Acoustic and Thermodynamic Properties of Binary Mixtures of Ionic Liquid [Bmim][PF6] and 2-Pyrrolidone from T = (298.15to 323.15)K

The density (p), ultrasonic velocity (u) and refractive index (nD) of the binary mixtures of 1-Butyl 3-Methylimidazolium Hexafluoropasphate (Bmim [PF6]) and 2-pyrrolidone and those of pure liquids were measured using Anton Paar vibrating tube density and sound velocity meter (DSA 5000 M) and Dr. Kernchen Abbemat (Anton Paar, Austria) refractometer over the whole composition range as a function of temperature between 298.15 and 323.15 K in steps of 5K at atmospheric pressure. From the experimental data, the excess values of molar volumes (V_m^E), isentropic compressibility (k_s^E), acousticimpedance (ZE), free length (L_f^E), speeds of sound (uE), and deviations in refractive index (n_D) were calculated and fitted to a RedlichKister type equation. From the values of V_m^E, k_s^E, L_f^E, we observe that the inversion in the sign from negative to positive indicates decrease of interaction between [Bmim] [PF6] and 2-Pyrrolidone with the increase of IL concentration. Negative values of deviation in refractive index suggest that the ion-ion interactions between [Bmim+] and [PF_6^-] ions dominate at higher mole fractions of [Bmim] [PF6] in the mixture.