Absorption and Dispersion of Ultrasonic Waves

Abstract

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Ultrasonic Relaxation Study of 1-Alkyl-3-methylimidazolium-Based Room-Temperature Ionic Liquids: Probing the Role of Alkyl Chain Length in the Cation

“…Although a direct comparison of the obtained ultrasound absorption spectra was not possible, the comparison with the ultrasound velocity dispersion reported by Fukuda et al 18 1 This type of behavior has been observed in the past at similar temperatures for molecular organic liquids as well; 23,49,50,55 however, it should be noted that a separate determination of the shear viscosity behavior requires a separate study of the shear ultrasound wave because ultrasound relaxation spectra determined using a longitudinal wave are not suitable to investigate the microscopic origin of the shear viscosity (there are difficulties in the separation of the contributions of volume and shear viscosities to the ultrasonic relaxation). It is a general agreement that if α < α cl , the shear viscosity relaxation must be present.…”

“…In these highly attenuating conditions as well as in the border regions, particular care must be exercised because the relaxation processes cause not only absorption but also dispersion. 55,56 In the case of the four measured RTILs, the lowest f S8). Since (i) the relaxation frequency associated with the ultrasound absorption decreases with decreasing temperature (and vice versa), (ii) the α/α cl is small (in the range 1−3) and practically independent of temperature, and (iii) the temperature coefficient of ultrasound absorption is negative, the close similarity of ultrasound absorption in RTILs studied and associated molecular liquids is observed (the similarity was suggested already in our previous reports 1,2 ).…”

“…It is a general agreement that if α < α cl , the shear viscosity relaxation must be present. 1,2,5,23,49,50,55 Assuming that the shear viscosity relaxation is present and taking into account the simple Maxwell model, the real part η′(ω) of the complex (relaxing) shear viscosity can be expressed as…”

“…In other words, this is the same group according to liquid classification in relation to ultrasound absorption. 55,56 Since the relaxation frequency decreases with increasing pressure in associated molecular liquids, 55,57 we can suppose with the great probability that both RTILs studied to date and generally all aprotic RTILs behave similarly. Obviously, it is only supposition, but it is plausible.…”

“…3,58 In other words, the lower the temperature the higher the increase of the speed of sound with decreasing temperature (concave up curve) is observed, and it is typical behavior in dispersion regions. 54,55 In the case of [C 8 mim]-[PF 6 ], an additional argument for the existence of a relaxation region in the range of several MHz is connected with the shear relaxation spectra which indicate, for instance, the shear relaxation frequency is already around 20 MHz at 298.15 K. 44 Therefore, in practice it is always necessary to exercise caution whether the measured speed of sound in the given experimental conditions (lower temperature and/or high pressure) can be Because of the relatively large ultrasonic absorption in many RTILs, ultrasonic velocity is subject to a considerable dispersion. Fortunately, from the ultrasound absorption data, the frequency dependence of the ultrasonic velocity can be estimated.…”