Shape Modification of Water-in-CO₂ Microemulsion Droplets through Mixing of Hydrocarbon and Fluorocarbon Amphiphiles

An oxygen-rich hydrocarbon (HC) amphiphile has been developed as an additive for supercritical CO₂ (scCO₂). The effects of this custom-designed amphiphile have been studied in water-in-CO₂ (w/c) microemulsions stabilized by analogous fluorocarbon (FC) surfactants, nFG(EO)₂, which are known to form spherical w/c microemulsion droplets. By applying contrast-variation small-angle neutron scattering (CV-SANS), evidence has been obtained for anisotropic structures in the mixed systems. The shape transition is attributed to the hydrocarbon additive, which modifies the curvature of the mixed surfactant films. This can be considered as a potential method to enhance physicochemical properties of scCO₂ through elongation of w/c microemulsion droplets. More importantly, by studying self-assembly in these mixed systems, fundamental understanding can be developed on the packing of HC and FC amphiphiles at water/CO₂ interfaces. This provides guidelines for the design of fluorine-free CO₂ active surfactants, and therefore, practical industrial scale applications of scCO₂ could be achieved.