Effects of fluorocarbon surfactant chain structure on stability of water-in-carbon dioxide microemulsions. Links between aqueous surface tension and microemulsion stability

Formation and stability of water-in-carbon dioxide microemulsions are described with eight related fluorinated analogues of the anionic surfactant Aerosol-OT. The aim was to identify a structure-performance relationship in CO2 with surfactants of high surface chemical purity that can also be synthesized from readily available reagents. The most effective CO2-philes from this group were sodium bis(1H,1H-perfluoropentyl)-2-sulfosuccinate (di-CF4), sodium bis(1H,1H,2H,2H-perfluorohexyl)-2-sulfosuccinate (di-CF4H), and sodium bis(l-H,1H-perfluoroheptyl)-2-sulfosuccinate (di-CF6). All three of these compounds stabilized microemulsions at CO2 bottle pressure (57 bar) at 15 degreesC, with a w value ([water]/[surf]) of 10. A close correlation is demonstrated between limiting aqueous phase surface tension of a given surfactant at its critical micelle concentration, gamma(cmc), and its performance in water-CO2 microemulsions, as measured by the phase transition pressure P-trans. This finding has important implications for the rational design of CO2-philic surfactants.