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

TL;DR

This study investigates how the structure and viscoelastic properties of a C12EO6/H2O lyotropic mixture change with concentration and temperature, revealing micellar growth, network formation, and distinct relaxation mechanisms through rheological measurements.

Contribution

It provides a detailed rheological analysis of structural transitions in the isotropic phase of the mixture, identifying two relaxation processes and their temperature behaviors, which was not previously characterized.

Findings

Identification of two Maxwell relaxation mechanisms with different temperature dependencies.

Reptation likely causes the slower relaxation process.

Fast relaxation related to local micellar order remains temperature-independent.

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.