Adsorption equilibria of benzoic acid on silica gel from supercritical carbon dioxide

Supercritical fluids, especially carbon dioxide, are increasingly used as carriers for adsorption-desorption processes, particularly in the pharmaceutical industry. Nevertheless, equilibria data for such processes are rather limited. Therefore, in this work, the adsorption equilibria of benzoic acid onto non-modified silica gel from scCO₂ were evaluated by Supercritical Fluid Chromatography applying the Peak Maxima method. Solubility of benzoic acid in scCO₂ was enhanced by addition of 2-propanol.The effects of modifier content, temperature and pressure on the solute loading were investigated. Experimental data were best described by the cubic Hill isotherm model, which accounted for the change of curvature of the elution profiles observed as concentrations in the mobile phase increased. For the concentration range reached in this study (up to 6 mg/mL), adsorption of benzoic acid was favoured at low modifier contents, high temperatures and low pressures, conditions at which the solvating power of the modified scCO₂ decreased.