Synthesis and Properties of a Series of CO2 Switchable Surfactants with Imidazoline Group

Switchable surfactants are environment-friendly compounds, which can be separated from the system or lose surface activity after completing their function during one stage of a process. In order to study switchable properties of a kind of CO₂ switchable surfactant with an imidazoline group, four 2-alkyl-1-hydroxyethylimidazolines were synthesized by condensation of N-(2-hydroxyethyl)ethanediamine with dodecanoic, tetradecanoic, hexadecanoic, and octadecanoic acids. Then, the series of long-chain alkyl imidazoline compounds were reacted with dry ice to produce imidazolinium bicarbonates cationic surfactants. The critical micelle concentration (CMC) and surface tension at CMC (??_cmc) measured by the Wilhelmy plate technique show that these surfactants have excellent surface activity. The changes of conductivity before and after bubbling CO₂ show the conversion between imidazolines and imidazolinium bicarbonates cationic surfactants. Conductivity cycles indicated that these surfactants could be switched by CO₂ reversibly and repeating this three times. However, their switchable function on the emulsification-demulsification of water-alkane was dissatisfactory due to the emulsibility of amide which was hydrolyzed from 2-alkyl-1-hydroxyethylimidazoline. Therefore, the application of these switchable surfactants needed to be studied further.