Construction and regulation of thermo-responsive vesicular solution in catanionic surfactant systems

The aggregation behaviors in the binary systems of cationic Gemini surfactants with amide groups in hydrophobic chains (C₁₂A-C₂-AC₁₂) or spacer group (C₁₂-AC₂A-C₁₂) and anionic amino acid surfactant N-dodecanoylglutamic acid (C₁₂Glu) have been studied at two different pH values by adjusting the mixing ratio and temperature. The catanionic surfactant systems at pH 5.0 and 10.0 formed spherical micelles and vesicles at 25 ℃ by varying the mixing ratio, respectively. Upon increasing the temperature, the different temperature responsivenesses were realized in four vesicular solutions of catanionic surfactant systems. At pH 5.0 and 10.0, the vesicles formed in C₁₂A-C₂-AC₁₂&C₁₂Glu system were stable and maintained a bluish hue, while the vesicles in C₁₂-AC₂A-C₁₂&C₁₂Glu system tended to form the larger aggregates, accompanied by the change in solution from bluish to milky, or even precipitation with increasing temperature. Besides, the intermolecular hydrogen bonding of C₁₂Glu containing two carboxyl groups played an important role in vesicle formation and thermal stability. For C₁₂A-C₂-AC₁₂&C₁₂Glu and C₁₂-AC₂A-C₁₂&C₁₂Glu systems at pH 5.0, the hydrogen bonding of carboxyl groups in C₁₂Glu promoted the vesicle formation and thermal stability. In contrast, at pH 10.0, the carboxyl groups did not form hydrogen bonds and the vesicular solutions of C₁₂A-C₂-AC₁₂&C₁₂Glu and C₁₂-AC₂A-C₁₂&C₁₂Glu systems underwent dark blue or the transition from bluish to milky to precipitation, respectively. Moreover, these transition processes were thermally reversible. These thermo-responsive vesicular solutions can be elucidated on the basis of the temperature-induced variations in the strength of hydrogen bonds of the headgroups. The results suggested that the location of amide groups and hydrogen bonding between carboxyl groups could affect the formation and thermal stability of vesicular solutions.