Monomer Exchange and Concentration Fluctuations in Poly(ethylene glycol) Monoalkyl Ether/Water Mixtures. Dependence upon Nonionic Surfactant Composition

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 104; no. 21; pp. 4846 - 4856
• Main Authors: Telgmann, T, Kaatze, U
• Format: Journal Article
• Language: English
• Published: American Chemical Society 01.06.2000
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp994159d

The ultrasonic absorption spectra between 105 Hz and 2 × 109 Hz, the sound velocities at different frequencies, and the shear viscosities of aqueous solutions of the following nonionic surfactants have been measured:  C6E4, C6E5, C7E3, C8E4, C8E5, C10E4, and C12E5. Most C i E j /water systems have been considered at different temperatures between 12 and 40 °C and/or at different concentrations (0.01 mol/L ≤ c < 0.33 mol/L). The results are compared to those for the triethylene glycol monohexyl ether/water (C6E3/H2O) system reported previously. At solute concentrations around the critical micelle concentration the ultrasonic spectra show one relaxation term due to the formation/decay kinetics of oligomeric species. At higher surfactant content much more complicated spectra reveal the simultaneous presence of various processes. All systems are subject to local fluctuations in the micelle concentration which are described by the Bhattacharjee−Ferrell model here. The spectra for the shorter surfactants C6E4, C6E5, C7E3, and C8E5, as those for C6E3, in addition to the Bhattacharjee−Ferrell contribution exhibit a lower frequency Hill-type relaxation term which is attributed to the monomer exchange process. With the C8E4/water and the C10E4/water systems the monomer exchange equilibrium is reflected by two or one relaxation terms with discrete relaxation time, respectively. In the C12E5/water mixture, as a result of the small monomer concentration, contributions from the exchange process are missing in the spectra. The long chain micelle solutions (C8E5, C10E4, C12E5) also show contributions from an ultrahigh-frequency relaxation with relaxation time at around 0.25 ns. It is attributed to the chain rotational isomerization. The parameters of the different molecular mechanisms are discussed, particularly in view of the simultaneous action of the monomer exchange and the fluctuations in the micelle concentration. Also evaluated is the background contribution to the ultrasonic spectra, yielding a shear viscosity relaxation in the megahertz frequency range.