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.