Chemistry and physics of lipids vol:58 issue:1-2 pages:131-144
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 beta-crystallization. Upon melting the alpha-phase, recrystallization to beta-occurs very quickly. The present data do not indicate the intervention of a detectable amount of beta'-form during the transformation of alpha to beta. The beta'-form can be crystallized from the alpha-melt without the intervention of the beta'-form. No indications are found to support the existence of a liquid-crystalline structure in the alpha-melt: the slower the recrystallization to beta, the more isotropic the melt. The alpha-beta 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 beta from the alpha-melt as compared to a beta-crystallization from the isotropic melt, mainly results from the much faster nucleation, the latter being accelerated by the persistence of crystallites in the alpha-melt. Melting of the beta-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 beta-melting temperature. The beta'-form of tripalmitin and tristearin can only be obtained from the isotropic melt and is characterized by a typical beta'-2-wide angle X-ray diffraction pattern. The two endotherms, appearing in the DSC-thermogram upon melting beta'-2, cannot be correlated with two independent beta'-subforms (beta'-2 and beta'-1). Crystal perfection and crystallinity of the beta'-form is considerably influenced by the crystallization conditions.