SOLVENT POLARITY PARAMETERS OF SUPERCRITICAL CARBON-DIOXIDE AS MEASURED BY INFRARED-SPECTROSCOPY

The solvent polarity parameters of supercritical carbon dioxide were determined as a function of the temperature and pressure within the ranges 318-353 K and 5.85-28.5 MPa. An infrared (IR) spectroscopic technique has been employed to determine the values of pi* dipolarity/polarizability and the Lewis basicity parameter B(MeOD) by measuring the transitions in the IR absorption wavelength of functional groups (such as C = O and O-H...BA, where BA is a hydrogen-bond acceptor), in supercritical carbon dioxide. Ultraviolet/visible (UV/vis) spectroscopy was used to determine the value of the Kamlet-Taft's solvatochromic acidity parameter, alpha. The B(MeOD)-value was enhanced with density, but was very small at the temperatures and pressures examined, almost the same as that for hexane. The alpha-value tended to be dependent on density at lower pressures, but was remarkably smaller compared with those for alcohols. The pi*-value significantly increased with an increase in the density, becoming more negative at higher temperatures above 10 MPa. Furthermore, the E(T)(30)-value was estimated by using the determined pi*-value. A reliable E(T)(30)-value for supercritical carbon dioxide has been obtained.