Polarity, supercritical fluids

Electronic solvation dynamics in non-polar supercritical fluids... 

Ternary mixtures of additives, less polar modifiers (like methylene chloride), and non-polar supercritical fluids have not been extensively used as chromatographic mobile phases. A few measurements suggest that they do not produce the dramatic decrease in solute retention (compared to the additive in methanol) implied by the increases in solvent strength in Figure 8. 

The addition of comparatively less polar alcohols to solutions of acetylacetone in water shifts its keto/enol equilibrium in favour of the less polar m-enolic form (4b), which has been quantitatively rationalized in terms of so-called pairwise solute/solvent interactions [245], The keto/enol equilibrium of ethyl acetoacetate and acetylacetone has also been studied in polar supercritical fluids such as CHF3 (//= 1.65 D) and CCIF3 [fi = 0.50 D) [246], In polar trifluoromethane, the dipolar keto form was found to be favoured, although the change in the equilibrium constant with increasing sc-fluid density [i.e. increasing pressure) was quite minor. For ab initio calculations of the relative stabilities of various enols of acetylacetone in the gas phase, and theoretical calculations of keto/enol equilibria in aqueous solutions, see references [247] and [248], respectively. 

This suggests that the inclusion of density dependent local composition mixing rules can improve models of phase behavior in highly compressible polar supercritical fluid solutions. 

As with liquids, polar solutes are most soluble in polar supercritical fluids, although nominally nonpolar fluids can be remarkably good solvents for many moderately polar compounds. Carbon dioxide, for example, at higher pressures can exhibit solvating properties intermediate between pentane and aiethylene chloride. 

One limitation of carbon dioxide as an extractant is its polarity. In its supercritical state and at low densities, CO2 has a polarity close to that of hexane. Even at extremely high pressures the solubility parameter may not approach that which is required to solubilize and extract polar analytes. This limitation can be overcome by the use of another extraction fluid, which is more polar, or by adding a polar modifier to the CO2. The most commonly used modifier with CO2 has been methanol. Increased solubilities and recoveries of polar analytes have been reported when a polar modifier is added to a less polar supercritical fluid (66-68). The ability of the supercritical fluid to dissolve a particular analyte is not the only factor, which affects extraction efficiency. The degree to which the analyte partitions into the supercritical fluid fi om the solid-sample matrix depends greatly on the sorptive and active sites on the solid matrix and the polarity of the solute (64,69). The addition of a polar modifier or entrainer, such as methanol, to a supercritical fluid such as CO2, not only increases the solubility of polar analytes in the supercritical fluid, but also may help block sorptive sites on the surface of the sample matrix. 

Y-P Sun, CE Bunker. Twisted intramolecular charge transfer of ethyl p-(N,N-diethylaminojbenzoate in the gas phase and in the low-density non-polar supercritical fluids, a quantitative spectral resolution using principal component analysis and self modeling. J Chem Soc, Chem Commun 5, 1994. 

K Takahashi, CD Jonah. The measurement of an electron transfer reaction in a non-polar supercritical fluid. Chem Phys Lett 264 297, 1997. 

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