AGGREGATION OF AMPHIPHILIC MOLECULES IN SUPERCRITICAL CARBON-DIOXIDE - A SMALL-ANGLE X-RAY-SCATTERING STUDY

Highly soluble amphiphilic materials are shown to form aggregates in s upercritical CO2. The strategy for synthesis of these amphiphilic mole cules involves incorporating CO2-philic segments that, for this study, are perfluorinated alkyl chains. These CO2-philic regions function li ke the hydrocarbon tails of conventional surfactant molecules used in liquid organic solvents. Synthesis and characterization of three diffe rent CO2 amphiphiles are reported. Subsequent small angle X-ray scatte ring (SAXS) measurements were used to characterize the aggregation of these materials in supercritical CO2. Each of the three amphiphiles st udied showed a different type of aggregation behavior. A graft copolym er consisting of a CO2-philic backbone and CO2-phobic grafts associate d into a micellar structure in the presence of water to promote hydrog en bonding. These aggregates contain approximately 600 grafts in the c ore. The commercially available surfactant perfluoroalkylpoly(ethylene oxide), or F(CF2)(6-10)CH2CH2O(CH2CH2O)(3-8)H, forms classic reverse micelle structures having radii of about 84 Angstrom under the conditi ons of high pressure required to solubilize the material. A third amph iphile, the semifluorinated alkane diblock molecule F(CF2)(10)(CH2)(10 )H, may form small aggregates of at most 4 unimers per aggregate. The improved understanding of amphiphile aggregation in CO2 will aid:in th e development of new routes for polymer and organic synthesis in this relatively benign solvent.