Use of Equations of State and Coarse Grained Simulations to Complement Experiments: Describing the Interfacial Properties of Carbon Dioxide + Decane and Carbon Dioxide + Eicosane Mixtures

Abstract

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 (CO2) + n-decane (n-C10H22) at 344.15 K and carbon dioxide (CO2) + n-eicosane (n-C20H42) 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 SAFT-γ 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 CO2) 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 CO2, larger in eicosane than in decane, is detected along the interface.