Molecular dynamics study on the growth of structure I methane hydrate in aqueous solution of sodium chloride

The structure, thermodynamic, and kinetic properties of methane hydrates formed from the aqueous solution of sodium chloride are investigated based on molecular dynamics simulations. A three-phase molecular model consisting of a slab of methane hydrate phase, a slab of liquid water containing sodium chloride, and a gas phase of methane molecules is used. The decrease in the three-phase coexisting temperatures (by 2-3 K) at different pressures (10-100 MPa) for aqueous NaCl solutions (about 2 mol %) confirms the thermodynamic inhibition of NaCl. The growth rate of methane hydrates in NaCl solution is found to be half to one-third of that in pure water. The kinetic inhibition of NaCl is found to be a result of the reduced water repelling at the growing interface due to the strong hydration of ions. Individual ions or NaCl ion pairs can replace water molecules to participate in the formation of the cage structures. The distortion of water cages due to the presence of ions may result in a reduced fraction of occupation of methane in the cage cavities. Our results provide useful insights into the mechanism of growth of methane hydrates in seawater and the desalination.