Interfacial properties and foam performance of alpha olefin sulfonate and CO2 switchable aminopropyl methyl siloxane surfactant mixtures

CO₂-in-water (C/W) foams have attracted much attention given their eco-friendliness in recent years. However, limited surfactants were able to effectively adsorb at the supercritical CO₂–water (C–W) interface considering the weak solvent strength of CO₂, especially at high temperatures. Herein, attempts have been made to design suitable CO₂ foaming agents by evaluating the influence of proportions of anionic and cationic surfactants and the affinity between aminopropyl methyl siloxane (APSi) and CO₂. Through systematical foamability and foam stability experiments, together with the measurements of C–W equilibrium and dynamic interfacial tensions, the adsorption and stabilization mechanisms of sodium alpha olefin sulfonate (AOS)-APSi aqueous dispersions on the CO₂ foam films are revealed. Excellent foam properties were observed at AOS/APSi mass ratio of 9:1, in which initial foam height (h₀) and half-life period (t_1/2) first increased and then decreased with increasing pressure, whereas the interfacial tension decreased with increasing pressure. The results indicate that the interaction of anionic and cationic head-groups contributes to accelerating the surfactant adsorption rate from bulk to the C–W interface, enhancing foamability and stabilizing foam. Besides, the AOS/APSi mass ratio of 8:2 shows a good affinity for CO₂ at 15 MPa, of which h₀ is 26% higher and t_1/2 is 60% slower than AOS alone. Furthermore, the initial mean bubble area of both 9:1 and 8:2 AOS/APSi mixtures was around half that of AOS alone. This work broadens the design of novel surfactant methodologies including CO₂ foam, providing a theoretical guidance for the application of CO₂ on enhanced-oil-recovery technologies.