Molecular simulation of imperfect structure I CO2 hydrate growth in brine.

Abstract

In order to investigate the viability of carbon dioxide (CO₂) storage in seawater, molecular dynamics techniques were employed to study the dynamic evolution of CO₂ hydrate in saline water. The simulation was conducted under specific conditions: a temperature of 275 K, a pressure of 10 MPa and a simulated marine environment achieved using a 3.4 wt% sodium chloride (NaCl) solution. The total simulation time was 1000 ns. The results of the simulation indicate that the pre-existence of CO₂ hydrate crystals as seeds leads to rapid growth of CO₂ hydrate. However, analysis of the F3 and F4 order parameters reveals that the hydrate does not meet the standard values of the perfect structure I (sI) type, confirming the existence of an imperfect structure during the simulation. Additionally, the changes in the number of different phase states of water molecules during the hydrate growth process shows that there are always some liquid water molecules, which means some water molecules fail to form solid water cages. Further investigation suggests that the presence of Na⁺ and Cl^- hampers the hydrogen bonds between water molecules, resulting in incomplete cage structures. By analyzing the density variations in the system, it is observed that CO₂ hydrate, with a density of around 1.133 g cm^-3, forms rapidly, surpassing the average density of seawater. This density increase facilitates the efficient and swift containment of CO₂ on the seabed, thereby supporting the feasibility of the CO₂ storage theory.