Molecular origin of pressure-induced re-entrant phase separation of CO2–H2O mixtures

CO 2 and H 2 O are the predominant fluid components in the Earth’s interior, spanning from the crust to the mantle. Fluid–fluid immiscibility in binary CO 2 –H 2 O fluids is of particular interest owing to its implications for geochemical processes and industrial applications, including geological CO 2 sequestration. CO 2 –H 2 O fluids exhibit re-entrant phase separation upon compression, yet, the microscopic origin of this behaviour remains unclear, in contrast to well-studied temperature-induced re-entrant phenomena. Here, using molecular dynamics simulations, we reveal the microscopic mechanisms underlying pressure-induced re-entrant phase separation in CO 2 –H 2 O mixtures. CO 2 and H 2 O exhibit distinct behaviours near their critical solution pressures, which highlights the pivotal role of H 2 O in fluid–fluid separation. Specifically, H 2 O–H 2 O interactions provide the primary contribution to phase separation; strong short-range H 2 O–H 2 O attraction accelerates segregation of H 2 O molecules under high-density conditions, whereas the recovery of hydrogen bonding drives segregation under low-density conditions. A simple united-atom approach demonstrate that intermolecular interactions outweigh differences in molecular size in pressure-induced phase separation. Our findings identify the molecular origin of pressure-induced fluid–fluid separation, suggesting the potential for re-entrant phase behaviour in a broad range of other binary mixtures.