CRYSTAL GROWTH & DESIGN
Cocrystal Formation in Solution: In Situ Solute Concentration Monitoring of the Two Components and Kinetic Pathways
2009 VOL. 9, NO. 8 3376–3383
Emilie Gagniere,† Denis Mangin,† Franc¸ois Puel,*,† C. Bebon,† Jean-Paul Klein,† Olivier Monnier,‡ and Eric Garcia§ UniVersite´ de Lyon, UniVersite´ Lyon 1, LAGEP, UMR 5007 CNRS-CPE, 43 Bd du 11 NoVembre 1918, F-69622 Villeurbanne, France, SANOFI AVENTIS Recherche, F-34000 Montpellier, France, and SANOFI AVENTIS Chimie, F-30390 Aramon, France ReceiVed September 11, 2008; ReVised Manuscript ReceiVed March 20, 2009
ABSTRACT: The purpose of this work was to investigate the kinetic pathways in the phase solubility diagram of a cocrystallization process in solution. A cocrystal model composed of an active pharmaceutical ingredient (carbamazepine, CBZ) and a vitamin (nicotinamide, NCT) was selected. Batch experiments were performed in a stirred vessel. The process monitoring was carried out using in situ ATR-FTIR spectroscopy, which provided estimates of the solute concentrations of both CBZ and NCT at different temperatures. In the working field, it was possible for CBZ/NCT cocrystals and CBZ crystals to develop separately or simultaneously. The concentration profiles plotted in the phase diagram showed the kinetic pathways of the cocrystallization process. Their analysis indicated which solid form was in suspension and what the proportion of each solid phase was. Several experiments were in particular performed in the working domain leading to the invariant composition solution in equilibrium with both CBZ/NCT and CBZ crystals. Temporal information on the kinetic pathways and supersaturation levels is essential for finding the optimal operating conditions of the cocrystallization process. Introduction Cocrystals are a class of compounds, which have been researched for approximately half a decade. These materials offer huge potential to the pharmaceutical industry. Indeed, cocrystallization could be a new way to isolate an active pharmaceutical ingredient (API) which crystallizes with difficulty in its pure solid form or in its salt form. Moreover, cocrystallization is able to improve powder end-used properties such as solubility, melting point, crystallinity, hygroscopicity, and physical and chemical stability. In this work, carbamazepine/nicotinamide (CBZ/NCT) cocrystal was the model substance. CBZ and NCT molecules give equimolecular (1:1) cocrystals (CBZ/NCT) involving a homosynthon.1 The cocrystallization of carbamazepine-nicotinamide has been thoroughly investigated,1-5 and the generation of CBZ/ NCT cocrystals has been achieved through bulk solution crystal growth,1-3 crystal growth in deliquescent conditions,4 melt crystallization,5 and with the use of a polymer acting as heteronuclei.6 Most of the crystallization routes of CBZ/NCT have led to the form I of CBZ/NCT. Its crystal structure, obtained from crystals grown from solution, has been recorded2 (refcode in the Cambridge Structural Database is UNEZES7). The crystal system is monoclinic. Since the two molecules possess the functional group amide, the structure of the CBZ/ NCT cocrystals is characterized by N-H · · · OdC hydrogen bounds.3 This structure is considered to be the stable form.6 A second crystalline form obtained from the melt is regarded as metastable and named form II.5 A third polymorphic form may also have been present but was not clearly identified.6 The phase solubility diagram of CBZ/NCT cocrystals in ethanol was presented and explained by solubility product and solution complexation.1,8 CBZ/NCT cocrystals may develop in the * Corresponding author. Mailing address: LAGEP CPE Lyon UCBL1, 43 Bd du 11 Novembre 1918, F-69622 Villeurbanne cedex, France. Phone: +33 4 72 43 18 34. E-mail:
[email protected]. † Universite´ de Lyon. ‡ SANOFI AVENTIS Recherche. § SANOFI AVENTIS Chimie.
presence of either CBZ crystals or NCT crystals. A first paper has highlighted the importance of the kinetic aspects since the cocrystallization of CBZ/NCT may be in competition with the crystallization of CBZ or NCT crystals.9 In addition to the knowledge of the thermodynamic data of the system, information about the kinetic pathway is also essential in order to operate and optimize the cocrystallization process. CBZ/NCT cocrystallization has been previously monitored with an in situ video probe.9 As the crystal habits of CBZ crystals and CBZ/NCT cocrystals were different, their detection and their evolution in the slurry were easily monitored. However, this technique provides qualitative information, and it is difficult to obtain quantitative results. The technique is also limited when the crystal concentration exceeds 10% weight. In order to complete our previous in situ video observation of the solid phases, a quantitative monitoring in concentrated slurry (solid content up to 30% weight) can be carried out using an in situ ATR-FTIR spectroscopy probe.10 In this work, the crystallization pathway was monitored by following the solute concentrations of both CBZ and NCT molecules which were measured thanks to an in situ ATR-FTIR spectroscopy probe dipped in the slurry. Materials and Methods Raw Materials. Anhydrous carbamazepine CBZ (99.6% purity grade) and nicotinamide NCT (99.3% purity grade) were purchased from the Quimdis Company, stored under ambient conditions and used as received. Solid state forms were identified by X-ray powder diffraction. The more stable polymorphs of these raw materials were used: form III (P-monoclinic lattice) for carbamazepine11 and form I (monoclinic lattice) for nicotinamide.1 Absolute ethanol was of 99.7% grade and was purchased from Carlo Erba reagents. Phase Solubility Diagram. Two graphical representations of the isothermal phase diagram of the ternary system (CBZ, NCT and ethanol) exist, with orthogonal8 and triangular axis,12,13 respectively. For practical purposes, the representation with orthogonal axis was used. We have shown in a previous paper that this phase solubility diagram could be split into 4 domains, from the standpoint of thermodynamics,
10.1021/cg801019d CCC: $40.75 2009 American Chemical Society Published on Web 06/23/2009
Cocrystal Formation in Solution
Figure 1. The phase diagram of the CBZ/NCT system at 25 °C (solid line ) CBZ solubility curve; dotted line ) CBZ/NCT solubility curve; square symbol ) solubility measurement point; cross symbol ) point A corresponding to the intersection of CBZ and CBZ/NCT solubility curves; diamond symbol ) initial experimental conditions). Reprinted with permission from ref 9. Copyright 2009 Elsevier B.V. and that it could be divided into 6 domains for kinetic reasons, since metastable solid forms could also be temporally present. This subdivision is shown in Figure 1 and was confirmed by an in situ video observation: • Domain I: A solution represented by a point in this domain is undersaturated with respect to CBZ crystal and CBZ/NCT cocrystal. No crystalline phase is able to develop. • Domain IIa: A solution in this domain is undersaturated with respect to CBZ/NCT cocrystals and supersaturated with respect to CBZ crystals. Only the CBZ solid phase is able to nucleate and grow. • Domain IIb: A solution in this domain is supersaturated with respect to CBZ crystals and CBZ/NCT cocrystals. CBZ and CBZ/NCT solid phases are able to develop together. With regard to the respective solubilities, CBZ/NCT is the metastable phase whereas CBZ is the stable one. • Domain III: A solution in this domain is supersaturated with respect to CBZ and CBZ/NCT. CBZ and CBZ/NCT crystals are able to nucleate and grow together. A thermodynamically stable suspension represented by a point in this domain contains CBZ and CBZ/NCT crystals in equilibrium with a solution of composition given by the invariant point A. Thus, from the standpoint of thermodynamics, the CBZ and CBZ/ NCT solid phases are both stable in this domain.12 • Domain IVa: A solution in this domain is supersaturated with respect to CBZ and CBZ/NCT. CBZ and CBZ/NCT crystals are able to nucleate and grow together temporarily. With regard to the solubilities, CBZ is the metastable phase, whereas CBZ/NCT is the stable one. • Domain IVb: A solution in this domain is undersaturated with respect to CBZ and supersaturated with respect to CBZ/NCT. Only the CBZ/NCT solid phase is able to nucleate and grow. Experimental Setup. The batch cooling cocrystallization experiments were carried out in a 1 L glass vessel (Figure 2). The reactor was equipped with a condenser, cooled with cold water, to prevent the loss of ethanol by evaporation. Four stainless steel baffles were used in the reactor, in conjunction with a speed controlled mechanical stirrer. A high efficiency radial flow 45° pitched blade turbine was used to obtain good homogeneity of the slurry. Surface cooling was ensured by means of heat transfer through the jacket wall. The temperature of the suspension was measured using Pt100 probes, and controlled by manipulating the set point temperature of a 1 kW heating bath containing water. In situ solute concentration measurement was performed using the mid-infrared spectrometer Prote´ge´ 460 manufactured by Nicolet, which was equipped with an ATR immersion probe manufactured by the Axiom Analytical Corporation. The probe, which comprises a ZnSe conical reflection internal element, was swept with a slight flow of dried air at room temperature. A microcomputer allowed the recording of the infrared spectra, which were performed with the
Crystal Growth & Design, Vol. 9, No. 8, 2009 3377 software Omnic. Several samples of solution were also withdrawn in order to confirm the measurement of CBZ concentration by UV spectroscopy. Batch Operating Conditions. The vessel was initially filled with ethanol. CBZ and NCT were then added in order to reach the concentrations desired. The suspension was kept at 45 °C until complete dissolution was achieved. The stirring rate was set to 350 rpm to ensure a specific power input of 0.35 W · kg-1. Mid-infrared spectra were recorded in situ during the run, and the calculation of the solute concentrations was performed off line using the calibration model. From time to time, a few milliliters of solution were withdrawn from the crystallizer by means of a pipet equipped with a filter. The sample was immediately diluted to measure CBZ solute concentration by UV absorption. The clear solution was cooled at a constant rate of 20 °C · h-1 until it reached 25 °C. An isothermal step of several hours (4 to 9 h) was then applied until CBZ solubility and/or CBZ/NCT solubility were reached. For all the experiments, crystal nucleation and growth occurred only on this isothermal plateau. The slurry was filtered and washed in a pressure filtration apparatus at ambient temperature. Drying was finally performed at 60 °C. The solid phase was systematically analyzed by the XRD and observed under the SEM. The main operating conditions of the reported experiments are given in Table 1. The different experiments were located in the different parts of the phase diagram (see Figure 1). Since the NCT solubility is higher than that of both CBZ and CBZ/NCT, the solution in each experiment was always undersaturated with respect to NCT. Conditions in domains beyond NCT solubility were not investigated. In Situ CBZ and NCT Solute Concentration Measurements with ATR-FTIR Spectroscopy. Fourier transform infrared spectroscopy FTIR coupled with attenuated total reflectance (ATR) probe was reported to be efficient for monitoring the solute concentration profile during batch solution crystallization experiments.14-16 The use of chemometric spectral analysis software, taking into account the temperature of the spectral data, was carried out by Boukerche et al.17 and Henry et al.18 The measurement of concentrations in solution of two different species, such as the main dissolved product and one of its impurities, was also reported.19 Our objective with this ATR-FTIR spectroscopy technique was to measure concomitantly CBZ and NCT solute concentrations in the slurry. In order to monitor these solute concentrations during both the cooling and the isothermal steps, the temperature was involved in the calibration model. The calibration procedure needed to process the spectral data consists of four parts: (i) building a calibration pattern; (ii) recording the calibration spectra of appropriate solutions with known concentration; (iii) modeling the relationship between relevant spectral data and the concentration(s) using chemometric spectral analysis; (iv) validating the in situ ATR-FTIR concentration measurements using suspensions and solutions which were not used to establish the calibration data set. The calibration was performed in both supersaturated and undersaturated domains (see Figure 3). The primary nucleation could be avoided in the different supersaturated solutions thanks to the use of a quick cooling rate of 20 °C · h-1. In order to avoid risky extrapolation of the calibration model during the crystallization runs, the calibration data set covered the whole following field study: CBZ solute concentration (mol · L-1 solvent) [CBZ] ∈ [0.06-0.21]; NCT solute concentration (mol · L-1 solvent) [NCT] ∈ [0.09-0.25] and the temperature (°C) T ∈ [25-40]. A spectral measurement resulted from the average of 32 scans at a resolution of 8 cm-1 over a wavenumber region of 4000-650 cm-1. The calibration data were obtained as follows. The reactor was filled with ethanol at 25 °C. A background spectrum of the agitated solution was then recorded. A known amount of NCT was added. The mixture was heated to 45 °C. A known amount of CBZ was added, and we waited for complete dissolution of the two solids. Spectra were recorded at 40, 35, 30, and 25 °C. Afterward, the solution was heated up and a new known amount of CBZ was introduced into the vessel to obtain spectral data related to another concentration. IR spectra were collected again with varying temperature values. The procedure was then carried out as before, and repeated until the upper CBZ solute concentration limit was reached. The CBZ concentration difference between two cooling cycles was about 0.02 mol · L-1. For each NCT concentration, 8 series of 4 spectra were obtained. The procedure was repeated by increasing the NCT concentrations by 0.02 or 0.04 mol · L-1 until the upper NCT solute concentration limit was reached. Around 150 calibration spectra (expressed as absorbance) were acquired. The
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Figure 2. Experimental batch cooling crystallization setup equipped with an in situ ATR-FTIR spectroscopy probe.
Figure 3. Range of variation of the calibration data in the phase diagram (diamond symbols ) calibration points). Table 1. Operating Conditions of the Batch Cocrystallization Experiments
run no.
location of the starting point in the phase diagram
initial CBZ solute concn (mol · L-1 solvent)
initial NCT solute concn (mol · L-1 solvent)
17 25 43 39 45 36
IVa IVa IVa III III IIb
0.157 0.136 0.157 0.193 0.205 0.193
0.199 0.230 0.222 0.154 0.154 0.104
spectra were processed using TQAnalyst, the spectral analysis software supplied with the Nicolet spectrometer in order to build the calibration model. Several quantitative methods were tested to relate the CBZ and NCT solute concentrations and the temperature to the spectral data. A pretreatment method (i.e., first derivative) was applied on the spectra. It emerged that the standard error of calibration was minimized using partial least squares (PLS). The concentration values used for the validation procedure were different from those used to establish the calibration model, although they belonged to the calibration field. The assessment of the accuracy of the calibration PLS model was performed, comparing in a clear solution, ten predicted values of the CBZ and NCT solute concentrations to the corresponding known
concentrations (see Figure 4). Table 2 summarizes the main statistical values of error. For CBZ and NCT concentrations, the average relative prediction errors were lower than 1.3% and the maximal relative prediction errors for the lowest concentrations measured were about 6.5%. In terms of process monitoring, it is a satisfactory result. Finally, in order to take into account the temperature dependency of the spectral data, it was necessary to include the temperature as a parameter in the calibration model. Nevertheless, the temperature estimates using the calibration model were indicative, since the temperature was also measured by a Pt 100 Probe. Offline CBZ Solute Concentration Measurement by UV Absorption. The calibration model with ATR-FTIR spectroscopy described above was validated in a clear solution by UV absorption with an UV-vis Cary 50 spectrophotometer. ATR-FTIR spectroscopy is known to be almost insensitive to the presence of particles. Nevertheless, some deviations have sometimes been detected.19 Thus, CBZ solute concentration measurements carried out with the in situ FTIR spectroscopy were compared with an ex situ analytical method. UV absorption at 286 nm was chosen, since the measurement of CBZ solute concentration was not sensitive to the presence of NCT solution.1 Each sample of filtered suspension was diluted with ethanol (volumetric dilution factor ) 2500). The relative uncertainty of the UV analysis was assessed at 1.8%. X-ray Powder Diffraction. Powder diffraction patterns of solid phases were recorded with a scanning X-ray diffractometer (Bruker D8 Advance) using Cu KR radiation (λ ) 1.54 Å), tube voltage of 33 kV, and tube current of 45 mA. The intensities were measured at 2-theta values from 4.5° to 50° at a continuous scan rate of 10°/min with a position sensitive detector aperture at 3° (equivalent to 0.5°/min with a scintillator counter). X-ray powder diffraction patterns obtained were compared to the ones calculated from the crystal structure reported in the Cambridge Structural Database.7
Results and Discussion Comparison between ATR-FTIR and UV Measurements. Figure 5 presents the time variations of the CBZ solute concentration profile obtained from 2 experiments (runs #43 and 45) using the in situ and ex situ measurement methods. In both cases, the accordance between the two measurements is fully acceptable, which validates the in situ ATR-FTIR monitoring. These runs were conducted in domains III and IVa of the phase solubility diagram. Crystallization Pathway in Domain IVa. Three runs (#17, 25 and 43) were located in domain IVa. Figure 6 shows the estimates of the temperature and solute concentrations of both
Cocrystal Formation in Solution
Crystal Growth & Design, Vol. 9, No. 8, 2009 3379
Figure 4. FTIR predicted concentrations of CBZ and NCT against real values (open circle ) calibration data; solid circle ) validation data; dotted line ) bisector).
Figure 5. Time variations of the CBZ solute concentration (diamond symbol ) ex situ UV absorption; solid line ) in situ ATR-FTIR spectroscopy). Table 2. Statistical Evaluation of the Accuracy of the Predictions Computed for the Validation of the Calibration PLS Model (10 Points) predicted variable [CBZ] [NCT] temp
av absolute error -1
0.0018 mol · L 0.0019 mol · L-1 0.41 °C
av relative error 1.33% 1.11% 1.2%
CBZ and NCT achieved by using the FTIR calibration model for run #25. As shown in Figure 6, run #25 can be divided into two stages. During stage 1, the CBZ and NCT solute concentrations of the clear solution were constant despite the cooling profile. This demonstrates that the calibration correctly took into account the temperature influence on the spectral data. No
Figure 6. Time variations of the temperature (black solid line) and of the solute concentrations (blue ) CBZ; red ) NCT) from the FTIR calibration model for run #25.
maximal absolute error -1
0.0039 mol · L 0.0049 mol · L-1 1.08 °C
maximal relative error
std deviation
6.5% 5.4% 4.3%
0.0010 mol · L-1 0.0016 mol · L-1 0.37 °C
crystallization mechanism occurred during the cooling. Stage 2 started at 25 °C with the drop of CBZ and NCT solute concentrations. This decrease was attributed to the consumption of CBZ and NCT by the crystalline growth. We could also clearly demonstrate that these concentration decrease rates were identical for the two molecular species. This stoichiometric consumption of CBZ and NCT corresponded to the cocrystallization of CBZ/NCT cocrystals. The in situ video observation presented in a previous study also showed the unique presence of cocrystals (needlelike habit) and the absence of CBZ crystals (prismlike habit), albeit temporarily.9 This stage ended when the CBZ and NCT solute concentrations reached progressively stable values. These values remained constant with time and corresponded to the solubility product of the cocrystal in solution at 25 °C. These solubility values given by IR spectroscopy were consistent with those previously obtained by the gravimetric method.9 The XRD pattern of the final solid confirmed that only cocrystals were formed. The two other runs (#17 and 43) presented similar temperature and solute concentration profiles. Since the cocrystallization occurred at a constant temperature close to 25 °C, the evolution of the CBZ and NCT solute concentrations was plotted in the phase diagram (see Figure 7) for these three runs. We observed that the cocrystallization pathways were parallel to the bisecting
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Figure 7. Solute concentration pathways for runs #17, #25 and #43 in the phase solubility diagram at 25 °C.
Gagniere et al.
Figure 9. Time variations of the temperature (black solid line) and of the solute concentrations (blue ) CBZ; red ) NCT) for run #39.
Figure 10. Solute concentration pathway for run #39 in the phase solubility diagram at 25 °C. Figure 8. Time variations of the cocrystal CBZ/NCT supersaturation ratio for runs #17, #25 and #43 at 25 °C. Table 3. Domain IVa: Supersaturation Ratio and Nucleation Time at the Primary Nucleation Point run no.
CBZ/NCT supersaturation ratio
nucleation time (min)
17 25 43
1.54 1.55 1.62
74.2 63.2 57.1
line, which is in accordance with the stoichiometric consumption of the two molecular species by the cocrystal growth. Moreover, as predicted by the thermodynamics, the operation ended on the solubility curve of the cocrystals. The supersaturation ratio was calculated according to its definition given in the literature8 (Table 3). Figure 8 shows the time variation of the supersaturation profile. In accordance with the classical theory of primary nucleation, nucleation occurred earlier with higher initial supersaturation. The sharp decrease of supersaturation, within about 20 min, down to a value of approximately 1.1, was due to the quick growth of nuclei. In a second stage, the supersaturation rate decreased slowly. The system progressively reached equilibrium as the cocrystal growth rate declined. The three other operating conditions tested did not reveal any significant differences in the desupersaturation profiles. It should also be noted that, at the equilibrium, the calculated supersaturation ratio oscillated above and below a
value of 1.0 (from 0.95 to 1.03). The accuracy of the IR calibration model explains such uncertainty. Crystallization Pathway in Domain III. Two runs (#39 and 45) were performed in domain III. As shown in Figures 9 and 10 for run #39, the crystallization pathway in domain III unfolded in four stages. During the first stage of cooling (from E to D), solute concentrations of the clear solution remained constant. No crystallization phenomenon occurred. The solution became more and more supersaturated. The second stage (from D to C) started with the simultaneous drops of CBZ and NCT solute concentrations and with the onset of a burst of nuclei, which was visible to the naked eye. The slopes of concentration decrease (see Figure 9) were identical for the two molecular species and corresponded to the stoichiometric consumption of CBZ and NCT. This is confirmed by a concentration pathway parallel to the bisecting line in the phase solubility diagram (see Figure 10). It turned out to be the primary nucleation and growth of CBZ/NCT cocrystals. This stage ended quickly and the third stage (from C to B) started when the CBZ concentration decrease became higher than the NCT ones (see Figure 9). Consequently, the concentration pathway deviated from the bisector and deflected to the bottom on the phase solubility diagram. The occurrence of CBZ crystal primary nucleation was expected, since previous observations with an in situ video probe in domain III have shown a latter nucleation of prismlike CBZ crystals in the presence of a needlelike CBZ/NCT cocrystal
Cocrystal Formation in Solution
Crystal Growth & Design, Vol. 9, No. 8, 2009 3381 Table 4. Domain III: Supersaturation Ratio and Nucleation Time at the Primary Nucleation Points of CBZ/NCT Cocrystals and CBZ Crystals primary nucleation of CBZ/NCT cocrystals
primary nucleation of CBZ crystals
CBZ/NCT CBZ maximum maximum supersaturation supersaturation nucleation run no. ratio ratio time (min) 39 45
Figure 11. Solute concentration pathway for run #45 in the phase solubility diagram at 25 °C.
population.9 CBZ crystals and CBZ/NCT cocrystals grew together. At the end of this stage, the NCT concentration remained constant. IR spectroscopy and UV absorption measurements indicate that the concentrations of CBZ and NCT were close to the cocrystal solubility product. In the last stage (from B to A), we noticed a relative decrease of 7% of the CBZ solute concentration and a relative increase of 6% of the NCT solute concentration until stable values were obtained (see Figure 9). These final CBZ and NCT solute concentrations corresponded to a solution saturated with the two solid forms in the slurry: CBZ/NCT cocrystals and CBZ crystals. In the phase diagram (see Figure 10), the concentration pathway reached the invariant point named A. The XRD pattern on the final dried crystals confirmed the presence of the two solid forms, which were the two stable forms at that point. We could also highlight the fact that the crystallization pathway began in domain III (at point E), and then continued into domain IIb (to point B). In this final stage from point B to point A, the cocrystal solubility curve was crossed over, although the measurement was not accurate enough to show it. Thus, the crystallization pathway had slightly entered domain IIa, inducing partial dissolution of the cocrystals until point A. Hence, the slight increase of the NCT concentration can be explained by the release of NCT, induced by cocrystal partial dissolution. The CBZ molecules released by the cocrystal dissolution were consumed by the growth of the CBZ crystals in suspension until reaching point A. The pathway in domain IIa is close to the solubility curve of the cocrystals. This suggests that the limiting stage is the CBZ growth, and that the cocrystal dissolution kinetics is not limiting, otherwise, the crystallization pathway would have been closer to the CBZ solubility curve. However, this observation does not lead us to any conclusions about the relative values of the intrinsic kinetics of the cocrystal dissolution and the CBZ crystal growth, since the global kinetics observed are also dependent on the concentrations of the two solid phases. The other run (#45) presented similar crystallization pathways (see Figure 11) and the final state reached was always point A. This confirms that a cocrystallization started in domain III converges to the invariant point A. It could also be noted that the pathway through domain IIa observed for the two runs is not systematic and should depend on the initial conditions of concentrations and temperature. Table 4 reports the nucleation time and the supersaturation values at the nucleation points of CBZ/NCT cocrystals and CBZ crystals. As shown in Figure 12, the time variations of the
1.53 1.52
1.76 1.82
73.1 77.2
nucleation time (min) 81.2 87.2
supersaturation profiles present interesting features. For the two runs carried out, the primary nucleation of CBZ/NCT cocrystals started, even though the CBZ supersaturation ratio was fifteen to twenty percent greater than the CBZ/NCT supersaturation ratio. Despite a lower supersaturation, the primary nucleation frequency of CBZ/NCT was greater than that of CBZ. A possible explanation lies in the fact that there may be a better affinity with the solvent of the CBZ/NCT nuclei than with that of the CBZ nuclei. The nucleation of CBZ crystals occurred about ten minutes later. Since the primary nucleation frequency of CBZ in the bulk was lowered, the presence of the cocrystal form in suspension may favor a nucleation of CBZ crystals on the surface of CBZ/NCT solid form. Experimental observations not presented here have already shown some possible epitaxial relationships between the two solid forms. The CBZ/NCT supersaturation profiles in domain III were similar to the ones observed in domain IVa: a sharp decrease to a value of about 1.1, and then a slower decrease. The CBZ supersaturation profiles showed the same trend, but the return to the equilibrium took longer. This may have been due to a lower overall growth rate of CBZ crystals combined with a less numerous population of CBZ particles. The calculated supersaturation ratios in the final part of the runs oscillated above or below the value of 1.0. With the assumption of a kinetic pathway in domain IIa, the supersaturation ratio of cocrystals should be under 1.0. Nevertheless, the values are close to unity or slightly above. As already discussed in the previous part of this section, the accuracy of the FTIR calibration model was not enough to discriminate values close to the solubility. Crystallization Pathway in Domain IIb. Only one experiment (run #36) was kept in order to describe the crystallization pathway in domain IIb. Figure 13 presents the time variations of CBZ and NCT solute measurements. There is a certain level of discrepancy between the CBZ solute concentration profiles obtained with the in situ and ex situ measurement methods. Several trials were performed in domain IIb, and these two measurements frequently showed a progressive deviation which increased with time. This effect was due to the gradual fouling on the optical crystal of the ATR probe. This fouling was attributed to the start of crystallization of CBZ crystals by a heterogeneous primary nucleation. In run #36, a very slight fouling was observed on the probe, allowing reliable measurements. The other runs were rejected, since the measured values were not in accordance with the process. For instance, CBZ solute concentration values increased during the crystallization instead of decreasing. In domains III and IVa previously studied, CBZ/NCT cocrystals nucleated first and this fouling on the probe was never detected. During the first stage of the cooling profile, the solution was clear and the CBZ and NCT solute concentrations remained constant. Close to 25 °C, a nucleation occurred in the stirred reactor. A CBZ solute concentration fall was, at the same time, detected by UV absorption, and later on, by IR spectroscopy. However, NCT solute concentration values
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Figure 12. Time variations of the supersaturation ratio of CBZ/NCT cocrystals and of CBZ crystals for runs #39 and #45 at 25 °C.
Figure 13. Time variations of the temperature (black solid line) and of the solute concentrations (red ) NCT; blue ) CBZ by in situ FTIR spectroscopy; diamond symbol ) CBZ by ex situ UV absorption) for run #36.
were stable. This difference in the time evolutions of CBZ and NCT solute concentrations was attributed to the nucleation and growth of CBZ crystals. A simultaneous nucleation of CBZ/NCT cocrystals and CBZ crystals had already been reported with a video probe under similar conditions.9 Nevertheless, in this run, the time evolution of NCT concentration appeared constant. Thus, CBZ/NCT crystallization was not detected, either because it did not actually occur or because the change in the NCT concentration was too low to be detected by IR analysis. This operation ended when CBZ solute concentration reached progressively stable values. The final CBZ solute concentration values obtained by IR spectroscopy and by UV absorption were in accordance with solubility values at 25 °C obtained by a gravimetric method.9 The XRD pattern on the final solid confirmed the presence of only the CBZ crystalline lattice. The evolution of the CBZ and NCT solute concentrations was plotted in the phase diagram (see Figure 14) for run #36. Its crystallization kinetic pathway went in a vertical direction corresponding to the development of a CBZ crystal population. No significant deviation from this vertical line could be noticed. The occurrence of a metastable CBZ/NCT crystal population would have modified this pathway (a first deviation toward lower NCT concentration in domain IIb followed by a deviation toward higher NCT concentration in domain
Figure 14. Solute concentration pathway for run #36 in the phase solubility diagram at 25 °C.
IIa), but the final point located on the CBZ solubility curve would have been the same. Conclusion The model system CBZ/NCT was used to investigate the crystallization kinetic pathways in the phase solubility diagram. Depending on the position in the phase diagram, CBZ/NCT cocrystals and CBZ crystals were able to develop and compete against each other. With the benefit of an in situ IR spectroscopy, both CBZ and NCT solute concentrations could be monitored in the slurry. These measurements have led to the plotting of the kinetic pathways in the phase diagram. The supersaturation, with respect to the different species able to crystallize, could then be plotted versus time. These experiments were complementary to the ones already published in the literature wherein only the starting and the ending points in the phase diagram were known. The analysis of the CBZ and NCT concentration profiles in solution allowed us to determine which solid form nucleated and what the proportion was of each solid phase present in suspension. A stoichiometric consumption of CBZ and NCT results from the development of CBZ/NCT cocrystals. An increase of this consumption ratio allows detection of the nucleation and growth of a second population of CBZ crystals. The calculated supersaturation profile showed a sharp desupersaturation stage as soon as the CBZ/NCT cocrystals started growing, followed by a slow return to equilibrium. This
Cocrystal Formation in Solution
information on the kinetic pathways and supersaturation levels versus time is essential for finding the optimal operating conditions of the cocrystallization process. Acknowledgment. Financial support for this study provided by Sanofi-Aventis is gratefully acknowledged. In addition, the authors are indebted to Jean Pierre Valour for his help in the experimental part of this study and to Ruben Vera for the X-ray diffractometry measurements and advice (Centre de Diffractome´trie Henry Longchambon, Universite´ Lyon 1, Villeurbanne). The authors express their gratitude to Eric and Sheila Bower for their helpful suggestions with the English.
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CG801019D
