Growth of Structure I Carbon Dioxide Hydrate from Molecular Dynamics Simulations

The growth of carbon dioxide hydrates and the mechanism are studied using molecular dynamics simulations. The melting temperatures at different pressures are determined using a three-phase molecular model consisting of a liquid carbon dioxide phase, a liquid water phase, and a solid hydrate phase. The growth simulations are then performed at a temperature (260 K) below the melting point. It is found that the growth rate of CO2 hydrates slightly decrease with pressure, from 0.132 m/s at 3 MPa to 0.096 m/s at 100 MPa under a supercooling of 25 K. The growth mechanism of CO2 hydrates is found to be largely similar to that of methane hydrates. One remarkable difference is the occurrence of the unprecedented transient 4151062 water cages (consisting of hydrogen bonding network of one rectangle, ten pentagons, and two hexagons) at the growing interface. Such cages may stay at the interface for as long as 15 ns and are transformed to the typical 512 and 51262 cages of structure I as the crystallization proceeds. The transformations of 4151062 cages to 512 and to 51262 cages are illustrated based on our simulations.