Specific ion effects on the electrophoretic mobility of small,highly charged peptides: A modeling study

In this work, we use coarse‐grained modeling to study the free solution electrophoretic mobility of small highly charged peptides (lysine, arginine, and short oligos thereof (up to nonapeptides)) in NaCl and Na₂SO₄ aqueous solutions at neutral pH and room temperature. The experimental data are taken from the literature. A bead modeling methodology that treats the electrostatics at the level of the nonlinear Poisson Boltzmann equation developed previously in our laboratory is able to account for the mobility of all peptides in NaCl, but not Na₂SO₄. The peptide mobilities in Na₂SO₄ can be accounted for by including sulfate binding in the model and this is proposed as one possible explanation for the discrepancy. Oligo arginine peptides bind more sulfate than oligo lysines and sulfate binding increases with the oligo length.     

Approximate expression for the electrophoretic mobility of a spherical colloidal particle covered with an ion-penetrable uncharged polymer Layer

An approximate analytic expression is derived for the electrophoretic mobility of a charged spherical colloidal particle covered with an ion-penetrable uncharged polymer layer in an electrolyte solution by taking into account the relaxation effects. This expression is applicable for all values of zeta potentials at large a(aca. 30), where is the Debye–Huckel parameter and a is the radius of the particle core. A simple expression for the ratio of the electrophoretic mobility of a polymer-coated particle to that of a bare particle without a polymer layer is also given.     

Effects of fixed-charge distribution and pH on the electrophoretic mobility of biological cells

The electrophoretic mobility of biological cells is investigated theoretically. In particular, the effects of the distribution of the charges in the surface layer and the pH of bulk liquid phase on the mobility of cells are examined. The former includes the fixed charges due to the dissociation of the functional groups and the charges due to the penetrated electrolyte ions. The present analysis extends previous results in that the fixed charges are distributed nonuniformly across the surface layer of a cell. It is found that the distribution of the fixed charges in the surface layer has a significant effect on its electrophoretic mobility. Thus, assuming that the fixed charges are homogeneously distributed in the surface layer of a cell may lead to a significant deviation.     

Surface charging behaviors of electrocatalytic interfaces with partially charged chemisorbates

The surface charge is a key concept in electrochemistry. Mathematically, the surface charge is obtained from a spatial integration of the volume charge along a particular direction. Ambiguities thus arise in choosing the starting and ending points of the integration. As for electrocatalytic interfaces, the presence of chemisorbates further complicates the situation. In this minireview, I adopt a definition of the surface charge within a continuum picture of the electric double layer. I will introduce surface charging behaviors of firstly ordinary electrochemical interfaces and then electrocatalytic interfaces featuring partially charged chemisorbates. Particularly, the origin of nonmonotonic surface charging behaviors of electrocatalytic interfaces is explained using a primitive model. Finally, a brief account of previous studies on the nonmonotonic surface charging behavior is presented, as a subline of the spectacular history of electric double layer.     

Effect of Electrolyte Concentration on the Stern Layer Thickness at a Charged Interface

The chemistry and physics of charged interfaces is regulated by the structure of the electrical double layer (EDL). Herein we quantify the average thickness of the Stern layer at the silica (SiO₂) nanoparticle/aqueous electrolyte interface as a function of NaCl concentration following direct measurement of the nanoparticles’ surface potential by X‐ray photoelectron spectroscopy (XPS). We find the Stern layer compresses (becomes thinner) as the electrolyte concentration is increased. This finding provides a simple and intuitive picture of the EDL that explains the concurrent increase in surface charge density, but decrease in surface and zeta potentials, as the electrolyte concentration is increased.     

Toward understanding the role of the electric double layer structure and electrolyte effects on well-defined interfaces for electrocatalysis

The interplay between the structure and composition of the electric double layer and the surface charge controls the electrocatalytic activity of reactions central to decarbonization of chemical fuels and materials. The employed electrolyte can affect the charge distribution and the electric field on the interface, which also alters the local pH and ordering of the water-solvent network. Additionally, the electrolyte plays a key role in stabilizing or destabilizing the adsorbed intermediates via non-covalent bonds, or poisons the surface and induces surface reconstruction, affecting the reactivity of the active sites positions. Herein, we discuss, from an experimental perspective, electrolyte effects on different interfacial properties relevant to electrocatalysis.     

Range of Validity of a Simplified Model for Diffuse Charge Dynamics

The modelling of electrochemical processes often requires the solution of the Poisson‐Nernst‐Planck (PNP) equations. In complex geometries, such as porous electrodes, that is challenging due to the presence of disparate length scales, ranging from the Debye screening length (～nm) to the device length scale (～cm). To overcome this difficulty, one often assumes that the electric double layer (EDL) is at quasi‐equilibrium to construct a simplified model that accounts for ion diffusion in the electro‐neutral bulk of the electrolyte while replacing the EDLs with appropriate boundary conditions. Various researchers have demonstrated that such an approach is valid in the asymptotic limit of a thin EDL and moderate electrode potentials. In this note, we explore the range of validity of this approximation by considering a one‐dimensional electrolytic cell with blocking electrodes subjected to a step change and time‐periodic alternations in the electrodes’ potentials by calculating the errors associated with the approximate approach as functions of the EDL thickness and electric field frequency and intensity. Additionally, we delineate numerical instabilities associated with the numerical solutions of the bulk equations with the nonlinear boundary condition peculiar to this problem.     

Effect of matrix chain length on the electrophoretic mobility of large linear and branched DNA in polymer solutions

We study the effect of matrix chain molecular weight Mw and concentration c on the electrophoretic mobility micro of large linear and star-like, branched DNA in polymer solutions. Polyethylene oxide (PEO) with narrow molecular weight distributions form the main focus of this study. For PEO concentrations ranging from one half the overlap concentration, c*, to 3c*, the effective drag coefficient, zeta is identical with (mu0/mu) - 1, satisfies the following approximate scaling relationship, zeta approximately cMw(0.7). Here, mu0 is the electrophoretic mobility in free solution. While the concentration dependence is consistent with predictions from the transient entanglement coupling (TEC) model, the molecular weight dependence is significantly weaker. Although a similar dependence of mobility on Mw can be predicted when nonentangling collisions are the dominant source of drag, a model based on these collisions alone cannot reproduce the experimental observations. We also find that the architecture of large DNA does not affect either the concentration dependence or molecular weight dependence of the electrophoretic mobility.     

The structure of the electric double layer: Atomistic versus continuum approaches

This article reviews recent forays in theoretical modeling of the double layer structure at electrode/electrolyte interfaces by current atomistic and continuum approaches. We will briefly discuss progress in both approaches and present a perspective on how to better describe the electric double layer by combining the unique advantages of each method. First-principles atomistic approaches provide the most detailed insights into the electronic and geometric structure of electrode/electrolyte interfaces. However, they are numerically too demanding to allow for a systematic investigation of the electric double layers over a wide range of electrochemical conditions. Yet, they can provide valuable input for continuum approaches that can capture the influence of the electrochemical environment on a larger length and time scale due to their numerical efficiency. However, continuum approaches rely on reliable input parameters. Conversely, continuum methods can provide a preselection of interface structures and conditions to be further studied on the atomistic level.     

Solution of the Poisson-Boltzmann Equation for a Cylindrical Particle with a Limited Length： Functional Theoretical Approach

Introduction The electrostatic potentialΨis the most importantproperty for the electrical double layer( EDL) of acharged particle in an electrolyte solution[1—4]. Thispotential is characterized by the so-called Poisson-Bolt-zmann(PB) equation. The PB equation is a second-or-der nonlinear differential equation with a constant coef-ficient, except a flat-plate model, which cannot besolved analytically by the traditional method. To ourknowledge, apart from the numerical solution to thisequa…     

Fluorination effect of activated carbon electrodes on the electrochemical performance of electric double layer capacitors

The surface of phenol-based activated carbon (AC) was fluorinated at room temperature with different F₂:N₂ gas mixtures for use as an electrode material in an electric double-layer capacitor (EDLC). The effect of surface fluorination on EDLC electrochemical performance was investigated. The specific capacitance of the fluorinated AC-based EDLC was measured in a 1 M H₂SO₄ electrolyte, in which it was observed that the specific capacitances increased from 375 and 145 F g⁻¹ to 491 and 212 F g⁻¹ with the scan rates of 2 and 50 mV s⁻¹, respectively, in comparison to those of an unfluorinated AC-based EDLC when the fluorination process was optimized via 0.2 bar partial F₂ gas pressure. This enhancement in capacitance can be attributed to the synergistic effect of increased polarization on the AC surface, specific surface area, and micro and mesopore volumes, all of which were induced by the fluorination process. The observed increase in polarization was derived from a highly electronegative fluorine functional group that emerged due to the fluorination process. The increased surface area and pore volume of the AC was derived from the physical function of the fluorine functional group.     

Effect of pH on the electrophoretic mobility of a particle with a charge-regulated membrane in a general electrolyte solution

The electrophoretic motion of an entity comprised of a rigid, uncharged core covered by a charge-regulated membrane which simulates a biological cell, in a general a:b electrolyte solution is analyzed. The membrane carries a fixed charge which arises from the dissociation of the acidic functional group HA. We show that the higher the concentration of cations in the bulk liquid phase, the lower the absolute Donnan potential, _D, and the lower the concentration of functional group, N₀, the lower the _D. Also, the higher the pH, the higher the absolute electrical potential, and the greater the N₀, the lower the pH. The absolute mobility of a cell, μ, increases with pH, but decreases with the increase in the friction coefficient of the membrane phase, γ. For a fixed total number of HA, if γ is large, μ/μ_s is less than unity, μ_s being the mobility of the corresponding rigid particle, and it decreases with the thickness of membrane d, and the inverse is true if γ is small. For a medium γ, the variation of μ/μ_s as a function of d has a local maximum, and depending upon d, it can be either greater or less than unity.     

抗衡离子对硫酸盐表面活性剂气/液界面性质影响的分子动力学模拟

采用全原子分子动力学方法研究了抗衡离子为第一主族离子(Li⁺、Na⁺、K⁺、Rb⁺和Cs⁺)的十二烷基硫酸盐表面活性剂的气/液界面性质. 通过分析体系中各组分的密度分布曲线, 考察表面活性剂单分子层在界面的聚集形态, 并利用径向分布函数分析了表面活性剂极性头基与抗衡离子间的相互作用. 研究结果表明: 随着抗衡离子半径的增大, 不同体系的界面水层厚度依次增加, 表面活性剂极性头基与抗衡离子形成的Stern和扩散层厚度也相应增加. 但表面活性剂吸附层的抗衡离子缔合度以及体系表面张力却随抗衡离子半径的增大而减小. 研究表明抗衡离子的差异对十二烷基硫酸盐表面活性剂气/液界面性质有很大影响.     

表面电荷及外电场对液固界面热阻的影响

采用非平衡分子动力学方法模拟了外电场及固体表面电荷对水与固体间界面热阻的影响. 结果表明,外加电场平行于界面时, 其对界面热阻几乎没有影响, 而垂直于界面时, 界面热阻将随着电场强度的增大而减小. 壁面带正电荷或负电荷都将使得界面热阻减小. 界面热阻与表面电荷密度及电场强度均满足二次函数关系. 模拟结果表明施加外电场和表面电荷是控制液固界面热阻的有效方法.     

Examination of the role of ion size in determining double layer properties on the basis of a generalized mean spherical approximation

A new model for the diffuse double layer which accounts for the effects of ion size and solution permittivity is described. It is then used to estimate the potential drop across the diffuse layer at negative charge densities for the cases that Li⁺ and Cs⁺ are the electrolyte cations. The potential drop in the Li⁺ system is considerably smaller than that in the Cs⁺ system at 1 M, and both values are smaller than the value predicted by the Gouy–Chapman model. As the electrolyte concentration decreases these differences become smaller so that at 0.01 M, the present model predicts that the diffuse layer potential drop is approximately 90% of the Gouy–Chapman estimate. The results of the model are used to examine the differences in inner layer structure at mercury electrodes with Li⁺ and Cs⁺ ions at the outer Helmholtz plane, and to reconsider the question of the specific adsorption of Cs⁺ at negative-charge densities.     

Influence of membrane layer properties on the electrophoretic behavior of a soft particle

The influence of the physical properties of the membrane layer of a soft particle, which comprises a rigid core and a porous membrane layer, on its electrophoretic behavior, is investigated. Because that influence was almost always neglected in the previous studies, the corresponding results can be unrealistic. The applicability of the model proposed is verified by the available theoretical and experimental results. The electrophoretic mobility of the particle under various conditions is simulated through varying the dielectric constant, the thickness, and the drag coefficient of the membrane layer, and the bulk ionic concentration. We show that under typical conditions, the deviation in the electrophoretic mobility arising from assuming that the dielectric constant of the membrane layer is the same as that of the bulk liquid phase can be in the order of 50%. In addition, the thicker the membrane layer and/or the higher the bulk ionic concentration, the larger the deviation. If the surface of the core of the particle is charged, as in the case of inorganic particles covered by an artificial membrane layer, the deviation at constant core surface potential is larger than that under other types of charged conditions. However, if the surface of the core is uncharged, as in the case of biocolloids, then that deviation becomes negligible. These findings are of fundamental significance to theoreticians in their analysis on the electrokinetic behaviors of soft particles, and to experimentalists in the interpretation of their data.     

Pressure effects on the structural,electronic, and optical properties of Sin@SWCNTs

Well‐ordered single, double/four parallel, three/four‐strands helical chains, and five‐strand helical chain with a single atom chain at the center of Si nanowires (NWs) inside single‐walled carbon nanotubes (Si_n@SWCNTs) are obtained by means of molecular dynamics. On the basis of these optimized structures, the structural evolution of Si_n@SWCNTs subjected to axial stress at low temperature is also investigated. Interestingly, the double parallel chains depart at the center and transform into two perpendicular parts, the helical shell transformed into chain, and the strand number of Si NWs increases during the stress load. Through analyzis of pair correlation function (PCF), the density of states (DOS), and the z‐axis polarized absorption spectra of Si_n@SWCNTs, we find that the behavior of Si_n@SWCNTs under stress strongly depends on SWCNTs' symmetry, diameter, as well as the shape of NWs, which provide valuable information for potential application in high pressure cases such as seabed cable. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Molecular dynamics simulation studies of the conformation and lateral mobility of a charged adsorbate biomolecule: implications for estimating the critical value of the radius of a pore in porous media

The conformations, the values of the lateral transport coefficient of a charged biomolecule (desmopressin) in the adsorbed layer and in the liquid layers above the adsorbed layer, the potential energies of the interaction between the biomolecules located in different liquid layers with the charged solid surface and with the biomolecules in the adsorbed layer, the potential energies of the interaction between water molecules in the hydration layers surrounding the conformations of the biomolecules in different layers, as well as the structure and number of hydration layers between the different conformations of desmopressin, were determined by molecular dynamics simulation studies. The results show that the lateral mobility of the adsorbed desmopressin is approximately equal to zero and the value of the lateral transport coefficient of the biomolecule in the liquid layers located above the adsorbed layer increases as the distance of the liquid layer from the charged solid surface increases. But the values of the lateral transport coefficient of the biomolecule in the liquid layers above the adsorbed layer are lower in magnitude than the value of the transport coefficient of desmopressin along the direction normal to the charged solid surface in the liquid phase located above the vacant charged sites of the solid surface, and these differences in the values of the transport coefficients have important implications with respect to the replenishment of the biomolecules in the inner parts of a channel (pore), the overall rate of adsorption, and the form of the constitutive equations that would have to be used in macroscopic models to describe the mechanisms of mass transfer and adsorption in the pores of adsorbent media. Furthermore, a novel method is presented in this work that utilizes the information about the sizes of the conformations of the biomolecule in the adsorbed layer and in the liquid layers above the adsorbed layer along the direction that is normal to the charged solid surface, as well as the number and size of the hydration layers along the same direction, and could be used to estimate the value of the lower bound of the linear characteristic dimension of a pore (i.e., pore radius) in porous adsorbent media (e.g., porous adsorbent particles; skeletons of porous monoliths) in order to realize effective transport and overall adsorption rate.     

Temperature Effects on the Capacitance of an Imidazolium‐based Ionic Liquid on a Graphite Electrode: A Molecular Dynamics Simulation

Temperature‐dependent electric double layer (EDL) and differential capacitance–potential (C_d–U) curves of the ionic liquid 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BMIM⁺/PF₆^?) were studied on a graphite electrode by molecular dynamics simulations. It was found that all C_d–U curves were asymmetric camel‐shaped with higher C_d at negative polarization, attributed to the specific adsorption of BMIM⁺. In addition, the maxima of C_d at the negative polarization decrease monotonically with temperature due to the thicker EDL, whereas at the positive polarization they gradually increase from 450 to 550 K and decrease at 600 K. Such temperature effects at positive polarization may be understood in terms of the competition between two aspects: the weakening specific adsorption of BMIM⁺ allows more effective screening to the positive charge and overall increasing EDL thickness. Although the former dominates from 450 to 550 K, the latter becomes dominant at 600 K.     

Salient features of the electrical double layer structure on mechanically renewed gold-silver electrodes in aqueous solutions of a surface-inactive electrolyte

The behavior of electrodes, which are made of binary Au-Ag alloys (Ag content 1–15 at %) and renewed by mechanical cutting in aqueous solutions of sodium fluoride, is studied with the aid of cyclic voltammetry and impedance methods. It is established that, in the region of potentials corresponding to ideal polarizability, the differential capacitance of the electrical double layer rapidly changes with time elapsed after the renewal of the surface of the electrodes. The change in the capacitance is brought about by the exit of silver atoms into a surface layer. The implication is that silver is the surface-active component in these alloys. The rate of the surface segregation of silver atoms depends on the electrode potential. The segregation rate substantially increases upon going into the region that corresponds to positive charges of the silver electrode surface and to the beginning of adsorption of atomic oxygen on the electrode. Based on phenomenological models, a method for processing capacitance curves is realized, which links experimentally observed time effects to variations that occur in the surface composition, and assumptions concerning the mechanism of relaxation processes that are responsible for the observed time effects are put forth. Explicit data on the effect, which is exerted by mechanical renewal on the composition of the surface layer of Au-Ag alloys at different distances from the interface with a vacuum, are obtained with the aid of an x-ray photoelectron spectroscopy method. It is established that the surface layer (～0.5 nm) is enriched by silver atoms as compared with the alloy’s bulk.