An examination of the vapour-liquid interface of associating fluids using a SAFT-DFT approach

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Abstract
With a realistic description of the free energy of bulk fluids, it is now possible to make accurate predictions at the molecular level for the phase behaviour of systems as complex as aqueous solutions of amphiphiles, reacting and associating fluids, polymers, and electrolytes (e.g. using the statistical associating fluid theory SAFT). A quantitative molecular description of the interfacial properties of inhomogeneous fluids, including surface tension and adsorption is much less common. In this work we first hope to improve the general understanding of the effect of association on the vapour-liquid interface. The vapour-liquid interface of the inhomogenous associating fluid is examined by combining the SAFT and density functional theory (DFT) approaches. A simple SAFT-HS representation is employed as it incorporates all of the essential physics of associating fluids and provides a good representation of the vapour pressure and coexisting phases. In this simplified SAFT approach the bulk fluid is represented as a hard-core reference, the association is treated with Wertheim's first order perturbation theory (TPTI), and a van der Waals mean-field approximation is used for the dispersive attractive interactions. In order to keep the representation of the bulk fluid and interface at the same level of approximation we use the van der Waals theory for non-uniform fluids, which is a DFT at the level of a local density approximation (LDA); the correlations are neglected in the attractive non-local term. The vapour-liquid interface of model systems with one, two and four bonding sites are examined for varying degrees of association. As expected, a stronger site-site interaction is generally found to sharpen the interface (decrease the interfacial thickness) and increase the surface tension. In the case of a dimerizing (single site) fluid a limiting behaviour is reached for full association (saturation) where the molecular species are dimers. After an in depth analysis of the effect of association on the vapour-liquid interface, we highlight the strengths of our simple SAFT-DFT approach by making some quantitative comparisons with experimental surface tensions for selected systems including water and replacement refrigerants.