Molecular dynamics study of swelling patterns of Na/Cs-montmorillonites and hydration of interlayer cations

We report on a molecular dynamics study of swelling patterns of an Na-rich/Cs-poor montomorillonite and a Cs-montomorillonite. The recently developed CLAYFF force field is used to predict the basal spacing as a function of the water content in the interlayer. The simulations reproduce the swelling patterns of the Na and Cs-montomorillonite, suggesting a mechanism of its hydration different from that of the montomorillonite. In the meanwhile, we find that the differences in size and hydration energy of Na and Cs ions have strong implications for the structure and the internal energy of interlayer water. In particular, our results indicate that the hydrate difference in the presence of coexistent Na and Cs has a larger influence on the behavior of clay-water system. For Na-rich/Cs-poor montomorillonite, the hydration energy values of Na ions and water molecules each have a dramatic increase compared with those in Na-montomorillonite on the interlayer spacing, and the hydration energy values of Cs ions and water molecules decrease somewhat compared with those in Cs-montomorillonite.     

Swelling of K+, Na+ and Ca2+-montmorillonites and hydration of interlayer cations: a molecular dynamics simulation

This paper performs molecular dynamics simulations to investigate the role of the monovalent cations K, Na and the divalent cation Ca on the stability and swelling of montmorillonite. The recently developed CLAYFF force field is used to predict the basal spacing as a function of the water content in the interlayer. The simulations reproduced the swelling pattern of these montmorillonites, suggesting a mechanism of their hydration different (K+ 相似文献   

A molecular dynamics study of uranyl hydration

Molecular dynamics calculations were carried out in order to investigate the hydration structure of uranyl in aqueous solution. The CF1 model of flexible water molecules is used. This model allows one to investigate a hydrolysis reaction for water molecules in the first uranyl hydration shell. Charge redistribution effects on hydrolysis products are also taken into account. We found five ligands in uranyl hydration shell, which is of bipyramidal pentacoordinated structure. The charge redistribution effects resulted in ligands of four water molecules and one hydroxyl, which was found closer to uranium than the other ligands.     

Diffusion activation energy versus the favourable energy in two-order-parameter model:A molecular dynamics study of liquid Al

In the present work, we find that both diffusion activation energy E_a(D) and E_a(S^ex) increase linearly with pressure and have the same slope (0.022±0.001 eV/GPa) for liquid Al. The temperature and pressure dependence of excess entropy is well fitted by the expression -S^ex(T,P)/k_B=a(P)+b(P)T+c(P)exp(E_f/k_BT), which together with the small ratio of E_f/k_BT leads to the relationship of excess entropy to temperature and pressure, i.e. S^ex≈-cE_f/T, where c is about 12 and E_f (=Δ E-PΔV) is the favourable energy. Therefore, there exists a simple relation between E_a(S^ex) and E_f, i.e. E_a(S^ex)≈cE_f.     

高温下钙蒙脱石膨胀特性的分子动力学模拟

高温下蒙脱石的膨胀特性在核废料深部封存、二氧化碳封存及页岩气开发等应用中有着重要影响,但相关机理尚不明确.本工作使用分子动力学模拟为技术手段计算5 MPa和298—500 K等条件下,1.40—4.00 nm晶面间距(d)的一系列饱和钙蒙脱石的膨胀压力.以模拟所得的数值结果为依据,基于水化效应、双电层效应和离子关联效应等模型推演膨胀压力随温度与d的变化规律,并与相应的实验数据进行对比.模拟结果表明,当d较小时,因为高温会弱化水化力的强度,钙蒙脱石膨胀压力震荡的幅度降低,同时水化力作用的d的范围减小.当d较大时,因为高温强化离子关联效应,膨胀压力降低,同时双电层力的作用的d的范围增加.在较高温度和较大d时,膨胀压力为收缩力,阻碍膨胀.这些膨胀压力的变化规律与前期钠蒙脱石体系的研究类似.然而,通过对比两种蒙脱石体系的模拟结果,发现两种体系存在显著的差异—钙蒙脱石比钠蒙脱石更难膨胀到较大的d.此模拟结果与前人实验观测的结果相符.我们进一步将此差异归于钙蒙脱石的离子关联效应要远大于钠蒙脱石.有别于分子模拟中对于离子关联效应的精确描述,连续化的Poisson-Boltzmann方程因为忽略了离子关联效应,从而无法表达出与两种体系模拟结果都相吻合的膨胀压力变化规律.     

高温下钙蒙脱石膨胀特性的分子动力学模拟

高温下蒙脱石的膨胀特性在核废料深部封存、二氧化碳封存及页岩气开发等应用中有着重要影响,但相关机理尚不明确.本工作使用分子动力学模拟为技术手段计算5 MPa和298—500 K等条件下,1.40—4.00 nm晶面间距(d)的一系列饱和钙蒙脱石的膨胀压力.以模拟所得的数值结果为依据,基于水化效应、双电层效应和离子关联效应等模型推演膨胀压力随温度与d的变化规律,并与相应的实验数据进行对比.模拟结果表明,当d较小时,因为高温会弱化水化力的强度,钙蒙脱石膨胀压力震荡的幅度降低,同时水化力作用的d的范围减小.当d较大时,因为高温强化离子关联效应,膨胀压力降低,同时双电层力的作用的d的范围增加.在较高温度和较大d时,膨胀压力为收缩力,阻碍膨胀.这些膨胀压力的变化规律与前期钠蒙脱石体系的研究类似.然而,通过对比两种蒙脱石体系的模拟结果,发现两种体系存在显著的差异—钙蒙脱石比钠蒙脱石更难膨胀到较大的d.此模拟结果与前人实验观测的结果相符.我们进一步将此差异归于钙蒙脱石的离子关联效应要远大于钠蒙脱石.有别于分子模拟中对于离子关联效应的精确描述,连续化的Poisson-Boltzmann方程因为忽略了离子关联效应,从而无法表达出与两种体系模拟结果都相吻合的膨胀压力变化规律.     

Molecular dynamics and quantum chemical approaches in the study of the hydration of protonated cyclohexyldiamines

Explicit hydration of the neutral and charged cyclohexylamine and of the cyclohexyldiamine isomers in their mono- or diprotonated forms is investigated through classical molecular dynamics (MD) simulations in aqueous solutions combined with DFT calculations in amine–water complexes. The MD studies performed in the monoamines reveal that the structure of the hydration shell around the neutral amino group (NH₂) is quite distinct from that around the charged one (NH₃⁺). On average, the number of water molecules surrounding the two groups is calculated to be ～2 and 3–4, respectively. The variation of the hydration structure prompted by the groups’ proximity is discussed based on the data found for the mono- and diprotonated diamines. To have a more detailed picture of the water molecules’ arrangement around the amino groups and of the amine–water hydrogen bonds, geometry optimisations in hydrates with up to six water molecules are carried out at the B3LYP/aug-cc-pVDZ level. Complexation energies are also computed. The main findings emerging from these calculations are found to be very helpful to rationalise the mutual influence of the amino groups and therefore to better elucidate the MD findings. The complementary nature of the two research methods is emphasised as an excellent tool in order to closely examine the hydration of polyamines, as exemplified for the cyclohexyldiamines.     

Pt(111)表面低能溅射现象的分子动力学模拟

利用嵌入原子方法的原子间相互作用势，通过分子动力学模拟，详细研究了贵金属原子在Pt (111)表面的低能溅射现象.模拟结果显示：对于垂直入射情况，入射原子的质量对Pt (11 1)表面的溅射阈值影响不大.当入射原子的能量小于溅射阈值时，入射原子基本以沉积为主 ；当入射原子的能量大于溅射阈值时，溅射产额随入射原子能量的增加而线性增大；当入射 原子能量达到200 eV时，各种入射原子的溅射产额都达到或接近1，此时入射原子主要起溅 射作用.溅射原子发射的角分布概率和溅射花样与高能溅射相类似.研究表明：与基于二体碰 撞近似的线性级联溅射理论不同，当入射原子能量大于溅射阈值时，低能入射原子的溅射产 额正比于入射原子的约化能量和入射原子与基体原子的质量比.通过对低能入射原子的钉扎 能力分析，提出了支配低能溅射的入射原子反射物理机理. 关键词： 分子动力学模拟、低能溅射     

低密度Ar熔化及结晶的分子动力学模拟

采用有位移力的ＬｅｎｎａｒｄＪｏｎｅｓ（１２－６）势对微正则系综下低密度Ａｒ系统（简约数密度为ρ＝０．８５）的一级相变过程进行了细致的分子动力学模拟,发现Ａｒ系统的熔化过程是原子的崩塌过程,结晶过程和理想的完整晶体不同,是一活化过程：形核长大过程随温度的降低进行,原来均匀分布在系统中的自由体积呈集中分布,由此系统达到了更稳定的结晶状态。     

石墨烯纳米孔道内受限水介电常数影响因素的分子动力学模拟

受限水的介电特性对于石墨烯电容器的储能效率有着重大的影响.因此本文使用分子动力模方法,研究了不同石墨烯纳米孔道宽度(0.812～10 nm)下内受限水介电常数的分布及其影响因素.结果表明,在石墨烯纳米孔道内部受限水双电层结构可分为空隙层、界面层和体相层三个部分.其整体的介电常数随孔道大小的降低而线性减少.这是由内部体相层的宽度的降低而导致的.此外电极电压大小和石墨烯-水相互作用参数ε_CO的强弱也会显著改变电极表面的双电层(Electric Double Layers, EDLs)结构.其中电压的增大使得介电常数分布的震荡的程度也随之增加,最终导致了整体介电常数的减小.与之类似,亲水态的石墨烯表面(高ε_CO)下受限水分布的震荡程度也显著增加,这导致了整体介电常数的降低.     

纳米沟槽表面黏着接触过程的分子动力学模拟研究

采用大规模分子动力学方法研究了刚性球型探头与具有不同纳米沟槽基体表面的黏着接触过程,探讨了表面沟槽结构对载荷-位移曲线、接触引力和拉离力以及材料转移的影响规律.研究结果表明:在相同的压入深度下,与原子级光滑表面的黏着接触过程相比,刚性探头与具有纳米沟槽结构基体表面的接触压力较小,接触加载过程中的引力作用范围较大,并伴随载荷的多次跳跃,且接触引力和拉离力均有减小;当沟槽深度相同时,随着沟槽宽度的增大,接触引力和拉离力逐渐减小,当沟槽宽度逐渐接近探头与光滑表面的接触直径时,接触引力和拉离力又逐渐增大,趋于接近探头与光滑表面的接触过程;当沟槽宽度相同时,随着沟槽深度的增大,接触引力相对减小,拉离力变化不大.     

A molecular dynamics simulation study on the cavitation inception of water with dissolved gases

The promotion/prevention mechanism of dissolved gases on cavitation inception is essential for many high-tech industries and research. In the present study, large-scale molecular dynamics simulations are performed to investigate the effects of water cavitation caused by different gas types by using nitrogen and oxygen gases with TIP4P/2005 water. The cavitation inception behaviour is analyzed via Mean First Passage Time method. Water with dissolved gases has a higher nucleation rate and is easier to cavitate than pure water. At the same gas concentration, the cavitation of water with nitrogen is promoted to a greater extent than that with oxygen. The number and energy of hydrogen bond (HB) are further calculated by the Acceptor-Hydrogen-Donor method to explain this promotion mechanism. The number and energy of HB in water with gases decrease compared with those in pure water. The introduction of gases weakens the HB network and promotes cavitation inception because of weaker interactions between gas and water molecules. A model is developed to describe the relationship between nucleation rate and HB energy. Gas molecules assemble on the surface of bubbles during water cavitation, which may decrease the free energy of bubble surface, maintain the existing bubble, and contribute to the growth process.     

Investigation of the structural and dynamic basis of kinesin dissociation from microtubule by atomistic molecular dynamics simulations

Kinesin is a molecular motor that can step processively on microtubules via the hydrolysis of ATP molecules. An important factor characterizing the processivity of the kinesin motor is its dissociation from the microtubule. Here, using all-atom molecular dynamics simulations, we studied the dissociation process of the kinesin head in weak-microtubule-binding or ADP state from tubulin on the basis of the available high-resolution structural data for the head and tubulin. By analyzing the simulated snapshots of the structure of the head-tubulin complex we provided detailed structural and dynamic information for the dissociation process. We found that the dissociation of the head along different directions relative to the tubulin exhibits very different dynamic behaviors. Moreover, the potential forms or energy landscapes of the interaction between the head and tubulin along different directions were determined. The studies have important implications for the detailed molecular mechanism of the dissociation of the kinesin motor and thus are critical to the mechanism of its processivity.     

A molecular dynamics study of the structural change differences between Au225 and Au369 clusters on MgO surfaces at low temperature

The differences in structural change between Au₂₂₅ and Au₃₆₉ clusters with their (111) facets supported on MgO(100) surfaces at 5 K are studied by using molecular-dynamics simulations with the atomic interchange potentials of the Au/MgO interface. The parameters are obtained from the ab initio energies using the Chen-Möbius inversion method. Analyses of the pair distribution functions show that the two Au clusters use different deformation processes to adjust the distances between the interface atoms, owing to the misfit between the atom distances among the clusters and the substrates. The local structural changes are identified by atomic density profiles.     

Au-Pd共晶纳米粒子熔化行为的分子动力学研究

采用分子动力学方法结合嵌入原子势, 对Au-Pd共晶纳米粒子的热稳定性进行了模拟研究. 计算结果表明: Au-Pd纳米粒子的熔点明显高于Au单质纳米粒子而低于Pd纳米粒子. 通过计算Lindemann指数发现Au-Pd共晶纳米粒子中的Au原子首先熔化, 然后带动Pd原子的熔化; 熔化所经历的温度区间明显要宽于单质纳米粒子.     

氧化硅团簇切削单晶硅粗糙峰的分子动力学模拟研究

应用分子动力学模拟方法研究了氧化硅团簇在不同的切削 深度下切削单晶硅粗糙峰的过程, 考察了切削过程中粗糙峰和氧化硅团簇形态变化、团簇的受力状况、粗糙峰原子配位数和温度分布等. 模拟结果表明: 切削深度小于0.5 nm时, 被去除的材料以原子或者原子簇形式存在, 并黏附在颗粒表面被带走; 当切削深度增大至1 nm时, 材料的去除率增大, 并形成大的切屑. 在切削过程中, 由于压力和温度的升高, 粗糙峰切削区域的单晶硅转变为类似Si-Ⅱ相和Bct5-Si相的过渡结构, 在切削过程后的卸载阶段, 过渡结构由于压力和温度的下降转变为非晶态结构.     

温度及深度对钛中氦泡释放过程影响的分子动力学研究

利用分子动力学模拟方法对温度及He泡深度给金属Ti内He泡的体积、压强和释放过程等带来的影响进行了研究. 首先, 通过研究室温下He泡在金属Ti内不同深度处的状态, 得到He泡的形状、压强、体积等物理量随其深度的变化规律. 发现He泡压强随其深度增加逐渐变大, 体积则逐渐减小, 但当He泡深度增大到2.6 nm时, 二者均维持在某个固定值附近. 然后对包含有He泡的Ti体系在温度作用下的演化过程进行了模拟, 发现不同深度处He泡从金属Ti内释放出来所需要的临界温度有很大差别, 总体来看He泡越深, 释放所需的临界温度越高. 但不同温度下He原子的释放速率没有明显差别, 释放过程几乎均为瞬间完成. 最后通过对He泡内部压强和其上方金属Ti薄层的抗张强度进行统计对比, 阐述了金属Ti 体内He泡的释放机制: 当He泡内部压强大于其上方Ti薄层抗张强度时, He泡就会将Ti 薄层撕裂, 从而使He原子得到释放.     

A molecular dynamics study on iridium

In this study, molecular dynamics simulations are performed by using a modified form of Morse potential function in the framework of the Embedded Atom Method (EAM). Temperature-and pressure-dependent behaviours of bulk modulus, second-order elastic constants (SOEC), and the linear-thermal expansion coefficient is calculated and compared with the available experimental data. The melting temperature is estimated from 3 different plots. The obtained results are in agreement with the available experimental findings for iridium.