Fluid Phase Equilibria 227 (2005) 225–238 Simultaneous prediction of interfacial tension and phase equilibria in binary mixtures An approach based on cubic equations of state with improved mixing rules Andr´ es Mej´ ıa a,∗ , Hugo Segura a , Lourdes F. Vega b , Jaime Wisniak c a Departamento de Ingenier´ ıa Qu´ ımica, Universidad de Concepci´ on, Concepci ´ on, Chile b Institut de Ci` encia de Materials de Barcelona (ICMAB-CSIC), Consejo Superior de Investigaciones Cient´ ıficas, Campus de la U. A. B., 08193 Bellaterra, Barcelona, Spain c Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel Received 19 May 2004; received in revised form 25 October 2004; accepted 25 October 2004 Abstract Vapor–liquid interfacial tensions of miscible mixtures have been predicted by applying the gradient theory to an improved Peng–Robinson equation of state. The modified Huron–Vidal mixing rule model has been considered for fitting vapor–liquid equilibrium data of miscible polar and non-polar mixtures and, then, for predicting the interfacial tension of these mixtures. According to results, an accurate and globally stable fitting of the vapor–liquid equilibrium data results on a physically coherent prediction of interfacial tensions in the full concentration range. In addition, we present a criteria based on the geometry of the grand potential function along the interface for assessing the predictive value of the GT. Calculations for subcritical binary mixtures are presented and compared to experimental data and the Parachor method for demonstrating the potential of the unified approach suggested in this work. © 2004 Elsevier B.V. All rights reserved. Keywords: Interfacial tension; Gradient theory; Excess Gibbs energy models; Mixing rules; Vapor–liquid equilibrium 1. Introduction The interfacial tension (σ ) is an important physical property in several industrial applications because the per- formance of many engineering processes depends on the behavior of fluid–fluid and solid–fluid interfaces [1]. Typical processes that exhibit a strong dependence on the magnitude of σ are the adhesion of thin films over surfaces, the stability of foams, and the generation of drops and bubbles. On a larger scale, the efficient recovery of oil from wells, the production of foods and drugs, the quality of paints and anticorrosives, the phase behavior in porous media, etc., constitute additional examples where the interfacial behavior plays a central role. ∗ Corresponding author. Tel.: +56 41204197; fax: +56 41247491. E-mail address: amejia@diq.udec.cl (A. Mej´ ıa). Due to its importance, a theoretical approach able to pre- dict σ as a function of temperature, pressure and concentra- tion is valuable from a practical viewpoint. One of the most successful approaches for fluid mixtures is the gradient the- ory (GT), originally developed by van der Waals in 1894 and reformulated later by Cahn and Hilliard [2]. In their ap- proach Cahn and Hilliard developed a theory that describes a continuous evolution of the Helmholtz energy along the interface, from which σ can be calculated. The physical re- liability of such an approach was checked using the regular solution theory for qualitatively describing the properties of planar interfaces. Since then, several authors have systemat- ically improved the approach for predicting σ , both for pla- nar and curved interfaces. Special attention has been paid to a reliable modeling of the Helmholtz energy and differ- ent equations of state (EOS) have been considered for the purpose [3]. The potential advantage of such an approach is 0378-3812/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2004.10.024