Effects of Fluorocarbon Surfactant Chain Structure on Stability of Water-in-Carbon Dioxide Microemulsions. Links between Aqueous Surface Tension and Microemulsion Stability

Abstract

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 CO 2 with surfactants of high surface chemical purity that can also be synthesized from readily available reagents. The most effective CO 2 -philes from this group were sodium bis(1 H,1 H -perfluoropentyl)-2-sulfosuccinate (di-CF4), sodium bis(1 H,1 H,2 H,2 H -perfluorohexyl)-2-sulfosuccinate (di-CF4H), and sodium bis(1 H,1 H -perfluoroheptyl)-2-sulfosuccinate (di-CF6). All three of these compounds stabilized microemulsions at CO 2 bottle pressure (57 bar) at 15 °C, 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, γ cmc, and its performance in water−CO 2 microemulsions, as measured by the phase transition pressure P trans . This finding has important implications for the rational design of CO 2 -philic surfactants.