1 Surface Tension of Binary Mixtures Including Polar Components Modeled by the Density Gradient Theory Combined with the PC-SAFT Equation of State 1 Václav Vinš 2 , Barbora Planková, Jan Hrubý Institute of Thermomechanics AS CR, v. v. i., Dolejškova 1402/5, 182 00 Prague 8, Czech Republic Abstract In this study, we use the Cahn-Hilliard density Gradient Theory (GT) for predicting the surface tension of various binary mixtures at relatively wide temperature ranges and test the application of the GT for predictions of homogeneous nucleation. The GT was combined with two physically based equations of state (EoS), namely the Perturbed-Chain (PC) Statistical Associating Fluid Theory (SAFT) and its modification for polar substances the Perturbed- Chain Polar (PCP) SAFT. The GT applied to the planar phase interface was employed to predict the interfacial tension for various quadrupolar (CO 2 and benzene) and dipolar (difluoromethane, i.e. R32; pentafluoroethane, i.e. R125; and 1,1,1,2-tetrafluoroethane, i.e. R134a) substances and for five binary mixtures including polar components (n-decane + CO 2 , benzene + CO 2 , R32 + R125, R32 + R134a, R134a + R125). The PCP-SAFT EoS combined with the GT provides more accurate results for both the quadrupolar and dipolar substances than the original PC-SAFT EoS. Besides the planar phase interface, the GT was also applied to the spherical phase interface simulating a critical cluster occurring in homogeneous nucleation of droplets. Carbon dioxide was considered, because it has a relatively high quadrupole moment and because of its relevance to natural gas processing. Application of the PCP-SAFT EoS provides a significant improvement compared to the PC-SAFT EoS, and it is clearly superior to the 1 Paper presented at the 19 th European Conference on Thermophysical Properties, August 28 – September 1, 2011, Thessaloniki, Greece 2 Corresponding author, email: vins.vaclav@seznam.cz, telephone: +420 266 053 152, fax: + 420 286 584 695