A microscopic study on the nucleation of cage structures and interfacial growth characteristics during methane hydrate synthesis process

Molecular dynamics method is used to explore the microscopic mechanism of methane hydrate synthesis process. The three important stages of methane hydrate synthesis are characterized. The diffusion coefficient of the dissolution and diffusion stage, the nucleation stage, and the growth stage are calculated to be 4.6 × 10−11 m2/s, 1.57 × 10−14 m2/s, and 6.97 × 10−12 m2/s, respectively. To reveal the features of the dissolution and diffusion process, the number variation of methane molecules is plotted from time and space perspectives. In the nucleation process, the structure of rings, semi-cages, and cages is identified. It is found that the number of ring structures starts to increase at 5 ns while for semi-cages and cages, they increase at 8 and 14 ns. During the growth process, a method for defining the solid–liquid interface region is proposed based on the F4 order parameter. The interface dynamic parameters such as the position, movement rate, interface morphology, and interface region thickness of the solid–liquid interface region are analyzed. The results of interface dynamics indicate that the moving rate of solid–liquid interface is not constant. The thickness of the solid–liquid interface region is about 1/2 of the lattice constant of methane hydrates. The relationship between the number of hydrate-like particles suggests that the cavity occupancy ratio is less than 100%. Besides, it was discovered that the hydrate seeds have an important positive influence on methane hydrate synthesis. The simulated system with 2 × 2 × 3 hydrate seeds has more stable cage structures and higher F4 values than the other systems with thinner seeds.