Characterization of water/supercritical CO2 microemulsion by UV-visible spectroscopy and dynamic light scattering

A water/supercritical CO(2) microemulsion (W/scCO(2) microE) 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 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) microE, respectively. The polarities of the aqueous cores in the Winsor-IV W/scCO(2) microE 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) microE increased drastically with W(0)(c) but gradually with a decrease in the CO(2) density. The increase in the diameter was a result of the aggregation of the microE droplets and the fusion promoted by the increase in W(0)(c) and the decrease in the CO(2) density, in addition to the swelling of reversed micelles.