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 of fluorination. As well as being able to stabilize water-CO₂ interfaces, these surfactants must also be economical, prevent bioaccumulation and strong adhesion, deactivation of enzymes, and be tolerant to high salt environments. Recently, an ion paired catanionic surfactant with environmentally acceptable fluorinated C₆ tails was found 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 constituent surfactants alone did not stabilize W/CO₂ μEs, this was the first demonstration of surfactant synergistic effects in W/CO₂ microemulsions. The aim of this new study is to understand the origin of these intriguing effects by detailed investigations of nanostructure in W/CO₂ microemulsions using high-pressure small-angle neutron scattering (HP-SANS). These HP-SANS experiments have been used to determine the headgroup interfacial area and volume, aggregation number, and effective packing parameter (EPP). These SANS data suggest the effectiveness of this surfactant originates from increased EPP and decreased hydrophilic/CO₂-philic balance, related to a reduced effective headgroup ionicity. This surfactant bears separate C₆F₁₃ tails and oppositely charged headgroups, and was found to have a EPP value similar to that of a double C₄F₉-tail anionic surfactant (4FG(EO)₂), which was previously reported to be one of most efficient stabilizers for W/CO₂ μEs (maximum W₀ = 60-80). Catanionic surfactants based on this new design will be key for generating superefficient W/CO₂ μEs with high stability and water solubilization.

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

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.

Hybrid CO2-philic surfactants with low fluorine content

The relationships between molecular architecture, aggregation, and interfacial activity of a new class of CO(2)-philic hybrid surfactants are investigated. The new hybrid surfactant CF2/AOT4 [sodium (4H,4H,5H,5H,5H-pentafluoropentyl-3,5,5-trimethyl-1-hexyl)-2-sulfosuccinate] was synthesized, having one hydrocarbon chain and one separate fluorocarbon chain. This hybrid H-F chain structure strikes a fine balance of properties, on one hand minimizing the fluorine content, while on the other maintaining a sufficient level of CO(2)-philicity. The surfactant has been investigated by a range of techniques including high-pressure phase behavior, UV-visible spectroscopy, small-angle neutron scattering (SANS), and air-water (a/w) surface tension measurements. The results advance the understanding of structure-function relationships for generating CO(2)-philic surfactants and are therefore beneficial for expanding applications of CO(2) to realize its potential using the most economic and efficient surfactants.

Super-efficient surfactant for stabilizing water-in-carbon dioxide microemulsions

The fluorinated double-tailed glutarate anionic surfactant, sodium 1,5-bis[(1H,1H,2H,2H-perfluorodecyl)oxy]-1,5-dioxopentane-2-sulfonate (8FG(EO)(2)), was found to stabilize water-in-supercritical CO(2) microemulsions with high water-to-surfactant molar ratios (W(0)). Studies were carried out here to obtain detailed information on the phase stability and nanostructure of the microemulsions by using a high-pressure UV-vis dye probe and small-angle neutron scattering (SANS) measurements. The UV-vis spectra, with methyl orange as a reporter dye, indicated a maximum attainable W(0) of 60 at 45 and 75 °C, and SANS profiles indicated regular droplet swelling with a linear relationship between the water core nanodroplet radius and W(0). This represents the highest water solubilization reported to date for any water-in-CO(2) microemulsion. Further analysis of the SANS data indicated critical packing parameters for 8FG(EO)(2) at the microemulsion interface >1.34, representing approximately 1.1 times the value for common aerosol-OT in water-in-heptane microemulsions under equivalent conditions.