Sound propagation in water–ethanol mixtures at low temperatures. I. Ultrasonic velocity

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Abstract
Ultrasonic velocity measurements in ethanol aqueous solutions from +30 to −40 °C over the entire composition range and in the frequency range 10–70 MHz are presented. The results, in combination with previous determinations up to +80 °C, allow us to follow the peculiar behavior of the adiabatic compressibility of this system in a very large temperature range extending from a region where the anomalies of pure water tend to disappear, down to the supercooled region where these anomalies are noticeably enhanced. A ‘‘normal’’ and an ‘‘anomalous’’ contribution to the isothermal compressibility of water, as derived from these data, is compared with previous evaluations and discussed in the framework of the existing theoretical models. The compressibility of the diluted solutions is interpreted in terms of stabilization of water into ordered, less dense, and more rigid structures around the solute. We show that a simple two‐state model based on this concept and on the hypothesis of a structural breakdown at the concentration of maximum sound absorption can explain the main experimental features of the volumetric properties of these solutions.