Revisions to standard theory provide clues to understanding phase behavior of clathrate hydrates

Clathrate hydrates are crystalline compounds formed when hydrophobic guest molecules are trapped in lattices of water. The interest of industry in hydrates is growing and natural gas hydrates are expected to be an energy resource. Hydrates are also predicted to be a storage medium for various sorts of gases, including the greenhouse gas carbon dioxide. The content of these trapped molecules, however, can vary considerably in different thermodynamic conditions. The content of the gases inside clathrate hydrates, and the structures occurring in nature, have long been the main targets both of theoretical investigation and experimental measurements.

Tanaka et al. put forth a new theory that looks to describe the phase behaviors of hydrocarbon and noble gas clathrate hydrates. Revising a statistical mechanical theory originating from van der Waals and Platteeuw (vdWP), the group evaluated the free-energies of the guest species and water under a constant pressure. They found for the first time that minute differences in guest molecule size can lead to profound differences in the phase behaviors of such molecules.

The theory allowed the authors to explain why the temperature dependence of the solubility in liquid water in contact with gaseous methane is opposite to cases where a clathrate hydrate is present. It provides a way to evaluate the phase boundaries that hydrates share both with water and guest molecules without introducing empirical parameters. In addition, the theory uses less computational power than direct simulations by several orders of magnitude so that it can be used in practical applications.

Hideki Tanaka, an author on the paper, said he hopes the findings will spark renewed interest in using the revised vdWP theory in novel ways. The group looks to extend their theory further to handle clathrate hydrates at both cryogenic temperatures and under extreme pressure.

Source: “On the phase behaviors of hydrocarbon and noble gas clathrate hydrates: Dissociation pressures, phase diagram, occupancies, and equilibrium with aqueous solution,” by Hideki Tanaka, Takuma Yagasaki, and Masakazu Matsumoto, The Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5044568 .