The vast potential of carbon dioxide (CO 2 ) as an environmentally clean, abundant, and tunable solvent is now being realized on a variety of fronts, including cleaning protocols in microelectronics and garment care industries, coatings, and polymer production and processing. 1 To take advantage of CO 2 's benefits, however, it is often necessary to confront an important limitation, namely its low capacity for solubilizing many materials, including water. 2 One way this issue has been addressed is through the use of small-molecule fluorosurfactants to aid the dispersion of water-in-CO 2 (W/C). 3 Unfortunately, only a handful of surfactants have proven capable of water uptake in neat CO 2 . Here, we report initial findings on chemically homogeneous anionic phosphate fluorosurfactants that enable significant water uptake within a continuous CO 2 phase, through the formation of W/C microemulsions without the aid of a cosurfactant. 4 The double-tailed surfactants are of two general structural types: a form containing two fluorinated chains (1) and a "hybrid" form 3a containing one fully hydrocarbon chain and one fluorinated chain (2). Two such surfactants, which can be prepared in a straightforward manner in high purity, 5 are shown below.Ionic solutes generally possess low solubility in compressed CO 2 . 2 This was indeed the case for surfactant 1, which was insoluble at 1 wt % at temperatures from 25 to 65°C and pressures up to 380 bar. 6 Surfactant 2 proved soluble at 1 wt % under rather mild pressures (for example, cloud points at 60 and 27°C occurred at 225 and 119 bar, respectively). It is not clear why the hybrid surfactant 2 has such appreciable "dry" solubility in CO 2 , although self-assembly into reverse micelles, perhaps with internal cores incorporating the hydrocarbon chain, is one possibility. Addition of water to either surfactant in CO 2 allowed for the formation of clear, single-phase solutions. Cloud point profiles of 2.5 wt % surfactant solutions in CO 2 with varied water/surfactant molar ratios (W o ) 7 are shown in Figure 1, where homogeneous solutions existed at pressures above the curves and heterogeneous, phase-separated solutions existed below the curves. For surfactant 1, the cloudpoint behavior followed essentially linear trends for water loadings spanning from W o ) 11 up to at least 45 (for W o ) 45 there were equivalent weights of water and surfactant). The cloud-point pressure ranges observed here were comparable to those determined for W/C microemulsion-forming fluorosurfactants at similar temperatures and concentrations (Figure 1a). 3 The phase behavior for surfactant 2 was not straightforward. At W o ) 11, cloud-point pressures curved upward as temperatures decreased, while for W o ) 17 and 34, cloud-point pressures decreased rather linearly ( Figure 1b). Opposite trends were recently reported for a cationic perfluoropolyether surfactant in CO 2 . 8 Considering the high analytical purity of the surfactant and the reproducibility of the cloud-point measurements ((0.5 bar), we sp...
Anionic phosphodiester surfactants, possessing either two fluorinated chains (F/F) or one hydrocarbon chain and one fluorinated chain (H/F), were synthesized and evaluated for solubility and self-assembly in liquid and supercritical carbon dioxide. Several surfactants, of both F/F and H/F types and having varied counterions, were found to be capable of solubilizing water-in-CO2 (W/C), via the formation of microemulsions, expanding upon the family of phosphate fluorosurfactants already found to stabilize W/C microemulsions. Small-angle neutron scatteringwas used to directly characterize the microemulsion particles at varied temperatures, pressures, and water loadings, revealing behavior consistent with previous results on W/C microemulsions.
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