Effect of surfactants on the interfacial tension between supercritical carbon dioxide and polyethylene glycol

The effects of various surfactants on the interfacial tension between supercritical CO2 and 600 MW polyethylene glycol (PEG) are reported at 45 degrees C on the basis of measurements with a novel tandem variable-volume pendant drop tensiometer. The interfacial tension of the CO2-polyethylene glycol binary system decreases from 9.0 dyn/cm at 85 bar to 3.1 dyn/cm at 300 bar, primarily because of the increase in the density and likewise the free energy density of the CO2 phase. This result is predicted quantitatively with a gradient model using the lattice fluid equation of state. The experimental results for the effects of three surfactants on the interfacial tension are described by a modified Winsor R theory in terms of the molecular interactions on each side of the interface and the surfactant solubility in each phase. At 276 bar, the addition of 1 wt % ammonium carboxylate perfluoropolyether (PFPE) surfactant reduces the interfacial tension from 3.2 to 2.1 dyn/cm and the inter facial area of the surfactant is 437 Angstrom(2)/molecule. In contrast, surface activity is not observed for a hydrocarbon polyether (Brij30), since it favors the organic phase, or for a high molecular weight fluoropolymer (PolyFOA), since it favors the CO2 phase. Because PFPE is interfacially active, it stabilizes PEG-in-CO2 microemulsions.