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Linear solvation energy relationships (LSERs) are used to probe the changes in mobile and stationary phase properties of a carbon dioxide-based mobile phase and a polymeric stationary phase under near-critical conditions. Four mobile phase modifiers are compared with respect to dipolarity/polarizability, hydrogen bond donating and accepting ability, and other intermolecular interactions as a function of temperature. As temperature nears the mixture critical point, the differences in these properties between the mobile and stationary phases change to reflect the growing heterogeneity in mobile phase component distribution at the chromatographic interface. The stationary phase loses many of its original characteristics and takes on characteristics typical of the mobile phase modifier due to preferential adsorption of the modifier at the surface of the stationary phase. [ABSTRACT FROM AUTHOR]

The temperature dependence on selectivity using modified carbon dioxide mobile phases with silica-based packed columns is found to be primarily related to the hydrogen bonding character of the analytes. Selectivity between hydrogen bond acceptor compounds is enhanced at temperatures above the mixture critical temperature. Selectivity for these compounds is further enhanced using hydrogen bond donating modifiers, such as alcohols. Selectivity between hydrogen bond donor/acceptor compounds is enhanced at lower temperatures, below the mixture critical temperature, regardless of the modifier used. Analysis of the enthalpic contributions to retention indicate that the main driving force behind these variations in temperature and modifier effects are the differences in solute heat of adsorption on the silica stationary phase. [ABSTRACT FROM AUTHOR]