Effect of Fluorocarbon and Hydrocarbon Chain Lengths in Hybrid Surfactants for Supercritical CO₂

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.