Surface tension of chain molecules through a combination of the gradient theory with the CPA EoS

Despite the interest in systems containing non-associating compounds such as alkanes and fluoroalkanes or associating compounds like alkanols, their vapor-liquid interfaces have received little quantitative attention. Aiming at modeling the interfacial tensions of several families of chain molecules, a combination of the density gradient theory of fluid interfaces with the Cubic-Plus-Association (CPA) equation of state was developed. The density gradient theory is based on the phase equilibria of the fluid phases separated by the interface, for what an adequate equation of state is required. In this work, the series of the n-alkanes, n-perfluoroalkanes and n-alkanols were studied in a broad temperature range. It will be shown that even for non-associating compounds such as the n-alkanes, CPA can improve considerably the estimation of vapor pressures and coexisting phase densities, which are important for the accuracy of the surface tensions obtained through the density gradient theory. The surface tensions of the compounds studied were accurately described with a global average deviation of 0.5%. As it will be shown, the combination of the density gradient theory with the CPA EoS also allowed very good predictions of the surface tensions of some binary mixtures. A discussion on the regression and selection of pure component parameters is also reported. (C) 2008 Elsevier B.V. All rights reserved.