Monomer exchange and concentration fluctuations of micelles. Broad-band ultrasonic spectrometry of the system triethylene glycol monohexyl ether/water

At four temperatures between 17.5 and 40 degrees C and several surfactant c oncentrations, the ultrasonic absorption spectra in the frequency range fro m 100 kHz to 2 GHz, the sound velocities, and the shear viscosities have be en measured for aqueous solutions of triethylene glycol monohexyl ether. At solute concentrations smaller than or close to the critical micelle concen tration (cmc), the broad-band spectra reveal one relaxation region reflecti ng the formation/decay kinetics of oligomeric species. The spectra of the m ore concentrated solutions show two different relaxations, one subject to a small the other to a broad distribution of relaxation times. The former re laxation is due to the monomer exchange between micelles and the suspending liquid. The parameters of this process largely follow the predictions of t he Teubner-Kahlweit theory, which is based on the Aniansson-Wall model of m icelle kinetics. Close to the cmc, however, the principal relaxation rate s hows a different behavior, which is assumed to also result from the action of oligomeric species. The relaxation time distribution is considered a con sequence of fluctuations in the local micelle concentration. The second rel axation, which extends over a significantly broader frequency range, can be well represented by the Bhattacharjee-Ferrell model of critical concentrat ion 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 fluctuatio ns as to be due to variations in the local concentration of micelles.