Single Crystal Growth Kinetics of Two Polymorphs of Piracetam

The crystal growth of single crystals of the metastable and stable polymorph of piracetam (PCM) in ethanol and isopropanol has been investigated using the rotating disk (RD) technique. A number of seed crystals were attached onto a disk that was rotated in a supersaturated solution. The crystal growth rate in length and width directions was measured for each individual crystal, and the growth rates were also extracted for the (011) crystal facet. The metastable FII form was found to exhibit a faster growth rate than the thermodynamically stable FIII form. The impact of solvents, supersaturation, temperature, and crystal seed properties was studied. Within a range of experimental conditions, the growth kinetics of both polymorphs were strongly affected by the temperature and, to a lesser degree, by the solvent choice. For both polymorphs, faster growth rates were obtained in ethanol than in isopropanol. The growth rate exhibits a fairly linear increase with increasing supersaturation for both polymorphic forms, but the slope is different. By fitting the Burton Cabrera Frank (BCF) and Birth and Spread (B + S) theories to the growth rate data at different supersaturations and temperatures in ethanol, a lower interfacial energy and a longer mean diffusion distance on the surface is obtained for the metastable polymorph compared to the stable form. The crystal growth rates measured in the present study are in good agreement with the data previously determined using isothermal desupersaturation experiments (ISD), and so are the fundamental parameters estimated by the BCF and the B + S theories.