Controlling the Crystal Morphology and Polymorphism of 2,4

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Controlling the Crystal Morphology and Polymorphism of 2,4-Dinitroanisole Chagit Denekamp, Olga Meikler, Michael Zelner, Kinga Suwinska, and Yoav Eichen Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01199 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 14, 2018

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Crystal Growth & Design

Controlling the Crystal Morphology and Polymorphism of 2,4-Dinitroanisole Chagit Denekamp1*, Olga Meikler1,2, Michael Zelner2, Kinga Suwinska3, Yoav Eichen2* 1

RAFAEL, Advanced Defense Systems LTD, POB 2250 Haifa 3102101, Israel

2

Schulich Faculty of Chemistry, Technion – Israel Institute of Technology, Technion City Haifa

3200008, Israel 3

Faculty of Mathematics and Natural Sciences, Cardinal Stefan Wyszynski University in Warsaw,

Wóycickiego 1/3, PL-01 938 Warsaw, Poland

ABSTRACT

Nitroaromatic 2,4-dinitroanisole (DNAN) was used as a model compound for the study of surface poisoning effects in crystallization. In addition to the expected solvent effect it is found that the concentration of solutions and presence of additives control the formation of polymorphs and forms. In some cases, aromatic and/or nitro containing additives interact with DNAN, probably competing with intrinsic intramolecular interactions that allow the formation and growth of DNAN crystals. It is also shown that single-crystal-to-single-crystal phase transitions, take place between two β-forms and from the β- to the α-form. This β- to α- transformation is most probably possible because of a similarity between the two crystal structures. Hence, the barrier for this transformation is rather low. 1 ACS Paragon Plus Environment

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Nevertheless, the formation of α-DNAN in the typically β- morphology introduces strain in the crystals, causing defects. It is also shown that the orientation of the molecules against the unit cell faces for the two α-forms (blocks and needles) is markedly different.

Keywords: DNAN, Morphology, Polymorphism, Crystallization, Surface poisoning.

Introduction Controlling the morphology (polymorphism, shape and size) of powders is of significance for many industrial applications as these properties influence the fluidity of both the powder and its compositions.1,2,3 In pigments, small morphological modifications can alter color strength, hue and light-fastness, which are all dependent not only on the size of the crystals but also on their shape and relative crystal-face distribution. 4,5,6 In pharmacy, crystal morphology plays a major role in the control of the kinetics of dissolution and drug delivery. 7,8,9,10 In polymer-bonded explosives, the influence of powder morphology on sensitivity is reported repeatedly. 11 , 12 Controlling crystal morphology is therefore a ubiquitous challenge and thus draws the attention of many research groups. One way to gain control over crystal morphology is by using molecular additives. These molecules are added to the solution, affecting crystallization through interactions with specific faces of the growing crystals. The first choice for such an additive is a co-solvent used in the crystallization process. Differences in the interactions between the solvent and the surface of the growing crystal may account for their anisotropic growth. 13 In some cases, additives disrupt the growth of the crystals to such extent that certain phases are not expressed. An important role of polar or protic additives is in stabilizing or competing on hydrogen bond interactions. For example, the effect of co-

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solvents was demonstrated on the crystal growth of alkyl-gluconamides by the addition of methanol to water, as a crystal-growth medium. Crystals of alkyl-gluconamides are characterized by a head-totail packing, with opposing facets being hydrophobic and hydrophilic (as revealed from wetting experiments).14 Methanol was found to preferably wet the hydrophilic faces, limiting crystal growth mainly to the hydrophobic facets. The use of additives that are similar in shape to the crystallizing component in order to control crystal growth was also demonstrated, for example in the case of N-(E-cinnamoyl)-(S)-alanine.15 The authors show that by using such additives, they were able to induce preferential growth of specific faces. These findings were explained by the interaction with the functional groups present on the respective surfaces. 16 Another example of inhibition of crystal growth in a directionallypreferred manner was demonstrated on N-(2-acetamido-4-nitrophenyl) pyrrolidene (PAN) by using an additive that is structurally similar to the crystallizing component.17 This compound crystallizes in two known polymorphs, one of which is the centrosymmetric, plate-shaped, α-form and the other is the metastable, needle-shaped, β-form. Different additives, structurally similar to PAN, were tested for their ability to act as crystal-growth inhibitors and phase directors. Furthermore, some of these additives were examined in both their monomeric and polymeric forms, to see whether the latter increases the directing efficiency. It is reported that several additives lead to preferential formation of the β- phase, but in the shape of triangles rather than needles. The authors suggest that this is due to growth inhibition by the additives. Moreover, polymeric forms of the inhibitors proved even more effective than their respective monomers. 18,19 Another study on the morphology of paracetamol shows selective solubility enhancement of some faces over others, leading to preferential growth of some facets over others.20 Earlier related studies already explored effects of additives on paracetamol crystals.21 This study focuses on the rate of growth of a particular crystal face due to supramolecular



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hydrogen bonds between the functional groups exposed at the [001] face (amides and hydroxyls) and C=O acceptors. It was claimed that interaction between these groups and carbonyl or hydroxyl groups of metacetamol cause face-growth inhibition. A study on specific face growth rate in super saturation was also carried out on non-polar crystals of paraffin, in which the addition of ppm-scale di-n-octadecyl amine, (C18H37)2NH, was studied.22 It is shown that with increasing concentrations of the amine impurity, the [110] face growth rate is decreased.23 In a more recent study the effects of various anions and cations on the growth of dl-alanine was investigated.24 It is found that at normal super saturation conditions of DL-alanine in water, crystallization is exceptionally slowly. Furthermore, there exists a large dead super saturation zone within which the polar c-axis virtually does not grow. The existence of the dead zone was attributed to strong water affinity and thus adsorption to the -NH3 rich +c end and -COO−rich -c end. It is observed that both polar faces undergo a revival of solvent-poisoned dead growth in the presence of additives, such as Mg2+ and SO42– ions. Meekes and coworkers found polymorphic self-poisoning in the crystallization of the 7RMN steroid.25 To explain the differences in growth mechanism on the opposite polar faces, it is proposed that the [010] face and adjacent faces are covered by a monolayer or several layers of the metastable polymorph. This leads to surface roughening and blocking of growth. A method for polymorphic control, gel-induced crystallization, was demonstrated by Trout and coworkers. 26 The crystallized polymorphic form of carbamazepine is evidently sensitive to the polymer mesh size in polyethylene glycol diacrylate gels. As mentioned above, crystal morphology is also relevant in the field of nitro aromatics, some of which are energetic materials. 27 , 28 An interesting example is shown in the case of 1,3,5trinitrobenzene. Two new stable crystal forms of this known compound were reported. These elusive forms have been obtained by the use of the additive tris-indane. 29 An extensive study on


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polymorphism in 4-amino-3,5-dinitrobenzamide reveals the existence of four polymorphs and one hydrate. Each polymorph forms different types of hydrogen-bonding networks. Forms I, II, III and the hydrate adopt different types of sheet structures (Form I, corrugated; Form II, crinkled; Form III and the hydrate, planar) while Form IV adopts a herringbone packing arrangement. 30 Cocrystallization of 3,5-dinitrobenzamide and 4-amino-3,5-dinitrobenzamide in a 1:1 ratio from methanol resulted in the formation of Form I along with Form II as concomitant polymorphs. Crystallization of 4-amino-3,5-dinitrobenzamide from methanol and many other solvents gave Form II. Form IV is obtained by the crystallization of amino-3,5-dinitrobenzamide from a benzene solution by a slow evaporation method.

2,4-Dinitroanisole, DNAN

Nitroaromatic 2,4-dinitroanisole (DNAN) is an energetic material that is lately used as a substitute for 2,4,6-trinitrotoluene (TNT) in melt pour that are less sensitive replacements for composition B. Crystalline DNAN is known to have at least two polymorphic structures, referred to as α-form31 and β-form, 32,33 both monoclinic with space group P21/n (α-form with Z=8, Z'=2, β-form with Z=4, Z'=1). Calorimetry studies of DNAN reveal that the melting point of the α-form (96.65°C) is higher than that of the β-form (87.45°C). The β-form shows yet another phase transition at -8.65°C. Upon this transformation, the β angle of the β-form changes from 90.190° (24.85°C) to 96.716° (-173.15°C), and the length of the b axis shows negative thermal expansion, changing from 13.741(5) to 13.810(5) Å respectively. 34 The authors suggest that this thermal behavior relates to a dynamic disorder around the nitro groups. Since both polymorphic transformations and thermal-induced 5 ACS Paragon Plus Environment


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volume variation can increase the formation of voids and defects in melt pour compositions it is important to understand how to control these process, especially for long-term aging.35,36 In the present work, we focus on the control of morphology and crystal growth of DNAN by varying the environment around the forming crystal, either by controlling the concentration or by adding surface-poisoning substances.

Experimental Materials 2,4-Dinitroanisloe (DNAN) was purchased from Alfa-Aesar (98% purity) and used without further purification. Analytical grade solvents and materials were purchased from various vendors and used without further purification unless noted. Apparatus Differential scanning calorimetry measurements were conducted on a differential scanning calorimeter (DSC1, Mettler Toledo), using aluminum crucibles with perforated covers. Optical microscopy was measured using a LV 100ND microscope (Nikon Eclipse), equipped with a hot stage unit (HS82, Mettler Toledo). X-ray diffraction data of single crystals of DNAN (for both α- and β-forms) were collected on a Bruker AXS D8 VENTURE dual source diffractometer in the ω- and φ-scan modes using graphite monochromator and CuKα (λ=0.71073 Å) radiation. Data processing was performed using the APEX3 package. Data were corrected for the absorption effect using SADABS program. 37 The structures were solved by direct methods and refined by the full matrix least-squares using SHELXTL 38 and WinGX programs. 39 Molecular graphics was performed using the Mercury program.40


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For the α-form of DNAN two data collections were performed: one for the boat-shaped crystal and the second one for needle-shaped crystal. Both crystals have the structure of the reported α-form,20,21,23 i.e. monoclinic P21/n. The unit cell parameters, at -173 °C were a=8.586(2), b=12.653(2), c=15.268(3) Å, β=97.079(7)° (boat-shape crystals) and a=8.602(2), b=12.654(3), c=15.268(3) Å, β=97.18(1)° (needle-shape crystals), and are the same at 2σ level. The arrangement of the molecules is also the same as shown in Figure 1.

Figure 1: Superposition of the packing diagrams in the boat-shaped crystal (yellow) and the needleshaped crystal (light blue). View along the b crystallographic axis.

Two data collections were performed for the same β-form crystal: one at 0 °C and the second one -123 °C, i.e. above and below the phase transition described by Takahashi et al.34 Both crystals have the same structure as reported by Takahashi, i.e. monoclinic P21/n. The unit cell parameters are: a=3.9521(1), b=13.7231(4), c=15.4403(5) Å, β=91.129(2)° and a=3.8164(1), b=13.8039(4), c=15.3331(5) Å, β=96.715(1)° at 0 and -123 °C respectively, Figure 2.



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Figure 2: Superposition of the packing diagrams in the two β-form at 0°C (orange) and -123°C (light green). View along the b crystallographic axis.

Crystallization experiments: Closed systems: 0.3 gr DNAN was dissolved in the appropriate solvent and stirred at 70°C to reach total dissolution. The hot solution was filtered through a 0.22 µm Teflon filter and placed in screwcap glass vials. The solution was allowed to reach room temperature (22°±2°C). Open systems: Samples were prepared as above described but placed in open screw-cap glass vials. The solution was allowed to reach room temperature (22°±2°C) and evaporate slowly until crystals are formed. Glassware: all glassware was cleaned by immersing it in a saturated KOH/IPA solution for 48 hrs. Experiments were performed at least three times to ensure consistency of results. Results were very consistent when one phase dominated. However, in mixtures, deviations were observed. We attribute these deviations to cumulative effects stemming from the many factors, not all of them controllable, involved in the process.


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Results and discussion Calorimetry Differential scanning calorimetry (DSC) measurements of commercial DNAN reveal an endothermic phase transition at 96.3°C, corresponding to the melting of the α-phase, Figure 3. The assignment of α-DNAN is also supported by single crystal X-ray diffraction (references for comparison presented above). Some of the crystal structures of α- and β-DNAN show a disordered nitro group at one of the two molecules in the unit cell. It is not clear whether this disorder represents two co-existing structures or the diffraction quality. Cooling of a liquid sample of DNAN in the DSC pan results in an exothermic phase transition at ~50°C, corresponding to re-crystallization of the super-cooled melt. This was previously used to calculate the entropy of crystallization.41 The present work focuses on directing polymorphism, hence it was interesting to observe that a second heating step leads to a melting process at 86.7°C, indicating that β-DNAN is the predominant polymorph that crystallizes from the super-cooled melt of DNAN. The β-DNAN is the kinetic polymorph and is more likely to be formed upon fast cooling, in the absence of solvent. The preferential formation of β-DNAN from the melt is interesting from a practical point of view as DNAN is used in melt-cast compositions of energetic materials. If the β-DNAN is formed during this process from the melt, it may undergo phase transition into α-DNAN with time. Polymorphic phase transitions in casted products are undesired since they may undergo volume changes and form defects, consequently increasing the sensitivity of the explosive.



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Figure 3: DSC curve measured for commercial DNAN. An endothermic melting point at 96.3°C upon heating, an exothermic crystallization step at 50.8°C upon cooling and a second melting point at 86.7°C during a second heating step, indicating that β-DNAN is crystallized from liquid DNAN.

Crystallization Takahashi and Tamura34 reported the temperature-controlled crystallization of α- and β-DNAN from ethanol. They describe that upon heating a DNAN solution in ethanol to 40°C, filtering and cooling, α-DNAN is formed after three days. However, heating to 70°C followed by one day crystallization results in the formation of β-DNAN. In the present work it is found that upon heating an ethanol solution of DNAN (0.02 g/mL) to 70°C, followed by filtering and cooling, the solution crystallizes into a mixture of α- and β- polymorphs. After a while in solution, however, only α-DNAN is


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observed. This β- to α- transformation seems to be accelerated by evaporation. Various additional solvents were tested, most of which direct the formation of α-DNAN or mixtures of the two polymorphs. Some examples are presented in Table 1. In all cases presented in the Table, samples were left for crystallization for seven days either under slow or fast evaporation. However, as solubility is different in each solvent, crystallization occurred at different times after the dissolution, ranging between few hours and few days.

Table 1: Examples of solutions (0.02 g/mL) from which DNAN was crystallized1 Solvent Ethanol Ethanol Methanol Methanol IPA Acetone Toluene Chlorobenzene p-Xylene o-Xylene 1. 2. 3.

Shape Needles Blocks Needles Blocks Needles Blocks Irregular no crystallization3 Irregular no crystallization3

Polymorph2 β+α α β+α α β+α α α α -

Method Closed beaker (slow evaporation) Evaporation Closed beaker (slow evaporation) Evaporation Closed beaker (slow evaporation) Evaporation Evaporation Evaporation Evaporation

Saturation values (g/mL) at 22°C: IPA: 0.009 ; ethanol: 0.017; methanol: 0.036; acetone: 0.500. Assignments of pure polymorph by X-ray, mixtures by DSC. At higher concentration (0.15 g/mL, saturation) α is formed.

Preliminary experiments lead to the understanding that by varying the crystallization conditions one can affect both the polymorphism and shape of DNAN. For example, the morphology of the crystals obtained from xylene solutions, as well as from other aromatic solvents, are either irregular or needle-like, regardless of the polymorph. Crystals obtained from acetone or from high



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concentrations of DNAN in alcohols (methanol, ethanol and iso-propanol) are α-blocks, boat-like grains, Figure 4 right side. In contrast, crystallization from lower concentrations of DNAN in several solvent lead to the formation of α- or β- phases in the form of long threads, as detailed ahead, Figure 4 left.

Figure 4: The shape of DNAN crystals. Left: long threads or needles. Right: boat-like blocks.

Concentration effect in crystallization The accelerated β- to α- transformation during evaporation led to the understanding that the concentration concentration of the solution affects the crystallization process. Indeed, crystallization from diluted solutions solutions (see

Table 2) affords β-DNAN as long delicate threads, Figure 4 left. These β-DNAN crystals are stable in solution (sealed vial) for at least weeks at room temperature (~22°C), showing no sign for β- to α12 ACS Paragon Plus Environment

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phase transition. Moreover, the isolated dry solid material appears to be stable as β-DNAN for at least weeks below 22°C.

Table 2: Concentration effect in the crystallization of DNAN.* Solvent

Concentration [g/mL]

Shape

Polymorph

Methanol

0.013

No crystallization

-

0.0578**

mixture***

α+β mixtures***

0.13

Blocks

α

0.013

Needle/Thread

β

0.02

Blocks

α

0.13

Blocks

α

IPA

* **

***

Evaporation done under open vial slow evaporation conditions. The concentration of DNAN in methanol that is equivalent to 0.013 g/mL in IPA in terms of percentage from saturation. Upon six repetitions, each experiment provides a different ratio of α+β mixtures as well as different crystal shapes.

The dependence of the predominant polymorph on the concentration of DNAN in the crystallization solution implies an effect of self-poisoning on polymorph selection. Yet another effect that can be observed from the table is solubility and nucleation. Concentration of 0.013 g/mL DNAN in IPA corresponds to 0.0578 g/mL in methanol in terms of its solubility. When leaving to crystalize these two solutions one can observe the formation of needle-like β- crystals in IPA while in methanol mixtures of needle-like and boat-like α- and β- crystals are formed. This implies that upon nucleation the β- polymorph is formed in both solvents, at different concentrations and temperatures. Moreover, if the nucleation temperature is high enough to allow β- to αtransformation it is observed in the phase distribution. 13 ACS Paragon Plus Environment


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Additionally, exposing dry β-DNAN crystals to temperatures higher than 50°C results in the full transformation to α-DNAN while retaining the typical needle-like morphology of β-DNAN. This appears to be a solid-to-solid phase transition. According to the X-ray structural analysis the αDNAN crystals of the boat-like morphology are elongated in crystallographic a direction, Error! Reference source not found..

Figure 5: The morphology of a boat-like α-DNAN crystal. Left: simulated morphology and the orientation of the unit cell, view along [011] crystallographic direction. Right: a photograph of the crystal with indexed faces.


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Figure 6: The morphology of a needle-like α-DNAN crystal: a photograph of the crystal with indexed faces.

Indexing the faces for the α-DNAN crystals of the needle-like morphology was more challenging. While opaqueness was clearly observed only in dry crystals. In all cases, α-needles taken from solutions exhibit high degree of mosaicity. Despite the fact that both types of crystals have exactly the same structure (Figure 1), the long direction of the crystal is NOT the a crystallographic direction but it is close to the [104] one, Figure 6. The β-DNAN crystal in both high- and lowtemperature phases has the needle-like morphology and is elongated in the crystallographic a direction, Figure 7.



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Figure 7: The morphology of a needle-like β-DNAN crystal. Top left: simulated morphology and the orientation of the unit cell, view along [011] crystallographic direction at 0°C. Top right: simulated morphology and the orientation of the unit cell, view along [011] crystallographic direction at -123°C. Bottom: a photograph of the crystal with indexed faces (both data collections were done on the same crystal). The orientation of the molecules against the unit cell faces is presented in Table 3: Dihedral angles between the r.m.s. molecular plane of DNAN and unit cell faces

α-form molA molB β-form (0°C) Mol β-form (-123°C) Mol

(100)

Dihedral angle (°) (010)

(001)

36.8 41.8

54.3 56.0

84.6 74.4

25.3

65.1

86.9

21.3

69.5

78.2


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Figure 8: The orientation of molecules in DNAN crystals viewed down a crystallographic axis. Left: α-form. Middle: β-form at 0°C. Right: β-form at -123°C. . For both β-forms the orientation of the molecules in the unit cell is very similar. This explains the reversibility of the single-crystal-to-single-crystal phase transition taking place between the two βforms. On the other hand, the orientation of molecules in the unit cell of the α-form is different.

Table 3: Dihedral angles between the r.m.s. molecular plane of DNAN and unit cell faces

α-form molA molB β-form (0°C) Mol β-form (-123°C) Mol

(100)

Dihedral angle (°) (010)

(001)

36.8 41.8

54.3 56.0

84.6 74.4

25.3

65.1

86.9

21.3

69.5

78.2



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Figure 8: The orientation of molecules in DNAN crystals viewed down a crystallographic axis. Left: α-form. Middle: β-form at 0°C. Right: β-form at -123°C.

The crystals that are formed upon heating dry β-DNAN retain their morphology but become opaque and lusterless, unlike the original bright and light-transmissive crystals, Figure 9, an indication of the imperfect transformation of the needle as the phase shifts from the β- to the α-form.

α/β-DNAN mixture

→

pure α-DNAN


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Figure 9: Left: needles of a mixture of α- and β-DNAN. Right: needles of pure α-DNAN formed by heating the sample to 50-70°C. The resulting crystals are opaque and lusterless.

Long-thread shaped α-DNAN crystals were also subjected to heating in a hot-stage microscope. The melting at 97°C provided additional support to the identification of the polymorph. Evidently, there is a self-poisoning effect in the crystallization of DNAN as the predominant phase is concentration-dependent. Concentration effect is also revealed in the rate of β → α transformation in solution. The β polymorph, crystallized from IPA, undergoes transformation into the α-form within 24 h when the initial concentration is 0.02 g/mL. The same transformation is finalized in less than 5 h if the initial concentration of DNAN in IPA is 0.75 g/mL.

Controlling the crystal growth of DNAN using additives Solvents are not the only substances able to influence crystal growth. As detailed in the introduction, it is also possible to add low concentrations of substances that interact with the surface of a developing crystal and suppress the growth of polymorphs or specific crystal faces, thus leading to different morphology. In the case of DNAN, potential crystallization controlling additives are nitroaromatics. For example, the interaction between DNAN and TNT is revealed from the DSC curves of their mixtures. A sample of TNT subjected to a heating-cooling-heating treatment, undergoes first melting at 82.7°C, crystallization at 58.0°C and second melting at 82.4°C. TNT, like DNAN, exhibits significant overcooling before crystallization from the melt.



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58.0°C

Heat → cool → heat

82.4°C

82.7°C

Figure 10: DSC curve measured for TNT. An endothermic melting point at 82.7°C upon heating, an exothermic crystallization step at 58.0°C upon cooling and a second melting point at 82.4°C during a second heating step, indicating that the initial polymorph is re-crystallized from the melt.

Unlike DNAN, the second melting occurs at the same temperature as the first one, indicating that it re-forms the same polymorph from the melt.42 In the present work a 1:1 mixture of TNT and DNAN was subjected to a heating-cooling-heating cycle, resulting in melting at 60.4°C upon the first heating step and a second melting step at 53.0°C during the second heating cycle. According to the DSC trace, the crystallization of the eutectic 1:1 mixture does not produce a clear exothermic peak and is probably inhomogeneously broadened, Table 4.


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Table 4: Thermal behavior of eutectic mixtures of DNAN and TNT, indicating the interaction between the two molecules. Material TNT 2,4-DNAN 2,4-DNAN:TNT 1:1 2,4-DNAN:TNT 1:2 2,4-DNAN:TNT 2:1

m.p. [°C] 82.7 96.7 60.2 60.8 84.9, 61.1

Crystallization signal [°C] 58.0 50.9 29.9, 15.7 28.6

Re-melt signal [°C] 82.4 86.7 53.4 52.7, 62.7 53.1, 75.2

DSC measurements suggest significant interaction between DNAN and TNT. Therefore, TNT is a good candidate for our study on crystallization control agents for DNAN. Hence, DNAN/TNT mixtures were dissolved in methanol and crystallization was carried out using a three-layer technique.43 The bottom layer in this process is water, the intermediate layer is a 1:1 mixture of water and methanol and the top layer is a solution containing 0.02 g/mL DNAN and 10-50% TNT (w/w with respect to DNAN) in methanol. Crystallization of DNAN in this three-layer setup afforded long threads of DNAN, resembling the needles of β-DNAN. Nonetheless, DSC and XRD measurements indicate the presence of only α-DNAN, despite the unexpected β-phase like morphology and the high concentrations. Crystallization of DNAN under the same conditions in the absence of TNT affords only boat-like α-DNAN crystals. When DNAN/TNT mixtures in methanol are left standing for prolonged time, TNT also crystallizes out of solution. However, there is no evidence for co-crystallization or epitaxial crystallization, Figure 11. It was possible to follow the crystallization of DNAN from methanol in the presence of TNT under the microscope, observing the elongation of the thread without significant crystallization towards the perpendicular direction, Figure 12.



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Figure 11: Crystallization of DNAN from methanol in a three layered setup in the presence of different amounts of TNT. At high concentrations of TNT (50%) the two compounds crystallize separately into long threads (DNAN) and round particles (TNT).

Figure 12: Following the growth of a DNAN needle under a microscope in the presence of methanol and TNT.

The major modes of interaction between DNAN and other nitro-aromatics, such as TNT, are expected to be -C-H•••π, π-π and most importantly -C-

H•••O=N-


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interactions with the relatively acidic hydrogen atoms of the methyl group of TNT. Yet another possible effect that may be responsible for selective surface poisoning is the interaction of polar TNT with surface dipoles existing on the growing faces of DNAN.

Benzotrifuraxan, BTF, is also known to form co-crystals with several nitroaromatic compounds,44 one of which is DNAN.45 Indeed, addition of 5% (w/w with respect to DNAN) BTF to a 0.02 g/mL DNAN solution in methanol results in the formation of long threads of α-DNAN, much like the effect of TNT but at lower concentrations. Addition of 5% BTF to DNAN in IPA (0.02 g/mL) results in the formation of the β-polymorph. Table 5 depicts the effect of BTF and some other additives on the crystallization of DNAN. Solutions of 0.02 g/mL DNAN in MeOH or IPA were heated to 70°C, filtered, and 5% w/w of the respective additive was added. Mixtures were left at room temperature for slow evaporation for three days and the solvent decanted. Other additives that were applied in the same manner are nitrotoluene isomers. It is found that in methanol o-nitrotoluene directs the formation of boat-like α-DNAN while p-nitrotoluene suppresses crystallization completely.

Table 5: Effect of additives on crystallization. Solvent

Shape

Polymorph (by DSC) 23

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Reference 0.02 g/mL BTF (5%)1 TATB (5%)1,2 TNT (5%)1 3,5 DNAN (5%)1 1. 2.

MeOH IPA MeOH IPA MeOH IPA MeOH IPA MeOH IPA

Mixture Mixture Mixture Needles Blocks Needles Needles Needles Needles Needles

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α+β α+β α+β β α α+β α+β α+β α + β (~80% α) α + β (~90% β)

Percentage refers to w/w% with respect to 2,4-DNAN TATB= 2,4,6-triamino-1,3,5- trinitrobenzene

Table 6: Effect of additives on crystallization of low concentration of DNAN in IPA in sealed vials.

Reference 0.013 g/mL Nitromethane (5%)1 Nitroethane (5%)1 Nitropropane (5%)1 RDX2 (5%)1 PETN3 (5%)1 Acetone (3%)4 Acetone (6%)4 Acetone (14%)4 1. 2. 3. 4.

Shape Needles Needles Needles Needles Needles Needles Needles No crystallization No crystallization

Polymorph (by DSC) β β α/β 60:40 α/β 30:70 α/β 30:70 α/β 40:60 α/β 55:45 -

Time to crystallization overnight 1 month overnight 1 month overnight overnight 4 days 1 month 1 month

Percentage refers to w/w% with respect to 2,4-DNAN RDX=Cyclotrimethylenetrinitramine PETN= Pentaerythritol tetranitrate Percentage refers to v/v% with respect to solvent (IPA)

At low concentrations (< 0.013 g/mL,


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Table 6) crystallization may occur after long periods. In the presence of some additives such as nitromethane and acetone, crystallization is completely suppressed. Other additives only prolong the crystallization process. Table 6 presents the effect of additives after a day and after four days.

Conclusions In this work, DNAN was used as a model compound for the study of surface poisoning effects in crystallization. Three major variables were found to affect crystallization: a) concentration of DNAN in the crystallization solution, b) the presence of additives in the crystallization solution and c) solidsolid transformations. It is found that low concentrations of DNAN in IPA afford the β- polymorph of DNAN. Under these conditions the growth of the more stable α- polymorph is suppressed and the β- needles are stable for long periods at low temperatures. At higher concentration, the β- polymorph undergoes transformation into the α- phase. Two processes can occur: at highly concentrated solutions and suitable temperatures blocks of α-DNAN crystallize out of solution. Another process involves the formation of α-DNAN crystals that retains the needle shape of the originally formed βpolymorph. This phase transition also occurs in dry crystals. The β- to α- transformation is most probably possible because of a similarity between the two crystal structures. Hence, the barrier for this transformation is low.46 Nevertheless, the formation of α-DNAN in the typically β- morphology induces stain into the crystals leading to defects. It is also shown that the orientation of the molecules against the unit cell faces for the two α-forms (blocks and needles) is clearly different. The effects of additives on the crystallization process are also shown here. Aromatic and/or nitro containing additives interact with DNAN, competing with intrinsic intramolecular interactions that allow the formation of DNAN crystals.



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As mentioned above, the detailed modes of interaction between the growing DNAN crystals and their surroundings are not fully understood yet and certainly deserve additional study.

Several possible explanations can account for the observed polymorph preference depending on solvent, additives, temperature and time. Both crystal nucleation and growth can be directed by either thermodynamic or kinetic effects. It is conventional that the formation of metastable polymorphs is kinetically controlled by the higher rate of nucleation and/or crystal growth. However, Belenguer, Sanders and co-workers 47 suggested that in some systems the stability of small crystallites depends on their surroundings’ dipole moment. This is explained in terms of cooperative interactions between specific surfaces and solvent/additives molecules, affecting the surface tension of the crystallites and thus their stability. As a result, they observe a crossover in polymorphism with crystal size in specific solvents. These findings are also supported by DFT calculations, describing the different solvents by their dielectric constant. It is also possible, however, that the effect of solvent or additive is through surface poisoning via specific non-covalent interactions between surface molecules and their surrounding molecules. This mechanism is more difficult to support with calculations. The diversity of results, in the case of DNAN, suggests that several mechanisms are operative. The effects of crystal size and morphology resemble the findings of Belenguer and Sanders. However, we cannot exclude that interactions on the surface block specific facets from being expressed.

Acknowledgement We acknowledge the Pazy Foundation for financial support.


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For Table of Contents Use Only

Controlling the Crystal Morphology and Polymorphism of 2,4-Dinitroanisole Chagit Denekamp1*, Olga Meikler1,2, Michael Zelner2, Kinga Suwinska3, Yoav Eichen2*

1

RAFAEL, Advanced Defense Systems LTD, POB 2250 Haifa 3102101, Israel

2

Schulich Faculty of Chemistry, Technion – Israel Institute of Technology, Technion City Haifa 3200008, Israel

3

Faculty of Mathematics and Natural Sciences, Cardinal Stefan Wyszynski University in Warsaw, Wóycickiego 1/3, PL-01 938 Warsaw, Poland



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The study of surface poisoning effects in the crystallization of nitro aromatic 2,4-dinitroanisole reveals that both its concentration in solution as well as the presence of crystal face poisoning additives control the resulting polymorph and morphology. While the β- phase appears only as long threads, the α-phase appears both as long threads and as boat-like blocks.


Controlling the Crystal Morphology and Polymorphism of 2,4

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