New sights on derived behaviors of methane hydrate molecular structure in Na+/Cl- ions invading process

Abstract

The method of non-equilibrium molecular dynamics simulation (NEMD) was used to investigate the Na+/Cl- migration characteristics in methane hydrate (MH) phase under a static electric field. The molecular structure derived behaviors of MH were analyzed in this study. A hydrate-solution two-phase system was set up and the electric field of 0.5 V/nm perpendicular to the interface was applied to promote ions migration into MH phase. The results showed that a series of random semi-open cages composed of Na+/Cl- ions and water molecules containing Na+/Cl- ions, which are essentially metastable, formed rapidly. Then, only a few parts of cages could recovery the original structure after ions migration process. Finally, the migration of Na+/Cl- caused hydrate cages breakage. Therefore, the Na+/Cl- migration process into MH phase was defined as ions invading in this study. In contrast, Cl- will cause greater distortion of the MH cage structures. What’s more, the variations in free energy during ions invading process was also calculated. It was found that Na+/Cl- have no adsorption effect at the hydrate surface, and the electrostatic field makes the Na+/Cl- easier to invade into the MH phase. According to the free energy during ion invasion process, Na+/Cl- ions have no adsorption effect on hydrate surface, and the electrostatic field makes Na+/Cl- ions more easily invade into MH phase. Moreover, the electrostatic field has weak effect on hydration shell of Na+/Cl- in solution phase. The positions and orientation angle distribution of water molecules around the ions in hydrate phase are similar to that in the solution phase, indicating that the ions in hydrate phase are still keep in hydration state. These MD-based findings, particularly those involving hydrate molecular structure, may be instructive for the coexistence of MH and ions in sea water.