Insights into Adsorptive Drug Loading on Silica Aerogels from Supercritical CO₂

This work gives an insight into the process of adsorptive drug loading in silica aerogels from supercritical CO₂ (scCO₂). A prevalent method for such loading entails bringing the aerogels into contact with the drug-scCO₂ solution followed by slow or isothermal depressurization. Our previous work established that drug precipitates during depressurization. Consequently, the adsorptive drug loading measured by the prevalent method includes precipitation. An alternative method is presented for accurate measurement of adsorption avoiding precipitation. To illustrate the disparity between the two methods, the concentration dependence of drug loadings in scCO₂, obtained by both methods, is compared for fenofibrate, ibuprofen, and benzoic acid. A mechanism is proposed, and a mathematical model is developed for comparing the adsorptive drug loadings by both methods. The loading obtained by the present method is found to be much less than that by the prevalent method, though both correlate well with the formalism of the Langmuir adsorption equation with the same equilibrium constant at low concentrations. Equations are developed for the concentration dependence of drug loadings measured by both methods at all concentrations in scCO₂.