Highly viscous wormlike micellar phases formed from the mixed AOT/C14DMAO/H2O system

The phase behavior and rheological properties of an anionic surfactant, bis(2-ethylhexyl) sulfosuccinate (AOT), mixed with a zwitterionic tetradecyldimethylamine oxide (C14DMAO) in aqueous solutions, were studied at different ratios, R = wAOT/(wC14DMAO+wAOT)wAOT/(wC14DMAO+wAOT). When R = 1, the 6.0 wt% AOT solution is two-phase with dense vesicles as the lower phase. With an increase of C14DMAO fraction (decreasing R) at a total concentration of 6.0 wt%, the lower vesicle-phase (Lαv-phase) extends to generate a single Lαv-phase. Then the Lαv-phase turns into a viscoelastic wormlike micellar phase and finally rod-like or spherical C14DMAO micelles. The wormlike micellar solutions (from R = 0.3 to 0.2) are highly viscoelastic, indicating the formation of rigid network structures. The rheological properties of the viscoelastic solutions exhibit a typical Maxwell characteristic at low and intermediate oscillatory frequencies. A pronounced temperature effect on the wormlike micellar structures can be observed by rheological studies. With an increase in temperature, the samples become less structured due to shortening of the micelles. After introducing certain additives, e.g., octanol and divalent metal ions, a transition from wormlike micellar phases to birefringent Lαv-phases was observed.

The wormlike micellar phase forms at low R value in the AOT/C14DMAO/H2O system at room temperature.Figure optionsDownload high-quality image (72 K)Download as PowerPoint slideResearch highlights
► Reports of wormlike micellar phases formed by anionic surfactants are comparatively rare and have not been found in AOT-participated systems.
► The viscoelasticity of the wormlike micellar phases formed in the AOT/C14DMAO/H2O system is very high, which is scarcely found in mixed systems containing anionic surfactants with short tails.
► A phase transition from wormlike micelles to vesicles can be realized through modulating the interactions between the hydrophilic groups and hydrophobic chains by introducing cosurfactants and divalent metal ions.