Elasticity and Stability of Clathrate Hydrate: Role of Guest Molecule Motions

Abstract

Molecular dynamic simulations were performed to determine the elastic constants of carbon dioxide (CO ₂) and methane (CH ₄) hydrates at one hundred pressure–temperature data points, respectively. The conditions represent marine sediments and permafrost zones where gas hydrates occur. The shear modulus and Young’s modulus of the CO ₂ hydrate increase anomalously with increasing temperature, whereas those of the CH ₄ hydrate decrease regularly with increase in temperature. We ascribe this anomaly to the kinetic behavior of the linear CO ₂ molecule, especially those in the small cages. The cavity space of the cage limits free rotational motion of the CO ₂ molecule at low temperature. With increase in temperature, the CO ₂ molecule can rotate easily, and enhance the stability and rigidity of the CO ₂ hydrate. Our work provides a key database for the elastic properties of gas hydrates, and molecular insights into stability changes of CO ₂ hydrate from high temperature of ~5 °C to low decomposition temperature of ~−150 °C.