Structural Transition in the Isotropic Phase of the C${}_{12}$EO${}_6$/H${}_2$O Lyotropic Mixture: A Rheological Investigation

Abstract

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.

References in corpus (2)

• Connectivity of the Hexagonal, Cubic, and Isotropic Phases of the C${}_{12}$EO${}_6$/H${}_2$O Lyotropic Mixture Investigated by Tracer Diffusion and X-ray Scattering
• High-frequency rheological behaviour of a multiconnected lyotropic phase