Characterization of Water/Supercritical CO2 Microemulsion by UV-visible Spectroscopy and Dynamic Light Scattering

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A water/supercritical CO2 microemulsion (W/scCO2 μE) with methyl orange (MO) and sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate, 8FS(EO)2, was characterized by means of UV-visible absorption spectral and dynamic light scattering (DLS) measurements. Visual observation of the scCO2 mixtures revealed transparent and reddish scCO2 phases with and without separated excess water; they were identified as Winsor-II and Winsor-IV W/scCO2 μE, respectively. The polarities of the aqueous cores in the Winsor-IV W/scCO2 μE were examined by observing the spectral shift of the absorbance maximum of MO. It was observed that with an increase in the water-to-surfactant molar ratio (W0c), the measured absorbance maximum shifted from 418 to 423 nm, which suggests that the polarity of the environment surrounding the MO molecules was methanol-like, and the polarity increased with W0c. The hydrodynamic diameter of Winsor-IV W/scCO2 μE increased drastically with W0c but gradually with a decrease in the CO2 density. The increase in the diameter was a result of the aggregation of the μE droplets and the fusion promoted by the increase in W0c and the decrease in the CO2 density, in addition to the swelling of reversed micelles.