Concentration fluctuations in water–methanol mixtures as studied by ultrasonic absorption

Abstract

Measurements of the ultrasonic absorption and velocity, and viscosity have been carried out in the system methanol–water at 0, 15, and 25 °C at frequencies from 10 to 50 MHz. The data show the existence of two maxima in the excess absorption as a function of composition. The existence of the two maxima are predicted by the Romanov–Solovyev theory relating the absorption to concentration fluctuations. The magnitude of the excess absorption is calculated at 25 and 0 °C from density, vapour pressure, and diffusion data. New experimental data on density and vapour pressure are presented for this system at 0 °C. The calculated excess absorption is in reasonable agreement with the measured. This indicates that any specific localized intermolecular interactions present in this system could be responsible for at best a smaller portion of the excess ultrasonic absorption, as longer range concentration fluctuations account for essentially all of the excess. Ultrasonic data on the systems methanol–octanol and n-propanol–water are also presented in the course of examining the generality of these phenomena.