CO₂-responsive surfactants with tunable switching pH

One of the major challenges in applying CO₂-responsive surfactants concerns their tunable switchability and robustness under operating conditions. We hypothesize that combining monoethanolamine (MEA) with long-chain fatty acids (LCFAs) of variable chain lengths through electrostatic attraction could develop a series of CO₂-responsive surfactants with tunable switching pH.

The tunability of switching pH for this group of surfactants was demonstrated by in situ probing of the CO₂-responsive characteristics at the oil/water interface using dynamic interfacial tension (IFT) measurements. Two protocols were applied to distinguish interfacial response and solution response. The key importance of interfacial response was demonstrated by two essential applications of CO₂-responsive surfactants: demulsification of stable emulsions, and alternation of the interfacial properties of ultra-heavy crude oil-water interfaces.

The switching pH of the CO₂-responsive surfactants was controlled by the hydrocarbon chain length of LCFAs. More importantly, their switching behaviour was found to be different at the interface and in the bulk solution, which is attributed to the enhanced molecular interactions at the interface. Since most applications require surfactants to be switched at the interface, it is thereby most appropriate to determine the switching pH through their interfacial responses.