Anisotropic reversed micelles with fluorocarbon-hydrocarbon hybrid surfactants in supercritical CO2

Abstract

Previous work (M. Sagisaka, et al. Langmuir 31 (2015) 7479-7487), showed the most effective fluorocarbon (FC) and hydrocarbon (HC) chain lengths in the hybrid surfactants FCm-HCn (sodium 1-oxo-1-[4-(perfluoroalkyl)phenyl]alkane-2-sulfonates, where m = FC length and n = HC length) were m and n = 6 and 4 for water solubilization, whereas m 6 and n 6, or m 6 and n 5, were optimal chain lengths for reversed micelle elongation in supercritical CO. To clarify why this difference of only a few methylene chain units is so effective at tuning the solubilizing power and reversed micelle morphology, nanostructures of water-in-CO (W/CO) microemulsions were investigated by high-pressure small-angle neutron scattering (SANS) measurements at different water-to-surfactant molar ratios (W) and surfactant concentrations. By modelling SANS profiles with cylindrical and ellipsoidal form factors, the FC6-HCn/W/CO microemulsions were found to increase in size with increasing W and surfactant concentration. Ellipsoidal cross-sectional radii of the FC6-HC4/W/CO microemulsion droplets increased linearly with W, and finally reached ∼39 Å and ∼78 Å at W = 85 (close to the upper limit of solubilizing power). These systems appear to be the largest W/CO microemulsion droplets ever reported. The aqueous domains of FC6-HC6 rod-like reversed micelles increased in size by 3.5 times on increasing surfactant concentration from 35 mM to 50 mM: at 35 mM, FC6-HC5 formed rod-like reversed micelles 5.3 times larger than FC6-HC6. Interestingly, these results suggest that hybrid HC-chains partition into the microemulsion aqueous cores with the sulfonate headgroups, or at the W/CO interfaces, and so play important roles for tuning the W/CO interfacial curvature. The super-efficient W/CO-type solubilizer FC6-HC4, and the rod-like reversed micelle forming surfactant FC6-HC5, represent the most successful cases of low fluorine content additives. These surfactants facilitate VOC-free, effective and energy-saving CO solvent systems for applications such as extraction, dyeing, dry cleaning, metal-plating, enhanced oil recovery and organic/inorganic or nanomaterial synthesis.

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

“…Tensiometric data are displayed in Figure 1 and interfacial properties (CMC and γ CMC ) estimated from these data are listed in Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 was found to decrease with increasing total fluorine content, as expected. [9][10][11][12][13][14][15] The lowest surface tension and CMC are respectively lower by ~9 mN/m and ~100 times than the parent anionic single FC-tail surfactants with a sodium counterion. These results clearly demonstrate synergistic effects of surfactant anion-cation pairing on surface activity and surface tension lowering in water.…”

“…A detailed description of the experimental apparatus and procedures was introduced in earlier papers. 9,[14][15][16] Formation of W/CO 2 µEs was investigated by FT-IR spectroscopy with a pressure cell (volume: 1.5 cm 3 ), connected to the HP-apparatus mentioned above. The FT-IR spectra were measured with a FT-IR spectrometer (JASCO Co., FT/IR-4700).…”

“…In contrast with the poor solubilizing power of HC surfactants, some anionic fluorinated surfactants having perfluoropolyether (PFPE), double fluorocarbon (FC) and FC-HC hybrid tailstructures were reported to be highly soluble in CO 2 and surface-active at the W/CO 2 interface, promising formation of W/CO 2 µEs. [9][10][11][12][13][14][15] In the cases of the PFPE surfactant (PFPECOONH 4 ), the hybrid surfactant FC6-HC4 and the double perfluorooctyl tail surfactant 8FG(EO) 2 (Figure S1 in supporting information), the W 0 max values were found to be ~20, ~80 and ~60, respectively. 9,14,15 Along with the efforts to explore and develop CO 2 -philic surfactants, applied research into using W/CO 2 μEs has also been conducted for nanoparticle (NP) synthesis 16 , enzymatic reactions 17 , dry cleaning 18 , and extraction 19 .…”

“…(As such W 0 max represents the Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 maximum number of water molecules which can be solubilized by one surfactant molecule). The studies aiming to find efficient CO 2 -philic solubilizers started in the 1990s 4 and continue nowadays [5][6][7][8][9][10][11][12][13][14][15] . Therefore, exploring CO 2 -philic hydrocarbon (HC) surfactants for scCO 2 remains an important task.…”

Extraction with Water‐in‐Carbon Dioxide Microemulsions: A Case Study on Steviol Glycosides

“…Surfactants that act as a bridge between water and CO 2 have been developed and studied during the past two decades (Consani and Smith, 1990;Girard et al, 2016;Peach and Eastoe, 2014;Sagisaka et al, 2003). Many surfactants have been shown to create microemulsions of water in CO 2 (Clarke and Harrison, 1997;Eastoe et al, 1997;Lee et al, 2003;Sagisaka et al, 2018). Water-in-scCO 2 microemulsions are reversed micelles with a highpolarity aqueous core.…”