Crystallization and crystal structure of the β phase of oleic acid (cis-9-octadecenoic acid) have been investigated with morphological observation, X-ray diffraction, and DSC (differential scanning calorimeter). The morphology of growing crystals of the β phase depends significantly on supercooling. It was found that the β phase could be classified into at least two solid modifications, β1 (mp 16.3 °C) and β2 (mp 16.0 °C). The crystal structure analysis of the β1 phase has been performed. The unit cell belongs to a triclinic system of P1̄, and the asymmetric unit contains two crystallographically independent molecules, A and B. The molecular layer is a unique interdigitated structure, where the methyl group of molecule A and the carboxyl group of molecules B (or vice versa) are located on the same plane. For the conformation around the cis-CC bond, both molecules approximate to trans−cis−trans conformation, the first case for a cis-monounsaturated fatty acid. The methyl and carboxyl side chains together form a T∥ subcell. On the basis of its crystal structure, it was speculated that the β1 phase has a unique surface structure at the (001) face. The factors for the characteristic properties of the β phase were also discussed.

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