We report our observations of the transient formation of the translational and rotational defects in the host lattice of methane hydrates. We perform molecular dynamics simulations of a fully occupied methane hydrate system and find that the hydrogen bonds of a water molecule can be thermally broken, and the water molecule can then rotate more freely. We observe the formation of transient Bjerrum defects around the water molecule, and the two hydrogens of the water molecule can be interchanged when the transient defects anneal. The 95% confidence interval for the rate of the hydrogen interchange is estimated to be 1.3 × 105–2.4 × 105 s–1 per water molecule, and the 95% confidence interval for the associated free energy of activation is estimated to be 38.2–39.4 kJ/mol at 270 K. We also observe the transient formation of vacancy-interstitial water defects. The formation and annealing of these vacancy-interstitial defects can result in the interchange of two or three neighboring water molecules on the gas hydrate lattices. The 95% confidence interval for the rate of the formation of transient vacancy-interstitial water defects as a result of the water interchanges is estimated to be 6.7 × 102–1.6 × 104 s–1 per water molecule, and the 95% confidence interval for the associated free energy of activation is estimated to be 44.1–51.3 kJ/mol at 270 K.

Gas diffusion is considered a rate-limiting step in the formation of gas hydrates, yet its molecular mechanisms remain unclear. In this work, we present the molecular mechanisms of the CO2 cage-to-cage transport in gas hydrates, as directly observed from molecular dynamics simulations performed at elevated temperatures. We found that at least one water vacancy is required for the CO2 molecules to pass through five-membered water rings, while only the distortion of the local ring structure is required for the CO2 molecules to pass through the six-membered water rings. We used the transition-state theory to estimate the relevant kinetic parameters associated with the CO2 diffusion in gas hydrates. The calculated free energy of activation is about 44 ± 6 kJ/mol, and the diffusion coefficient is in the range of 1.0 × 10–16∼2.0 × 10–14 m2/s, for the CO2 diffusion at 270 K, in close agreement with previous experiments. This work suggests that the presence of empty cages is crucial for the CO2 cage-to-cage transport in gas hydrates.

Although the melting of gas hydrates tends to be heterogeneous in most cases, in principle, this process can also be homogeneous when surface melting is properly inhibited. In this work, we investigated the molecular mechanisms of the homogeneous melting of superheated methane hydrates by means of molecular dynamics simulations. The homogeneous melting processes were found to be stochastic with varied induction times. We observed the formation of structural defects within the hydrogen-bonded water lattices of hydrate crystals during the induction times. The methane molecules were found to be relatively more stable within the gas hydrate phases, which might be responsible for the high stability of the superheated metastable methane hydrates. Although the melting processes involve the collective motion of water and methane molecules, the current work suggests that the migration and aggregation of methane molecules are critical in initiating the homogeneous melting of gas hydrate crystals.