Thermodynamic equilibrium, metastable zone widths, and nucleation behavior in the cooling crystallization of gestodene–ethanol systems

• The solubility data of the two polymorphs (forms I and II) of gestodene were measured.
• The metastable zone widths of both forms were determined by FBRM.
• Nucleation kinetics parameters were evaluated to understand the nucleation behavior.

A systematic investigation of nucleation behavior for the batch cooling crystallization of unseeded gestodene–ethanol solutions was carried out. The solubilities of the two polymorphs (forms I and II) of gestodene in ethanol were gravimetrically measured between 268.15 and 333.15 K under atmospheric pressure of 0.10 MPa. In addition, the metastable zone widths (MSZWs) of the gestodene–ethanol solutions were determined by the polythermal method combined with the focused beam reflectance measurement (FBRM®) technique. Moreover, polymorphic forms of the grown crystals were identified by X-ray powder diffraction (XRD) and optical microscope. Experimental results indicated that the measured MSZWs were dependent on numerous technological parameters, including cooling rate, saturation temperature, and agitation intensity. With variation of the nucleation temperature and cooling rate, forms I, II, and a mixture of the two forms were crystallized from ethanol solution. The nucleation kinetic parameters were estimated from MSZW data using the self-consistent Nývlt-like approach. Due to the high solubility of form I in ethanol at the corresponding temperature range, the stronger solute–solvent interactions confirmed that the nucleation of form I had a greater activation energy than that of form II.