Molecular Dynamics Study of Structural and Transport Properties of Silver Iodide Using Effective Charges

The superionic conductor, solid state, and body-centered cubic structure, silver iodide at room temperature, has been studied via molecular dynamics simulations. The calculated results using pairwise Coulomb-Buckingham potential, zero pressure on the sample, a semi-rigid model system of 1000 Ag and 1000 I ions, (NVE) as a statistical ensemble, and an effective charge of Z=0.63 for the pairs Ag-Ag and I-I, were found to be consistent with experimental data and one study using Z=0.60, different potential, and simulation software. For the pair Ag-I, there is a discrepancy due to the high silver ion diffusion. The calculated value of the diffusion constant of the silver ion is greater than iodide ion. The dynamic transport properties (mean square displacement, velocity autocorrelation function) results indicated typical behavior reported by other authors, using different potentials in their DM simulations for iodine and silver ions.     

银团簇结构与特性的分子动力学研究

用分子动力学方法模拟了银团簇的结构与力能学.计算模拟中使用了一种基于第一性原理的原子间互作用多体势函数.通过分子动力学模拟确定了银微团簇(原子个数3～13)的稳态结构;模拟了原子个数为13～141的银FCC晶体结构理想球形团簇的力能学,发现球形银团簇形成三雏紧密结构;计算了平均结合能,给出了结合能随团簇原子数N的变化图,发现随N增大团簇结合能逐渐接近块材的数值.     

Extra Silver Atom Triggers Room‐Temperature Photoluminescence in Atomically Precise Radarlike Silver Clusters

Luminescent metal clusters show promise for applications in imaging and sensing. However, promoting emission from metal clusters at room temperature is a challenging task owing to the lack of an efficient approach to suppress the nonradiative decay process in metal cores. We report herein that the addition of a silver atom into a metal interstice of the radarlike thiolated silver cluster [Ag₂₇(S^tBu)₁₄(S)₂(CF₃COO)₉(DMAc)₄]?DMAc ( NC1 , DMAc=dimethylacetamide), which is non‐emissive under ambient conditions, produced another silver cluster [Ag₂₈(AdmS)₁₄(S)₂(CF₃COO)₁₀(H₂O)₄] ( NC2 ) that displayed bright green room‐temperature photoluminescence aided by the new ligand 1‐adamantanethiol (AdmSH). The 28th Ag atom, which hardly affects the geometrical and electronic structures of the Ag–S skeleton, triggered the emission of green light as a result of the rigidity of the cluster structure.     

Microscopic Diffusion Dynamics of Silver Complex‐Based Room‐Temperature Ionic Liquids Probed by Quasielastic Neutron Scattering

Quasielastic neutron scattering is used to probe the microscopic diffusion dynamics of the hydrogen‐bearing cations of two different silver complex‐derived room‐temperature ionic liquids, [Ag(propylamine)₂⁺][Tf₂N^?] (Tf=trifluoromethanesulfonyl) and [Ag(1‐pentene)⁺][Tf₂N^?]. In the temperature range from 300 to 340 K, analysis of the scattering momentum transfer dependence of the data provides evidence for three distinct diffusion components. The slowest component describes the long‐range cationic translational diffusion. A possible link between the microscopic diffusion parameters and the structural features of the cations comprising these two ionic liquids is discussed.     

Heterogeneities in Polymer Structural and Dynamic Properties in Graphene and Graphene Oxide Nanocomposites: Molecular Dynamics Simulations

The effect of graphene (G) and graphene oxide (GO), used as the nanofiller in polymer nanocomposites (NC), on the structural and dynamic properties of polymer chains, has been studied by means of molecular dynamics (MD) simulations. Two polymers, i.e., poly(propylene) and poly(vinyl alcohol), are employed as matrices to cover a wider range of polymer–filler interactions. The local structural properties, e.g., density profile, average R _g, and end‐to‐end distance as well as dynamic properties, e.g., estimated translational and orientational relaxation times, of polymer chains are studied. In addition, the interaction energies are estimated between polymers and nanofillers for different hybrid systems using MD pullout simulations. Strong heterogeneities in polymer structural and dynamic properties have been observed such that chains are more oriented and exhibit slower dynamics in the vicinity of the nanofillers (G and GO) as compared to bulk. It is also found that the orientation of polymer chains at the interface is more influenced by the nanofiller in such a way that the more oriented polymer chains are observed in G‐based NC for both polymers. However, the immobilization of polymer chains at the interface proves to be very much dependent on the polymer–filler interactions.

     

Molecular Modeling of the Three‐Dimensional Structure of Human Sphingomyelin Synthase

Sphingomyelin synthase (SMS) produces sphingomyelin and diacylglycerol from ceramide and phosphatidylcholine. It plays an important role in cell survival and apoptosis, inflammation, and lipid homeostasis, and therefore has been noticed in recent years as a novel potential drug target. In this study, we combined homology modeling, molecular docking, molecular dynamics simulation, and normal mode analysis to derive a three‐dimensional structure of human sphingomyelin synthase (hSMS1) in complex with sphingomyelin. Our model provides a reasonable explanation on the catalytic mechanism of hSMS1. It can also explain the high selectivity of hSMS1 towards phosphocholine and sphingomyelin as well as some other known experimental results about hSMS1. Moreover, we also derived a complex model of D609, the only known small‐molecule inhibitor of hSMS1 so far. Our hSMS1 model may serve as a reasonable structural basis for the discovery of more effective small‐molecule inhibitors of hSMS1.     

Controlled Folding,Motional, and Constitutional Dynamic Processes of Polyheterocyclic Molecular Strands

General design principles have been developed for the control of the structural features of polyheterocyclic strands and their effector‐modulated shape changes. Induced defined molecular motions permit designed enforcement of helical as well as linear molecular shapes. The ability of such molecular strands to bind metal cations allows the generation of coiling/uncoiling processes between helically folded and extended linear states. Large molecular motions are produced on coordination of metal ions, which may be made reversible by competition with an ancillary complexing agent and fueled by sequential acid/base neutralization energy. The introduction of hydrazone units into the strands confers upon them constitutional dynamics, whereby interconversion between different strand compositions is achieved through component exchange. These features have relevance for nanomechanical devices. We present a morphological and functional analysis of such systems developed in our laboratories.     

All‐Atom Molecular Dynamics Simulation of Structure and Diffusion of Hydrophilic Antistatic Agents in Polypropylene

Glycerol monostearate (GMS) is an additive widely used in plastic industry for its good ability to improve the wettability and antistatic property of polymer surfaces. Based on GMS, we propose five additives of different polarity by attuning the number of oxyethyl groups. All‐atom molecular dynamics simulations of these additives in polypropylene (PP) matrix are carried out at temperatures of 300 K, 350 K and 400 K. Detailed molecular conformations are obtained and analyzed. Due to the gauche effect of the dihedral angles, the polar parts of these additives form helix structures. The diffusion coefficient of the additives depends on their molecular conformations and decreases monotonously with increasing polarity. These results are expected to be helpful in rational design of hydrophilic antistatic agents in polymeric materials.     

Transport Properties of Fluids in Micropores by Molecular Dynamics Simulation

The transport properties of fluid argon in micropores, i.e. diffusivity and viscosity, were studied by molecular dynamics simulations. The effects of pore width, temperature and density on diffusivity and viscosity were analyzed in micropores with pore widths from 0.8 to 4.0 nm. The results show that the diffusivity in micropores is much lower than the bulk diffusivity, and it decreases as the pore width decreases; but the viscosity in micropores is significantly larger than the bulk one, and it increases sharply in narrow micropores. The diffusivity in channel parallel direction is obviously larger than that in channel perpendicular direction. The temperature and density are important factors that obviously affect diffusivity and viscosity in micropores.     

Structural Changes in Block Copolymer Solutions under Shear Flow as Determined by Non‐Equilibrium Molecular Dynamics

Summary: A non‐equilibrium molecular dynamics computer simulation on microsegregated solutions of symmetrical diblock copolymers is reported. As the polymer concentration increases, the system undergoes phase transitions in the following order: body centered cubic (BCC) micelles, hexagonal (HEX) cylinders, gyroid (GYR) bicontinuous networks and lamellae (L), which are the same morphology reported for block copolymer melts. Structural classification is based on the patterns of the anisotropic static structure factor and characteristic 3‐dimensional images. The systems in the BCC micellar (ρσ³ = 0.3) and HEX cylindrical (ρσ³ = 0.4) phases were then subjected to a steady planar shear flow. In weak shear flow, the segregated domains in both systems tend to rearrange into sliding parallel close‐packed layers with their normal in the direction of the shear gradient. At higher shear rates, both systems adopt a perpendicular lamellar structure with the normal along the neutral direction. A further increase in the shear rate results in a decrease in lamellar spacing without any further structural transitions. Two critical shear rate values that correspond to the demarcation of different structural behaviors were found.

Shear‐induced BCC‐LAM phase transition.     

四组分高能体系结合能和力学性能的分子动力学模拟

用分子动力学(MD)方法模拟研究了下列4种四组分高能混合体系的结合能和力学性能: 聚叠氮缩水甘油醚(GAP)/硝化甘油(NG)/1,2,4-丁三醇硝酸酯(BTTN)/二硝基偶氮氧化二呋咱(DNOAF)、GAP/ NG/ BTTN/三氢化铝(AlH₃)、聚乙二醇(PEG)/NG/BTTN/DNOAF和PEG/NG/BTTN/AlH₃. 结果表明, 在三组分粘合剂中加进DNOAF和AlH₃, 结合能均较大, 依次为45.35, 56.02, 48.75和65.96 kJ/kg, 预示体系稳定性和相容性均较好. 组分间的相互作用主要是非键力, 且含AlH₃体系的静电力更大, 其余体系以van der Waals力较大. 静态力学分析表明, 在4种混合体系中, PEG/NG/BTTN/AlH₃的拉伸模量、体模量(K)、剪切模量(G)、K/G 和柯西压(C₁₂－C₄₄)值均较大, 预示该体系的刚性、塑性和延展性均较好, 这可能与PEG的醚O和AlH₃的缺电子桥键之间存在特殊的配位键作用有关.     

Dynamics of Intact Immunoglobulin G Explored by Drift‐Tube Ion‐Mobility Mass Spectrometry and Molecular Modeling

Collision cross‐sections (CCS) of immunoglobulins G1 and G4 have been determined using linear drift‐tube ion‐mobility mass spectrometry. Intact antibodies and Fc‐hinge fragments present with a larger range of CCS than proteins of comparable size. This is rationalized with MD simulations, which indicate significant in vacuo dynamics between linked folded domains. The IgG4 subclass presents over a wider CCS range than the IgG1 subclass.     

Speciation and Structural Properties of Hydrothermal Solutions of Sodium and Potassium Sulfate Studied by Molecular Dynamics Simulations

Aqueous solutions of salts at elevated pressures and temperatures play a key role in geochemical processes and in applications of supercritical water in waste and biomass treatment, for which salt management is crucial for performance. A major question in predicting salt behavior in such processes is how different salts affect the phase equilibria. Herein, molecular dynamics (MD) simulations are used to investigate molecular‐scale structures of solutions of sodium and/or potassium sulfate, which show contrasting macroscopic behavior. Solutions of Na?SO₄ exhibit a tendency towards forming large ionic clusters with increasing temperature, whereas solutions of K?SO₄ show significantly less clustering under equivalent conditions. In mixed systems (Na_xK_2?xSO₄), cluster formation is dramatically reduced with decreasing Na/(K+Na) ratio; this indicates a structure‐breaking role of K. MD results allow these phenomena to be related to the characteristics of electrostatic interactions between K⁺ and SO₄^2?, compared with the analogous Na⁺?SO₄^2? interactions. The results suggest a mechanism underlying the experimentally observed increasing solubility in ternary mixtures of solutions of Na?K?SO₄. Specifically, the propensity of sodium to associate with sulfate, versus that of potassium to break up the sodium–sulfate clusters, may affect the contrasting behavior of these salts. Thus, mutual salting‐in in ternary hydrothermal solutions of Na?K?SO₄ reflects the opposing, but complementary, natures of Na?SO₄ versus K?SO₄ interactions. The results also provide clues towards the reported liquid immiscibility in this ternary system.     

荧光银簇的制备、性能及应用研究进展

银簇是一类粒径为1-2 nm的银纳米材料,其除具有特殊的光、电、磁及催化等性能,还具有低毒、强荧光、生物相容性等特殊的物理、化学等特性。因此被广泛应用于生物检测、环境监测等领域。以银簇为研究对象,对银簇的制备、银簇的性能和银簇的应用进行综述。总结了荧光银簇的制备、性能和应用方面的最新研究进展。     

Deconvolution of Molecular Weight Distributions Using Dynamic Flory‐Schulz Distributions

Summary: The deconvolution of molecular weight distributions (MWDs) may be useful for obtaining information about the polymerization kinetics and properties of catalytic systems. However, deconvolution techniques are normally based on steady‐state assumptions and very little has been reported about the use of non‐stationary approaches for the deconvolution of MWDs. In spite of this, polymerization reactions are often performed in batch or semi‐batch modes. For this reason, dynamic solutions are proposed here for simple kinetic models and are then used for deconvolution of actual MWD data. Deconvolution results obtained with dynamic models are compared to deconvolution results obtained with the standard stationary Flory‐Schulz distributions. For coordination polymerizations, results show that dynamic MWD models are able to describe experimental data with fewer catalytic sites, which indicates that the proper interpretation of the reaction dynamics may be of fundamental importance for kinetic characterization. On the other hand, reaction dynamics induced by modification of chain transfer agent concentration seem to play a minor role in the shape of the MWD in free‐radical polymerizations.

This Figure illustrates that MWDs obtained at unsteady conditions should not be deconvoluted with standard steady‐state Flory‐Schulz distributions.     

氟化钾团簇的相变、成核和重结晶的分子动力学模拟

我们利用Born-Mayer-Huggins相互作用势函数对(KF)_N(N=108，256，500和864)团簇进行了分子动力学(MD)模拟。为了避免周期性边界条件对相变、成核和重结晶的干扰作用，对体系采用了自由边界。基于MD模拟结果，对团簇的熔化温度、熔化焓、扩散系数、成核速率、固液界面自由能、临界核大小等进行了计算和讨论。在对(KF)₈₆₄双晶团簇的热退火模拟中，观察到了固态的重结晶和晶粒的生长。经典的成核理论成功地解释了(KF)₈₆₄双晶团簇的重结晶MD模拟结果。     

Effects of Moisture on the Mechanical Properties of Microcrystalline Cellulose and the Mobility of the Water Molecules as Studied by the Hybrid Molecular Mechanics–Molecular Dynamics Simulation Method

A hybrid molecular mechanics–molecular dynamics simulation method has been performed to study the effects of moisture content on the mechanical properties of microcrystalline cellulose (MCC) and the mobility of the water molecules. The specific volume and diffusion coefficient of the water increase with increasing moisture content in the range studied of 1.8–25.5 w/w%, while the Young's modulus decreases. The simulation results are in close agreement with the published experimental data. Both the bound scission and free‐volume mechanisms contribute to the plasticization of MCC by water. The Voronoi volume increases with increasing moisture content. It is related to the free volume and the increase enhances the mobility of the water molecules and thus increases the coefficient of diffusion of the water. Moreover, with increasing moisture content, the hydrogen bonding per water molecule between MCC–water molecules decreases, thus increasing the water mobility and number of free water molecules. © 2019 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 454–464     

An Iterative Method for Producing Equilibrated Symmetric Three‐Arm Star Polymer Melts in Molecular Dynamics

Melts of symmetric three‐arm stars are generated using a novel iterative method. In this method, an equilibrated low molecular weight configuration is used to generate progressively higher molecular weights by affine scaling and equilibration. At each stage in the progression, the synthetically lowered entanglement density allows bypassing of the exponentially large relaxation times of branched polymers. The quality of equilibration was assessed by measuring the mean dimensions, distribution of dimensions, and internal length scales of the polymers. The total time required to generate the progression of equilibrated configurations was seen to scale as the Rouse time of the highest molecular weight.

     

Characterisation of N‐(Octadecyl)‐1,8‐naphthalimide Monolayer Compression Using Molecular Dynamics and Experimental Approaches

The development of luminescent surfaces is an active area of supramolecular chemistry, particularly for the development of new sensing platforms. One particularly useful surface deposition method is the Langmuir–Blodgett technique where organic amphiphilic fluorophores (e.g. 1,8‐naphthalimides) can form ordered monolayers at an air–water interface before being deposited onto solid supports. The ability to simulate monolayer formation and consequently develop predictability over film formation would allow for significant advances in the luminescent materials field where synthesis might be directed by simulation data. Here, we compare pressure‐area isotherms of N‐(octadecyl)‐1,8‐naphthalimide determined experimentally, using the Langmuir–Blodgett technique, and computationally, using three different simulation techniques. We find that all three simulation techniques are able to describe the liquid‐condensed/liquid‐expanded region of the isotherm, and that the isotherms are highly similar in this region, although the NγT ensemble performs best. Experimental isotherms showed film formation properties that align with the simulation data, suggesting that simulations are a viable means to direct synthesis. Investigation of the underlying structural details disclosed by the simulations reveals the compression‐induced ordering at atomic‐level detail, which will allow prediction of how functionalisation of the naphthalimides will alter the monolayer compression and mounting process.