On the Kinetics of the Formation of Small Micelles. 1. Broadband Ultrasonic Absorption Spectrometry

At 25 °C the ultrasonic absorption spectra between 100 kHz and 2 GHz, the sound velocities, and the shear viscosities of aqueous solutions of n -heptylammonium chloride have been measured at the critical micelle concentration (cmc) and also at concentrations above the cmc. The term cmc denotes a concentration range rather than a well-defined value here. The absorption spectra reveal two relaxation regions, one at frequencies around a few megahertz and the other one at around 1 GHz. The low-frequency relaxation may be attributed to the exchange of surfactant monomers between the micelles and the suspending phase. In contrast to the theoretical predictions from the accepted models of the kinetics of micelle formation, however, the low-frequency process is subject to a relaxation time distribution. The width of the distribution function is particularly large near the cmc. Close to the cmc, in addition, the relaxation time and relaxation amplitude do not follow the theoretically predicted dependences upon concentration. In conformity with previous findings, the high-frequency relaxation seems to be partly due to the mechanism of chain isomerization in the micellar cores. However, estimations show that the process of rotational isomerization cannot fully account for the measured facts. It is assumed that, at least at solute concentrations in the cmc region and slightly above the cmc, a variety of geometrically less defined supramolecular structures exist in the solutions of amphiphiles rather than globular micelles that follow a comparatively narrow size distribution. This idea is supported by the shear viscosity data.