At four temperatures between 17.5 and 40 °C and several surfactant concentrations, the ultrasonic absorption spectra in the frequency range from 100 kHz to 2 GHz, the sound velocities, and the shear viscosities have been measured for aqueous solutions of triethylene glycol monohexyl ether. At solute concentrations smaller than or close to the critical micelle concentration (cmc), the broad-band spectra reveal one relaxation region reflecting the formation/decay kinetics of oligomeric species. The spectra of the more concentrated solutions show two different relaxations, one subject to a small the other to a broad distribution of relaxation times. The former relaxation is due to the monomer exchange between micelles and the suspending liquid. The parameters of this process largely follow the predictions of the Teubner−Kahlweit theory, which is based on the Aniansson−Wall model of micelle kinetics. Close to the cmc, however, the principal relaxation rate shows a different behavior, which is assumed to also result from the action of oligomeric species. The relaxation time distribution is considered a consequence of fluctuations in the local micelle concentration. The second relaxation, which extends over a significantly broader frequency range, can be well represented by the Bhattacharjee−Ferrell model of critical concentration fluctuations. For the solution of critical composition, the relaxation rates of local energy fluctuations derived from the ultrasonic spectra are confirmed by the (static) shear viscosity data. We discuss these fluctuations as to be due to variations in the local concentration of micelles.

The ultrasonic absorption spectra between 100 kHz and 2 GHz of a variety of poly(ethylene glycol) monoalkyl ether/water (C i E j /H2O) mixtures are discussed to indicate common trends in the nonionic surfactant solutions. The systems considered in this study include 13 surfactants with different alkyl chains and/or number of ethylene glycol groups, extending from C2E1 to C12E5. Despite a multitude of different shapes of the ultrasonic spectra, depending on the C i E j molecule, the temperature, and the mixture composition, it is found that all spectra can be analytically represented by a linear superposition of some relaxation spectral terms. Particularly interesting are the term that is related to the monomer exchange between micelles and the suspending phase and the terms that reflect the fluctuations in the local concentration near a critical demixing point. The parameters of the latter reveal a mechanism of fluctuation controlled monomer exchange. Its relaxation rate is largely controlled by diffusion; its amplitude is correlated with the monomer concentration of surfactants. The treatment includes solutions of low molecular weight C i E j (i ≤ 4) which are not normally considered micellar systems, for which (rather high) critical micelle concentration values may be derived from the ultrasonic absorption data.