Polymorphism of linoleic acid (cis-9, cis-12-Octadecadienoic acid) and alpha-linolenic acid (cis-9, cis-12, cis-15-Octadecatrienoic acid)

Crystallization and polymorphic properties of linoleic acid (cis-9, cis-12- Octadecadienoic acid) (LA) and alpha -linolenic acid (cis-9, cis-12, cis-15 -Octadecatrienoic acid) (alpha -LNA) have been studied by optical microscop y, differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The DSC analyses presented three polymorphs in LA, and two polymorphs in alpha -LNA. The XRD patterns of the higher- and lower-temperature forms in LA an d alpha -LNA showed orthorhombic O'(parallel to) + O-like and O'(parallel t o) subcell, which were similar to those of alpha- and gamma -forms of mono- unsaturated fatty acids, respectively. From the solvent crystallization of LA and alpha -LNA in acetonitrile, single crystals of the higher temperatur e polymorphs have been obtained. The crystal habits of truncated rhombic sh ape were also similar to those of alpha -forms of the mono-unsaturated fatt y acids. The enthalpy and entropy values of fusion and dissolution of the a lpha -forms of LA, alpha -LNA and oleic acid showed that the two values dec reased with increasing number of the cis-double bond. (C) 2000 Elsevier Sci ence Ireland Ltd. All rights reserved.