An Equation-of-State Contribution for Polar Components: Dipolar Molecules Joachim Gross Chair for Separation Technology, Delft University of Technology, 2628 CA Delft, The Netherlands Jadran Vrabec Institut fu ¨r Technische Thermodynamik und Thermische Verfahrenstechnik, Universita ¨t Stuttgart, 70550 Stuttgart, Germany DOI 10.1002/aic.10683 Published online October 6, 2005 in Wiley InterScience (www.interscience.wiley.com). Accounting for dipolar interactions in a physically based equation of state (EOS) can substantially improve the modeling of phase equilibria of real mixtures. An EOS contri- bution for dipolar interactions of nonspherical molecules is developed based on a third-order perturbation theory. Molecular simulation data for vapor–liquid equilibria of the two-center Lennard–Jones (2CLJ) plus pointdipole fluid is used to determine model constants of the EOS. The resulting model is compared to simulation data of pure dipolar nonspherical molecules and their mixtures and an excellent agreement is found. The proposed dipole term is applied to real substances with the perturbed-chain statistical associating fluid theory (PC-SAFT) EOS and it is confirmed that accounting for dipolar interactions not only reduces the binary interaction parameter, but also improves the description of pure component and mixture phase equilibria. Literature values for the dipole moment can thereby be used and no further dipole-related pure component parameter has to be adjusted. This constitutes an advantage over earlier approaches, where dipole-related parameters were fitted to pure component data or to mixture data. © 2005 American Institute of Chemical Engineers AIChE J, 52: 1194 –1204, 2006 Introduction Many of the newer thermodynamic models applied in sci- ence and engineering practice are derived from statistical me- chanical fluid theories. The description of long-range interac- tions, whether from polar charge distribution or from permanent (ionic) charges, however, remains challenging. Al- though appropriate theories are available for dilute conditions, the behavior of dense polar or ionic fluids is subject to exten- sive research. Molecular simulations have long since played an important role in evaluating fluid theories but they may also more directly aid in bridging the gap to polar systems, as a previous investigation—among many other examples—target- ing on quadrupolar molecules has shown. 1 There are two prominent routes toward a description of dipolar interactions. One is given through integral equations and the other is through perturbation theories, where a known nonpolar reference fluid is defined and the dipolar contribution to the intermolecular interactions is considered as a perturba- tion. Perturbation theories for polar fluids converge slowly and are thus commonly given as third-order expansions written in a Pade ´ approximation, as first suggested by Stell et al. 2,3 Simple engineering-like expressions for the involved pair correlation integrals were proposed by Rushbrooke et al. 4 for fluids exhib- iting hard repulsion and later, also accounting for ionic charges, by Henderson et al. 5 Gubbins and Twu 6 elaborated multipolar and nonspherical components, and their mixtures, and derived simple expressions for fluids with a Lennard–Jones (LJ) refer- ence potential. Those equation-of-state (EOS) contributions Correspondence concerning this article should be addressed to J. Gross at J.Gross@wbmt.tudelft.nl. © 2005 American Institute of Chemical Engineers 1194 AIChE Journal March 2006 Vol. 52, No. 3