Monika Geppert-Rybczyńska

Acoustics as a Tool for Better Characterization of Ionic Liquids: A Comparative Study of 1-Alkyl-3-methylimidazolium Bis[(trifluoromethyl)sulfonyl]imide Room-Temperature Ionic Liquids

Acoustic properties of three (1-ethyl-, 1-butyl-, and 1-octyl-) 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imide room-temperature ionic liquids are reported and discussed. The speeds of sound in RTILs were measured as a function of temperature in the range 288-323 K by means of a sing around method. The densities and isobaric heat capacities were determined from 288.15 to 363.15 K and from 293.15 to 323.15 K, respectively. The related properties, like isentropic and isothermal compressibilities, isobaric coefficients of thermal expansion, molar isochoric heat capacities, and internal pressures, were calculated. It was found that for some ionic liquids, temperature dependence of isobaric coefficients of thermal expansion is small and negative. All investigations were completed by the ultrasound absorption coefficient measurements in 1-ethyl- and 1-octyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imide as a function of frequency from 10 to 300 MHz at temperatures 293.15-298.15 K. The ultrasound absorption spectra indicate relaxation frequencies in the megahertz range.

Speed of Sound and Ultrasound Absorption in Ionic Liquids

A complete review of the literature data on the speed of sound and ultrasound absorption in pure ionic liquids (ILs) is presented. Apart of the analysis of data published to date, the significance of the speed of sound in ILs is regarded. An analysis of experimental methods described in the literature to determine the speed of sound in ILs as a function of temperature and pressure is reported, and the relevance of ultrasound absorption in acoustic investigations is discussed. Careful attention was paid to highlight possible artifacts, and side phenomena related to the absorption and relaxation present in such measurements. Then, an overview of existing data is depicted to describe the temperature and pressure dependences on the speed of sound in ILs, as well as the impact of impurities in ILs on this property. A relation between ions structure and speeds of sound is presented by highlighting existing correlation and evaluative methods described in the literature. Importantly, a critical analysis of speeds of sound in ILs vs those in classical molecular solvents is presented to compare these two classes of compounds. The last part presents the importance of acoustic investigations for chemical engineering design and possible industrial applications of ILs.