CO₂-Responsive Imidazoline-Based Surfactants for Tunable Interfacial Activity and Controllable Emulsification

Switchable surfactants are an emerging class of environmentally friendly compounds that can be reversibly triggered between active and inactive states in response to external stimuli, enabling on-demand or on-command control over the interfacial properties. In this work, a type of imidazoline-based CO₂-switchable surfactants was synthesized and systematically investigated. Structural characterization was performed by using Fourier transform infrared and ¹H NMR spectroscopy, confirming the successful formation of long-chain alkyl imidazolines. Their surface activity and critical micelle concentration (CMC) were evaluated by measuring surface tension and interfacial tension, revealing strong adsorption behavior and efficient micellization. Conductivity and NMR analyses demonstrated the reversible transition between neutral and ionic forms under alternating CO₂/N₂ bubbling, confirming the robustness and repeatability of the switching process. The emulsification/demulsification cycles were repeated over multiple switching operations, where oil/water emulsions were stabilized by their bicarbonate salt forms with long-term stability, whereas the neutral forms induced rapid demulsification. These findings not only establish the fundamental interfacial behavior of imidazoline-based surfactants but also highlight their potential applications in enhanced oil recovery, chemical separation, drug delivery, and sustainable process design.