Probing the instability of surface structure on solid Hydrates: A microscopic perspective through experiment and simulation.

Quasi–liquid layer (QLL) on the hydrate surface is discovered to possess multilayered structures. The regions of QLL near gaseous interface exhibit fluidity, while the majority at QLL–hydrate interface behaves similarly to bound water. [Display omitted] • Evaluating surface structure characteristics of nano-thick hydrate QLL by AFM measurement and MD simulation. • QLL fluidity increases with a steeper hydrate surface and higher temperature. • QLL distribution is related to the surface morphology of polycrystal hydrate. • QLL exhibits interface–dependent behavior, with outermost layer demonstrating highest instability. The surface structural characteristics of clathrate hydrates play a significant role in mass transfer, interfacial properties, and mechanical stability during the hydrate growth process. However, the microstructure of hydrate surfaces remains unclear and controversial. In this study, a modified Atomic Force Microscopy (AFM) system was employed to confirm the presence of a quasi–liquid layer (QLL) on the solid tetrahydrofuran (THF) hydrate surface. Subsequently, sensitivity analyses were conducted on factors affecting the instability (fluidity) of QLL, including relative position, inclination angle, environmental temperature, and surface morphology. The results indicate that higher temperatures enhance the instability of the QLL surface, and altering the slope of the hydrate surface leads to an increase in the average thickness of the QLL due to the driving forces of gravity and surface tension, resulting in increased fluidity. Additionally, the surface morphological structure formed by Oswald ripening in polycrystalline THF hydrates affects the thickness of QLL. Furthermore, molecular dynamics simulations reveal that water molecules on the hydrate surface exhibit a multilayered structure, with the QLL region near the gas phase showing greater instability, while the region closer to the hydrate exhibits solid–phase characteristics, consistent with experimental findings. [ABSTRACT FROM AUTHOR]