The absorption spectra and velocity of ultrasonic longitudinal waves are reported for some aqueous solutions of the triethylene glycol monohexyl ether (${\mathrm{C}}_6$${\mathit{E}}_3$) nonionic surfactant, with special emphasis on the region near the consolute point. The excess ultrasonic absorption spectra from 5 to 160 MHz are satisfactorily fitted by an equation with two relaxation times. Samples near the consolute point did not show the absorption and velocity anomalies as expected in binary critical mixtures. The sound absorption behavior in both micellar and critical samples is discussed from the point of view of the existing aggregate equilibria and of the current theories for critical sound propagation, respectively. The peculiar concentration dependence of the sound velocity at different temperatures is explained as being due to a competition between the compressibility of water and the apparent compressibility of the surfactant.

• Received 4 April 1994

DOI:https://doi.org/10.1103/PhysRevE.50.4817