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

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 W0, 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 (η0) of this system was 1858 Pa s−1 at W0 = 14. Upon increasing the 12-Ph-12 concentration to 300 mmol L−1, η0 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 W0 = 10, with a relaxation time (τR) of 0.11 s. At W0 = 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.