Unveiling the CO2 hydrate phase diagram from computer simulation: Locating the hydrate–liquid–vapor coexistence and its upper quadruple point

Carbon dioxide (CO2) hydrates hold promising applications in capturing and separating CO2 for climate change mitigation. Understanding their behavior at the molecular level is, therefore, essential, and computer simulations have become powerful tools for exploring their formation and stability, providing valuable insights into their underlying mechanisms. In this work, we perform molecular dynamics simulations to compute the three-phase coexistence line involving the stability region where CO2 is in the vapor phase: CO2 hydrate-liquid water-vapor. This computation was previously inaccessible using the traditional three-phase direct coexistence technique. To achieve this, we employ a novel solubility-based method, which allows us to accurately evaluate the coexistence line. Our results exhibit excellent agreement with experimental data and, for the first time, accurately reproduce the hydrate-liquid-vapor equilibrium line of the CO2-water phase diagram. Finally, we have determined the upper quadruple point (Q2), where the four phases, namely hydrate, liquid water, liquid CO2, and vapor, coexist. Our pioneering result for the Q2 value shows remarkable agreement with experimental observations, validating the accuracy of our findings and representing a significant milestone in the field of gas hydrate research.