Water-in-CO₂ Microemulsions Stabilized by an Efficient Catanionic Surfactant

Area of Science:

• * Supramolecular chemistry and materials science.
• * Physical chemistry of interfaces and microemulsions.

Background:

• * Efficient and eco-friendly surfactants are crucial for water-in-supercritical CO2 microemulsions (W/CO2 μEs).
• * Existing surfactants face challenges like bioaccumulation and limited salt tolerance.
• * Recent work identified synergistic effects in catanionic surfactants for W/CO2 μEs stabilization.

Purpose of the Study:

• * To elucidate the nanostructural origins of synergistic effects in catanionic surfactants for W/CO2 μEs.
• * To understand how surfactant structure influences microemulsion stability and water solubilization.
• * To investigate the role of headgroup ionicity and effective packing parameter (EPP).

Main Methods:

• * High-pressure small-angle neutron scattering (HP-SANS) to probe W/CO2 microemulsion nanostructure.
• * Determination of key structural parameters: headgroup interfacial area, volume, aggregation number, and EPP.
• * Comparative analysis with previously studied fluorinated surfactants.

Main Results:

• * HP-SANS data revealed increased EPP and decreased hydrophilic/CO2-philic balance.
• * Reduced effective headgroup ionicity was identified as a key factor in surfactant effectiveness.
• * The catanionic surfactant exhibited performance comparable to highly efficient double-tail anionic surfactants.

Conclusions:

• * Synergistic effects in catanionic surfactants significantly enhance W/CO2 μEs stability.
• * Optimized surfactant design, focusing on EPP and ionicity, is key for high water solubilization.
• * These novel catanionic surfactants offer a promising pathway for advanced W/CO2 μEs applications.