Prediction of the vapor-liquid interfacial tension of nonassociating and associating fluids with the SAFT-VR density functional theory

The SAFT-VR DFT Helmholtz free energy density functional [Gloor, G. J.; Jackson, G.; Blas, F. J.; Martin del Rio, E.; de Miguel, E. J. Chem. Phys. 2004, 121, 12740] is used to describe the vapor-liquid interface of nonassociating and associating molecules ranging in size from small molecules to long chains. The functional, which is based on the statistical associating fluid theory for attractive potentials of variable range (SAFT-VR) description of the homogeneous fluid [Gil-Villegas, A.; Galindo, A.; Whitehead, P. J.; Mills, S. J.; Jackson, G.; Burgess, A. N. J. Chem. Phys. 1997, 106, 4168], is constructed by partitioning the free energy density into a reference term (which incorporates all of the short-range interactions and is treated locally) and an attractive perturbation (which incorporates the long-range dispersion interactions). This functional accounts explicitly for the correlations between the segments using a density-averaged correlation function incorporated into the perturbative term in a similar way to that proposed by Toxvaerd. The SAFT-VR DFT formalism is used to examine the vapor-liquid inter-facial tension of a number of pure components, including: n-alkanes; small associating molecules, such as water;linear alkan-1-ols; and some selected replacement refrigerants. In the case of the hydrocarbons and other weakly polar substances, the surface tension can be predicted from intermolecular parameters derived in the usual way from the bulk fluid phase equilibria (vapor-pressure and saturated liquid density). By contrast, when one describes the interfacial properties of associating compounds, it is important to include the surface tension data as well as the bulk vapor-liquid phase equilibria in developing the intermolecular potential model. This provides a means of determining the balance between the dispersive and associative (hydrogen bonding) contributions to the intermolecular potential. The use of interfacial data in the refinement of the potential model allows one to obtain a reliable set of parameters, which can be used to predict the bulk and interfacial properties of mixtures for a broad range of thermodynamic conditions.