Monomer exchange and concentration fluctuations in poly(ethylene glycol) monoalkyl ether/water mixtures. Dependence upon. nonionic surfactant composition

The ultrasonic absorption spectra between 10(5) Hz and 2 x 10(9) HZ. the so und velocities at different frequencies, and the shear viscosities of aqueo us solutions of the following nonionic surfactants have been measured: C6E4 , C6E5, C7E3, C8E4, C8E5, C10E4, and C12E5. Most CiEj/water systems have be en considered at different temperatures between 12 and 40 degrees C and/or at different concentrations (0.01 mol/L less than or equal to 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 simul taneous presence of various processes. All systems are subject to local flu ctuations in the micelle concentration which are described by the Bhattacha rjee-Ferrell model here. The spectra for the shorter surfactants C6E4, C6E5 , C7E3, and C8E5, as those for C6E3, in addition to the Bhattacharjee-Ferre ll contribution exhibit a lower frequency Hill-type relaxation term which i s attributed to the monomer exchange process. With the C8E4/water and the C 10E4/water systems the monomer exchange equilibrium is reflected by two or one relaxation terms with discrete relaxation time, respectively. In the C1 2E5/water mixture, as a result of the small monomer concentration, contribu tions from the exchange process are missing in the spectra. The long chain micelle solutions (C8E5, C10E4, C12E5) also show contributions from an ultr ahigh-frequency relaxation with relaxation time at around 0.25 ns. it is at tributed to the chain rotational isomerization. The parameters of the diffe rent molecular mechanisms are discussed, particularly in view of the simult aneous action of the monomer exchange and the fluctuations in the micelle c oncentration. Also evaluated is the background contribution to the ultrason ic spectra, yielding a shear viscosity relaxation in the megahertz frequenc y range.