Unusual viscoelastic behavior of aqueous solutions of fluorocarbon–hydrocarbon hybrid surfactant and its morphological transformations

The mechanisms of the viscoelastic behavior and morphological transformations of aggregates of aqueous solutions of a sulfate-type hybrid surfactant (F8EH3OS), whose molecules have a hydrocarbon chain (propyl group) and a fluorocarbon chain (perfluorooctyl group), have been investigated. A 160 mM aqueous solution of F8EH3OS is a transparent gel and exhibits high viscoelasticity. The zero-shear viscosity of aqueous F8EH3OS solutions is maximum at a concentration of 160 mM. The frequency dependence of the dynamic viscoelasticity and freeze-fracture transmission electron microscopy observations of the 160 mM F8EH3OS solution indicate the formation of wormlike micelles. The high viscoelasticity of the 160 mM solution is attributed to the formation of wormlike micelles and these solutions are called micelle gels in this paper. F8EH3OS forms aggregates of various morphologies, such as spherical or spheroidal micelles, rod-like and wormlike micelles, vesicles, and lamellar phases, in aqueous solutions. A hydrocarbon surfactant (H8EH3OS), in which the fluorocarbon chain in F8EH3OS is replaced by a hydrocarbon chain, does not exhibit high viscoelasticity at any concentration. This unusual viscoelastic behavior is caused by the formation and disruption of micelle gels, and the fluorocarbon chain in F8EH3OS is essential for the formation of micelle gels. 1H NMR experiments suggest that the conformation of the hydrocarbon chain in F8EH3OS changed with surfactant concentration. This conformational change leads to transformation of the aggregates formed from F8EH3OS by changing the critical packing parameter. The hybrid nature of the hydrophobic part of F8EH3OS causes the unusual viscoelastic behavior and the formation of various morphologies.