Electrostatic Strength Research Articles

Morphological Response of a Spherical Polyelectrolyte Brush to Solvent Quality and Electrostatic Interaction Strength

We study surface morphologies of a spherical polyelectrolyte brush in the presence of trivalent counterions using molecular dynamics simulations. Solvent quality and electrostatic interaction strength are varied to generate a series of structures. Through a careful analysis on snapshots of morphologies, shape factor of tethered chains, and monomer–monomer pair correlation function we find a nonmonotonic dependence of surface morphology on electrostatic strength, which represents clearly the electrostatic correlation effect mediated by the multivalent counterions. Due to the very importance of counterions, we further study the correlation effect by classifying counterions into four states, calculating the monomer–counterion pair correlation function and diffusion coefficient of counterions. Our simulation results clearly demonstrate that ordered patterns can be induced by the electrostatic correlation effect in the presence of trivalent counterions, which is absent in the system with monovalent ions. Also,...

Rodlike Counterions near Charged Cylinders: Counterion Condensation and Intercylinder Interaction.

We study a system composed of like-charged cylinders and dumbbell-like counterions, with the focus laid on the role of the internal structure of counterions, using MonteCarlo simulations. The dumbbell ions are found to exhibit novel counterion condensation behavior governed by their length. Effective electrostatic interactions mediated between charged parallel cylinders also turn out significantly different from the case of pointlike ions, as a result of the complex interplay between the spatially separated charge distribution in the dumbbell counterions, their orientation, and the curvature of the charged cylinder. We show that at a weak-to-moderate electrostatic coupling strength, where effective like-charge interactions are usually found to be repulsive, the intercylinder interaction can become attractive and display a distinctive sensitivity to the cylinder curvature and dumbbell size, proving the significant effect of ion structure.

First principles study of the structural, electronic, and optical properties of Sn2+-doped ZnO–P2O5 glasses

The atomic-structural, electronic, and optical properties of Sn-doped ZnO–P2O5 glasses are examined using combined semi-empirical and density functional theory calculations. From model structures for two different ZnO–P2O5 glass compositions, it is found that these glasses consist of nearly ideal tetrahedral PO4 unit and largely distorted ZnO4 and ZnO5 units. The concentration of ZnO5 units is calculated to increase and exceed that of ZnO4 as the amount of ZnO in the glass is increased. Also, the concentration of terminal oxygen atoms shared by two Zn neighbors is largely increased for 70ZnO:30P2O5 (70:30) glass compared to 60:40 glass, which can be rationalized by electrostatic bond strength calculations. Regarding medium-range order, calculations show that Q1 and Q2 structures (where Qn refers to structures containing n bridging oxygen atoms on PO4 tetrahedra) are dominant for the 60:40 glass, while Q0 and Q1 prevail for the 70:30 glass, in good agreement with experimental observations. For the Sn-doped ZnO–P2O5 glass, various Sn coordination geometries are found, with trigonal pyramidal SnO3 and largely distorted pyramidal SnO4 configurations being dominant and a small fraction of two-fold and five-fold coordinated Sn configurations also present. From combined cluster and glass model calculations, two-fold coordinated Sn are found to exhibit slightly smaller optical transition energy than three-fold and four-fold coordinated Sn. However, local variations in coordination structures throughout the glass likely result in a broad singlet transition (S0→S1) around 5.4 eV for all the different Sn configurations. A triplet transition (S0→T1) driven by spin-orbit coupling can occur around 4.5 – 4.8 eV, but the magnitude of this transition is predicted to be much smaller than the spin-allowed singlet transition.

Evaluation of SF6-alternative gas C5-PFK based on arc extinguishing performance and electric strength

C5-PFK (C5-perfluoroketone, C5F10O) is under wide consideration as an environmentally-friendly alternative gas to SF6 in high-voltage applications, because of its superior insulation performance. The aim of this work is to study theoretically the arc extinguishing performance and electric strength of C5-PFK.The arc extinguishing performance of C5-PFK was evaluated by analyzing and comparing the thermophysical properties of C5-PFK, SF6, CF4, CO2 and N2 plasmas. It was difficult to obtain the species formed in C5-PFK plasmas because of the complex C5-PFK molecular decomposition process. In this work, the decomposition process of C5-PFK and the related species were analyzed by the bond energy analysis method. For the species for which parameters such as the partition function and the enthalpy of formation were not available, computational chemistry methods were used to obtain the required data. The collision integrals were calculated using the phenomenological potential model. Using these results, the local thermodynamic equilibrium composition at temperatures from 300 to 30 000 K at 1–10 atm of pure C5-PFK was calculated by the method of minimization of the Gibbs free energy, and the corresponding transport coefficients were calculated by Chapman–Enskog method. Through the comparison of the thermophysical properties, it was found that C5-PFK had similar characteristics to SF6, with large peaks in specific heat below 4500 K, indicating potentially good thermal interruption capability. However, the specific heat peak at 7000 K corresponding to CO decomposition may detract from the thermal interruption capability. Specific heat peaks at higher temperatures are associated with the breaking of double or triple bonds, and should be avoided if possible in the new alternative gases.The electric strength of C5-PFK was assessed using the molecular electrostatic potential, which can be accurately calculated or measured, and gives strong insights into important characteristics of the molecule. Based on the analysis of the molecular surface electrostatic potential and electric strength of C5-PFK, SF6, CF4, CO2, and N2, it is found that the positive potential area of the molecular surface has a strong correlation with the electric strength and is expected to be one of the predictors of electric strength. To verify this phenomenon, 36 kinds of particles were used for the correlation analysis. The correlation coefficient between the positive potential area and electric strength is up to 0.9 which means strong correlation.

Interactions between Polyelectrolyte Gel Surfaces

Surfaces formed by charged polymeric species are highly abundant in both synthetic and biological systems, for which maintaining an optimum contact distance and a pressure balance is paramount. Here we investigate interactions between surfaces of two same-charged and highly swollen polyelectrolyte gels, using extensive molecular dynamic simulations and minimal analytical methods. The external-pressure responses of the gels and the polymer-free ionic solvent layer separating two surfaces are considered. Simulations confirmed that the surfaces are held apart by osmotic pressure resulting from excess charges diffusing out of the network. At a threshold pressure, the counterion-induced osmotic pressure in the gel interiors becomes nearly equal to that in solvent gap, and the gels begin to deform. At the threshold pressure, the distance between surfaces is equivalent to the electrostatic screening length imposed by excess charges. Both the solvent layer and pressure dependence are well described by an analytical model based on the Poisson–Boltzmann solution for low and moderate electrostatic strengths. Scaling descriptions for the threshold pressure and critical distance compare well with the simulations. The threshold pressure decreases with increasing polymerization degree of network chains, similar to sparsely grafted polyelectrolyte brushes. As the electrostatic coupling strength is increased, the deviations from the analytical model also increase due to condensed counterions. Our results are of great importance for systems where charged gels or gel-like structures interact in various solvents, including systems encapsulated by gels and microgels in confinement.

Bonding and structure in network glasses

The aim of this paper is to foster greater use of the concepts of electrostatic bond strength and bond valence in the interpretation of glass structure. Methods of applying these concepts to diffraction data to investigate the atomic bonding environments in network glasses are presented, and illustrated using neutron diffraction results on lead gallate glasses, which show that gallium atoms are tetrahedrally coordinated by oxygen, and lead atoms have an average oxygen coordination number between three and four. It is of great value to consider the oxygen anion-centred coordination numbers, as well as the cation-centred coordination numbers. Due to the requirement for charge balance, the dominant coordination for oxygen in lead gallate glasses is 3-coordinated, with both OGa2Pb and OGaPb2 motifs. A small fraction (~1–2%) of the oxygen atoms are bonded to four cations, either as OGa2Pb0+2 or OGaPb1+2. By means of a valence sum function, it is shown that that the number of long (~2.8Å) Pb-O bonds is not significant, and thus the number of Pb atoms on symmetric sites (with a stereochemically inactive lone pair of electrons) is not significant. It is shown that, even though glasses and crystals obey the same bonding rules, the structure of a glass can differ from that of a closely related crystal.

Electrostatics, structure prediction, and the energy landscapes for protein folding and binding.

While being long in range and therefore weakly specific, electrostatic interactions are able to modulate the stability and folding landscapes of some proteins. The relevance of electrostatic forces for steering the docking of proteins to each other is widely acknowledged, however, the role of electrostatics in establishing specifically funneled landscapes and their relevance for protein structure prediction are still not clear. By introducing Debye-Hückel potentials that mimic long-range electrostatic forces into the Associative memory, Water mediated, Structure, and Energy Model (AWSEM), a transferable protein model capable of predicting tertiary structures, we assess the effects of electrostatics on the landscapes of thirteen monomeric proteins and four dimers. For the monomers, we find that adding electrostatic interactions does not improve structure prediction. Simulations of ribosomal protein S6 show, however, that folding stability depends monotonically on electrostatic strength. The trend in predicted melting temperatures of the S6 variants agrees with experimental observations. Electrostatic effects can play a range of roles in binding. The binding of the protein complex KIX-pKID is largely assisted by electrostatic interactions, which provide direct charge-charge stabilization of the native state and contribute to the funneling of the binding landscape. In contrast, for several other proteins, including the DNA-binding protein FIS, electrostatics causes frustration in the DNA-binding region, which favors its binding with DNA but not with its protein partner. This study highlights the importance of long-range electrostatics in functional responses to problems where proteins interact with their charged partners, such as DNA, RNA, as well as membranes.

Ionomer Self-Assembly in Dilute Solution: A Molecular Dynamics Study

Self-assembly of ionomer chains in dilute solution is studied by classical molecular dynamics. Based on a bead-spring ionomer model, the simulation approach captures formation of cylindrical, bundle-like [1] aggregates with a hydrophobic core region, a surface layer of charged anionic headgroups and a halo of counterions (protons) in the surrounding aqueous phase. Ionomer aggregation enforces the stretching and stiffening of ionomer chains, which is caused mainly by the repulsion of anionic headgroups. The strengths of hydrophobic and electrostatic interactions are varied to explore their effect on the aggregation process. Strong hydrophobicity of the ionomer main chain results in greater aggregate sizes. Figure 1a shows the result of the changes in the aggregate size, <k>, over the range of explored hydrophobicity. Strong electrostatic interaction of ionic headgroups decreases the aggregate size, results are shown in figure 1b. Simulations also show the spontaneous formation of networks of ionomer bundles. Results of simulations will be discussed in the context of experimental studies on the formation of rodlike structures in ionomer solutions [2-3] that form the mechanically stable skeleton of polymer electrolyte membranes such as Nafion [4]. Moreover, implications of simulation results for water sorption phenomena [5], proton conductivity [6], membrane degradation [7] and membrane failure will be discussed. Figure caption: Figure 1. Changes of the average aggregate size, <k>, as a function of variation in strengths of hydrophobic interactions (a) and electrostatic interactions (b). Strength of hydrophobic interactions are controlled through variation of strength of excluded volume interaction of hydrophobic monomeric beads while the electrostatic strength is varied through changes in the Bjerrum length.

Weak polyelectrolytes in the presence of counterion condensation with ions of variable size and polarizability

.Light scattering and viscometric measurements on weak polyelectrolytes show two important aspects of counterion condensation, namely, non-monotonic variation in the polyelectrolyte size with the increase in the electrostatic strength, and, monovalent counterion selectivity in determining the nature of collapse transition at high electrostatic strengths. Here, we present a self-consistent variational theory for weak polyelectrolytes which includes the effects of the polarizability of monovalent counterions. Our theory reproduces several experimental findings including non-monotonic conformational size with the variation in the electrostatic strength and a shift from a continuous to a discontinuous collapse transition with the increase in the dipole strength of condensed ions. At low dipole strength and high electrostatic strength, our theory predicts a series of solvent quality driven size transitions spanning the re-entrant poor, theta and good solvent regimes. At high dipole strength, the size remains that of a compact globule independent of solvent quality. The dipole strength of the ion-pair formed due to counterion condensation, which depends on the size and polarizability of the monovalent counterions, is found to be an important molecular parameter in determining the nature of collapse transition, and the size of the collapsed state at high electrostatic strength.

Study of gold nanorods–protein interaction by localized surface plasmon resonance spectroscopy

In this paper, gold nanorods’ (GNRs) interaction with different proteins (i.e. carbonic anhydrase, lysozyme, ovalbumin and bovine serum albumin (BSA)) at physiological pH is investigated using localized surface plasmon resonance (LSPR) spectroscopy. We observe that the incubation of these proteins at different concentrations with cetyltrimethylammonium bromide-capped GNRs of three aspect ratios induces dramatic changes in the extinction spectra of the nanoparticles. In particular, we correlate the position and shape of the longitudinal LSPR peaks to the ability of the proteins to specifically interact with GNRs’ surface. The different types of behaviour observed are explained by the exposed molecular surface area of the proteins’ cysteine residues as modelled on the basis of their respective X-ray crystallographic data structures. Cysteine is the only amino acid that exhibits an SH group that is well known to have a strong affinity to gold. The presence and the accessibility of such a residue may explain the protein binding to GNRs. The isoelectric point of the proteins is also an important characteristic to take into account, as the electrostatic strength between GNRs and protein explains some of the cases where aggregates are formed.

Pauling bond strength, bond length and electron density distribution

The power law regression equation, = 1.46(<ρ(r c)>/r)−0.19, relating the average experimental bond lengths, , to the average accumulation of the electron density at the bond critical point, <ρ(r c)>, between bonded pairs of metal and oxygen atoms (r is the row number of the M atom), determined at ambient conditions for oxide crystals, is similar to the regression equation R(M–O) = 1.41(ρ(r c)/r)−0.21 determined for three perovskite crystals at pressures as high as 80 GPa. The pair are also comparable with the equation = 1.43( /r)−0.21 determined for oxide crystals at ambient conditions and = 1.39( /r)−0.22 determined for geometry-optimized hydroxyacid molecules that relate the geometry-optimized bond lengths to the average Pauling bond strength, , for the M–O bonded interactions. On the basis of the correspondence between the equations relating <ρ(r c)> and with bond length, it seems plausible that the Pauling bond strength might serve a rough estimate of the accumulation of the electron density between M–O bonded pairs of atoms. Similar expressions, relating bond length and bond strength hold for fluoride, nitride and sulfide molecules and crystals. The similarity of the expressions for the crystals and molecules is compelling evidence that molecular and crystalline M–O bonded interactions are intrinsically related. The value of <ρ(r c)> = r[(1.41)/ ]4.76 determined for the average bond length for a given coordination polyhedron closely matches the Pauling’s electrostatic bond strength reaching each the coordinating anions of the coordinated polyhedron. Despite the relative simplicity of the expression, it appears to be more general in its application in that it holds for the bulk of the M–O bonded pairs of atoms of the periodic table.

Folding and stability of helical bundle proteins from coarse‐grained models

We develop a coarse-grained model where solvent is considered implicitly, electrostatics are included as short-range interactions, and side-chains are coarse-grained to a single bead. The model depends on three main parameters: hydrophobic, electrostatic, and side-chain hydrogen bond strength. The parameters are determined by considering three level of approximations and characterizing the folding for three selected proteins (training set). Nine additional proteins (containing up to 126 residues) as well as mutated versions (test set) are folded with the given parameters. In all folding simulations, the initial state is a random coil configuration. Besides the native state, some proteins fold into an additional state differing in the topology (structure of the helical bundle). We discuss the stability of the native states, and compare the dynamics of our model to all atom molecular dynamics simulations as well as some general properties on the interactions governing folding dynamics.

High‐Energy Density Dielectrics and Capacitors for Elevated Temperatures: Ca(Zr,Ti)O3

The Ca(Zr1−xTix)O3 (CZT) solid solution system is a linear dielectric that is of interest for high‐temperature power capacitor applications. This dielectric was synthesized by conventional solid‐state processes and prototyped into single‐layer CZT capacitors, which were fabricated with interdigitated electrodes cofired between dielectric layers ~10‐μm thick. The dielectric properties of these capacitors were extensively investigated as a function of Zr/Ti ratio on the CaZrO3 side of the solid solution. The electrostatic energy density and breakdown strength of the CZT capacitors were investigated by measuring polarization–electric field curves as a function of temperature from room temperature to 250°C. The Ca(Zr0.80Ti0.20)O3 capacitors show high electrostatic energy density of 4 J/cm3 at 250°C. Highly accelerated life testing (HALT) was also performed on these dielectrics, and we quantified the thermal stimulated depolarization current (TSDC) to access the major point defects in the CZT system.

Folding and Stability of Helical Bundle Proteins from Coarse-Grained Models

We develop a coarse-grained model where solvent is considered implicitly, electrostatics is included as short-range interactions and side-chains are coarse-grained to a single bead. The model depends on three main parameters: hydrophobic, electrostatic and side-chain hydrogen bond strength. The parameters are determined by considering three levels of approximations and characterizing the folding for three selected proteins (training set). Nine additional proteins (containing up to 126 residues) as well as mutated versions (test set) are folded with the given parameters. In all folding simulations, the initial state is a random coil configuration. Besides the native state, some proteins fold into an additional state differing in the topology (structure of the helical bundle). We discuss the stability of the native states, and compare the dynamics of our model to all atom molecular dynamics simulations as well as some general properties on the interactions governing folding dynamics.

Edge dislocation effect on optical properties in wurtzite ZnO

The effect of edge dislocations on optical properties in wurtzite ZnO is studied using a k·p multiband Hamiltonian model. An edge dislocation is modeled as a negatively charged line due to its electron-acceptor nature with an elastic strain field due to the lattice distortion around the dislocation. The electrostatic potential strength of the negatively charged dislocation depends on the filling fraction, which describes the fraction of acceptor sites occupied by trapped electrons along the dislocation line. To understand the effect of electrostatic potential strength, the filling fraction has been varied in this work. Using the calculated energy levels and wave functions for electrons and holes from the k·p multiband Hamiltonian, the spontaneous emission spectrum has been obtained as a function of dislocation density and filling fraction. The calculated results are compared with available experimental photoluminescence data. The band edge peak intensity decreases significantly with increasing dislocation density. It is found that the electrostatic potential strength does not affect the band edge peak emission, but it generates deep level emissions. For low filling fractions, corresponding to high temperature, the most commonly observed green luminescence is found. For a high filling fraction, corresponding to low temperature, the green luminescence shifts to red luminescence, which is consistent with experimental observation.

Modeling of sorption of bisphenol A in sludge obtained from a membrane bioreactor process

This work deals with the modeling of bisphenol-A (BPA) adsorption in sludge obtained from an immersed membrane activated sludge process. Batch experiments were carried out in Erlenmeyer shake flasks containing either activated sludge or inactivated sludge (200 mL of sludge at 15 g L −1 of suspended solids) in the presence of different BPA concentrations (0.45–18.2 mg BPA L −1) in order to distinguish physicochemical sorption and biosorption processes. Then, mathematical models were applied to describe the sorption phenomenon followed by the determination of kinetic of BPA removal. The applicability of the Langmuir isotherm for BPA removal allowed determining the adsorption rate constant ( K L = 0.370 L g −1) and the maximum adsorption capacity ( q m = 0.833 mg g −1). The Elovich kinetic model which takes into account of the instantaneous sorption described very well the process. The reaction rate constant for BPA removal was 0.194 g mg −1 min −1. The relatively low enthalpy value (Δ ad H° < 200 kJ mol −1) indicated that during adsorption process, very low interactions occurred between BPA and sludge, such as Van Der Waals attraction or electrostatic strengths.

Transversal electrostatic strength of patterned collector affecting alignment of electrospun nanofibers

Conventional electrospinning produces randomly ordered nanofibers, however many potential applications require precisely defined morphology and anisotropic properties of nanofibrous materials. In this work, a collector consisting of two segments has been used to prepare uniaxially ordered nanofibers. Optimized collector design comes from both analytical calculations of electrostatic strength in the vicinity of the electrodes and from experimental verification by polyvinyl alcohol nanofibers fabrication. The transversal electric strength is a key factor in achieving very well aligned nanofibers. The results presented here help provide a better understanding of the electrically induced alignment mechanism.

Charge Renormalization, Thermodynamics, and Structure of Deionized Colloidal Suspensions

In charge-stabilized colloidal suspensions, highly charged macroions, dressed by strongly correlated counterions, carry an effective charge that can be substantially reduced (renormalized) from the bare charge. Interactions between dressed macroions are screened by weakly correlated counterions and salt ions. Thermodynamic and structural properties of colloidal suspensions depend sensitively on the magnitudes of both the effective charge and the effective screening constant. Combining a charge renormalization theory of effective electrostatic interactions with Monte Carlo simulations of a one-component model, we compute osmotic pressures and pair distribution functions of deionized colloidal suspensions. This computationally practical approach yields close agreement with corresponding results from large-scale simulations of the primitive model up to modest electrostatic coupling strengths.

Electrical characterization of an inductively coupled gaseous electronics conference reference cell

Plasma parameters and particularly the external electrical operational parameters are examined with respect to mode transition and hysteresis. The external electrical parameters such as antenna current (Ic) and antenna voltage (Vc) amplitudes are measured after the matching network and discussed for an inductively coupled argon discharge. A wide range of discharge conditions by varying applied power (up to 150 W at 13.56 MHz), gas pressure (0.7–7.5 Pa), and electrostatic coupling strength are measured for a gaseous electronics conference radio frequency reference cell. The effect on the power coupling efficiency by varying electrostatic coupling strength is studied via implementing two distinct grounded Faraday shields, in addition to the original nonshielded condition. A brief discussion on the evolution of floating potential, plasma potential, electron density, and electron energy distribution function with power and pressure is also presented mainly in context to mode transitions and hysteresis. Relatively smooth transitions in the plasma parameters and in the external electrical parameters are measured close to H to E mode transition region. Contrary to plasma parameters, however, the reverse transition from E to H mode was found to be abrupt in external parameters. The plasma parameters are measured using a commercial Langmuir probe, whereas the antenna currents and voltages are measured using a homebuilt Rogowski coil and capacitor divider, respectively.

Electric tunable of electron spin polarization in hybrid magnetic–electric barrier structures

Electron spin-dependent transport properties have been theoretically investigated in two-dimensional electron gas (2DEG) modulated by the magnetic field generated by a pair of anti-parallel magnetization ferromagnetic metal stripes and the electrostatic potential provided by a normal metal Schottky stripe. It is shown that the energy positions of the spin-polarization extremes and the width of relative spin conductance excess plateau could be significantly manipulated by the electrostatic potential strength and width, as well as its position relative to the FM stripes. These interesting features are believed useful for designing the electric voltage controlled spin filters.