Microstructure And Dynamic Properties Of Fluid (Methanol And Water Etc) By Molecular Dynamic Simulation

Molecular simulation can provide the microscopic details structural and dynamic properties of a system. Compared with the real experiments, computer simulation techniques are more direct, which are often termed'computer experiments'. The main contents of this dissertation are as follows.Molecular dynamics simulations have been carried out for mixtures of Lennard-Jones fluids. Self-diffusion coefficients and mutual diffusion coefficients have been obtained for Ar and Ar/Kr mixture. The temperature dependences of the self-diffusion coefficients are investigated. The diffusion coefficients calculated from the Green-Kubo integral over velocity correlation function and from the Einstein relation for the mean square displacement are equivalent. The value of thermodynamic factor obtained from the radial distribution functions is 1.03, and the mixture of Ar/Kr system is near ideal solute. The relationship between diffusion coefficient of Ar or Ar/Kr system and temperature is in good accordance with Arrhenius behavior, the activity energy fitted from self-diffusivity data of Ar, self-diffusivity data of Kr and mutual-diffusion for Ar-Kr system is 1278J/mol, 1340J/mol and 1296J/mol respectively.A series of constant-temperature molecular dynamics simulations of water from ambient to supercritical conditions, were carried out to investigate the structure, geometry and life-time of hydrogen-bond. The Simple Point Charge model is used for water. It has been found that, with increasing temperature, the distance of bonding O…H increases while the bonding O…O separation almost remains unchanged. The average number of hydrogen bonds in water decreases with increasing temperature and decreasing density. The pair correlation functions and the orientation distribution of hydrogen-bond angle are almost unaffected by decreasing the density at the same temperature. The thermal motion of the bonding H atoms has been enhanced by increasing temperature, and form in average bigger O…O-H orientational angle. The hydrogen-bond lifetimes decrease remarkably with increasing temperature.Molecular dynamics simulations of liquid water were performed at 258K and a density of 1.0g/cm3 under various applied external electric field, ranging from 0~1010V/m. The influence of external field on structural and dynamics properties of water was investigated. The simple point charge (SPC) model and the flexible simple point charge model are used for water molecules. An enhancement of the water hydrogen bond structure with increasing strength of the electric field has been deduced from the radial distribution functions and the analysis of hydrogen bonds structure. With increasing field strength, water system has a more perfect structure, which is similar to ice structure. However, the electrofreezing phenomenon of liquid water has not been detected since the self-diffusion coefficient was very large. The self-diffusion coefficient decreases remarkably with increasing strength of electric field, and the self-diffusion coefficient is anisotropic.A series of constant-temperature molecular dynamics simulations of aqueous NaCl solutions at different salt concentrations, ranging from 0.87 mol/L to 4.1 mol/L, were carried out to investigate the structure and the dynamical properties. It is found that significant structure changes occur in the oxygen-oxygen radial distribution function of water molecules and no well-defined second hydration shell is found around a central water molecule with increasing ion concentration. The orientation of water molecules in the first hydration shell can be seen in visualizations of the equilibrium configurations of the solution. The hydration numbers of positive ions are found to decrease and that of negative ions increase gradually at higher ion concentrations. The residence times of water molecules near ions show an increasing trend as the ion concentration is increased. An increase of ion concentration leads to an enhancement of the oscillations in the microscopic dynamics of the molecules and the self-diffusion coefficients of both ions and solvent molecules decrease with ion concentration.Molecular dynamics simulations of liquid methanol were performed at 298K and a density of 0.78g/cm3 by using the three-site OPLS potential model. It is found that the vaporization heat and the self-diffusion coefficient of the methanol are agreed well with the experimental data. The microscopic structure of methanol molecules can be seen in visualizations of the equilibrium configurations. The hydrogen bonding statistics show that the fractions of molecules having two hydrogen bonds and the average number of hydrogen bonds per methanol molecule are 77.39% and 1.902 respectively. The peak position in the distribution curve of the hydrogen-bond angle is 12.2o, the fractions of the hydrogen-bond angle less than 12.2o is 57.39%. The hydrogen bonded methanol molecules are nearly linear distribution. The mean lifetime of the hydrogen bonds is 12.6ps.Molecular dynamics simulations of liquid methanol were performed at 298K and a density of 0.78 g·cm-3 under various applied external electric field, ranging from 0~1010 V·m-1. The methanol is described by using the three-site OPLS potential model. The influence of the external field on structural and dynamics properties of methanol was investigated. An obvious structural change can be detected by the snapshots of methanol microscopic configuration without and with an electric field of 1.0×1010 V·m-1. An enhancement of the methanol hydrogen bond structure with increasing strength of the electric field has been deduced from the radial distribution functions and the analysis of hydrogen bonds structure. The self-diffusion coefficient decreases with increasing strength of electric field, and the self-diffusion coefficient is anisotropic.Molecular dynamics simulations of single ions (Na~+ or Cl~-) in methanol were performed at 298 K and a density of 0.786 6 g/cm3. The solvation structure and dynamical properties were analyzed in detail by comparing with aqueous systems. The snapshots of relative orientations of methanol in the first solvation shell were obtained. It is shown that the methanol molecules orient with their oxygen atoms pointing toward the cation in the Na~+ shell, whereas orient with their hydrogen atoms pointing toward the anion in the Cl~- shell. The solvation structure was studied by the orientation distribution and the residence time of methanol molecules in the first solvation shell. In comparison with aqueous systems, the angle between the ion-oxygen vector and the plane of methanol molecule in the first shell decreases remarkably, and with a higher tendency to planarity. The residence times of methanol molecules are much bigger than that of water molecules due to the difference between the hydrogen-bonded structures of solvents.Molecular dynamics simulations of methanol and water mixtures were performed to investigate the thermodynamic, the micro-structural and the dynamic properties. The excess heat of mixing increases with increasing methanol mole fraction firstly, and then decreases with increasing methanol mole fraction, the minimum is at xM=0.34 (the methanol mole fraction), which is agreed well with the experimental data. The coordination number of water molecules in the first coordination shell of the methanol molecule decreases with the increasing methanol mole fraction, the coordination number decreases linearly with increasing xM.