ULTRASONIC AND HYPERSONIC RELAXATIONS OF MONOHYDRIC ALCOHOL WATER MIXTURES

Ultrasonic absorption spectra have been measured between 150 kHz and 1.6 GHz for mixtures of water with ethanol, the propanol isomers, and all isomers of butanol at 25-degrees-C. With the exception of the systems exhibiting a miscibility pp (n-butanol, isobutyl alcohol, sec-butyl alcohol), the measurements have been performed over the complete composition range. Various relaxation functions have been fitted to the measured spectra, among them such reflecting stoichiometrically well-defined chemical reactions and, alternatively, functions based on conception of fluctuations in the composition of the binary liquids. The frequency dependent excess absorption of the complete set of mixtures can be well represented assuming a sum of a Debye-type term with discrete relaxation time and a Fixman-Kawasaki term. The latter reflects a distribution of relaxation times as resulting from concentration fluctuations including critical behavior. The parameters of these relaxation terms are discussed to yield information on the underlying molecular mechanisms. One result is the finding of the fluctuation correlation length in the completely miscible systems to not exceed some molecular diameters. This result is important for our idea of the molecular behavior of these liquids. It is also relevant to our discussion of frequency-dependent measurements of nuclear magnetic relaxation rates and also of dielectric spectroscopy.