Calculation of surface properties of pure fluids using density gradient theory and SAFT-EOS

The Cahn-Hilliard theory was combined with three equations of state (EOS) ( the Peng-Robinson (PR), the Sanchez-Lacombe (SL) lattice fluid model, and t he original Statistical Associating Fluid Model) in order to describe both the phase behaviour and the surface properties of different types of molecu les, namely nonpolar substances (n-alkanes and aromatic compounds), alcohol s and water. Experimental surface tensions for nonpolar compounds could be correlated ac curately and comparably by all EOS, adjusting one temperature-independent i nfluence parameter. Despite the limitation in the critical region, the Stat istical Association Fluid Theory (SAFT)-EOS successfully predicts the satur ated liquid density and the degree of hydrogen bonding for methanol. The im plementation of the SAFT-EOS into the Cahn-Hilliard framework leads to a us eful possibility to calculate surface tensions which are in satisfactory ag reement with experimental data, if the temperature-independent influence pa rameter was fitted to experimental surface tensions. The non-ideal behaviour of water is reflected in its phase behaviour and al so in its surface tension. Some noticeable improvements of the association model 4C (four association sites) over the model 3B (three association site s) are found for the calculation of liquid-vapour equilibrium, the monomer concentration and the surface tension of water. Unfortunately, the SAFT-app roach for associating compounds did not generally result in accurate calcul ations, especially near the critical region. Under comparable conditions th e SAFT-EOS gives better surface tensions of water at low temperatures than the Associated-Perturbed-Anisotropic-Chain Theory (APACT)-EOS. At high temp eratures, the opposite behaviour was observed. (C) 2000 Elsevier Science B. V. All rights reserved.