Habit Changes of Sodium Bromate Crystals Grown from Gel Media Rositza I. Petrova and Jennifer A. Swift* Department of Chemistry, Georgetown University, 37th and O Streets NW, Washington, DC 20057-1227, USA Received June 28, 2002;
CRYSTAL GROWTH & DESIGN 2002 VOL. 2, NO. 6 573-578
Revised Manuscript Received August 19, 2002
ABSTRACT: Agarose, gelatin, and silica gel media have been surveyed as appropriate matrixes for the crystal growth of sodium bromate (NaBrO3). From pure aqueous solution, NaBrO3 crystallizes with a characteristic tetrahedral habit. We have demonstrated that a greater variety of habits (e.g., cubic, tetrahedral, polyhedral, and dendritic) can be reliably obtained from the growth in these gel media by controlling the solute concentration and the gel density. Agarose and silica matrixes suppress the nucleation as the gel density increases, while increasing gelatin concentrations seem to enhance crystal nucleation rates. These observations support the general hypothesis that the gel media influences the relative growth rates by controlling the solute nucleation and diffusion; however, little can be concluded about the possible chemical interaction between the polymeric backbone and the solute. Introduction The habit of a crystalline material can have a strong influence on its physical properties, such as the ease and effectiveness of solid/liquid separations, bulk packing efficiency, and flow characteristics.1 However, we do not yet have a complete understanding of how to control macroscopic crystal shape. Crystal habit is determined by the relative growth rates along different crystallographic directions. The slower the growth rate, the larger the face. The growth rate of each individual crystal face is determined by a combination of internal (intermolecular bonds, defects, symmetry) and external (supersaturation, temperature, solvent, impurities) factors. It is well-known that altering the external growth parameters can affect crystal growth rates, but such changes do not necessarily change the habit, because many crystal faces can be affected simultaneously by changes in solution conditions. The introduction of “tailor-made” impurities2 to the growth solution can be a reliable way to alter crystal shape provided interactions are quite different on different crystal surfaces. The extent of habit modification in the presence of impurities depends largely on the strength, specificity, and concentrations of the impurities present. Unfortunately, for some applications in which crystal purity is an issue, impurities may not be the most desirable means to effect habit modification. This paper details our efforts to discern whether crystal growth in gel media provides a rational alternative route toward habit modification. As a test of this theory, we initially focused on sodium bromate, NaBrO3, a salt that crystallizes in the cubic space group P213 (a ) 6.70717 (5)).3 Though NaBrO3 crystals are chiral,4 they usually adopt an achiral tetrahedral growth habit with predominantly (111) faces when grown from aqueous solution. Interestingly, the isostructural salt sodium chlorate is well-known to grow in a cubic form terminated with (100) faces. Holcomb et al.5 rationalized that, in principle, conditions should exist such that either * To whom correspondence should be addressed. Tel: (202) 6875567. Fax: (202) 687-6209. E-mail:
[email protected].
habit is possible for the two salts. In studying the aqueous solution-grown habit of NaBrO3 as a function of supersaturation and temperature, they were able to identify solution conditions under which one of three distinct habits predominatesscubes, polyhedra, and tetrahedra (Figure 1). In general, growth at elevated temperatures appeared to favor the deposition of cubes and polyhedra. Inoue and Nishioka6 later also examined the habit of NaBrO3 grown in the presence of ∼1% acetic acid impurity. They reported that the effect of the impurity is generally more significant at low supersaturations. For crystallization at room temperature, a slight twist on the conventional solution-based crystal growth methods is to use gel media. For over a century, gel matrixes have been used as effective environments for growing crystals of sparingly soluble salts with appreciable size7-9 or to obtain crystals of high quality.10-12 Gels are typically two-component phases, a liquid phase (the growth solution) and a macroporous solid phase. The solid phase consists of polymeric chains, either chemically linked (e.g., silica, polyacrylamide) or physically entangled (e.g., agarose, gelatin). Growth in gel media also offers several practical benefits over solution growthsthe suppression of convection currents and turbulence decreases the frequency of collisions that can lead to unstable nuclei, and growth rates are reduced because solute is supplied to the crystal surface only by diffusion. The soft gel framework is almost always assumed to be chemically inert and therefore able to support crystal growth within the macropores without exerting additional forces on them. Crystal nucleation rate is believed to be dependent on the pore size, while crystal growth is determined by the connectivity between the pores.9,13 We were interested in developing gel media as a general method for effecting habit changes in crystals. Other reports of gel-grown crystals have demonstrated that even simple salts can adopt very different growth habits in gel media than from pure solution growth. For example, Doxsee et al.14 showed that silver bromide adopts unusual habits by growth from poly(vinyl chlo-
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Figure 1. Ideal crystal habits of sodium bromate. (Left) cubic: all (001) faces; (middle) polyhedral: mixture of (001), (111), and possibly higher Miller Index faces; and (right) tetrahedral: predominantly (111) with smaller (110) faces.
ride) gels. Helically shaped silica crystals have also been obtained by sol-gel polymerization in cholesterol-based organogel matrixes.15 Our study focuses on surveying the crystal habit of NaBrO3 grown from an assortment of standard gel media as a function of solute supersaturation and gel density. Agarose, gelatin, and silica gels were selected because of their vastly different monomeric structures, to try to establish whether the chemical composition of the gel, its physical properties (density and pore size), and/or solute supersaturation play a discernible role in crystal habit formation. Experimental Section Materials. Sodium bromate, 99+% (Aldrich), agarose (Type I, Sigma), gelatin (Type B, Sigma), sodium silicate solution (F ) 1.390 g cm-3, Aldrich), acetic acid (EM Science), and o-phosphoric acid (Fisher Scientific) were used without further purification. Water was purified by passage through two Barnstead deionizing cartridges followed by distillation. Gel Preparation. Thermoreversible agarose and gelatin gels were prepared by heating the aqueous suspension of sodium bromate and the gelling substance to 80 °C in a water bath until clear solutions were obtained. Upon slow cooling to room temperature, the solutions formed a gel. Over a period of a few days, crystals of visible size grew in these matrixes. Silica gel was prepared by neutralization of sodium silicate solution. The silicate solution was diluted with water and heated to 80 °C. The solution was later acidified with either 1 M acetic or o-phosphoric acid to pH 6 and stirred for few seconds after which sodium bromate was added. Stirring continued until the sodium bromate dissolved, and then the gel was slowly cooled to room temperature. Over a period of typically a few hours, crystals of visible size grew in these silica matrixes. It is important to add the sodium bromate after the sodium silicate solution and acid have been mixed; otherwise, a white coagulate is formed. All gels were prepared in standard test tubes with a diameter of 25 mm and had a total gel volume of 20 mL. Crystal growth in all cases occurred at room temperature (23 ( 1°). All gel and sodium bromate concentrations listed in the text are given in wt %. Gelation Temperature Determination. The optical rotation measurements were performed on a Rudolph Instruments polarimeter (DigiPol DP781) at λ ) 589 nm in a water-jacket optical cell with a path length of 10 cm. The cooling rate was 0.5 °C/min.
Results and Discussion Habits Obtained from Pure Aqueous Solution. To determine the effect of the gel matrix and supersaturation, the growth of NaBrO3 crystals was systematically investigated at different concentrations of the gelling substance and the salt. Habits of crystals grown from pure aqueous solution serve as the reference point for all gel-growth studies. The solubility of NaBrO3 in water at 25 °C is 28.29 wt %.16 Reproducing earlier work, we found that the habits of NaBrO3 crystals grown from pure aqueous solution at room temperature varied widely as a function of supersaturation. Aqueous
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solutions with low supersaturation ([NaBrO3] < 28%) routinely generated tetrahedral crystals, in agreement with previous reports. Growth from more highly supersaturated solutions ([NaBrO3] > 36%) tended to yield a mixture of tetrahedrons, polyhedrons, and cubes (Figure 2). The appearance of plates/cubes is most easily explained by an increased nucleation rate at high supersaturation. The plates observed are always smaller than the tetrahedrons and polyhedrons, suggesting that nucleation of the cubic form can occur at high supersaturation, but that once material is deposited from solution and the supersaturation level decreases, nucleation and growth of tetrahedral and/or polyhedral forms are favored. Growth from Agarose Gel. Agarose is a naturally occurring polysaccharide derived from agar and composed of alternating 1,3-linked β-D-galactopyranose and 1,4-linked 3,6-anhydro-R-L-galactopyranose. At high temperatures, agarose adopts a random coil structure, but as the temperature is reduced, the molecules adopt helical conformations. At the gelling temperature, the helices aggregate to form rod-like fibers with a diameter of few nanometers.17,18 Arnott et al.19 studied the tertiary structure of agarose by polarimetry and X-ray diffraction, and proposed a left-handed double helix model with 3-fold symmetry and a 1.90-nm pitch. The pore size of the gel depends on the agarose concentration; 0.3 and 0.5% (w/v) agarose gels have estimated average pore sizes of 1000 and 810 nm, respectively.20,21 Studies on the effect of various salts on the coil-helix transition of agarose have found that the anions can stabilize or destabilize the gel state by preferential adsorption of either denaturant or water molecules to the agarose macromolecules.22,23 In general, the pore size increases with increasing ionic strength.20,24 We investigated the growth of NaBrO3 (28-32%) in agarose matrixes with gel concentrations ranging between 0.1 and 0.5%. Agarose gels with a density higher than 0.5% were found to severely inhibit and/or prevent crystal growth, while solutions less than 0.1% failed to gel aqueous solution at room temperature. NaBrO3 concentrations higher than 32% resulted in crystal nucleation before the gel was properly set, while solute concentrations less than 28% failed to yield crystals over a period of several months. Typically, under all agarose gel conditions studied, only a few crystals per gel grew within 2-5 days, far fewer than what is typically observed from pure solution growth. This observation is consistent with the widely held notion that nucleation is suppressed in gel media. Varying the NaBrO3 concentrations between 28 and 32% did not make a discernible difference in the shapes of the crystals obtained, but varying the gel concentration did have a noticeable effect. The crystals grown in low-density gels (i.e., 0.1-0.2%) tended to adopt tetrahedral habits (Figure 3). Small (110) faces were observed at the vertices of the tetrahedrons; however, cubic (100) faces are usually very small or completely absent in the range of solute concentrations examined (Figure 4). As the gel density increases, the NaBrO3 morphology changes from tetrahedral to elongated tetrahedral to dendritic. Figure 5 shows the morphology of two representative crystals grown in 0.5% agarose gel. In both cases, first dendrites
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Figure 2. Crystal growth of NaBrO3: (a) from low (36%) supersaturation solutions yields different crystal habits. Scale bar ) 500 µm.
Figure 3. Crystals grown from agarose gel containing 30% NaBrO3. Agarose concentration: (a) 0.1%, (b) 0.2%, (c) 0.3%.
Figure 4. Close-up view of a crystal facet. Crystal was grown from 0.3% agarose and 28% NaBrO3. Scale bar ) 500 µm.
develop and crystals continue to grow on top of one another until they obtained a “pine-tree” shape. These polycrystalline monoliths can reach sizes of up to several centimeters. The tips of these dendritic crystals always express geometries consistent with tetrahedral (111) facets. Dendritic growth is a fairly common phenomena for dense gel-growth and has been observed previously for PbS crystals grown from silica gel.25 We note that crystals grown from agarose gels at all concentrations routinely appear slightly cloudy and opaque. This may be due in part to polycrystallinity, but it is also certainly due to the inclusion of small quantities of agarose polymer that become trapped in the NaBrO3 matrix during growth. When the surfaces of agarose-grown crystals were cut away and the crystal centers were redissolved in pure aqueous solution, the solutions clearly indicated the presence of agarose polymer. Additional evidence for interaction between agarose and NaBrO3 can be seen from the gelation temperature, which can be measured by monitoring the optical rotation of a gel as it slowly cools. Under our experimental cooling rates, the gelation temperature of
a pure 5% agarose solution was 32 °C. The addition of 20% NaBrO3 to the agarose solution reduced the gelation temperature by several degrees to about 25 °C. Growth from Gelatin Gel. Gelatin is a protein obtained by the denaturation of collagen. Its exact chemical composition depends on the collagen source, but as a general rule every third residue is a glycine, the smallest amino acid. At high temperature, it exists in a coil conformation, and upon cooling below 30 °C it forms triple left-handed helices that are stabilized by hydrogen bonds.26,27 The triple helices, randomly distributed in space, form a three-dimensional porous network. Tightly bound, structured water also plays an integral part in the network formation. Halberstadt et al.5 have observed that the cell walls of gelatin are smooth and relatively free of pores, properties considered beneficial for increased nucleation. Saltzman et al. 28 determined the pore size of a 10% gelatin gel to be approximately 10 nm. The minimum concentration of gelatin required to form a gel is 2% in the absence of NaBrO3 and 3% in its presence. The addition of NaBrO3 decreases the gelation temperature of the gelatin by only a few degrees. When sodium bromate crystals were grown from gelatin gel with concentrations of 5.0, 7.5, and 10.0%, significantly increased nucleation of NaBrO3 was observed compared to that in agarose gel. Crystals tended to grow in clusters, and the number of clusters increased with increasing gelatin and/or NaBrO3 concentrations (Figure 6). The clusters grown in gelatin fall apart when they are harvested from the gel, suggesting that crystals have nucleated separately, not through epitaxial growth on one another. Inspection under higher magnification (50×) revealed that the clusters
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Figure 5. Sodium bromate crystals grown from 0.5% agarose gel.
Figure 6. NaBrO3 crystals grown from gelatin gel. NaBrO3 concentration 30%. Gelatin concentration: (a) 5.0%, (b) 7.5%, (c) 10.0%.
Figure 7. NaBrO3 crystals grown in 10% gelatin containing 30% bromate: (a) crystals from the center of the cluster, scale bar 50 µm; (b) crystals from the periphery of the cluster, scale bar 500 µm.
are composed of small crystals with a range of different morphologiesscubic, tetrahedral, and polyhedral (Figure 7). The diversity of shapes observed in the center of the cluster can probably be explained by solute depletion in the region around the fast nucleating crystals, which alters the growth rate of different faces. Interestingly, crystals grown at the periphery of the cluster tend to be much larger than those in the center and more commonly have morphologies terminated by large (100) faces. Growth at the highest NaBrO3 concentration examined (32%) yielded dendritic “pine-tree” habits, similar to those observed in agarose. Growth from Silica Gel. Silica gels are traditionally formed by polymerization reactions of either sodium metasilicate (via neutralization) or tetramethoxysilane or tetraethoxysilane (via hydrolysis). The microstructure of the gel depends on a variety of factors including the silica content and the pH.7,13,21 The polymerization starts with the formation of rings, which subsequently grow by the addition of monomers to form beads.21 These beads grow or aggregate and finally a three-
dimensional network is developed. Halberstadt et al.7 reported that silica gels formed from 0.2 M Na2SiO3 (at pH 5) exhibited a broad distribution of pore sizes ranging between 100 nm and 4 µm. This is a similar concentration to what is used in our present study. A narrower pore size distribution and thicker cell walls can be obtained in denser gels. For example, Moreno et al.13 observed pore size distribution ranging from 50 to 150 nm for 1.06 g cm-3 prepared at pH 6. As a general word of caution, because sodium salt and/or alcohol byproducts are generated in these polymerization reactions, one may also need to consider whether these species have any effect on the habits of crystals grown in their presence. Our silica gels were prepared by neutralizing silicate solutions with either acetic or o-phosphoric acid to a pH of 6. In preparing the silica gels, we found that the sodium bromate could only be added to the gel after it had partially polymerized in the presence of acid, or else an undesirable white coagulate formed. This requirement meant that to solubilize the sodium bromate,
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Figure 8. Crystals grown from silica gel: (a) 30% NaBrO3, F ) 1.02 g cm-3, and (b) 28% NaBrO3, F ) 1.03 g cm-3. Table 1. Summary of Crystal Habits Obtained from Different Gel Mediaa low supersaturation aqueous solution agarose gel gelatin gel silica gel a
high supersaturation
optically transparent?
T
C, T, P
Y
T C, T, P T
D D C, T, P
N sometimes Y
C ) cubic; D ) dendritic; P ) polyhedral; T ) tetragonal.
elevated temperatures were required. Since the polymerization rate also increases with temperature,21 our effective range of gel densities was limited to approximately 1.02-1.03 g cm-3. In these silica gel matrixes, the first visible NaBrO3 crystals appeared after only a few hours, a much shorter time than the crystal observation time seen in the other gel matrixes previously described. Under low gel density and sodium bromate concentrations higher than 28%, crystals developed as nearly perfect tetrahedrons (Figure 8). Under these silica gel conditions, the crystals obtained generally have a similar or slightly better optical quality than crystals grown from pure aqueous solution. An increase in the gel density or a decrease in sodium bromate concentration led to a general reduction in the number of crystals and an increase in the appearance of (100) faces. Taking into account the results of Inoue and Nishioka6 study of acetic acid’s effect on the NaBrO3 morphology, we also prepared silica gels by neutralization with o-phosphoric acid. Similar crystal habits were obtained. We rationalized that in the case of NaBrO3 crystal growth in silica gel, the gel density and solute supersaturation play a more significant role in determining the crystal habit than the presence of neutralizing acid or polymerization byproducts. Conclusions In our survey of sodium bromate crystal growth from different gel media, we have tried to understand the observed habit changes in terms of supersaturation and/ or the chemical and physical structures of the gels based on what is known from existing literature. Several qualitative observations can be made from these studies (Table 1). Crystal growth from pure aqueous solution produced tetragonal crystals at low supersaturations, and a mixture of habits at higher supersaturations. In comparison, different habits of sodium bromate crystals can be reproducibly accessed by growth in agarose, gelatin, and silica gels. Of the three gels investigated,
only gelatin appears to enhance nucleation rates; increasing concentrations of agarose and silica tended to decrease overall nucleation rates. Growth from silica gel yielded crystals with good optical properties, while those grown from agarose showed that the gel is routinely occluded in the crystal matrix. Growth from low density agarose or silica gel most closely resembles the growth conditions from aqueous solutionsit is here that we observe tetrahedral crystals nearly exclusively. At the opposite end of the spectrum at high agarose concentrations, the nucleation rate decreases and a transformation to dendritic growth is observed. Dendritic habits are also observed in gelatin matrixes when the solute concentration is high (32%), despite the fact that increasing gelatin concentrations tend to increase nucleation which should quickly reduce supersaturation. In dense silica gels or at intermediate concentrations of gelatin and solute, a variety of habits is observed (cubic, polyhedral, tetragonal), a fact that is likely explained by competition for solute in the regions where many nuclei are present. Unfortunately, we do not know how or to what extent the gel’s chemical backbone directly interacts with different NaBrO3 crystal faces to additionally influence nucleation and growth rates. It is certainly possible that the gel polymers may have different affinities for different sodium bromate surfaces. For example, the density of sodium ions on (001) NaBrO3 surfaces (1 per 22.41 A2) is lower than that on (111) surfaces (1 per 25.88 A2). Modeling the molecular contacts and/or calculating the energies of polymer-surface interactions is fairly challenging, since presumably solvent molecules mediate such interactions. Despite this uncertainty, the similarity of crystal habits observed in these widely different gels tends to support the hypothesis that the gel matrix influences the relative crystal growth rates and associated habits most effectively by regulating the supersaturation via controlled nucleation and solute diffusion. Acknowledgment. The authors are grateful for the financial support provided by the Petroleum Research Fund administered by the American Chemical Society (36457-G5) and the Henry Luce Foundation. References (1) Myerson, A. S. Handbook of Industrial Crystallization; Butterworth-Heinemann: Boston, 2002. (2) Weissbuch, I.; Popovitz-Biro, R.; Lahav, M.; Leiserowitz, L. Acta Crystallogr. 1995, B51, 115-148. (3) Abrahams, S. C.; Bernstein, J. L. Acta Crystallogr. 1977, B33, 3601-3604. (4) Pagni, R. M.; Compton, R. N. Cryst. Growth Des. 2002, 2, 4, 249-253. (5) Holcomb, E. R. C.; Inoue, T.; Nishioka, K. J. Cryst. Growth 1996, 158, 336-339. (6) Inoue, T.; Nishioka, K. J. Cryst. Growth 2000, 212, 507511. (7) Halberstadt, E. S.; Henisch, H. K.; Nickl, J.; White, E. W. J. Coll. Interface Sci. 1969, 29, 469-471. (8) Arora, S. K. Prog. Cryst. Growth Charact. 1981, 4, 345378. (9) Henisch, H. K. Crystals in Gels and Liesegang Rings; Cambridge University Press: Cambridge [England], New York, 1988. (10) Provost, K.; Robert, M. C. J. Cryst. Growth 1995, 156, 112120.
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(11) Lorber, B.; Sauter, C.; Ng, J. D.; Zhu, D. W.; Giege, R.; Vidal, O.; Robert, M. C.; Capelle, B. J. Cryst. Growth 1999, 204, 357-368. (12) Lorber, B.; Sauter., C.; Robert, M. C.; Capelle, B.; Giege, R Acta Crystallogr. 1999, D55, 1491-1494. (13) Moreno, A.; Juarez-Martinez, G.; Hernandez-Perez, T.; Batina, N.; Mundo, M.; McPherson, A. J. Cryst. Growth 1999, 205, 375-381 (14) Doxsee, K. M. C., R. C.; Chen, E.; Myerson, A. M.; Huang, D. J. Am. Chem. Soc. 1998, 120, 585-586. (15) Ono, Y.; Nakashima, K.; Sano, M.; Hojo, J.; Shinkai, S. Chem. Lett. 1999, 1119-1120. (16) Ricci, J. E.; Linke, W. F. J. Am. Chem. Soc. 1947, 69, 10801083. (17) Djaburov M.; Clarck, A. H.; Rowlands, D. W.; Ross-Murphy, S. B. Macromolecules 1989, 22, 180-188. (18) Whytock, S.; Finch, J. Biopolymers 1991, 31, 1025-1028. (19) Arnott, S.; Fulmer A.; Scott, W. E. J. Mol. Biol. 1974, 90, 269-284.
Petrova and Swift (20) Maaloum, M.; Pernodet, N.; Tinland, B. Electrophoresis 1998, 19, 1606-1610. (21) Robert, M.-C.; Vidal, O.; Garcia-Ruiz, J.-M.; Otalora, F. In Crystallization of Nucleic Acids and Proteins, 2nd Ed.; Ducruix, A., Giege, R., Eds.; Oxford University Press: New York, 1999; Chapter 6. (22) Piculell, L.; Nilsson, S. J. Phys. Chem. 1998, 93, 5596-5601. (23) Piculell, L.; Nilsson, S. J. Phys. Chem. 1989, 93, 5602-5611. (24) Waki, S.; Harvey, J. D. Biopolymers 1982, 21, 1909-1926. (25) Garcia-Ruiz, J. M. J. Cryst. Growth 1986, 75, 441-453. (26) Djabourov, M.; Leblond, J.; Papon, P. J. Phys. Paris 1988, 49, 319-332. (27) Pezron, I.; Djabourov, M.; Bosio, L.; Leblond, J. J. Polym. Sci. Polym. Phys. Ed. 1990, 28 1823-1839 (28) Saltzman, W. M.; Radomsky, M. L.; Whaley, K. J.; Cone, R. A. Biophys. J. 1994, 66, 508-515.
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