Surface properties and phase behavior of Gemini/conventional surfactant mixtures based on multiple quaternary ammonium salts

The mixing properties of Gemini surfactants are significantly important to the large-scale application of these surfactants. In this study, we studied the mixing properties of multiple quaternary ammonium salts (MQAS) with cationic surfactant DTAB, anionic surfactant SDS, nonionic surfactant Triton X-100 and anionic biosurfactant rhamnolipid, respectively. Moreover, the phase behavior and aggregation properties of MQAS/SDS mixtures in water were examined by ocular observations and DLS. For equilibrium surface properties, unlike MQAS/DTAB and MQAS/TX-100 mixtures, the equimolar cationic/anionic mixtures (MQAS/SDS and MQAS/RL mixtures) have a lower critical micelle concentration than either pure component. This is always attributed to the formation of cationic surfactant complexes through the electrostatic attraction of hydrophilic groups. Results of dynamic surface properties show that MQAS/FX-100 mixtures exhibit the best dynamic surface activity among these surfactant mixtures, resulting from the good dynamic surface activity of DC-100 and the weak interaction between MQAS and TX-100. Considering the large size of cationic/anionic surfactant complexes, the MQAS/SDS and MQAS/RL mixtures have a smaller R-1(/2) value, but the gamma(m) values of equimolar cationic/anionic mixtures are lower than either pure components. This suggests that the formation of cationic/anionic surfactant complexes reduces the electrostatic adsorption barrier. In the anionic-rich side, the MQAS/SDS mixtures are prone to form vesides, while the addition of a small amount of MQAS will induce the micelle-to-vesicle transition, which can be regulated by the composition of mixtures. Meanwhile, the increase of temperature induces the transition of precipitate to vesicles. But the micelle-to-vesicle or vesicle-to-micelle transitions induced by the increase of temperature have not been observed at the studied temperature range. (C) 2019 Elsevier B.V. All rights reserved.