Structure and Crystallization Behavior of the β Phase of Oleic Acid, страница 4

Figure 3 Melting behavior of the β phase.

Similar differences were observed in X-ray diffraction profiles, as shown in Figure 4. The specimen crystallized at 11.0 °C showed very weak reflections due to long spacings for 2θ < 10°. These reflections increased in intensity after standing at 15.5 °C for 2 days. Reflections in the region of 15 < 2θ < 30° also became sharper with some of them being split into doublets. However, the overall diffraction pattern has not changed much.

Figure 4 X-ray powder diffraction patterns of β₁ and β₂.

From the above experimental results, we conclude that the β phase of oleic acid can be classified into two phases, β₁ (mp 16.3 °C) and β₂ (mp 16.0 °C). We have already reported a complex solution−crystallization mechanism of n-fatty acids accompanying a solid-state phase transition from a metastable phase to a stable phase.^19,20 We now infer that a similar phenomenon also takes place during the melt crystallization of the β phase, i.e., the stable phase β₁ was generated via a metastable state β₂. At first the crystal grows as the β₂ phase followed by the transformation of the β₂ phase to the β₁ phase through a solid-state phase transition. The observation that only a portion near the growing end melts at 16.0 °C (Figure 3) supports this interpretation. We therefore considered that the crystal grows as β₁ only in the vicinity of the melting point (16.3 °C).

Crystal Structure of β₁. The experimental conditions and the cell parameters are summarized in Table 1, and the coordinates of non-hydrogen atoms are listed in Table 2.

Table 1: Crystallographic Data of Oleic Acid β₁ Phase

Table 2: Fractional Coordinates of Non-H Atoms and Equivalent Isotropic Temperature Factors with Estimated Standard Deviations in Parentheses