Crystallization and transformation mechanisms of α, β‐ and γ‐polymorphs of ultra‐pure oleic acid

Crystallization and transformation mechanisms of ultrapure (99.999%) oleic acid were examined by Differential Scanning Calorimetry (DSC) and X‐ray diffraction. X‐ray diffraction spectra revealed three different polymorphs newly named α, β and γ, which differ from each other most significantly in the short spacing spectra. α and β were found to be equivalent to the previous data which Lutton named low and high melting polymorphs, whereas γ was newly identified in the present study. DSC studies have clarified the thermodynamic stability of the three polymorphs in a range of temperature from ‐20 to 16.2 C. β is always most stable, whereas α and γ are metastable, undergoing a reversible first‐order solid‐state transformation at ‐2.2 C (on heating). DSC also showed that the crystallization behaviors are strongly dependent on the polymorphs; α crystallizes at a much higher rate than β; despite the fact that they have close melting points (α, 13.3 C; β, 16.2 C). It was demonstrated for the first time that the above peculiar polymorphic behaviors of oleic acid are quite different from those of stearic acid, a saturated fatty acid with the same carbon chain length.