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.