(119z) Insights into the Structure Stability of CH4-C2H6 Hydrates Using Molecular Dynamic Simulation

The structure of CH₄-C₂H₆ hydrates changes with gas composition and pressure, but the mechanisms of the hydrate stability and the effect of these influencing factors are unclear. In this work, the molecular dynamic simulation is employed to explore the structure stability of CH₄-C₂H₆ hydrates in sI and sII structures. The cage occupancy ratio of CH₄ and C₂H₆ molecules in the small and large hydrate cages is determined using the thermodynamic models. The average rotation angle, mean square displacement (MSD), and radial distribution function (RDF) of H₂O molecules are applied to evaluate the stability of hydrate cages. It is revealed that the CH₄-C₂H₆ hydrates are more stable in sI structure when the composition of CH₄ is 20 mol%, whereas the sII structure is more stable with a CH₄ composition of 90 mol%. This is consistent with the theoretical calculation and also the experimental observations. The impact of pressure on the structure change of CH₄-C₂H₆ is detected. The stability of sI CH₄-C₂H₆ hydrate cages with a CH₄ composition of 90 mol% increases with pressure, whereas the sII hydrate cages become less stable when the pressure is elevated. The influence of pressure on the hydrate stability varies with the gas composition. When the CH₄ composition is 20 mol%, the stability of sI structure CH₄-C₂H₆ hydrates hardly changes with pressure, while that of sII hydrates is enhanced with the increasing pressure. The rotational and translational motion of CH₄ and C₂H₆ molecules is greatly influenced by the cage occupancy. The motion of CH₄ molecules is affected by the C₂H₆ molecules in the neighboring hydrate cages, which is reflected by the change in amplitude of fluctuation in the average rotation angle and MSD curves, as well as the RDF of C-C pair between CH₄ and C₂H₆ molecules. It is also observed that the frequency of fluctuations in the average rotation angle and MSD curves of CH₄ molecules is larger than that of C₂H₆ molecules. This is a result of the larger potential energy of CH₄-H₂O pair than that of C₂H₆-H₂O pair due to the different size and polarizability of CH₄ and C₂H₆ molecules.