Buffering the Aqueous Phase pH in Water-in-CO2 Microemulsions

The addition of organic and inorganic buffers to nanometer size water-in-CO2 microemulsion droplets stabilized by ammonium perfluoropolyether (PFPE-NH4) results in an increase in pH from 3 to values of 5−7. The effects of temperature, pressure, buffer type, buffer concentration, ionic strength, and CO2 solubility on the pH inside water-in-CO2 microemulsions and on biphasic water−CO2 systems were measured by the hydrophilic indicator 4-nitrophenyl-2-sulfonate and were predicted accurately with thermodynamic models. In both systems, modest buffer loadings result in a steep pH “jump” from 2.5 pH units. Further increases in pH require large amounts of base to overcome buffering due to the carbonic acid−bicarbonate equilibrium. A pH approaching neutrality was obtained in w/c microemulsions with approximately 1.5 mol kg-1 NaOH. At high buffer loadings, the effects of temperature and pressure on pH values are negligible.