Nearcritical and supercritical ethanol as a benign solvent: polarity and hydrogen-bonding

Nearcritical (NC) and supercritical (SC) ethanol may offer novel media for both chemical reactions and separations as a replacement for environmentally undesirable organic solvents. We investigated the dipolarity/polarizability, hydrogen-bond donating acidity and accepting basicity in terms of Kamlet–Taft solvatochromism parameters π¹, α and β in saturated liquid ethanol from 25 to 225 °C and in gaseous and SC ethanol at 250 °C as a function of pressure. Reichardt’s E_T(30) scale was determined for ethanol under the same conditions. NC and SC ethanol has a wide range of solvent strength, which can be readily and continuously tuned by temperature and pressure. Liquid ethanol becomes nearly nonpolar as the temperature increases towards its critical point. The dipolarity/polarizability for SC ethanol ranges from gas-like to nonpolar liquid-like with increasing pressure. On the other hand, ethanol maintains significant hydrogen-bond donating acidity even under the supercritical conditions at 250 °C and at pressures up to 18.7 MPa. The hydrogen-bond accepting basicity, however, is considerably weakened at elevated temperatures. These well-established solvent parameters greatly improve our understanding of hot compressible ethanol, and allow us to explore the feasibility of using it in a variety of benign processes.