Modeling of Interfacial Growth and Structural Processes and Dynamics of sII Gas Hydrate Systems using Molecular Dynamics and Geometric Techniques

Gas hydrates are inclusion compounds comprising a backbone of water molecules that enclose guest molecules in separate cages. Each volume of hydrate contains 160 volume equivalents of gas. Initially, large scale gas hydrate research was centered around the flow assurance problems they cause in the extraction and transportation of petroleum and its derivatives. Naturally occurring gas hydrates are also studied to satisfy global energy demand: estimates put the total energetic capacity of hydrate reserves at twice that of other fossil fuel reserves combined. Their potential use in the removal of carbon from the atmosphere, carbon capture and storage, and for energy exploitation makes gas hydrates a prime candidate for climate change mitigation research. Characterizing the interfacial properties of a material that requires intense formation conditions and sublimates rapidly in standard atmosphere is experimentally challenging. Controlling sample purity and system homogeneity is often difficult in devices designed to measure interfacial properties. It is also costly to properly clean apparatus after using additives to study new ones. Finally, rapidly testing different guest molecules, external pressures, surface effects, and extracting meaningful, fundamental information about the atomic and molecular interfacial behaviors is nearly impossible without critical sample disruption. This work uses molecular dynamics as implemented in the Large-scale Atomic/Molecular Massively Parallel Simulator to characterize the temperature and pressure effects on the interfacial tension, energy of interfaces, and growth rate of natural gas hydrates to overcome the complexity of experimentally understanding their performance in extreme environments and prove that theoretical modeling, prediction, and advanced characterization techniques can explain the structural and transport properties that govern hydrate in engineering applications. This project studies the impact of the surface coverings and additives on surface energy, attachment strength, and creation of new nucleation sites in sII gas, and examines the thermal transport across the hydrate/water, hydrate/gas, and hydrate/ice interfaces by calculating the thermal conductivity across the interface. This project also uses geometric analysis techniques to understand the dynamic behavior of the crystalline interfaces, including what specific polyhedral faces tend to face the melting phase and what type of pre-melting dynamics and interfacial thicknesses exist in these systems. It is essential to understand the underlying mechanisms occurring at the molecular level and the nanoscopic behaviors leading to macroscopic properties, thereby clarifying behaviors and phenomena that dominate potential applications of these structures. Our work has shown that there is excellent agreement between sI methane hydrates and experimental values, as well as for sII natural gas hydrates. The hydrates nucleate preferentially with film-shaped nucleation, then cap-shaped, lens-shaped, and homogeneous nucleation. We have confirmed the presence of a novel pre-melting layer at the interface between the structures. We have been able to produce temperature and pressure correlations of surface tension for engineering applications. The excess entropy, adsorption, radial pair distribution function, and charge distribution at the interface were calculated to confirm our findings. The molecular dipole at the interface indicates novel organization of molecules that affects further nucleation and governs the use of surface coverings and additives to control behavior. The interfacial thickness has been characterized to show interface expansion and contraction at certain conditions, as well as showing the fine atomic phenomena that are at play in controlling macroscale properties.