Non-equilibrium characteristics and regulation methods of methane hydrate nucleation on the substrate surface

Abstract

The synthesis process of methane hydrate exhibits strong non-equilibrium phenomena at all stages, with the nucleation stage being the most prominent. In this study, we used molecular dynamics simulations to investigate the spontaneous synthesis of methane hydrate on solid surfaces, focusing on the mass transfer non-equilibrium characteristics during the nucleation stage. From the overall perspective of nucleation events, we applied the Onsager hypothesis and introduced normalized autocorrelation functions to analyze the fluctuation-dissipation characteristics of hydrate cage structures on different substrate surfaces. We also examined the relationship between the orderliness of water molecules forming the cages and the solubility and diffusion of guest molecules. By comparing the effects of temperature and the presence of SDS and THF additives, we regulated the non-equilibrium characteristics during nucleation. The results indicate that nucleation on aluminum surfaces exhibits stronger non-equilibrium characteristics; there is a synergy between the components forming the hydrate and the overall fluctuation characteristics; lowering the temperature inhibits the non-equilibrium characteristics of the nucleation process but also reduces the number of nucleation events; the addition of additives effectively enhances the non-equilibrium characteristics, with the effect being more pronounced when SDS is in the gas phase, shortening the fluctuation period and relaxation time by 90.2 % and 66.3 %, respectively. This study demonstrates the mass transfer-driven non-equilibrium characteristics of the spontaneous nucleation of methane hydrates on solid surfaces and provides regulatory methods, contributing to a deeper understanding of the synthesis mechanism of hydrates.