Sound propagation in water–ethanol mixtures at low temperatures. II. Dynamical properties

Publisher Website
Google Scholar
Abstract
Measurements of the ultrasonic absorption coefficient over the entire concentration range of ethanol aqueous solutions in the frequency range 10–250 MHz and for a temperature interval extending from +30 to −40 °C are presented. Large peaks, which increase noticeably as temperature and frequency decrease, are observed in the attenuation vs composition plot. A detailed analysis of these results and of the velocity dispersion data shows that the frequency spectra are better accounted for by the concentration fluctuations theory of Romanov and Solov’ev (RS) which assumes a distribution of relaxation times rather than by the quasichemical theories which consider single or double relaxation processes. We also show that the absorption and velocity expressions as given by RS can be put in such a form to allow the experimental data as a function of concentration, temperature, and frequency to be displayed on two single ‘‘universal’’ plots. An analysis of the extracted RS theory parameters indicates that the large attenuations are mainly related to the small mutual diffusion coefficients of the solutions. At a molecular level the observed dynamical behavior is associated to the forming and breaking up of alcohol–water aggregates whose lifetimes at the peak concentration range from about 10−9 to 10−7 s on going from higher to lower temperatures.