Prediction of surface tension of ionic liquids by molecular approach Seyyed Alireza Mirkhani a, ⁎, Farhad Gharagheizi a, ⁎, Nasrin Farahani b , Kaniki Tumba c a Department of Chemical Engineering, Buinzahra Branch, Islamic Azad University, Buinzahra, Iran b Department of Chemistry, Buinzahra Branch, Islamic Azad University, Buinzahra, Iran c Department of Chemical Engineering, Mangosuthu University of Technology, Durban, South Africa abstract article info Article history: Received 23 October 2012 Received in revised form 21 November 2012 Accepted 23 November 2012 Available online 12 December 2012 Keywords: Surface tension QSPR model Ionic liquids LSSVM Validation techniques Originally, Quantitative Structure Property Relationship (QSPR) models for the surface tension of ionic liquids are developed based on molecular descriptors. A large data set of 930 experimental surface tension data points for 48 ionic liquids is applied to derive the model. Seven descriptors are selected by genetic function approximation to relate the surface tension of ionic liquids to their corresponding anions and cation structures. To capture the nonlinear nature of surface tension, a model based on Least-Squared Supported Vector Machine (LSSVM) is also developed. The derived models are authenticated with several statistical validation techniques. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Recently, ionic liquids (ILs) as the new generation of conductive materials find their way in the innovative industrial and chemical appli- cations. The term ionic liquid or more specifically room temperature ionic liquid (RTIL) refers to the salts entirely composed of ions and have a melting point below the normal boiling point of water. Same as other salts, ionic liquids possess very negligible vapour pressures even at conditions well above room temperature [1]. In addition, low-toxicity as well as non-volatility is the other novel characteristics of ILs. By possessing aforementioned qualities, ionic liquids are promising candi- dates to supersede convenient organic solvents in industrial applications. Ionic liquids consisted of low-symmetry, large and unreactive cation such as phosphorus or sulphur containing ring and an anion that largely controls its physical and chemical properties. Altering in the cation and anion combinations permits the physical, chemical and biological prop- erties of ionic liquids to be tailored for specific applications, as largely manifested by the task-specific ionic liquids (TSILs). One of the recent applications of ILs is their employment in the processes such as extraction and multiphasic homogeneous catalytic reactions which are mainly governed by interfacial phenomena. The mentioned type of reactions occurs in the two-phase systems; one phase contains the reactant and products and the other, the immiscible one, act as the catalyst solvent. Such processes occur at the interface between the IL and the overlying aqueous or organic phase, and are dependent on the accessibility of the material to the surface and the transfer of material across the interface. More exhaustive studies of the surface-related properties are required to enhance our insight into the mechanisms behind these processes. The available surface tension data of ionic liquids are very limited in comparison of possible ionic liquids (≈10 15 ). To “tailor” an ionic liquid with the desired properties, it is important to have a rational estimation of its properties e.g. surface tension prior to the synthesis in the absence of the experimental data. For this purpose, prediction methods which provide accurate estimations of desired properties are essential. Models based on parachors, group contribution methods or corre- sponding state theories (CST) are widely applied for the prediction of the surface tension of the ionic liquids [2]. The foundation of parachor-based models are an empirical formula originally proposed by MacLeod [3], which relates density to the surface tension via temperature-independent Eq. (1): σ 1 4 ¼ K ρ ð1Þ Sugden [4] modified the original formula by multiplying each side of the expression by the molecular weight, M w , to give a constant K.M w labelled as parachor, P ch : P ch ¼ KM W ¼ M W ⋅σ 1 4 ρ : ð2Þ Sugden [4] also proposed that the parachor of a compound is an additive property which can be described by the sum of its parachor contribution. Journal of Molecular Liquids 179 (2013) 78–87 ⁎ Corresponding authors. Fax: +98 21 77 92 65 80. E-mail addresses: seyyed.alireza.mirkhani@gmail.com (S.A. Mirkhani), fghara@gmail.com (F. Gharagheizi). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2012.11.018 Contents lists available at SciVerse ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq