Abstract
Numerical simulations of the nucleation process of methane hydrate have been carried out using the molecular dynamics technique. A mixture of water and methane, consisting of 46 rigid water molecules and eight spherical methane molecules, placed arbitraily in a cubic cell of length 12.0 A, has been used as a model system to simulate the nucleation process of the hydrate mainly at a temperature of 275 K. The intermolecular interactions are described by the modified MCY potential model for water-water, and the simple Lennard-Jones potential model both for guest-guest and water-guest. As a result, the nucleation process is considered to comprise of three successive stages. In the first stage, the guest molescules undergo a dispersive process. The evidence of the transition from the first stage into the second lies in the fact that the distribution of the guests reaches nearly that in the hydrate. In the second stage, water molecules form the hydrate cavities surrounding the guests. Then, in the third stage, the hydrate structure becomes steady. It is found that the key for successful formation of the hydrate structure is that the distribution of the guests approaches that in the hydrate, since the nucleation process in much easier to evolve from the second stage into the third. Moreover, the hydrate structure is one of the most stable among all the configurations of the model system as it has the lowest systematical potential.
