Polymorphism of pos. I. occurrence and polymorphic transformation

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Polymorphic behavior of 1,3‐rac‐palmitoyl‐stearoy 1‐2‐oleoylglycerol, 99.9% purity (POS) was examined by X‐ray diffraction (XRD), differential scanning calorimetry (DSC), solubility measurements and optical microscopy in comparison with 1,3‐dipalmitoyl‐2‐oleoylglycerol (POP) and 1,3‐distearoyl‐2‐oleoylglycerol (SOS). Melt crystallization and solvent crystallization were examined for the occurrence of metastable and stable polymorphs. The number of independent polymorphs was four; α,δ, pseudoβ′ andβ. The lowest melting form, α, was identical to that commonly observed in POP and SOS lowest melting forms. As to the highest melting form,β, the XRD shortspacing pattern was identical toβ1 of POP and SOS. This is consistent with crystal habit:β single crystals of POS showed the same shape as those of β1 of POP and SOS. However, the melting point ofβ (POS), 35.9°C, was lower than those ofβ1 of POP, 36.7°C, and of SOS, 43.0°C. Correspondingly, solubility ofβ of POS was lower than that of β1 of POP below about 13°C, but higher above 13°C. POS did not possessβ2, which is the second stable form in POP and SOS. Two forms of6 and pseudoβ′ occurred, the latter being more stable. The structural properties ofδ showed thatδ is not identical toγ previously observed in POP and SOS. Transformation behavior from the metastable to stable polymorphs of POS showed some differences from those of POP and SOS.