Abstract:

Stimuli-responsive wormlike micelles（WLMs）represent a frontier in viscoelastic surfactant research, enabling on-demand modulation of rheological properties. Among various triggers, CO₂ has emerged as an attractive alternative to conventional pH regulation due to its ready availability, environmental benignity, and reversible removal without salt accumulation. This review systematically addresses CO₂-responsive WLMs, focusing on molecular design principles, assembly mechanisms, and rheological characteristics. The response behaviour hinges on the reversible protonation of organobases—primarily tertiary amines—upon CO₂ sparging, which generates cationic or pseudo-gemini surfactants that self-assemble into entangled networks. Conversely, CO₂ removal by inert gas purging or mild heating reverses the protonation, dismantling the micellar network and restoring low viscosity. Three prototypical systems are delineated:（i）binary mixtures of anionic surfactants with amine-based hydrotropes, which form CO₂-switchable pseudo-gemini species through electrostatic linkages;（ii）single-component long-chain polyamines, where CO₂ induces a vesicle-to-wormlike transition via mixed protonated species and bicarbonate counterions, a pathway unattainable by simple acid titration; and（iii）ultra-long-chain tertiary amine surfactants, which enable reversible sol-gel transitions at ambient conditions using only CO₂ and air. Notably, CO₂ can also destroy pre-formed anionic WLMs by converting carboxylate headgroups into neutral acid forms, thereby reducing viscosity—a distinct “turn-off” mechanism. In summary, these systems exhibit robust cyclability, precise rheological switching, and structure-property correlations governed by molecular packing parameters and counterion effects. The findings underscore the critical role of protonation pathways and ionic speciation in dictating WLM formation, offering a versatile platform for applications in oilfield chemistry, drug delivery, and smart materials.

Key words: viscoelastic surfactant, wormlike micelles, CO₂-responsive, self-assembly, sol-gel transition