Molecular Dynamics Simulation of CO2 Hydrate Growth and Intermolecular Weak Interaction Analysis

The growth of carbon dioxide (CO2) hydrate in CO2 aqueous solution (box A), and in CO2 aqueous solution with structure I (sI) CO2 hydrate cell as seed (box B) under the same conditions of 275 K and 10 MPa, were simulated respectively. The total simulation time is 5000 ns. The nucleation process is very slow, so it is difficult to observe CO2 hydrate growth in box A without CO2 hydrate seed, CO2 just accumulates to form bubbles, while H2O still remains liquid, the F4 order parameter remains about –0.1. By contrast, beautiful CO2 hydrate in box B grows rapidly, CO2 is trapped in the cages formed by H2O molecules, F4 rises from 0.12 to 0.7 due to the formation of sI hydrate. The density of CO2 hydrate in box B is as high as 1.25 g/cm3, higher than that of seawater, which is beneficial to its permanent sealing on the seabed. In box B, in accordance with the independent gradient model based on Hirshfeld partition (IGMH) analysis, a H2O molecule forms four hydrogen bonds with surrounding H2O molecules, CO2 only has van der Waals interaction with surrounding water cages. In the actual large-scale CO2 storage process, especially for the rapid storage of CO2 in the form of hydrate, it is strongly recommended to pre-add CO2 hydrate crystals into the solution to achieve the purpose of rapid growth of CO2 hydrate.