A SAFT–DFT approach for the vapour–liquid interface of associating fluids

Abstract

We present a density functional theory (DFT) based on the statistical associating fluid theory (SAFT) bulk free energy to describe the behaviour of inhomogeneous associating molecular fluids, with a special emphasis on the vapour–liquid interface. The molecules are described in terms of hard-sphere (HS) segments which interact through an arbitrary intermolecular potential. Off-centre square-well bonding sites are incorporated to mediate the association between molecules. The approach presented here is based on a perturbation theory about a HS reference fluid under the local density approximation (LDA); the contributions due to chain formation and association are also considered at the local density level. The Helmholtz free energy functional due to the dispersive attractive interactions is treated at the mean-field level by ignoring the correlations. This description is essentially a ‘van der Waals’ theory of non-uniform fluids. The incorporation of a SAFT free energy of the bulk associating fluid represents the simplest extension of the theory to deal with inhomogeneous associating systems (SAFT–DFT). The resulting functional is used to investigate the effect of the range of the attractive interactions on the interfacial properties, such as the density profile and surface tension, for two different intermolecular potential models, namely the Yukawa and square-well. This simple SAFT–DFT approach is also used to make quantitative comparisons with the experimental surface tension for water.