The type and magnitude of the molecular interactions that occur at interfaces are responsible for many of the phenomena that are observed in nature. Formation of micelles by amphiphilic surfactant molecules in aqueous solutions (like soaps and cosmetics), the nature of the lipid bilayers that form cell membranes or the stability of colloids in emulsions (such as milk and paintings) are just some examples. Moreover, the interfacial tension is a key property to determine the miscibility of a mixture. Accurate knowledge of the behaviour of a fluid at the interface is of crucial for a proper understanding of all these phenomena. However the modelling of interfacial properties is still a difficult task, due to the inhomogeneous nature of the system.

In this work we examine the density profiles and surface and interfacial tension of several fluids of industrial interest. The modelling is done coupling a density functional theory (DFT) with a molecular SAFT-type equation of state (SAFT-VR) [1]. The functional is constructed by partitioning the free energy density into a reference term, described by the use of the local density approximation (LDA), and an attractive perturbation (which incorporates the long-range dispersion interactions). This SAFT-VR DFT approach [2] is used to describe the vapour-liquid interface of several non-associating and associating molecules ranging in size from small molecules to long chains, with particular attention to n-alkanes, carbon dioxide and water. They are studied at different thermodynamic conditions, reproducing some typical values observed in several crude oil fields. Surface tension and profile density results are compared with molecular simulations and experimental data in order to check the accuracy of the method. Finally, the theory is extended to the treatment of binary mixtures and some preliminary results for n-alkane mixtures and CO2-alkane systems are presented. A comparison with the density gradient theory (DGT) is made, highlighting the advantages and disadvantages of the two methodologies.

[1]. Gil-Villegas A.; Galindo, A.; Whitehead, P.J.; Mills, S.J.; Jackson, G.; Burguess, A.N. J. Chem. Phys. 106, 4168-4186 (1997).

[2]. Gloor, G.J.; Jackson G.; Blas F.J.; del Rio E.M.; de Miguel, E.; J. Chem. Phys. 121, 12740-12759 (2004)