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Molecular simulation study of the microstructures and properties of pyridinium ionic liquid[HPy][BF_(4)]mixed with acetonitrile

The microstructures and thermodynamic properties of mixed systems comprising pyridinium ionic liquid[HPy][BF_(4)]and acetonitrile at different mole fractions were studied using molecular dynamics simulation in this work.The following properties were determined:density,self-diffusion coefficient,excess molar volume,and radial distribution function.The results show that with an increase in the mole fraction of[HPy][BF_(4)],the self-diffusion coefficient decreases.Additionally,the excess molar volume initially decreases,reaches a minimum,and then increases.The rules of radial distribution functions(RDFs)of characteristic atoms are different.With increasing the mole fraction of[HPy][BF_(4)],the first peak of the RDFs of HA1-F decreases,while that of CT6-CT6 rises at first and then decreases.This indicates that the solvent molecules affect the polar and non-polar regions of[HPy][BF_(4)]differently.

Molecular Dynamics Study on Configuration Energy and Radial Distribution Functions of Ammonium Dihydrogen Phosphates Solution

Molecular dynamics simulations were carried out to study the configuration energy and radial distribution functions of mmonium dihydrogen phosphate solution at different temperatures. The dihydrogen phosphate ion was treated as a seven-site model and the ammonium ion was regarded as a five-site model, while a simple-point-charge model for water molecule. An unusually local particle number density fluctuation was observed in the system at saturation temperature. It can be found that the potential energy increases slowly with the temperature from 373 K to 404 K, which indicates that the ammonium dihydrogen phosphate has partly decomposed. The radial distribution function between the hydrogen atom of ammonium cation and the oxygen atom of dihydrogen phosphate ion at three different temperatures shows obvious difference, which indicates that the average H-bond number changes obviously with the temperature. The temperature has an influence on the combination between hydrogen atoms and phosphorus atoms of dihydrogen phosphate ion and there are much more growth units at saturated solutions.

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）, cyclotetramethylenetetranitroamine （HMX）, cyclotrimethylenetrinitramine （RDX）, and N-nitrodihydroxyethylaminedinitrate （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. Using 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 intermoleenlar interaction is. In addition, the force field energy at different temperature was also analyzed to predict Tm of the four systems.

Insight into Capture of Greenhouse Gas （CO2） based on Guanidinium Ionic Liquids

Quantum mechanics and molecular dynamics are used to simulate guanidinium ionic liquids. Results show that the stronger interaction exists between guanidine cation and chlorine anion with interaction energy about 109.216 kcal/mol. There are two types of spatial distribution for the title system： middle and top. Middle mode is a more stable conformation according to energy and geometric distribution. It is also verified by radial distribution function. The continuous increase of carbon dioxide （CO2） does not affect the structure of ionic liquids, but CO2 molecules are always captured by the cavity of ionic liquids.

Molecular Dynamics Simulation for the Binary Mixtures of High Pressure Carbon Dioxide and Ionic Liquids

Molecular dynamics simulation with an all-atom force field has been carded out on the two binary sys- tems of [bmim][PF6]-CO2 and [bmim][NO3]-CO2 to study the transport properties, volume expansion and micro- structures. It was found that addition of CO2 in the liquid phase can greatly decrease the viscosity of ionic liquids （ILs） and increase their diffusion coefficient obviously. Furthermore, the volume expansion of ionic liquids was found to increase with the increase of the mole fraction of CO2 in the liquid phase but less than 35% for the two simulated systems, which had a significant difference with CO2 expanded organic solvents. The main reason was that there were some void spaces inter and intra the molecules of ionic liquids. Finally, site to site radial distribution functions and corresponding number integrals were investigated and it was found that the change of microstructures of ILs bv addition CO2 had a great influence on the orooerties of ILs.

A Mathematical Model for Designing Optimal Shape for the Cone Used in Z-flow Type Radial Flow Adsorbers

Nonuniform flow distribution along the radial direction usually exists in a Z-flow type radial flow adsorber,which will decrease the utilization of adsorbent and the switching time and may result in operating safety problems in cryogenic air separation.In order to improve the uniformity of the flow distribution along the radial direction in the adsorber,a differential equation is derived through pressure drop analysis in the Z-flow type radial adsorber with a cone in the middle of the central pipe.The differential equation determines the ideal cross-sectional radii of the cone along the axis.The result shows that the cross-sectional radius of the cone should gradually decrease from 0.3 m to zero along the axis to ensure that the process air is distributed uniformly in the Z-flow type radial flow adsorber and the shape of the cone is a little convex.The flow distribution without the cone in the central pipe is compared under different bed porosities.It is demonstrated that the proposed differential equation can provide theoretical support for designing Z-flow type radial flow adsorbers.

MONTE CARLO CALCULATION OF ENERGY DEPOSITION BY DELTA RAYS AROUND ION TRACKS

The radial distribution of dose around the path of a hemp ion has been studied by a Monte Carlo transport analysis of the delta rays produced along the track of a heavy ion based on classical binary collision dynamics and a single scattering model for the electron transport process. Result comparisons among this work and semi-empirical expression based delta ray theory of track structure, as well as other Monte Carlo calculations are made for 1, 3 MeV protons and several heavy ions. The results of the Monte Carlo simulations for energetic heavy ions are in agreement with experimental data and with results of different methods. The characteristic of this Monte Carlo calculation is a simulation of the delta rays theory of track structure.

Vapor-Liquid Equilibrium Simulation of Binary and Ternary Mixtures of CH_4,C_2H_4 and iso-C_4H_(10)

The vapor-liquid equilibrium(VLE) properties for the binary and ternary mixtures of CH4,C2H4 and isoC4H10 are of great importance in the recovery of ethylene from mixture containing CH4 and C2H4 with iso-C4H10 as solvent.Hence,Gibbs ensemble Monte Carlo(GEMC) simulations were used to estimate vapor-liquid equilibrium for the binary and ternary mixtures of CH4,C2H4 and iso-C4H10 with the united atom potential NERD model.The selected simulation conditions are based on the experiment in the literature.The results of this work were shown to be in satisfactory agreement with available experimental data and predictions of Peng-Robinson equation of state.The structure of simulated liquid phase is also characterized by radial distribution function(RDF),which contributes to further understanding of the VLE curve of these systems.RDF is not sensitive to the pressure and temperature range.With the increase of pressure or the decrease of temperature,the molecules tend to gather together.

Study on Non-Newtonian Behaviors of Lennard-Jones Fluids via Molecular Dynamics Simulations

Using nonequilibrium molecular dynamics simulations, we study the non-Newtonian rhe-ological behaviors of a monoatomic fluid governed by the Lennard-Jones potential. Both steady Couette and oscillatory shear flows are investigated. Shear thinning and normal stress effects are observed in the steady Couette flow simulations. The radial distribution function is calculated at different shear rates to exhibit the change of the microscopic struc- ture of molecules due to shear. We observe that for a larger shear rate the repulsion between molecules is more powerful while the attraction is weaker, and the above phenomena can also be confirmed by the analyses of the potential energy. By applying an oscillatory shear to the system, several findings are worth mentioning here： First, the phase difference between the shear stress and shear rate increases with the frequency. Second, the real part of complex viscosity first increases and then decreases while the imaginary part tends to increase mono- tonically, which results in the increase of the proportion of the imaginary part to the real part with the increasing frequency. Third, the ratio of the elastic modulus to the viscous modulus also increases with the frequency. These phenomena all indicate the appearance of viscoelasticity and the domination of elasticity over viscosity at high oscillation frequency for Lennard-Jones fluids.

Modeling for mean ion activity coefficient of strong electrolyte system with new boundary conditions and ion-size parameters

A rigorous approach is proposed to model the mean ion activity coefficient for strong electrolyte systems using the Poisson-Boltzmann equation. An effective screening radius similar to the Debye decay length is introduced to define the local composition and new boundary conditions for the central ion. The crystallographic ion size is also considered in the activity coefficient expressions derived and non-electrostatic contributions are neglected. The model is presented for aqueous strong electrolytes and compared with the classical Debye-Hfickel （DH） limiting law for dilute solutions. The radial distribution function is compared with the DH and Monte Carlo studies. The mean ion activity coefficients are calculated for 1：1 aqueous solutions containing strong electrolytes composed of alkali halides. The individual ion activity coefficients and mean ion activity coefficients in mixed sol- vents are predicted with the new equations.

Influence of filling atoms on radial collapse and elasticity of carbon nanotubes under hydrostatic pressure

Using molecular mechanics and molecular dynamics simulations, we focus on the influence of filling atoms on radial collapse and elasticity of single-walled carbon nanotubes(SWNTs). It is shown that the filled argon(Ar) and silicon(Si) atoms can effectively improve the resistance to high pressure and radial elasticity of SWNT, which may attribute to the strong repulsive force from the filled Ar(Si) atoms. However, due to the strong interaction of Cu atoms, filling Cu atoms deteriorate SWNT's radial elasticity. In addition, it is found that the phase transitions of the atoms filled in SWNT occur in the process of loading and unloading pressure, so that the electrical properties of the SWNTs filled with atoms change in the process of loading and unloading pressure. In view of the restorability of SWNT filled with Si atoms upon unloading, the filled SWNTs can be used to develop a new class of nano-electronic devices such as pressure sensor, relay and memory, etc.

All-atom and united-atom simulations of guanidinium-based ionic liquids

Ionic liquids (ILs) have been widely used in separation, catalysis, electrochemistry, etc., and one of the most outstanding characteristics is that ILs can be tailored and tuned for specific tasks. In order to design and make better use of ionic liquids, the structures and properties relationship is indispensable. Both molecular dynamics and Monte Carlo simulations have been proved useful to understand the behavior of molecules at the microscale and the properties of the system. However, the quality of such simulations depends on force field parameters describing the interactions between atoms. All-atom (AA) or the united-atom (UA) force fields will be chosen because of the demand for more exact results or the lower computational cost, respectively. In order to make a systematic comparison of the two force fields, molecular simulations for four kinds of acyclic guanidinium-based ionic liquids (cations: (R2N)2C=N+<, anion: nitric or perchloric acid) were performed based on the AA and the UA force fields in this work. AA force field parameters were derived from our previous work (Fluid Phase Equilib., 2008, 272: 1-7), and the UA parameters were proposed in this work. Molecular dynamics simulation results for the AA and UA force fields were compared. Simulation densities are very similar to each other. Center of mass radial distribution functions (RDFs), site to site RDFs and spatial distribution functions (SDFs) were also investigated to depict the microscopic structures of the ILs.