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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.