Computational Studies of Chemical Systems: I. A Molecular Dynamics Simulation of Methane Hydrate; II. Theoretical Investigation of Water Loading on a Pyrophyllite (001) Surface.

摘要

This dissertation consists of two independent parts: Part I. methane hydrate, and Part II. water loading on a clay surface. In Part I (chapter 2-3), we conducted molecular dynamics simulations with non-polarizable force fields to study structural and thermal properties of methane hydrate. We show that the TIP4P/Ice and TIP4P/2005 model potentials do well in the description of the lattice constant and radial distribution functions. Yet they, together with SPC/E and TIP4P models, overestimate the thermal expansion coefficient due to the inadequate description of the non-linear response of lattice constant to temperature. We also show that TIP4P/Ice and TIP4P/2005 overestimate the decomposition temperature of methane hydrate from the experimental value by 50 K and 30 K respectively, while SPC/E gives a good estimation deviating by about 5 K. All these force fields are found to overestimate the thermal conductivity of methane hydrate, but they are able to describe the weak temperature dependence from 100 to 150 K and 225 to 270 K. It is also found that all initial structures used in the work have a proton ordering tendency, suggesting a potential role of proton arrangement in the temperature dependence of the thermal conductivity. In part II (chapter 4), we conducted dispersion-corrected density function theory (DFT-D) and classical force field calculations to study the water loading on a pyrophyllite (001) surface. We disclose low-energy binding motifs from one water molecule to six water molecules and reinterpret the hydrophobic nature of the pyrophyllite surface from the point of view that a water molecule prefers to interact with other water molecules than to be bound on the surface. The force field approach, while providing a similar trend of the water binding to the DFT-D result, predicts some low-energy binding motifs which are not confirmed by the DFT-D calculation. It suggests a refinement of the force field to better describe the interfacial orientation of water on a clay surface.