Microheterogeneity of monohydric alcohol/water mixtures. Extended view of ultrasonic relaxation

Recently measured broadband ultrasonic spectra of binary mixtures of water with various monohydric alcohols are discussed with respect to precritical behaviour of the liquids. It is shown that the frequency‐dependent excess absorption can indeed be empirically represented by a very few parameters if a special relaxation function is used. Moreover, the coupling of the acoustic field to the heterogeneous structure can be alternatively considered. The interest is particularly focussed on visco‐inertial and thermal conductivity effects which have so far been taken into account in acoustically more heterogeneous systems like emulsions, suspensions, and biological tissues only. Though the shape of the spectra of the alcohol/water mixtures excellently agrees with that predicted by the theories of frequency‐dependent visco‐inertial and thermal conductivity losses some discrepancies in the parameter values result if these theoretical models are fitted to the measured data. It is, however, suggested that such effects, in addition to the previously considered fluctuations in concentration, contribute to the precritical ultrasonic spectra. This view involves not only fluctuations of mass but also special modes of impuls and energy transport to be important at internal interfaces of the micro‐heterogeneous liquids.