Dynamic and Interfacially Reinforced Surfactant for Ionic Liquid Microemulsions in Supercritical CO2.

The development of efficient, environmentally benign surfactants for stabilizing microemulsions in supercritical carbon dioxide (scCO2) remains a significant challenge for enhanced oil recovery (EOR), owing to the historical reliance on fluorinated compounds and the poor stability of conventional surfactants. Herein, we report a fluorine-free supramolecular surfactant system based on a β-cyclodextrin (β-CD) and polyethylene glycol (PEG) conjugate (denoted β-CD-PEG). This conjugate is further functionalized to enable dynamic covalent cross-linking at the interface. The surfactant system leverages host-guest chemistry to anchor the ionic liquid [Bmim][Pro] via inclusion complexation, while the PEG chain ensures scCO2 solubility. The optimized microemulsion achieves an ultralow interfacial tension of 1.8 mN/m and forms a rigid, elastic interfacial film (G' = 15.3 mN/m) through dynamic covalent cross-linking, ensuring long-term stability (>30 days). Core-flooding experiments demonstrate a reduction in the minimum miscibility pressure by 2.50 MPa and a 17.2% incremental oil recovery. The β-CD-PEG surfactant system also exhibits enhanced selectivity for extracting heavy oil components, leading to a viscosity reduction of up to 45.1% for heavy crude. This work establishes a paradigm for stabilizing complex fluid interfaces in scCO2 using supramolecular, dynamic, and fluorine-free surfactants, with significant implications for green and efficient chemical EOR processes.