Computer simulation of electron transfer processes across the electrode|electrolyte interface: a treatment of solvent and electrode polarizability

A polarizable molecular dynamics model for adiabatic electron transfer across the electrode|electrolyte interface is presented. The electronic polarizability of the water and of the metal electrode is accounted for by a dynamical fluctuating charge algorithm, image charges, and the Ewald summation adapted for a conducting interface. The effects of the solvent electronic polarizability are studied by computing the diabatic and adiabatic free energy curves for both polarizable and non-polarizable water models. This represents the first effort to compute the adiabatic free energy curves from simulation for a fully polarizable electrochemical system.     

First-principles simulations of the electrode|electrolyte interface

We review a direct dynamics method for the simulation of metal|water interfaces. The occupancy of on-top binding sites for water in this model as applied to a (100) surface of ‘copper' is very sensitive to potential. We suggest that this may account for some previously unexplained features of X-ray data on water structure and noble metal|water interfaces. We discuss the problem of statistical fluctuations on the occupancy of such tightly bound water molecules in such simulations. Though the problem is not too serious for charged interfaces, the problem of accounting for fluctuations at zero charge can be quite formidable, as we illustrate for the (100) surface of copper.     

Molecular dynamics simulation of the water|nitrobenzene interface

The structure and dynamics of the neat water|nitrobenzene liquid|liquid interface are studied at 300 K using molecular dynamics computer simulations. The water is modeled using the flexible SPC potential, and the nitrobenzene is modeled using an empirically determined nitrobenzene potential energy function. Although nitrobenzene is a polar liquid with a large dielectric constant, the structure of the interface is similar to other water|non-polar organic liquid interfaces. Among the main structural features we describe are an enhancement of interfacial water hydrogen bonds, the specific orientation of water dipoles and nitrobenzene molecules, and a rough surface that is locally sharp. Surface roughness is also characterized dynamically. The dynamics of molecular reorientation are shown to be only mildly modified at the interface. The effect due to the polarizable many-body potential energy functions of both liquids is investigated and is found to affect only mildly the above results.     

Structure and dynamics of water at water|Pt interface as seen by molecular dynamics computer simulation

Five molecular dynamics computer simulations were performed to study the structural and dynamical properties of water next to uncharged and charged Pt surfaces. The results show that the structure of a water layer adsorbed on the metal surface is very sensitive to the details of the water–metal potential. While patches of short-living hexagonal ice-like structure are observed in the adsorbed water layer next to the uncharged Pt(111) surface, a square lattice solid-like structure is seen for the layer on top of the uncharged Pt(100) surface. The orientational ordering for the following two layers of water next to uncharged Pt is displaying a preference towards the orientations that are characteristic of hexagonal ice-I, while water is liquid-like in these layers. In the presence of a high value external electric field water reorients and undergoes a layering transition.     

Simultaneous separation of mercury and lignosulfonate from aqueous solution using supported liquid membrane

This paper presents an experimental investigation on facilitated and simultaneous transport of mercury and lignosulfonate (LS) through a flat sheet supported liquid membrane (SLM) having Nylon 6,6 as support, trioctylamine (TOA) as carrier and dichloroethane as solvent. The experiments were performed at various operating conditions such as strip phase concentration, feed pH, carrier concentration and feed concentration to find the best set of parameters that would yield the maximum separation of pure mercury as well as its mixture with LS. The experiments were performed in co-transport mode using NaOH as the strip phase. It was observed that extraction of mercury as well as its mixture increases with increase in concentration of NaOH up to a certain limit 0.1 M NaOH in case of pure solution and 0.2 M NaOH in case of mixture). Feed phase pH significantly affects the mercury separation process. However, initial feed concentration does not affect the extraction process appreciably. Separation of mixture of mercury and LS behaves in a similar way as their pure solution; however the extraction is low in comparison to pure solution. The extraction of mercury from its pure solution is about 81% in 1 h. The extraction of mercury and LS from their mixture is about 52.6% and 50.2%, respectively at optimum condition and in a period of 2 h.     

分子动力学模拟无定形BCN体系结构特征

为研究无定形BCN材料的原子结构特征, 采用分子动力学方法模拟了高温下无定形BCN体系的动力学行为, 计算并分析了引入C原子对体系的径向分布函数(RDF)和配位数的影响. 模拟结果表明, C原子对BCN体系的RDF曲线的峰位置及形状影响很小, 但使其偏径向分布函数(PRDF)的一些峰向右偏移; 引入的C原子部分占据了原来N原子的位置, 使得B-N的配位数降低. 在模拟条件下, 未观察到BN相与C相分离.     

甘油水溶液氢键特性的分子动力学模拟

为了研究低温保护剂溶液的结构和物理化学特性, 以甘油为保护剂, 采用分子动力学方法, 对不同浓度的甘油和水的二元体系进行了模拟. 得到了不同浓度的甘油水溶液在2 ns内的分子动力学运动轨迹, 通过对后1 ns内运动轨迹的分析, 得到了各个原子对的径向分布函数和甘油分子的构型分布. 根据氢键的图形定义, 分析了氢键的结构和动力学特性. 计算了不同浓度下体系中平均每个原子(O和H)和分子(甘油和水)参与氢键个数的百分比分布及其平均值. 同时还计算了所有氢键、水分子之间的氢键以及甘油与水分子之间的氢键的生存周期.     

室温离子液体[bmim][BF4]和水混合物的计算机模拟研究

针对室温离子液体[bmim][BF₄]和水的混合物, 用分子动力学模拟研究了溶液的微观组成与浓度的关系. 模拟结果表明：该混合物中各组分间的径向分布函数随[bmim][BF₄]摩尔分数的增加呈有规律的变化；在此基础上计算了溶液的局部组成以及组分间的缔合因子, 进一步考察了各组分间的相互作用情况；另外基于Kirkwood-Buff理论估算了混合物的偏摩尔体积、等温压缩因子以及活度系数对浓度的偏导数, 对于理解离子液体与水之间的交互作用具有一定的参考意义.     

室温离子液体[bmim][BF_4]和水混合物的计算机模拟研究

针对室温离子液体[bmim][BF4]和水的混合物,用分子动力学模拟研究了溶液的微观组成与浓度的关系.模拟结果表明:该混合物中各组分间的径向分布函数随[bmim][BF4]摩尔分数的增加呈有规律的变化;在此基础上计算了溶液的局部组成以及组分间的缔合因子,进一步考察了各组分间的相互作用情况;另外基于Kirkwood-Buff理论估算了混合物的偏摩尔体积、等温压缩因子以及活度系数对浓度的偏导数,对于理解离子液体与水之间的交互作用具有一定的参考意义.     

基于粗粒化模型对有机溶剂的分子动力学模拟

在基于Boltzmann分布对四种基本构象进行Monte Carlo取样后, 通过与全原子模型的范德华势比较得到了Gay-Berne (GB)参数. 又在对用量化计算得到的分子体系的电势进行电荷、偶极矩和四极矩的拟合后, 得到了电多极展开势(EMP)参数. 利用得到的粗粒化参数, 基于粗粒化模型, 对CHCl₃及四氢呋喃(THF)两种有机溶剂进行了分子动力学模拟(MDS), 并将结果同全原子模拟进行了比较. 计算结果表明用粗粒化模型从整体上能重复全原子模型的模拟结果, 但在某些细节的计算与全原子模型有偏差, 其原因可能是目前工作仅考虑了单位点情况, 为此今后在对具有复杂结构的分子进行粗粒化模拟时还应考虑合理放置及增加相互作用位点.     

Structure and dynamics of La(III) in aqueous solution – An ab initio QM/MM MD approach

Ab initio QM/MM MD simulations have allowed to clarify some of the ambiguities arising from various studies on the hydrated La(III) ion. Both nine- and ten-coordinated hydrates co-exist and interchange in a dissociative process on the nano- or even subnanosecond scale, and thus much faster than any other trivalent main group or transition metal ions. The weak ion–ligand bond (53 N/m) supplies a reasonable explanation for it. The simulation results for La(III) are also compared to those for the isoelectronic ions Cs(I) and Ba(II) obtained by the same ab initio MD procedure, leading to conclusions on the influence of central ion charge on structural and dynamic properties of hydrate complexes.     

Aggregation of Glycerol Induced by Carbon Nanotubes in Aqueous Solution and Its Influencing Factors

Carbon nanotubes(CNTs) have received wide application and investigation because of their unique electronic, chemical and mechanical properties. But the self-aggregation of CNTs limits their practical application and study. In order to disperse CNTs effectively, polymers, such as polyglycerol and its derivatives, are adopted as dispersants in view of their strong interaction with CNTs. In order to understand the interaction between CNTs and glycerol in water in detail, a series of simulations has been conducted to investigate the interaction between them and analyze the influences of CNTs diameter and temperature. All the analyses indicate that the glycerol molecules are prone to aggregate around CNTs with the addition of CNTs. This is mainly due to hydrophobic interaction. It is confirmed that this aggregation is influenced by CNTs diameter and the temperature to some degree. This work will establish the basis for the exploration of polyglycerol and its derivatives interacting with CNTs and provide an invaluable guide to seek for emergent dispersants for CNTs.     

Molecular dynamics study of host–guest interactions in cyclodextrins: methodology and data analysis for a comparison with solution data and the solid-state structure

A general method is proposed to model the behavior of cyclodextrins (CDs) and of their inclusion compounds through energy minimizations and molecular dynamics (MD) simulations at a constant temperature. In this way, the formation of a host–guest compound is obtained starting from many trial geometries with the guest outside the CD cavity without any a priori assumption. The MD simulation results are analyzed through two functions: (i) the similarity maps of the root-mean-square distances between instantaneous conformations found in the MD runs to recognize different families of conformers; (ii) the pair distribution function PDF, yielding the probability density of finding appropriate atom pairs as a function of their distance at equilibrium. As an example, the inclusion compound formed by β-CD and (−)-menthol-β-d-glucoside is investigated. The lowest-energy inclusion compound is in good agreement with the results of single-crystal X-ray analysis, while at room temperature the MD runs show a closely similar arrangement with thermal fluctuations. In this case, the PDF between diagnostic hydrogen atoms of β-CD and of the guest molecule are fully consistent with the experimental NOE results obtained from NMR measurements in solution.     

Computer simulation of trifluoromethane properties with ab initio force field

Intermolecular interaction potentials of the trifluoromethane dimer in 15 orientations have been calculated using the Hartree‐Fock (HF) self‐consistent theory and the second‐order Møller‐Plesset (MP2) perturbation theory. Single point energies at important geometries were also calibrated by the coupled cluster with single and double and perturbative triple excitation [CCSD(T)] calculations. We have employed Pople's medium size basis sets [up to 6‐311++G(3df,3pd)] and Dunning's correlation consistent basis sets (up to aug‐cc‐pVQZ). Basis set limit potential values were obtained through well‐studied extrapolation methods. The calculated MP2 potential data were employed to parameterize a 5‐site force field for molecular simulations. We performed molecular dynamics simulations using the constructed ab initio force field and compared the simulation results with experiments. Quantitative agreements for the atom‐wise radial distribution functions and the self‐diffusion coefficients over a wide range of experimental conditions can be obtained, thus validating the ab initio force field without using experimental data a priori. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011     

Liquid properties of dimethyl ether from molecular dynamics simulations using Ab Initio force fields

We have used molecular dynamic simulations to study the structural and dynamical properties of liquid dimethyl ether (DME) with a newly constructed ab initio force field in this article. The ab initio potential energy data were calculated at the second order Møller‐Plesset (MP2) perturbation theory with Dunning's correlation consistent basis sets (up to aug‐cc‐pVQZ). We considered 17 configurations of the DME dime for the orientation sampling. The calculated MP2 potential data were used to construct a 3‐site united atom force field model. The simulation results are compared with those using the empirical force field of Jorgensen and Ibrahim (Jorgensen and Ibrahim, J Am Chem Soc 1981, 103, 3976) and with available experimental measurements. We obtain quantitative agreements for the atom‐wise radial distribution functions, the self‐diffusion coefficients, and the shear viscosities over a wide range of experimental conditions. This force field thus provides a suitable starting point to predict liquid properties of DME from first principles intermolecular interactions with no empirical data input a priori. © 2012 Wiley Periodicals, Inc.     

Molecular Dynamic Simulation of Melting Points of Trans-1,4,5,8-tetranitro-1,4,5,8-tetraazadacalin (TNAD) with Some Propellants

Molecular dynamic simulation was employed to predict the melting points Tm of TNAD/HMX, TNAD/RDX, TNAD/DINA, and TNAD/DNP systems (tans-1,4,5,8-tetranitro-1,4,5,8-tetraazadacalin (TNAD), dinitropiperazine (DNP), cyclotetramethylenete-tranitroamine (HMX), cyclotrimethylenetrinitramine (RDX), and N-nitrodihydroxyethy-laminedinitrate (DINA)). Tm was determined from the inflexion point on the curve of mean specific volume vs. temperature. The result shows that the Tm values of TNAD/HMX, TNAD/RDX, and TNAD/DINA systems are 500, 536, and 488 K, respectively. The TNAD/DNP system has no obvious Tm value, which shows the system is insoluble. Us-ing Tm, the solubility of the four systems was analyzed. The radial distribution functions of the four systems were analyzed and the main intermolecular forces between TNAD and other energetic components are short-range interactions. The better the solubility is, the stronger the intermolecular interaction is. In addition, the force field energy at different temperature was also analyzed to predict Tm of the four systems.