Molecular insights into the stability of CO2 hydrate in variable-diameter nano-slit

Abstract

Most literatures focused on the stability of CO₂ hydrate in uniform-diameter nano-slits, while variable-diameter nano-slits may be more prevalent in the realistic environments for CO₂ storage processes. This study employs molecular dynamics (MD) simulations to systematically explore the effects of various factors (disordered water, slit size, and wettability) on hydrate stability in variable-diameter nano-slits. It is revealed that the variable-diameter nano-slit affects the activity of disordered water, rather than altering the hydrate crystal structure, to dominate the hydrate stability. The presence of disordered water decreases hydrate stability, with the melting point dropping from 282 K to 269 K when disordered water in large slits, and further declining to 267 K when it in small slits. The Vertical surfaces in the nano-slits induce heterogeneity in disordered water activity, and its proportion dominates the further depression of melting points for hydrates in large slits. The slit surface-water interaction caused by the wettability variation hardly impact the hydrate stability, whereas the differences in nanobubbles distribution significantly influence the stability of hydrates in the small slits. This study firstly offers valuable insights into the complex interactions affecting CO₂ hydrate stability in more realistic porous media environments, uncovering the mechanisms by which geometric heterogeneity and interfacial behavior influence hydrate stability.