3801
J. Phys. Chem. 1994,98, 3801-3808
Infrared Spectroscopic and Chemical Etching Study on the Crystallization Process of the B and E Forms of Stearic Acid: Roles of Dislocations in Single Crystals Fumitoshi Kaneko,. Hirotoshi Sakashita, and Masamichi Kobayashi Department of Macromolecular Science, Faculty of Science, Osaka University, Toyonaka Osaka 560, Japan Masao Suzuki Research Laboratory, Nippon Oil and Fats Company, Amagasaki, Hyogo 580, Japan Received: September 13, 1993; In Final Form: January 20, 1994'
Crystallization processes of B and E forms of stearic acid were investigated by means of infrared spectral measurement and microscopic observation. Polymorphic and polytypic transformations took place during the crystal growth in a solution. The single-layered polytypeof the E form was generated at first, and the overgrowth of the double-layered polytype of E took place on the (001) face of the single-layered one. The polymorphic transformation from E to B occurred for specific single crystals. The single crystal of B grew as a single-layered polytype in the initial stage, consuming crystals of the unchanged E crystals through solution-mediated phase transformation. In the chemical etching experiment, the crystals that had been transformed to B quickly exhibited many etch pits due to screw dislocations on the (001) face, whereas there were very few pits on the unchanged crystals. This fact strongly suggests that the E B transition would proceed in one single crystal along the spiral structure due to screw dislocations.
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Introduction The aggregation state of lipids and related long-chain compounds changes sensitively with environmental conditions; this property closely relates to the function of these compounds in biological tissues and industrial materials. Knowledge about the mechanism of structure formationprocesses and structure changes is important for understanding the properties of these compounds profoundly. Although a large number of studies have been done on the polymorphism of long-chain compounds, there are few studies that treat the molecular mechanism of crystallization process. Usually, the crystallization process is pictured as two steps: nucleation and subsequent growth of nuclei. However, our recent study showed that the crystallization process of specific crystalline phases cannot be described with such a simple picture. Even in simple long-chain molecules such as n-fatty acids, the crystal growth process consists of several elementary processes including solid-state phase transitions. Single crystals sense small energy differences among polymorphic phases and change the growth mechanism. We indicated that the following points are the cause for the complex mechanism: (1) The relative stability of each polymorphic phase changes during crystal growth from a nucleus to a bulk crystal. (2) Crystal faces of some metastable phases inducethe heterogeneous nucleationof more stable phases. In this paper, we show that lattice defects also play a significant role in crystallization. Crystals contain various types of defects, such as point defects, screw and edge dislocations, stacking faults, structural disorder, and so on. Tht structures, concentration, and dynamic properties of these defects have a considerable effect on the mechanical properties, topochemical reactions, and crystal growth.1s2 It is well-known that a certain crystal possessing some growth steps due to screw dislocationson its crystal faces can grow even at low supersaturations through a spiral growth mechanism. Although it is expected that some defects play important roles in solid-state phase transitions, this problem has not yet been studied in detail for molecular crystals. There are various crystal structures for stearic acid, which is one of the most abundant n-fatty acids in nature.3~~ There are
* Abstract
published in Advance ACS Abstracts, March 1, 1994.
five polymorphic phases: triclinic A2 and A3 forms adopting the triclinic subcell (TIL) with a parallel arrangement of hydrocarbon zigzag planes5 and monoclinic B,6 C,7 and E8-10 forms of the orthorhombic subcell(Ol) with a perpendicular arrangement of zigzag planes. The A, B, and E forms occur only by solution crystallization, whereas the C form is formed by both solution and melt crystallization. Furthermore, two polytypes, singlelayered (Mon) and double-layered (Orth 11) structures, exist in the B and E forms,11-14 which resemble each other in structure and crystal habit (Figure 1 and Table 1). Both the Band E forms adopt a lozenge-shaped plate crystal having an acute interedge angle of 7S0, where the hydrocarbon chains tilt 2 7 O toward the b, axis of the O1 subcell (the setting of the subcell axes a,, b,, and c, is made in accordancewith theorthorhombic polyethylene), but the acyl chains take a gauche conformation at the C r C 3 bond in the B form contrary to the all-trans conformation in E. In the previous paper, we clarified that crystallization of the B form does not occur directly from solution but through a heterogeneous nucleation on the (001) face of the E crystal generated at the first stage and through a subsequent solid-state phase transition from E to B.15 Although single crystals of E never change spontaneously,they start to transform to B as soon as the heterogeneous nucleation of B begins. Since most of the molecule maintains its original arrangement during the transition (except the carboxyl group and its vicinity), the crystal habit and transparency remain unchanged. We expect that the phase boundary between the B and E regions proceeds into the E region in one bimolecular layer through a succession of the structural changes at the boundary (Figure IC). However, the mechanism whereby the B region spreads into the interior of the E crystal is still uncertain. The moving boundary hardly propagates across the lamellar interface. Since interlamellar interactions are weak compared with intralamellar interactions, the local structural change around the carboxyl group on the E B transition can occur without inducing a rearrangementof adjacent layers. Indeed, we found that the transition was interrupted in some crystals. It is well-known that many screw dislocationsin single crystals of long-chain compounds are generated during crystal growth.lG1* We postulate that the E B transition proceeds to the flunked layers along the spiral structure of bimolecular layers which are
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0 1994 American Chemical Society
3802 The Journal of Physical Chemistry, Vol. 98, No. 14, 1994
Kaneko et al. I
a B(Mon)
(b)
Mon
a E(Monl
b B(0rthIll
(C1
Orth I/
~~
,
b
€(Or th Ill
,
E B Figure 1. (a) Crystal structures of B(Mon), B(0rth 11), E(Mon), and E(0rth 11). (b) Schematic representation of Mon and Orth I1 polytypes. (c) Boundary between E and B.
TABLE 1: Cell Parameters of B ( M o ~ ) B(0rth ,~ 11),13 E(Mon),’ and E(Orth I I ) l r of Stearic Acid B(Mon) B(0rth 11) E(Mon) E(0rth 11) space group P21/a Pbca W a Pbca a, A 5.581 7.404 5.603 7.359 b, A 7.386 5.591 7.360 5.609 c, A 49.33 87.66 50.789 88.42 B, dcg 117.24 119.40
v,A3
Z
1810 4
3629 8
1825 4
3649 8
caused by a screw dislocation. One of the subjects of this paper is to confirm this postulate. We will show that the following phenomena are also related to screw dislocations: only specific single-crystal specimens of E can be transformed to B, and the frequencyof Occurrence of B and the rate of the E B transition depend on supersaturation during the nucleation of E. We also discuss the growth mechanism of the double-layered polytype (Orth 11). Although the thermodynamicand mechanical properties of single- and double-layered polytypes have been investigated by means of Raman and Brillouin spectroscopies as well as by observation of morphological changes of the B form of stearic acid,l1Jsz1the process of forming the Orth I1 structure is still ambiguous. In particular, it has been impossible to obtain single crystals of E(0rth 11) reproducibly. Therefore, there was no detailed information about the crystal structure and physi-
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cochemical properties of E(0rth 11) when we started this study. To clarify this process of polytype formation in long-chain compounds is fundamentally important for lipid science. In this study we followed the crystallization processes of the B and E forms through infrared spectral measurement and microscopicobservation,and investigated the dislocations formed in the grown single crystals through chemical etching experiments.‘
Experimental Section Samples. A high-puritysample (guaranteed more than 99.9%) of stearic acid was supplied from Nippon Oil and Fats Co. The solvents employed were n-hexane (>97%), acetone (>99%), methanol (>99.6%), and 2-butanone (>99%) purchased from Nakarai Tesque Inc. IR Measurement. Polarized infrared spectra were taken with a Jasco FT/IR-8300 and Jasco Janssen Micro FTIR spectrometers equipped with an MCT detector and a wire-grid polarizer. The resolution was set at 2 cm-1. For measurement of fine single crystals, the specimen was sandwiched in a pair of KBr plates with a small amount of hexachlorobutadiene (HCB) in order to suppress the scattering of the incident radiation. Microscopic Observation. The sample was observed with a reflective Normarsky interferencecontrast microscope (Olympus IMT2-NIC) and a polarized microscope (OlympusSZH-ILLK). n-Hexane was used as an etching agent.
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Crystallization of B and E Forms of Stearic Acid
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3803
TABLE 2 Incidence of Each Polymorphic Form for DHexane Solutions
6.0 6.0 5.0 5.0
4.3 4.2 4.0
3.1 6.6 4.6 4.5 4.5 4.5 4.5
3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 1.o
1.o 0.5
0.5 0.5 0.5
20-1
rr
6°C
c, loo EM^, 83 EM^, 80
B,, 17 B M ~20 ,
EM^, 50 EM^, 20 EM^, 80 A, 100 E ~ o a 71 , EM^, 11
42 B M ~60 , boa, 20
Eorthrr, 35
BEorthn, 6
c , 100 c , 100
A, 100 A, 50
h a n29 , h i m , 24
B E M ~8, B E M ~20 ,
iL
I500
B E M ~24 ,
C, 50
c , 100 * BE denotcs a transition state from E to B.
Classification of Single Crystals. We can easily identify the A and C forms by observing their crystal habits (A, needle-like; C, lozenge-shaped plate with an interedgeangleof 5 5 O ) . However, it is impossible to distinguish B from E through visual observation of the crystal morphology, because they have the same crystal habit with an acute interedge angle of 75O. Hence, grown single crystals were measured via IR spectroscopy to identify their polymorph and polytype. The characteristics of the IR bands are described in the previous paper," the in-plane O - C 4 deformation band (E, 688 cm-l; B, 648 cm-l), and the characteristic patterns of the progression bands due to CH2 wagging modes. Two polytypic structures can be identified using the methyl symmetric deformation b,(CH3) band. In the Orth I1 type, the 1382- and 1370-cm-1 bands polarize in the a, and 6,directions, respectively, while in the Mon type, the 1376-cm-l band is polarized a, and the 1371-cm-1 band is polarized b,. Crystallization. Solution crystallization was carried out using the following two procedures. ( 1 ) Cooling at a Constant Rate. The frequency of occurrence of each polymorph and polytype was investigated under various conditions. Cooling rate was varied in the range of 0.5-4 *C/h, and the temperature was controlled within f0.1 "C. Crystals were filtered off within a few hours after their occurrence. Mostly, well-grown single crystals (edge length about 2 mm) were obtained. ( 2 ) Crystallization by Stirring. An excess amount of stearic acid was added to n-hexane. This solutionwas stirred for several days at 20 OC. Undissolved fine crystals exhibited the characteristics of B(0rth 11). The solution saturated with B(0rth 11) was filtered with a membrane filter (pore size
