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Density (ρ), refractive index (n D ), and speed of sound (u) values are measured for the binary mixture of 1butyl-3-methylimidazolium tetrafluoroborate (1) and N-vinyl-2-pyrrolidinone (2) over the entire range of mole fractions at temperatures from T = (298.15 to 323.15) K under atmospheric pressure. Using the basic experimental data, various acoustic and thermodynamic parameters are calculated and are discussed in terms of molecular interactions between the present investigated binary system. The excess values are fitted to the Redlich−Kister polynomial equation to estimate the binary coefficients and standard deviation between the experimental and calculated values. Further, the molecular interactions in the binary mixture system are analyzed using the experimental FT-IR spectrum recorded at room temperature.

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Density, speed of sound, and viscosity values were measured for different binary mixture concentrations of 1-butyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim][NTf 2 ]) + N-methylaniline (NMA) and 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][Pf 6 ]) + NMA systems at temperatures from T = 298.15 to 323.15 K under atmospheric pressure. The excess values of molar volume, molar isentropic compressibility, intermolecular free length, and viscosity were calculated. The partial molar volumes and partial molar isentropic compressibilities at infinite dilutions were also estimated. The Redlich−Kister equation was utilized to evaluate coefficients and standard deviations. The mechanism of effective packing between [Bmim][NTf 2 ]/ [Bmim][Pf 6 ] and NMA was represented by the negative excess molar volumes. In both the studied systems, the occurrence of strong chemical interactions was represented by the negative excess intermolecular free length values. The present work clearly showed that the simultaneous analysis of excess properties could provide a comprehensive description of molecular interactions occurring in the binary mixtures, specifically if the component molecules have different sizes. Further, the high viscosities of the ionic liquids were decreased by mixing with low viscous NMA. Hence, the investigated systems could be more suitable for their possible applications in the transport mechanism and absorption of gasses.

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