CO2–water/brine interfacial tension: Widom-line based two-regime correlations and thermodynamics consistency

The interfacial tension (IFT) between CO2 and water/brine exhibits significant pressure dependence along isotherms. As pressure increases, the IFT undergoes a noticeable transition from linear decrease to a pseudo-plateau stage, attributable to the phase transition of CO2 from gas to liquid. In the subcritical regime (below 304.15 K), this transition is well-explained by thermodynamics, given the distinct pressure responses of gas and liquid CO2 at the interface. However, under supercritical conditions (above 304.15 K)—increasingly relevant for CO2 utilization and storage—it is challenging to confirm the same transition, as “gas” and “liquid” lose their clear definitions. This study addresses this challenge by introducing the Widom line as the transition boundary between “gas-like” and “liquid-like” pseudo-phases in the supercritical regime. For the first time, the consistency of the subcritical gas-to-liquid transition and the supercritical gas-like to liquid-like transition at interfaces has been confirmed from macroscopic and microscopic perspectives. The Widom line identifies the changes in interfacial configurations between water molecules and CO2 molecules, supporting the dominant confinement roles of the subcritical phases and contributing to the integrity of the thermodynamic rules. Based on this, an IFT correlation is developed, applicable to both subcritical and supercritical conditions, which achieves high accuracy over broad pressure and temperature ranges and exhibits extensibility to similar interfaces, such as CH4–water interfaces. The success in using the Widom line to interpret IFT transitions suggests thermodynamic continuity across subcritical and supercritical states. It highlights the fundamental similarity between supercritical pseudo-phases and subcritical phases.