Atomic-Scale Insights into the Phase Behavior of Carbon Dioxide and Water from 313 to 573 K and 8 to 30 MPa

We performed molecular dynamics (MD) simulations of CO₂ + H₂O systems by employing widely used force fields (EPM2, TraPPE, and PPL models for CO₂; SPC/E and TIP4P/2005 models for H₂O). The phase behavior observed in our MD simulations is consistent with the coexistence lines obtained from previous experiments and SAFT-based theoretical models for the equations of state. Our structural analysis reveals a pronounced correlation between phase transitions and the structural orderliness. Specifically, the coordination number of Ow (oxygen in H₂O) around other Ow significantly correlates with phase changes. In contrast, coordination numbers pertaining to the CO₂ molecules show less sensitivity to the thermodynamic state of the system. Furthermore, our data indicate that a predominant number of H₂O molecules exist as monomers without forming hydrogen bonds, particularly in a CO₂-rich mixture, signaling a breakdown in the hydrogen bond network's orderliness, as evidenced by a marked decrease in tetrahedrality. These insights are crucial for a deeper atomic-level understanding of phase behaviors, contributing to the well-grounded design of CO₂ injection under high-pressure and high-temperature conditions, where an atomic-scale perspective of the phase behavior is still lacking.