Molecular dynamics simulation of CO2 hydrate growth in salt water

Abstract

To achieve the “carbon peaking and carbon neutrality goals”, carbon capture, utilization, and storage (CCUS) is a research hotspot. In our work, molecular dynamics simulation method is used to simulate the CO2 hydrate growth in NaCl aqueous solution with pure structure I CO2 hydrate as seed, at a temperature of 275 K, a pressure of 10 MPa, and a total simulation time of 5000 ns. It was found that not all water molecules participated in the formation of hydrate. The F3 and F4 order parameters and cage numbers proved the formation of structure I CO2 hydrate. According to independent gradient model based on Hirshfeld partition (IGMH) analysis, one water molecule could form four H-bonds with adjacent water molecules in a perfect hydrate; single Cl− could form multiple H-bonds with adjacent water molecules; but the specific water molecule which was near the Cl− might not form four H-bond with its adjacent water molecules, resulting in the inability to form a perfect hydrate and thus water molecules remain liquid; Na+ does not participate in the formation of hydrates. In addition, Na+ and Cl− always come in pairs, so we do believe that NaCl always act as hydrate inhibitor, regardless of its concentration. This is the mechanism how hydrate inhibitor works and plays a significant guiding role in developing highly effective hydrate inhibitors. From the perspective of binding energies of H-bonds, we believe that HOH···Cl− is stronger than HOH···OH2. The density of CO2 hydrate is higher than that of seawater, which could help CO2 hydrate sink spontaneously on the seabed.