  We present measurements of the steady shear viscosity, the longitudinal
elastic modulus and the ultrasonic absorption in the one-phase isotropic liquid
region of the nonionic surfactant C12E8 aqueous solutions. The overall results
support the presence of two separated intervals of concentration corresponding
to different structural properties. In the surfactant-rich region the
temperature dependence of the steady shear viscosity follows an equation
characteristic of glass-like systems. The ultrasonic absorption spectra show
unambiguous evidence of viscoelastic behaviour described by a Cole-Cole
relaxation formula. In the water-rich region the behaviour of the measured
quantities are more complex and reflect the presence of dispersed aggregates
whose size increases with temperature and concentration. An additional low
frequency contribution is also observed, which is ascribed to the exchange of
water molecules and/or surfactant monomers between the aggregates and the bulk
solvent region.
