Solvent effect on tolbutamide crystallization induced by compressed CO2 as antisolvent

The aim of this work is to investigate the crystallization of tolbutamide induced by the addition of compressed carbon dioxide, with a particular focus on the role of the liquor solvent on the product characteristics. Crystals morphology and sizes were documented by microscopy and laser diffraction, respectively; since tolbutamide exists in four polymorph forms, characterizations by powder X-rays diffraction, differential scanning calorimetry and Raman spectroscopy were carried out. When processed from acetone or ethyl acetate, the drug crystallizes as polyedres and in a crystal lattice typical of Form III. If ethanol is added to acetone, Form I appears in the powder and becomes predominant for a content of 29% (in mol) and above; at the same time, mean particles size decreases. However, ethanol improves the solubilization of tolbutamide in the formed CO₂–solvent mixture, and is thus not favourable to a good yield of production. Mixtures of acetone with poor solvents such as diethyl ether and water were tested out; both enable the recovery of a mixture of Forms I and III, but with no significant improvement in sizes or yields compared with pure acetone or acetone–ethanol mixtures. Finally, the comparison with crystals obtained by evaporation indicates that the solvent itself was the main cause of the crystal phase observed, rather than the supercritical treatment.