Comparative study of physicochemical properties of two imidazolium-based bis(trifluoromethanesulfonyl)imides functionalized with a hydroxyl or ether group in the cation, based on ultrasound speed and ultrasound absorption spectroscopy measurements

Ether- and hydroxyl-functionalized in the cation ionic liquids (ILs) are task-specific ILs with attractive properties and applications. However, the influence of both functional groups on the physicochemical, structural, or thermodynamic properties, as well as the accurate determination of key thermodynamic properties still remains a challenge. We report, for the first time, ultrasound speed and absorption data for two aprotic, single-site, in the cation functionalized ILs, namely : 1-(2-hydroxyethyl)-3-methylimidazolium and 1-(2-methoxyethyl)-3-methylimidazolium bis (trifluoromethanesulfonyl)imides. In both cases, the ultrasound absorption spectra (frequency and temperature range 10–250 MHz and 293.15–323.15 K, respectively) resemble those observed for non-functionalized 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. However, absorption magnitude in non-relaxation low frequency region increase significantly in the case of functionalization with a hydroxyl group and much less in the case of ether group; at 293.15 K, by about 140 % and by about 40 %, respectively. Moreover, spectra show frequency and temperature dependencies (in both cases negative). Additionally, the density, isobaric specific heat capacity, thermal conductivity, kinematic and dynamic (shear) viscosity across a wide temperature range of the chosen ILs were investigated. Based on achieved results and acoustic method all related thermodynamic parameters (i.a. compressibility and isochoric heat capacity) were indirectly determined, and conclusions about the internal structure of the researched ILs were made. The functionalization, by hydroxyl or ether group, efficiently changes the properties in relation to non-functionalized base IL, causing also an increase in ultrasound speed, viscosity, heat capacity, and internal pressure for both ILs. Generally, the hydroxyl group alters the structure more noticeably (most probably by formation of additional hydrogen bonds between –OH groups of cations) than the ether group. Conversely, no effect of functionalization on thermal conductivity was observed. • Study of ultrasound speed and absorption in single-site functionalized ionic liquids. • Consistent thermodynamic data were indirectly determined by acoustic measurements. • Hydrogen bonds effectively dominate properties of ionic liquids. • Hydroxyl groups modify ionic liquids' macroscopic properties more than ether groups. • Only the thermal conductivity is not affected by cation functionalization.