Characterization of the quasi-liquid layer on gas hydrates with molecular dynamics simulations

Abstract

Understanding the properties of the surficial quasi-liquid layer (QLL) is a prerequisite in proper handling of gas hydrates, a potential alternative future energy resource. Despite of its critical importance, the characterization of QLL has long been known to be highly challenging. In this work, the evolution of QLL during hydrate decomposition was systematically investigated by Molecular Dynamics simulations. Using the instant displacement of water molecules as a measure of the fluidity of the molecular layer adjacent to hydrate surfaces, QLL thickness was accurately measured using the dynamic properties of water molecules for the first time. The QLL thickness obtained by this new molecular dynamics-based approach takes amorphous water molecules close to hydrate surfaces into account, which was often ignored in the previous results based on the static structural order parameters. The variation of QLL thickness throughout the hydrate decomposition process at different temperatures and pressures was then evaluated. The thickness of QLL was found to increase with elevated temperature and pressure, owing to the varied changes of the amorphous and hydrate-like molecular content in this important layer. The results regarding the dynamics of the QLL with molecular resolution improved the current understanding on the stability of gas hydrate and shed new light on the physical fundamentals relevant to future hydrate exploration as well as anti-hydrate materials design.