Abstract: During the mixing of fats and oils, solid solution systems, eutectic systems, and molecular compounds (MC) are formed due to the different intermolecular interactions of triglycerides. MC have attracted much attention due to the unique β-crystal form with a double-chain-length structure, but the crystalline properties and formation mechanism have not yet been fully clarified. In this study, mixtures of the symmetric triglycerides 1,3-dipalmitoyl-2-oleoyl-sn-glycerol (POP) and 1,3-dioleoyl-2-palmitoyl-sn-glycerol (OPO) in different proportions were investigated for thermodynamic behavior, polymorphism, and crystal morphology transformation using differential scanning calorimetry, two-dimensional multifunctional small-angle X-ray scattering, and polarized light microscopy. The results showed that under the same heating and cooling rates, all POP/OPO mixtures exhibited increased melting point and the transition of β’ to β crystal form, and only when the ratio between POP and OPO was 1:1 (m/m), did separate crystallization peaks and melting peaks appear. Meanwhile, the melting temperature was significantly increased, and the crystal form was converted from β’ to β. The 1:1 mixture showed a double-chain-length structure and prominent crystalline properties of MC. In addition, the crystallization rate had a significant effect on the formation of MC. For this mixture, slower cooling rates were more favorable to the formation of MC, while too fast cooling rates (≥ 20 ℃/min) resulted in successive crystallization of samples depending on melting points, as well as eutectic phenomena, which had a detrimental effect on the formation of MC.

Key words: binary mixtures of symmetrical triglycerides 1,3-dipalmitoyl-2-oleoyl-sn-glycerol and 1,3-dioleoyl-2-palmitoyl-sn-glycero; cooling rate; thermodynamic behavior; polymorphism; microstructure