Molecular Dynamics Simulation of CO2 Hydrate Growth in NaCl Aqueous Solution

Climate change has brought enormous adverse outcomes to biological activities around the world. The main reason is that too much CO2 has been released into the atmosphere. In recent years, storing CO2 in the form of CO2 hydrate is a research hotspot, the main purpose of which is to reduce carbon emissions to mitigate the greenhouse effect. In this work, we use the molecular dynamics simulation method to study the growth of CO2 hydrate in NaCl aqueous solution with the assumption of induction of CO2 sequestration in the ocean. The temperature is 275 K and the pressure is 10 MPa in this work. Under these conditions, stucture I type (sI- type) CO2 hydrate with a density of about 1150 kg/m3 formed within a very short period of time. The simulation results show that during hydrate growth, Na+ and Cl- are "driven" together and the water molecules remain liquid in this region, where they are near Na+ and Cl-. From the independent gradient model (IGM) based on Hirshfeld partition (IGMH) analysis, Na+ does not bond with any ions/molecules, which hinders the formation of water cages and thus inhibits hydrate growth; Cl- forms multiple H bonds with neighboring H2O molecules and can participate in the formation of water cages. However, it is worth noting that not all Cl- and the nearby water molecules can form either a five-membered ring or a four-membered ring; even some water molecules and Cl- cannot form a closed ring. Therefore, it is impossible to determine whether the water molecules near the Cl- are all in liquid or solid state.