A differential scanning calorimetry study of the crystallization kinetics of tristearin-tripalmitin mixtures

A comprehensive study of the isothermal crystallization kinetics of tripalmitin-tristearin mixtures was carried out using DSC, with data fitted to the Avrami equation. Polymorphs were identified by subsequent melting of samples in the differential scanning calorimeter, with additional confirmatory information obtained from wide-angle X-ray diffraction. It was found that alpha-, beta'-, and beta-forms require small (< 1.0 degrees C), moderate (3.5-8.5 degrees C), and large (9.0-13.0 degrees C) amounts of subcooling below their respective polymorph melting temperatures for nucleation to occur. Concurrent crystallization of beta and beta' polymorphs was not observed. The beta polymorphs exhibited sharper heat flow exotherms than beta', due to the higher crystallization driving forces experienced. Analysis of apparent induction times shows that the activation free energy of nucleation for the beta-form is significantly higher than for the beta'-form. Samples rich in either species crystallized faster (both shorter apparent induction times and sharper peaks) than samples with equivalent compositions. Driving-force arguments do not fully explain this behavior, strongly suggesting that mass transfer resistances (greatest for equivalent compositions) have a significant effect on kinetics. Multiple crystallization events were observed for 50-80% tristearin samples between 56 and 60 degrees C and were attributed to a demixing of tripalmitin-rich and tristearin-rich beta phases, in line with established phase diagrams.