Anti-solvent crystallization and transformation of thiazole-derivative polymorphs—I: effect of addition rate and initial concentrations

Abstract

Polymorphous crystals of thiazole-derivative (BPT) were crystallized by adding water (an anti-solvent for the solute) to methanol solutions of BPT. At 323K, the crystallization of the polymorphs was found to depend on the water-addition rate and the initial concentration of the BPT solution. At low initial concentrations, BH crystallized independent of the addition rate of water. However, at higher initial concentrations, the D form crystallized in addition to the BH form, especially at low water-addition rates. The increase in the proportion of the BH form with increasing water-addition rate may have resulted from an increase in the amount of water at the nucleation zone around the water droplet. The appearance of the D form with higher initial concentrations may be due to the hindrance of the hydration of the BPT molecule. The crystallized form was found to transform to several different forms, and the transformation characteristics are dependent upon the initial concentration of BPT. The transformation from the BH to the A form was observed at low initial concentrations. The transformation rate appeared to decrease with the addition rate and the initial concentration. With higher initial concentrations, both the BH and D forms transformed to the A form. At much higher concentrations, the D form transformed into the BH form and the transformation rate decreased with the addition rate. With very low addition rates, the nucleation of the D form occurred preferentially even at low methanol composition.