The effect of surfactant tail structure on the stability of a water/supercritical CO2 microemulsion (W/ScCO2 mu E) was examined for various fluorinated double-tail anionic surfactants of different fluorocarbon chain lengths, F(CF2)(n) (n = 4, 6, 8, and 10), and oxyethylene spacer lengths, (CH2CH2O)(m/2) (m = 2 and 4). The phase behavior of the water/surfactant/CO2 systems was studied over a wide range Of CO2 densities from 0.70 to 0.85 g/cm(3) (temperatures from 35 to 75 degrees C and pressures up to 500 bar) and corrected water-to-surfactant molar ratios (W-0(c)). All of the surfactants yielded a W/ScCO2 mu E phase, that is, a transparent homogeneous phase with a water content larger than that permitted by the solubility of water in pure CO2. With increasing W-0(c), a phase transition occurred from the mu E phase to a macroemulsion or a lamella-like liquid crystal phase. The maximum W-0(c) value was obtained at a tail length of 12-14 angstrom, indicating the presence of an optimum surfactant tail length for W/ScCO2 mu E formation.