study, we focus on the effect of the frequency of applied electric fields (100 Hz-1 kHz) on the alignment of silica particles settling at the bottom of a silica glass vessel. In digital imaging analyses for pearl chains of silica particles, it is confirmed that surface distances between two neighboring particles decrease but the number of particles in a pearl chain increases as the frequency of the applied electric field is lowered from 1 kHz to 100 Hz. More interestingly, electrical conductance measurements suggest that the induced ionic polarization of EDL around silica particles at the bottom of the silica vessel is enhanced as the frequency is lowered from 1 kHz to 100 Hz, whereas the ionic polarization around isolated silica particles in uniform dispersions is alleviated by the relaxation of ionic concentration in the EDL as a result of the diffusion of counterions. This curious phenomenon can be explained by considering that the ionic polarization of EDL of silica particles at the bottom of a vessel is affected by the electro-osmosis of the silica surface at the bottom of the vessel.     

Effects of Applied Electric Fields on Drop—Interface and Drop—Drop Coalescence*

In this work, coalescence of a single organic or aqueous drop with its homophase at a horizontal liquid interface was investigated under applied electric fields. The coalescence time was found to decrease for aqueous drops as the applied voltage was increased, regardless of the polarity of the voltage. For organic drops, the coalescence time increased with increasing applied voltage of positive polarity and decreased with increasing applied voltage of negative polarity. Under an electric field, the coalescence time of aqueous drops decreases due to polarization of both the drop and the flat interface. The dependency of organic drop-interface coalescence on the polarity of the electric field may be a result of the negatively charged organic surface in the aqueous phase. Due to the formation of a double layer, organic drops are subjected to an electrostatic force under an electric field, which, depending on the field polarity, can be attractive or repulsive. Pair-drop coalescence of aqueous drops in the organic phase was also studied. Aqueous drop-drop coalescence is facilitated by polarization and drop deformation under applied electric fields. Without applied electric fields, drop deformation increases the drainage time of the liquid film between two approaching drops. Therefore, a decrease in the interfacial tension, which causes drop deformation, accelerates drop-drop coalescence under an electric field and inhibits drop coalescence in the absence of an electric field.     

Ionization of water in interfacial electric fields: An electrochemical view

High electric fields promote ionization of water, yet relatively little is known about this topic due to the difficulty of generating such fields. The high field capability of field emitter tips enables study of ionization in water layers. Results from this work include ionization fields, water layer morphology, dielectric properties, coadsorbate interactions, cluster distributions of hydrated hydronium ions H⁺(H₂O)_m, and field ionization images. These experimental results, combined with theoretical findings, are interpreted in the context of four examples from electrochemistry; double layer structure, hydrogen oxidation, CO oxidation, and oxygen reduction; to reveal the research frontier in interfacial ionization of water.     

A new lattice-based theory for hydrogen-bonding liquids in uniform electric fields

We propose a new lattice-based, mean-field theory for predicting alignment of molecular dipoles and hydrogen bonds in liquids subject to uniform electric fields. The theory is presently restricted to liquids whose molecules possess one (proton) donor and one acceptor sites each, and wherein the H-bond axis is collinear with the dipole moments of the bonded molecules. The final expressions for hydrogen bond stoichiometry and polarization are free of lattice parameters, are interpretable using simple phenomenological arguments, and reduce to known limiting forms. The theory is applied to understand the internal structure of hydrogen cyanide in the liquid state at different electric fields.     

Dynamics of the Electric Double Layer: Analysis in the Frequency and Time Domains

No rigorous theory of electrokinetic phenomena is conceivable without properly accounting for double layer polarization under the action of external fields. Since processes leading to such polarization need a finite time to develop, an analysis of the behavior of the quantities of interest (potential and ion concentration profiles, particle or fluid velocity, and so on) as a function of time should be extremely illustrative. In this work, we analyze how those quantities evolve in the nanosecond to microsecond time range after the application of an electric field. The network method is proposed (in which, essentially, an electric circuit simulator program is used to solve the differential equations involved, after their proper interpretation in terms of fluxes and forces) to gain information about the evolution with time of the potential, counterion, and co-ion perturbations, the particle velocity, and the fluid velocity profile. The performance of the method is first ckecked in the frequency domain, for which rigorous solutions exist, and then the procedure is used in the time domain. Reasons are discussed for the observed time dependencies of the analyzed quantities. Copyright 2000 Academic Press.     

离子交换树脂悬浊液的介电弛豫谱研究

研究了D354阴离子交换树脂分散在不同浓度KCl溶液中的悬浊液的频率域介电谱,发现在测量频率为106～107 Hz处出现了显著的介电弛豫现象,得出了介电常数、电导率以及弛豫时间随KCl溶液浓度的特异的变化关系,理论分析表明,该弛豫是一个以界面极化为主的非单一极化机制的弛豫过程,进而利用Maxwell-Wagner界面极化理论和双电层性质解释了该体系的特异介电行为,得到了树脂悬浊液在外加交变电场下的离子迁移和聚集信息,并确定了该树脂在静态平衡下双电层中对离子的相对离子强度.     

Electrophoresis of solid particles at large Peclet numbers

A theory of concentration polarization of a thin electrical double layer (DL) on a spherical particle is developed for the regime of large Peclet numbers which is realized in strong electric fields. In this regime, the concentration field arising outside DL is estimated under influence of diffusion and convection. According to the theory developed, polarization of DL at large Peclet numbers causes a change in the Stern potential, the formation of a dipole moment and the long-range potential. The diffuse layer deviates strongly from spherical symmetry and electroneutrality, and the screen of the surface charge is provided not only by the diffuse atmosphere but also by the charge induced in the convective-diffusion layer. The effect of electric field on the induced charge gives rise to the additional electroosmotic slip, that was called "secondary electroosmosis". Thus, a nonlinear additional term for the Smoluchowski formula of electrophoretic velocity is based on the changes of zeta-potential and on the secondary electroosmotic slip. The comparison of theory with experimental results revealed considerable fitting.     

The role of new functional groups in the surface layer of LDPE during its high-voltage contact polarization

New functional groups containing Al-C bonds and Al₂O₃ molecules are formed in the surface layer of the polymer during depositing of aluminum on polyethylene films. This effect is absent when Au is deposited on the films. For capacitors with Al and Au electrodes, processes of high-voltage contact polarization and conductivity are studied at various configurations of the external electric field. If Al-C organometallic bonds and Al₂O₃ or, for example, Al-O-C bonds are formed in the surface layer during application of high electric fields, a number of dielectric “anomalies” are observed. They manifest themselves as the fact that an increase in the amount of cycles of a bipolar saw-tooth voltage is accompanied by a marked increase in the residual surface charge density. At such a configuration of the external field, a decrease in its frequency (for region, where dE/dt > 0) leads to a marked reduction in high-voltage dielectric permittivity to negative values. This phenomenon is explained by the fact that, along with the external field, an internal field appears owing to formation of the space charge near metal/polymer boundaries.     

Non-Gaussian statistics of electrostatic fluctuations of hydration shells

This paper aims to understand the statistics of the electric field produced by water interfacing a non-polar solute of nanometer dimension. We study, by numerical simulations, the interface between SPC/E water and a Kihara solute, which is a hard-sphere core with a Lennard-Jones layer at its surface. The distribution of the interfacial electric field is monitored as a function of the magnitude of a point dipole placed close to the solute-water interface. The free energy surface as a function of the electric field projected on the dipole direction shows a cross-over with increasing dipole magnitude. While it is a single-well harmonic function at low dipole values, it becomes a double-well surface at intermediate dipole moment magnitudes, transforming into a single-well surface again, with a non-zero minimum position, at still higher dipoles. This transformation, reminiscent of a discontinuous phase transition in bulk materials, has a broad intermediate region where the interfacial waters fluctuate between the two minima. This region is characterized by intense field fluctuations, with non-Gaussian statistics and variance far exceeding expectations from the linear-response approximation. The excited state of the surface water is found to be lifted above the ground state by the energy required to break approximately two hydrogen bonds. This state is pulled down in energy by the external electric field of the solute dipole, making it readily accessible to thermal excitations. The excited state is a surface defect in the hydrogen-bond network, creating a stress in the nearby network, but otherwise relatively localized in the region closest to the solute dipole.