A Density Functional Theory for Vapor–Liquid Interfaces of Mixtures Using the Perturbed-Chain Polar Statistical Associating Fluid Theory Equation of State

A density
functional theory based on the perturbed-chain polar
statistical associating fluid theory (PCP-SAFT) is extended to mixtures.
The Helmholtz energy functional is suitable for inhomogeneous fluid
phases and is here applied to vapor–liquid systems. The attractive
branch of the van der Waals attractions are treated with a perturbation
theory of first order in a nonmean field approach. The radial distribution
function is considered in the Percus–Yevick closure for chain
fluids with a three-fluid theory approach. This perturbation term
is brought to consistency with the PCP-SAFT equation of state using
the approach of Gloor et al. [Gloor, G. J.; Jackson, G.; Blas, F.
J.; del Rio, E. M.; de Miguel, E. J. Chem. Phys. 2004, 121, 12740]. The approach is applied
to vapor–liquid interfaces of mixtures and the resulting surface
tension is compared to experimental data. Binary mixtures with two
subcritical components as well as mixtures with one species above
its critical point, for the considered temperature, are considered.
The density functional theory is applied to these systems with unaltered
pure component parameters of the PC-SAFT or PCP-SAFT model. The binary
interaction parameter was determined to bulk-phase vapor–liquid
data (temperature, pressure, composition) only. Excellent agreement
was found for the surface tension of mixtures without any model parameter
adjusted to interfacial properties.