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

摘要

Hybrid surfactants containing both fluorocarbon (PC) 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 (W-0). 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 similar to 17 mM, <= 400 bar, temperature <= 75 degrees C, and W-0=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 (W-0 similar to 80 at 45 degrees C and 350 bar) with a hydrocarbon length, m, of 4. The W-0 value of 80 is the highest for a HC-FC hybrid surfactant, matching the highest value reported for a PC 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 W-0. 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 W-0 (17 mM and W-0 = 20), the longest reversed micelles were obtained for FC6-HC6 where a mean aspect ratio of 6.3 was calculated for the aqueous cores.