Molecular dynamics study on the mechanisms of ultrafine bubbles in CO₂ hydrate formation

Abstract

The accelerating rise in atmospheric CO₂, driven by anthropogenic emissions, necessitates urgent mitigation strategies. Among carbon capture and storage (CCS) technologies, CO₂ hydrate-based methods offer a promising pathway for efficient sequestration, storage, and utilization. However, the inherently slow kinetics of hydrate nucleation and growth limit their practical application. This study explores the use of various nanobubbles (NBs), including hydrogen, nitrogen, oxygen, and carbon dioxide, as stable, nanoscale gas cavities that act as novel promoters to enhance CO₂ hydrate formation, using molecular dynamics (MD) simulations. The results demonstrate that under optimal thermodynamic conditions, the presence of NBs significantly enhances hydrate formation. This enhancement is attributed to the hydrophobic NB surfaces acting as nucleation spots, promoting local concentration gradients and accelerating clathrate formation kinetics, while reducing the likelihood of random nucleation events in the bulk phase. Due to their smaller molecular sizes, hydrogen and nitrogen NBs further facilitate hydrate formation by diffusing into the solution from the NB core. However, lower temperature, as a primary sub-optimal thermal condition, reduce molecular mobility and suppress these mechanisms, thereby hindering hydrate growth. At elevated pressures, NBs exhibit a dual role, both promoting and inhibiting hydrate formation, and the comparison with non-nanobubbled samples reveals a pressure-dependent shift in the dominant nucleation mechanism from NB-induced interfacial ordering to bulk-phase interactions.