Water molecules migration at oil–paper interface under the coupling fields of electric and temperature: a molecular dynamics study

Moisture is an important factor affecting the insulation properties of transformers. Due to the limitations of macroscopic experimental methods, the diffusion of water at oil–paper interface cannot be accurately measured. Therefore, molecular dynamics method was used in this work to establish oil–paper layer model of 10⁵ atoms. Through jointly analysing the aggregation degree, diffusion coefficient, free volume as well as radial distribution function of water molecules, the diffusion mechanism of water molecules at oil–paper interface was studied. The results show that when the initial water content in paper was high, water molecules would accumulate at oil–paper interface to form the local high-water region during heating. The polarisation of the electric field strengthened the hydrogen bonding interaction between water molecules and increased the probability of occurrence of the high-water region. Meanwhile, electric field reduced the free volume and diffusion coefficient of water molecules and rendered its diffusion coefficient anisotropic. What’s more, when the electric field was combined with the temperature field, the electric field played a leading role in the diffusion of water molecules while the temperature field was less affected. Diffusion coefficients of water molecules at different temperatures from molecular dynamics simulations were well consistent with experimental results, which verified the rationality of the model.     

Fractional variant of Stokes–Einstein relation in aqueous ionic solutions under external static electric fields

Both ionic solutions under an external applied static electric field E and glassy-forming liquids under undercooled state are in non-equilibrium state.In this work,molecular dynamics(MD)simulations with three aqueous alkali ion chloride(NaCl,KCl,and RbCl)ionic solutions are performed to exploit whether the glass-forming liquid analogous fractional variant of the Stokes–Einstein relation also exists in ionic solutions under E.Our results indicate that the diffusion constant decouples from the structural relaxation time under E,and a fractional variant of the Stokes–Einstein relation is observed as well as a crossover analogous to the glass-forming liquids under cooling.The fractional variant of the Stokes–Einstein relation is attributed to the E introduced deviations from Gaussian and the nonlinear effect.     

Interface water-induced hydrophobic carbon chain unfolding in water

The folding and unfolding of the carbon chain, which is the basic constitutional unit of polymers,are important to the performance of the material. However, it is difficult to regulate conformational transition of the carbon chain, especially in an aqueous environment. In this paper, we propose a strategy to regulate the conformational transition of the carbon chain in water based on the all-atom molecular dynamics simulations. It is shown that the unfolded carbon chain will spontaneously collapse into the folded state, while the folded carbon chain will unfold with an external electric field. The regulation ability of the electric field is attributed to the electric field-induced redistribution of interface water molecules near the carbon chain. The demonstrated method of regulating conformational transition of the carbon chain in water in this study provides an insight into regulating hydrophobic molecules in water, and has great potential in drug molecule design and new polymer material development.     

Anion–water interactions of weakly hydrated anions: molecular dynamics simulations of aqueous NaBF4 and NaPF6

In aqueous ionic solutions, both the structure and the dynamics of water are altered dramatically with respect to the pure solvent. The emergence of novel experimental techniques makes these changes accessible to detailed investigations. At the same time, computational studies deliver unique possibilities for the interpretation of the experimental data at the molecular level. Here, using molecular dynamics simulations, we demonstrate how competing mechanisms can explain the seemingly contradictory statements about the structure and dynamics of ion-coordinated solvent in aqueous solutions of two interesting and technologically important electrolytes, NaBF₄ and NaPF₆. While the static structural data (i.e. radial, radial-angular and spatial distribution functions, as well as hydrogen bonding statistics) unequivocally point at very weak anion–water hydrogen bonding in both salts, dynamic analyses (in particular, orientational anisotropy decay and solvent residence times) reveal quite significant retardation of water rotation and mobility due to solute coordination. Additionally, rotational immobilisation of coordinated solvent molecules is clearly unrelated to the hydrogen bond strength between them, as demonstrated by the interatomic oxygen–oxygen distance distributions for coordinated and bulk water.     

Modulation of intra- and inter-sheet interactions in short peptide self-assembly by acetonitrile in aqueous solution

Besides our previous experimental discovery(Zhao Y R, et al. 2015 Langmuir, 31, 12975) that acetonitrile(ACN)can tune the morphological features of nanostructures self-assembled by short peptides KIIIIK(KI4K) in aqueous solution,further experiments reported in this work demonstrate that ACN can also tune the mass of the self-assembled nanostructures.To understand the microscopic mechanism how ACN molecules interfere peptide self-assembly process, we conducted a series of molecular dynamics simulations on a monomer, a cross-β sheet structure, and a proto-fibril of KI4 K in pure water, pure ACN, and ACN-water mixtures, respectively. The simulation results indicate that ACN enhances the intra-sheet interaction dominated by the hydrogen bonding(H-bonding) interactions between peptide backbones, but weakens the inter-sheet interaction dominated by the interactions between hydrophobic side chains. Through analyzing the correlations between different groups of solvent and peptides and the solvent behaviors around the proto-fibril, we have found that both the polar and nonpolar groups of ACN play significant roles in causing the opposite effects on intermolecular interactions among peptides. The weaker correlation of the polar group of ACN than water molecule with the peptide backbone enhances H-bonding interactions between peptides in the proto-fibril. The stronger correlation of the nonpolar group of ACN than water molecule with the peptide side chain leads to the accumulation of ACN molecules around the proto-fibril with their hydrophilic groups exposed to water, which in turn allows more water molecules close to the proto-fibril surface and weakens the inter-sheet interactions. The two opposite effects caused by ACN form a microscopic mechanism clearly explaining our experimental observations.     

Properties of aqueous NaCl solutions in external electromagnetic fields

Molecular dynamics simulations are performed to investigate the effects of an external electromagnetic field on aqueous NaCl electrolyte solutions at a temperature of 298 K. The simulations assume that the electromagnetic waves propagate in the x-axis direction with a frequency of 50, 100 or 200 GHz. The intensity of the electromagnetic field is assumed to vary in the range 0.1–0.3 V/Å. The simulations are conducted for two different NaCl solution concentrations, namely 1 m and 5 m (molality). The results indicate that as the intensity of the electromagnetic field is increased, the re-orientation of the water molecules becomes more pronounced and the diffusion coefficient of the aqueous NaCl solution increases. It is also shown that in both the low-concentration and high-concentration solutions, the diffusion coefficients of water molecules and ions increase as the frequency and intensity of the electromagnetic field increase.     

一种可控的Ioffe型冷分子表面微电阱

囚禁于阱中的粒子(原子或分子)可获得更长的相互作用时间,因而在精密测量中可获得更高的分辨率.阱中的粒子与外界隔离,从而可以被冷却到更低的温度.因此原子(或分子)阱已广泛应用到许多研究领域.然而中心电场强度为零的势阱会导致粒子发生非绝热跃迁,这是原子或分子损失的主要来源.该损失曾是制备原子玻色-爱因斯坦凝聚的最后一道障碍.本文提出了一种可控的Ioffe型表面微电阱,其电场强度处处不为零,可有效避免分子的非绝热损失.另外,通过调节电压等参数,势阱中心电场强度以及势阱中心距芯片表面的高度可以在较大范围内调节,例如在本文参数下,势阱中心电场强度可在0.15—5.5 kV/cm变化,势阱中心高度可在6.0—17.0μm变化.本文通过有限元软件计算了芯片表面微电阱的电场分布,并用Monte Carlo模拟验证了该方案的可行性.该表面微电阱不仅可用于分子芯片的集成,而且可用于表面量子简并气体的制备.为精密测量、量子计算、表面冷碰撞和冷化学等领域提供了一个平台.     

DFT studies of surface‐enhanced Raman spectroscopy of 1,4‐benzenedithiol–Au2 complex under the effect of static electric fields

In this work, we demonstrate that the applied electric‐field strength and orientation can multiply modulate the Raman intensity and vibrational wavenumber of small molecule–metal complex, 1,4‐benzenedithiol–Au₂ (1,4BDT–Au₂), by density functional theory and time‐dependent density functional theory simulations. The polarizabilities are changed by the applied electric fields, leading to enhanced specific vibrational intensity and shifted vibrational wavenumber of the surface‐enhanced Raman scattering effect. The applied electric fields perturb the bonds and angles of the 1,4BDT–Au₂ complex. Owing to this reason, the peaks of Raman spectra related to these structures exhibit distinguishable responses in quasi‐static field (low‐frequency oscillating electric field). We use the visualized method of charge difference density to show that the electric fields tune the traditional excited state to pure charge‐transfer excited state. The charge‐transfer resonance transition produces enhanced Raman intensities for non‐totally symmetric modes and totally symmetric modes. These simulation results of the function of static electric field provide new guidance for the surface‐enhanced Raman scattering measurements. Copyright © 2015 John Wiley & Sons, Ltd.     

外电场下PbO作硫化剂交联聚乙烯分子结构及特性

本文研究外加电场对离子交联聚乙烯的微观结构和能量特性的影响,使用分子模拟法建立PbO作硫化剂的交联聚乙烯分子模型,通过半经验法对模型进行几何优化同时施加沿X轴方向的外电场,计算离子交联聚乙烯的分子总能量、偶极矩、极化率、分子轨道能量、能隙、红外光谱、电荷分布并进行分析.得出结论:随着外加电场的上升,交联聚乙烯分子结构会发生变化,当外加电场过大时本文所建交联聚乙烯分子离子盐桥的S-Pb-S键将会发生断裂形成自由基.外加电场会使分子内部的电荷从交联聚乙烯碳链端部向离子盐桥转移,盐桥处Pb原子电荷量不断积累,到达临界点时将会断裂形成电荷量较大的Pb自由基,在外加电场的作用下进一步影响交联聚乙烯分子的稳定性导致其性能下降,研究结果对交联聚乙烯的电树枝老化生成提供参考.     

Dehydration process in NaCl solutions under various external electric fields

Ionic motions at solid-liquid interface in supersaturated NaCl solutions have been investigated by molecular dynamics (MD) simulation for understanding crystal growth processes. The density profile in the vicinity of the interfaces between NaCl(100) and the supersaturated NaCl solution was calculated. Diffusion coefficients of water molecules in the solution were estimated as a function of distance from the crystal interface. It turned out that the structure and dynamics of the solution in the interfaces was different from those of bulk solution owing to electric fields depending on the surface charge. Therefore, the electric field was applied to the supersaturated solutions and dehydration phenomenon occurring in the process of the crystal growth was discussed. As the electric field increased, it was observed that the Na⁺ keeping strongly hydration structure broke out by the electric force. In supersaturated concentration, the solution structure is significantly different from that of dilution and has a complicated structure with hydration ions and clusters of NaCl. If the electric fields were applied to the solutions, the breakout of hydration structure was not affected with increasing the supersaturated ratio. This reason is that the cluster structures are destroyed by the electric force. The situation depends on the electric field or crystal surface structure.     

First-principles molecular dynamics simulation of biased electrode/solution interface

First-principles molecular dynamics simulations have been carried out for water in contact with Pt(1 1 1) surface. To apply negative bias potential to the water/Pt interface, excess electrons were added to our slab model using the recently developed computational scheme called “effective screening medium (ESM)”. Water molecules located away from the surface reoriented themselves to screen the electric field, but they responded differently near the surface. Water molecules nearest to the surface, forming a distinct layered structure with the hydrogen atom directed to the surface, increased the density with increasing field. On these bases, we discuss microscopic aspects of the electric double layer.     

Bound water: Evidence from and implications for the dielectric properties of aqueous solutions

Broadband dielectric spectra of a variety of aqueous solutions are evaluated as to indications of water that may be considered bound. Static permittivity decrements due to depolarizing internal electric fields, from kinetic depolarization, as well as from dielectric saturation are discussed. The latter effect reflects the preferential orientation of water permanent dipole orientations within strong Coulombic field of small ions, especially multivalent cations. Such water may be considered bound even though rapid rotations around the orientation of the electric dipole moment are definitely possible and also a fast exchange of water molecules between the hydration region and the bulk may take place. Water exhibiting large dielectric relaxation times, as typical for regions with large local concentration of foreign matter, may also be named bound. However, no clear evidence for interaction energies exceeding the hydrogen bond energy of pure water has been found. Rather enhanced relaxation times at low water content reflect the small concentration of hydrogen bonding sites and thus low probability density for the formation of a new hydrogen bond. Potential interferences of the water relaxation with relaxations from other molecules or from ionic structures are mentioned briefly.     

分子光学及其应用前景

随着原子光学的快速发展，一门新兴的有关研究中性分子与电场、磁场和光场等物质相互作用及其冷却、囚禁、操控与应用的学科——“分子光学”正在逐步形成．文章首先就分子光学的学术内涵及其研究内容作一简单类比与讨论；其次，就冷分子束的产生与超冷分子样品的实验制备、超冷分子物理与光谱学、非线性与量子分子光学的研究及其最新进展进行简要综述．最后，就分子光学的应用前景进行了展望．     

Computer simulations of dynamic crossover phenomena in nanoconfined water

In order to study dynamic crossover phenomena in nanoconfined water we performed a series of molecular dynamics (MD) computer simulations of water clusters adsorbed in zeolites, which are microporous crystalline aluminosilicates containing channels and cavities of nanometric dimensions. We used a sophisticated empirical potential for water, including the full flexibility of the molecule and the correct response to the electric field generated by the cations and by the charged atoms of the aluminosilicate framework. In addition, the full flexibility of the aluminosilicate framework was included in the calculations. Previously reported and new simulations of water confined in a number of different types of zeolites in the temperature range 100-300 K and at various coverage are discussed in connection with the experimental data. Dynamic crossover phenomena are found in all the considered cases, in spite of the different shape and size of the clusters, even when the confinement hinders the formation of tetrahedral hydrogen bonds for water molecules. Hypotheses about the possible dynamic crossover mechanisms are proposed.     

Effects of high-frequency and high-voltage pulsed electric field parameters on water chain formation

Separation methods utilizing high-frequency and high-voltage pulsed DC electric fields have been used extensively in the oil and petroleum industries, where the occurrence of water-in-oil dispersions is highly unwelcome because of physical constraints and the high maintenance costs required to treat these dispersions. This paper reports the results of studies of the effects of applied electric field parameters, including electric field strength, frequency, and duty ratio, on water chain formation in water-in-oil emulsions. The investigations were performed in a rectangular Perspex^® cell. The results of the studies show that dipole–dipole forces dominate the process of water chain formation. At low electric field strength, frequency, or duty ratio, dipole–dipole forces are negligible; therefore, the process of water chain formation and aqueous drop coalescence are inconspicuous. However, at high electric field strength, frequency, or duty ratio, significant dipole–dipole forces give rise to water chain formation and aqueous drop coalescence. At extremely high electric field strength, frequency, or duty ratio, aqueous drops are excessively polarized and disintegrate, inhibiting the processes of water chain formation and aqueous drop coalescence. The optimum electric field parameters for separation of water-in-oil dispersions are as follows: electric field strength, 3.80 kV cm⁻¹; frequency, 4.0 kHz; and duty ratio, 0.65.     

On the possibility of sorting ortho and para water molecules during diffusion in nanopores

The possibility of spatial sorting of ortho and para water molecules at filtering water vapor in a nanoporous medium is justified. Three factors are indicated, which in combination can cause sorting: Knudsen character of motion of molecules in nanopores, inhomogeneous surface electric fields in pores, and water molecule sorting over rotational states in an inhomogeneous electric field, recently implemented in experiments with molecular beams.     

Effect of an electric field on dewetting transition of nitrogen-water system

We investigate the influence of an external electric field on the dewetting behavior of nitrogen-water systems between two hydrophobic plates using molecular dynamics simulations. It is found that the critical distance of dewetting increases obviously with the electric field strength, indicating that the effective range of hydrophobic attraction is extended. The mechanism behind this interesting phenomenon is related to the rearrangement of hydrogen bond networks between water molecules induced by the external electric field. Changes in the hydrogen bond networks and in the dipole orientation of the water molecules result in the redistribution of the neutral nitrogen molecules, especially in the region close to the hydrophobic plates. Our findings may be helpful for understanding the effects of the electric field on the long-range hydrophobic interactions.     

Atomistic hybrid DSMC/NEMD method for nonequilibrium multiscale simulations

A multiscale hybrid method for coupling the direct simulation Monte Carlo (DSMC) method to the nonequilibrium molecular dynamics (NEMD) method is introduced. The method addresses Knudsen layer type gas flows within a few mean free paths of an interface or about an object with dimensions of the order of a few mean free paths. It employs the NEMD method to resolve nanoscale phenomena closest to the interface along with coupled DSMC simulation of the remainder of the Knudsen layer. The hybrid DSMC/NEMD method is a particle based algorithm without a buffer zone. It incorporates a new, modified generalized soft sphere (MGSS) molecular collision model to improve the poor computational efficiency of the traditional generalized soft sphere GSS model and to achieve DSMC compatibility with Lennard-Jones NEMD molecular interactions. An equilibrium gas, a Fourier thermal flow, and an oscillatory Couette flow, are simulated to validate the method. The method shows good agreement with Maxwell–Boltzmann theory for the equilibrium system, Chapman–Enskog theory for Fourier flow, and pure DSMC simulations for oscillatory Couette flow. Speedup in CPU time of the hybrid solver is benchmarked against a pure NEMD solver baseline for different system sizes and solver domain partitions. Finally, the hybrid method is applied to investigate interaction of argon gas with solid surface molecules in a parametric study of the influence of wetting effects and solid molecular mass on energy transfer and thermal accommodation coefficients. It is determined that wetting effect strength and solid molecular mass have a significant impact on the energy transfer between gas and solid phases and thermal accommodation coefficient.     

Dipolar response and hydrogen-bond kinetics in liquid water in square-wave time-varying electric fields

Non-equilibrium molecular dynamics simulations of liquid water have been performed at 298 K in the presence of external time-varying electric fields, approximating a square wave, of varying peak intensity (0.005–0.1 V/Å) in the microwave to far-infrared frequency range (20–500 GHz). Significant non-thermal field effects were noted in terms of dipolar response and acceleration of hydrogen-bond kinetics. The coupling between the total dipole moment and the external field has been investigated and autocorrelation functions (ACFs) of both the total dipole moment and the average of the individual molecular dipole moment along the laboratory axis of the applied fields exhibited coupling, with the former showing a stronger coupling and the latter showing coupling to lower magnitude fields. The maximum alignment achieved has been computed as a function of field intensities and frequencies: the lower frequencies show a greater maximum alignment as the system had more time within each field cycle to respond. The normalised probability distribution and the hydrogen-bond ACFs have been computed: the ACF showed a clear effect over shortening the hydrogen-bond relaxation time. The field effects over the molecules’ transitions from four to five hydrogen bonds have been computed. There was an enhancement of fewer molecules undergoing transitions and a dampening for a larger proportion of molecules, depending on the external fields’ periods.     

Effects of ion atmosphere on hydrogen-bond dynamics in aqueous electrolyte solutions

We have performed a series of molecular dynamics simulations of aqueous NaCl and KCl solutions at different concentrations to investigate the effects of ion atmosphere on the dynamics of water-water hydrogen bonds at room temperature. The average number of hydrogen bonds per water molecule decreases with increase of ion concentration. The dynamics of hydrogen-bond breaking is found to accelerate somewhat and that of hydrogen-bond structural relaxation, which occurs at a longer time scale, is found to slow down with increasing ion concentration for both NaCl and KCl solutions.