Monomer Exchange and Concentration Fluctuations in Poly(ethylene glycol) Monoalkyl Ether/Water Mixtures. Toward a Uniform Description of Acoustical Spectra

The ultrasonic absorption spectra between 100 kHz and 2 GHz of a variety of poly(ethylene glycol) monoalkyl ether/water (CiEj/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 CiEj 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 CiEj (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.

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