Adsorptive crystallization of benzoic acid in aerogels from supercritical solutions

In this work, adsorption and crystallization of benzoic acid in different porous carriers (silica aerogel, MCM41, Trisopor glass, zeolite) from supercritical CO₂ solutions is studied. The main purpose is to reveal the influence of the adsorptive properties of the carrier on the crystallization behavior of the solute. Therefore, both adsorption and crystallization processes are studied as a function of carrier's surface properties. Adsorption of the solute and CO₂ is measured in situ using a magnetic suspension balance, whereas crystallization is realized in a high pressure view cell. The carriers could be loaded with up to ∼35 wt.% of benzoic acid, depending on the nature and the amount of the functional groups of the carrier. The size of the benzoic acid particles obtained inside the aerogel matrix depends on the crystallization conditions and is in the range of ∼20 nm-50 μm. The crystallinity of the particles was studied and it is shown that the physical state of the loaded benzoic acid inside the pores of aerogels is influenced by benzoic acid-aerogel surface interactions: strong interactions favor the amorphous form, weak interactions favor crystalline particles. The amorphous form of benzoic acid is shown to be stable over a long time period. Thus, silica aerogels can be used for stabilizing amorphous forms of organic compounds, which can be used for instance in pharmaceutical applications for the improvement of drug bioavailability.