Polymorphism of Cis-Unsaturated Fatty Acid Amide: Oleamide

The polymorphism of oleamide, an amide derivative of oleic acid, the most abundant naturally occurring cis-unsaturated fatty acid, was studied mainly by differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD) using a high-purity sample. The melt-crystallized sample showed two reversible solid-state phase transitions, revealing three crystalline phases, A–C, in order from the low-temperature side. The three solid phases gradually transformed to another solid phase, D, by holding them below 35 °C; D reverted only to C when heated above 45 °C. The B → C reversible transition was accompanied by a large transition enthalpy amounting to about one-third of the heat of fusion of the C phase. A low-purity oleamide showed no polymorphism. The XRD profiles in the long-spacing region suggested that all four phases formed the segregated lamellar structure, in which the methyl and amide groups were separated.