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

Abstract

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.

Recent advances in supercritical fluid dyeing

“…9 Water/supercritical carbon dioxide reverse micellar systems are stable microemulsions of water dispersed in the continuous supercritical phase, 58 which are generally obtained employing environmentally unacceptable fluorocarbon-tail surfactants. 59 This is probably the main reason why the reverse micellar coloration of natural textiles was practically abandoned after 2008. 60 To the author's knowledge, the only recent application of this technique in the area of textiles is the removal of impurities from grey cotton fabrics.…”

“…Among these, the solubilisation of polar dyes in water/supercritical carbon dioxide reverse micellar systems and the synthesis of novel reactive disperse dyes have been considered to be the most promising 9 . Water/supercritical carbon dioxide reverse micellar systems are stable microemulsions of water dispersed in the continuous supercritical phase, 58 which are generally obtained employing environmentally unacceptable fluorocarbon‐tail surfactants 59 . This is probably the main reason why the reverse micellar coloration of natural textiles was practically abandoned after 2008 60 .…”

Multicharge zwitterionic molecules: Hydration, kosmotropicity and anti-fouling potential

“…[18] The results have also shown a powerful ability of DC to promote surfactant aggregation, which is important in processes where an ultra-low surface tension is required such as in the formation of scCO2 in water microemulsions. [54] Considering the data presented in Table 1 on CMC reduction upon addition of salts, it might be tempting to conclude that the character and solution behaviour of ZWSs, particularly in the case of short chain C2S, is mainly determined by their "salt" nature, i.e. by presence of ionic groups, quaternary ammonium and bromide, making them similar to simple TMAB salt.…”

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

“…For model fitting data analysis, the W/CO 2 µE droplets were treated as ellipsoidal particles with a Schultz distribution in core radii 31 . The polydispersities in ellipsoid radii were fixed at 0.3 as found in spherical D 2 O/CO 2 µEs with the double FC-tail surfactants (polydispersity = 0.17-0.40) 12,13,25 . Full accounts of the scattering laws are given elsewhere 12,13,25,32 .…”