Effect of methane concentration on the formation pathways of methane hydrate near hexagonal ice surfaces

Molecular dynamics simulations are performed to investigate the heterogeneous nucleation of methane hydrate near ice surfaces over a range of initial methane concentrations (3.0-15.0 mol%) at 250 K and 50 MPa. Across all concentrations studied, the presence of ice enhances methane hydrate nucleation, albeit via distinct mechanisms depending on methane availability. At low initial methane concentrations (<5.8 mol%), ice growth precedes hydrate formation; the advancing ice front concentrates methane in the remaining liquid, triggering hydrate nucleation once the local concentration approaches some threshold (4-6 mol%). The formation of hydrate then consumes methane, enabling further ice growth. Therefore, we observed a coupled growth mechanism regulated by local methane availability. At intermediate concentrations (5.8-6.3 mol%), ice growth is suppressed, yet nucleation remains promoted due to modifications in the nucleation potential near the ice interface, which lower the free-energy barrier and reduce the critical nucleus size. At high concentrations (>6.3 mol%), spontaneous hydrate formation emerges via collective stochastic nucleation, occurring randomly throughout the aqueous phase. In all cases, hydrate nuclei preferentially form at a distance from the ice interface or in the bulk liquid; direct nucleation on the ice surface is rare and observed only in the presence of cubic ice domains arising from stacking faults in the hexagonal lattice. These results reveal the complex interplay between methane concentration, ice growth, and hydrate nucleation, providing mechanistic insights into the dynamic behavior of clathrate formation near ice interfaces.