Fourier‐transform infrared spectra of fatty acid salts—Kinetics of high‐oleic sunflower oil saponification

Abstract

Sodium, lithium, and calcium soaps obtained by saponification of high-oleic sunflower oil were studied by Fourier-transform infrared spectroscopy. Spectra of crude mixtures containing soap, glycerin, residual alkali, and triacylglycerols were compared to those of pure soaps obtained from fatty acids. The infrared spectra of crude soaps showed the same characteristic bands as pure ones. The absorption bands of asymmetric (ω 2 ) and symmetric (ω 1 ) stretching vibrations of the carboxylate group indicated that the metal-oxygen bonds of these soaps had an ionic character whose strength differed from one cationic counterion to another. Once the characteristic absorption bands of the soaps were assigned, a kinetics study of saponification was performed. Saponification by sodium, anhydrous lithium, and calcium hydroxides was an autocatalytic reaction, characterized by an Sshaped kinetics curve, whereas saponification by aqueous lithium hydroxide was stoichiometric. The structure of the metal-oxygen bond played a role in the kinetic mechanisms.Paper no. S1345 in JSD 6, 305-310 (October 2003).KEY WORDS: Alkaline-earth soaps, alkaline soaps, higholeic sunflower oil, infrared spectroscopy, kinetics of saponification, saponification.Among all the oleaginous plants having a high-oleic acid content, only sunflower is grown on a large scale in open fields throughout the world. The high-oleic sunflower oil (HOSO) obtained from these plants is available for food uses as well as for industrial ones. The properties of HOSO are linked to their high content of oleic acid (18:1): stability vs. oxidation, fluidity, and a monounsaturated substrate. Its vegetable origin is a commercial asset for many applications, commercial as well as technical. Soap manufacture from vegetable oils is quite well known and used, but few quantitative studies have been done on the saponification reaction itself. Soaps with a high-oleic acid content once were manufactured from tallow, but because of the spread of the bovine spongiform encephalopathy and the importance of oleaginous plant farming in the northern hemisphere, e.g., sunflower cultivation, a more accurate study of saponification is now needed to produce alkaline or alkaline-earth soaps easily from vegetable oils.Study of the infrared spectra of soaps allows us to compare crude soaps, obtained from the saponification of triacylglycerols (TAG) of vegetable oils, with those obtained by more traditional means [the neutralization of fatty acids (1,2), a double decomposition reaction between an alkaline soap and a metallic salt (3), or the fusion of fatty acids (3)]. The choice of metallic counterion in the soap is essential; physiochemical properties of the final product, and thus its uses, depend on it. Moreover, Fourier-transform infrared (FTIR) spectroscopy enables one to compare the kinetics of saponification of HOSO based on the metallic hydroxide used: sodium, lithium, or calcium hydroxide. The aim of this study was thus to characterize crude soaps and then to evaluate the kinetics of the sapo...

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