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 C12EO6/H2O 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 × 104 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 (τ1 ≈ 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 (τ2 ≈ 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 C12EO8 (which forms smaller micelles), where only the fast mechanismrelated to local orderis detected.