X-ray diffraction and spectroscopic studies of oleic acid-sodium oleate

The sodium oleate-oleic acid (1:1)complex (NaHOl(2)) is characterized using X-ray diffraction, FT-IR photoacoustic spectroscopy. FT-Raman spectroscopy. and DSC. The special arrangement of hydrogen-bonded pairs of carboxylic acid and carboxylate groups into unique 'head-group' is supported by frequency shifts and partial or total disappearance of characteristic vibrations of carboxylic acid dimer and of carboxylate groups. The well-ordered state of hydrocarbon chains is demonstrated by the existence of sharp Raman bands in the C-C stretching region (1000-1150 cm(-1)) and other conformationally sensitive modes. The FT Raman results: suggest that the transition at about 32 degreesC involves the cooperative melting of methyl- and carboxyl:carboxylate-sided hydrocarbon chains. From the X-ray diffraction data it is clear that this transition is associated with the disintegration of the hydrogen-bonded carboxylate-carboxylic acid complex, followed by the separate formation of oleic acid and sodium oleate. The packing of hydrocarbon chain in the acid-soap complex is different from the parent oleic acid ol sodium oleate. The hydrocarbon chains in the NaHOl(2) form more stable packing (O-parallel to subcell) in comparison to that of oleic acid. A temperature composition phase diagram is presented. (C) 2001 Published by Elsevier Science Ireland Ltd. All rights reserved.