Reverse worm-like micelles formed by an equi-charged mixture of cationic gemini surfactant and anionic single-chain surfactant in cyclohexane

Abstract

An equi-charged mixture of cationic gemini surfactant, C,C′-bis(sodium dodecyldimethylammonium bromide)-p-benzenedimethylene (12-Ph-12), and anionic surfactant, sodium laurate (SL), can be dissolved in cyclohexane to form reverse worm-like micelles. The samples, with their different surfactant concentrations and water contents, were studied using rheological measurements. The minimum value of W ₀, the molar ratio of water to total surfactants, was 9 for the equally charged system of 12-Ph-12 (200 mmol L⁻¹)/SL. The maximum value of zero-shear viscosity (η ₀) of this system was 1858 Pa s⁻¹ at W ₀ = 14. Upon increasing the 12-Ph-12 concentration to 300 mmol L⁻¹, η ₀ increased rapidly to 2200 Pa s⁻¹. The dynamic rheology showed the typical visco-elastic response of a thread-like micelle solution at 12-Ph-12 (200 mmol L⁻¹) and W ₀ = 10, with a relaxation time (τ _R) of 0.11 s. At W ₀ = 14, a characteristic of a surfactant gel was shown, where the elastic modulus (G′) and the viscous modulus (G″) were independent of frequency. Non-linear visco-elasticity showed a strain-softening response at high strains.