We report surface tension measurements, coexisting densities, concentration profiles along the interfacial region, surface activities, and relative Gibbs adsorption isotherms for binary mixtures of carbon dioxide (CO 2 ) + n-decane (n-C 10 H 22 ) at 344.15 K and carbon dioxide (CO 2 ) + n-eicosane (n-C 20 H 42 ) at 323.15 K over a pressure range from 0.1 MPa to 10.35 MPa. The results are obtained by employing a broad approach that integrates experiments with both theory and molecular simulations to gain an enhanced multiscale description of the interfacial region. Measurements are based on the use of a high-pressure pendant drop tensiometer coupled to a high-pressure densimeter. Theoretical modeling is carried out using the Square Gradient Theory based on a version of the Statistical Associated Fluid Theory (SAFT-VR Mie) equation of state. At the molecular level, Molecular Dynamics is employed and molecules are represented by the SAFTγ coarse-grained force field. The novelty here is that both the theory and the simulations uniquely share the same underlying intermolecular potentials, hence the experimental data are employed to verify and inform in the same way both the theory and simulations. Reassuringly, theory, experiments, and molecular simulations agree with each other in the description of the bulk phase equilibria and interfacial tension. It is observed that for both mixtures, the interfacial tension decreases as the pressure (or the liquid mole fraction of CO 2 ) increases. Furthermore, there is quantitative agreement between the theoretical predictions and the results obtained from the molecular simulations of surface activities, concentration profiles along the interfacial region, and relative Gibbs adsorption isotherms at the interfaces. A remarkable high excess adsorption of CO 2 , larger in eicosane than in decane, is detected along the interface.

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The accurate description of the phase equilibria and interfacial behavior of the ternary mixture H2O + CO2 + CH4 is of fundamental importance in processes related with enhanced natural gas recovery, CO2 storage, and gas-oil miscibility analysis. For this reason, the physical understanding and theoretical modeling of this remarkably complex mixture, in a wide range of thermodynamic conditions, constitutes a challenging task both for scientists and engineers. This work focuses on the description of the interfacial behavior of this mixture, with special emphasis on several regions that yield different scenarios (vapor–liquid, liquid–liquid, and vapor–liquid–liquid equilibria) and in pressure and temperature ranges related with the practical applications previously mentioned. A comparison between three alternative approaches has been performed: atomistic Monte Carlo simulations (MC), coarse grained molecular dynamics (CG-MD) simulations, and density gradient theory (DGT) have been used to characterize the interfacial region, describing in detail complex phenomena, including preferential adsorption and wetting phenomena even in the ternary triphasic region. Agreement between the results obtained from different methods indicate that the three alternative approaches are fully equivalent to analyze the interfacial behavior. It has been also found that the preferential adsorption of CO2 over H2O interface is greater if compared to CH4 in all conditions characterized. In fact, we have also demonstrated that CH4 under triphasic conditions has very limited influence on the complete wetting of the binary system H2O + CO2.

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Empirical thermal cohesion functions, R(T r ), are frequently used in conventional equations of state (EOS) for fitting the vapor pressures of pure fluids. Accurate vapor pressure predictions are required for correlating and/or predicting the phase equilibrium and interfacial tension of multicomponent mixtures. This is the case for the Redlich-Kwong-Soave and Peng-Robinson models, two well-established models for engineering applications. In this work, we demonstrate that, in the case of pure fluids, the R(T r ) function can potentially predict multiple mechanically stable critical points, thus affecting the global topology of phase equilibrium predictions. A detailed analysis, based on the consistency of the prediction of the Joule-Thomson inversion curve, reveals that these predictions are not reliable from a physical point of view. In fact, conventional cubic EOS are able to predict multiple Joule-Thomson inversion curves, a behavior symptomatic of the prediction of multiple stable critical points for pure fluids. Similar pitfalls have been detected in theoretically based EOS such as SAFT and the model proposed by Johnson et al.

Density gradient theory (DGT) and molecular-dynamics (MD) simulations have been used to predict subcritical phase and interface behaviors in type-I and type-V equal-size Lennard-Jones mixtures. Type-I mixtures exhibit a continuum critical line connecting their pure critical components, which implies that their subcritical phase equilibria are gas liquid. Type-V mixtures are characterized by two critical lines and a heteroazeotropic line. One of the two critical lines begins at the more volatile pure component critical point up to an upper critical end point and the other one comes from the less volatile pure component critical point ending at a lower critical end point. The heteroazeotropic line connects both critical end points and is characterized by gas-liquid-liquid equilibria. Therefore, subcritical states of this type exhibit gas-liquid and gas-liquid-liquid equilibria. In order to obtain a correct characterization of the phase and interface behaviors of these types of mixtures and to directly compare DGT and MD results, the global phase diagram of equal-size Lennard-Jones mixtures has been used to define the molecular parameters of these mixtures. According to our results, DGT and MD are two complementary methodologies able to obtain a complete and simultaneous prediction of phase equilibria and their interfacial properties. For the type of mixtures analyzed here, both approaches have shown excellent agreement in their phase equilibrium and interface properties in the full concentration range.

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