Phase behavior of CO₂-expanded fluorinated microemulsions

The formation of CO₂-expanded, fluorinated reverse microemulsions is demonstrated for the system of perfluoropolyether (PFPE) surfactant (CIPFPE-NH₄, MW = 632) and PFPE oil (PFPE, MW = 580). The phase behavior of this system is examined as a function of temperature (25-45°C), pressure, CO₂ concentration, and water to surfactant molar ratios (W₀ = 10 and 20). Visual observations of one-phase behavior consistent with reverse microemulsion formation are further supported by spectroscopic measurements that establish the existence of a bulk water environment within the aqueous core. Microemulsion formation is not observed in the absence of CO₂ for this PFPE surfactant/PFPE oil system, and a CO₂ content greater than 70 mol % is required to induce microemulsion formation. Over the range of water loadings and temperatures investigated, the lowest cloud point pressure is observed at 46 bar (5 wt % ClPFPE-NH₄ in PFPE oil, W₀ = 20,x_CO(2) = 0.7, T = 25°C). In the regions where one-phase behavior is observed, the cloud point pressures increase with temperature, water loadings, and CO₂ content. The driving forces of microemulsion formation in the CO₂-expanded fluorinated solvent are discussed relative to traditional reverse microemulsions and CO₂-continuous microemulsions.