Application of the SAFT-VR density functional theory to the prediction of the interfacial properties of mixtures of relevance to reservoir engineering

The SAFT-VR DFT formalism, which has recently been extended to mixtures [F. Llovell et al., J. Chem. Phys. 133 (2010) 024704], is applied to the interfacial properties of a wide variety of binary mixtures that are of relevance to enhanced oil recovery and carbon dioxide sequestration. The approach is based on the statistical associating fluid theory for attractive potentials of variable range (SAFT-VR) and includes a formal density functional theory (DFT) treatment, where the different terms of the equation of state are separated in order to describe the short and long-range interactions of the inhomogeneous fluid. The structural correlations between the particles are taken into account following the ideas of Toxvaerd [S. Toxvaerd, J. Chem. Phys. 64 (1976) 2863] with the application of appropriate van der Waals mixing rules. Binary mixtures of short and long chain n-alkanes, n-alkanes + carbon dioxide, water + carbon dioxide and water + n-alkanes are investigated over wide ranges of temperature and pressure. The molecular parameters for each pure compound are obtained by comparison with vapour-liquid equilibrium data. A binary parameter is used to account for the unlike dispersive energy interactions and provide a representation of the bulk fluid phase equilibrium properties of the mixtures. No additional intermolecular parameters are required in the implementation of the DFT for the interfacial properties of the systems. The predictions for the vapour-liquid and liquid-liquid interfacial tensions are generally found to be in good quantitative agreement with experimental data. Selected density profiles exhibiting noteworthy adsorption features (such as preferential surface adsorption or depletion) are highlighted. The degree of hydrogen bonding along the interface is also reported for the aqueous mixtures studied.