Fluoro-surfactants at air/water and water/CO2 interfaces

Aqueous phase behaviour and water-in-carbon-dioxide (w/c) microemulsion for mation were studied with various fluorinated sulfosuccinate surfactants. Fo r aqueous solutions of two different compounds surface coverages measured b y neutron reflection and surface tension were consistent, giving reliable v alues for mean areas per molecule at the air/water (a/w) interface. At 20 d egreesC and pressures above about 250 bar, seven different surfactants were found to be effective at stabilising w/c microemulsions. With reference to recent work (J. Eastoe, A. Downer, A. Paul, D. C. Steytler and E. Rumsey, Prog. Colloid Polym. Sci., 2000, 115, 214) it is possible to identify a str ucture-performance relationship for these surfactants in water-CO2 systems. Comparison of the phase behaviour shows that sodium bis(1H,1H-nonafluorope ntyl)-2-sulfosuccinate (di-CF4) forms microemulsions at the lowest pressure , e.g. for w=20 ([water](added)/[surf]) at 30 degreesC the w/c phase was st able down to 120 bar. High-pressure FTIR spectroscopy indicates a fraction of the added water partitions out of microemulsion droplets, thereby satura ting the bulk CO2. Furthermore, high-pressure small-angle neutron scatterin g (SANS) is characteristic of a simple spherical droplet structure in the m icroemulsions. SANS data also indicate temperature-induced changes in radiu s, and this is consistent with partitioning of water to maintain saturation of the bulk CO2. For three different surfactants it has been possible to c ompare adsorption at a/w and w/c surfaces, highlighting differences in pack ing requirements to stabilise these two different interfaces.