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 $\omega = 6 \times 10^4$ 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 ($\tau _1 \simeq 10^{-4}$ 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 ($\tau _2 \simeq 10^{-6}$ 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.