Preparation and mechanism of pH and temperature stimulus-responsive wormlike micelles

Abstract

A quaternary ammonium salt surfactant with tertiary amine groups (C16MIMBr) was prepared by the reaction of bromohexadecane with N, N, N, N-tetra methylpropane diamine, and the structure of the product was demonstrated by 1H NMR. A viscoelastic wormlike micellar system was constructed with C16MIMBr as the main agent and potassium bromide as the counterion salt, and the ratio of surfactant to counterion was optimized. It was shown that the viscosity of the system was maximum when the molar ratio of the C16MIMBr-KBr system was 1:4, and the rheological test also indicated that the viscoelasticity was best at 1:4. The pH-stimulated responsiveness and temperature-stimulated responsiveness of the C16MIMBr-KBr system were investigated by rheology and cryo-transmission electron microscopy, and the response mechanism was investigated and analyzed. It is shown that when the system is acidic, the positive charges on both sides of the surfactant lead to strong electrostatic repulsions within and between the surfactant molecules, resulting in a cone-like structure of the surfactant molecules and the formation of spherical micelles with very low viscosity. When the system is deprotonated under alkaline conditions, only one end of the surfactant head group is positively charged, and the inter-and intra-molecular electrostatic repulsive forces are greatly weakened, resulting in the intertwined growth of micelles and the formation of wormlike micelles. The temperature response mechanism analysis of the C16MIMBr-KBr system shows that at low temperature, only a small amount of cation can cross-mix with micelles to form a curved vesicle structure due to the low solubility of the system, resulting in a low viscosity of the system. As the temperature rises to 60 °C, the water solubility of the system is enhanced, and it desorbs from the vesicles into the solution. According to the equation of critical accumulation parameter, the critical accumulation parameter of the system decreases and the transition from vesicles to wormlike micelles occurs, leading to an increase in system viscosity.