Rheological properties of polyoxyethylene cholesteryl ether wormlike micelles in aqueous system

Polyoxyethylene cholesteryl ether (ChEO(20)) nonionic surfactant self-assembles into spherical micelles above the critical micelle concentration in water. An ordering of micelles takes place with an increase in surfactant concentration and forms a micellar cubic phase with the space group Pm3n at ∼30%. Cocamid methyl MEA (designated as C-11S) cosurfactant is soluble at the palisade layer of the ChEO(20) micelle as a result; the curvature of the aggregates tends to decrease and favors sphere-to-rod transition. The axial length of the rod increases with C-11S concentration, and after a certain concentration, elongated micelles entangle with each other, forming a transient network of wormlike micelles. Viscosity increases by 5 orders of magnitude. The zero-shear viscosity (η(0)) versus C-11S concentration curve shows a peak, and the position of the peak shifts toward the right (at higher concentration of C-11S) when the concentration of ChEO(20) in water is increased from 10 to 15%. On the other hand, the peak position shifts toward the left with a decrease in the ethylene oxide (EO) chain of the surfactant, i.e., in the ChEO(15) system. Viscosity increases only slightly with a longer EO chain ChEO(30) system, and it does not show any viscoelastic properties. These wormlike micelles exhibited viscoelastic behavior and could be described by the Maxwell mechanical model with a single stress relaxation mode that is sensitive to temperature. Viscosity and relaxation time were first increased and then decreased, but the plateau modulus increased continuously upon heating. These observations revealed that micelles first grew with temperature and then branched. Dynamic rheology and small-angle X-ray scattering (SAXS) further support the rheology data.