水在不均匀纳米通道内自扩散性质的分子动力学研究

本文用分子动力学(MD)方法模拟了受限水在不均匀润湿性微通道中的自扩散性质.通过考察不均匀润湿通道内水的自扩散行为,发现在微通道中水的扩散性质表现出明显的尺度效应,随着通道高度的增加水的扩散增强.更重要的是,由于通道高度的不同,通道内的不均匀润湿段对水的扩散与均匀通道相比有不同的影响.当通道高度为0.8 nm时,不均匀润湿通道内水的扩散增强;当通道高度为1.0 nm时,不均匀润湿通道内水的扩散减弱;而当通道高度达到1.2 nm时,不均匀润湿通道内水的扩散基本相同.     

Anisotropic self-diffusion of fluorinated poly(methacrylate) in metal-organic frameworks assessed with molecular dynamics simulation

Utilizing the periodically structured metal–organic framework(MOF) as the reaction vessel is a promising technique to achieve the aligned polymer molecular chains, where the diffusion procedure of the polymer monomer inside MOF is one of the key mechanisms. To investigate the diffusion mechanism of fluorinated polymer monomers in MOFs, in this paper the molecular dynamics simulations combined with the density functional theory and the Monte Carlo method are used and the all-atom models of TFMA(trifluoroethyl methacrylate) monomer and two types of MOFs, [Zn_2(BDC)_2(TED)]n and[Zn_2(BPDC)_2(TED)]n, are established. The diffusion behaviors of TFMA monomer in these two MOFs are simulated and the main influencing factors are analyzed. The obtained results are as follows. First, the electrostatic interactions between TFMA monomers and MOFs cause the monomers to concentrate in the MOF channel, which slows down the monomer diffusion. Second, the anisotropic shape of the one-dimensional MOF channel leads to different diffusion speeds of monomers in different directions. Third, MOF with a larger pore diameter due to a longer organic ligand, [Zn_2(BPDC)_2(TED)]n in this paper, facilitates the diffusion of monomers in the MOF channel. Finally, as the number of monomers increases, the self-diffusion coefficient is reduced by the steric effect.     

Self-consistent Shaw optimized model potential: Application to the determination of structural and atomic transport properties of liquid alkali metals by molecular dynamics simulations

The ‘first-principles’ fully non-local and energy-dependent optimized model potential (OMP) derived by Shaw is developed further. In contrast to Shaw's original paper, OMP parameters are derived in a self-consistent manner that does not rely on knowledge of experimental values of the ionization and cohesive energies. To our knowledge, this is the first time that this method has been used for effective potential calculations. In an application to liquid Li, Na, and K alkali metals, we used OMP pseudopotential-based interactions between ions to carry out standard molecular dynamics simulations. In the calculations, the ionic structure for the liquid state was first checked at a temperature near the melting point. Similar accurate calculations, but for atomic transport properties, predict the temperature dependence of the self-diffusion coefficients. The theoretical results obtained are in overall agreement with available experimental measurements. Thus, one can have some confidence in the ability of the optimized model potential to give a good representation of the physical properties of these alkali ions in the liquid environment.     

Molecular dynamics study on water self-diffusion in aqueous mixtures of methanol,ethylene glycol and glycerol: investigations from the point of view of hydrogen bonding

Molecular dynamics simulations have been performed to investigate the aqueous binary mixtures of alcohols, including methanol, ethylene glycol (EG) and glycerol of molalities ranging from 1 to 5 m at the temperatures of 273, 288 and 298 K, respectively. The primary purpose of this paper is to investigate the mechanism of water self-diffusion in water-alcohol mixtures from the point of view of hydrogen bonding. The effects of temperature and concentration on water self-diffusion coefficient are evaluated quantitatively in this work. Temperature and concentration to some extent affect the hydrogen bonding statistics and dynamics of the binary mixtures. It is shown that the self-diffusion coefficient of water molecules decreases as the concentration increases or the temperature decreases. Moreover, calculations of mean square displacements of water molecules initially with different number n of H-bonds indicate that the water self-diffusion coefficient decreases as n increases. We also studied the aggregation of alcohol molecules by the hydrophobic alkyl groups. The largest cluster size of the alkyl groups clearly increases as the concentration increases, implying the emergence of a closely connected network of water and alcohols. The clusters of water and alcohol that interacted could block the movement of water molecules in binary mixtures. These findings provide insight into the mechanisms of water self-diffusion in aqueous binary mixtures of methanol, EG and glycerol.     

Molecular dynamics study of self-diffusion in Zr

We employed a recently developed semi-empirical Zr potential to determine the diffusivities in hcp and bcc Zr via molecular dynamics simulation. The point defect concentration was determined directly from molecular dynamics (MD) simulation rather than from theoretical methods using T = 0 calculations. Our MD simulation indicates that the diffusion proceeds via the interstitial mechanism in hcp Zr, and both vacancy and interstitial mechanisms contribute to diffusivity in bcc Zr. The agreement with the experimental data is excellent for hcp Zr and rather good for bcc Zr at high temperatures, but there is considerable disagreement at low temperatures.     

Al表面的"类液"结构及其自扩散通道

应用分子动力学方法,采用嵌入势模型在熔点下和熔点上对Al(001),(110)和(111)表面的原子结构和自扩散现象进行研究.发现这些表面的第一层原子在低于熔点时,Al(110)面在700±10 K,Al(001)面在 860±10 K,Al(111)面在 930±10 K呈现明显自扩散且最终转变为"类液"结构,而其余各层仍保留有序状态.对这种"类液"结构进行均方位移、结构有序参数、径向分布函数和z向粒子密度分析,发现其结构和扩散行为与熔化的Al表面不同,并能在一定温度区间稳定存在.在"类液 关键词： 表面结构 分子动力学 自扩散     

Critical anomaly and finite size scaling of the self-diffusion coefficient for Lennardben Jones fluids by non-equilibrium molecular dynamic simulation

We use non-equilibrium molecular dynamics simulations to calculate the self-diffusion coefficient, D, of a Lennard-Jones fluid over a wide density and temperature range. The change in self-diffusion coefficient with temperature decreases by increasing density. For density ρ^* = ρσ³ = 0.84 we observe a peak at the value of the self-diffusion coefficient and the critical temperature T^* = kT/ε = 1.25. The value of the self-diffusion coefficient strongly depends on system size. The data of the self-diffusion coefficient are fitted to a simple analytic relation based on hydrodynamic arguments. This correction scales as N^-α, where α is an adjustable parameter and N is the number of particles. It is observed that the values of α < 1 provide quite a good correction to the simulation data. The system size dependence is very strong for lower densities, but it is not as strong for higher densities. The self-diffusion coefficient calculated with non-equilibrium molecular dynamic simulations at different temperatures and densities is in good agreement with other calculations from the literature.     

Molecular dynamics study for the thermal conductivity of diatomic liquid

The thermal conductivity of diatomic liquids was analyzed using a nonequilibrium molecular dynamics (NEMD) method. Five liquids, namely, O₂, CO, CS₂, Cl₂ and Br₂, were assumed. The two-center Lennard-Jones (2CLJ) model was used to express the intermolecular potential acting on liquid molecules. First, the equation of state of each liquid was obtained using MD simulation, and the critical temperature, density and pressure of each liquid were determined. Heat conduction of each liquid at various liquid states [metastable (ρ=1.9ρ_cr), saturated (ρ=2.1ρ_cr), and stable (ρ=2.3ρ_cr)] at T=0.7T_cr was simulated and the thermal conductivity was estimated. These values were compared with experimental results and it was confirmed that the simulated results were consistent with the experimental data within 10%. Obtained thermal conductivities at saturated state were reduced by the critical temperature, density and mass of molecules and these values were compared with each other. It was found that the reduced thermal conductivity increased with the increase in the molecular elongation. Detailed analysis of the molecular contribution to the thermal conductivity revealed that the contribution of the heat flux caused by energy transport and by translational energy transfer to the thermal conductivity is independent of the molecular elongation while the contribution of the heat flux caused by rotational energy transfer to the thermal conductivity increases with the increase in the molecular elongation. Moreover, by comparing the reduced thermal conductivity at various states, it was found that the increase of thermal conductivity with the increase in the density, or pressure, was caused by the increase of the contribution of energy transfer due to molecular interaction.     

Molecular simulation of liquid crystals

This article reviews recent progress in the computer simulation of liquid crystals at the molecular level. It covers the use of simple rigid-body models of the constituent molecules, and more detailed modelling via atomistic force fields. Bulk mesophases, inhomogeneous systems, and interfaces are discussed. Recent progress in calculating elastic properties and dynamics is summarised. As well as presenting an overview, some specific topics of recent interest are highlighted: the biaxial nematic phase, chiral phases, ionic liquid crystals, and charge-transfer systems.     

温度对有机物传热影响的分子动力学模拟及微观机理研究

热导率是表征物质导热性能的一个重要物性参数.通过分子模拟从微观角度揭示有机物分子液体导热机理并计算热导率具有重要的理论意义和应用价值.通过非平衡态分子动力学模拟方法,分别模拟了庚烷、己醛、2-己酮和己醇在263～363 K的热传导过程并得到了热导率.4种有机物在263～363 K下热导率的计算值与实验值的相对平均偏差分别小于5.40%,5.46%,4.29%和7.80%,表明模拟结果与实验结果基本一致.热流分解和原子热路径的结果表明,对总热流有显著贡献的库仑相互作用项、范德华相互作用项和扭转角项都随着温度的升高而减小,这使得4种有机物的热导率随着温度的升高而降低.同时研究表明温度的升高增大了分子的原子振动,加速了分子运动,降低了模拟体系的质量密度.本文为温度对液体热传导影响提供了微观解释和理论依据.     

Molecular dynamics study of self-diffusion in stoichiometric B2-NiAl crystals

Self-diffusion parameters in stoichiometric B2-NiAl solid state crystals were estimated by molecular statics/dynamics simulations with the study of required simulation time to stabilise diffusivity results. An extrapolation procedure to improve the diffusion simulation results was proposed. Calculations of volume diffusivity for the B2 type NiAl in the 1224–1699?K temperature range were performed using the embedded-atom-model potential. The results obtained here are in much better agreement with the experimental results than the theoretical estimates obtained with other methods.     

异质原子对液体气泡成核影响的分子动力学模拟

采用分子动力学方法模拟了池沸腾中液体层加入异质原子对气泡成核的影响.分析了异质原子能量参数对液体起始气泡成核时间和温度的影响及其机理.结果表明,当异质原子能量参数小于液氩能量参数时,液体起始气泡成核时间缩短,起始温度降低.当异质原子剂能量参数大于液氩能量参数时,液体起始气泡成核时间增加,起始温度升高.异质原子在壁面上的吸附及在液体中的扩散行为影响固液界面性质,较大能量参数的异质原子扩散系数较小,更多能量参数较大的原子吸附在固体表面上使得壁面势能壁垒增加,导致沸腾时间延迟,液体需要吸收更多的热量克服势能壁垒,进而提高沸腾起始温度.能量参数较小的异质原子扩散系数较大,异质原子更容易分散到液体中,使得壁面附近液体层势能减小,液体层更容易气泡成核行为.     

Molecular dynamics simulations of the penetration lengths: application within the fluctuation theory for diffusion coefficients

Mutual diffusion in condensed phases is a theoretically and practically important subject of active research. One of the most rigorous and theoretically advanced approaches to the problem is a recently developed approach based on the concept of penetration lengths (Physica A 320 (2003) 211; Physica A 322 (2004) 151). In the current study, a fast molecular dynamics scheme has been developed to determine the values of the penetration lengths in Lennard–Jones binary systems. Results deduced from computations provide a new insight into the concept of penetration lengths. It is shown for four different binary liquid mixtures of non-polar components that computed penetration lengths, for various temperatures and compositions, are consistent with those deduced from experiments in the framework of the formalism of the fluctuation theory. Moreover, the mutual diffusion coefficients obtained from a coupled fluctuation theory and molecular dynamics scheme exhibit consistent trends and average deviations from experimental data around 10–20%.     

Properties of a soft-sphere liquid from non-Newtonian molecular dynamics

A soft-sphere, inverse-12 liquid is simulated in both the isokinetic-isochoric and the isokinetic-isobaric ensemble using nonequilibrium molecular dynamics. The simulation for the isobaric ensemble is discussed in detail. The non-Newtonian characteristics of the liquid are clearly demonstrated; namely, the shear-rate-dependent pressure and density (shear dilatancy), the shear-rate-dependent shear viscosity (shear thinning), and evidence of normal pressure differences. For the first time, it is clearly shown that a significant component of isobaric shear thinning is due to shear dilatancy. The isochoric and isobaric results are checked for consistency. Simple empirical relations for the equation of state and transport properties of the fluid are presented.This is a publication in part of the National Institute of Standards and Technology (formerly NBS) and is not subject to copyright.     

改性活性炭内丙酮吸附的动力学模拟

本文通过在活性炭孔表面植入不同摩尔比例的羰基、羟基和羧基实现孔的改性,并采用分子动力学模拟研究了丙酮分子在改性活性炭孔内的动力学特性.模拟结果表明:改性后,孔内丙酮分子自扩散系数显著降低,其中羰基和羧基改性孔的丙酮分子扩散系数随着官能团的增多逐渐降低,但羟基改性孔的分子扩散系数并未呈现单调递减的趋势.植入官能团摩尔比例相同时,羧基对丙酮分子扩散的影响最高,羟基次之,羰基最低.总结得出改性孔内丙酮分子扩散主要受势能平滑度,自由体积分数以及分子与孔表面相互作用能三大因素的影响.     

纳米多晶铜微观结构的分子动力学模拟

基于EAM镶嵌原子势函数,采用分子动力学模拟了零温下纳米多晶铜的微观结构.首先用Voronoi几何方法构造了5个纳米多晶铜数值模型,在300K弛豫50ps并退火至0K.然后分析零温下弛豫模型的径向分布函数、原子能量、配位数、原子Voronoi体积、以及本征应力分布 关键词： 纳米多晶铜 微观结构 分子动力学     

纳米Cu颗粒等温晶化过程的分子动力学模拟研究

采用分子动力学方法和镶嵌原子势,模拟了500个Cu原子(简称Cu₅₀₀) 组成的纳米颗粒的等温晶化过程.利用修正的均方位移、键对分析技术和内在结构(IS) 等方法对该过程中的结构和动力学行为进行分析研究.结果显示:与块体金属不同的是, Cu₅₀₀纳米颗粒在某一温度保温时,其晶化时间并不是一个定值, 而是存在一个统计分布,并且保温温度越低其晶化时间的分布范围越广, 最长晶化时间越长.在低温晶化时, Cu₅₀₀经历了一系列中间构型的转变才达到晶态, 表现出多步晶化的特征.文章作者研究了颗粒的初始构型对晶化进程的影响, 发现颗粒的初始结构特征和能量状态对其随后的晶化过程有着重要的影响, 同一温度下,颗粒初始构型的IS能量越低其晶化时间越长,这一点在低温时尤其明显.     

聚乙烯/银纳米颗粒复合物的分子动力学模拟研究

通过分子动力学模拟对聚乙烯/银纳米颗粒复合物的结构、极化率和红外光谱、热力学性质、力学特性进行计算, 分析其随模拟温度和银颗粒尺寸的变化规律. 模拟结果表明: 聚乙烯/银纳米颗粒复合物为各向同性的无定形结构, 温度升高可提高银纳米颗粒的分散均匀性; 银纳米颗粒表面多个原子层呈现无定形状态, 并在银颗粒和聚乙烯基体的界面形成电极化层, 界面区域随颗粒尺寸和温度的增加分别减小和增加; 与聚乙烯体系相比, 聚乙烯/银纳米颗粒复合物的极化率高很多, 且随温度的升高和银颗粒尺寸的减小而增大; 银颗粒尺寸直接影响界面电偶极矩的强度和振动频率, 红外光谱峰强度和峰位随颗粒尺寸发生变化; 聚乙烯/银纳米颗粒复合物具有比聚乙烯体系更高的等容热容和与聚乙烯体系相反的负值热压力系数, 热容随颗粒尺寸的变化较小, 但随温度的升高而明显减小, 具有显著的温度效应; 热压力系数随温度的变化较小, 但随颗粒尺寸的增加而减小, 具有明显的尺度效应, 温度稳定性更好; 聚乙烯/银纳米颗粒复合物的力学特性表现出各向同性材料的弹性常数张量, 具有比聚乙烯体系更高的杨氏模量和泊松比, 并且都随温度的升高和银颗粒尺寸的增大而减小, 加入银纳米颗粒可有效改善聚乙烯的力学性质. 关键词： 分子动力学模拟 聚合物纳米复合物 纳米颗粒