Practice-based experimental reviews have unambiguously shown that the experimental efforts for determining and understanding in detail the dependence of the interfacial tension (IFT) of the H2O + CO2 system at high pressure are limited by the well-known mass density inversion. This phenomenon entails that the CO2-rich phase becomes denser than the H2O-rich phase. Additionally, there are often inconsistencies among the existing literature data, thereby making it challenging to propose predictive models to complement densities and IFT in the regions where experimental measurements are difficult or even impossible to access. In this contribution, the mass density inversion effect on IFT is corroborated by coarse-grained molecular dynamics simulations employing the SAFT-γ-Mie force field, combined with the density gradient theory. The mass density inversion due to gas enrichment is revealed to be an important switch that controls the slope in the IFT curve and also the conformation of minimum and maximum accumulation on the interfacial population of species, which interestingly implies simultaneous desorption and adsorption along the interfacial zone.

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This work focuses on the application of a two-way approach, where Molecular Dynamics (MD) simulations and the Square Gradient Theory (SGT) have been used for describing the phase and interface behavior of binary and ternary Lennard-Jones (LJ) mixtures, along a condition of three-phase equilibrium. The unequivocal correspondence between MD and SGT has been achieved by using the global phase diagram of binary mixtures composed by equally sized Lennard-Jones molecules, from which representative molecular parameters for Type-I, Type-II, and Type-III systems have been determined. The so selected binaries have been used then to scale the behavior of a ternary mixture characterized by complex phase equilibrium patterns. For the case of the theoretical SGT approach applied to the Lennard-Jones equation of state was used for predicting phase equilibrium and interfacial properties. In addition the corresponding MD simulations of these macroscopic properties have been conducted for the LJ potential by using equivalent molecular parameters and conditions than in the theoretical approach. Excellent agreement has been observed between the predictions obtained from theory and simulations. Particularly, our results concerning the characterization of the three phase line of a binary Type-III mixture indicate that the bulk liquid (α) and the bulk gas (G) regions are sharply separated by a bulk liquid region (β) for all the explored temperature, pressure, and concentration conditions. The structural analysis of these bulk phases reveals that a secondary liquid phase (β) perfectly wets the liquid-gas interface (α-G), as previously found for Type-II mixture [A. Mejía and L. F. Vega, J. Chem. Phys. 124, 244505 (2006)]. The exploration along the three-phase line for the ternary mixture shows good agreement between SGT and MD. Particularly, we observed the specific influence of a third component in the phase and interface behavior. From all the previous results, we conclude that the SGT applied to an EoS with appropriate mixing rules produces reliable predictions of the properties of ternary mixtures.

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