Molecular Modeling of Interfacial Structure, Kinetics and Processes of sII Gas Hydrate Systems for Engineering Applications

Gas hydrates are crystalline compounds composed of a hydrogen bonded backbone of water molecules enclosing guest molecules in cages stabilized by Van der Waals forces. Initially, large scale gas hydrate research was centered around flow assurance problems in hydrocarbon extraction, transportation, and processing. Interest then grew in the usage of hydrates to store hydrocarbons and hydrogen, for desalination processes, and for carbon capture and storage. Their potential use in carbon capture and storage and for energy exploitation makes gas hydrates an excellent candidate for further research. Characterizing the material properties of a crystal that requires extreme formation conditions and sublimates rapidly in standard atmosphere is experimentally challenging. While the equilibrium formation conditions lie in the megapascal range for gas hydrates, experimental conditions of formation and engineering environment involve extreme pressures in the gigapascal range. Controlling sample purity and system homogeneity is often difficult in devices designed to measure interfacial properties because they are not designed for extreme conditions and stresses. Finally, testing different guest molecules, external pressures, surface effects, and extracting fundamental information about molecular interfacial behaviors is nearly impossible without critical disruption of the structure stability. This project circumvents these issues by using molecular dynamics (MD) simulations with high experimental fidelity to study gas hydrates and provide methods to prototype new systems for implementation of efficient green technology in extreme formation environments, where the high-pressure driving force ensures the crystal formation. It is essential to understand the fundamental physical and chemical mechanisms at the molecular level and the nanoscopic behaviors leading to macroscopic properties to clarify phenomena that govern potential applications of these structures. This work uses MD 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 under extreme environments. Our work has shown that there is excellent agreement between sI methane hydrates and experimental values, with the interfacial energy decreasing with temperature. Preliminary results show that the sII natural gas hydrates show the same behavior. Our work has shown that 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. This information yields data to determine which conditions favor or hinder hydrate formation in applications of interest. This project studies molecular origins of macroscale, real world behavior of important energy-related materials. The impact of surface coverings is crucial in pipeline and natural gas infrastructure, and in storage technologies, but its molecular behavior is not sufficiently characterized. Using MD, statistical mechanics, and machine learning to develop an efficient workflow, interfacial behavior can be studied rapidly, effectively evaluating new green energy solutions. This provides guidance for future material development in extreme subsea environments, engineering applications, and harsh nucleation conditions.