Tubular Structure Agglomerates of Calcium Carbonate Crystals

Growth of vaterite with novel morphologies directed by a collodion ... Fabrication of Tubular Structure Agglomerates of Calcium Carbonate by Using Col...
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Tubular Structure Agglomerates of Calcium Carbonate Crystals Formed on a Cation-Exchange Membrane Masakazu Takiguchi, Koichi Igarashi, Masayuki Azuma, and Hiroshi Ooshima* Graduate School of Engineering, Osaka City UniVersity, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 7 1611-1614

ReceiVed January 24, 2006; ReVised Manuscript ReceiVed April 27, 2006

ABSTRACT: Crystallization of calcium carbonate was carried out by using 1 M calcium chloride and 1 M sodium carbonate and a specially designed crystallizer equipped with a cation-exchange membrane. The crystallizer is a jacketed cylindrical glass tube that was divided into two compartments by a cation-exchange membrane and placed horizontally. Calcium carbonate crystals grew on the surface of the cation-exchange membrane in the sodium carbonate solution side and formed self-assembled tubular agglomerates vertically against the membrane surface. The tube was named the CC-tube. The inner and outer diameters of the CC-tube obtained after 9 days of crystallization at 50 °C was about 15 and 30 µm, respectively. The length was about 200-400 µm. Most of the CC-tubes were composed of small calcite crystals. The growing site of the tube was concluded to be the interface between the membrane and the tube rather than the top of tube. 1. Introduction There are many examples indicating that inorganic crystals are the structural materials of organisms. The seashell that is composed of calcium carbonate crystals and proteins is a typical example of such biominerals, in which the polymorphism, size, shape, and orientation of calcium carbonate crystals are controlled.1-10 The mantle, which covers the whole visceral mass, secretes the shell. Namely, a three-layer film of the organic substance called the periostracum is secreted from the mantle. The middle layer is responsible for secretion of the outer prismatic shell layer through processes of vacuolization and antrum formation. Crystals of calcium carbonate (calcite) grow in the antrum.11 The film plays an important role in the precipitation of calcium carbonate crystals and the construction of the shell structure. Tong et al. investigated constructive interactions of the organic/inorganic matrix with decalcified and deproteinized shells to understand how soluble proteins and organic matrixes influence the conformation and structure of crystals composing the shell.7 They observed a figure that the prismatic layer is divided into discrete compartments and showed that each compartment is responsible for the column crystal growth, spatial orientation, and size. They also suggested that the organic matrix ploughed in the column crystals is involved in providing a nucleation point and inducing the nucleation process of calcite. The details of the formation of the shell has not been made clear yet, but one of the important roles of the membrane (film) should be a controlled release of calcium or/and carbonate ions.12 The controlled release of the ions should be important to prevent random deposition of calcite crystals as observed in a rapid mixing of two solutions individually containing calcium ions and carbonate ions. Another role of the membrane may be a provision of nucleation sites at the surface of the membrane. In the present study, we attempted to crystallize calcium carbonate on a cation-exchange membrane. Our aim was not to ascertain the mechanism of biomineralization, but to artificially utilize or mimic the mechanism of biomineralization, even if it is very primitive. One of the roles of the membrane is permitting a controlled release of calcium ions. The interface * Corresponding author. Tel: +81-6-6605-2700. Fax: +81-6-6605-2701. E-mail: [email protected].

Figure 1. Schematic diagram of the crystallizer.

between the membrane and the solution may play an important role in nucleation. The material and structure of the membrane must affect the deposition of crystals and the structuring of the crystalline solid. These effects cannot be expected in conventional batch reactive crystallization in which two solutions separately containing cations and anions are directly mixed. A cation-exchange membrane was first used by Iijima et al. for crystallization of octacalcium phosphate to investigate the formation mechanism of tooth enamel in vitro, where the membrane was used to control the diffusion rate of ions.13-15 As a result, they obtained long ribbonlike crystals of octacalcium phosphate. We report the peculiar structure of agglomerates of calcium carbonate crystals. 2. Materials and Methods Calcium chloride dihydrate and sodium carbonate used were of reagent grade (Wako Pure Chemicals Co. Ltd., Japan). A cationexchange membrane, Nafion N-117, was purchased from DuPont, USA. The membrane is a perfluorosulfonic acid/PTFE copolymer, and the typical thickness is 183 µm. It was rinsed with distilled water before use. Calcium chloride and sodium carbonate were individually dissolved in deionized water and degassed to avoid the generation of bubbles during crystallization. Figure 1 presents a schematic diagram of the crystallizer adopted in the present study. The crystallizer was horizontally placed as shown in Figure 1. The crystallizer is composed of two jacketed cylindrical glass tubes, a membrane, silicon rubber sheets, and a horseshoe-shaped clamp. The inner diameter of the glass tube is 20 mm. A cationexchange membrane was tightly fixed to the glass vessels by silicon rubber sheets and a horseshoe-shaped clamp.

10.1021/cg060045x CCC: $33.50 © 2006 American Chemical Society Published on Web 06/02/2006

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Figure 2. (a) The SEM image of the tubular aggregate of calcium carbonate formed on the cation-exchange membrane. (b) The magnified image of the rectangular area in panel a.

Figure 3. Calcium carbonate tubes obtained on the cation-exchange membrane by crystallization at 50 °C for 1 day. (a) This tube may be composed of vaterite. (b) This tube may be on the way toward transformation of vaterite to calcite.

Figure 4. (a) The calcium carbonate tubes formed on the cation-exchange membrane at 50 °C after 14 days. (b) The magnified image of the rectangular area in panel a. The calcium chloride and the sodium carbonate solutions were placed in each compartment. The initial concentrations of sodium carbonate and calcium chloride were 1.0 M; this concentration was arbitrarily adopted only to get a sufficient driving force for the penetration of calcium ions. The pH of the sodium carbonate solution was adjusted to pH 12.0 with 5 N NaOH to allow the equilibrium between carbonate and bicarbonate to shift completely to the carbonate-ion side. The crystallization was carried out at 50 °C without stirring. Calcium ions transferred to the sodium carbonate solution through the cationexchange membrane, and they reacted with carbonate ions as soon as they reached the sodium carbonate side. Calcium carbonate crystals grew on the surface of the cation-exchange membrane. Crystals were recovered after a given crystallization time, in which care was taken to prevent separation of crystals from the membrane. Recovered crystals were gently rinsed with distilled water three times and dried in air for 1 day at room temperature. The crystals coated with gold were observed with a scanning electron microscope (SEM) (SHIMADZU EPM-810). On the other hand, crystals growing in the crystallizer were directly observed with a digital optical microscope (KEYENCE VHX-100).

3. Results and Discussion Figure 2 shows SEM images of calcium carbonate crystals grown on the cation-exchange membrane. Those were recovered after crystallization for 9 days at 50 °C. A cylindrical tube composed of small calcium carbonate crystals was observed on the membrane. The inner and outer diameters were about 15 and 30 µm, respectively, and the length of the tube was 200400 µm. All crystals that formed on the surface of the membrane were not cylindrical tubes; cubic and rectangular calcite crystals also existed, as can be seen in Figure 2a. Figure 2b shows a magnified image of the white rectangle part in panel a. The calcium carbonate crystals constituting the cylindrical tube seemed to be calcite. The size of the component crystals was about 4-8 µm. We named the cylindrical tube the calcium carbonate-tube (CC-tube). The formation of the CC-tubes was reproducible. These were observed everywhere on the surface

Tubular Agglomerates of Calcium Carbonate

Figure 5. The possible mechanism of the growth of the CC-tubes: (a) The CC-tubes grown at the top of the tube. (b) The growth of the tube proceeds in the bottom of the tubes.

of the membrane, although not homogeneously scattered on the surface. All tubes had a similar dimension except for the length. Figure 3 shows the CC-tubes obtained after a 1-day crystallization. The surface of the tube was not coarse but smooth compared with that in Figure 2. The small crystals on the tubes may be vaterite. Figure 3b shows another view on the membrane. Both a smooth part and a coarse part were observed on the surface. This tube may be on the way toward transformation of vaterite to calcite. This observation, however, does not mean that the calcite tubes are always formed from vaterite tubes because most of the crystals on the surface of the membrane

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were determined to be calcite by powder XRD analysis even in the beginning of crystallization. Figure 4 shows SEM images of the CC-tubes obtained after a 14-day crystallization. In Figure 4a, many CC-tubes could be observed. The size of the small component crystals on the tube was 5-8 µm. The texture of the tubes was coarser than that of the CC-tube shown in Figure 2, although we could not explain the cause. Figure 4b presents a magnified image of the white rectangle part in panel a. We could recognize that a tube was broken to two pieces and confirmed that the middle structure is also hollow. In addition to the tubes, typical calcite crystals were observed. We investigated the mechanism of the CC-tube formation, in particular, details about the growing site of the tube. We can make two possible hypotheses about it. One is that the tubes may grow at the top of tubes as illustrated in Figure 5a. Calcium ions permeate through the cation-exchange membrane and diffuse in the tube from the surface of the membrane to the top of the tubes to react with carbonate ions. Another hypothesis is that growth of the tubes may proceed at the bottom of the tubes, namely, at the interface between the membrane and the tube as illustrated in Figure 5b. In this case, the tubes should be pushed up. We conducted an experiment to examine the mechanism. Namely, the CC-tubes growing on the membrane were directly observed with a digital microscope. Figure 6 shows images of the tubes after a 7-, 8-, 9-, and 10-h crystallization, respectively. The lower white part in the figure shows silicon rubber, which is out of focus. A Y-shaped tube, which is indicated by a white arrow, and three straight tubes are observed in Figure 6. These tubes grew longer with elapsed time. It should be noted,

Figure 6. The growth process of calcium carbonate tube on the membrane. Calcium carbonate was crystallized from 1 M CaCl2 and 1 M Na2CO3 solutions at room temperature and observed by a digital microscope. Crystallization times were (a) 7 h, (b) 8 h, (c) 9 h, (d) 10 h, respectively.

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Figure 7. (a) SEM image of the cylindrical tube mildly dried in a desiccator for 3 days. (b) The magnified image of the rectangular area in panel a.

however, that the size of the upper part of the Y-shape tube, the V-shape part, did not change. This fact indicates that growth of the tubes proceeds at the bottom of tubes because if the growth proceeds at the top of the tube as illustrated in Figure 5a the V-shape part should become longer. Furthermore, it was found from Figure 6 that the tube had a cap on the top. Figure 7 presents a SEM image of a CC-tube having a cap. The cap was hemispheric and consisted of small crystals. The cap should disrupt a release of calcium ions through the hollow of the tubes. This supports the above conclusion that the growing site of the CC-tube is not at the top but at the bottom of the tube. However, many tubes we observed with an electron microscope did not have the caps as shown in Figures 2-4. It may be explained as follows. As mentioned in the experimental section, the tubes were dried in air for 1 day before observation with SEM. If the drying time was not long enough to remove the whole water kept in the tube, the residual water should be blown out under a high vacuum during gold coating before SEM observations. At the time, the cap or/and the interface between membrane and tube might be broken. Indeed, the tube presented in Figure 7 was recovered by drying on silica gel in a desiccator for 3 days. We discussed above a growth mechanism of the tube. However, we did not explain why such a tube structure is formed. We tried to speculate on this mechanism as follows. It seems to be true that the membrane acts as a kind of template. We infer that the tubular structuring of agglomerates (the CCtubes) is caused by the presence of the surface of the membrane where calcium ions cannot penetrate. Namely, the membrane may be not homogeneous, but there may be spots where the penetration of calcium ions is blocked. It might be a designed characteristic of the membrane or might be an imperfection accidentally formed during manufacturing of the membrane. Any rate, if there would be such spots, crystals would grow avoiding those spots. The edge of the spot would remain as a spring of calcium ions even after the other surface of the membrane is covered with crystals. As a result, the formation of crystals would continue around the spot and may form a dome that grows into a CC-tube. 4. Conclusions We attempted to crystallize calcium carbonate by using a specially designed crystallizer shown in Figure 1, in which calcium ions gently were in contact with carbonate ions through a cation-exchange membrane, Nafion N-117. Calcium carbonate

crystals grew on the surface of the cation-exchange membrane and formed cylindrical tubes standing vertically against the membrane surface. We named the tube the CC-tube. The CCtubes were agglomerates of small crystals mainly consisting of calcite, which were formed by self-assembly. It has not been known that calcium carbonate crystals form tubular agglomerates by self-assembly. We could also observe a CC-tube that seemed to be on the way toward transformation from vaterite to calcite. We investigated the formation mechanism of the CC-tube, in particular, details about the growing site of the tube. We made two possible hypotheses about it. One is that the tube may grow at the top of the tube as illustrated in Figure 5a. Another one is that the tube may grow at the bottom of the tube as illustrated in Figure 5b. From an in-situ observation of the growing tube with a digital microscope, we concluded that the growing site of the tube is the interface between the membrane and the tube as illustrated in Figure 5b and that the tube is pushed up. This growth mechanism was supported by the existence of a cap on the top of tube. The reason the tube-structured agglomerates were formed was shortly discussed, although it was based only on speculation. References (1) Weiner, S.; Hood, L. Science 1975, 190, 987-989. (2) Sudo, S.; Fujikawa, T.; Nagakura, T.; Ohkubo, T.; Sakaguchi, K.; Tanaka, M.; Nakashima, K.; Takahashi, T. Nature 1997, 387, 563564. (3) Sarashina, I.; Endo, K. Am. Mineral. 1998, 83, 1510-1515. (4) Samata, T.; Hayashi, N.; Kono, M.; Hasegawa, K.; Horita, C.; Akera, S. FEBS Lett. 1999, 462, 225-229. (5) Chateigner, D.; Hedegaard, C.; Wenk, H.-R. J. Struct. Geol. 2000, 22, 1723-1735. (6) Kono, M.; Hayashi, N.; Samata, T. Biochem. Biophys. Res. Commun. 2000, 269, 213-218. (7) Tong, H.; Hu, J. M.; Ma, W. T.; Zhong, G. R.; Yao, S. N.; Cao, N. X. Biomaterials 2002, 23, 2593-2598. (8) Dauphin, Y.; Cuif, J. P.; Doucet, J.; Salome´, M.; Susini, J.; Willams, C. T. J. Struct. Biol. 2003, 142, 272-280. (9) Suzuki, M.; Murayama, E.; Inoue, H.; Ozaki, N.; Tohse, H.; Kogure, T.; Nagasawa, H. Biochem. J. 2004, 382, 205-213. (10) Miyamoto, H.; Miyoshi, F.; Kohno, J. Zool. Sci. 2005, 22, 311315. (11) Checa, A. Tissue Cell. 2000, 32, 405-416. (12) Simkiss, K.; Wilbur, K. M. Biomineralization: Cell Biology and Mineral Deposition; Academic Press: London, 1989. (13) Iijima, M.; Moriwaki, Y. J. Cryst. Growth 1989, 96, 59-64. (14) Iijima, M.; Moriwaki, Y. J. Cryst. Growth 1991, 112, 571-579. (15) Iijima, M.; Moriwaki, Y. J. Cryst. Growth 1999, 198/199, 670-676.

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