Thermal and structural properties of sn‐1,3‐dipalmitoyl‐2‐oleoylglycerol and sn‐1,3‐dioleoyl‐2‐palmitoylglycerol binary mixtures examined with synchrotron radiation X‐ray diffraction

Abstract

Thermodynamic and polymorphic behavior of POP (sn-1,3-dipalmitoyl-2-oleoylglycerol) and OPO (sn-1,3-dioleoyl-2-palmitoylglycerol) binary mixtures was examined using differential scanning calorimetry and conventional and synchrotron radiation X-ray diffraction. A molecular compound, β C , was formed at the 1:1 (w/w) concentration ratio of POP and OPO, giving rise to two monotectic phases of POP/compound and compound/OPO in juxtaposition. β C has a long-spacing value of 4.2 nm with a double chainlength structure and the melting point of 31.9°C. A structural model of the POP-OPO compound is proposed, involving the separation of palmitoyl and oleoyl chain leaflets in the double chainlength structure. In the polymorphic occurrence of the POP-OPO mixtures, the POP fraction transformed from α to β′ with no passage through γ, then transformed to β. The presence of OPO in POP promoted the β′-β transformation of POP during the melt-mediated crystallization. JAOCS 74, 1213-1220 (1997).KEY WORDS: Binary mixture, differential scanning calorimetry, phase behavior, polymorphism, synchrotron radiation X-ray diffraction, triacylglycerol.Triacylglycerols (TAG) are employed in oils, margarine, cream, chocolate, and other edible fats products, and the fat structures are the major factors to determine their physical properties, such as texture, plasticity, morphology, and so on (1-4). Polymorphism is of the highest importance in the physical properties of TAG. Polymorphism of TAG has been elucidated at a molecular level for pure systems (5-7). In the practical systems, however, the fats are present in multicomponent states consisting of different types of TAG species. The first step in approaching the multicomponent systems is to study the binary systems of major TAG components, which reveal the nature of molecular interactions between the component materials at macroscopic and microscopic dimensions.Various types of phase diagrams of the binary mixtures of TAG were reviewed by Small (8), Knoester et al. (9), Timms (10), and Rossell (11). As a typical mixture system, tristearin-tripalmitin (SSS-PPP) mixtures have been studied by many researchers (11-13), where metastable α and β′ forms exhibited continuous solid solutions, yet eutectic nature was revealed in the most stable β form. The formation of the molecular compound was suggested in the mixture of sn-1,3-dipalmitoyl-2-oleoylglycerol and sn-1,3-dioleoyl-2-palmitoylglycerol (POP-OPO) (P, palmitoyl; O, oleoyl) by Moran (14) who indicated two binary systems, in juxtaposition, of POP/compound and compound/OPO. In this mixture, the compound was estimated to form at an equal concentration ratio. Most recently, Engstrom (15), Koyano et al. (16), and Minato et al. (17) reported precise analyses of the mixture systems of sn-1,3-distearoyl-2-oleoylglycerol and 1,2-distearoyl-3-oleoylglycerol (SOS/SSO) (S, stearoyl), sn-1,3-distearoyl-2-oleoylglycerol and sn-1,3-dioleoyl-2-stearoylglycerol (SOS/OSO), and sn-1,3-dipalmitoyl-2-oleoylglycerol and 1,2-dipalmitoyl-3-oleoylglycerol (POP/PPO), r...

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