The strong influence of alkyl tail length on the aggregation and viscoelasticity of carboxylate gemini surfactants with a p-dibenzenediol spacer in aqueous solution

This paper reports a study on the aggregation and rheological behavior of the family of O, O’-bis(sodium 2-alkylcarboxylate)-p-dibenzenediol (referred to as Cmϕ2Cm, m = 10, 12, 14, respectively) in aqueous solution using dynamic light scattering, 1H NMR and rheology measurements. The results showed that all three surfactants formed large network-like aggregates at low concentrations. However, C10ϕ2C10 formed small compact micelles simultaneously but neither C12ϕ2C12 nor C14ϕ2C14 did. These network-like aggregates were transformed into the wormlike micelles with increasing the surfactant concentration. The length of alkyl tails was found to strongly affect the viscoelasticity of wormlike micellar solutions. From C10ϕ2C10, C12ϕ2C12 to C14ϕ2C14 in turn, the system developed rapidly from the viscous fluid to typically viscoelastic solution and then to a solid-like gel. The scaling exponents of the concentration dependence of both zero-shear viscosity (η0) and plateau elastic modulus (G′∞) greatly exceeded the theoretic predictions, showing fast micellar growth and strong entanglements between the wormlike micelles. For C14ϕ2C14 that had the longest alkyl tails in this series, the wormlike micelles formed at 140 mmol L−1 were quite long and the micellar reptation dominated over the scission and recombination. This system yielded a viscosity as high as 2.20 × 104 Pa⋅s at 25 °C.