Fatty acid polymorph identification by infrared

IR spectra in the 7.5–25.0 µm region are shown to be unique for polymorphic modifications of long chain saturated fatty acids of even carbon number and also for the individual fatty acids. IR spectra are presented for the A‐, B‐, C‐ and E‐forms of stearic acid and for the C‐form of myristic, palmitic, arachidic and behenic acid and the differences discussed. X‐ray determined crystal long spacings for the A‐, B‐, C‐ and E‐forms of a series of homologous even carbon‐numbered acids are presented and compared with literature values. The formation of the different polymorphic forms are found to be temperature‐concentration related and affected little by polarity of the solvent. Wideline nuclear magnetic resonance second moment values are given for the polymorphic forms of stearic acid.