Computer Simulation Of Water System's Structural And Dynamical Properties

In this thesis, computer application technology and physics theory are used together to carry out the computer simulation study on water system's structural and dynamical characteristics. This study achieves the practical application of computer methods in physics research and obtains some satisfactory results.This thesis can be divided into five main parts. The first two chapters are the introduction for the basic knowledge and principles, and the last three chapters foucus on our research project and describes the main work we carried out.The first chapter describes the water system's nature and microstructure, and then describes the structure and physical characteristics of ice——the solid phase of water. On this basis, this section describes in detail two singular structure of the water system,which are formed under special conditions——the confined water and gas hydrate.The second chapter mainly describes the application development of the computer simulation methods in physics and a number of computer simulation theories, among which, the lattice dynamics simulations and molecular dynamics simulation methods are related in detail.The third chapter is based on the previous two chapters. According to the physical characteristics of water systems and computer simulation methods, we optimize the simulation model for type I and II gas hydrate, as well as confined water in carbon nanotubes.And then we code the physics parameters, describing them from physics language to computer language. Through the running of the computer programs, the actual physical problems are abstracted into some binary data. According to the ice rules, we put forward effective algorithm to place the hydrogen atoms disorderedly on hydrate lattice. Through comparison of advantages and disadvantages of the output using several different models, we choose the TIP4P water molecule model to study the water system. On this basis, we define the modeling operations on gas hydrate system and confined water system in carbon nanotubes. To study some features of the physical system, we usually need to consider some physical processes and physical parameters. The second half of this section is, according to the definition of some physical parameters and physical processes, and to express in computer languaget. These statements are not only to comply with the true meaning of the laws of physics, but also fulfill requirements of the computational simulation.This chapter provide computer technology support for the chapters left.The main content in chapter four is using the lattice dynamics method to discuss structure and dynamic properties of the gas hydrate. The simulation results and the the experimental results are compared. According to the content In the last chapter, this chapter will pursue comparative study for type Iand type II gas hydrate,.first of all, relatively simple inert gas hydrates are researched. We discussed differences between the type I and II hydrate when the same kind of gas molecules are filled. We found that the dodecahedron structure of gas hydrates plays an important role in the whole system.it has the strongest coupling with the guest molecules. Filling single atom gas molecules of different size into the same lattice structure of water and found that when effective diameter ratio between the guest molecules and the advantageous cages is above 0.745, he crystal structure can exist stably. When the methane gas was filled in type II hydrate, the above requirement is fulfilled. So we preliminarily estimates of the existence of type II methane hydrate. And further research on diatomic molecules of nitrogen gas hydrate is carried out. Simulation result revealed the excitation derived from internal vibration of diatomic molecules. This property does not exist in single-atom gas hydrate. This provide convenience for further study on methane molecules in gas hydrate. Calculating the lattice energy when nitrogen molecules take on different orientations in the cage structure. We found that there is no minimum potential energy orientation for nitrogen molecules, and the maximum fill rate of nitrogen in water lattice is obtained, proving that double-filled structure of nitrogen stability exists. At last, we discussed the more complex structure of methane hydrate, revealed the relationship between the lattice energy and gas filling rate. Methane hydrate of type II are thought to exist in nature and in the large cage double gas may be filled.The fifth chapter discusses the molecular dynamics calculation of water molecules confined in single-walled carbon nanotubes. Based on the modeling system we build In the third chapter for the carbon nanotube and its surrounding water molecules, we discuss the confined water's dynamiccal and static properties from the aspect of nanotube's diameter and chirality. For the static properties of confined water, we mainly study its phase transition structure under ordinary pressure and high pressure in the carbon nanotubes with different diameters and chirality. We found the water molecules'forbidden area inside carbon nanotubes and this area is reduced from 2.8 ? to 2.4? with increasing pressure. Under the same pressure, with the increasing of the diameter of carbon nanotubes, the formation of ordered ice nanotubes show a more complex structure, and the phase transition temperature decreases. This is exactly the opposite with the theoretical prediction results.For the same nanotube, the confined water molecules show a more dense distribution structure under the higher pressure. For the (9,9) and (10,10) tubes, ice nanotube and a water chain are found in them. Through the analysis of the distribution of dipole moment of water molecules, we found the interactions between water molecules and tube wall have a greater impact on the static structure of confined water than that of the hydrogen bonds between water molecules. For different chiral carbon nanotubes, as long as the diameter is similar size, its internal water molecules phase transition behavior is the same. In addition, we calculated the average number of hydrogen bonds of water molecules in a variety of different chirality and diameter carbon nanotubes and found that the average number of hydrogen bonds is less than the state of bulk water, and the smaller the diameter is, the more defects the hydrogen bonds have. It is only the diameter of carbon nanotubes affect the phase transition properties of confined water, chirality of carbon nanotubes do not affect it. Transfer rate of confined water were calculated in different diameter armchair and zigzag carbon nanotube, we found the hydrogen bonds between water molecules have a greater impact on the dynamic transport properties of confined water than that of the interactions between water molecules and tube wall. As the diameter increases, the same chiral carbon tube's transfer flux of water molecules increases, but the transfer rate decreases. Within the same diameter carbon nanotubes, water molecules in the armchair-tube intend to have larger transfer rate than the zigzag type That is to say the dynamic properties of confined water in carbon nanotubes are not only determined by the diameter of carbon nanotubes, but also by the tube chirality.