Viscoelastic behavior of aqueous solutions of a polyoxyethylene-nonionic-amphiphile surfactant

We present extensive measurements of shear viscosity, sound velocity, and ultrasonic absorption in aqueous solutions of the polyoxyethylene nonionic amphiphile C₁₂H₂₅(OCH)₂(CH₂)₆OH (i.e., C₁₂E₆) in a wide temperature range (5--55 ^∘{}C) and in a concentration range extending from the critical one up to the hexagonal mesomorphic phase. The static shear viscosity rapidly increases with concentration, and presents maxima as a function of temperature. The sound velocity exhibits an apparently complex behavior that can be interpreted as being due to a regular elastic contribution from the micellar moiety. Data on sound absorption in most concentrated solutions show unambiguous evidence of a viscoelastic behavior. Despite the qualitative similarity of our results with those observed in microemulsions of reversed micelles, the underlying interpretation is different. In particular, our analysis leads us to conclude that percolation phenomena due to increased connectivity cannot explain the sound velocity behavior. Rather, the sound velocity behavior can be related to the formation of long micellar structures which, on increasing temperature, become softer, likely resembling entangled polymeric solutions in a good solvent.