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Water-in-CO₂ Microemulsions Stabilized by Fluorinated Cation–Anion Surfactant Pairs

Sagisaka, Masanobu, Saito, Tatsuya, Yoshizawa, Atsushi, Rogers, Sarah E., Guittard, Frédéric, Hill, Christopher, Eastoe, Julian, Blesic, Marijana
Langmuir 2019 v.35 no.9 pp. 3445-3454
Fourier transform infrared spectroscopy, carbon dioxide, emulsions, micelles, neutron diffraction, solubilization, solvents, stabilizers, surface tension, surfactants
High-water-content water-in-supercritical CO₂ (W/CO₂) microemulsions are considered to be green, universal solvents, having both polar and nonpolar domains. Unfortunately, these systems generally require environmentally unacceptable stabilizers like long and/or multifluorocarbon-tail surfactants. Here, a series of catanionic surfactants having more environmentally friendly fluorinated C₄–C₆ tails have been studied in terms of interfacial properties, aggregation behavior, and solubilizing power in water and/or CO₂. Surface tensions and critical micelle concentrations of these catanionic surfactants are, respectively, lowered by ∼9 mN/m and 100 times than those of the constituent single fluorocarbon-tail surfactants. Disklike micelles in water were observed above the respective critical micelle concentrations, implying the catanionic surfactants have a high critical packing parameter, which should be suitable for the formation of reverse micelles. Based on visual observation of phase behavior and Fourier transform infrared spectroscopic and small-angle neutron scattering studies, one of the three catanionic surfactants tested was found to form transparent single-phase W/CO₂ microemulsions with a water-to-surfactant molar ratio of up to ∼50. This is the first successful demonstration of the formation of W/CO₂ microemulsions by synergistic ion-pairing of anionic and cationic single-tail surfactants. This indicates that catanionic surfactants offer a promising approach to generate high-water-content W/CO₂ microemulsions.