Water-in-CO2 Microemulsions Stabilized by an Efficient Catanionic Surfactant

To facilitate potential applications of water-in-supercritical CO2 microemulsions (W/CO2 μEs)efficient and environmentally responsible surfactants are required with low levels fluorination. As wellas being able to stabilize water-CO2 interfaces, these surfactants must also be economical, prevent bioaccumulation and strong adhesion, deactivation of enzymes, and also be tolerant to high salt environments.Recently, an ion paired catanionic surfactant with environmentally-acceptable fluorinated C6-tails wasfound to be very effective at stabilizing W/CO2 μEs with high water-to-surfactant molar ratios (W0) up to~50 (Sagisaka, M. et al. Langmuir, 2019, 35, 3445–3454). As the cationic and anionic constituentsurfactants alone did not stabilize W/CO2 μEs, this was the first demonstration of surfactant synergisticeffects in W/CO2 microemulsions. The aim of this new study is to understand the origin of these intriguingeffects by detailed investigations of nanostructure in W/CO2 microemulsions using high pressure smallangle neutron scattering (HP-SANS). These HP-SANS experiments have been used to determine theheadgroup interfacial area and volume, aggregation number and effective packing parameter (EPP). TheseSANS data suggest the effectiveness of this surfactant originates from increased EPP and decreasedhydrophilic/CO2-philic balance, related to a reduced effective headgroup ionicity. This surfactant bearsseparate C6F13-tails and oppositely-charged headgroups, and was found to have a EPP value similar tothat of a double C4F9-tail anionic surfactant (4FG(EO)2), which was previously reported to be one of mostefficient stabilizers for W/CO2 μEs (maximum W0 = 60-80). Catanionic surfactants based on this newdesign will be key for generating super-efficient W/CO2 μEs with high stability and water solubilization.