Molecular Gibbs Surface Excess and CO₂-Hydrate Density Determine the Strong Temperature- and Pressure-Dependent Supercritical CO₂-Brine Interfacial Tension

In CO₂ geological storage, the interfacial tension (IFT) between supercritical CO₂ and brine is critical for the storage capacitance design to prevent CO₂ leakage. IFT relies not only on the interfacial molecule properties but also on the environmental conditions at different storage sites. In this paper, supercritical CO₂-NaCl solution systems are modeled at 343-373 K and 6-35 MPa under the salinity of 1.89 mol/L using molecular dynamics simulations. After computing and comparing the molecular density profile across the interface, the atomic radial distribution function, the molecular orientation distribution, the molecular Gibbs surface excess (derived from the molecular density profile), and the CO₂-hydrate number density under the above environmental conditions, we confirm that only the molecular Gibbs surface excess of CO₂ molecules and the CO₂-hydrate number density correlate strongly with the temperature- and pressure-dependent IFTs. We also compute the populations of two distinct CO₂-hydrate structures (T-type and H-type) and attribute the observed dependence of IFTs to the dominance of the more stable, surfactant-like T-type CO₂-hydrates at the interface. On the basis of these new molecular mechanisms behind IFT variations, this study could guide the rational design of suitable injecting environmental pressure and temperature conditions. We believe that the above two molecular-level metrics (Gibbs surface excess and hydrate number density) are of great fundamental importance for understanding the supercritical CO₂-water interface and engineering applications in geological CO₂ storage.