Temperature Dependence of Structure and Dynamic Properties of Oleic Acid γ and α Phases Studied by FTIR Spectroscopy

The temperature dependence of the structure and dynamic properties of oleic acid gamma and alpha phases has been studied by FTIR spectroscopy. By observing the characteristic bands due to the cis-olefin, carboxy, and methyl groups as well as the CH2 rocking-CH2 twisting progression bands, the structural change of each portion of oleic acid corresponding to the gamma -> alpha phase transition has been elucidated in detail. For the analysis of the progression bands, the simple-coupled oscillator model has been employed. It has been found that an enhanced mobility of the methyl terminal occurs as the temperature increases in the gamma phase, which operates as pre-disordering prior to the sudden and drastic structural changes at the gamma -> alpha phase-transition point. Meanwhile, the carboxy group undergoes a gradual and localized structural change in the gamma phase. The structural change of carboxy group in the gamma phase is likely to be caused by an equilibrium shift between two different states of dimerizecl carboxy groups. Drastic structural changes at the phase-transition point primarily take place in the portion from the cis-olefin group to the methyl group, which is fully consistent with previous studies by X-ray and Raman spectroscopy. It is clearly shown that the CH2 rocking-CH2 twisting progression bands can be employed as a probe for sensing the states of the methyl-sided and carboxy-sided chains separately, which is applicable for the structural studies of a wide range of fats and lipids systems containing unsaturated acyl chains.