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Effect of Fluorocarbon and Hydrocarbon Chain Lengths in Hybrid Surfactants for Supercritical CO2
- Sagisaka, Masanobu, Ono, Shinji, James, Craig, Yoshizawa, Atsushi, Mohamed, Azmi, Guittard, Frédéric, Rogers, Sarah E., Heenan, Richard K., Yan, Ci, Eastoe, Julian
- Langmuir 2015 v.31 no.27 pp. 7479-7487
- aggregation behavior, carbon dioxide, droplets, micelles, neutrons, perfluorocarbons, solubilization, surfactants, temperature
- Hybrid surfactants containing both fluorocarbon (FC) and hydrocarbon (HC) chains have recently been shown to solubilize water and form elongated reversed micelles in supercritical CO₂. To clarify the most effective FC and HC chain lengths, the aggregation behavior and interfacial properties of hybrid surfactants FCm-HCn (FC length m/HC length n = 4/2, 4/4, 6/2, 6/4, 6/5, 6/6, and 6/8) were examined in W/CO₂ mixtures as functions of pressure, temperature, and water-to-surfactant molar ratio (W₀). The solubilizing power of hybrid surfactants for W/CO₂ microemulsions was strongly affected by not only the FC length but also by that of the HC. Although the surfactants having short FC and/or HC tails (namely, m/n = 4/2, 4/4, and 6/2) did not dissolve in supercritical CO₂ (even at ∼17 mM, ≤400 bar, temperature ≤ 75 °C, and W₀ = 0–40), the other hybrid surfactants were able to yield transparent single-phase W/CO₂ mixtures identified as microemulsions. The solubilizing power of FC6-HCm surfactants reached a maximum (W₀ ∼ 80 at 45 °C and 350 bar) with a hydrocarbon length, m, of 4. The W₀ value of 80 is the highest for a HC-FC hybrid surfactant, matching the highest value reported for a FC surfactant which contained more FC groups. High-pressure small-angle neutron scattering measurements from FCm-HCn/D₂O/CO₂ microemulsions were consistent with growth of the microemulsion droplets with increasing W₀. In addition, not only spherical reversed micelles but also nonspherical assemblies (rodlike or ellipsoidal) were found for the systems with FC6-HCn (n = 4–6). At fixed surfactant concentration and W₀ (17 mM and W₀ = 20), the longest reversed micelles were obtained for FC6-HC6 where a mean aspect ratio of 6.3 was calculated for the aqueous cores.