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

Aqueous phase behaviour and water-in-carbon-dioxide (w/c) microemulsion formation were studied with various fluorinated sulfosuccinate surfactants. For aqueous solutions of two different compounds surface coverages measured by neutron reflection and surface tension were consistent, giving reliable values for mean areas per molecule at the air/water (a/w) interface. At 20 degreesC 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 structure-performance relationship for these surfactants in water-CO2 systems. Comparison of the phase behaviour shows that sodium bis(1H,1H-nonafluoropentyl)-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 stable down to 120 bar. High-pressure FTIR spectroscopy indicates a fraction of the added water partitions out of microemulsion droplets, thereby saturating the bulk CO2. Furthermore, high-pressure small-angle neutron scattering (SANS) is characteristic of a simple spherical droplet structure in the microemulsions. SANS data also indicate temperature-induced changes in radius, and this is consistent with partitioning of water to maintain saturation of the bulk CO2. For three different surfactants it has been possible to compare adsorption at a/w and w/c surfaces, highlighting differences in packing requirements to stabilise these two different interfaces.