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Formation of Crystalline Hollow Whiskers as Relics of Organic Dissipative Structures Published as part of a virtual special issue of selected papers presented at the 9th International Workshop on the Crystal Growth of Organic Materials (CGOM9). Damien Martins,† Torsten Stelzer,‡ Joachim Ulrich,‡ and Gerard Coquerel†,* † ‡
Unite de Cristallogenese, SMS UPRES EA 3233, Universite de Rouen, F-76821 Mont Saint Aignan Cedex, France. Martin-Luther-Universit€at Halle-Wittenberg, Zentrum f€ur Ingenieurwissenschaften/TVT, D-06099 Halle (Saale), Germany
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ABSTRACT: This work presents an original general route to obtain faceted tubular organic crystals (of several tens of micrometers in section) by putting the compound inside a thermal gradient. These hollow whiskers grow spontaneously when the thermal gradient exceeds a certain threshold, the highest temperature being close to the melting point, the lowest temperature being RT. This phenomenon could be explained by heat dissipation and transport of matter thanks to sublimation mechanisms and/or capillarity, i.e. by convective heat transfer. The open system is characterized by energy transport, entropy creation and spontaneous apparition of anisotropy. That is the reason why these hollow whiskers can be considered as relics of dissipative structures (Prigogine, I. Wiley: New York, 1967).
1. INTRODUCTION Hollow crystals, also called hollow needles, hollow whiskers, tubular crystals, etc., result from a particular crystal growth. Numerous examples are reported in the literature, most of them related to inorganic materials, such as ZnO,1 GeO2,2 CdSe, or CdS.3 Rare cases concern organic material. Indeed, only two active pharmaceutical ingredients (API) have been known to form hollow whiskers, dexamethazone acetate sesquihydrate4 and more recently 5-methyl-5-(40 -ethylphenyl)hydantoin.5 Different mechanisms were proposed to explain this particular crystal growth.6 Two crystal growth routes can be distinguished: (i) by vapor transport (i.e., sublimation); (ii) via liquid-phase diffusion-limited crystallization. Eddleston and Jones7 reported the general formation of tubular crystals by slow solvent evaporation via the second method for a large class of organic compounds. This study reports the formation of crystalline hollow whiskers with the following organic compounds: glycine, salicylic acid, saccharin, barbital, phenobarbital, various hydantoin derivatives, succinic acid, caffeine, anhydrous theophylline, acetanilide, and acetamide. This nonexhaustive list shows the variety of compounds able to form microtubular crystals and highlights the general character of the mechanism. These hollow acicular crystals generally exhibit a cavity bigger than those previously encountered in the dedicated literature. This property could be of interest to prepare microcontainers for drug delivery devices.8 r 2011 American Chemical Society
2. EXPERIMENTAL SECTION All compounds employed were purchased from Aldrich and used as received. The method used to prepare hollow crystals was similar for each compound. A small amount of powder was put on a fine microscope cover slip to form a small heap of matter (glass slides used could have different size and thickness). Then the cover slip was set down on a Kofler bench or a heating plate. The glass slide prevented risk of contamination of and from the Kofler bench surface. The sample was warmed close to its melting point. Due to its poor thermal conductivity coefficient, the powder was in a thermal gradient leading to the growth of hollow acicular crystals (Figure 1). Scanning electron microscopy (SEM) pictures were obtained with a JEOL JCM-5000 NeoScope instrument (secondary scattering electron) at an accelerated voltage between 10 and 15 kV. Glass slides with hollow needles were stuck on an SEM stub with gloss carbon and coated with gold to reduce electric charges induced during analysis with a NeoCoater MP-19020NCTR. Optical microscopy pictures were obtained with a HIROX KH-7700 digital microscope system. Lens used is the MX-2016Z. X-ray powder diffraction analyses were carried out on a D8 Discover Bruker system. The instrument is equipped with an X-ray tube containing a copper anticathode, (40 kV, 40 mA, KR1 radiation = 1.5406 Å, KR2 Received: March 7, 2011 Revised: April 12, 2011 Published: April 20, 2011 3020
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Figure 1. Schematic drawing of organic crystalline hollow whisker formation by application of an appropriate thermal gradient on a powder.
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Figure 3. Crystalline hollow whisker growth routes envisaged. Left route, growth at the particle tip; right route, growth from the root. Notice that the heap of powder retracts to offer the minimum contact with the hot base.
salicylic acid. The distinctive characteristic of these tubular crystals is to present an extremely fine wall, circa several tens of nanometers. The crystal of glycine in Figure 2c has a hexagonal cross section with a thick wall, 310 μm, and an irregular inner channel whose diameter ranges from 510 μm. The hollow needle of (()5-ethyl-5-methylhydantoin (Figure 2d) exhibits a similar morphology. It is interesting to note how well faceted are the external crystal walls, with sharp edges. This is in contrast with the central cavities of the particles, which are quite irregular. Two movies showing the hollow whiskers of glycine and salicylic acid growing entitled “Formation of glycine whiskers 2” and “Formation of salicylic acid whiskers” are available. They are accelerated 10 times, and the real duration is 2 min for both movies. The hollow crystals of glycine, saccharin, salicylic acid, and hydantoin were studied in more detail to investigate the reproducibility and the formation mechanism. However, tests were also carried out with other organic compounds (barbital, phenobarbital, succinic acid, caffeine, anhydrous theophylline, and acetanilide) and have shown similar results. This proves that the mechanism of hollow crystal formation is somewhat general and valid for many covalent organic materials. Figure 2. Scanning electron microscopy images of hollow crystals of various organic compounds: (a) saccharin; (b) salicylic acid; (c) glycine; (d) (()5-ethyl-5-methylhydantoin (12H). radiation = 1.5444 Å), and mounted with an angular detector, Lynx eye . The scan step was fixed at ∼0.04° with a counting time of 0.5 s/step over an angular range 3°30°. The system is monitored with Diffract. plus XRD commander software version 2.6.1. Gas chromatography analyses were carried out on a Thermo Fisher Scientific Trace GC Ultra chromatograph equipped with a split/splitless injector and a flame ionization detector operating at 250 °C. Helium was used as carrier gas at an average velocity of 25 cm s1. The split flow was adjusted to 30 mL min1. Column (β-dex120, 30 m 0.25 mm 0.25 μm) was purchased from Supelco, Bellefonte. Chiral analyses were performed at 145 °C.
3. RESULTS The scanning electron microscopy (SEM) pictures in Figure 2 show hollow crystals of saccharin, salicylic acid, glycine and (() 5-ethyl-5-methylhydantoin prepared by annealing of the powder on a Kofler bench. Crystals of saccharin have a needle shape with a rectangular cross section and channel size ranging between 30 and 60 μm (Figure 2a). Figure 2b shows a faceted hollow needle of
4. DISCUSSION Heating a powder sample on a Kofler bench or on a hot plate is a daily test in numerous laboratories. Some of these compounds (glycine, salicylic acid, saccharin, etc.) are used as standards of melting point for calibration. So it is rather surprising that, to our knowledge, these experiments and these morphologies have not been reported before. Indeed, it is a phenomenon perceptible to naked eyes. In the case of salicylic acid, the needles can reach 5 mm, or even more. This phenomenon seems to be limited to covalent chemicals. Some additional restrictions can be enumerated, in particular (i) a relatively high melting point (above 115 °C, that is, the temperature corresponding to the positive candidate exhibiting the lowest melting point: acetanilide), (ii) a good thermal stability allowing it to reach fusion without chemical degradation, and (iii) the possibility to sublimate the compound. Our results show that sublimation should preferably occur close to the triple point and the presence of a liquid phase can favor this phenomenon. From these facts, it could be possible to find a general mechanism for formation of hollow crystalline whiskers. Two possible growth routes were envisaged. The first mechanism is a growth at the tip of the tubular particle (left route in Figure 3). Thanks to the 3021
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Figure 4. Crystal growth of hollow whiskers of salicylic acid during annealing at ca. 155 °C: (a) t = 0; (b) t = þ20 s; (c) t = þ30 s; (d) t = þ40 s.
driving force, here the heat flow, leading to a sufficient gradient of temperature, that is, for a gradient (∂T/∂z) above a certain threshold, the matter, sublimated or liquid, could be transported inside the hollow crystal by convection phenomena, and when the matter reaches the extremity of the needle, it feeds the growth of the hollow whiskers. This can explain the open channel and it is consistent with the formation of branched particles. The second mechanism is a growth from the root (right route in Figure 3). According to this latter hypothesis, the sublimated or liquid matter is transported by the heat flow up to the heap surface. Then this matter can crystallize when the temperature decreases, for example, at the needle base. These crystals grow from the root, like hair, fed from their bases. But this mechanism does not explain well the open channel of the whiskers, and it is not consistent with the morphologies observed. Experiments were carried out to prove that the whiskers grow at the tip of the particle. A small amount of salicylic acid powder was annealed on a heating plate around 155 °C. Phenomena were recorded with the HIROX optical microscope with a 80 magnification. Some hollow needles were observed during their growth. Pictures obtained are presented in Figure 4. Striations are visible on the needle surface, perpendicular to the direction of crystal growth. These lines, parallel between them, are probably marks of “growth episodes” left by the variations of the whisker growth rate, as geological strata. These steps, which do not move while the crystal is growing, highlight the hollow whiskers' growth at the tip of the elongated particle. A movie entitled “Formation of salicylic acid whiskers 2” is available. The movie is accelerated 10 times, and the real duration is 4 min. The same tests were carried out in a closed system under low gas pressure. The hollow whisker growth phenomena were quasi-inhibited under partial vacuum. The same observations were made
Figure 5. Branched hollow whiskers observed by SEM: (a) (()5-ethyl5-methyl hydantoin; (b) caffeine with DMSO at 195 °C; (c) succinic acid; (d) detail of panel c, (e) detail of panel d, white circle; (f) detail of panel d, black circle.
for glycine annealed at ca. 245 °C under low gas pressure. These results seem to prove that the tubular crystal growth is based upon a gas convection effect, which can be partially inhibited by working under partial vacuum. This growth mode, based on sublimation in a large thermal gradient and in an open system, is so completely different from the plate sublimation technique, developed by Karl,9 which operate in a closed system, under partial vacuum, and using a small thermal gradient (ΔT from 0.5 to 5 °C per cm). This latter method is suitable to purify organic compounds (or component donoracceptor complexes) leading to well-faceted full crystals in the sublimation zone after a period of between several days and 1 month. Positive tests were performed using single crystals of glycine to determine whether it is possible to reproduce the phenomenon with compact materials. Text and videos are available as Supporting Information. Careful observation of the hollow whiskers permits one to see that some needles are sometimes branched. It seems that during the tubular crystal growth, a part of the sublimated (less likely liquid) matter can abruptly change the growing direction. This phenomenon is probably due to macrodefects on the hollow needle rim. For example, Figure 5a exhibits five small needles of (()5-ethyl-5-methylhydantoin (12H) grown on a bigger one. The median diameter of these needles is variable from one to another, but it is obvious that the two 3022
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Crystal Growth & Design bigger ones on the right part of the picture are hollow. The morphologies and orientations of the branched hollow needles are also compound dependent. Indeed, Figure 5b,c shows branched hollow needles of caffeine and succinic acid, respectively, with characteristic morphologies. In the case of succinic acid, the crystalline ramifications are (quasi)perpendicular to the mother hollow needle, forming a real hollow faceted tubular network, whereas in the case of caffeine, needle stars with many branches oriented at random are frequently encountered. In all cases, every branch is hollow and exhibits the same shape as the stem crystal. This indicates that the growth mode is the same for the small branches as for the mother hollow needle. That favors the mechanism of matter transport inside the channel and so the crystal growth at the extremity, that is, at the tip of the needle. Figure 5d is a part of the succinic acid crystal network. The hollow whiskers highlighted with a circle are enlarged in Figure 5e,f. On the one hand, Figure 5e shows the tip of a mother hollow whisker. Small needles are visible on the surface. They are oriented in all directions and on all the mother whisker faces. This type of needle is probably due to “classical” sublimation of succinic acid, that is to say, without gaseous transport inside the channel. On the other hand, Figure 5f represents magnification of a branched hollow whisker. This one is perpendicular to the mother whiskers and is unmistakably hollow. Figure 5c,d shows clearly the preferred orientation of the branches with regards to the stem whisker. These elements seem to prove the feed of the branches by matter transport directly from the inside of the mother hollow whisker channel. This hollow crystal growth by matter transport in a sufficient gradient of temperature is a fast process. Indeed, only few minutes are necessary to obtained hollow needles from an organic powder. This phenomenon can even be boosted by addition of an appropriate solvent during the heating. This variant demonstrates a continuum with the growth mechanism in solution previously described by Eddleston and Jones.7 In the case of (anhydrous) caffeine, the compound exhibits a melting point around 228 °C at room pressure. If a small amount of caffeine is heated on a Kofler bench at ca. 195 °C, a relatively slow tubular crystal growth can be observed. Upon addition of one droplet of dimethyl sulfoxide (hereafter DMSO, boiling point ∼189 °C at room pressure) directly on the powder, this phenomenon is significantly accelerated. Indeed, the solvent rapidly dissolves the organic compound, just before being evaporated. The hollow crystal growth of caffeine is thus boosted by a faster transport of matter due to the combination of DMSO evaporation and the thermal gradient. Figure 5b illustrates hollow needles obtained by this process and a movie entitled “Formation of caffeine whiskers þ” is available. The movie is accelerated 10 times, and the real duration is 8 min. In the same way, it is possible to obtain needles at low temperature (i.e., in a small temperature gradient) by adding a droplet of solvent exhibiting a high vapor pressure. So whiskers of acetamide (Tf = 79 °C) were obtained by using a single droplet of a mixture water/ethanol to help and to increase the driving force sufficient to create and to sustain the dissipative structures. As aforementioned, the growing rate, intrinsically bonded to the gradient of temperature, is the key parameter of this process. Indeed, the hollow needles do not grow at a unique temperature corresponding to the melting point but on a temperature range specific to the compound. So the thermal gradient could be slightly modified, which will accelerate or slow the growing rate of these tubular structures. Figure 6 illustrates several hollow
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Figure 6. Hollow whiskers of (()5-ethyl-5-methylhydantoin by SEM: (ac) needles obtained by fast process; (df) needles obtained by slow process.
needles obtained with (()5-ethyl-5-methylhydantoin. The three first pictures (panels a, b, and c) show three hollow needles obtained at a fast growing rate, that is to say, with a 90 °C per cm gradient, the highest temperature being close to Tf = 150 °C. It can be noted that the three tubular crystals exhibit an irregular wall surface with thin walls and therefore a large internal channel. Although (()5-ethyl-5-methylhydantoin crystallizes as a conglomerate, apparent single crystals contain almost no enantiomeric excess. This is due to a “multiepitaxy” phenomenon between the two chiral molecules.10 Nevertheless, the presence of two enantiomeric phases could have explained the particular morphology observed because of the competition between the two enantiomers. In a second experiment, powder of (()5-ethyl5-methylhydantoin was heated to complete fusion and then the molten phase was slowly recrystallized at 140 °C during ca. 1 h. Figure 6d,e,f exhibits three hollow needles obtained by this second process. It is obvious that the general morphology of the tubular crystal is totally different compared with the first faster process. The crystal growth rate has been decreased, leading to external needle surfaces perfectly smooth. Besides, the walls appear thicker at the needle tip. It is even not far from being closed at the extremity in some cases (Figure 6d,e). This is because of the drop in thermal gradient and consequently a slower matter transport rate. Thanks to the molecular stability after the fusion of this hydantoin, gas chromatography 3023
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Figure 8. Schematic drawing of a hollow crystal obtained by sublimation according to the Iwanaga model (adapted from the original drawing of ref 12).
Figure 7. Scanning electron microscopy pictures of hollow crystals tips: (a) glycine; (b) detail of panel a, white circle; (ce) saccharin; (f) schematic of skeletal crystal growth.
analyses were carried out to measure the enantiomeric excess of these hollow whiskers. Although the general morphology can be modified by the growth rate, it does not change the enantiomeric excess of the hollow particles which remains quasi-nil. The operating conditions, that is, the heating rate, the final temperature, and the amount of powder, can lead to particular morphologies. In the case of crystalline hollow whiskers of glycine, obtained from a native powder sample corresponding to structurally pure R form, striations are visible inside the channel (Figure 7a,b). These striations, similar to winglets, are consistent with a growth at the extremity (Figure 3) via a transport of gaseous matter inside the crystal channel. In addition, X-ray powder diffraction analysis on a heated powder sample with hollow needles revealed a mixture of R form and β form (it is difficult to harvest the hollow whiskers only). Metastable monoclinic β polymorph is well-known to crystallize by sublimation.11 It seems to confirm the role of the vapor phase in the growth mechanism. Interesting crystal morphologies were also observed for saccharin. Figure 7c shows an “ear” shaped needle. This morphology is characterized by (i) a long crystal body (∼700 μm) with no evolution of the inner channel diameter and (ii) a stepwise narrowing of the crystal head, leading to a tip. More exotic morphologies also were observed for saccharin. Figure 7d,e exhibit an amazing “stairs” shape; also with a constant body until a narrowing of the crystal head, each nodule appears hollow on the image. These types of morphologies are not consistent with the growth model proposed by Iwanaga et al.12 Indeed, according to this
Figure 9. Scannning electron microscopy of salicylic acid sublimated: (a) enlarged down part of needles on gloss carbon; (b,c) detail of open needles.
model, crystal growth fed by sublimation leads to a widening of the construction because “After the growth of an initial needle, many c-whiskers grow on its side surface by the two dimensional nucleation. These c-whiskers develop into needles and again c-whiskers grow on their side surface” (ref 12, p 439440), as represented in Figure 8. All these phenomena lead to the formation of a cavity in the center of the crystalline aggregate, with a smaller base than the head of the needle. To confirm this predictive model, needles of salicylic acid were obtained by sublimation. A few milligrams of powder were sublimated at 160 °C in a glass tube and salicylic acid crystals were grown on metallic stub (with gloss carbon) placed in the growth zone. A typical example of the needles obtained after a growth time of a few minutes is shown in Figure 9. The general morphology of the needles is 3024
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That is due to the low thermal conductivity of the air (∼0.026 W m1 K1). Based on this interpretation, the possibility to elongate further the hollow whiskers by using a better heat conductor medium such as helium was predicted. Because this noble gas exhibits a thermal conductivity 5 times greater than that of the air, it was postulated that gaseous molecules will be transported farther before condensation. Saccharin hollow needles obtained in the same closed glass tube filled with helium on the one hand and air on the other hand were compared after using the same experimental conditions (Figure 10). As expected, the general needle size (length and width) obtained in helium is higher than that obtained in air. These experiments confirm that the hollow whisker growth is fed by gaseous matter transport involving convection phenomena. This hollow needle growth, which can be perceived as a simply aesthetic or anecdotic phenomenon, can also turn out to be problematic. When considering DSC analyses of organic compounds with pierced lids, the DSC profile might be biased because part of the powder can be transported leading to a poor contact with the hot base of the crucible.
Figure 10. Saccharin hollow whiskers obtained by annealing at ca. 228 °C and observed with a HIROX optical microscope in polarized light: (ac) needles obtained in helium environment; (df) needles obtained in air environment. Magnification, 40; scale size, 2 mm.
different from those obtained by an annealing in a gradient of temperature at room pressure. As described by Iwanaga, the needle base is clearly smaller than the rest of the particle (Figure 9a). Moreover, the channels of the needles are open along their full lengths. Indeed one face of the tubular crystal appears split (Figure 9b,c). In all cases, these experiments prove that the general mechanism of hollow crystal growth presented in this paper cannot be described by the usual simple sublimation phenomenon, but additional parameters must be considered. As a consequence, a new mechanism of hollow crystal growth is proposed here. A scheme of a growing hollow saccharin crystal is illustrated in Figure 7f. During the annealing in the thermal gradient, the heat flow is dissipated by convection. The heat crosses the powder heap and a part of the energy is transferred to the powder, which partly sublimates or melts. The chimney-like structures design themselves to provide heat dissipation and matter transport. Under these conditions, these columns are directly fed from the inside; this can lead to a narrowing at the extremity, by exhausting the flow of matter, when the thermal gradient and the temperature decrease. So, the morphologies observed in this work are the remaining skeletons of dissipative structures. Indeed, such structures appear for systems submitted to specific energy and matter flows, (i.e., far from thermodynamic equilibrium), which show a spatial and a time-resolved organization. As aforementioned, the hollow whisker growth mechanism seems to be essentially based on gaseous matter transport in a thermal gradient. The limiting factor of the hollow needle growth is the fast cooling as the tubular crystal recedes from the heat source, leading to a rapid solid condensation of the compound.
5. CONCLUSION An original process for hollow crystal formation applicable to a wide range of covalent compounds is presented in this work. This phenomenon consists simply in placing a layer of some millimeters of powder in a thermal gradient whose limits are (i) the melting point of the component and (ii) the ambient temperature. The similarities in the behaviors of a dozen organic compounds having very different chemical structure, crystal habits, and melting points lead to the conclusion of a general mechanism. The experiments highlight the role of the heat flow as the driving force of the mechanism. The concomitant evaporation of some appropriated solvent can also help the driving force. This study shows that the particles grow at their upper tips. The hollow crystal formation is characterized by a matter transport inside the tubular particles by convective heat transfer. These elongated hollow crystals are then relics of dissipative structures. Experiments are in progress to fully control the process to test whether microcontainers can be built up using this technology. ’ ASSOCIATED CONTENT
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Supporting Information. Movies in avi format, a description of the experiment depicted in the movies, and a figure capturing the evolution of the crystals. This material is available free of charge via the Internet at http://pubs.acs.org.
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Web Enhanced Feature. Movies of the formation of glycine, salicylic acid, and caffeine whiskers in avi format are available.
’ AUTHOR INFORMATION Corresponding Author
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[email protected] . Phone: þ33 2 35 52 29 27. Fax: þ33 2 35 52 29 27.
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