Phase properties of binary mixtures of petroselinic acid (PSA, cis-(omega - 12)-octadecenoic acid)/oleic acid (OA, cis-(omega - 9)-octadecenoic acid) and asclepic acid (APA, cis-(omega - 7)-octadecenoic acid)/OA were studied with differential scanning calorimetry (DSC) and X-ray diffractometry (XRD). The three unsaturated fatty acids examined differ only in the position of a double bond, having the same number of aliphatic carbon atoms, n(c) = 18. The phase diagrams of the two binary mixture systems showed eutectic properties; neither intermiscible solid solution nor compound formation systems appeared. In the full range of concentration ratios of the two mixture systems except for eutectic points, heating DSC thermograms revealed two large endothermic peaks, which correspond to the melting of high-temperature stable forms. Small endothermic peaks were also detected, corresponding to polymorphic transformations in the crystals of either component. At the eutectic points (PSA/OA around 35/65, APA/OA around 50/50), a single melting peak appeared due to overlapping of the two melting thermograms. The XRD spectra also evidenced the formation of the eutectic system, showing the existence of individual polymorphic forms which were observed in the pure systems. In a solid phase of the PSA/OA mixture, the XRD long and short spacing patterns showed the coexistence of alpha-form (OA) and low-melting (LM) form (PSA) in a high-temperature region, and of gamma-form (OA) and LM form (PSA) in a low-temperature region. Due to the polymorphic transformation from gamma-form to alpha-form of the oleic acid crystal fraction present in the eutectic mixture, small DSC endothermic peaks were detectable at the OA concentrations above 10%. The gamma --> alpha-transformation temperature decreased on approaching the eutectic point in the case of rapid crystallization, but increased when the rate of crystallization was lowered at all concentrations. This indicates partial mixing of the molecules. In the APA/OA mixture system, the DSC heating thermograms revealed multiple endothermic peaks on heating: small endothermic peaks corresponding to the transformations of gamma --> alpha (OA), gamma --> alpha (APA), and alpha --> alpha' (APA), and large peaks of the melting of alpha-form OA and alpha'-form APA. The rapid crystallization lowered the gamma --> alpha transformation temperature, most remarkably near the eutectic points. With decreasing rate of crystallization, the transition temperature was raised, and two transitions, gamma --> alpha in APA and gamma --> alpha in OA, could be differentiated clearly. The effects of the rate of crystallization on the transformation behavior indicated the partial mixing of the molecules in the two binary systems; it was more enhanced in the APA/OA mixture than in PSA/OA mixture. In summary, the present study proved that sparing interactions between the positional isomers of cis monounsaturated fatty acids caused the formation of eutectic mixture systems, even in the case that the position of double bond differs only by three (PSA-OA) or two (APA-OA) carbon atoms. This indicates the critical importance of the position of the cis-double bond in the aliphatic chain-chain interactions in the cis unsaturated fats and lipids.
