Investigation on thermophysical and excess properties of binary mixtures of imidazolium based ionic liquids at temperatures (293.15 to 323.15) K: III [Cnmim][PF6] (n = 4, 6, 8) + THF

• Density, speed of sound and refractive index were measured for ionic liquid mixtures.
• Derived volumetric, acoustic and optical properties were calculated from the measured ρ, u and nD respectively.
• Redlich–Kister polynomial was used to fit the excess functions.
• Excess molar volume estimated using the PFP theory.
• Refractive index mixing rules were applied to the experimental refractive index.

As part of ongoing research focusing on volumetric, acoustic and optical properties of non-aqueous binary liquid mixtures including ionic liquids, here we report, densities, ρ, speeds of sound, u   and refractive indices, nDnD of room temperature ionic liquids (RTIL) and their binary mixtures with tetrahydrofuran (THF) over the entire composition regime within the temperature range of (293.15 to 323.15) K at 5 K intervals at atmospheric pressure. The ILs studied here were, 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]), 1-hexyl-3-methyl imidazolium hexafluorophosphate ([C6mim][PF6]) and 1-octyl-3-methylimidazolium hexafluorophosphate ([C8mim][PF6]). The excess molar volumes, VmE and excess molar isentropic compressibilities, KS,mE are negative whereas refractive index deviations, ΔϕnD   are positive over the entire mole fraction range and at all temperatures studied. The excess properties were fitted to the Redlich–Kister polynomial equation by the method of least squares with all points weighed equally to derive the binary coefficients and to estimate the standard deviation. The effect of alkyl chain length of ionic liquids on the excess properties has been discussed in terms of interstitial accommodation and ion–dipole interaction between the components of the mixtures. Refractive indices of the mixtures were predicted through several mixing rules and compared with the experimental values by means of the standard deviation (SD) which are in excellent agreement. The Prigogine−Flory−Patterson (PFP) theory has been applied to interpret the VmE results.