The influence of NaCl ions on hydrate structure and thermodynamic equilibrium conditions of gas hydrates

Concentrated brines are known to be very good inhibitors for gas hydrate formation, and gas hydrates can be used in the desalination of seawater under the assumption that salinity is zero in gas hydrates. However, its molecular mechanism has not been understood yet. We examined this by classical molecular dynamics simulations. The simulations results showed that a pair of Na+–Cl− ions can be included in a unit cell of the hydrates with the hydrate structure being stable. However, the equilibrium pressure was larger than that of pure hydrates. Therefore, it is concluded that gas hydrates may not be salt-free completely, i.e., 0.07 wt.% salinity at least. Furthermore, Na+ ions have a smaller effect on the crystal structure deformation and the equilibrium pressure of the hydrates than Cl− ions, as 4 Na+ ions may be contained in a unit cell of the hydrates. Meanwhile, massive salt ions approach to concentrate on the surface of the hydrates. The simulation results are consistent with the available desalination experimental results utilizing gas hydrate method.

► Molecular mechanisms of desalination utilizing gas hydrates are understood.
► Gas hydrates may be not salt-free completely, 0.07 wt.% salt at least.
► More Na+ but few Cl− ions can be contained in gas hydrates.
► Massive salt ions approach to concentrate on the surface of the hydrates.
► Some experimental phenomena can be explained by the simulation results.