There is a renewed interest on the characterization of the interfacial properties of carbon dioxide (CO2) – linear and aromatic hydrocarbon mixtures in the context of oil reservoir technologies, both in terms of enhanced oil recovery and ultimate CO2 sequestration. Specifically, it has been repeatedly recognized that the wetting behavior and interfacial phenomena are crucial governing factors in the overall performance of any process based on these mixtures. Consequently, accurate thermo-physical properties have to be conveniently known over a wide range of temperature, pressure, and concentrations. However, in spite of the apparent simplicity of the mixture, the prediction of the required phase equilibria and interfacial properties is still challenging, especially at high temperatures and pressures. In fact, experimental results suggest that these mixtures may exhibit immiscibility gaps, homogeneous azeotropy, three and four phase equilibrium and barotropy (or density inversion), among other types of phase behavior. The interfacial behavior is expected to follow these anomalities, however, it has been much less explored. No unique empirical approach seems to be trustworthy of extrapolating experimentally determined behavior to extreme thermodynamic conditions nor of describing concentration profiles along the interface, surface activity, relative Gibbs adsorption isotherms and interfacial tension with any degree of confidence.
In this work, we employ a concerted approach that includes experimental determinations and theoretical modelling in order to unequivocally characterize the interfacial properties of CO2 + toluene + heptane ternary mixtures and its CO2 binary mixtures (CO2 + toluene and CO2 + heptane).
Experimental determinations are carried out on a combined device that includes a high-pressure vibrating tube densimeter and a high-pressure pendant drop tensiometer. Measurements of mass density and interfacial tension for CO2 + hydrocarbon mixture (toluene and heptane) are carried out at different volume compositions of toluene (i.e., 0, 25, 50, 75, 100 %v/v) at the isothermal condition of 344.15K and from 0.1 to 8 MPa.
The theoretical modeling of the interfacial properties was carried out by employing the Square Gradient Theory (SGT) using the Statistical Associated Fluid Theory (SAFT VR Mie) equation of state (EoS).
According to the results, the experimental bulk phase equilibrium densities and interfacial tensions obtained are in very good agreement with the theoretical modeling. By seamlessly combining experimental and modelling approaches we are able to simultaneously predict phase equilibrium and interfacial properties. For the systems and conditions studied we do not observe mass nor molar barotropic inversion; the interfacial tensions decrease as the pressure (or liquid mole fraction of carbon dioxide) and/or as the toluene volume fraction decreases. The surface relative Gibbs adsorption of the species along the interfacial region is reported; carbon dioxide is adsorbed along the interfacial region, whereas toluene and heptane do not exhibit any special adsorption activity. The adsorption of carbon dioxide increases with pressure.
