Thermal and surface study of octadecanoic acid reactions with iron and magnesium

Abstract

Reactions of octadecanoic acid with magnesium, iron, iron(III) oxide powders and stainless steel gauze were examined from ambient temperature to 600 °C. Product analyses were carried out principally by using Fourier transform IR spectroscopy, scanning electron microscopy, X-ray diffraction, electron microprobe and gas chromatography. The variations in Δ H values were observed by means of differential scanning calorimetry in air with a few runs completed in argon to obtain comparison data. The magnesium soap of the octadecanoic acid formed in air at around 165 °C, and supplemental IR work on the products of reaction of bulk magnesium oxide with octadecanoic acid, confirmed that the soap arose mainly through reduction of the oxide film 200 – 400 Å thick on the metal. From about 290 to 450 °C dehydrogenation occurred and magnesium oxide was present as a reaction product at the highest temperatures. Although less active, bulk iron(III) oxide showed similar temperature zones of activity to those of the iron powder which supported an iron(III)-iron(II) oxide film approximately 100 Å thick. The product of the reaction of 23.7 wt.% iron(III) oxide with octadecanoic acid when quenched from 300 °C to ambient temperature showed a carbonyl stretch at 1590 cm -1 as evidence of soap formation. The loci of reaction initiation on both iron oxide and magnesium oxide may be sites of octahedral symmetry amenable to direct or coordination transfer of anion species such as OH -.