Composition and Temperature Dependence of Excess Properties of Binary Mixtures of Imidazolium Based Ionic Liquids: II ([C n mim][PF 6 ]) + Propylamine Zuber Vaid 1 • Utkarsh U. More 1 • Ramesh L. Gardas 2 • Naved I. Malek 1 • Sushma P. Ijardar 3 Received: 17 September 2014 / Accepted: 22 November 2014 / Published online: 1 April 2015 Ó Springer Science+Business Media New York 2015 Abstract The experimental data of densities, q, speeds of sound, u, and refractive indices, n D , for pure ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate ([C 4 mim][PF 6 ]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([C 6 mim][PF 6 ]) and 1-octyl-3-methylimidazolium hexafluorophosphate ([C 8 mim][PF 6 ]) and their binary mix- tures with propylamine were measured by an Anton Paar DSA 5000 densimeter and Abbemat 300 refractometer, respectively. The measurements were performed as a function of concentration and at temperatures between 293.15 and 313.15 K and at 0.1 MPa. From these measured values, the excess molar volumes V E m , excess molar isentropic compress- ibilities K E S;m , and refractive index deviations D u n D were derived and correlated with the Redlich–Kister polynomial equation. The values of V E m and K E S;m are negative and decrease with increase in temperature whereas the D u n D are positive and increase with temperature for all studied systems. V E m and K E S;m decrease with increasing alkyl chain length of ILs. The variations in excess properties are discussed with respect to increases in temperature, addition of –CH 2 groups in alkyl chain of ILs and the strength of solute-solvent interac- tions existing in the ILs ? amine mixtures. V E m was also predicted from the Prigogine– Flory–Patterson theory. Keywords Ionic liquid  Excess molar volume  Excess molar isentropic compressibility  Refractive index deviation  Prigogine–Flory–Patterson theory & Naved I. Malek navedmalek@yahoo.co.in & Sushma P. Ijardar sushmaijardar@yahoo.co.in; sushmaijardar@csmcri.org 1 Applied Chemistry Department, S. V. National Institute of Technology, Surat 395 007, India 2 Department of Chemistry, Indian Institute of Technology Madras, Chennai, Tamil Nadu, India 3 Salt and Marine Chemicals Division, CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific & Industrial Research (CSIR), Gijubhai Badheka Marg, Bhavnagar 364 002, India 123 J Solution Chem (2015) 44:718–741 DOI 10.1007/s10953-015-0325-1