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

Abstract

Abstract The temperature dependence of the structure and dynamic properties of oleic acid γ and α 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 γ → α 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 γ phase, which operates as pre-disordering prior to the sudden and drastic structural changes at the γ → α phase-transition point. Meanwhile, the carboxy group undergoes a gradual and localized structural change in the γ phase. The structural change of carboxy group in the γ phase is likely to be caused by an equilibrium shift between two different states of dimerized 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.