The pH within PFPE reverse micelles formed in supercritical CO₂

We report quantitative pH determinations within the water pool of reverse micelles derived from the surfactant ammonium carboxylate perfluoropolyether (PFPE) formed in supercritical CO₂. Toward this end, we use a fluorescent probe, Cl-NERF, that exhibits a pH-sensitive excitation spectrum and locates preferentially within the aqueous core region of the reverse micelle. Sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (commonly know as aerosol-OT or AOT) reverse micelles were formed in liquid n-heptane and used to calibrate the PFPE/CO₂ system. We report the effect of CO₂ continuous phase pressure and micelle water loading on the pH within the PFPE water pool. We compare the measured water core pH to the value computed assuming CO₂ saturation within the PFPE water pool and determine that the PFPE micelle provides a 0.5 pH unit barrier to CO₂ partitioning into the micelle water pool.