Solid-State Low Temperature → Middle Temperature Phase Transition of Linoleic Acid Studied by FTIR Spectroscopy

Temperature-dependent FTIR spectra of linoleic acid were measured over a temperature range of −80 to −30 °C to explore solid-state low-temperature (LT) → middle-temperature (MT) phase transition of linoleic acid. Band assignments were made mainly for the 900–760 cm–1 region, where the bands due to cis-olefin group’s C–H out-of-plane mode and the progression bands due to CH2 rocking–CH2 twisting modes are expected to appear. The structural evolutions during the LT → MT phase transition of linoleic acid were investigated by analyzing the progression bands with the simple-coupled oscillator model to elucidate the features of acyl chain influenced with two cis-CC bonds linked through one CH2 group. Structure changes in the LT → MT transition of linoleic acid were compared with those in the γ → α transition of oleic acid. It has been found that the former is rather localized around the cis-diene group, while the latter occurs in the larger portion between the cis-olefin group to the methyl end. It is also suggested that the conformational change in the cis-olefin group on the LT → MT transition is on a lesser scale than that on the γ → α transition.