Growth of Structure I Carbon Dioxide Hydrate from Molecular Dynamics Simulations

The growth of carbondioxide hydrates and the mechanism are studied using molecular dynamics simulations. The melting temperatures at different pressures are determined using a three phase molecular mode 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 occurence of the unprecedented transient 4(1)5(10)6(2) 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 5(12) and 5(12)6(2) cages of structure I as the crystallization proceeds. The transformations of 4(1)5(10)6(2) cages to 5(12) and to 5(12)6(2) cages are illustrated based on our simulations.