STRUCTURE AND TRANSFORMATION IN POLYMORPHISM OF PETROSELINIC ACID (CIS-OMEGA-12-OCTADECENOIC ACID)

Polymorphic behavior of petroselinic acid (cis-ω-12,Δ-6-octadecenoic acid) was examined by differential scanning calorimetry (DSC), X-ray diffractometry (XRD), vibrational spectroscopy, and optical microscopy. To examine the effects of the position of a cis double bond on the polymorphism, a comparison was made to oleic acid (ω-9) and asclepic acid (ω-7) that are two positional isomers of octadecenoic acids. Two polymorphs were observed in petroselinic acid: a high-melting (HM) form and a low-melting (LM) form, the melting points being 30.5°C (HM) and 28.5°C (LM). The two forms always crystallized at the same time from the melt phase, yet the concentration of the HM form increased as the crystallization temperature increased. The thermodynamic stability of the two polymorphs was determined based on the solubility data and the transformation behavior: the HM form is more stable than the LM form above 18.7°C, and vice versa below 18.7°C, since Gibbs free energies of the two forms have the same value at 18.7°C. An irreversible solid-state transformation was observed; the LM form converted slowly to the HM form above 18.7°C, but the HM → LM transition did not occur below that temperature. Single crystals of the LM form were grown only from acetonitrile solution, but the HM form was grown both from acetonitrile solution and from the melt just below its melting point. The XRD short spacing patterns of the LM form quite resembled those of the B form of stearic acid (octadecanoic acid) which is a typical saturated fatty acid. Vibrational spectroscopic data showed that the acyl chains in the LM form were packed in an O⊥ subcell, as in the B form of stearic acid. The occurrence of the O⊥ subcell in cis-unsaturated acids was observed for the first time in this polymorph. The HM form revealed a triclinic parallel packing (T∥) of the aliphatic chains. Hence, the aliphatic packing converted from O⊥ (LM) to T∥ (HM). No interfacial melting was observed in the two forms, unlike oleic and asclepic acids, in both of which the aliphatic chain segment between the double bond and the methyl end group became disordered on heating below the melting point. Consequently, thermal and molecular structural behavior of the polymorphism of petroselinic acid is largely different from those of oleic acid and asclepic acid. This leads us to speculate that the polymorphism of the cis-unsaturated fatty acids is critically influenced by the position of the cis double bond in the aliphatic chain. © 1990 American Chemical Society.