Water in Supercritical Carbon Dioxide Microemulsions: Spectroscopic Investigation of a New Environment for Aqueous Inorganic Chemistry

Abstract

In this paper, we present spectroscopic evidence for the formation of water in supercritical carbon dioxide (scCO2) microemulsions stabilized by an ammonium carboxylate perfluoro polyether (PFPE) surfactant. FTIR spectroscopy has been employed to determine the existence of “bulk” (hydrogen-bonded) water (H2O and D2O) at the core of the microemulsions, and to distinguish between this and the presence of “free” (monomeric) water dissolved in the scCO2 and builds on preliminary results described elsewhere (Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624). Cloud point studies confirm that optically transparent and thermodynamically stable microemulsion solutions are formed. We have investigated the utility of these microemulsions as novel environments for reaction chemistry. In particular, we have shown that, using the PFPE surfactant, an aqueous solution of potassium permanganate (KMnO4) may be dispersed in scCO2, leading to a purple-colored solution with concentration on the order of 5 × 10-4 M, as detected by UV−vis absorption spectroscopy. Moreover, aqueous sodium nitroprusside (Na2[Fe(CN)5(NO)]) and potassium dichromate (K2Cr2O7) are also shown to be soluble in the water/PFPE/scCO2 microemulsions and to undergo simple aqueous inorganic reactions with gaseous reactants such as H2S and SO2. Methyl orange has been used to investigate the presence of a carbonic acid microenvironment in the water/PFPE/scCO2 microemulsions.