Phase transitions of saturated triglycerides

The a--> P phase transition of Tristearin (SSS) has been investigated both by Differential Scanning Calorimetry and Small Angle X-Ray Diffraction as a function of temperature at the same heating rate ( 2 deglmin ) in order to elucidate either this transition occurs in the solid state or is melt mediated. INTRODUCTION iriglycerides display a complex monotropic polymorphism. They frequently exhibit from 3 to 6 crystalline varieties, the occurence of which depends on the nature of the fatty acid chains esterified on the glycerol molecule, crystallization procedure (melt cooling or solvent mediation), and samples purity. These crystals are made by the stacking of triglyceride layers, the thickness of which depends on the length and insaturation of the fatty acid chains, and their angle of ti lt wi th respect t o the basal planes formed by the methyl end groups (figure 1 ). These thicknesses. also called "long spacings", can be measured by Small Angle X-Ray Diffraction at 0 < q <0.9Al . The lateral packing of the aliph?tic chains (crystalline subcell) (figure 2 ) is characterized by "short spacings" at 0.9 < q < 1.8 A-'. Transitions between these polymorphic varieties are irreversible and only possible when leading to the formation of more stable species (monotropism). The very fast kinetics observed for transitions between short chain compounds didn't allow to decide whether true melting takes place or whether the transitions occur in the solid state [ I I. However, monotropism implies that these transitions occur via the melting of the most stable species. In order to investigate these mechanisms, slower transitions between polymorphic species of a long chain triglyceride (tristearin:SSS) have been studied. One of them will be presented here. Fiaure 1 Examples of bilayer Figure 2 Schematic representation of possible chain packings of triglyceride packings (long long chains compounds: (a) Hexagonal packing, (bl Onhorhombic spacings). packing (O,), fc) Triclinic packing (T,). (short spacings). Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1993825 JOURNAL DE PHYSIQUE IV MATERIAL AND METHODS Tristearin has been synthesized and purified at the laboratory (purity 99.9 %) 121. The a-->P transition is recorded by DSC (DSC-4 Perkin-Elmer calorimeter) at a heating rate of +2OC/min. 10 mg of melted tristearin is placed in a sealed aluminium pan and rapidly quenched from 90 to 20°C to get a form. For X-Ray Diffraction, a form has been prepared by rapid quenching of the melt from 90°C to 5OC. 20 frames of 45 seconds are recorded as a function of temperature in the q=0.005 (0=0.04O) to q=1.855 a-1 (0=13.73O) range using D24 high flux of L.U.R.E as X-Ray source (1.608 A), while heating at 2 deg/min the sample contained in a thin capillary tube 1.4mm, 0.01 mm walls. Heating is monitored by Eurotherm 902P temperature controller using 2x0.2 mm type E thermocouple as a sensor. In order to minimize the temperature deviations from linear increase versus time during heating, thermocouple was located between the sample and the heating device rather than very close to the sample. The thermal gradient created by this set-up resulted in a temperature lag between recorded and sample temperature which has been evaluated to be about + 1 OC. A 1024 channels gas-filled one-dimensional position sensitive detector is used for data collection. Fiaure 3 Transitions between polymorphic Table 1 Thermal and structural features varieties of SSS. of SSS polymorphs [31. RESULTS AND DISCUSSION Trisaturated triglycerides generally crystallize under 3 main polymorphs called a, P ' , and P. Alpha, which is the less stable form, is obtained by rapid quenching of the melt. Chains are vertical and hexagonaly packed with almost statistical orientation. The slow melting of this unstable species leads to the formation of the most stable variety P, the subcell of which is triclinic parallel (TI/), while anneaiing of the supercooled liquid down to close to the melting point of a yields 0' species. This last exhibits a melting point intermediate between a and P and orthorhombic perpendicular (01) subcell. Both P' and j3 present tilted chains. The main features of these SSS polymorphs are summarized in Table 1 131. Possible transitions between SSS polymorphs are summarized in figure 3. The process of a > P transition of SSS has been investigated by both X-Ray Diffraction and Differential Scanning Calorimetry as a function of temperature at the same heating rate. Figure 4 represents the a > P transition of SSS recorded at 2 deglmin by DSC. The melting of a, which starts at 53.4OC (max: 54.g°C), is immediately followed by a broad exotherm spreading until about 64OC. The overlapping of a melting and subsequent P recrystallization results in a complex recording in which both events partially neutralize each other. The small exothermic bump observed at about 61 OC corresponds t o the melting of some P' variety transiently formed between 55 and 61 OC and its subsequent and immediate recrystallization into p. The melting of this last polymorph finally occurs at about 71 OC (max: 72.1 OC). Fioure 5 Evolution of long (a) and short (b) spacings during the a-->P-->liquid transitions of SSS. Heating rate= 2 deglmin. Short and long spacings are presented as 3D plots versus temperature. A 0 0 0