Retention characteristics of silica materials in carbon dioxide/methanol mixtures studied by inverse supercritical fluid chromatography.

In this work, inverse supercritical fluid chromatography was applied to characterize the surface of four silica materials (three commercial Kromasils and one silica aerogel) from chromatographic retention data. Retention factors at various pressures (150-300 bar), temperatures (25-60 °C) and modifier concentrations (5-20 vol.% methanol in CO) for a set of representative 17 solutes were correlated with the solute properties by the linear solvation energy relationships (LSER). Two types of the LSER models were identified based on different criteria. Firstly, a generally valid model with two descriptors concerning dipolarity/polarizability and solute hydrogen-bonding acceptor ability was constructed. Secondly, a group of specific models for each particular silica material was proposed. According to the statistical analysis of the modeling results, the acid-basic interactions were demonstrated to have a major contribution to the retention for all studied silicas. The intensity of these interactions decreases with increasing methanol concentration in the mobile phase, possibly due to the mixed mechanism of competitive adsorption of the modifier on silanol groups and modification of mobile phase property. Moreover, retention factors measured under constant conditions (p, T, methanol concentration) for a pair of the materials were found to be proportional in logarithmic scale implying the transferability of the adsorption free energies and the adsorption constants across four studied silica materials.