CO₂-switchable foams stabilized by a long-chain viscoelastic surfactant

Smart foams sensitive to external stimulation have gained increasing attention recently. However, reversibly switchable CO₂ foams have been less documented. In this work, a novel kind of CO₂-switchable foams was developed using a long-chain cationic surfactant, N-erucamidopropyl-N,N-dimethylammonium bicarbonate (UC₂₂AMPM⋅H⁺), as both the foaming agent and stabilizer. The foams can be rapidly transformed between stable and unstable states at ambient temperature with CO₂/NH₃·H₂O as the triggers. The foaming properties and switchable performance were examined by a combination of confocal microscopy, cryogenic transmission electron microscopy, and rheological techniques. The results demonstrated that the enhanced foam stability in the presence of CO₂ is attributed to the high bulk phase viscosity and gas/liquid surface viscosity, resulting from the entanglement of wormlike micelles (WLMs) formed from UC₂₂AMPM⋅H⁺. When NH₃·H₂O is added, the network structure of WLMs is disrupted, and the bulk phase viscosity and surface viscosity subsequently drop, consequently leading to an ultimate foam destabilization. Such a CO₂-sensitive viscoelastic surfactant could not only be used to fabricate smart CO₂ foams but can also enable CO₂ to play dual roles as both the dispersed phase, as most gases do, and an "activator" to protonate long-chain tertiary surfactants into cationic analogs to form viscoelastic WLMs to stabilize foams.