Polytypic Transition of n-Hexatriacontane during Solution

Infrared spectroscopic study of polytypic effects on the crystal-growth mechanism of n-hexatriacontane (n-C36H74). Hideki Kubota , Fumitoshi Kaneko , ...
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CRYSTAL GROWTH & DESIGN

Polytypic Transition of n-Hexatriacontane during Solution Crystallization

2004 VOL. 4, NO. 2 369-375

Hideki Kubota,† Fumitoshi Kaneko,*,† Tatsuya Kawaguchi,† and Masatsugu Kawasaki‡ Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka, 560-0043, Japan, and JASCO Corporation, Ishikawa-cho 2967-5, Hachioji, Tokyo, 192-8537, Japan Received August 13, 2003;

Revised Manuscript Received November 21, 2003

ABSTRACT: The crystallization process of n-hexatriacontane (n-C36H74) was investigated by means of micro-FTIR spectroscopy. From the results of in-situ oblique IR transmission measurements, it was confirmed that single crystals of the M011 modification exhibited a polytypic transformation during solution growth. At the initial stage of the crystallization, single crystals appeared as the single-layered polytype (Mon). The overgrowth of the double-layered polytype (Orth II) took place subsequently on the (001) face of the single crystal. After that, the single crystal gradually transformed into a single crystal of the Orth II type through a solution-mediated phase transition. It was suggested that the transformation from Mon to Orth II was caused by a drop in the supersaturation of the solution. Introduction One of the important features of normal long-chain compounds, such as n-alkanes, n-alcohols, n-fatty acids, is the great diversity of solid-state structures. Almost all of these compounds possess two or more solid phases, depending on crystallization conditions, thermal history, and so on. The solid phases usually form layered structures,1 which can be categorized with two concepts, “polymorphism” and “polytypism”. Polymorphism refers to the structure variation within one layer, with respect to molecular conformation, subcell structure, inclination of hydrocarbon chains, and so on. On the other hand, polytypism is caused solely by the difference in the stacking mode of the layers and can be considered as the higher order structural variation of a polymorph.2 The concept of polytype was historically first applied to minerals and synthetic inorganic compounds, and also organic compounds nowadays. As for long-chain compounds, polytypic structures have been found in n-alkanes,3-5 n-alcohols,6,7 n-fatty acids,8-10 and cis-unsaturated fatty acids.11 It can be recognized that polytypism is a phenomenon generally observed in long-chain compounds. n-Hexatriacontane (n-C36H74) has various crystal modifications as with other n-alkanes.12 It is confirmed that there are at least five polymorphic phases as summarized in Table 1: four monoclinic modifications13-15 and an orthorhombic one.16 The M011 modification, which is the most frequently encountered oblique polymorph in solution crystallization and considered as the most stable at the room temperature, has two polytypic structures, the single-layered structure Mon13 and the double-layered structure Orth II (Figure 1).3,17,18 Each layer consists of hydrocarbon chains inclining toward the bs axis of the O⊥ subcell (the setting of the * To whom correspondence should be addressed: Department of Macromolecular Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka, 560-0043, Japan. Tel: +81-6-6850-5453. Fax: +81-6-6850-5288. E-mail: [email protected]. † Osaka University. ‡ JASCO Corporation.

Figure 1. Single-layered (Mon) and double-layered (Orth II) polytypic structures. Table 1. Cell Parameters of the Modifications of the n-C36H74 M01113 M10114 M01214 M20115 Orth I16 Orth II3 space group a b c (Å) β (deg) V (Å3) Z

P21/a 5.57 7.42 48.35 119.1 1746 2

P21/a 7.84 4.96 48.35 108.9 1746 2

P21/a 5.12 7.42 48.35 104.4 1746 2

P21/a 8.85 4.96 48.35 124.2 1750 2

Pca21 7.42 4.96 95.14 90 3500 4

Pbca 7.40 5.58 84.60 90 3492 4

subcell axes as, bs, and cs is made in accordance with the orthorhombic polyethylene19) by 27°, as shown in Figure 2. The unit cell of the Orth II type is comprised of two layers related to the 2-fold screw axis along the normal of the interfaces between layers. To deepen the understanding of the physicochemical properties of n-alkanes, it is necessary to elucidate how each polytype is generated on crystallization. Although several studies on the polytypism of n-alkanes have been carried out, the occurrence conditions of each polytype are still uncertain. This is because the Mon and Orth II are almost impossible to distinguish with morphological observation and powder X-ray diffraction methods. The single crystals of Mon and Orth II are identical in crystal shape: lozenge-shaped crystals with an interedge angle of 72°. Since Orth II has a 2-fold screw axis along the c axis, the (00l) reflections due to long spacings are observed only at the even number of l according to the extinction rule. Therefore, the Orth

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Figure 2. Relationship between the O⊥ subcell and the main lattice of the M011 modification of n-C36H74.

II exhibits a series of (00l) reflections at almost the same 2θ angles, despite its twice repeating period. Contrary to X-ray diffraction, vibrational spectroscopy is very sensitive to small structural variation, and the difference in polytypic structures is clearly reflected in vibrational spectra. In particular, these polytypic structures, Mon and Orth II, can be easily distinguished with the oblique transmission IR measurement.20 In this study, to clarify the growth mechanism of each polytype, we made in-situ observations on a growing crystal of n-C36H74 in solution with a micro-FTIR spectrometer that we had developed for the oblique transmission measurement.21 Oblique Transmission IR Method The oblique transmission measurement is a simple and effective technique of IR measurement for the threedimensional structural study on the various solid states.23-26 Although the ordinary transmission polarized infrared spectroscopy is useful to obtain information about the orientation of functional group, the obtained information is only about the projection of the transition moments onto the basal plane of the specimen. For example, it cannot distinguish the two inclination modes, P and Q, shown in Figure 3a. However, the two arrangements give quite different intensity changes upon tilting the specimen with respect to the propagation direction of the incident radiation as shown in Figure 3b,c. Therefore, the three-dimensional arrangement of functional groups can be determined unambiguously. The detail will be described in a later section.

Kubota et al.

Figure 3. Schematic representation of the oblique transmission method. (a) Two inclination modes, P and Q. The black and white bars indicate the molecular axis of P and Q, respectively. The black and white arrows denote transition moments perpendicular to the molecular axis of black and white, respectively. (b) and (c) Arrangements of the molecular axis and transition dipole moment in an oblique sample setting; (b) the bands increase in intensity, (c) the bands decrease in intensity.

Experimental Section Sample Preparation. A high-purity sample (guaranteed more than 99.9%) of n-C36H74 was purchased from GL Sciences Inc. The solvent employed was cyclohexane (>98%) purchased from Nacalai Tesque Inc. An excess amount of n-C36H74 was added to cyclohexane. This solution was stirred for several days at 20.0 °C. The saturated supernatant solution was filtered with a membrane filter (pore size 50 µm), the in-situ IR measurement was initiated. IR spectra were measured with a micro-FTIR spectrometer equipped with a condenser system for oblique transmission measurement (Figure 4b).21 With the combination of the optical elements (an ellipsoidal mirror, a plane mirror, and polarizer) and a rotary stage, we can choose an arbitrary incident direction, which is very effective for the specimens whose orientation cannot be adjusted. For polarized IR measurement, a wire-grid polarizer was mounted on the rotary stage of the condenser system. The p-polarized radiation, whose electric vector was parallel to the plane of incidence, was applied to a lozenge-shaped single crystal along its obtuse bisectrix for studying polytypic structure, and along its acute bisectrix for measuring the crystal thickness. The angle of incidence was ca. 20° in the air. All IR spectra were measured with 2 cm-1

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Crystal Growth & Design, Vol. 4, No. 2, 2004 371

Figure 4. Optical arrangement of the in-situ IR measurement. (a) Optical cell used for the in-situ measurements. (b) Condenser and objective system of the IR microscope for the oblique transmission method.

Figure 5. Schematic representation of the sample setting for the oblique transmission measurement. The electric vector is parallel to the plane of incidence. (a) The acute bisectrix is parallel to the plane of incidence (R and β). (b) The obtuse bisectrix is parallel to the plane of incidence (γ and δ). resolution. The number of accumulation cycles for measuring each spectrum was 64.

Results and Discussion CH2 Rocking Modes in Mon and Orth II. The wellgrown single crystal of the M011 modification is a lozenge-shaped plate crystal with an interedge angle of 72°. The hydrocarbon chain inclines toward the obtuse bisectrix of the crystal, and accordingly the projections of the as and bs axes of the O⊥ subcell on the basal plane are parallel to the acute and obtuse bisectrices, respectively.13 Concerning the O⊥ subcell, the CH2 rocking mode gives rise to two intense bands at 730 and 720 cm-1, whose transition moments are parallel to the as and bs axes of the O⊥ subcell, respectively.20,22 In oblique transmission measurements, the former band appears with the incidence along the acute bisectrix, and the latter band with the incidence along the obtuse bisectrix. The bs component at 720 cm-1 band is useful for identifying each polytype, as shown in Figures 6 and 7. When oblique transmission spectra are measured with two incident directions along the obtuse bisectrix, the crystal of Mon shows a significant intensity change of the 720 cm-1 band between the two incident directions (Figure 6a), because the bs axis inclines to only one direction. On the other hand, there is no intensity change for the crystal of Orth II where two opposite inclinations appear alternately (Figure 7a). The as component at 730 cm-1 shows no particular changes between Mon and Orth II, as shown in Figures 6b and 7b, because the as axis is parallel to the basal plane of single crystals both for Mon and Orth II. Since the intensity of the 730 cm-1 bands depends solely on

Figure 6. Oblique transmission spectra of Mon. (a) Spectra taken with the sample setting where the obtuse bisectrix is set parallel to the plane of incidence. (b) Spectra taken with the sample setting where the acute bisectrix is set parallel to the plane of incidence.

the thickness of specimens, we used this band as a measure of crystal thickness. Solution Crystal Growth of n-C36H74. Figure 8 shows the results of the in-situ oblique transmission measurement at 15.2 °C. Immediately after its occurrence, a single crystal showed a clear intensity difference of the bs component of the CH2 rocking mode [rb(CH2)] at 720 cm-1 between the two incident directions, γ and δ, which suggests the single-layered struc-

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Figure 7. Oblique transmission spectra of Orth II. (a) Spectra taken with the sample setting where the obtuse bisectrix is set parallel to the plane of incidence. (b) Spectra taken with the sample setting where the acute bisectrix is set parallel to the plane of incidence.

ture Mon. Although the crystal morphology remained unchanged, the intensity ratio between γ and δ gradually approached unity as the crystal grew. Finally, the intensity difference completely disappeared, which suggests the double-layered structure Orth II. The occurrence of Orth II was supported by the spectral changes of the methyl symmetric deformation [δ(CH3)] bands around 1380 cm-1, as shown in Figure 9. The frequencies of the δ(CH3) bands are sensitive to the polytypic structure. It has been confirmed that the δ(CH3) bands are observed at 1374 and 1372 cm-1 in Mon and at 1381 and 1369 cm-1 in Orth II.27,28 In particular, the 1381 cm-1 band is a good indicator of the Orth II structure. At the inception of crystallization, only the 1374 and 1372 cm-1 bands assigned to Mon were observed. As the crystal grew, the 1381 cm-1 band due to Orth II gradually increased in intensity and the Mon components became obscure. Finally, only the bands due to Orth II were observed. These spectral changes are another indication of the polytypic transition from Mon to Orth II during crystal growth. To obtain more detailed information about the crystal growth mechanism, the time dependence of crystal thickness, solute concentration, and the Mon-Orth II ratio within a single crystal was estimated from the spectral changes. The crystal thickness, h(total), was estimated from the absorbance of the ra(CH2) band at 730 cm-1. For solution concentration, the integrated

Figure 8. Spectral changes in the CH2 rocking region during the crystal growth of M011 modification at 15.2 °C.

intensity of the band due to solute around 720 cm-1 was used. On the basis of the assumption that the Mon and Orth II regions form a layered structure, the fraction of the Mon region in a single crystal of an intermediate state, x, was estimated with the following equation

x)

(1 + R)(1 - r) (1 - R)(1 + r)

(1)

where R is the intensity ratio of the rb(CH2) band at 720 cm-1 between the two incident directions (Figure 6a) for a pure Mon crystal, and r is the intensity ratio for an intermediate state. The thickness of each polytype in a single crystal can be estimated from h(total) and x.

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Figure 9. Spectral changes in the CH3 symmetric deformation region at 15.2 °C.

Figure 10 shows the time dependence of the overall crystal thickness, the thickness of each polytype, and the supersaturation for the crystallization at 15.2 °C. Here, the supersaturation, σ, was defined as

σ≡

C - Ceq Ceq

(2)

where Ceq is the concentration at saturation with respect to Orth II, and C is the measured concentration estimated from the IR band due to the solute. However, a drastic structural change started 60 min after the beginning of the measurement. Although the crystal continued to grow while its morphology was maintained, the thickness of Mon started to decrease, when the supersaturation decreased to ca. 0.08. On the other hand, the Orth II component kept growing at a rapid pace until the crystal transformed to a complete single crystal of Orth II. Structural changes under other crystallization conditions were also investigated. Figures 11 and 12 show the results of the crystallization at 15.9 and 16.9 °C, respectively. In 15.9 and 16.9 °C, the Orth II region had been already generated when the crystal reached the size suitable for IR measurement. As the initial supersaturation decreased in the following order, 1.75 (at 15.2 °C), 1.41 (at 15.9 °C), and 0.98 (at 16.9 °C), the growth of Orth II was enhanced from the early stage of crystallization. In any case, the dissolution of Mon and the conspicuous growth of Orth II was confirmed immediately after the supersaturation declined into ca. 0.08. It is suggested that the decrease in the supersaturation accelerates the Mon-Orth II transition. From the results of the in-situ IR measurement on a growing single crystal, we infer the following crystallization process. At the inception of the solution crystallization at high supersaturations, the M011 modification appeared as the single-layered structure, Mon, exclusively. The double-layered structure, Orth II, was generated through a heterogeneous nucleation on the (001) face of the single crystal of Mon when the supersaturation decreased to a certain level. Finally, the Orth II portion grew through a solution-mediated phase transition from Mon to Orth II at low supersaturation.2,29-32 Heterogeneous Nucleation of Orth II on the (001) Surface of Mon. The above experimental results indicate that Mon and Orth II are thermodynamically

Figure 10. Time dependence of overall crystal thickness, h (top), thickness of each polytype (middle), and supersaturation (bottom). The measurement was carried out at 15.2 °C.

metastable and stable, respectively. Therefore, the solution crystallization of n-C36H74 can be regarded as an example of the crystallization obeying Ostwald’s step rule. The growth of each polytype depended significantly on supersaturation. We infer that the growth rates of Mon and Orth II change with supersaturation as shown in Figure 13. It is considered that, at high supersaturation, a single crystal appears as Mon for the advantage of surface energy over Orth II. The crystal continues to grow as Mon while the supersaturation is high (the right side of the region I of Figure 13), but the growth of Orth II becomes pronounced with the decrease of supersaturation (region II). Finally, the solution-

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Figure 11. Time dependence of thickness of each polytype (top: filled circle, Mon; open circle, OrthII) and supersaturation of solution (bottom). The measurement was carried out at 15.9 °C.

mediated transformation from Mon to Orth II takes place (region III). The occurrence of Orth II took place through the heterogeneous nucleation on the Mon crystal. In the beginnings of its growth, Orth II always appeared coexistent with Mon in a single crystal. The heterogeneous nucleation on the basal plane, i.e., the (001) face of the Mon substrate is considered to have the advantage of the following. (1) The (001) face is covered with methyl terminals. The arrangement of methyl terminals is essentially the same between Mon and Orth II. For a new layer taking the Orth II-type stacking on the (001) face, there are few differences between Mon and Orth II substrates as to the geometrical structure at the interface, which leads to comparable adhesion energies at nucleus/substrate interface. Therefore, we can conjecture that the heterogeneous nucleation of Orth II on the Mon substrate takes place with the ease of the 2D nucleation on the Orth II substrate. (2) The cohesive energy at the lamellar interface was estimated to be almost the same between Mon and Orth II.33 It seems that the change from the Mon-type stacking mode to the Orth II-type brings about no large change of the lamellar interfacial energy. In the original study by Shearer and Vand,13 they were able to grow single monoclinic crystals of M011 modification. Presum-

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Figure 12. Time dependence of thickness of each polytype (top: filled diamond, Mon; open diamond, OrthII) and supersaturation of solution (bottom). The measurement was carried out at 16.9 °C.

Figure 13. Concentration dependence of the growth rates of Mon and Orth II. (Region I) The growth rate of metastable Mon surpasses that of Orth II. (Region II) The growth rate of Orth II begins to surpass that of Mon. (Region III) Only Orth II continues growing.

ably, they did this by using a higher supersaturation with a large volume so that the supersaturation did not decrease much and did not allow Orth II to form. To clarify how the solution-growth mechanism of n-C36H74 is affected by supersaturation in detail, we are now conducting an in-situ observation on a growing single crystal under a constant supersaturation.

Polytypic Transition of n-Hexatriacontane

Conclusion The crystallization process of the M011 polymorph of n-C36H74 was followed by FTIR spectroscopy. The structural changes that occurred on a single crystal during solution growth were confirmed by an in-situ observation using an IR microscope designed for the oblique transmission measurements. Although a single crystal appears as Mon, the domain of Orth II is generated on the crystal and grows. Finally, the crystal becomes Orth II through a solution-mediated phase transition mechanism. These processes are closely connected with the variation of supersaturation. It has been demonstrated that the in-situ observation with oblique transmission IR spectroscopy is useful for the systems for which the diffraction method and morphological observation are not effective. Acknowledgment. This work was supported by a Grant-in Aid for Scientific Research from the Ministry of Education and Culture of Japan (No.09554033). References (1) Kitaigorodsky, A. I. Molecular Crystals and Molecules; Academic Press: New York, London, 1973; Chapter 1. (2) Aquilano, D.; Sgualdino, G. In Crystallization Processes in Fats and Lipid Systems; Garti, N., Sato, K., Eds.; Marcel Dekker: New York, 2001; Chapter 1. (3) Kobayashi, M.; Kobayashi, T.; Ito, Y.; Chatani, Y.; Tadokoro, H. J. Chem. Phys. 1980, 72, 2024. (4) Kobayashi, M.; Sakagami, K.; Tadokoro, H. J. Chem. Phys. 1983, 78, 6391. (5) Aquilano, D. J. Cryst. Growth 1977, 37, 215. (6) Amelinckx, S. Acta Crystallogr. 1955, 8, 530. (7) Amelinckx, S. Acta Crystallogr. 1956, 9, 16. (8) Verma, A. R. Proc. R. Soc. A 1955, 228, 34. (9) Inaoka, K.; Kobayashi, M.; Okada, M.; Sato, K. J. Cryst. Growth 1988, 87, 243. (10) Kobayashi, M.; Kobayashi, T.; Itoh, Y.; Sato, K. J. Chem. Phys. 1984, 80, 2897.

Crystal Growth & Design, Vol. 4, No. 2, 2004 375 (11) Kaneko, F.; Kobayashi, M.; Kitagawa, Y.; Matuura, Y.; Sato, K.; Suzuki, M. Acta Crystallogr. 1992, C48, 1054. (12) Keller, A. Philos. Mag. 1961, 8, 329. (13) Shearer, H.; Vand, V. Acta Crystallogr. 1956, 9, 379. (14) Sullivan, P.; Weeks, J. J. Res. NBS 1970, A 8, 203. (15) Reynhardt, E.; Fenrych, J.; Basson, I. J. Phys. Condens. Matter 1994, 6, 7605. (16) Teare, P. W. Acta Crystallogr. 1959, 12, 294. (17) Boistelle, R.; Simon, B.; Pepe, G. Acta Crystallogr. 1976, B 32, 1240. (18) Kobayashi, M.; Tadokoro, H. J. Chem. Phys. 1977, 66, 1258. (19) Bunn, C. W. Trans. Faraday Soc. 1939, 35, 482. (20) Kaneko, F.; Sirai, O.; Miyamoto, H.; Kobayashi, M.; Suzuki, M. J. Phys. Chem. 1994, 98, 2185. (21) Kubota, H.; Kaneko, F.; Kawaguchi, T.; Kawasaki, M. Vib. Spectrosc. 2003, 31, 11. (22) Snyder, R. G. J. Mol. Spectrosc. 1961, 7, 116. (23) Garton, A.; Carlsson, D. J.; Wiles, D. M. Appl. Spectrosc. 1981, 35, 432. (24) Fina, L. J.; Koenig, J. L. J. Polym. Sci. B 1986, 24, 2509. (25) Kaneko, F.; Ishikawa, E.; Kobayashi, M.; Suzuki, M. Mol. Cryst. Liq. Cryst. 1998, 316, 175. (26) Kaneko, F.; Ishikawa, E.; Kobayashi, M.; Suzuki, M. Spectrochim. Acta A 2004, 60, 9. (27) Kobayashi, M.; Kobayashi, T.; Ito, Y.; Sato, K., Bull. Mine´ ral. 1986, 109, 171. (28) Kobayashi, M.; Kobayashi, T.; Cho, Y.; Kaneko, F. Makromol. Chem. Mocromol. Symp. 1986, 5, 1. (29) Rinaudo, C.; Aquilano, D.; Boistelle, R. Acta Crystallogr. A 1979, 35, 992. (30) Sato, K.; Kobayashi, M.; Morishita, H. J. Cryst. Growth 1988, 87, 236. (31) Sato, K.; Suzuki, M. J. Am. Oil Chem. Soc. 1986, 63, 1356. (32) Kaneko, F.; Sakashita, H.; Kobayashi, M.; Suzuki, M. J. Phys. Chem. 1994, 98, 3801. (33) Kobayashi, M.; Kobayashi, T.; Cho Y.; Kaneko, F. Makromol. Chem., Macromol. Symp. 1986, 5, 1. (34) Kobayashi, M.; Kobayashi, T.; Itoh, Y.; Sato, K. J. Phys. Chem. 1987, 91, 2273. (35) Sato, K.; Kobayashi, M. In Crystals; Karl, N., Ed.; Springer: Berlin and Heidelberg, 1991.

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