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...

Polymorphic crystallization of SOS (sn-1,3-distearoyl-2-oleoyl glycerol) has been studied with a time-resolved synchrotron radiation X-ray diffraction method (SR-XRD) using pure samples. SOS has five polymorphs:  α, γ, β‘, β2, and β1. Emphasis was placed on analyzing the events of melt-mediated transformation, in which the crystallization of more stable forms (γ, β‘, and β2) was induced by rapid melting of the less stable forms. This mode of crystallization is closely related to a long running dispute concerning a “memory effect” possibly occurring in the polymorphic crystallization of triacylglycerols from liquid. Two types of thermal processes were applied:  (a) a rapid heating soon after the least stable α form was crystallized from liquid and (b) a rapid heating after thermal annealing of the α form for 20 min before its melting. The SR-XRD measurements were monitored at a time interval of 10 s. The main results are as follows:  (1) the formation of lamellar ordering of SOS occurred more rapidly than that of subcell packing, as exhibited in the earlier occurrence of SR-XRD long spacing spectra in comparison to the short spacing spectra, (2) the α-melt mediation without thermal annealing unveiled the formation of two types of liquid crystalline structures having long spacing values of 5.1 and 4.6 nm, (3) the γ form directly occurred in the α-melt mediation involving the thermal annealing, which generated γ-embryos in the solid phase of α to serve as seed materials, and (4) the formation of the second most stable β2 form of SOS was obtained after quite long incubation of the β‘-melt mediation, yet the most stable β1 did not crystallize. The present study has given deeper insights of the polymorphism of SOS in comparison to previous studies with optical, calorimetric, and conventional XRD techniques:  in particular the presence of the liquid crystalline form and the formation of γ-embryos were directly observed for the first time.

The phase behavior of thesn‐1,3‐dipalmitoyl‐2‐oleoylglycerol (PPP‐POP) binary mixture system was studied by powder X‐ray diffraction with synchrotron radiation and by differential scanning calorimetry. The results showed that the immiscible phases were observed in metastable and in the most stable forms. In particular, synchrotron X‐ray diffraction enabled us to reveal the monotectic nature of α as a kinetic phase behavior. The equilibrium phase diagram of the PPP‐POP mixture is divided into two regions. In POP concentration ratios below 40%, solid‐state transformation from α to β was observed, indicating that the α‐β transition of PPP was promoted in the presence of POP. By contrast, the polymorphic transition proceeds from α to β through the occurrence of the intermediate β′ form at POP concentration ratios above 50%.

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