Supercritical fluid crystallization of adipic acid using urea as habit modifier

The crystal morphology of adipic acid mediated by the action of urea as additive has been investigated using the supercritical antisolvent precipitation (SAS). The process was performed using acetone and carbon dioxide as solvent and antisolvent, respectively. The effect of urea concentration in the liquid solution and of precipitation pressure on the crystal habit was investigated; the products were characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRPD), differential scanning calorimetry (DSC), and infrared spectrometry (FT-IR). Urea was found to be very effective in modifying the crystal habit of adipic acid from needle-like shape with a length of several hundreds of micrometers to prismatic shape. Operating at 40 degrees C, 12 MPa, and a urea/adipic acid mass ratio of 12.5% prismatic crystals with a mean size of about 7.5 mu m and a very narrow crystal size distribution were obtained. The habit modification was attributed to the preferential adsorption of urea to the fastest growing crystal face of the needle crystal, which inhibited the crystal growth along the corresponding direction.