STRUCTURE AND CRYSTALLIZATION BEHAVIOR OF THE BETA-PHASE OF OLEIC-ACID

Crystallization and crystal structure of the beta phase of oleic acid (cis-9-octadecenoic acid) have been investigated with morphological ob servation, X-ray diffraction, and DSC (differential scanning calorimet er). The morphology of growing crystals of the beta phase depends sign ificantly on supercooling. It was found that the beta phase could be c lassified into at least two solid modifications, beta(1) (mp 16.3 degr ees C) and beta(2) (mp 16.0 degrees C). The crystal structure analysis of the beta(1) phase has been performed. The unit cell belongs to a t riclinic system of P1, and the asymmetric unit contains two crystallog raphically independent molecules, A and B. The molecular layer is a un ique interdigitated structure, where the methyl group of molecule A an d 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 molecu les approximate to trans-cis-trans conformation, the first case for a cis-monounsaturated fatty acid. The methyl and carboxyl side chains to gether form a TII subcell. On the basis of its crystal structure, it w as speculated that the beta(1) phase has a unique surface structure at the (001) face. The factors for the characteristic properties of the beta phase were also discussed.