Physical Properties of Two Isomers of Conjugated Linoleic Acid

Thermal properties, powder X-ray diffraction patterns and FT-IR absorption spectra of crystals of two isomers of conjugated linoleic acid (CLA), 9- cis , 11- trans -CLA (c9t11), 10- trans , 12- cis -CLA (t10c12) were examined. To search for polymorphic modifications, we carefully performed crystallization from melt and solution phases, and isolated one type of crystalline form in c9t11 and t10c12. The melting temperature ( T m ) was 14.9 °C, enthalpy of fusion (Δ H ) was 38.7 kJ/mol, and entropy of fusion (Δ S ) was 134 J/mol K for c9t11, and T m = 19.8 °C, Δ H = 35.6 kJ/mol and Δ S = 122 J/mol K for t10c12. The X-ray diffraction and FT-IR measurements indicated O ⊥ subcell packing in the crystals of c9t11 and t10c12, and long spacing values of 4.22 nm for c9t11 and 3.88 nm for t10c12. The unique molecular structures of the two isomers of CLA are discussed in comparison to the polymorphism of oleic acid, petroselinic acid, elaidic acid and linoleic acid, all of which are unsaturated fatty acids having the same carbon number of 18 as that of the two CLA isomers.