Molecular-level insights into the structure stability of CH4-C2H6 hydrates

Abstract

The structure of CH4-C2H6 hydrates changes with gas composition and pressure. This work employs molecular dynamic simulation to explore the stability variation of CH4-C2H6 hydrates in sI and sII structures. The rotational and translational motions of water molecules reveal that the CH4-C2H6 hydrates are more stable in sI structure when the CH4 composition is 20 mol%, whereas sII hydrates are more stabilized with 90 mol% CH4. Cage occupancy greatly influences the motion of gas molecules. The motion of gas molecules is affected by the C2H6 molecules in the neighboring hydrate cages, reflected by the fluctuation amplitude of the rotational and translational curves. The fluctuation frequency in the rotational motion curves of C2H6 molecules is smaller than that of CH4, due to the higher probability of C2H6 getting close to the cage water molecules. The impact of pressure on the structure stability of CH4-C2H6 hydrates varies with gas composition.