In Situ Monitoring of Stirring Effects on Polymorphic Transformations

Sep 12, 2012 - In situ monitoring of CBZ polymorphic outcome was performed using custom-built experimental setup for simultaneous measurement of inten...
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In Situ Monitoring of Stirring Effects on Polymorphic Transformations during Cooling Crystallization of Carbamazepine Katarzyna Sypek,† Iain S. Burns,† Alastair J. Florence,‡ and Jan Sefcik†,* †

Department of Chemical and Process Engineering, University of Strathclyde, Glasgow, U.K. Strathclyde Institute of Pharmacy and Biomedical Science, University of Strathclyde, Glasgow, U.K.

‡

ABSTRACT: The influence of experimental conditions on polymorphic outcome and transformations during cooling crystallization of carbamazepine (CBZ) from anhydrous ethanol has been investigated. Stirring was found to be the most important controlling factor for the initial polymorphic outcome in solutions prepared using commercial CBZ powder. For quiescent conditions, a few large crystals of the metastable trigonal α form (II) initially appeared, undergoing subsequent slow transformation into the stable P-monoclinic β form (III). Under sufficiently vigorous stirring, the induction times observed were clearly defined by the onset of turbidity, which was due to formation of a large number of small form III prismatic crystals. In experiments with solutions prepared by using recrystallized CBZ, significantly shorter induction times were observed under stirring conditions: a large number of small form II needleshaped crystals initially formed at the onset of turbidity and then relatively rapidly transformed to small crystals of form III. This indicates a possible effect of impurities in the commercial CBZ powder inhibiting rapid nucleation of form II under stirring conditions. In situ monitoring of CBZ polymorphic outcome was performed using custom-built experimental setup for simultaneous measurement of intensity of transmitted and scattered light. Distinct pathways in two-variable intensity plots were observed for formation of each respective polymorph under stirring conditions. The same monitoring technique should be readily applicable to other systems.

1. INTRODUCTION Crystallization of polymorphs is a complex process involving a sequence of competitive steps of nucleation and growth of two or more possible crystalline forms of a given compound.1−4 Differences in the molecular packing arrangements within polymorphs influence a variety of physiochemical properties including solubility and dissolution rate, compaction and flow, and stability and may affect bioavailabilty.1 Therefore, obtaining the desired polymorphic form of a drug compound is crucial for pharmaceutical manufacturing,5,6 as highlighted by the widely studied example of the unexpected appearance of a new, more thermodynamically stable form of ritonavir.6,7 Formation of a metastable polymorph is influenced by the relative kinetics of nucleation and subsequent growth (and transformation) of the metastable phase with respect to the more stable phase(s). Factors controlling rates of these processes can be divided into two groups: (i) process related factors, such as stirring, seeding, and temperature/cooling rate; (ii) composition related factors, such as solute concentration, solvents, pH, and additives. Investigations of polymorphic crystallization often focus on the effects of the solvent, solution composition, and temperature profile.8,9 According to Ostwald’s rule, the stable form will crystallize preferably at lower supersaturation and the metastable form(s) at higher supersaturation. A metastable form can sometimes be obtained using a relatively poor solvent for a given compound.5 The effect of solvent on crystallization has been extensively studied,10 and © 2012 American Chemical Society

the solvent influence is typically unique for each system. The effect of solvent can be crucial for the relative nucleation rates and thermodynamic stability of polymorphs.11Concentration control was found to be superior to temperature control for polymorphic control in the crystallization process of paracetamol.12 Seeding or heteronucleation are well-known to promote crystallization of the desired (typically stable) form and in accordance with Ostwald’s rule can be used to avoid the metastable form.13 The influence of shear flow on nucleation and growth has been extensively studied in polymer crystallization. The alignment of polymer molecules with the flow direction controls the orientation of primary nuclei and thus promotes crystal growth in the direction perpendicular to the flow14 so that at given shear rate, only molecules with larger than “critical orientation molecular weight” will be aligned with the flow. In polymer melts, shear flow causes acceleration of crystallization kinetics leading to a change in the resulting morphology.14 For colloidal and protein suspensions a rotational shearing field can strongly influence nucleation processes, either stimulating or suppressing crystallization15 depending on the protein system and the shear rate. The effect of stirring as a controlling factor has been studied in competitive crystallization of two forms of Received: April 25, 2012 Revised: August 14, 2012 Published: September 12, 2012 4821

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Figure 1. Schematic representation of light-scattering experimental setup.

acid.16 The stable β polymorph was preferentially obtained both for fast cooling with agitation and for slow cooling without stirring. The authors analyzed the stability of the obtained metastable α form as a function of process flow conditions and suggested that agitation of α form slurries may stabilize this form by disruption of nucleation sites for the β form. This hypothesis is different from that proposed by Roelands and co-workers,17 who studied a similar system and suggested that liquid−liquid phase separation influenced the observed polymorphic outcome. Recently it has become increasingly clear that crystal nucleation in a variety of systems does not necessarily follow the classical pathway via small embryos with the same density, molecular arrangements, and surface free energy as the macroscopic crystalline phase. Several nonclassical nucleation mechanisms have been reviewed in the recent literature.18,19 According to one theory the appearance of crystals is preceded by creation of a metastable liquid phase within the original supersaturated solution. This liquid−liquid phase transition results in density and concentration gradients that will eventually trigger nucleation.20 Liquid−liquid microphase separation (in contrast to bulk phase separation well-known in crystallization as the oiling-out phenomenon21) has been proposed to play role in antisolvent crystallization of D,Lvaline22 and was also observed in molecular solutions in the absence of any solid phases.23 Another nonclassical crystallization mechanism24 is based on a mesocrystal formation step via self-assembly of primary colloidal scale particles. Spontaneous nanoemulsification (Ouzo effect) is another well-known example of nonclassical nucleation phenomenon.25,26 Carbamazepine (CBZ) is a drug used as anticonvulsant to treat epilepsy and trigeminal neuralgia. In pharmaceutical research, it has been extensively studied as a model compound for solid-form discovery and control because it is known to form five different nonsolvated polymorphs27−29 as well as a dihydrate30 and a large number of crystalline solvates31−33 and cocrystals.34,35 Published data36 show that for CBZ the Pmonoclinic β form (III) is the most stable at room temperature, the trigonal α form (II) and the C-monoclinic form (IV) are metastable at room temperature, and the triclinic γ form (I) is the most stable at temperatures exceeding 130 °C. No relative stability data is currently available for form V. Crystallization methods of CBZ show that form IV can be obtained during evaporation from methanol in the presence of hydroxypropyl cellulose37 or drying over phosphorus pentoxide at room

temperature,38 both with the use of additives in relatively complex processes. Form V, in which CBZ molecules pack with an alternative catemeric hydrogen-bonded motif to the other four polymorphs, was obtained by reverse sublimation onto a seed crystal of form II 10,11-dihydrocarbamazepine.39 In one solvent screening of CBZ with the use of cumene (isopropyl benzene),40 the authors obtained three polymorphs (with form I at temperatures over 80 °C). We note that while form II is generally referred to as a polymorph, it also contains large voids (unfavorable) that have been shown to contain nonstoichiometric amounts of disordered solvent. Hence there is the possibility that this form is to some degree solvent stabilized, possibly in a form of a nonstoichiometric solvate.41,42 An extensive experimental screen of CBZ forms, involving 66 solvents under 5 different sets of conditions using an automated parallel crystallization has been reported by Florence and coworkers that also included a crystal structure prediction study for thermodynamically feasible forms.27 Based on lattice energy, the relative stability of CBZ polymorphs at 0 K was predicted to be III > I > IV > II. It was concluded that higher supersaturation, and thus fast crystallization rate, favored form II, which is the least stable structure of all four polymorph structures (having the lowest lattice energy and density). The effect of the solvent in CBZ crystallization and preferential nucleation has been studied extensively.10,43 Crystallizing CBZ from a single solvent, ethanol, has been reported to yield two forms: form III by slow evaporation from supersaturated solutions at room temperature and form II by cooling the solution to 5 °C and holding it at that temperature for a few hours.36 Unfortunately, no details on stirring conditions were reported. Triclinic form I can be obtained from solution above 80 °C and by reverse sublimation and thermal transformation of form III. However, no effect of stirring conditions on crystallization outcome has been established in previous studies. In fact, in many cases the crystallization methods reported in literature do not give details of relevant process parameters, such as vessel dimensions or stirring conditions. Since CBZ crystallization from anhydrous ethanol was found in our laboratory to show effects of stirring conditions on the formation of the respective polymorphs, we have set out to carefully investigate these effects in order to obtain better understanding and control of effects of stirring on the polymorphic outcome.

L-glutamic

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over all angles. It is well-known that the angle dependence of lightscattering intensity is influenced by the size and shape of the scattering structures. As a consequence, the ratio between the attenuation coefficient and 90° scattering intensity should be sensitive to changes in crystal morphology as well as size. This flexible instrument is capable of monitoring both attenuation and scattering within a nonstandard sample holder under a range of flow conditions. This is beyond the capabilities of most commercial light-scattering instruments and allows real-time identification of product morphology during crystallization. Flexibility and low cost are further advantages of this experimental approach which should be readily applicable to other systems. The obtained crystal slurries were vacuum filtered with 0.45 μm PTFE filters, dried, and characterized with microscopy and powder XRD. Transmission foil XRPD data were collected on a Bruker AXS D8-Advance transmission diffractometer equipped with θ/θ geometry, primary monochromatic radiation (Cu Kα1, λ = 1.54056 Ǻ ).46 Data were collected in the range 4°−35° with a 0.015° 2θ step size and compared with reference patterns.27

2. MATERIALS AND METHODS Solutions of carbamazepine (Sigma-Aldrich; form III powder stored at 4 °C) in ethanol (Sigma-Aldrich; 99.8% v/v) at concentrations from 3 to 5 mg per 100 mL of ethanol were prepared by dissolution with magnetic stirring in a glass sample bottle inside an incubator (Stuart Scientific, Incubator S.I.60) at 56 °C. Clear undersaturated solutions were filtered with syringe filters, using 0.45 μm PTFE (Nalgene) or 0.1 μm Anotop 10 (Whatman) filters. All equipment was kept in the incubator at 56 °C to avoid contact with cold surfaces leading to undesirable nucleation. Filtered solutions were placed in identical glass sample bottles (Chromacol; ID = 12.5 mm × 36 mm height, neck ID = 11 mm × 8 mm height, nominal volume 4.4 mL, total volume 5.1 mL when completely filled), stirred on magnetic stirrer (magnetic mixer HANNA Instruments HI200M) with identical magnetic stirrer bars (Flea micro magnetic spin bar LxD 12 mm × 3 mm), and cooled to 21 °C. Solubility data for CBZ (form III) in ethanol were taken from the literature;44,45 two data points at 24 and 35 °C were checked experimentaly by gravimetry, and they agreed with the previously published data of Liu and co-workers45 (which were slightly lower than values of Qu and co-workers44). Recrystallization of CBZ was done by cooling of CBZ ethanolic solutions prepared at 56 °C to room temperature while stirring, followed by growth of crystals for around 2 h. The resulting product was filtered (vacuum Millipore filtration setup with membrane filters HVLP 0.45 μm pore size), dried in an oven at 50−70 °C for 3 h, and stored in a desiccator at ambient temperature. Induction time experiments were performed by preparing solutions as described above and filtering with syringe filters (0.45 μm PTFE or 0.1 μm Anotop 10) before removing the sample bottle from the incubator and allowing it to cool to room temperature (stabilized at 21 °C). The crystallization process was monitored either by microscopy and powder XRD on samples withdrawn from crystallizing suspensions, filtered and dried, or using the light scattering setup described below. The temperature evolution in solutions was monitored by a thermocouple inserted and carefully sealed in the sample bottle filled with pure ethanol under conditions identical to those used in crystallization experiments. A custom-built setup for simultaneous measurement of both attenuation coefficient and 90° light scattering intensity under stirring conditions was developed in our laboratory (see Figure 1). A helium− neon laser (Thorlabs HRP170; λ = 632.8 nm) was passed through the stirred solution in the sample bottle. Scattered light was detected at 90° using an avalanche photodiode (Hamamatsu, C5460); an aperture was placed close to the sample so that light scattered from the glass surfaces would not reach the detector. A beam sampler (Thorlabs BSF10A) was used to direct a reflection of the laser beam on to a photodiode (Thorlabs PDA36A) to measure the reference intensity. An identical photodiode was used to measure the intensity of the beam transmitted through the sample; a neutral-density filter was used to avoid saturation of the detector. The photodiode had to be placed close to the cylindrical sample bottle, where the beam-waist was narrowest. Although the focusing effect of the curved vessel could have been avoided if a rectangular sample holder had been used, this would have led to different conditions of mixing and heat-transfer compared with the other experiments. Signals were preamplified and were digitized by a data-acquisition card (National Instruments PCI-6221). The experiments were conducted in a darkened room, and the constant background levels on the detectors were subtracted from the signals prior to further data analysis. The measured scattering intensity was normalized by the reference intensity to compensate for drifts in the laser output power. The attenuation coefficient, τ, was determined from measurements recorded during crystallization using the modified Beer−Lambert law:

3. RESULTS AND DISCUSSION Polymorphic structure of CBZ samples of form III (commercial powder) and form II (prepared in house by cooling crystallization without stirring) were confirmed by powder XRD (Figure 2).36

Figure 2. Powder XRD patterns of CBZ form III (commercial) and form II (prepared in house by cooling crystallization without stirring).

3.1. Effect of Stirring. Our investigation was motivated by observations of stirring effects on the polymorphic outcome of cooling crystallization of CBZ from anhydrous ethanol. As described above, CBZ solutions of 3.5 g of CBZ and 5 g of CBZ per 100 mL of ethanol (using commercial CBZ source) were used, starting from equilibrated undersaturated solutions prepared at 56 °C in an incubator. In order to induce crystallization, sample bottles were transferred from the incubator to the laboratory bench (ambient temperature 21 °C) and kept there either under quiescent conditions or under magnetic stirring. In quiescent solutions, long needle-like form II crystals (Figure 3a) appeared after hours to days (depending on solution concentration). If left in solution, they gradually transformed over several days to a few relatively large form III crystals (presumably via solution-mediated transformation) (Figure 3b). Under stirring conditions, there was a clear onset of significant turbidity at times ranging from 30 min to several hours (see also Figure 7). The time elapsed until this abrupt rise in turbidity was taken to be the induction time, where many small crystals of form III appeared.

⎛I⎞ τL = − ln⎜ ⎟ ⎝ I0 ⎠ where I is the transmitted intensity, I0 is the reference intensity, and L is the optical path length. When molecular absorption is negligible, the attenuation coefficient is a measure of the total scattering integrated 4823

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Figure 3. Microscope images of CBZ crystals produced under quiescent and stirring conditions from solutions prepared using commercial CBZ: (a) quiescent, large form II crystals slowly appear; (b) quiescent, form II crystals are gradually transforming into a few large form III crystals; (c) stirring conditions, small form III crystals.

It is important to note that supersaturated solutions were filtered through submicrometer size filters before being placed in carefully cleaned bottles in order to rule out contamination with macroscopic solid impurities. The effect of the filter used was found not to be significant, as indicated by tests where the same solutions were split into two batches, one filtered with 0.45 μm PTFE filter and the other one with 0.1 μm Anotop filter. No differences of polymorphic outcomes (form II in quiescent solutions and form III under stirring) or average induction times were observed whether using any of these filters or indeed none at all. 3.2. Effect of Cooling Rate, Solution Concentration, Air−Liquid Interface, and Purification by Recrystallization. In order to investigate the dependence of polymorphic outcome on the cooling profile, three different cooling profiles have been applied: (a) slow cooling, gradual lowering of temperature in the incubator by 5 °C every 20 min from 56 to 21 °C; (b) fast cooling, moving the bottle from the incubator to the laboratory bench where the controlled laboratory temperature (21 °C) was exponentially approached in about 30 min; (c) ice cooling, bottles with crystallizing solutions were moved to an ice−water bath at 4 °C, where they remained for the whole duration of the crystallization process. A range of CBZ concentrations between 3 and 5 g of CBZ per 100 mL of ethanol was investigated. It was found that while faster cooling resulted in earlier appearance of crystals (as expected), it did not influence the polymorphic outcome in quiescent systems. Irrespective of cooling profile, well-developed form II crystals slowly appeared under quiescent conditions and then underwent transformation to large form III crystals over an extended period of time. In contrast, stirring resulted in well-defined onset of turbidity due to appearance of many small crystals, which were typically of form III, except at the shortest induction times, where small form II crystals appeared initially and then relatively rapidly transformed to form III crystals (see below for further details). Therefore we can observe that stirring appears to be the key factor influencing the initially obtained polymorphic form in cooling crystallization from solutions of commercial CBZ in anhydrous ethanol, within the range of cooling rates and CBZ concentrations investigated here. In order to investigate other possible parameters impacting the initial polymorphic outcome, we focused on the role of air− liquid interfaces and soluble impurities. The issue of air−liquid interface possibly assisting nucleation of a certain polymorph was studied in a series of experiments using either partially or completely filled bottles for solutions with CBZ concentrations of 3.5 g of CBZ per 100 mL of ethanol. The capacity of the sample bottles was 5.1 mL when completely full. Solutions were prepared at 56 °C as described above and were cooled to 21 °C while being stirred with the speed ranging from 600 to 1500

rpm. During stirring, the resulting vortex had a maximum height of about 10 mm with 3.3 mL of solution in the sample bottle. No vortex was observed for the completely filled bottle because the volume of air trapped under the bottle cap was less than 0.3 mL. Comparison of results for full bottles (5.1 mL) and partially filled bottles (3.3 mL) showed that induction times for full bottles were lower on average but with a higher standard deviation, although the resulting polymorphic outcome did not correlate with any particular set of air−liquid interface conditions.47 The issue of possible soluble impurities in the commercial CBZ powder was addressed through purification by recrystallization as described in the Materials and Methods section. It was found that using recrystallized CBZ had a significant effect on the observed induction times. Induction times for solutions prepared using recrystallized CBZ were less than 30 min (for 3.5 g of CBZ per 100 mL of ethanol), as compared with typical induction times of an hour or more with a much wider range from 30 min up to 3 h for the commercial CBZ, as seen in Figure 4.

Figure 4. Distribution of induction times for 3.5 g of CBZ per 100 mL of ethanol solutions under stirring conditions using two sources of CBZ: commercial and recrystallized.

Interestingly, for the solutions prepared with the recrystallized CBZ, it was consistently observed that the initial turbidity was due to many small form II crystals, which then relatively rapidly transformed into form III crystals. In contrast, in solutions prepared from commercial CBZ, the initial turbidity was typically due to many small form III crystals, without any detectable amount of form II present. As we noted above, while sometimes the initial turbidity in solutions prepared from commercial CBZ was found to be due to form II crystals, invariably this was observed at the shortest induction times (less than 30 min when the final solution temperature was 21 °C) and especially when solutions were cooled to below 20 °C. 4824

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Figure 5. Samples withdrawn from turbid suspensions under stirring from solutions prepared using recrystallized CBZ at turbidity onset and later times as indicated showing gradual transformation of metastable form II into stable form III.

The transformation of form II to form III as visualized in Figure 5 was also monitored by powder XRD of samples withdrawn from turbid suspensions, filtered, and dried, as shown in Figure 6. It can be clearly seen that observed powder XRD patterns are in qualitative agreement with visual observations and microscopic images above.

Figure 7. Induction times for various solution concentrations. Open symbols, form II observed initially; closed symbols, form III observed initially (stirring conditions using solutions prepared from commercial CBZ). Circles indicate stirring conditions; triangles correspond to quiescent conditions. Figure 6. Powder XRD data from dried samples illustrating transformation of turbid suspension of form II crystals into form III in cooling crystallization using recrystallized CBZ under stirring conditions. Samples were taken at the onset of turbidity and 30 and 80 min thereafter.

crystals (few and relatively large, so that suspensions are not appreciably turbid) was taken as the induction time, as presented in Figure 7 using open triangle symbols. In order to address why stirring results in apparent bypassing of nucleation of metastable form II in solutions prepared using commercial (but not recrystallized) CBZ in cooling crystallization with end point temperature of 21 °C, we consider several potential hypotheses: (1) Heterogeneous primary nucleation, for example, due to solid impurities in solution, enhanced by circulation of impurities and improved mass transfer due to stirring; this is unlikely since nanofiltration was shown to have no effect on the solid form appearing initially and therefore solutions can be reasonably assumed to be free of such impurities (or at least the initial polymorphic outcome not to be affected by them). (2) Secondary nucleation, where initially formed crystals are subject to attrition/breakage or breed further nuclei; this would be most likely to enhance nucleation of form II since this form appears from unstirred (quiescent) solutions first, while instead enhanced nucleation of form III is observed under stirring conditions. Perhaps one could speculate that interactions of hypothetical initial few unnoticed form II crystals with the magnetic stirrer or resulting flow field lead to nucleation of stable form III without form II ever being present in measurable quantities. Interestingly, however, this was not the case for recrystallized CBZ where large numbers of form II crystals appeared first under stirring. (3) A two-step nucleation mechanism, where colloidal scale precursors are coalesced/aggregated by turbulent shear;51,52

Additional seeding experiments were conducted to exclude the possibility of form II seeding when using recrystallized CBZ, and the same behavior was observed after adding a few commercial form III crystals to supersaturated solutions of recrystallized CBZ under stirring. Since it is well know that crude CBZ may contain low levels (on the order of 0.1% or more) of chemically similar impurities,48−50 it is possible that these may inhibit nucleation of form II under stirring conditions. Induction time observations are summarized in Figure 7, where triangle and circle symbols correspond to induction times from quiescent and stirred systems, respectively. Open symbols correspond to form II observed at the onset of turbidity, while filled symbols correspond to form III observed initially. It can be seen that there is a significant distribution of induction times, especially for data originating from solutions prepared with commercial CBZ. The widespread distribution of values is partly because the data reported here are aggregated from experiments in partially and fully filled bottles as well as at several stirring speeds. However, the main purpose of the plot is to delineate regions in the time−concentration space where unique patterns of behavior in terms of polymorphic outcome are observed. To complete the picture, under quiescent conditions, the time of appearance of the first visible form II 4825

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when commercial CBZ was used. In Figure 9, we show microscopic images of crystals in samples withdrawn shortly after the onset of turbidity from stirred solutions of prepared commercial and recrystallized CBZ, with form III and form II crystals present, respectively. Plotting the results in terms of attenuation coefficient against the scattering intensity (Figure 10) shows an interesting trend

although such precursors have not been directly observed in these solutions, it may be that impurities in commercial CBZ inhibit formation of colloidal precursor species favoring rapid nucleation of metastable form II observed under stirring conditions from solutions prepared from recrystallized CBZ. 3.3. In Situ Monitoring of Polymorphic Transformation. The development of turbidity and transformation from metastable to stable form was studied in supersaturated CBZ ethanolic solutions under stirred conditions using the custombuilt light scattering instrument. Figure 8 shows the changes in

Figure 10. Attenuation coefficient vs scattering intensity data plotted together from several experimental repetitions.

Figure 8. In situ measurements of 90° light scattering intensity (red circles) and attenuation coefficient (blue triangles) during crystallization under stirring conditions. Filled symbols, commercial (COM) CBZ; open symbols, recrystallized (REC) CBZ; crossed symbols, pure ethanol solvent.

distinguishing both pathways. For recrystallized samples, the data tend toward higher attenuation coefficients for a given scattering intensity. This is consistent with the presence of the larger form II crystals, whose scattering intensity would be more biased toward small angles than for the small form III crystals (cf. Figure 9). The two pathways subsequently converge, which is attributed to the transformation of form II crystals into form III. This demonstrates that our instrument provides an automatic means to monitor CBZ polymorphic transformation in situ. The slight decrease in scattering intensity toward the end of the process may be connected to the growth of larger crystals or agglomeration of smaller form III crystals.

attenuation coefficient and scattering intensity while solutions were allowed to cool to the laboratory temperature (21 °C) under stirring. It can be seen that in the case of solutions prepared using either commercial or recrystallized CBZ samples, the development of turbidity and scattering occurs simultaneously; however the nucleation times differ significantly depending on the CBZ source used (see above). When recrystallized CBZ was used, the turbidity developed quickly (within the first 10−30 min), while commercial CBZ source led to longer induction times from 30 min up to 3 h. Recrystallized samples were found to go through two stages of the development of turbidity, and that is consistent with previous observations, where the first stage corresponds to appearance of metastable needle-like form II crystals (cf. Figure 6). This form then starts to transforms into stable form III over next 10−30 min. However, only the stable form III is observed

4. CONCLUSIONS Stirring was found to be the key controlling factor for the polymorphic outcome in CBZ cooling crystallization from anhydrous ethanol solutions prepared by using the commercial CBZ powder. For quiescent conditions, long needle-like crystals of form II were formed from clear solutions and then

Figure 9. Samples withdrawn from stirred solutions after the initial solid phase appears. (a) solutions prepared using commercial CBZ showing form III crystals; (b) solutions prepared using recrystallized CBZ showing form II crystals. 4826

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(9) Hilfiker, R.; Berghausen, J.; Blatter, F.; Burkhard, A.; De Paul, S. M.; Freiermuth, B.; Geoffroy, A.; Hofmeier, U.; Marcolli, C.; Siebenhaar, B.; Szelagiewicz, M.; Vit, A.; von Raumer, M. J. Therm. Anal. Calorim. 2003, 73, 429−440. (10) Threlfall, T. Org. Process Res. Dev. 2000, 4, 384−390. (11) Kitamura, M. Cryst. Growth Des. 2004, 4, 1153−1159. (12) Nagy, Z. K.; Chew, J. W.; Fujiwara, M.; Braatz, R. D. J. Process Control 2008, 18, 399−407. (13) Beckmann, W.; Otto, W.; Budde, U. Org. Process Res. Dev. 2001, 5, 387−392. (14) Somani, R. H.; Hsiao, B. S.; Nogales, A.; Srinivas, S.; Tsou, A. H.; Sics, I.; Balta-Calleja, F. J.; Ezquerra, T. A. Macromolecules 2000, 33, 9385−9394. (15) Penkova, A.; Pan, W. C.; Hodjaoglu, F.; Vekilov, P. G. Ann. N.Y. Acad. Sci. 2006, 1077, 214−231. (16) Cashell, C.; Corcoran, D.; Hodnett, B. K. J. Cryst. Growth 2004, 273, 258−265. (17) Roelands, C. P. M.; ter Horst, J. H.; Kramer, H. J. M.; Jansens, P. J. AIChE J. 2007, 53, 354−362. (18) Vekilov, P. G. Cryst. Growth Des. 2010, 10, 5007−5019. (19) Erdemir, D.; Lee, A. Y.; Myerson, A. S. Acc. Chem. Res. 2009, 42, 621−629. (20) Vekilov, P. G. Cryst. Growth Des. 2004, 4, 671−685. (21) Bonnett, P. E.; Carpenter, K. J.; Dawson, S.; Davey, R. J. Chem. Commun. 2003, 698−699. (22) Variny, M.; de Miguel, S. A.; Moore, B. D.; Sefcik, J. J. Dispersion Sci. Technol. 2008, 29, 617−620. (23) Rankin, S. E.; Sefcik, J.; McCormick, A. V. J. Phys. Chem. A 1999, 103, 4233−4241. (24) Niederberger, M.; Colfen, H. Phys. Chem. Chem. Phys. 2006, 8, 3271−3287. (25) Sitnikova, N. L.; Sprik, R.; Wegdam, G.; Eiser, E. Langmuir 2005, 21, 7083−7089. (26) Ganachaud, F.; Katz, J. L. ChemPhysChem 2005, 6, 209−216. (27) Florence, A. J.; Johnston, A.; Price, S. L.; Nowell, H.; Kennedy, A. R.; Shankland, N. J. Pharm. Sci. 2006, 95, 1918−1930. (28) Edwards, A. D.; Shekunov, B. Y.; Forbes, R. T.; Grossmann, J. G.; York, P. J. Pharm. Sci. 2001, 90, 1106−1114. (29) Arlin, J.-B.; Price, S. L.; Florence, A. J. Chem. Commun. 2011, 7074−7076. (30) Liu, W. J.; Wei, H. Y.; Black, S. Org. Process Res. Dev. 2009, 13, 494−500. (31) Johnston, A.; Johnston, B. F.; Kennedy, A. R.; Florence, A. J. CrystEngComm 2008, 10, 23−25. (32) Florence, A. J.; Johnston, A.; Price, S. L.; Nowell, H.; Kennedy, A. R.; Shankland, N. J. Pharm. Sci. 2006, 95, 1918−1930. (33) Fleischman, S. G.; Kuduva, S. S.; McMahon, J. A.; Moulton, B.; Walsh, R. D. B.; Rodriguez-Hornedo, N.; Zaworotko, M. J. Cryst. Growth Des. 2003, 3, 909−919. (34) Porter, W. W.; Elie, S. C.; Matzger, A. J. Cryst. Growth Des. 2008, 8, 14−16. (35) ter Horst, J. H.; Cains, P. W. Cryst. Growth Des. 2008, 8, 2537− 2542. (36) Grzesiak, A. L.; Lang, M. D.; Kim, K.; Matzger, A. J. J. Pharm. Sci. 2003, 92, 2260−2271. (37) Quist, F.; Mooney, J.; Kakkar, A. K. Colloids Surf., B 2008, 62, 319−323. (38) Harris, R. K.; Ghi, P. Y.; Puschmann, H.; Apperley, D. C.; Griesser, U. J.; Hammond, R. B.; Ma, C. Y.; Roberts, K. J.; Pearce, G. J.; Yates, J. R.; Pickard, C. J. Org. Process Res. Dev. 2005, 9, 902−910. (39) Harrison, W. T. A.; Yathirajan, H. S.; Anilkumar, H. G. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2006, 62, 240−242. (40) Getsoian, A.; Lodaya, R. M.; Blackburn, A. C. Int. J. Pharm. 2008, 348, 3−9. (41) Cruz Cabeza, A. J.; Day, G. M.; Motherwell, W. D. S.; Jones, W. Chem. Commun. 2007, 1600−1602. (42) Fabbiani, F. P. A.; Byrne, L. T.; McKinnon, J. J.; Spackman, M. A. CrystEngComm 2007, 9, 728−731.

further slowly transformed to a few large form III crystals. In the case of sufficiently vigorous stirring, the induction times observed (30−300 min for 3.5 g of CBZ per 100 mL of ethanol at end point temperature 21 °C) were clearly defined by the onset of turbidity which was due to formation of a large number of small form III crystals. Neither the cooling rate nor solution concentration was found to significantly affect the polymorphic outcome for these conditions. In contrast to the commercial CBZ solutions, in experiments with solutions prepared by using recrystallized CBZ, significantly shorter induction times (below 30 min) were observed under stirring conditions. Under those circumstances, a large number of small form II needle-shaped crystals were initially formed at the onset of turbidity under stirring conditions and then relatively rapidly transformed to small crystals of form III. These observations lead to the hypothesis that some (presently unknown) impurities in the commercial CBZ powder inhibit rapid nucleation of form II under stirring conditions, while seeding experiments excluded the possibility of form II seeding when using recrystallized CBZ. In situ simultaneous measurements of transmitted and scattered intensities provided us with direct means of monitoring polymorphic transformation during CBZ crystallization. It has been confirmed that, under stirring conditions, there are two separate pathways of CBZ crystallization depending on whether the commercially sourced CBZ has been purified by recrystallization. The results show that our in situ measurement setup can be successfully used not only to monitor the induction time (which could be done with a single measurable) but also to distinguish between different polymorphs formed and their mutual transformation. This capability is based on distinct morphologies of the two polymorphs, which leads to different scattering patterns and in turn to distinct fingerprints for each polymorph when multiple measurables are recorded and plotted against each other. The same monitoring technique should be readily applicable to other systems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Polymorphism in Pharmaceutical Solids, 2nd ed.; Brittain, H. G., Ed.; Informa Healthcare: New York, 2009. (2) Mullin, J. W. Crystallization, 4th ed.; Butterworth-Heinemann: Oxford, U.K., 2001. (3) Davey, R. J.; Garside, J. From Molecules to Crystallizers: An Introduction to Crystallization; Oxford University Press: Oxford, U.K., 2000. (4) Jones, A. G. Crystallization Process Systems; ButterworthHeinemann: Oxford, U.K., 2002. (5) Muller, M.; Meier, U.; Wieckhusen, D.; Beck, R.; Pfeffer-Hennig, S.; Schneeberger, R. Cryst. Growth Des. 2006, 6, 946−954. (6) Bauer, J.; Spanton, S.; Henry, R.; Quick, J.; Dziki, W.; Porter, W.; Morris, J. Pharm. Res. 2001, 18, 859−866. (7) Morissette, S. L.; Soukasene, S.; Levinson, D.; Cima, M. J.; Almarsson, O. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 2180−2184. (8) Florence, A. J. In Polymorphism in Pharmaceutical Solids; 2nd ed.; Brittain, H. G., Ed.; Informa Healthcare: New York, 2009; pp 139− 184. 4827

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Crystal Growth & Design

Article

(43) Kelly, R. C.; Rodriguez-Hornedo, N. Org. Process Res. Dev. 2009, 13, 1291−1300. (44) Qu, H.; Louhi-Kultanen, M.; Kallas, J. Int. J. Pharm. 2006, 321, 101−107. (45) Liu, W.; Dang, L.; Black, S.; Wei, H. J. Chem. Eng. Data 2008, 53, 2204−2206. (46) Florence, A. J.; Baumgartner, B.; Weston., C.; Shankland, N.; Kennedy, A. R.; Shankland, K.; David, W. I. F. J. Pharm. Sci. 2003, 92, 1930−1938. (47) Sypek, K. Investigation of the effects of flow on crystallisation process. Ph.D. thesis, University of Strathclyde, Glasgow, U.K., 2011. (48) Degenhardt, M.; Watzig, H. J. Chromatogr., A 1997, 768, 113− 123. (49) Dzodic, P.; Zivanovic, L.; Protic, A.; Zecevic, M.; Jocic, B. Chromatographia 2009, 70, 1343−1351. (50) Thomas, S.; Mathela, C. S.; Agarwal, A.; Paul, S. K. J. Pharm. Biomed. Anal. 2011, 56, 423−428. (51) Waldner, M. H.; Sefcik, J.; Soos, M.; Morbidelli, M. Powder Technol. 2005, 156, 226−234. (52) Sefcik, J.; Grass, R.; Sandkuhler, P.; Morbidelli, M. Chem. Eng. Res. Des. 2005, 83, 926−932.

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