Electrostatic Responses Research Articles

Mechanical deformation affects the counterion condensation in highly-swollen polyelectrolyte hydrogels.

Polyelectrolyte gels can generate electric potentials under mechanical deformation. While the underlying mechanism of such a response is often attributed to changes in counterion-condensation levels or alterations in the ionic conditions in the pervaded volume of the hydrogel, the exact molecular origins are largely unknown. By using all-atom molecular dynamics simulations of a polyacrylic acid hydrogel in explicit water as a model system, we simulate the uniaxial compression and uniaxial stretching of weakly to highly swollen (i.e., between 60-90% solvent content) hydrogel networks and calculate the microscopic condensation levels of counterions around the hydrogel chains. The counterion condensation under deformation is highly non-monotonic. Ionic condensation around the constituting chains of the deformed hydrogel tends to increase as the chains are stretched. This increase reaches a maximum and decreases as the chains are strongly stretched. The condensation around the collapsed chains of the hydrogel is weakly affected by the deformation. As a result, both compressing and stretching the model hydrogel lead to an overall increase in the counterion condensation. The effect vanishes for weakly swollen hydrogels, for which most ions are already condensed. The simulations with single, stretched polyelectrolyte chains show a qualitatively similar response, suggesting the effect of chain elongation on the ionic distribution throughout the hydrogel. Notably, this deformation-induced counterion condensation phenomenon does not occur in a polyelectrolyte solution at its critical concentration, indicating the role of hydrogel topology constraining the chain ends. Our results indicate that counterion condensation in a deforming polyelectrolyte hydrogel can be highly heterogeneous and exhibit a rich behaviour of electrostatic responses.

Electric Fields and Charge Separation for Solid Oxide Fuel Cell Electrodes.

Activation losses at solid oxide fuel cell (SOFC) electrodes have been widely attributed to charge transfer at the electrode surface. The electrostatic nature of electrode–gas interactions allows us to study these phenomena by simulating an electric field across the electrode–gas interface, where we are able to describe the activation overpotential using density functional theory (DFT). The electrostatic responses to the electric field are used to approximate the behavior of an electrode under electrical bias and have found a correlation with experimental data for three different reduction reactions at mixed ionic–electronic conducting (MIEC) electrode surfaces (H2O and CO2 on CeO2; O2 on LaFeO3). In this work, we demonstrate the importance of decoupled ion–electron transfer and charged adsorbates on the performance of electrodes under nonequilibrium conditions. Finally, our findings on MIEC–gas interactions have potential implications in the fields of energy storage and catalysis.

Molecular Modeling of Ionic Liquids: Force‐Field Validation and Thermodynamic Perspective from Large‐Scale Fast‐Growth Solvation Free Energy Calculations

AbstractIn molecular modelling of novel solvents such as ionic liquids, it is common to scale atomic charges to improve the experiment‐simulation agreement for some selected properties. As these liquids are designed to solvate solutes, whether the solvation thermodynamics could be correctly described is of utmost importance. Therefore, we present a comprehensive large‐scale calculation of solvation free energies via nonequilibrium fast‐switching simulations for a spectrum of molecules in ionic liquids, the atomic charges of which are scaled to maximize the prediction‐experiment correlation. Further, the density‐derived choice is compared with the solvation‐thermodynamics‐derived one. When the scaling factor is decreased, the density exhibits a monotonically decreasing behavior due to weaker inter‐molecular interactions produced by scaled atomic charges. However, solvation free energies do not show consistent monotonic behaviors, which are caused by competing electrostatic and vdW responses to the scaling‐parameter variation. More intriguingly, the solvation‐free‐energy‐derived scaling factor is generally slightly higher than the density‐derived one. We further calculate partition coefficients ortransfer free energies of solutes from water to ionic liquids to provide another thermodynamic perspective of charge scaling. Another central result is the detailed evaluation of the widely used force fields for bonded and vdW terms, i.e., the GAFF derivatives.

Study of Electrostatic Field Responses of Locally Marketed Face Masks in the Philippines

The face mask is the first line of defense against infectious particulates and droplets that may cause illness. Currently in the Philippines, the wearing of face mask is compulsory whenever citizens leave their residences as mandated by the government to mitigate the spread of COVID-19. The wearing of face masks has become a new normal among Filipinos. This created market opportunities for different types which became commonly and immediately available for purchase. This study aimed to differentiate the effectiveness of locally available face masks in terms of electrostatic filtration capability. Twelve different types of face masks grouped into five categories – surgical, fabric, N95 variants, foam type, and novelty type – were evaluated. Electrostatic fields were measured from each face mask including pore sizes via scanning electron microscopy. Moreover, by utilizing the estimated charge and mass of the SARS-CoV-2 virion, the transmission rate was simulated using COMSOL Multiphysics®. It was observed that face masks with negatively charged materials combined with small pore sizes afforded less particle transmission. The results of this study are of timely significance in potentially laying out public awareness in the selection and utilization of face masks that can provide foremost shielding against viral transmission.

Electrostatic responses of anisotropic dielectric films

We study the electrostatic responses (i.e. retardation effects due to the propagation of electromagnetic waves are ignored) of a linear homogeneous and anisotropic (LHA) dielectric film to an arbitrary external electrostatic potential. A set of algebraic equations has been established to calculate the polarisation charges induced in the film. In our derivation, the idea is exploited that a physical boundary can be looked upon as a region of rapid variation in polarisation rather than a simple geometric separation. With this no boundary conditions are needed in solving the relevant electrostatics problem. Our approach makes it clear that the responses consist of two contributions, one arising from the very presence of surfaces while the other existing even in an infinite medium. In light of the results, we discuss graphene plasma waves under the influence of a LHA dielectric film such as a few-layer hexagonal boron nitride. It is found that the dispersion of these waves is strongly affected by the anisotropy at wavelengths comparable to the film thickness.

Theoretical analysis and modeling of the electrostatic responses of the vibrational and NMR spectroscopic properties of the cyanide anion

The nature of the electrostatic responses of the vibrational and NMR spectroscopic properties is analyzed theoretically for the cyanide anion and the cyanide ligand to the heme. It is shown that the dependence of the CN stretching frequency on the applied electric field (the origin of the Stark tuning rate) is sufficiently large and linear, suggesting that the cyanide anion and ligand are good probes of the electrostatic situation around them. On the basis of the electron density analysis, it is clarified that the behavior of electrons induced by molecular vibrations plays an important role in this response. Electrostatic interaction models that employ both the vector and scalar components on atomic sites are constructed to consistently explain the changes in the spectroscopic properties induced by uniform electric field and by hydrogen-bond formation with water. It is shown that the vibrational and NMR spectroscopic properties are affected differently by the electric fields and electrostatic potentials on the atomic sites, suggesting that these probes play mutually complementary roles in elucidating the electrostatic situations around the CN group.

Detecting Dynamic Manipulation of the Space Charge Region in Doped Ceria with Scanning Thermo-Ionic Microscopy

A new generation of high performance energy storage and conversion devices is instrumental to commercial adoption of renewable, but intermittent, energy sources. Developing new materials for these devices often relies on bulk characterization techniques, such as electrochemical impedance spectroscopy, which are inherently unable to spatially resolve electrochemical phenomena and material properties. Conversely, scanning probe methods (SPM) have long been employed to probe local electrochemistry and material properties; however, relating these measurements to macroscopic performance can be challenging. We aim to bridge this gap by using SPM as a localized in operando witness during macroscopic measurements. Workers have recently employed Scanning Thermo-Ionic Microscopy (STIM), a strain-based SPM that leverages mechano-thermo-chemical coupled effects to detect local ionic concentrations. Briefly, STIM operates by inducing thermal stresses in the material from a periodic temperature perturbation at the scanning probe tip. The strain related to these thermal stresses drives ionic motion and is detected by deflection of the probe tip at a higher order harmonic of the temperature perturbation. This allows the ionic response to be distinguished from electrostatic and electromechanical responses, which occur at the temperature perturbation frequency. Previous ex-situ measurements demonstrated the potential of STIM to image grain boundary effects in doped ceria, possibly due to space charge regions. More recently, STIM was implemented point-wise to study manipulation of the space charge region under static polarization. Though promising, intensive hardware demands prevented imaging with STIM, thus ultimately reducing the impact of s-SPM implementation. Herein we demonstrate the use of data science enhanced SPM to overcome hardware limitations and extend our work to in operando, dynamic polarization studies. Furthermore, the effect of adsorbed surface species on STIM response is examined by controlling temperature, gas environment, and relative humidity.

Strategy for Modeling the Electrostatic Responses of the Spectroscopic Properties of Proteins.

For better understanding and more efficient use of the spectroscopic probes (vibrational and NMR) of the local electrostatic situations inside proteins, appropriate modeling of the properties of those probes is essential. The present study is devoted to examining the strategy for constructing such models. A more well-founded derivation than the ones in previous studies is given in constructing the models. Theoretical analyses are conducted on two representative example cases related to proteins, i.e., the peptide group of the main chains and the CO and NO ligands to the Fe2+ ion of heme, with careful treatment of the behavior of electrons in the electrostatic responses and with verification of consistency with observable quantities. It is shown that, for the stretching frequencies and NMR chemical shifts, it is possible to construct reasonable electrostatic interaction models that encompass the situations of hydration and uniform electric field environment and thus are applicable also to the cases of nonuniform electrostatic situations, which are highly expected for inside of proteins.

Design and modeling of highly tunable filters based on the linear electro-optic effect in strained silicon waveguides

Design and modeling of wavelength tunable filters based on the linear electro-optic (Pockels) effect in strained silicon waveguides is presented. The structure is based on the silicon grating-assisted contradirectional couplers. The basic idea is to utilize a silicon nitride (Si3N4) layer on top of the silicon waveguides to induce a relatively large Pockels effect in the waveguides by breaking the centrosymmetry of the Si crystal due to the surface strain. Wavelength tuning is obtained by applying DC voltages to the device contacts on top of the Si3N4 layer. Based on the presented structure, a wavelength tunable filter at the C-band communication window is designed and its characteristics were investigated by full-vectorial finite-element and finite-difference time-domain methods for electrostatic and optical responses. A 3.5-nm wavelength shift for an applied voltage of 1 V and a 7.5-nm shift for an applied voltage of 5 V are obtained with negligible loss. The presented device is superior to the previously reported structures in terms of tunability and loss.

Charge-modulated CO2 capture

Recently, the concept of using charge-modulation to manipulate the interaction of adsorbate molecules such as CO2 on certain sorbent materials has been advanced through a series of first principle computational predictions. The interactions switched on through charge-modulation shifting the Fermi level are in part, but by no means exclusively, electrostatic. In addition to electrostatics, the Fermi level shifting can be viewed as a way to modulate not only the position but also the identity and character of the frontier orbitals on the sorbent materials that engage with the adsorbate molecules. Thus, a rich new space for electrochemical modulation of surface-molecular interactions—guided by first principle computational modeling—suggests itself. Here we summarize the growing computational literature on switchable CO2 capture strategies. We also provide some new insights into contrasting electrostatic versus chemical responses to charge-modulation as exemplified by water versus CO2 on N-doped graphene surfaces, which suggest that this CO2 capture strategy could be water tolerant.

Electromagnetic zonal flow residual responses

The collisionless axisymmetric zonal flow residual calculation for a tokamak plasma is generalized to include electromagnetic perturbations. We formulate and solve the complete initial value zonal flow problem by retaining the fully self-consistent axisymmetric spatial perturbations in the electric and magnetic fields. Simple expressions for the electrostatic, shear and compressional magnetic residual responses are derived that provide a fully electromagnetic test of the zonal flow residual in gyrokinetic codes. Unlike the electrostatic potential, the parallel vector potential and the parallel magnetic field perturbations need not relax to flux functions for all possible initial conditions.

In Situ Probing of Ion Ordering at an Electrified Ionic Liquid/Au Interface.

Charge transport at the interface of electrodes and ionic liquids is critical for the use of the latter as electrolytes. A room-temperature ionic liquid, 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide (EMMIM TFSI), is investigated in situ under applied bias voltage with a novel method using low-energy electron and photoemission electron microscopy. Changes in photoelectron yield as a function of bias applied to electrodes provide a direct measure of the dynamics of ion reconfiguration and electrostatic responses of the EMMIM TFSI. Long-range and correlated ionic reconfigurations that occur near the electrodes are found to be a function of temperature and thickness, which, in turn, relate to ionic mobility and different configurations for out-of-plane ordering near the electrode interfaces, with a critical transition in ion mobility for films thicker than three monolayers.

Electrostatic and Allosteric Response of Myosin as a Mechanosensor

Allostery is fundamental to the functions of proteins, where a locally-applied input travels within the molecule to a distant region. While the allostery due to chemical input such as ligand binding has long been studied, the growing interest in mechanobiology prompts the study of the allostery due to mechanical input. One of the key proteins involved in mechanobiology is myosin. Myosin not only works as a motor, converting the chemical energy of the ATP hydrolysis to mechanical force, but also functions as a mechanosensor, changing the affinity for the ATP hydrolysis products and for the actin filament in response to the external force (load) that is applied to the so-called lever arm region. Since both the ATP-binding and the actin-binding regions are distant from the lever arm region, myosin is thought to be equipped with the allostery due to mechanical input. However, the underlying physical mechanism is unclear. By molecular dynamics simulation in combination with the replica exchange umbrella sampling, we investigated the response of myosin to the mechanical input applied to the lever arm. We found that the positional change of the basement of the lever arm (called converter region) induces significant electrostatic potential changes in the ATP-binding and the actin-binding regions. The mechanically-induced electrostatic responses we found are likely to be involved in the force generating function of the actin-myosin motor. We further show that allosteric response is caused by the large-scale rearrangement of the electrostatic bond network.

Controlling the motion of DNA in a nanochannel with transversal alternating electric voltages

A nanofluidic channel, with a pair of perpendicularly aligned nanoelectrodes, is proposed to electrically control the motion of DNA molecules. Using all-atom molecular dynamics simulations, we studied electrostatic responses of a charged DNA molecule in the nanochannel and investigated optimized operating conditions for controlling the DNA molecule. When the transversal electric field was periodically turned on and off, the DNA molecule was correspondingly immobilized on and released from the channel surface. Under simultaneously applied longitudinal biasing and transversal trapping electric fields, the DNA molecule moved forward in a ‘ratchet’-like fashion. It is expected that achieving the controlled motion of DNA in the channel can advance studies and applications of a nanochannel-based sensor for analyzing DNA (e.g., DNA sequencing).

Electrostrictive and electrostatic responses in contact mode voltage modulated scanning probe microscopies

Electromechanical response of solids underpins image formation mechanism of several scanning probe microscopy techniques including the piezoresponse force microscopy (PFM) and electrochemical strain microscopy (ESM). While the theory of linear piezoelectric and ionic responses are well developed, the contributions of quadratic effects including electrostriction and capacitive tip-surface forces to measured signal remain poorly understood. Here we analyze the electrostrictive and capacitive contributions to the PFM and ESM signals and discuss the implications of the dielectric tip-surface gap on these interactions.

Controlling Motion of DNA in a Nanochannel with Transverse Alternating-Electric-Voltages

One key requirement for fast and accurate DNA sequencing in nanopores or nanochannels is to control the DNA position and modulate the speed, which, however, still remains a big challenge. Because of drastically changed electric potentials across a nanochannel, the DNA motion is too rapid for a sensor to detect each DNA nucleotide. A nanofluidic channel, with a pair of perpendicularly aligned nanoelectrodes, is proposed to electrostatically control the motion of DNA molecules. Using all-atom molecular dynamics simulations, we studied electrostatic responses of a charged DNA molecule in the nanochannel and investigated optimized operating conditions for controlling the motion of the DNA. When the transverse electric field was periodically turned on and off, the DNA molecule was correspondingly immobilized on and released from the channel surface. Under simultaneously applied longitudinal biasing and transverse trapping electric fields, the DNA molecule moved forward in a “ratchet”-like fashion. It is expected that achieving the controlled motion of DNA in the channel can advance studies and applications of a nanochannel-based sensor for analyzing DNA (e.g., DNA sequencing).

Cloaking and magnifying using radial anisotropy

This paper studies the electrostatic responses of a polarly radially anisotropic cylinder and a spherically radially anisotropic sphere. For both geometries, the permittivity components differ from each other in the radial and tangential directions. We show that choosing the ratio between these components in a certain way, these rather simple structures can be used in cloaking dielectric inclusions with arbitrary permittivity and shape in the quasi-static limit. For an ideal cloak, the contrast between the permittivity components has to tend to infinity. However, only positive permittivity values are required and a notable cloaking effect can already be observed with relatively moderate permittivity contrasts. Furthermore, we show that the polarly anisotropic cylindrical shell has a complementary capability of magnifying the response of an inner cylinder.

Kelvin probe force microscopy and electrostatic force microscopy responses to the polarization in a ferroelectric thin film: Theoretical and experimental investigations

The ability of non-contact electric modes (Kelvin probe force microscopy (KPFM) and electrostatic force microscopy (EFM)) to provide quantitative information about the remnant polarization in a ferroelectric thin film is theoretically and experimentally investigated. The theoretical relationship between the microscopic signal and the polarization in the film is established: it is linear in the KPFM mode and parabolic in the EFM mode. The KPFM and EFM signals are predicted tip-shape, tip-surface distance, and poled area dimensions dependent so that an absolute value of the polarization cannot be extracted. Nevertheless provided these parameters are held constant, KPFM theoretically enables to investigate quantitatively the polarization dependence on any parameter relative to the film preparation conditions or to the poling process. It does not enable to investigate its dependence on poled area geometry. Based on these conclusions, experimental KPFM investigations have been carried out on a ferroelectric strontium barium niobate (SrxBa1−xNb2O6, SBN:100x) epitaxial thin film deposited on a platinum covered MgO substrate. Poled areas are created in the film by moving the biased tip in contact with the surface. KPFM images are dominated by the written polarization and not by injected or attracted extrinsic charges. The experimental decay of the KPFM signal with increasing tip-surface distance confirms the contribution of the volume of the lever/tip to the KPFM response. The spontaneous evolution of the written polarization as well as the polarization dependence on poling voltage and poling duration are investigated using KPFM, with the poled area dimensions as a parameter. For comparison, the macroscopic remnant polarization in the SBN film has been measured versus poling field from hysteresis cycles drawn using a macroscopic Pt dot as top contact. The significant difference observed between macroscopic and microscopic results can be accounted for by using a simple model of the inhomogeneous electric field generated by a poling tip. KPFM measurements do not alter the initial polarization of the film and can provide essential qualitative and quantitative information on the polarization in a ferroelectric thin film.

The electrostatic response of water to neutral polar solutes: implications for continuum solvent modeling.

Continuum solvation models are widely used to estimate the hydration free energies of small molecules and proteins, in applications ranging from drug design to protein engineering, and most such models are based on the approximation of a linear dielectric response by the solvent. We used explicit-water molecular dynamics simulations with the TIP3P water model to probe this linear response approximation in the case of neutral polar molecules, using miniature cucurbituril and cyclodextrin receptors and protein side-chain analogs as model systems. We observe supralinear electrostatic solvent responses, and this nonlinearity is found to result primarily from waters' being drawn closer and closer to the solutes with increased solute-solvent electrostatic interactions; i.e., from solute electrostriction. Dielectric saturation and changes in the water-water hydrogen bonding network, on the other hand, play little role. Thus, accounting for solute electrostriction may be a productive approach to improving the accuracy of continuum solvation models.

Ab initio molecular dynamics study of the Helmholtz layer formed on solid–liquid interfaces and its capacitance

We reveal the fundamental microscopic structure of the Helmholtz layer at a sodium solution–platinum interface using our original scheme based on ab initio molecular dynamics to decompose electrostatic responses in the electrochemical environment. The total potential variation is shown to mainly occur within “the contact layer”, a thin surface region with a low dielectric constant formed by the Pauli repulsion. This has not been previously considered by conventional electric double-layer models. An estimation of its capacitance is also achieved without the use of empirical parameters, where the value obtained mostly agrees well with previous experiments.