Structural Transition in the Isotropic Phase of the C12EO6/H2O Lyotropic Mixture: A Rheological Investigation

We study the structural changes occurring in the isotropic phase of the C 12 EO 6 /H 2 O lyotropic mixture (up to 35% surfactant weight concentration) upon increasing the concentration and temperature, from small individual micelles to an entangled network which subsequently becomes connected. High-frequency (up to ω = 6 × 10 4 rad/s) rheological measurements give us access to the viscoelastic relaxation spectrum, which can be well described by the sum of two Maxwell models with very different temperature behaviors: the slower one (τ 1 ≈ 10 - 4 s) is probably due to reptation, and its associated viscosity first increases with temperature (micellar growth) and then decreases after reaching a maximum (appearance of connections). The fast mechanism (τ 2 ≈ 10 - 6 s) remains practically unchanged in temperature and can be related to the relaxation of local micellar order, as observed at higher concentration in a previous investigation. This interpretation is confirmed by additional measurements in aqueous mixtures of the related surfactant C 12 EO 8 (which forms smaller micelles), where only the fast mechanism related to local order is detected.