CRYSTAL GROWTH & DESIGN
Crystallization in Polymorphic Systems: The Solution-Mediated Transformation of β to r Glycine
2003 VOL. 3, NO. 1 53-60
Elena S. Ferrari,* Roger J. Davey, Wendy I. Cross,† Amy L. Gillon, and Christopher S. Towler Crystals, Colloids and Interfaces Group, Department of Chemical Engineering, UMIST, P.O. Box 88, Manchester M60 1QD, United Kingdom Received July 31, 2002;
Revised Manuscript Received October 4, 2002
ABSTRACT: The crystallization and subsequent polymorphic transformation from the β to the R form of glycine has been studied using optical microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy techniques. The crystal structure and morphology of β glycine have been determined, and the influence of solvent, solubility, and process scale on its solvent-mediated transformation to the R form was quantified. It is concluded that for the low solubility environments used in this study, the rate-determining step in the transformation process is the dissolution rate of the metastable β polymorph. Introduction Polymorphism is a phenomenon related to the crystalline solid state.1 It occurs when a molecule packs in different ways giving rise to two or more crystal structures. Polymorphs can exhibit different mechanical, thermal, and physical properties, such as compressibility, melting point, solubility, and crystal habit, which can have a great influence on the bioavailability, filtration, and tableting processes of pharmaceutical, food, and specialty materials. Consequently, it is important to make sure that the desired polymorph is consistently obtained since problems can arise when a transformation between two forms occurs during the production process. According to Ostwald’s “Rule of Stages”,2 the metastable form should appear first during crystallization from solution and it should then transform into the stable form. The ability to control the transformation process is critical in order to ensure that the correct polymorph is produced; hence, it is essential to learn which experimental factors affect the transformation kinetics. This introduces the importance of the so-called “scale-up” problem.3-5 Often, a first trial for the crystallization process is carried out on a laboratory scale (up to 100 mL), but when the process is scaled up to production size, a different polymorphic form from the one desired can be obtained. This is due to the different experimental conditions (stirring, cooling regime, nucleation mechanism, etc.) that result from increasing the volume of the crystallizing system.6 In the work reported here, a study of the effects of solvent composition and experimental scale on the polymorphic transformation of β to R glycine has been carried out. Glycine exists in three distinct polymorphic forms: R, β, and γ. β is a metastable noncentric form first reported by Fisher in 1905.7 A crystallographic study of this form was carried out by Bernal in 19318 and by Ksanda et al. in 1938.9 The single crystal structure for this polymorph was published by Iitaka.10 β glycine has been found to transform rapidly into the R form in air or water, but the * To whom correspondence should be addressed. E-mail: elena.ferrari @umist.ac.uk. † Current address: GlaxoSmithKlein, Gunnels Road, Stevenage, United Kingdom.
crystals remained unchanged if kept in a dry environment.10 Albrecht and Corey11 were the first to report the structure of the R polymorph. They found that the R crystals were centrosymmetric bipyramids. The γ form of glycine is the stable form at room temperature, and it has a noncentric structure12,13 and transforms to R when heated above 165 °C13,14 causing caking during storage. The crystallization and relative stability of these three polymorphs have been recently studied by Allen et al.15 Experimental Section Materials. The glycine employed for the present study was obtained from SigmaAldrich (99%), ethanol (99.86%) was supplied by Hymana Limited, and distilled, deionized water was used in all experiments. Growth of a Single β Glycine Crystal. A single crystal of the β polymorphic form was grown using the liquid diffusion method. A small amount (∼1 mL) of a slightly undersaturated aqueous solution of R glycine at 25 °C was placed in a tube. Ethanol (∼2 mL) was then added carefully dropwise to form a layer on the glycine solution. A few small crystals of the β form (thin needles) appeared within minutes at the interface between the two liquids. The aqueous layer was carefully removed, using a disposable pipet, before the crystals could transform. They were then filtered, washed with ethanol, and dried in an oven at 65 °C. Transformation Experiments. Glycine was crystallized by drown-out in which variable volumes of ethanol and fixed volumes of an aqueous solution of glycine (5, 10, 91, and 200 mL, respectively) were rapidly mixed. Crystallization experiments were carried out using 50 mL (small scale) and 1000 mL (scale-up) glass-jacketed vessels in which temperatures were controlled via a circulating Haake waterbath for both crystallizers. A magnetic Teflon-covered stir bar and a magnetic stirrer were used for the small scale experiment. Agitation in the 1000 mL vessel was achieved using an overhead stirrer (a two-bladed glass impeller) and a Heidolph RZR-2000 motor (150-160 rpm). Each experiment was repeated at least three times, and the transformation times presented here are average values. The solubility of the R polymorph, at the selected temperature and water/ethanol proportion, was determined gravimetrically. Glycine was left in contact with the solvent mixture overnight, filtered, washed with ethanol, and left to dry for several hours in an oven at 65 °C. The solid obtained was weighed using an Ohaus Explorer balance (max ) 210 g, d ) 0.1 mg).
10.1021/cg025561b CCC: $25.00 © 2003 American Chemical Society Published on Web 11/05/2002
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Microscopy, Diffraction, and IR. The difference in morphologies of R and β forms meant that microscopic observation could give a good indication of the initial form to crystallize and the time taken for it to totally transform. Accordingly, samples of crystal slurry were taken from the appropriate crystallizer every 10 min during the transformation experiments and analyzed visually using a Zeiss Axioplan 2 polarizing optical microscope in order to estimate the transformation times. Infrared spectra of the crystals were collected with a Thermo Nicholet Avatar-360 Fourier transform infrared spectrometer (FT-IR) using the attenuated total reflection (ATR) method. A single reflection germanium crystal was employed with a spectral range between 4000 and 675 cm-1. Slurry samples were taken from the crystallizer, filtered, washed with ethanol, and dried in an oven at 65 °C for 10 min. Powder diffraction data for one of the transformation experiments were collected using a Scintag 2000 XDS diffractometer in the angular range of 10-20°. This instrument uses a copper radiation source, of wavelength 1.54060 Å, and an Ortec GLP Series, fitted with a high purity germanium semiconductor (operating at liquid nitrogen temperature). The detector is energy sensitive, and it is tuned to the wavelength of the copper source, eliminating the need for a monochromator. No filters were used during any of the experiments. The slurries (wet samples) were placed in a shallow aluminum cell (∼0.5 mL), which was connected to a waterbath in order to keep the temperature at the desired value. The crystal structure and morphology of a β form crystal (dimensions 0.5 mm × 0.03 mm × 0.02 mm) were determined at 150 K using a Nonious KCCD diffractometer.
Results Single Crystal Study of the β Form. In agreement with previous reports,10 single crystal X-ray diffraction (XRD) data for the β polymorph indicated that the fastgrowing needle axis is the b-axis. An image of a crystal for this form is shown in Figure 1a, from which it is evident that the two ends of the crystal are different. This is due to the fact that the crystal is noncentrosymmetric; hence, the two surfaces (010) and (01 h 0) have a different chemical nature as seen in Figure 1b. In the [010] zone, the needle is bounded by the low index forms: (100), (1 h 00), (001), (001 h ). Unfortunately, the crystals grown were too small to enable precise resolution of the morphological termination along the b-axis. The crystal structure obtained in this work is compared to that previously published by Iitaka10 for data taken at 283-303 K (Table 1). The distance between the nitrogen and the carbon atoms (N-CII, Figure 2a) was found to be slightly shorter than that published in 1960, although all of the other bond lengths are longer. The bond angles were found to differ from the values reported by Iitaka by approximately (0.5-1° (see Table 1). The main difference within the molecular structures was found to be the torsion angle between the nitrogen and one of the oxygen atoms. The value for the N-CIICI-OI angle calculated from our data was 24.41°, whereas for the previous structure it was 27.32°. The H-bonding motif in the crystal was found to be the same as described by Iitaka, with the nitrogen atom surrounded by four oxygen atoms, two of which were arranged roughly in tetrahedral directions (N- - -OI′ and N- - -OII′ in Figure 2b). Transformation Experiments. To obtain a transformation time between 10 min and a few hours, an initial set of experiments was carried out to assess the most suitable values for the water/ethanol ratio in the
Figure 1. (a) Single crystal of β glycine grown from water/ ethanol. (b) The molecular packing of β glycine in the unit cell. The (010) and (01 h 0) faces are also shown.
solvent mixture and the temperature. Two compositions of water/ethanol mixture were ultimately investigated, namely, 20:80 (v/v) and 9:91 (v/v) together with a temperature of 35 °C. Higher water contents in the solvents led to very short transformation times and ultimately the direct crystallization of the R form. Overall, these data indicated that the transformation is solution-mediated with the number of R crystals increasing with time and the β steadily disappearing. Table 2 summarizes the transformation times for each experiment carried out and their average values. These results are discussed in some detail below. Small Scale (50 mL). Figure 3 shows snapshots of the transformation of glycine crystallized from the 20: 80 v/v mixtures of water/ethanol. The image in Figure 3a was taken just after the complete addition of ethanol to the aqueous solution (t ) 0) and shows the initial formation of clusters of thin needlelike β crystals. After 10 min, a few crystals of the R form were observed
Solution-Mediated Transformation of β to R Glycine
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Table 1. Cell Parameters, Space Group, Bond Lengths, and Bond Angles for β Glycine
a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) space group N-CII (Å) CII-CI (Å) CI-OI (Å) CI-OII (Å) N- - -OI (Å) N-CII-CI (deg) CII-CI-OI (deg) CII-CI-OII (deg) OI-CI-OII (deg) N-CII-CI-OI (deg) N- - -OI′ (Å) N- - -OII′ (Å) CII-N- - -OI′ (deg) CII-N- - -OII′ (deg) N- - -OI′′ (Å) N- - -OII′′ (Å) CII-N- - -OI′′ (deg) CII-N- - -OII′′ (deg)
this work
Iitaka
5.0785 6.1923 5.3869 90.00 113.346 90.00 P21 1.4765 1.5360 1.2556 1.2467 2.7107 111.804 116.661 116.892 126.396 24.409 2.7565 2.8330 113.574 115.594 2.9531 2.9755 143.714 80.804
5.077 6.267 5.379 90.00 113.12 90.00 P21 1.484 1.521 1.233 1.257 2.700 110.8 117.8 115.9 126.2 27.322 2.758 2.833 113.3 115.4 3.002 3.022 144.9 82.6
(Figure 3b). Transformation for this first mixture took about 40 min, as shown in Figure 3d. Powder diffraction data were collected on the slurry samples for this experiment in order to verify which polymorphic form had crystallized. The related three-dimensional graph is shown in Figure 4. At t ) 0, only one peak at ∼18° was observed, which was assigned to the (001) reflection of the β polymorph. The second diffraction pattern, taken after 10 min, continued to be dominated by this main peak at 18°, but in addition, a small peak at ∼19° is evident, which was assigned to the (100) reflection of the R form. The relative intensity of these two peaks changed as the transformation progressed. The intensity of the β reflection dropped toward zero, whereas the peak at 19° increased. After 60 min, only the diffraction peak corresponding to the (100) reflection of the R polymorph was observed. Quick data acquisitions were also carried out between 24 and 27° in order to check if any γ crystals were present during the transformation. However, no peak at 25.5°, corresponding to the (110) reflection for this polymorph, was observed. It is clear that the progress of the transformation is also associated with certain characteristic changes in the vibrational spectra. These are shown with corresponding times in Figure 5, and the peak positions are summarized in Table 3. First, the weak band at about 3200 cm-1, assigned to the antisymmetric stretch of the NH3+ group, shifts, as expected,10 toward lower wavenumbers. Second, the intensity of the weak peak at ∼1660 cm-1 decreased with time and it was found to disappear once the transformation was complete. Other features were also found to move toward lower wavenumbers: the antisymmetric stretch of the carboxyl group at 1580-1590 cm-1, the deformation of the NH3+ group (1515-1500 cm-1), the band at ∼1445 cm-1 (whose intensity increased with time), the group of peaks between the range of 1134-900 cm-1, and the
Figure 2. (a) Molecule of glycine showing the atoms labels. The hydrogen atoms have been omitted. (b) The H-bond motif for β glycine. Table 2. Reaction Times for the Polymorphic Transformation of β Glycine to r Glycine (σ Is the Solution Supersaturation) water/ ethanol % 20:80 (σ ) 3.1) 9:91 (σ ) 3.8) 9:91 (σ ) 4.0)
a
50 mL scale (min) 30 30 40 180 200 210 90 150 140
avg time (min) 34 197
1000 mL scale
avg time
90 min 95 min 120 min
102 min
10 h < t > 24 ha
t > 10 h
127
See text.
O-CdO group bend at about 700 cm-1. The peak corresponding to the antisymmetric stretch for the COO- group was also found to split. No significant shift was observed for the symmetric stretch of the carboxylic group at 1409 cm-1 and the band at about 1333-1332 cm-1. These data indicate, in agreement with the optical microscopy and the powder XRD, a transformation time of about 40 min. When the ratio of ethanol in the crystallization solvent mixture was increased to 91%, the formation of
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Figure 3. Some snapshots taken at different times during the polymorphic transformation of glycine crystallized from a 20:80 (v/v) water/ethanol mixture (50 mL scale). Table 3. Molecular Vibration Frequencies (cm-1) Recorded during the Polymorphic Transformation (at Time t ) 0, t ) 15, and t ) 30 min) from β Glycine to r Glycine Crystallized from 20/80 (v/v) Water/Ethanol Mixture (50 mL Scale)a vibration type
t ) 0 min
t ) 15 min
t ) 30 min
-NH3+ antisym stretch -COO- antisym stretch
3176.23 1661.70 1589.83 1515.29 1446.05 1409.46 1333.66 1134.21 1117.28 1041.58 914.99 893.00 700.82
3177.10 1661.46 1589.09 1510.98 1444.08 1409.50 1333.18 1133.62 1116.97 1039.99 914.32 892.95 699.97
3161.62 1609.00 1575.74 1500.00 1443.30 1409.75 1331.33 1132.44 1111.58 1033.99 909.59 892.51 694.20
-NH3+ deformation -CH2 bend -COO- sym stretch -CH2 wag -NH3+ rock -C-N sym stretch -CH2 rock -C-C sym stretch O-CdO bend a
Figure 4. Powder diffraction spectra of the slurry for the polymorphic transformation of glycine crystallized from a 20: 80 (v/v) water/ethanol mixture (50 mL scale).
the first few R crystals, as observed by optical microscopy, was delayed to 30 min and the overall time for the β to R transformation increased to 3 h. For this experiment, the evolution of polymorphic forms was identified by FT-IR. The spectra are shown in Figure 6, and peak positions are recorded in Table 4. These
Sym, symmetric; antisym, antisymmetric.
data reveal identical changes to those in Figure 5 and Table 3. The transformation time of 200 min was found to be in agreement with the optical microscopy. Scale-Up (1000 mL). The two transformation experiments were also repeated in the 1000 mL crystallizer. The optical microscopy data are shown in Figure 7c and d, respectively. For both mixtures, the transformation time increased significantly on scale-up. For the water/ ethanol 20:80 (v/v) ratio, the formation of the first few R crystals was observed after 30 min and the complete transformation took 95 min.
Solution-Mediated Transformation of β to R Glycine
Crystal Growth & Design, Vol. 3, No. 1, 2003 57
Figure 5. ATR-FTIR spectra of the solid mixture taken at t ) 0, t ) 15, and t ) 30 min during the polymorphic transformation of glycine crystallized from a 20:80 (v/v) water/ethanol mixture (50 mL scale).
For the 9:91 (v/v) water/ethanol mixture, the R polymorph was observed only after 1 h. After 8 h, the crystal population roughly appeared to contain 50% of each polymorph. The system was therefore left overnight, and a sample of the slurry was taken the next day when the transformation was found to be complete. From this, the transformation time was estimated to be of the order of 16-24 h. Discussion It is clear from these data that the transformation times observed are highly sensitive to both the composition of the solvent mixture and the scale of the experiment: transformation times increase with increasing proportion of ethanol in the mixture and with increasing
experimental scale. Consider first the dependence of kinetics on solvent composition. Measured solubility data for the R polymorph show that the solubility of glycine in water/ethanol mixtures falls from about 1.8 g/L in an 80% ethanol solution to 0.43 g/L in a 91% solution. Given that the crystallization experiments are all performed with aqueous solutions of composition 204 g/L, this allows the initial supersaturations (σ ) ln Csolution/Csaturation where C is in g/L) to be estimated as 3.1 in the 80/20 (v/v) mixture and 3.8 in the 91/9 (v/v) mixture. Thus, the system with the higher initial supersaturation takes longer to transform. This must mean that the plateau supersaturation,6,16 which drives the solvent-mediated transformation process and controls the crystallization rate of the R form, decreases with increasing ethanol content. This in turn implies
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Figure 6. ATR-FTIR spectra of the solid mixture taken at t ) 0, t ) 30, t ) 90, t ) 150, and t ) 210 min during the polymorphic transformation of glycine crystallized from a 9:91 (v/v) water/ethanol mixture at σ ) 3.8 (50 mL scale).
that the overall transformation becomes increasingly controlled by the dissolution of the β form as the ethanol content increases.17 This conclusion is entirely consistent with the optical microscopy data, which reveal consistently longer induction times for the appearance of the first R crystals and larger final R crystals (compare Figure 7a,b and c,d) with higher ethanol contents. Both of these observations suggest that with increasing ethanol content and at higher supersaturation, the nucleation process becomes biased toward the β form. This effect was overcome by raising the σ value from 3.8 to 4.0 at a solvent ratio of 91/9 (v/v) (see Table 2), suggesting an increase of the nucleation rate for the R polymorph, which led to a reduction of the transformation time (from 200 min down to 150 min). The longer conversion time for the higher alcohol content system may be simply due to the relative nucleation and growth
kinetics of the two forms but could also be related to the action of ethanol either as a template for β or as an inhibitor for R. To check this, a series of experiments were carried out in which pure water, water-acetone, water-methanol, water-propanol, water-pentanol, and water-octanol were used as solvents. For the experiment involving only water, a saturated solution of glycine at 65 °C was crash-cooled to 5 °C. It was found that both polymorphs crystallized, although the β crystals were in large excess. The same result was obtained when methanol, propanol, or acetone were added to a saturated solution of glycine at 25 °C. When pentanol or octanol were added to the solution, two layers formed, because the two alcohols are not miscible with water. If the formation of β glycine was templated by the presence of the alcohol, one would expect crystals of this polymorph to grow at the interface between the
Solution-Mediated Transformation of β to R Glycine
Crystal Growth & Design, Vol. 3, No. 1, 2003 59
Figure 7. Comparing the crystal sizes grown during the transformation experiments: (a) 50 mL scale, 80% ethanol; (b) 50 mL scale, 91% ethanol; (c) 1000 mL scale, 80% ethanol; and (d) 1000 mL scale, 91% ethanol. The time reported on each image corresponds to the overall time of the transformation. Table 4. Molecular Vibration Frequencies (cm-1) Recorded during the Polymorphic Transformation (at Time t ) 0, t ) 90, and t ) 210 min) from β Glycine to r Glycine Crystallized from 9/91 (v/v) Water/Ethanol Mixture (50 mL Scale)a vibration type
t ) 0 min
t ) 90 min
t ) 210 min
-NH3+ antisym stretch -COO- antisym stretch
3180.16 1658.50 1592.83 1512.37 1444.27 1410.16 1334.23 1134.22 1117.59 1040.60 915.17 893.23 701.58
3175.66 1663.96 1590.67 1512.98 1445.82 1411.74 1333.64 1133.56 1117.36 1040.76 915.07 893.38 699.47
3157.21 1608.94 1576.89 1501.25 1443.28 1409.69 1331.45 1132.26 1112.14 1034.22 909.56 892.57 696.80
-NH3+ deformation -CH2 bend -COO- sym stretch -CH2 wag -NH3+ rock -C-N sym stretch -CH2 rock -C-C sym stretch O-CdO bend a
Sym, symmetric; antisym, antisymmetric.
two liquids. However, in this case, only crystals of the R form were found at the bottom of the vessels after 1 day. From these results, it can be concluded that the relative preference for β formation is a result only of the supersaturation that prevails upon mixing the solvents. The consequence of this effect is that a higher ethanol content results in an initial nucleation of β crystals that under conditions of low solubility, cannot dissolve fast enough to supply material to drive the
further nucleation and growth of the R form. To further corroborate this conclusion, the transformation was seeded with approximately 2% of R crystals (based on the total weight of glycine used for the experiment). Nevertheless, the transformation time was found to be unaffected by this seeding, a result that confirms that the dissolution of the β form plays a key, rate-determining role during the polymorphic transformation. Ideally, this should be confirmed by measurement of the solution composition of the liquid phase during the transformation. However, because of the extremely low solubility of glycine in these systems, this proved to be impractical either by in situ FT-IR or by ex situ UV/vis analysis. Given this overall mechanistic interpretation, the influence of scale, in this case, is thus related to enhanced convective mass transfer at the smaller scale together with increase in surface area of the β crystals due to the action of the magnetic stirrer. Both of these factors would increase the dissolution rate of the β form in the small scale experiments relative to the larger scale. Conclusions The study of the polymorphic transformation from the metastable to the R form of glycine revealed that the experimental factors significantly affecting the reaction are the temperature, the solubility of the metastable form, the method of stirring, and the scale of operation.
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It was found that increasing the temperature and the solubility of glycine in the solvent mixture reduced the transformation time from 3 h to 40-60 min. Scale-up of the process and the use of an overhead stirrer had the effect of increasing the transformation time, probably due to a change in convective mass transfer. Overall, the results obtained lead to the conclusion that for this transformation of β to R glycine, the ratecontrolling step in the process is the dissolution rate of the β form. Acknowledgment. This research project was funded (E.F.) by the EPSRC, under the ROPA scheme, and GlaxoSmithKlein provided a CASE award for C.T. Supporting Information Available: X-ray crystallographic information file (CIF) is available for β glycine single crystal. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) McCrone, W. C. Physics and Chemistry of the Solid State; Wiley: New York, 1965; Vol. II.
Ferrari et al. (2) Ostwald, W. F. Z. Phys. Chem. (Leipzig) 1897, 22, 289. (3) Yokota, M.; Takezawa, E.; Takakusaki, T.; Sato, A.; Takahashi, H.; Kubota, N. Chem. Eng. Sci. 1999, 54, 3831-3838. (4) Garside, J.; Davey, R. J. Chem. Eng. Commun. 1980, 4, 393-424. (5) Doki, N.; Kubota, N.; Sato, A.; Yokota, M.; Hamada, O.; Masumi, F. AIChE J. 1999, 45, 2527-2533. (6) Davey, R. J.; Blagden, N.; Righini, S.; Alison, H.; Ferrari, E. S. J. Phys. Chem. 2002, B 106, 1954-1959. (7) Fisher, E. Ber. Dtsch. Chem. Ges. 1905, 38, 2917. (8) Bernal, J. D. Z. Kristallogr. 1931, 78, 363. (9) Ksanda, C. J.; Tunell, G. Am. J. Sci. A 1938, 35, 173. (10) Iitaka, Y. Acta Crystallogr. 1960, 13, 35-45. (11) Albrecht, G.; Corey, R. B. J. Am. Chem. Soc. 1939, 61, 1087. (12) Iitaka, Y. Acta Crystallogr. 1958, 11, 225-226. (13) Iitaka, Y. Acta Crystallogr. 1961, 14, 1-10. (14) Weissbuch, I.; Leisorowitz, L.; Lahav, M. Adv. Mater. 1994, 6, 952-956. (15) Allen, K.; Davey, R. J.; Ferrari, E.; Towler, C.; Tiddy, G. J.; Jones, M. O.; Pritchard, R. G. Cryst. Growth Des. 2002, submitted for publication. (16) Davey, R. J.; Blagden, N.; Righini, S.; Alison, H.; Quayle, M. J.; Fuller, S. Cryst. Growth Des. 2001, 1, 59-65. (17) Cardew, P. T.; Davey, R. J. Proc. R. Soc. London Ser. A 1985, 398, 415-428.
CG025561B
