Synthesis, spectroscopic characterization and acoustic, volumetric, transport and thermal properties of hydroxyl ammonium based ionic liquids

•N-butyl-(N-hydroxyethyl) ammonium based protic ionic liquids (PILs) were synthesized.•Density, speed of sound, electrical conductivity and viscosity were measured for studied PILs.•Transport property data were fitted to Vogel–Tammann–Fulcher (VTF) equation.•FT-IR spectrum was helpful to explain the hydrogen bonding between ions.•Measured and derived properties were analyzed in terms of chemical structure of PILs.

In the present work, solvent-free synthesis of two hydroxyethyl ammonium-based ionic liquids (ILs) at room temperature was carried out namely, N-butyl-(N-hydroxyethyl) ammonium trifluoroacetate ([BHEA][TFA]) and N-butyl-(N-hydroxyethyl) ammonium nitrate ([BHEA][NO3]). The synthesized ionic liquids were characterized by various spectroscopic techniques such as 1H-NMR, 13C-NMR and FTIR. Furthermore, density (ρ), speed of sound (u), electrical conductivity (σ) and viscosity (η) have been measured within the temperature range from T = (303.15 to 343.15) K and at 0.1 MPa pressure. The measured density and viscosity values were fitted to the linear and Vogel–Tammann–Fulcher (VTF) equation, respectively. The temperature dependence conductivity of the measured ILs was fitted to a similar equation type of viscosity (VTF). Furthermore, the refractive index was measured at T = 303.15 K, in turn molar refraction (Rm) and free volume (fV) were calculated using the Lorentz–Lorenz equation. The thermodynamic properties such as thermal expansion coefficient (α), isentropic compressibility (βS) and intermolecular free length (Lf) were calculated by using the experimental values of density and speed of sound. The thermal decomposition temperature (Td) was investigated using TGA analysis. Finally, thermophysical properties of pure ILs were analyzed in terms of different kind of anions present in the ionic liquids.

Graphical abstractDensity, ρ (■) in kg · m−3, speed of sound, u (●) in m · s−1, dynamic viscosity, η (▴) in mPa · s, electrical conductivity, σ (♦) in S · cm−1of [BHEA][TFA] as the function of temperature and at 0.1 MPa pressure.Figure optionsDownload full-size imageDownload as PowerPoint slide