Marzena DZIDA1, Ma?gorzata MUSIA?2,3, Micha? ZOR?BSKI1, Edward ZOREBSKI1
1University of Silesia in Katowice, Institute of Chemistry, Katowice, Poland
2National Institute of Standards and Technology, Associate, Applied Chemicals and Materials Division, Boulder, United States
3University of Colorado, Boulder, United States
The speed of sound is a thermodynamic equilibrium property in the low frequency limit, where speed of sound does not depend on frequency and when the effects of absorption on speed of sound are small i.e. dissipative processes are neglected. The ionic liquids (ILs) are often dissipative or even strongly dissipative media, i.e. dispersion of ultrasound velocity and absorption occur. The results of ultrasound absorption of ILs under atmospheric pressure, which have been obtained up to now suggest that decreasing temperature and/or increasing pressure shift the relaxation region towards lower frequencies for aprotic ILs [1,2]. In addition, for some ILs, the ultrasonic absorption spectra indicate the occurrence of velocity dispersion already at frequencies of transducers operating in conventional ultrasonic devices. In such cases, measured values cannot be regarded as thermodynamic speed of sound (thermodynamic quantity); thus it is not possible to use Newton-Laplace equation. The obtained absorption spectra for ILs show small ratio of experimental to classical absorption approximately equal 2 and negative temperature absorption coefficient [1,2]. The values of classical absorption coefficient can be estimated by Stokes formula using readily available speed of sound, density and viscosity. This allows to determine roughly temperature and pressure range for which both of the above-mentioned conditions are fulfilled and to choose appropriate method of measurement (group or phase velocity method), and consequently to obtain the thermodynamic speed of sound. High pressure ultrasound absorption for ILs has never been measured yet. There are also simple methods of preliminary estimation of correctness of the obtained results of speed of sound. High classical absorption of ILs and large deviations from linearity of determined speed of sound dependence on temperature are indirect prove of existence of ultrasound velocity dispersion [1]. Comparison of group and phase velocity measured in the same sample also gives information about possible dispersion. If specified above experimental conditions are fulfilled, the acoustic method, although it is an indirect one, allows to obtain the most reliable thermodynamic data for the compressed liquids [3] and observe how subtle structural changes in ILs affect speed of sound [4,5].
[1] M. Dzida, E. Zor?bski, M. Zor?bski, M. ?arska, M. Geppert-Rybczy?ska, M. Chor??ewski,
J. Jacquemin, I. Cibulka, Chem. Rev. 2017, 117, 3883.
[2] E. Zorebski, M. Zor?bski, M. Musia?, M. Dzida, J. Phys. Chem. B 2017, 121, 9886.
[3] M. Musia?, M. Zor?bski, M. Dzida, J. Safarov, E. Zor?bski, E. Hassel, J. Mol. Liq. 2019, 276, 885.
[4] M. Dzida, M. Musia?, E. Zor?bski, M. Zor?bski, J. Jacquemin, P. Goodrich, Z. Wojnarowska, M. Paluch, J. Mol. Liq. 2019, 278, 401.
[5] M. Musia?, E. Zor?bski, M. Zor?bski, M. Dzida, J. Mol. Liq. 2020, 113188.