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

A water/supercritical CO2 microemulsion (W/scCO(2),mu E) with methyl orange (MO) and sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosticcinate, 8FS(EO)(2), was characterized by means of UV-visible absorption spectral and dynamic light scattering (DLS) measurements. Visual observation of the scCO(2) mixtures revealed transparent and reddish scCO(2) phases with and without separated excess water; they were identified as Winsor-II and Winsor-IV W/scCO(2) mu E, respectively. The polarities of the aqueous cores in the Winsor-IV W/scCO(2) mu E 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 (W-0(c)), 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 W-0(c). The hydrodynamic diameter of Winsor-IV W/scCO(2) mu E increased drastically with W-0(c) but gradually with a decrease in the CO2 density. The increase in the diameter was a result of the aggregation of the mu E droplets and the fusion promoted by the increase in W-0(c) and the decrease in the CO2 density, in addition to the swelling of reversed micelles.