Switchability and synergistic effect of a CO2-responsive surfactant with co-surfactants at an O/W interface: A molecular insight.

• Synergistic effect regulates the CO 2 -responsive mechanism. • DTAB does not interfere with the original CO 2 -responsive mechanism. • SDS favors the formation of a compact surfactant layer. • TX-100 enhances the interface activity of the mixture. • The distinct CO 2 -responsive mechanisms require a delicate formulation design. CO 2 -responsive surfactants are a promising technology with potential applications in various fields. Therefore, the design of their formulations must be carefully considered to ensure their effectiveness in each specific application. Herein, we employed molecular dynamics simulations to investigate an oil/water (O/W) interface stabilized by a surfactant mixture containing a CO 2 -responsive cationic primary surfactant and another co-surfactant. It has been discovered that formulating the CO 2 -responsive cationic surfactant with a typical anionic surfactant (sodium dodecyl sulfate, SDS) can lead to a distinct CO 2 -responsive mechanism of the surfactant mixture. To compare, the CO 2 -responsive mechanism remains unchanged if the CO 2 -responsive cationic surfactant is formulated with a typical cationic surfactant (dodecyl trimethylammonium bromide, DTAB). A nonionic co-surfactant can enhance the interface activity of the surfactant mixture but the synergistic effect is not strong enough to lead to a different CO 2 -responsive mechanism. The distinct phase behavior between two surfactant mixtures suggests that the CO 2 -responsive mechanism is dominated by the type of surfactants formulated with the CO 2 -switchable surfactant. [ABSTRACT FROM AUTHOR]