Langmuir 2002, 18, 3655-3658
3655
Effect of Anionic Starburst Dendrimers on the Crystallization of CaCO3 in Aqueous Solution: Size Control of Spherical Vaterite Particles Kensuke Naka,* Yasuyuki Tanaka, and Yoshiki Chujo* Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto, 606-8501, Japan Received August 23, 2001. In Final Form: January 23, 2002 Crystallization of CaCO3 in the presence and the absence of half-generation poly(amidoamine) (Gn.5 PAMAM) dendrimers was carried out by a double jet method to prevent heterogeneous nucleation at glass walls. The solution was kept at 25 °C under N2 for 4 days with gentle stirring. We found that the crystallization of CaCO3 in the presence of the anionic dendrimers resulted in the formation of spherical vaterite crystals, whereas rhombohedral calcite crystals were formed in the absence of additives. As the generation number of the PAMAM dendrimer increased from G1.5 to G3.5, the particle size of vaterite was decreased from 5.5 ( 1.1 to 2.3 ( 0.7 µm under the constant concentration (0.26 mM) of -COONa unit of the PAMAM dendrimers. With further increase in the generation number to G4.5, the particle size was not changed. The relationship between concentration of the dendrimer and the particle size in the same generation numbers of the PAMAM dendrimer was also studied under the same condition. As concentration of -COONa increased from 0.26 to 8.33 mM, the particle sizes of spherical vaterite were reduced from 5.5 ( 1.1 to 2.5 ( 0.6 µm.
Introduction Construction of organic-inorganic hybrid materials with controlled mineralization analogous to those produced by nature is a current interest for both organic and inorganic chemists to understand the mechanism of natural biomineralization process as well as to seek industrial and technological applications.1,2 In these mineralized tissues, crystal morphology, size, and orientation are determined by local conditions and, in particular, the presence of “matrix” proteins or other macromolecules. Due to the complexity of the natural biomineralization systems, much research has been conducted on model organic interfaces.3 Because the proteins that have been found to be associated with biominerals are usually highly acidic macromolecules, simple water-soluble polyelectrolytes, such as sodium salts of poly(aspartic acid) and poly(glutamic acid), were examined for the model of biomineralization in aqueous solution.4-6 Studies of inorganic crystallization in the presence of soluble synthetic polymers have shown that selectivity for certain crystal faces appears to be highly dependent on the secondary structure of the macromolecules.6 Poly(amidoamine) (PAMAM) dendrimers with carboxylate groups at the external surfaces termed half-generation or Gn.5 dendrimers have been proposed as mimics of anionic micelles or proteins.7,8 The starburst structures (1) Smith, B. L. Nature 1999, 399, 761. (2) Addadi, L.; Weiner, S. Proc. Natl. Acad. Sci. U.S.A. 1985, 82, 4110. (3) Naka, K.; Chujo, Y. Chem. Mater. 2001, 13, 3245, and references therein. (4) Sims, S. D.; Didymus, J. M.; Mann, S. J. Chem. Soc., Chem. Commun. 1995, 1031. (5) Gower, L. A.; Tirrell, D. A. J. Cryst. Growth 1998, 191, 153. (6) Levi, Y.; Albeck, S.; Brack, A.; Weiner, S.; Addadi, L. Chem. Eur. J. 1998, 4, 389. (7) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (8) Ottaviani, M. F.; Bossmann, S.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1994, 116, 661.
are disklike shapes in the early generations, whereas the surface branch cell becomes substantially more rigid and the structures are spheres.8 Due to unique and welldefined secondary structures of the dendrimers, the anionic starburst dendrimers should be a good candidate for studying inorganic crystallization. The interaction of the dendrimers with metal ions has been extensively examined.7-9 The PAMAM dendrimers were used as templates for formation of metal nanoparticles.9,10 In a previous communication,11 we found that crystallization of CaCO3 in the presence of a G1.5 PAMAM dendrimer with carboxylate groups at the external surface resulted in the formation of stable spherical vaterite crystals whereas rhombohedral calcite crystals were formed in the absence of additives. Calcium carbonate makes an attractive model mineral for studies in the laboratory, since its crystals are easily characterized and the morphology of CaCO3 has been the subject to control in biomineralization processes.12 The precipitation of CaCO3 in aqueous solution is also of great interest for industrial and technological applications. Pure CaCO3 has three anhydrous crystalline forms, i.e., calcite, aragonite, and vaterite. Calcite is more thermodynamically stable than the other two structures at room temperature and atmospheric pressure. Vaterite is thermodynamically the most unstable of the three crystal structures. It is well-known that vaterite transforms into the thermodynamically most stable calcite via a solventmediated process.13 Vaterite, however, is expected to be used for various purposes, because it has some features (9) Balogh, L.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 120, 7355. (10) (a) Zhou, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877. (b) Esumi, K.; Suzuki, A.; Aihara, N.; Usui, K.; Torigoe, K. Langmuir 1988, 14, 3157. (c) Imae, T.; Funayama, K.; Aoi, K.; Tsutsumiuchi, K.; Okada, M.; Furusaka, M. Langmuir 1999, 15, 4076. (11) Naka, K.; Tanaka, Y.; Chujo, Y.; Ito, Y. Chem. Commun. 1999, 1931. (12) (a) Lowenstam, H. A. Science 1981, 211, 1126. (b) Mann, S. Nature 1988, 332, 119. (13) Lopezmacipe, A.; Gomezmorales, J.; Rodriguezclemente, R. J. Cryst. Growth 1996, 166, 1015.
10.1021/la011345d CCC: $22.00 © 2002 American Chemical Society Published on Web 03/27/2002
3656
Langmuir, Vol. 18, No. 9, 2002
such as higher specific surface area, higher solubility, higher dispersion, and smaller specific gravity compared to the other two crystal systems. Spherical vaterite crystals have already been reported in the presence of divalent cations,14 a surfactant [bis(2-ethylhexyl)sodium sulfate (AOT)],13 poly(styrenesulfonate),15 poly(vinyl alcohol),16 and double-hydrophilic block copolymers.17 The control of the particle size of spherical vaterite should be important for application as pigments, fillers, and dentifrice. However, size control of spherical vaterite crystal by synthetic additives has been limited. One documented example is using poly(styrenesulfonate) in crystallization of spherical vaterite.15 They obtained monodispersed particles with 1.5 ( 0.3 µm diameter in the presence of poly(styrenesulfonate) (Mw ) 500 000). Authors described the influence of the molecular weight of poly(styrenesulfonate) on the particle size of CaCO3, but the range of the particle size was very small, i.e., from 0.8 ( 0.5 to 1.5 ( 0.3 µm. Here, we report on the effect of the generation number and concentration of the PAMAM dendrimers on the crystallization of CaCO3 in aqueous solution. We found that a wide range of particle sizes of vaterite, i.e., from 5.8 ( 1.8 to 1.5 ( 0.6 µm, were obtained by the PAMAM dendrimers.
Naka et al.
Figure 1. FTIR spectra of CaCO3 in the presence of (a) G1.5, (b) G3.5, and (c) G4.5 PAMAM dendrimers and (d) in the absence of the dendrimer. The -COONa unit of the PAMAM dendrimers were constant at 0.26 mM.
Experimental Section Materials. Half-generation poly(amidoamine) (G1.5, G3.5, and G4.5 PAMAM) dendrimers and poly(acrylic acid) (Mw ) 5100) were obtained from Aldrich. Calcium chloride and ammonium carbonate were purchased from WAKO Pure Chemical Industries, Ltd. Characterization. Scanning electron microscopic (SEM) measurements were carried out by a JEOL JSM-5310/LV at 15 kV. The X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectra were recorded on a Shimadzu XRD-6000 and a Perkin-Elmer System 2000, respectively. Crystallization of CaCO3. The precipitation of CaCO3 in the presence of the PAMAM dendrimers was carried out as follows. A solution of the dendrimer in 200 mL of distilled water was adjusted to pH 8.5 with aqueous NH3. Then 0.5 M CaCl2 aqueous solution (adjusted to pH 8.5) and 0.5 M (NH4)2CO3 aqueous solution (adjusted to pH 10.2) were injected via syringe into the reaction mixture at 25 °C under N2 with stirring. After a sudden increase in the turbidity of the solution, this solution was then kept at 25 °C under N2 for 4 days with gentle stirring. The crystalline CaCO3 was washed with water to remove contaminating dendrimers that were not involved in the crystals. Crystallization of CaCO3 in the absence of the dendrimer was carried out using 300 mL of aqueous solution containing 2.50 mmol of NaCl instead of the dendrimer solution.
Results and Discussion The precipitation of CaCO3 in the absence and the presence of the G1.5, G3.5, and G4.5 PAMAM dendrimers were carried out by a double jet method17a to prevent heterogeneous nucleation at the glass walls. The two reactants (CaCl2 and Na2CO3) are injected via capillaries into the reaction vessel under vigorous stirring. The two capillary ends are joined together so that a high local reactant concentration and thus extreme supersaturation is achieved at the moment when the two reactants leave the capillaries, which provides an immediate nucleation of CaCO3. The nuclei are then immediately transported (14) Brecevic, L.; Nothing-Laslo, V.; Kralji, D.; Popovic, S. J. Chem. Soc., Faraday Trans. 1996, 92, 1017. (15) Kawaguchi, H.; Hirai, H.; Sakai, K.; Nakajima, T.; Ebisawa, Y.; Koyama, K. Colloid Polym. Sci. 1992, 270, 1176. (16) Didymus, J. M.; Oliver, P.; Mann, S.; Devries, A. L.; Hauschka, P. V.; Westbroek, P. J. Chem. Soc., Faraday Trans. 1993, 89, 2891. (17) (a) Sedla´k, M.; Antonietti, M.; Co¨lfen, H. Macromol. Chem. Phys. 1998, 199, 247. (b) Co¨lfen, H.; Antonietti, M. Langmuir 1998, 14, 582. (c) Co¨lfen, H.; Qi, L. Chem. Eur. J. 2001, 7, 106.
Figure 2. X-ray diffraction patterns of CaCO3 in the presence of (a) G1.5, (b) G3.5, and (c) G4.5 PAMAM dendrimers and (d) in the absence of the dendrimer. The -COONa unit of the PAMAM dendrimers were constant at 0.26 mM.
to regions of lower CaCO3 concentration and can grow further. The CaCO3 crystal formation occurring after addition of CaCl2 and (NH4)2CO3 was readily observed by a sudden increase in the turbidity of the solution. The concentrations of the -COONa unit of the G1.5, G3.5, and G4.5 PAMAM dendrimers were constant at 0.26 mM, corresponding to 16, 4.0, and 2.0 µM of the G1.5, G3.5, and G4.5 PAMAM dendrimers, respectively. This solution was then kept at 25 °C under N2 for 4 days with gentle stirring. The crystalline CaCO3 was washed with water to remove contaminated dendrimers that were not involved in the crystals. The CaCO3 crystal phases of the obtained products in the presence of the PAMAM dendrimers were characterized by FTIR analysis.18 In the prsence of the PAMAM dendrimers, all three samples showed bands at 877 and 746 cm-1 indicating vaterite formation, while bands at 874 and 712 cm-1 assignable to calcite were scarcely observable (Figure 1a-c). The crystal phases of the obtained CaCO3 were further confirmed by XRD. In the presence of the PAMAM dendrimers, all the precipitates consisted entirely of (18) Xu, G.; Yao, N.; Aksay, I. A.; Groves, J. T. J. Am. Chem. Soc. 1998, 120, 11977.
Crystallization of CaCO3 in Aqueous Solution
Langmuir, Vol. 18, No. 9, 2002 3657
Figure 3. Scanning electron micrographs of CaCO3 in the presence of (a) G1.5, (b) G3.5, and (c) G4.5 PAMAM dendrimers and (d) in the absence of the dendrimer. The -COONa unit of the PAMAM dendrimers were constant at 0.26 mM.
vaterite (>99%) (Figure 2a-c). By contrast, the crystal phase of the CaCO3 obtained without dendrimer was calcite by IR spectroscopy, in which the bands at 874 and 712 cm-1 assignable to calcite were recognized (Figure 1d). The reflections of XRD are also characteristic for calcite (Figure 2d). These results indicated that the dendrimers affected morphology of CaCO3 crystallization. SEM observations showed that the crystals obtained in the absence of additives were rhombohedral (Figure 3d). In the presence of the PAMAM dendrimers, all three products were spherical (Figure 3a-c). Each shape of CaCO3 is a typical morphology for each polymorph. The particle sizes of the spherical vaterite crystals obtained in the presence of the PAMAM dendrimers depended on the generation numbers of the PAMAM dendrimers (Table 1). As the generation number of the PAMAM dendrimer increased from G1.5 to G3.5, the particle size of the spherical vaterite was decreased from 5.5 ( 1.1 to 2.3 ( 0.7 µm. With further increase in the generation number to G4.5, the particle size was not changed (2.3 ( 0.8 µm). The effect of the generation number on the size control of spherical vaterite crystal was not observed at the G4.5 PAMAM dendrimer. The relationship between concentration of the dendrimers and the particle sizes in the same generation numbers of the PAMAM dendrimer was also studied under the same condition as above. The results are also sum-
Table 1. Particle Size of Spherical Vaterite in the Presence of PAMAM Dendrimers particle sizea (µm) [-COONa] (mM)
G1.5
G3.5
G4.5
0.13 0.26 8.33
5.6 ( 1.4 5.5 ( 1.1 2.5 ( 0.6
5.8 ( 1.8 2.3 ( 0.7 1.5 ( 0.6
2.3 ( 0.8
a Average of the particle size was obtained directly from each SEM image by counting ca. 40 particles.
marized in Table 1. The crystal phases of all the obtained CaCO3 were pure vaterite by FTIR and XRD analyses. At the lower concentration of the G1.5 PAMAM dendrimer corresponding to 0.13 mM of -COONa, the particle size of the vaterite crystals was 5.6 ( 1.4 µm. As the concentration of -COONa increased from 0.26 to 8.33 mM, the particle sizes of the spherical vaterite were reduced from 5.5 ( 1.1 to 2.5 ( 0.6 µm. In the case of the G3.5 PAMAM dendrimer, the concentration of -COONa increased from 0.13 to 0.26 mM; the particle size also decreased from 5.8 ( 1.8 to 2.3 ( 1.8 µm. With further increase of the concentration to 8.33 mM, the particle size decreased to 1.5 ( 0.6 µm. These data indicated that the particle sizes of the spherical vaterite crystals depended on the concentration of the PAMAM dendrimers. The particle sizes by the lower generation (G1.5) were larger
3658
Langmuir, Vol. 18, No. 9, 2002
Naka et al.
Table 2. Precipitation of CaCO3 in the Absence and the Presence of Organic Additives additive additivesa
no G1.5 PAMAM dendrimer G3.5 PAPAM dendrimer PAA
[Ca2+]b (mM)
[Ca2+]/[-COONa ]b
yieldc (%)
polymorphsd
morphologye
3.26 4.25 9.00 8.33
0.51 1.08 1.00
51 61 30
