Crystallization and Transformation Behavior of Triacylglycerol Binary Mixtures Forming Molecular Compounds of POP/OPO, POP/rac-PPO, and POP/sn-PPO

Fat-based soft materials generally contain different triacylglycerol (TAG) species; hence, TAG mixtures' phase behaviors determine their important physical properties. TAG binary mixtures typically exhibit eutectic, monotectic, solid solution, and molecular compound (MC) forming phases; among them, the molecular interactions and the crystallization kinetics controlling the formation processes of MC-phases remain unexplored. We used thermal analysis, X-ray diffraction, and optical microscopy to examine the crystallization and transformation kinetics of the three MC-forming 1:1 binary TAG mixtures of 1,3-dipalmitoyl-2-oleoyl glycerol (POP)/1,3-dioleoyl-2-palmitoyl glycerol (OPO), POP/1,2-dipalmitoyl-2-oleoyl-rac-glycerol (rac-PPO), and POP/1,2- dipalmitoyl-3-oleoyl-sn-glycerol (sn-PPO). In the POP/OPO mixture, the MCPOP/OPO crystals of the most stable β form with double chain length stacking (β-2L) were crystallized by simple cooling treatments. In the POP/rac-PPO mixture, the β-2L form of MCPOP/rac-PPO was crystallized by only cooling at 0.1 °C/min, while metastable β′-2L formed at the cooling rates of 0.5 °C/min and 2.0 °C/min; then β′-2L melted and partially transformed into the β form during the heating processes. The POP/sn-PPO mixture showed different and significantly intricate polymorphic behavior. First, a single component of sn-PPO crystallized in β′ with triple chain length stacking (β′-3L) during cooling. This was followed by the crystallization of POP and MCPOP/sn-PPO of the β′-2L forms at slow cooling rates of 0.1 °C/min and 0.5 °C/min, and the α-2L forms at 2.0 °C/min. During heating, the metastable β′ or α forms melted, and MCPOP/sn-PPO transformed into the β form before melting. Observations of isothermal crystal growth rates by optical microscopy revealed that sn-PPO β-3L crystals grow faster than the other single TAGs or MCs, particularly above ∼18 °C. Differences in the crystallization kinetics between sn-PPO, POP, and MCPOP/sn-PPO may cause the separate crystallization while the POP/sn-PPO mixture cools.