Hybrid Fluorocarbon−Hydrocarbon CO2-philic Surfactants. 1. Synthesis and Properties of Aqueous Solutions
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Abstract
Six different hybrid fluorocarbon-hydrocarbon (F-H) sulfate and sulfonate surfactants, with variations in the relative F/H carbon chain length, have been synthesized and characterized in aqueous solution. These compounds have been targeted for potential activity in densified CO2. Tensiometric data and chemical analyses were consistent with surfactants of high chemical purity. Fluorination in terms of the F/H ratio exerts a strong control over all the surfactant physicochemical properties, including critical micelle concentrations (cmc's) and adsorption isotherms. One of these partially fluorinated surfactants (the sulfonate phi-F6H4) achieves very low surface tensions in water (gamma(cmc) approximately 19 mN m(-1)) more reminiscent of fully fluorinated double-chain compounds. Detailed 19F NMR studies revealed that omega'-CF3 groups can exhibit separate signals for monomeric and micellized forms, hence facilitating cmc determinations. Small-angle neutron scattering investigations confirmed the presence of ellipsoidal or extended disklike micelles, depending on the F-H chain asymmetry. For example, a symmetric hybrid F8H8 generates disklike micelles, whereas chain asymmetry in F8H4 or phi-F6H4 tends to drive cylindrical aggregation structures. These changes are consistent with variations in the surfactant packing parameter caused by the different chain F/H ratios. Hence, adsorption and aggregation are shown to respond in a predictable way to the molecular structure of these unusual surfactants.
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