Hybrid fluorocarbon-hydrocarbon CO2-philic surfactants. 2. formation and properties of water-in-CO2 microemulsions

Hybrid fluorocarbon-hydrocarbon (F-H) sulfate surfactants are shown to be efficient stabilizers in water-in-CO2 (w/c) microemulsions. The chain structure and F-H ratio affect the regions of P-T phase stability and aggregation structure in these w/c phases. High-pressure near-infrared spectroscopy and small-angle neutron scattering measurements of microemulsified water provide evidence for the stabilization of w/c microemulsion droplets. The relative lengths of the two chains were found to influence the favored aggregation structure: for symmetric chain surfactants (F8H8, F7H7) spherical reverse micelles are present, but for asymmetric chain surfactants (F7H4, F8H4) extended cylinder aggregates form. These changes in aggregation are consistent with different surfactant packing parameters owing to the controlled variations in molecular structure. Furthermore, the general order of w/c phase transition pressures (F8H8 < F7H7 and F8H4 < F7H4) is in line with estimations of surfactant fractional free volume, as proposed by Johnston et al. (J. Phys. Chem. B 2004, 108, 1962-1966). Studies of adsorption at the poly(dimethylsiloxane)-water interface are shown to be valuable for assessing the CO2-philicity of new surfactants. All in all, the symmetric F8H8 and F7H7 analogues are seen to be the most efficient compounds from this class for applications in CO2.