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

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.

Aggregation phenomena in water-alcohol solutions. Thermodynamic and dynamic studies

“…Details of these experiments are reported elsewhere [7,8,12]. We found that the x, T, and co dependence of c and a in the three investigated solutions are qualitatively similar, but rather anomalous.…”

Possible relation of water structural relaxation to water anomalies

“…The same approach was proposed for C P (T) (26) and α P (T) (25). Calorimetry, ultrasound (US) propagation, and hypersound Brillouin scattering (BS) experiments (24) were used to isolate the anomalous component in the bulk water response functions by measuring each ideal normal contribution separately. All of the apparently diverging functions fit a power-law form (the solid red line) analogous to that of ideal mode coupling theory χ A T ðT Þ ≈ jT =T × − 1j −θ and predict T × ∼ 225 ± 2 K, a value identical to that found in the confined water transport parameters that support the existence of a dynamic crossover (9,13).…”

“…[23][24][25][26][27]. In confined water in which C P (T), ρ(T), and α P (T) have been measurable inside the no-man's land (T ≥ 180 K), there is evidence of very different behavior.…”

“…Scattering experiments on bulk water, from the stable to the supercooled phase (14,18,(21)(22)(23)(24), indicate a strong dispersion, v = v(Q, ω), that on the basis of the previous equations is linked to the compressibilities.…”

“…In this conceptual framework, many studies have produced evidence for the existence of relaxing structures in the liquid, whose size increases as T decreases (14,18,(21)(22)(23)(24), and for their effects on the transport, configurational, and vibrational properties of the system. In the time domain, as T decreases the HB average relaxation time (〈τ〉) strongly increases (up to many orders of magnitude), and the parallel frequency domain is characterized by viscoelasticity and strong dispersions in sound propagation (9,18,19).…”