Phase transitions in oleic acid as studied by X-ray diffraction and FT-Raman spectroscopy

The solid-solid gamma' to alpha' transition in the melt crystallized oleic acid has been investigated using X-ray diffraction, Fourier transform (FT)- Raman spectroscopy and differential scanning calorimetry (DSC). The spectro scopic study indicates that in the gamma' to alpha' transition the conforma tion of the olefin group changes from skew-cis-skew' to skew-cis-trans. The observed splitting of many of the crystalline peaks in the wide angle regi on of X-ray pattern, including the fingerprint reflections could be simulat ed on the assumption that the unit cell of the melt crystallized gamma' pol ymorph is monoclinic with beta = 91 degrees and the other unit cell paramet ers are as given by Abrahamsson and Ryder-Nahringbauer for the solvent crys tallized gamma phase. In the X-ray diffraction studies the fingerprint refl ections are used to deduce the subcell structure for the carboxyl- and meth yl-sided chains in both phases. They indicate that the carboxyl-sided chain s keep a modified O'(parallel to) (orthorhombic) subcell, while the change in the conformation of the olefin group results in the transformation of th e subcell of the methyl-sided chains to modified M-perpendicular to (monocl inic). The methyl-sided hydrocarbon chains are partially disordered in the alpha' phase with a high content of gauche conformers, but the C-C bonds ne ar the methyl end are mostly in the tt conformation. (C) 2000 Elsevier Scie nce B.V. All rights reserved.