Application of the UNIFAC model for prediction of surface tension and thickness of the surface layer in the binary mixtures A.A. Rafati a,⇑ , A. Bagheri a , A.R. Khanchi b , E. Ghasemian c , Mojgan Najafi d a Department of Physical Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65174, Iran b Chemistry Research Department, School of Nuclear Science Research, Nuclear Science and Technology Research Institute, Tehran, Iran c Department of Chemistry, Faculty of Science, Ilam University, P.O. Box 69315516, Ilam, Iran d Department of Material Engineering, Hamedan University of Technology, Hamedan, Iran article info Article history: Received 25 September 2010 Accepted 1 December 2010 Available online 5 December 2010 Keywords: Surface tension Prediction Binary system Adsorption Thickness of surface layer abstract Surface properties of binary mixtures of (alkanol with acetonitrile) have been measured by surface ten- sion method at T = 298.15 K and atmospheric pressure. The UNIFAC method is used for calculation activ- ity coefficients of surface and bulk phases. Also, the surface tension has been predicted based on the Suarez method. This method combines a model for the description of surface tension of liquid mixtures with a UNIFAC group contribution method for the calculation of activity coefficient. Two techniques for calculation of molar surface areas, based on Paquette areas and Rasmussen areas are tested. On compar- ing the computed values of surface tension by the present approaches with experimental data, satisfac- tory results have been observed. In addition, the relative Gibbs adsorption and the surface mole fraction have been evaluated using this model. It is possible to calculate the thickness of liquid–vapor interfaces starting from surface tension data. A novel procedure is developed to obtain the thickness of liquid–vapor interfaces as a function of composition in binary systems. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Models for calculation of surface tensions are normally based on the assumption that the surface layer forms a separate phase with constant and uniform composition which is different from the com- position of the adjacent vapor and liquid bulk phases. The models all require pure component surface tensions for each component in the mixture. The models differ in the way in which the molar sur- face areas of the components are calculated and in the assumptions concerning the activity coefficients of the components both in the surface and in the bulk liquid phases [1]. Alcohols, either alone or in solutions (water, acetonitrile), are very widely used in the chem- ical, pharmaceutical and cosmetic industries [2–5]. Several different approaches have been used to predict the sur- face tension of mixtures which include the use of an equation of state together with the gradient theory [6], perturbation theory [7], the parachor [8], and also activity coefficient models such as Wilson, NRTL, and UNIFAC [9–12]. In this paper, we report experimental values of surface tension for binary mixtures of alkanol (methanol, ethanol, 2-propanol and 2-butanol) with acetonitrile obtained using the ring method and compared with predicted surface tension from the Suarez method [1,13]. The model used here to estimate and predict surface tension values for binary systems as a function of concentration and tem- perature relates the surface concentration of each component to the individual activity coefficients (in the liquid bulk phase and surface layer) and to the molar surface area of the components [14]. The activity coefficients were evaluated using the UNIFAC group contribution model [6,15]. Then, this model is used to derive surface mole (volume) fraction, surface activity coefficient and the relative Gibbs adsorption. In a new approach we describe an attempt at indirect measure- ment of the thickness of surface layer in binary mixtures. We com- pare these measurements with calculations of the lengths of molecules based on molecular modeling using Hyperchem soft- ware (MM + molecular mechanics method). In the present commu- nication, we used values of surface and bulk concentration and relative Gibbs adsorption in binary systems for calculation of thick- ness of the surface layer. Generally, a basic trend showing that in low concentration of alcohols (except methanol) the surface thick- ness increased with increasing alcohol concentration and this pro- cess to reach a maximum, then with more increasing alcohol concentration surface thickness decreased. 2. Materials and methods 2.1. Chemicals Acetonitrile (mass fraction > 0.999), methanol (mass frac- tion > 0.999), ethanol (mass fraction > 0.999) and 2-propanol (mass fraction, 0.995), were purchased from Merck and used without 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.12.003 ⇑ Corresponding author. Fax: +98 0811 8257407. E-mail addresses: rafati_aa@yahoo.com, aa_rafati@basu.ac.ir (A.A. Rafati). Journal of Colloid and Interface Science 355 (2011) 252–258 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis