Supercritical water spectroscopic studies

Supercritical fluids (SCFs) have long fascinated chemists and over the last 30 years this interest has accelerated. There is even a journal dedicated to the subject— the Journal of Supercritical Fluids. These fluids have many fascinating and unusual properties that make them useful media for separations and spectroscopic studies as well as for reactions and synthesis. So what is an SCF Substances enter the SCF phase above their critical pressures P and temperatures (Tc) (Figure 4.1). Some substances have readily accessible critical points, for example for carbon dioxide is 304 K (31 °C) and is 72.8 atm, whereas other substances need more extreme conditions. For example for water is 647 K (374 °C) and P is 218 atm. The most useful SCFs to green chemists are water and carbon dioxide, which are renewable and non-flammable. However, critical data for some other substances are provided for comparison in Table 4.1. In addition to reactions in the supercritical phase, water has interesting properties in the near critical region and carbon dioxide can also be a useful solvent in the liquid phase. Collectively, carbon dioxide under pressurized conditions (liquid or supercritical) is sometimes referred to as dense phase carbon dioxide. 

Studies of salt effects on reactions in supercritical water are continuing, both for mechanistic reasons and because salts will be present in any process. The kinetics of salt deposition from reactions and the morphology of the salts produced are also a subject of study [19]. Reactions of inorganic compounds are also being studied in supercritical water using spectroscopic techniques [20]. 

Spectroscopic studies were performed on water in supercritical CO2 microemulsions using an ammonium carboxylate PFPE surfactant (24). FTIR spec-toscopy was used to identify a bulk water phase within the microemulsion capable of solubilizing ionic species and supporting inorganic reactions. In addition, the UV-visible spectrum of the solvatochromic probe methyl orange indicated three microenvironments within the microemulsions (a) a polar microenvironment like that found in dry PFPE reverse micelles (b) bulk water microenvironment and (c) an acidic microenvironment due to CO2 dissolved in water. 

Further studies have demonstrated that PFPE-based surfactants can form microemulsions (with water cores) in supercritical CO2 (21). At higher water loadings, the CO2 was saturated with water and micelles began to solubilize water, which demonstrated bulk-like properties using spectroscopic probes. Although the PFPE-ammonium carboxylate surfactant was able to aggregate in CO2 at low water concentrations, a double-tailed surfactant, Mn(PFPE)2, was not soluble in CO2 without water. However, in the presence of water, Mn(PFPE)2-based micelles formed and the water core was able to ionize the manganese. 

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