The binary phase behavior of 1, 3-dipalmitoyl-2-stearoyl-sn-glycerol and 1, 2-dipalmitoyl-3-stearoyl-sn-glycerol

The binary phase behavior of purified 1, 3-dipalmitoyl-2-stearoyl-sn-glycerol (PSP) and 1, 2-dipalmitoyl-3-stearoyl-sn-glycerol (PPS) was investigated at a very slow (0.1 °C/min) and a relatively fast (3.0 °C/min) cooling rate. Mixtures with molar fractions of 0.1 increments were studied in terms of melting and crystallization, polymorphism, solid fat content (SFC), hardness and microstructure. Only the α-form of a double chain length (DCL) structure was detected for all mixtures in both experiments. The kinetic phase diagram, constructed using heating DSC thermograms, displayed two distinct behaviors separated by a singularity at the 0.5_PSP composition: a eutectic in the X_PSP ≤ 0.5 and a monotectic in the X_PSP ≤ 0.5 concentration region. The singularity was attributed to the formation of a 1:1 (mol:mol) molecular compound. Apart from the segment from 0.0_PSP to the eutectic point, X_E, the simulation of the liquidus line using a model based on the Hildebrand equation suggested that the molecular interactions are strong and tend to favor the formation of unlike pairs in the liquid state and that the miscibility is not significantly dependent on cooling rate. The kinetic effects are manifest in all measured properties, particularly dramatically in the X_PSP ≤ X_E concentration region. An analysis of induction time as measured by pulse nuclear magnetic resonance (pNMR) showed that PPS retards crystal growth, an effect which can explain the peculiarity of this concentration region. At both cooling rates, fit of the SFC (%) versus time curves to a modified form of the Avrami model revealed two common growth modes for all the mixtures. The polarized light microscope (PLM) of the PSP-PPS mixtures revealed networks made of spherulitic crystallites of size, growth direction and boundaries that are varied and sensitive to composition and cooling rate. The change in the microstructure and final SFC (%), particularly noticeable at compositions close to the eutectic, explain in part the differences seen in relative hardness.