Molecular Dynamics Simulation Study on the Growth of Structure II Nitrogen Hydrate.

Crystal growth of N hydrate in a three-phase system consisting of N hydrate, liquid water, and gaseous N was performed by molecular dynamics simulation at 260 K. Pressure influence on hydrate growth was evaluated. The kinetic properties including the growth rates and cage occupancies of the newly formed hydrate and the diffusion coefficient and concentration of N molecules in liquid phase were measured. The results showed that the growth of N hydrate could be divided into two stages where N molecules in gas phase had to dissolve in liquid phase and then form hydrate cages at the liquid-hydrate interface. The diffusion coefficient and concentration of N in liquid phase increased linearly with increasing pressure. As the pressure rose from 50 to 100 MPa, the hydrate growth rate kept increasing from 0.11 to 0.62 cages·ns·Å and then dropped down to around 0.40 cages·ns·Å once the pressure surpassed 100 MPa. During the hydrate formation, the initial sII N hydrate phase set in the system served as a template for the subsequent growth of N hydrate so that no new crystal structure was found. Analysis on the cage occupancies revealed that the amount of cages occupied by two N molecules increased evidently when the pressure was above 100 MPa, which slowed down the growth rate of hydrate cages. Additionally, a small fraction of defective cages including two N molecules trapped in 56 cages and three N molecules trapped 56 cages was observed during the hydrate growth.