Adsorption of fluoro surfactants at air-water and water-carbon dioxide interfaces

Abstract

Aqueous phase behaviour, and water-in-carbon dioxide microemulsion formation, were studied with two model fluorinated anionic surfactants, sodium bis(1H/,lH/-per-fluoropentyl)-2-sulfosuccinate (di-CF4) and sodium bis(1H,1H,5H-octafluoropentyl)-2-sulfosuccinate (di-HCF4). The properties are compared to the common, branched-chain, hydrocarbon surfactant Aerosol-OT (AOT). For the fluoro surfactants, surface excesses at the air-solution interface, measured by tensiometry and neutron reflection, agreed well, and the γ-ln(activity) curves were consistent with a pre-factor of 2 in the Gibbs equation. Both di-CF4 and di-HCF4 were effective at stabilising water-in-carbon dioxide (w/c) microemulsions. Pressure-temperature studies indicated that di-CF4 is the most effective, since it gave microemulsions at lower pressures, e.g. for w = 20 ([water]/[surf]) at 30 °C the w/c phase was stable down to 120 bar. High-pressure small-angle neutron scattering showed the microemulsions have a simple spherical droplet structure. In water-CC₂ mixtures these surfactants behave in a similar fashion to AOT in conventional hydrocarbon systems. Therefore, the fluoro-succinates represent a well-characterised model series of surfactants, which may be useful for understanding the effects of surfactant chemistry on w/c microemulsion phase properties.