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

Abstract The Cahn–Hilliard theory was combined with three equations of state (EOS) (the Peng–Robinson (PR), the Sanchez–Lacombe (SL) lattice fluid model, and the original Statistical Associating Fluid Model) in order to describe both the phase behaviour and the surface properties of different types of molecules, namely nonpolar substances (n-alkanes and aromatic compounds), alcohols and water. Experimental surface tensions for nonpolar compounds could be correlated accurately and comparably by all EOS, adjusting one temperature-independent influence parameter. Despite the limitation in the critical region, the Statistical Association Fluid Theory (SAFT)-EOS successfully predicts the saturated liquid density and the degree of hydrogen bonding for methanol. The implementation of the SAFT-EOS into the Cahn–Hilliard framework leads to a useful possibility to calculate surface tensions which are in satisfactory agreement with experimental data, if the temperature-independent influence parameter was fitted to experimental surface tensions. The non-ideal behaviour of water is reflected in its phase behaviour and also in its surface tension. Some noticeable improvements of the association model 4C (four association sites) over the model 3B (three association sites) are found for the calculation of liquid–vapour equilibrium, the monomer concentration and the surface tension of water. Unfortunately, the SAFT-approach for associating compounds did not generally result in accurate calculations, especially near the critical region. Under comparable conditions the SAFT-EOS gives better surface tensions of water at low temperatures than the Associated-Perturbed-Anisotropic-Chain Theory (APACT)-EOS. At high temperatures, the opposite behaviour was observed.

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