Volumetric, ultrasonic and spectroscopic studies of molecular interactions in binary mixtures of 1-butyl-3-methylimidazolium hexafluorophosphate with 2-propoxyethanol at temperatures from 298.15 to 323.15 K T. Srinivasa Krishna a , K.T.S.S. Raju b , M. Gowrisankar c , Anil K. Nain d, ⁎, B. Munibhadrayya e a Department of Physics, Vignan Institute of Technology & Science, Hyderabad 508284, Andhra Pradesh, India b Department of Chemistry, Andhra Loyola College, Vijayawada 520008, Andhra Pradesh, India c Department of Chemistry, J.K.C. College, Acharya Nagarjuna University, Guntur 522006, Andhra Pradesh, India d Department of Chemistry, Dyal Singh College, University of Delhi, New Delhi 110 003, India e Department of Physics, Sri Venkateswara College of Engineering, Bangalore 562157, Karnataka, India abstract article info Article history: Received 6 July 2015 Received in revised form 14 December 2015 Accepted 26 January 2016 Available online xxxx The densities, ρ ultrasonic speeds, u and refractive index n D of the binary mixtures of ionic liquid 1-butyl-3- methylimidazolium hexafluorophosphate [BMIM][PF 6 ] with 2-propoxyethanol, including those of pure liquids, have been measured over the entire range of composition at 298.15, 303.15, 308.15, 313.15, 318.15 and 323.15 K and atmospheric pressure. The IR spectra of these mixtures have also been recorded at 298.15 K. From the experimental data, excess molar volumes, V m E , excess isentropic compressibility, κ s E , deviations in ultra- sonic speed, Δu, excess molar isentropic compressibility, K s,m E , and deviations in refractive indices, Δ ϕ n D were cal- culated and fitted using the Redlich–Kister equation. The partial molar volumes, V m;1 and V m;2 , partial molar isentropic compressibilities, K s;m;1 and K s;m;2 , excess partial molar volumes, V E m;1 and V E m;2 and excess partial molar isentropic compressibilities, K E s;m;1 and K E s;m;2 over whole composition range; partial molar volumes, V ° m;1 and V ° m;2 , partial molar isentropic compressibilities, K  s;m;1 and K ° s;m;2 , excess partial molar volumes, V °E m;1 and V °E m;2 and excess partial molar isentropic compressibilities, K  E s;m;1 and K °E s;m;2 of the components at infinite dilution have also been calculated. The variations of these parameters with composition and temperature have been discussed in terms of intermolecular interactions prevailing in these mixtures. © 2016 Elsevier B.V. All rights reserved. Keywords: Density Ultrasonic speed Refractive index 1-Butyl-3-methylimidazolium hexafluorophosphate Molecular interactions 1. Introduction Ionic liquids (ILs) are a group of organic salts, which result from the combination of organic cations and inorganic anions, and they may be liquid at room temperature. The most commonly studied ILs contain the imidazolium cation with varying heteroatom functionality. In this study, we have used the ionic liquid formed by 1-butyl-3- methylimidazolium ion (BMIM + ) as cation and hexafluorophosphate ion (PF 6 − ) as anion. The addition of a co-solvent can improve the prop- erties of ionic liquids for their effective use in various chemical process- es. Among the various physicochemical properties of ILs, density has probably been the most widely studied because of its high significance and experimental accessibility [1–5]. In recent years the mixtures of ILs with other organic solvents have been the focus of many scientific in- vestigations [6–12]. Physicochemical properties of ethylene glycol derivatives in the aqueous or non aqueous solvents have been widely studied [13–19]. However, the refractive index of binary mixtures con- taining ionic liquids has received considerably little attention [20–23]. The physicochemical properties of mixtures of [BMIM][PF 6 ] with some organic solvents have been studied by various researchers [24–30]. However, the literature review indicates that the systematic volumetric, acoustic, optical and spectroscopic properties of binary mixtures of [BMIM][PF 6 ] with 2-propoxyethanol have not been reported earlier. 2- Propoxyethanol (ethylene glycol monopropyl ether) is useful in print- ing and other specialized coatings applications. Due to its fast evapora- tion and high solubility in water and active solvency make it ideal, for a variety of cleaning applications such as cleaning fluids, paints, coatings, and inks. In the present work, we will be investigating the molecular in- teractions of 1-butyl-3-methylimidazolium ion of [BMIM][PF 6 ] with hy- droxyl (−OH) group in 2-propoxyethanol and how these interactions affect the volumetric, acoustic, optical, spectroscopic properties and their derived parameters of [BMIM][PF 6 ] + 2-propoxyethanol binary mixtures. Journal of Molecular Liquids 216 (2016) 484–495 ⁎ Corresponding author. E-mail address: ak_nain@yahoo.co.in (A.K. Nain). http://dx.doi.org/10.1016/j.molliq.2016.01.085 0167-7322/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq