Phase Behavior of Binary Mixture System of Unsaturated Fatty Acids

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The phase behavior of oleic acid (OA ; cis-9 octadecenoic acid) /palmitoleic acid (POA ; cis-9-hexadecenoic acid) mixture under pressure was examined by using high pressure differential thermal analysis technique up to 200 MPa. The components in this mixture have the same DELTA chain (chain segment between double bond and carboxyl group) length, while the omega chain (aliphatic chain segment between double bond and methylene end group) length differs by 2 carbons. The temperature of solid-liquid phase boundary shift drastically toward high temperature as the pressure increases. On the other hand, the shape of the phase boundary remained almost unchanged regardless of the pressure.The mixture of POA/asclepic acid (APA ; cis-11-octadecenoic acid) is also examined. Although the miscibility of POA/APA is quite different from OA/POA,the pressure response of these mixtures are almost the same. That means that the characteristic features in solid phase of these mixtures are retained even under elevated pressure, at least up to 200 MPa.The volume change on the melting of molecular complex formed in the OA/POA mixture was calculated by the pressure dependence of the melting point, enthalpy change of melting, and pelting point. The reasonable value was obtained. This might be helpful to discuss the structure of this molecular compound, because the structure is unknown right now.