Synchrotron radiation investigations of the polymorphic transitions of saturated monoacid triglycerides. Part 1: Tripalmitin and tristearin

Abstract

Time-resolved X-ray diffraction studies of the polymorphic behaviour of tripalmitin and tristearin during crystallization and melting, have been performed using synchrotron radiation in combination with DSC. Both triglycerides show a large tendency towards β-crystallization. Upon melting the α-phase, recrystallization to β occurs very quickly. The present data do not indicate the intervention of a detectable amount of β′-form during the transformation of α to β. The β′-form can be crystallized from the α-melt without the intervention of the β′-form. No indications are found to support the existence of a liquid-crystalline structure in the α-melt: the slower recrystallization to β, the more isotropic the melt. The α-β transition cannot be described as a simple rearrangement of the molecules, but seems to involve a nucleation and crystal growth process. The high crystallization rate of β from the α-melt as compared to a β-crystallization from the isotropic melt, mainly results from the much faster nucleation, the latter being accelerated by the persistence of crystallites in the α-melt. Melting of the β-form involves a simultaneous increase in disorder in the lateral direction (in the a—b-plane) and along the c-axis of the crystal cell (lamellar arrangement). The present data do not support the persistence of a bilayer structure in the melt above the β-melting temperature. The β′-form of tripalmitin and tristearin can only be obtained from the isotropic melt and is characterized by a typical β′ 2-wide angle X-ray diffraction pattern. The two endotherms, appearing in the DSC-thermogram upon melting β′ 2, cannot be correlated with two independent β′-subforms ( β′ 2 and β′ 1). Crystal perfection and crystallinity of the β′-form is considerably influenced by the crystallization conditions.