Excess thermodynamic properties of binary mixtures of ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate) with alkoxyalkanols at several temperatures Amalendu Pal a, ⁎, Rekha Gaba a , Tejwant Singh b , Arvind Kumar b a Department of Chemistry, Kurukshetra University, Kurukshetra 136 119, India b Salt and Marine Chemicals Division, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR),G. B. Marg, Bhavnagar 364002, India abstract article info Article history: Received 5 December 2009 Received in revised form 16 March 2010 Accepted 24 March 2010 Available online 29 March 2010 Keywords: Ionic liquid Alkoxyalkanol Binary mixture Density Refractive index Physical properties such as densities, ρ, speed of sound, u, and refractive indices, n, have been measured for the binary mixtures of ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF 6 ] with diethylene glycol monomethyl ether (DEGMME), propylene glycol monomethyl ether (PGMME) and propylene glycol monoethyl ether (PGMEE) over the whole composition range at atmospheric pressure. Experimental densities have been used to estimate excess molar volume V m E . Changes in isentropic compressibility, Δκ S , have been estimated by using experimental speed of sound and density values. Experimental refractive indices values have been used to calculate the deviation in refractive indices, Δ φ n, and molar refraction, Δ x R. Excess properties are fitted to the Redlich–Kister polynomial equation to obtain the binary coefficients and the standard errors. Apparent molar volume and apparent molar compressibility at infinite dilution, V φ,i ∞ and K φ,i 0 have been calculated to gain information about solute–solvent interactions. The results are compared with our earlier studies concerning the binary mixtures of [bmim][PF 6 ] with alkoxyethanol or poly ethers. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Room temperature ionic liquids, ILs [1] are attracting a great interest from the scientific community as a replacement for conventional volatile organic solvents because of their unique physicochemical properties. In recent years these materials have been the focus of many scientific investigations [1–10]. ILs are finding place in many chemical processes such as organic and inorganic synthesis, liquid–liquid separations, and electrochemical applications, etc. [11–13]. Although the characterization of ILs as solvents, or reaction media, has been established for a large number of systems, detailed knowledge on the thermodynamic behaviour of the mixtures of ionic liquids with organic solvents is still limited [14–33]. The detailed knowledge of the thermodynamic behaviour of binary mixtures of ionic liquids with organic solvents is of immense importance for the design of any technological process. On the other hand, alkoxyalkanols are organic solvents that are amphiphilic in nature and have been used in many chemical processes. The range of application and versatility of ILs can be enlarged by dissolving in organic solvents. For potential use of these mixtures in various chemical processes it is of immense importance to understand their mixing behaviour and gather accurate physiochemical data. The thermodynamic properties of mixture of [bmim][PF 6 ] with some organic solvents have been studied by various groups [34–39] however systematic investigations of [bmim][PF 6 ] with alkoxyalkanols are very limited [31]. In continuation to our earlier work, this study examines how the change in molecular structure of alkoxyalkanols affects the volumetric, acoustic and derived properties of binary mixtures involving [bmim][PF 6 ]. Composition and temperature dependent trends in the excess properties, which can offer qualitative assessment of the solution behaviour are presented and analyzed. 2. Experimental 2.1. Materials Ionic liquid, 1-butyl-3-methyl imidazolium hexafluorophosphate, [bmim][PF 6 ](N 98.0 mol.%), was purchased from Fluka. IL obtained was further purified according to procedure described elsewhere [32]. Prior to the preparation of various binary mixtures, the IL was dried and degassed under vacuum at 60 °C for 48 h to remove moisture. Karl– Fisher analysis of the samples indicated that the water content was reduced to less than 200 ppm in IL. Diethylene glycol monomethyl ether (DEGMME), propylene glycol monomethyl ether (PGMME) and propylene glycol monoethyl ether (PGMEE) were of the same origin and purity as used in previous study [40,41]. Purity of the compounds was checked by comparing the density and speed of sound at the desired Journal of Molecular Liquids 154 (2010) 41–46 ⁎ Corresponding author. Tel.: +91 1744 239765; fax: +91 1744 238277. E-mail addresses: palchem@sify.com (A. Pal), rekhagaba@gmail.com (R. Gaba). 0167-7322/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2010.03.018 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq