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.

Density, ultrasonic velocity, viscosity, and electrical conductance of water/AOT/n-heptane microemulsions as a function of the volume fraction of the dispersed phase (0 < Q < 70) and of the molar ratio water/AOT = R (0 < R < 40) have been measured at 25 O C . The density and the isentropic compressibility of the micellar phase are derived from the experimental data. The volumetric properties, at high R, exhibit a trend toward an enhanced water-like character of the dispersed phase. At low R there is evidence for a structural change in the hydrogen-bonded network of water filling the droplets. Electrical conductivity data show the existence of a percolation threshold depending on R. Below the percolation threshold, the specific conductance qualitatively and quantitatively agrees with the prediction of the droplet charge fluctuation model. Comparison of the viscosity and the electric conductance behavior with 4 indicates that the processes for momentum and charge transfer, even if related to the droplets cluster formation, are different.

In order to elucidate the aggregation phenomena in alcoholic aqueous solutions, we present some experimental results from small-angle neutron scattering (SANS), elastic and quasielastic light scattering and ultrasonic spectroscopy. We find that, by increasing the alkyl chain length of the alcohol, the molecular aggregates change from small fluctuating wateralcohol groups to micelle-like structures. The role of temperature and concentration is also discussed.

By means of ultrasonic and Brillouin light-scattering techniques, we have performed extensive measurements of the attenuation coefficient and sound velocity in 2-butoxyethanol aqueous solutions over a large temperature range. In the more concentrated solutions, the frequency dependence of these properties is well accounted for by a double relaxation time equation thus excluding contributions from concentration fluctuations. This finding, as well as the overall behavior as a function of concentration and temperature, suggests the presence of alcohol aggregates beyond a characteristic concentration-temperature line resembling a critical micelle concentration curve in micellar systems. The similarity between the ultrasonic behavior and the type observed in nonionic surfactant aqueous solutions, as well as indications from other experimental sources, strongly supports the possibility that butoxyethanol aggregates are micellelike structures.

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.