Solvent Polarity Parameters of Supercritical Carbon Dioxide as Measured by Infrared Spectroscopy

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Abstract
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 π* dipolarity/polarizability and the Lewis basicity parameter BMeOD 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 partameter, α. The BMeOD-value was enhanced with density, but was very small at the temperatures and pressures examined, almost the same as that for hexane. The α-value tended to be dependent on density at lower pressures, but was remarkably smaller compared with those for alcohols. The π*-value significantly increased with an increase in the density, becoming more negative at higher temperatures above 10 MPa. Furthermore, the ET(30)-value was estimated by using the determined π*-value. A reliable ET(30)-value for supercritical carbon dioxide has been obtained.