Nucleation, nuclei stability, and crystal growth in supercooled organic melts of benzocaine and tolbutamide

The investigation of crystallization kinetics in supercooled melts has spanned more than a hundred years and is relevant to many areas of science and technology. Conventional techniques, however, limit the range of systems where this study is possible due to relatively slow crystallizers. As such, the nucleation kinetics in supercooled melt was accessed only for a few organic compounds.

Fast scanning calorimetry (FSC) allows us to greatly increase the range of molecules that can be supercooled by providing a cooling rate of thousands of degrees per second and beyond. FSC was extensively used to study nucleation and crystallization kinetics in polymers, but rather few applications of the method to small organic molecules are reported. Using the FSC technique, we have studied the nucleation and crystallization of benzocaine, a rapidly crystallizing molecule, from supercooled melt. The nucleation of benzocaine was observed at –70 °C (supercooling of about 160 degrees), which is more than 50 degrees below the glass transition of the compound. The crystallization half-times greatly depend on temperature and cover nearly five orders of magnitude.

Nucleation and crystal growth in the supercooled melt were also studied in tolbutamide. Compared to benzocaine, tolbutamide has lower critical cooling and heating rates, permitting the application of the two-stage nuclei development method based on FSC. In tolbutamide, the isothermal nucleation and crystallization kinetics were studied. A modified two-stage nuclei development technique was used to probe the nuclei stability and obtain an estimate of the lateral growth rate of the nuclei, providing a unique insight into the properties of crystal nuclei. The interplay between the nucleation, crystallization, and polymorphism of tolbutamide was also accessed.

The results of the measurements are discussed in the framework of the Classical Nucleation Theory.