Crossover behavior in micellar solutions with lower critical demixing point: Broadband ultrasonic spectrometry of the isobutoxyethanol-water system

The aggregation kinetics of isobutoxyethanol-water mixtures with lower critical demixing point has been investigated. Two types of kinetics have been observed, a diffusion-controlled formation of micellar species and the formation of a microheterogeneous liquid structure, governed by fluctuations in the local concentration. Ultrasonic attenuation spectra of isobutoxyethanol-water mixtures have been measured between 100 kHz and 2 GHz at 25 degreesC and at several concentrations, covering the complete composition range. With the mixture of critical composition measurements have been performed at some temperatures near the critical temperature (T-c=299.51 K). In addition to the asymptotic high frequency background contribution, the broadband spectra reveal a Bhattacharjee-Ferrell relaxation term due to critical concentration fluctuations, a restricted Hill term reflecting the monomer exchange between micelles and the suspending phase, and two Debye-type relaxation terms that are assigned to chemical relaxations. The relaxation rates of the Bhattacharjee-Ferrell term exceed those from static and dynamic light scattering (amplitude Gamma(0)=5.3x10(9) s(-1)), likely due to the effect of a second parallel pathway of relaxation in the ultrasonic field. The adiabatic coupling constant following from the amplitude in the ultrasonic spectrum agrees with that from a thermodynamic relation (g=1.3). The restricted Hill term displays the features of an extended Teubner-Kahlweit-Aniansson-Wall model of the micelle formation and decay kinetics in surfactant solutions with high critical micelle concentration (C=0.6 mol/l). The idea of a fluctuation controlled monomer exchange in aqueous solutions of poly(ethylene glycol) monoalkyl ether-water mixtures near the critical point is briefly discussed.