CRYSTALLIZATION AND PHASE-TRANSITION STUDIES OF TRIPALMITIN

The polymorphism of tripalmitin has been examined using X-ray diffraction and differential scanning calorimetry (DSC). Both the thermal characteristics of the different crystal structures and the kinetics of crystallization and phase transitions were investigated. The thermal characteristics of the crystalline states were found to be strongly influenced by the crystallization conditions. The α-phase crystallization takes place very rapidly with little undercooling (2°C) and the thermal characteristics are only slightly affected by the thermal conditions. For the β′- and β-phase, the crystallization kinetics are very strongly dependent on the degree of undercooling, which in turn results in considerable variations in thermal characteristics. For tripalmitin, the β′-phase can only be obtained properly from the isotropic melt within the temperature range of 44° to 50°C. The β′-phase cannot be crystallized under any circumstance from the α-phase, due to the kinetically much faster β-crystallization. Although their existence is generally accepted, no clear distinction can be made between the different subforms of the β′-phase (β′2 and β′1). The difference between both subforms seems to be rather a consequence of crystal perfection and crystallinity effects. The transition from β′2 to β′, cannot be described as a real phase transition. The β-phase can be obtained from the α-phase as well as from the β′-phase, the melt or by solvent crystallization. Similar to the β′-form the thermal characteristics of the β-phase can vary considerably. Again, the experimental data are not sufficient to prove the existence of multiple subforms. © 1990.