Study of Interactions in Supercritical Fluids and Supercritical Fluid Chromatography by Solvatochromic Linear Solvation Energy Relationships

Abstract

Linear solvation energy relationships were used to study the retention process in supercritical fluid chromatography (SFC) and to gain a better understanding of intermolecular interactions in supercritical fluids. Correlation of SFC retention data with a set of solute solvatochromic parameters, which are also applicable to gas and liquid chromatography, yields information regarding the relative contributions of dispersion, cavity formation, dipolar, and hydrogen-bonding processes to retention. Dispersion interactions and cavity formation processes dominate retention on an open tubular poly(dimethylsiloxane) stationary phase with pure carbon dioxide as the mobile phase. Dipolar interactions and hydrogen-bonding interactions are of decidedly less importance but do contribute significantly to retention. Based on prior solvatochromic studies of poly(dimethylsiloxane) and carbon dioxide, the changes in the regression coefficients with temperature and pressure are interpreted chemically. The relative importance of these contributions changes with temperature and pressure. As pressure increases, the carbon dioxide becomes more dense, and dispersion interactions between the solute and the mobile phase increase. A temperature increase at constant pressure decreases dispersion interactions with the stationary phase, as in gas chromatography, but also decreases dispersion interactions with the mobile phase, due to a decrease in carbon dioxide density. On the basis of the solvatochromic coefficients, carbon dioxide acts as both a Lewis base and a Lewis acid. The quality of fit for these correlations is very high and compares favorably with similar studies in gas chromatography and liquid chromatography, permitting the prediction of retention behavior from a solute's solvatochromic parameters.