Structure and Crystallization Behavior of the β Phase of Oleic Acid, страница 7

The β₁ phase forms a unique interdigitated structure as shown in Figure 6. Usually, fatty acids form a segregated lamellar structure, where the methyl terminals are segregated from the carboxyl groups.^21-23 Therefore, their lamellar interfaces solely consist of methyl groups. The γ phase of oleic acid is a typical example of this segregated structure. On the other hand, the lamellar interfaces of the β₁ phase consist of methyl and carboxyl groups. In more detail, there are two types of interfaces; one interface consists of the methyl groups of molecule A and the carboxyl groups of molecule B; the other consists of the carboxyl groups of molecule A and the methyl groups of molecule B. This nonsegregated lamellar structure has not been found in other cis-unsaturated fatty acids. Only the A-super form^24,25 and the A1 form²⁶ of n-saturated fatty acids form a nonsegregated structure.

Figure 6 ORTEP view of the β₁ phase. Thermal ellipsoids are plotted at the 50% probability level.

Another important point is the conformation around the carboxyl group. We have shown that the cis-olefin group of cis-unsaturated fatty acids can take various types of conformation.^27-30 The molecular mechanics calculation using MM2³¹ on cis-3-hexene supports this conclusion.¹² The conformational energy does not depend very sensitively on dihedral angles of both CC bonds linked to the cis-CC bond. So far, we have found skew−cis−skew type (erucic acid γ1 ²⁸ and petroselinic acid high-melting phase²⁸), skew−cis−skew‘ (oleic and erucic acids γ^7,29), and distorted skew−cis−skew‘ (about 160, 0, and −160° in the petroselinic acid low-melting phase³⁰). However, the conformations of molecules A and B are quite different from these conformations. The dihedral angles of the portion C(8)C(9)C(10)C(11) are 173.9, −0.7, and 173.0° for molecule A and 174.9, −0.6, 174.9° for molecule B. Therefore, both molecules A and B can be regarded as trans−cis−trans type.

As shown in Figure 7, polymethylene chains form two types of the triclinic parallel subcell (T), which are oriented in directions different from each other. One consists of the methyl side chains of molecule A and the carboxyl side chains of molecule B (subcell 1), and the other consists of the carboxyl side chain of molecule A and the methyl side chains of molecule B (subcell 2). The b_s axis of subcell 1 is parallel to the b axis of the main lattice, while the a_s and b_s axes of subcell 2 are not parallel to the basal (001) plane. There are systematic differences in subcell parameters between the two types of subcells, as listed in Table 4. The polymethylene chains incline toward the basal plane [(001) plane] by about 45° for both subcells 1 and 2. This inclination is rather large compared with those of n-fatty acids.

Figure 7 Two types of subcell structures and their orientations of oleic acid β₁.

Table 4: Subcell Parameters

^a This subcell is formed with the carboxyl side chains of A and the methyl side chains of B.^b This subcell is formed with the methyl side chains of A and the carboxyl side chains of B.