PRESSURE EFFECT ON TRANSFORMATION OF CIS-UNSATURATED FATTY-ACID POLYMORPHS .3. ERUCIC-ACID (CIS-OMEGA-9-DOCOSENOIC ACID) AND ASCLEPIC ACID (CIS-OMEGA-7-OCTADECENOIC ACID)

The effect of hydrostatic pressure on the polymorphic transformation and melting of erucic acid and asclepic acid was investigated by means of a high-pressure differential thermal analysis. The volume changes, DELTA-V, associated with the polymorphic transformation and melting at atmospheric pressure were evaluated by applying the Clausius-Clapeyron equation. The value of DELTA-V for the gamma-alpha-transformation of asclepic acid (5.3 cm3/mol) is well comparable to those obtained for the corresponding transformations of oleic acid (5.9 cm3/Mol) and palmitoleic acid (5.6 cm3/Mol). This result indicates that the extent of disordering in omega-chain resulting from the gamma-alpha-transformation is rather insensitive to the lengths of both omega-chain and DELTA-chain of cis-unsaturated fatty acids. On the other hand, a significantly large DELTA-V value (7.5 cm 3/Mol) was obtained for the gamma-1-alpha-1-transformation of erucic acid irrespective of its nature of order-disorder transformation similar to the gamma-alpha-transformation, reflecting the difference in the crystallographic structure between gamma, alpha and their counterparts gamma-1, alpha-1. For asclepic acid, the melt-crystallization under high pressures above about 110 MPa lead to the formation of new polymorphs which are not observed at atmospheric pressure. This demonstrates that the diversity of polymorphism observed in cis-unsaturated fatty acids is extended by applying high pressure.