Crystal Growth Kinetics of Piracetam Polymorphs in Ethanol and Isopropanol

The crystal growth kinetics of two different polymorphs of Piracetam have been investigated in ethanol and isopropanol. Isothermal seeded desupersaturation experiments were carried out at supersaturation ratios below 1.2 within the range of temperature 283-308 K. Liquid concentration was determined by in situ ATR-FTIR spectroscopy by a calibration-free method using principal component analysis. The power law equation and the BCF and B+S models were fitted to the experimental desupersaturation data by nonlinear optimization. The growth rates ranged 10^-7-10^-8 m/s, the growth rate order is clearly higher than unity, and the activation energies are in the range 39-66 kJ/mol for all the systems studied suggesting surface integration control. The growth of the metastable polymorph is faster than that of the stable form in both solvents. The crystal growth proceeds faster in ethanol than in isopropanol for both polymorphs. The solid-liquid interfacial energy is lower for the metastable form and is for both forms lower in ethanol than in isopropanol. The surface diffusion mass transfer rate is higher for the metastable form compared to the stable form and higher in ethanol than in isopropanol.