Facilitation of Hydrate Dissociation and Structural Evolution by Major Marine Anions under Static Electric Fields

Electric fields have been proven to be capable of significantly affecting the equilibrium state of hydrates. In this study, the thermodynamic properties and structural changes of methane hydrate (MH) in various anion solutions in an electric field at 0.7 V/nm were investigated by molecular dynamics simulations. The presence of anions significantly enhances the instability of methane hydrates under electric fields, leading to a staged dissociation process. First, the anions coexist with MH to form a temporary metastable structure under the action of an electric field. Then, the migration of anions causes the dissociation of nearby hydrates and the formation of flow channels in the hydrate layer, which leads to the complete dissociation of MH after a period. The promotive effects of F^-, Br^-, I^-, and Cl^- ions were close, while SO₄^2- was relatively weak. The anions are still in hydration shells in the MH phase, but the structure of the hydration shells differs slightly from that in solution (the coordination numbers of I^- and SO₄^2- ions increased). The migration resistances of multiple anions to cross the surface of the hydrate layer are similar. However, inside the hydrate phase, the anions with a larger radius have a higher migration resistance. It is difficult for SO₄^2- ions to migrate inside the hydrate phase, and they tend to form a metastable structure on the hydrate surface. Combining our previous studies, SrCl₂ solution has the best hydrate promotion under an electric field environment.