Transient Translational and Rotational Water Defects in Gas Hydrates

Abstract

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.

Electrochemical Conversion of CO2 and CH4 at Subzero Temperatures

“…Previous works have demonstrated that in gas hydrate crystal structures, the arrangement of their hydrogen bond network is “ice-like” but with slight distortions. Further stabilized by encaging a guest molecule, the cage structures in gas hydrate crystals are considered relatively rigid and the lattice water molecules are largely restricted from rotation and translation . The rigidity of the gas hydrate structure may influence different steps of the reaction mechanism.…”

Strengthening and weakening of methane hydrate by water vacancies

“…In reality, water cages-dominated crystalline clathrate hydrates in natural settings and laboratories are imperfect crystals, and inevitably contain defects including water vacancies, interstitial defects and grain boundaries (Roman-Perez et al, 2010;Vidal-Vidal et al, 2015Liang et al, 2016). Similar to other solid crystals, the properties of crystalline clathrate hydrates could be greatly influenced by those defects (Wu et al, 2015;Liang et al, 2016Liang et al, , 2017Lo et al, 2017;Cao et al, 2020Cao et al, , 2021. For example, it was revealed that water vacancy defects in clathrate hydrates promote the local hopping and diffusion behaviors of guest molecules across clathrate cages (Liang et al, 2016).…”

Status of Natural Gas Hydrate Flow Assurance Research in China: A Review

“…41−43 The main controlling factors for cage formation are the water− guest and water−water interactions. Moreover, the transient translational and rotational water defects in the crystalline structure can enhance the mass transfer of guest molecules 44,45 (Figure 5), while significant structural defects are presented during hydrate nucleation. 46 These microscopic insights further illustrate that hydrate nucleation is a stochastic process.…”