No abstract

Antisolvent crystallization is the most common method to study solvent effects on polymorphic crystallization. L‐Histidine (L‐his) served as a model substance. Acetonitrile, acetone, and methanol were selected as antisolvents, with water as solvent. The formation of L‐his polymorphs in antisolvent crystallization as a function of supersaturation, antisolvent volume fraction, and temperature was studied. The solubility of polymorph A of L‐his decreased with increasing volume fraction of antisolvent and increased with higher temperature. The metastable polymorph B of L‐his was obtained at higher antisolvent volume fraction and supersaturation. At lower antisolvent volume fraction and supersaturation, a mixture of polymorphs A and B was detected.

Solution-mediated polymorphic transformation (SMPT) of the metastable polymorph B of L-histidine (L-his) into the stable polymorph A in water-antisolvent solutions was performed. Methanol, acetone, and acetonitrile served as antisolvents. The SMPT was studied via nucleated batch antisolvent crystallization process by determining the change of the fraction of the stable polymorph A of L-his in the crystal phase with time during the crystallization process. The fraction of the stable polymorph A of L-his was assessed offline by Raman spectroscopy. The transformation time depended on the fractions of form A obtained at the initial stage of crystallization. The transformation rate of the metastable polymorph B into the stable polymorph A at lower antisolvent volume fraction was faster than at higher antisolvent volume fraction. The transformation time of polymorph B into polymorph A in water-acetonitrile solution was the shortest compared to the other solutions.