© Copyright 1998 by the American Chemical Society
VOLUME 102, NUMBER 2, JANUARY 8, 1998
LETTERS Two Martensitic Transitions in the Opposite Directions in Pentadecanoic Acid Fumitoshi Kaneko,* Junko Yano,† Hiroshi Tsujiuchi, and Kohji Tashiro Department of Macromolecular Science, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 560, Japan
Masao Suzuki Oleochemicals Laboratory, NOF Corporation, Ohama, Amagasaki 660, Japan ReceiVed: August 22, 1997; In Final Form: NoVember 18, 1997X
Solid-state phase transitions of pentadecanoic acid have been investigated with microscopic observation and micro-FTIR spectroscopy. Two martensitic phase transitions have been found; one is a transition from the B′ form to the A′ form, and the other is a transition from A′ to B′. These transitions are initiated by a mechanical stress. The product phases grow in a definite direction and are highly oriented. The occurrence of these transitions depends on temperature. Below 30 °C only the transition from B′ to A′ takes place, while above 30 °C the direction of the transition is inverted.
Introduction Fatty acids show various types of solid-state phase transitions by temperature change. For example, a large precessional displacement of hydrocarbon chains occurs during the B f C and E f C transitions of n-saturated fatty acids.1,2 For cisunsaturated fatty acids, a conformational disordering occurs on the γ f R and γ1 f R1 transitions.3-5 Recently we found a new type of transition in petroselinic acid, which can be recognized as a martensitic transition.6 A local stress triggers off the transition from the low-melting (LM) phase to the high-melting (HM) phase. By imposing a small mechanical stimulation, a domain of the single crystal transforms in an instant. The growth of the product phase is so fast that it cannot be followed with a naked eye, and the resultant transformed domain is highly oriented and shows clear extinction under cross-Nicol. Furthermore, there is a definite relation in the arrangement of the crystal axes between the mother and * To whom correspondence should be addressed. † Present address: Faculty of Applied Biological Science, Hiroshima University, Higashi-hiroshima, Hiroshima 724, Japan. X Abstract published in AdVance ACS Abstracts, December 15, 1997.
product phases. These features can be ascribed to the transition mechanism: cooperative displacements of numerous molecules accompanied by a shear deformation of unit cell.7,8 Petroselinic acid is a naturally occurring cis-monounsaturated fatty acid with rather unusual chemical and crystal structures.9-11 The polymorphism differs from that of other cis-monounsaturated fatty acids. The isomorphic structures of the LM and HM phases have not been found in other cis-monoenic acids yet. It is an important problem whether the martensitic transition is a phenomenon specific to petroselinic acid, which can be ascribed to the unique structure of the LM and HM phases. We sought another example of martensitic transition in fatty acids and found two martensitic transitions in pentadecanoic acid, between which the starting and resultant polymorphs interchange. Experimental Section Sample. The sample of pentadecanoic acid (>99.9% purity) was supplied by NOF corporation. Acetonitrile and n-hexane (>99% purity) were used as solvent for growing single crystals. Microscopic and Infrared Measurements. Single-crystal specimens were observed with a polarized microscope (Olympus
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Figure 1. Polarized infrared spectra of single-crystal specimens of the A′ form (a) and the B′ form (b) and the relationship between crystal morphology and subcell arrangement. Since the (111) plane of the O⊥ subcell is parallel to the basal plane of a single crystal in the B′ form, the O⊥ subcell projected onto this plane is somewhat distorted from a rectangle.
SZH-ILLK). Polarized infrared spectra were taken with a Jasco Janssen micro-FTIR spectrometer equipped with an MCT detector and a wire-grid polarizer. The resolution was set at 2 cm-1. Results and Discussion 1. Crystal Morphology and Subcell Arrangement of A′ and B′ Forms. There are three polymorphic phases in pentadecanoic acid, A′, B′, and C′.12 The C′ form exists only in the vicinity of the melting point, and at a room temperature we can obtain the A′13,14 and B′15,16 forms from solutions. Although both the A′ and B′ forms belong to triclinic systems (P1h), polymethylene chains are accommodated in the T| and O⊥ subcells for the A′ and B′ forms, respectively. Single crystals of the A′ and B′ forms show identical morphology, elongated hexagon, but the polymorph of a single crystal can be identified easily with IR spectra. Figure 1 shows the relationship between polarized IR spectra and crystal morphology. In the A′ form (Figure 1a), the 716 cm-1 CH2 rocking r(CH2) mode and the 1473 cm-1 CH2 scissoring δ(CH2) mode appear with polarization parallel and perpendicular to the long edge, respectively, suggesting that the skeletal plane of polymethylene chains is perpendicular to the long edge. For the B′ form, the 729 cm-1 ra(CH2) and 1472 cm-1 δa(CH2) modes show a maximum intensity with the polarization depicted with a solid line in Figure 1b, and the 720 cm-1 rb(CH2) and 1464 cm-1 δb(CH2) modes appear most intense with the polarization of a broken line. The two polarizations differ by about 80°, which is consistent with the crystal structure of the B′ form. The B′ form belongs to a triclinic system of space group P1h, and the polymethylene chains form the O⊥ subcell whose (111) plane is arranged to be parallel to the plane formed by methyl terminals. In this case the O⊥ subcell looks somewhat distorted in the view along the normal
of the flat crystal face; the projections of the as and bs axes of the O⊥ subcell onto this face intersect at 80°.16 The arrangement of the O⊥ subcell can be approximated as shown in Figure 2b. 2. B′ f A′ Transition. We found that the B′ form transformed to the A′ form by a mechanical stress at room temperature. When a single crystal of B′ was pressed with the end of a needle, a part of the crystal transformed to the A′ form in an instant (Figure 2a), as well as the LM f HM transition of petroselinic acid. The band of transformed region showed clear extinction under cross-Nicol, but its extinction angle is different from that of the unchanged A′ regions. The band runs toward the corner of 112° and makes an angle of 19° with the long edge of the elongated hexagon crystal. We have not found a transformed region running toward the corner of 118°. Clearly this martensitic transition occurs in a definite direction. The transformed region exhibited clear polarized IR spectra (Figure 3a). The r(CH2) and δ(CH2) bands appeared as singlet bands characteristic to the T| subcell. Comparing the spectra with those of a single crystal of A′ (Figure 1a), the polarization of the r(CH2) and δ(CH2) bands was interchanged. It is suggested that the skeletal plane of polymethylene chains in the transformed region is nearly parallel to the long crystal edge. These spectral changes suggest the subcell rearrangement depicted in Figure 3a on the B′ f A′ transition. 3. Relative Thermodynamical Stability of A′ and B′ Forms. Since the difference of free energy is the thermodynamic driving force for the phase transition, the A′ form must be thermodynamically stable at room temperature. The mechanical stress would be required to induce the nucleation of the A′ form in the surrounding mother phase B′. Using a solution-meditated phase transition, the relative thermodynamic stability of two polymorphs can be investigated. If two polymorphs coexist in a solution, the stable polymorph grows and the other begins to solve.
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Figure 2. Product-phase region generated by a mechanical stress: (a) B′ f A′ transition and (b) A′ f B′ transition.
Figure 3. Polarized infrared spectra of a transformed region and a schematic representation of subcell rearrangement during a transition: (a) B′ f A′ transition and (b) A′ f B′ transition.
We kept single crystals in solutions at least 2 days under stirring after the occurrence of single crystals by cooling. The polymorphs of the remaining single crystals were identified using a micro FT-IR spectrometer. Table 1 shows the results at various temperatures. The A′ form was predominantly obtained in the temperatures below 30 °C, but the B′ form prevailed in the higher temperatures. It is concluded that the free energies of the A′ and B′ forms cross around 30 °C (Figure 4) and that the A′ and B′ forms are stable in the lower and higher temperature regions, respectively. So far, we have not observed the occurrence of the B′ f A′ martensitic transition at temperatures above 30 °C, which is consistent with this conclusion. 4. A′ f B′ Transition. The A′ form becomes a metastable state above 30 °C. We inferred that the A′ f B′ martensitic transition would occur in this temperature region. We imposed a mechanical stress onto a single crystal of A′ at 40 °C; a band of the product phase appeared in an instant (Figure 2b). As
Figure 4. Gibbs energy relationship of A′ and B′ forms of pentadecanoic acid as a function of temperature. The dotted arrows mean the routes of two martensitic transitions.
well as the B′ f A′ transition, this martensitic transition proceeded toward the corner of 112°. The trail of this transition makes an angle of 64° with the long crystal edge.
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TABLE 1: Occurrence of A′ and B′ Forms of Pentadecanoic Acid at Various Temperatures temp (°C)
A′ (%)
B′ (%)
temp (°C)
A′ (%)
B′ (%)
25.8 28.5 29.6 31.0
100 100 88 33
0 0 12 67
32.0 36.5 41.6
0 0 0
100 100 100
The transformed region showed clear extinction under crossNicol and clear polarized IR spectra of B′. The r(CH2) and δ(CH2) modes split into doublets due to the O⊥ subcell, as shown in Figure 3b. The as and bs components of these modes appeared with polarizations perpendicular and parallel to the long crystal edge, respectively. These spectral data suggest the subcell rearrangement depicted in Figure 3b. As described above, these transitions were accompanied by a quick subcell rearrangement between O⊥ and T| and have a definite crystallographic relationship between mother and product phases. As to lipids-related long-chain compounds, such a quick martensitic transition has not been reported except one case in petroselinic acid, as far as we know. However, this study on a usual n-saturated fatty acid demonstrated the possibility of martensitic transitions in other long-chain compounds. Another important point is that a stress-induced transition in the reversed direction is also possible under certain conditions.
To clarify the mechanism of these transitions in detail, a structural study using X-ray diffraction methods is necessary. References and Notes (1) Kaneko, F.; Kobayashi, M.; Kitagawa, Y.; Matsuura, Y. J. Phys. Chem. 1992, 96, 7104. (2) Kaneko, F.; Shirai, O.; Miyamoto, H.; Kobayashi, M.; Suzuki, M. J. Phys. Chem. 1994, 98, 2185. (3) Kobayashi, M.; Kaneko, F.; Sato, K.; Suzuki, M. J. Phys. Chem. 1986, 90, 6371. (4) Kaneko, F.; Yamazaki, K.; Kobayashi, M.; Sato, K.; Suzuki, M. Spectrochim. Acta 1994, 50A, 1589. (5) Kaneko, F.; Yamazaki, K.; Kobayashi, M.; Kitagawa, K.; Matusura, Y.; Sato, K.; Suzuki, M. J. Phys. Chem. 1996, 100, 9138. (6) Kaneko, F.; Kobayashi, M.; Sato, K.; Suzuki, M. J. Phys. Chem. 1997, 101, 285. (7) Urusovskaya, A. A. Modern Crystallography IV. Physical Properties of Crystals; Shavslov, L. A., Ed.; Springer-Verlag: Berlin, 1988. (8) Raghavan, V.; Morris Cohen Treatise on Solid State Chemistry. Vol 5. Change of State; Plenum: New York, 1975. (9) Sato, K.; Yoshimoto, N.; Suzuki, M.; Kobayashi, M.; Kaneko, F. J. Phys. Chem. 1990, 94, 3180. (10) Kaneko, F.; Kobayashi, M.; Kitagawa, Y.; Matsuura, Y.; Sato, K.; Suzuki, M. Acta Crystallogr. 1992, C48, 1054. (11) Kaneko, F.; Kobayashi, M.; Kitagawa, Y.; Matsuura, Y.; Sato, K.; Suzuki, M. Acta Crystallogr. 1992, C48, 1057. (12) Kobayashi, M.; Kaneko, F. J. Disp. Sci. Technol. 1989, 10, 319. (13) Sydow, E. V. Acta Crystallogr. 1954, 7, 529; 1955, 8, 845. (14) Goto, M.; Asada, E. Bull. Chem. Soc. Jpn. 1980, 53, 2111. (15) Sydow, E. V. Acta Crystallogr. 1954, 7, 823. (16) Goto, M.; Asada, E. Bull. Chem. Soc. Jpn. 1984, 57, 1145. (17) Sato, K.; Kobayashi, M. Crystals 1991, 13, 65.
