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

Abstract

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 (W₀^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 W₀^c. The hydrodynamic diameter of Winsor-IV W/scCO₂ μE increased drastically with W₀^c 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 W₀^c and the decrease in the CO₂ density, in addition to the swelling of reversed micelles.