Raman spectroscopy of triolein under high pressures

Abstract

This article presents results of the high pressure Raman spectroscopy of triolein. Triolein, a triacylglyceride (TAG) of oleic acid, is an unsaturated fat, present in natural oils such as olive oil. As a basic food component and an energy storage molecule, it has considerable importance for food and fuel industries. To generate pressure in the experiment, we used a high-pressure cylindrical chamber with sapphire windows, presented in (R.M. Siegoczyński, R. Kościesza, D.B. Tefelski, and A. Kos, Molecular collapse – modification of the liquid structure induced by pressure in oleic acid, High Press. Res. 29 (2009), pp. 61–66). Pressure up to 750 MPa was applied. A Raman spectrometer in “macro”-configuration was employed. Raman spectroscopy provides information on changes of vibrational modes related to structural changes of triolein under pressure. Interesting changes in the triglyceride C‒H stretching region at 2650–3100 cm−1 were observed under high-pressures. Changes were also observed in the ester carbonyl (C˭ O) stretching region 1700–1780 cm−1 and the C‒C stretching region at 1050–1150 cm−1. The overall luminescence of the sample decreased under pressure, making it possible to set longer spectrum acquisition time and obtain more details of the spectrum. The registered changes suggest that the high-pressure solid phase of triolein is organized as β-polymorphic, as was reported in (C. Akita, T. Kawaguchi, and F. Kaneko, Structural study on polymorphism of cis–unsaturated triacylglycerol: Triolein, J. Phys. Chem. B 110 (2006), pp. 4346–4353; E. Da Silva and D. Rousseau, Molecular order and thermodynamics of the solid–liquid transition in triglycerides via Raman spectroscopy, Phys. Chem. Chem. Phys. 10 (2008), pp. 4606–4613) (with temperature-induced phase transitions). The research has shown that Raman spectroscopy in TAGs under pressure reveals useful information about its structural changes.