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Langmuir 2006, 22, 7760-7767
Formation of Stable Vaterite with Poly(acrylic acid) by the Delayed Addition Method Kensuke Naka,* Shu-Chen Huang, and Yoshiki Chujo* Department of Polymer Chemistry, Graduate School of Engineering, Kyoto UniVersity, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ReceiVed April 1, 2006. In Final Form: July 5, 2006 The crystallization of CaCO3 was examined by changing the addition time of poly(acrylic acid) (PAA) to an aqueous solution of calcium carbonate by selectively interacting with the crystal at different stages during the crystal-forming process. The precipitation of CaCO3 was carried out by a double jet method to prevent heterogeneous nucleation on glass walls, and the sodium salt of PAA was added by a delayed addition method. In the initial presence of PAA in an aqueous solution of calcium carbonate, PAA acted as an inhibitor for the nucleation and growth of crystallization. However, it was found that stable vaterite particles were successfully obtained by delaying the addition of PAA from 1 to 60 min. The vaterite particles were stable in the aqueous solution for more than 30 days, and the CaCO3 particles were formed by a spherulitic growth mechanism. It is suggested that PAA strongly binds with the Ca2+ ion on the surface of CaCO3 particles to stabilize the unstable vaterite form effectively. Upon changing the addition time of PAA, we found that CaCO3 particles were formed through different formation mechanisms in selectively controlled crystallization at different stages during the crystallization process.
Introduction Biomineralization is the process of producing inorganic minerals by living organisms. The construction of organicinorganic hybrid materials with controlled mineralization analogous to those produced by nature has recently received much attention with respect to understanding the mechanism of the biomineralization process.1-3 Calcium carbonate is one of the most studied systems because it is one of the major inorganic substances produced in biological organism as well as an important aspect in the development of new materials in many fields of applications. Calcium carbonate has three crystalline polymorphs, that is, vaterite, aragonite, and calcite in order of increasing thermodynamic stability.4,5 Calcite and aragonite are often found in biominerals, and vaterite is formed and stabilized by some organisms.1,6 The modeling of biomineralization is a promising approach to controlled crystal growth with the aim of seeking materials analogous to those made by natural organisms.2,7-9 Many efforts have focused on exploring the promoting effect of templates on crystal nucleation and growth. On the specific interactions, various synthetic polymers have been found to be potent inhibitors or habit modifiers for inorganic crystallization by adsorption onto the surfaces of the growing crystals, thus controlling their growth rate and habit through the strength and selectivity of this adsorption.10-15 * To whom correspondence should be addressed. E-mail: ken@ chujo.synchem.kyoto-u.ac.jp. (1) Addadi, L.; Weiner, S. Nature 1997, 389, 912. (2) Smith, B. L. Nature 1999, 399, 761. (3) Mann, S.; Ozin, G. A. Nature 1996, 382, 313. (4) Ku¨ther, J.; Seshadri, R.; Knoll, W.; Tremel, W. J. Mater. Chem. 1998, 8, 641. (5) Wei, H.; Shen, Q.; Zhao, Y.; Wang, D.-J.; Xu, D.-F. J. Cryst. Growth 2003, 250, 516. (6) Lowenstam, H. A. Science 1981, 211, 1126. (7) Naka, K.; Chujo, Y. Chem. Mater. 2001, 13, 3245. (8) Kato, T.; Sugawara, A.; Hosoda, N. AdV. Mater. 2002, 14, 869. (9) Co¨lfen, H. Curr. Opin. Colloid Interface Sci. 2003, 8, 23. (10) Co¨lfen, H.; Qi, L. Chem.sEur. J. 2001, 7, 106. (11) Ueyama, N.; Hosoi, T.; Yamada, Y.; Doi, M.; Okamura, T.; Nakamura, A. Macromolecules 1998, 31, 7119. (12) Manoli, F.; Dalas, E. J. Cryst. Growth 2001, 222, 293. (13) Kato, T.; Suzuki, T.; Amamiya, T.; Irie, T.; Komiyama, M.; Yui, H. Supramol. Sci. 1998, 5, 411.
In the crystallization of CaCO3, the final crystalline phase might arise through a multistage crystallization process initiated by the formation of amorphous precursors and crystalline intermediates that undergo subsequent phase transformations.5,16,17 The crystallization process is complicated, and there are many possible pathways in the particle-formation process, such as nanospherulites aggregation-induced crystallization or transformed by simple crystal growth, and so forth.18-23 Yu et al. reported the biomimetic crystallization of metal carbonate through a slow gas-liquid diffusion reaction at room temperature under the control of double-hydrophilic block copolymers (DHBCs) and observed the time-dependent self-assembly and growth of the superstructure development.24,25 The existence of several phases would enable an organism to control mineralization through intervention with kinetics. By selectively interacting with the mineral at different stages during the crystal-forming process, the organism may choose to manipulate both the polymorph and the orientation of the mineral to meet specific biological requirements. Although the presence of various synthetic additives has been extensively studied for inorganic crystallization, selective interaction of an organic matrix with the mineral at different stages of crystallization has not been examined in detail. Recently, we have reported a new concept for controlling crystalline polymorphs of CaCO3 by the interaction of a synthetic additive at different nucleation stages.18,26 Sodium (14) Gower, L. A.; Tirrell, D. A. J. Cryst. Growth 1998, 191, 153. (15) Yu, S.-H.; Co¨lfen, H. J. Mater. Chem. 2004, 14, 2124. (16) Peric´, J.; Vucˇak, M.; Krstulovic´, R.; Brecˇevic´, L.; Kralj, D. Thermochim. Acta 1996, 277, 175. (17) Co¨lfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350. (18) Keum, D.-K.; Naka, K.; Chujo, Y. Bull. Chem. Soc. Jpn. 2003, 76, 1687. (19) Co¨lfen, H.; Antonietti, M. Langmuir 1998, 14, 582. (20) Brecˇevic´, L.; No¨thig-Laslo, V.; Kralj, D.; Popovic´, S. J. Chem. Soc., Faraday Trans. 1996, 92, 1017. (21) Sta´vek, J.; Sˇ ´ıpek, M.; Hirasawa, I.; Toyokura, K. Chem. Mater. 1992, 4, 545. (22) Goldenfeld, N. J. Cryst. Growth 1987, 84, 601. (23) Andreassen, J.-P. J. Cryst. Growth 2005, 274, 256. (24) Yu, S.-H.; Co¨lfen, H.; Antonietti, M. J. Phys. Chem. B 2003, 107, 7396. (25) Yu, S.-H.; Co¨lfen, H.; Hartmann, J.; Antonietti, M. AdV. Funct. Mater. 2002, 12, 541. (26) Naka, K.; Keum, D.-K.; Tanaka, Y.; Chujo, Y. Bull. Chem. Soc. Jpn. 2004, 77, 827.
10.1021/la060874k CCC: $33.50 © 2006 American Chemical Society Published on Web 08/05/2006
Formation of Stable Vaterite with Poly(acrylic acid)
acrylate and a water-soluble radical initiator were used for selective interaction with the mineral at different stages during the crystal-forming process.26 While sodium acrylate does not affect the nucleation and growth of the crystals, poly(acrylate) affects crystal morphology by inhibiting the growth of particular crystal faces. Three different crystal polymorphs of CaCO3 (aragonite, vaterite, and calcite) were selectively induced by changing the addition time of the radical initiator. The influences of the sodium salt of poly(acrylic acid)s on the crystallization of CaCO3 to act as an inhibitor or modifier for crystal formation have been intensively investigated in recent years.13,27-34 However, our findings were the first reports of selectively controlling the crystallization of CaCO3 with in-situ polymerization of the acrylate monomer at different stages during the crystal-forming process. In this work, poly(acrylic acid) (PAA) for selectively interacting with the crystallization of calcium carbonate at different stages during the crystal-forming process was studied. The precipitation of CaCO3 was carried out by a double jet method to prevent heterogeneous nucleation on glass walls,35 and the sodium salt of PAA was added by a delayed addition method.26 The effects of different concentrations of calcium reagents were also examined. Although the initial presence of PAA acted as an inhibitor for crystal formation and no crystals were produced,13,27,29,32 vaterite particles were obtained by the delayed addition of PAA. It was found that the resulting vaterite particles were successfully stable in aqueous solution for more than 30 days, and the vaterite particles were formed by a spherulitic crystal growth mechanism.23 The stable vaterite particles would be applied as pigments, fillers, pastes, abrasives, and ceramics and also in the construction of new composite materials processed under aqueous conditions and assembled on the nanoscale. Experimental Section Materials. Poly(acrylic acid) (PAA) (Mw ) 5000), calcium chloride, and ammonium carbonate were obtained from Wako Pure Chemical Industries, Ltd. An aqueous solution of the sodium salt of PAA was prepared by mixing the same molar ratios of PAA and sodium hydroxide. Crystallization of CaCO3. The precipitation of CaCO3 was carried out by a double jet method35 as follows. A 0.1 M CaCl2 aqueous solution (adjusted to pH 8.5 with aqueous NH3) and a 0.1 M (NH4)2CO3 aqueous solution (adjusted to pH 10.0 with aqueous NH3) were injected via syringes into distilled water (adjusted to pH 8.5). An aqueous solution of the sodium salt of 0.1 M PAA was then added to the reaction mixture after incubation at 30 °C for several minutes (0, 1, 3, 20, or 60 min). This solution was then kept at 30 °C for 1 day with gentle stirring. The concentration of the sodium salt of PAA was constant in all experiments except for those specifically mentioned. The crystalline CaCO3 product was collected and washed with water several times and then dried under depressed conditions at room temperature. The crystallization of CaCO3 without additives was carried out by using an aqueous solution containing 1.37 mM NaCl. Measurements. The morphologies of CaCO3 crystals were observed by scanning electron microscopy (SEM, JEOL JSM-5600B (27) Zhang, S.; Gonsalves, K. E. Langmuir 1998, 14, 6761. (28) Balz, M.; Therese, H. A.; Li, J.; Gutmann, J. S.; Kappl, M.; Nasdala, L.; Hofmeister, W.; Butt, H.-J.; Tremel, W. AdV. Funct. Mater. 2005, 15, 683. (29) Boggavarapu, S.; Chang, J.; Calvert, P. Mater. Sci. Eng., C 2000, 11, 47. (30) Xu, G.; Yao, N.; Akasay, I. A.; Groves, J. T. J. Am. Chem. Soc. 1998, 120, 11977. (31) Yu, J.; Lei, M.; Cheng, B.; Zhao, X. J. Solid State Chem. 2004, 177, 681. (32) Naka, K.; Tanaka, Y.; Chujo, Y.; Ito, Y. Chem. Commun. 1999, 19, 1931. (33) Donnet, M.; Bowen, P.; Jongen, N.; Lemaıˆtre, J.; Hofmann, H. Langmuir 2005, 21, 100. (34) Kotachi, A.; Miura, T.; Imai, H. Chem. Mater. 2004, 16, 3191. (35) Sedla´k, M.; Antonietti, M.; Co¨lfen, H. Macromol. Chem. Phys. 1998, 199, 247.
Langmuir, Vol. 22, No. 18, 2006 7761 at 15 kV). The X-ray diffraction (XRD) was recorded with a Rigaku Mini Flex/AW in θ/2θ mode at room temperature. The 2θ scan data were collected at 0.01° intervals, and the scan speed was 1° (2θ)/ min. Fourier transform infrared (FTIR) spectra were recorded with a Perkin-Elmer 2000 spectrometer by a KBr pellet method. Thermogravimetric analysis (TGA) was measured on a TG/DTA 6200 (SEIKO Instruments, Inc.) at a heating rate of 10 °C/min under an air atmosphere.
Results Effect of Poly(acrylic acid) on the Crystallization of Calcium Carbonate. The crystallization of calcium carbonate by changing the addition time of the sodium salt of PAA was studied. The precipitation of CaCO3 was carried out by the double jet method to prevent heterogeneous nucleation on the glass wall of the flask.35 The two syringe tips were closely joined so that a local high concentration was achieved at the moment when the two reagents (CaCl2 and (NH4)2CO3) left the tips, which leads to the immediate nucleation of CaCO3. The nuclei were then immediately transported to regions of the homogeneous CaCO3 solution and grew continuously. After the two calcium and carbonate reagents were injected into the aqueous solution, the aqueous solution containing the sodium salt of PAA was added to the reaction mixture after incubation at 30 °C for several minutes (0, 1, 3, 20, or 60 min). For the addition time of 0 min, the sodium salt of PAA was added to the aqueous solution before the calcium reagents were injected. The experimental conditions and the results are summarized in Table 1. The yields of the obtained crystalline CaCO3 were 52.1 and 62.1% for 2.75 and 3.3 mM concentrations of calcium ions, respectively, when PAA was added after incubation for 1 min. The yields increased for the delayed addition time of 3 min and achieved almost the same values for further delays of the addition time. The yields showed no difference for addition times from 1 to 60 min at the higher calcium concentration of 5.5 mM. This is because the higher calcium concentration exhibits a faster rate of crystallization. The critical point of the sudden increase in the turbidity of the solution was observed before incubation for 1 min at the higher calcium concentration, whereas it was achieved after incubation for 1 min at the lower calcium concentration. The crystal phase of the obtained crystalline products was confirmed by XRD analysis (Figure s1, Supporting Information). The XRD pattern exhibits the characteristic reflections for vaterite (d spacing/2θ peak: 4.24 Å/20.9°, 3.58 Å/24.9°, 3.3 Å/27°, 2.73 Å/32.8°, 2.06 Å/43.9°, 1.86 Å/48.9°, 1.83 Å/49.8°, and 1.65 Å/55.7° corresponding to hkl 004, 110, 111, 112, 300, 302, 114, and 222, respectively)36 and calcite (d spacing/2θ peak: 3.86 Å/23°, 3.04 Å/29.4°, 2.5 Å/35.9°, 2.28 Å/39.5°, 2.09 Å/43.3°, 1.91 Å/47.6°, and 1.88 Å/48.4° corresponding to hkl 012, 104, 110, 113, 202, 018, and 116, respectively).37 Reflections of vaterite characteristics were found for all samples with the delayed addition of PAA. The crystal phase of CaCO3 obtained without polymer additive was calcite (runs 16 and 17 in Table 1). The crystal phase of the obtained CaCO3 products was also characterized by FTIR analysis (Figure s2, Supporting Information).30,38,39 A strong absorption occurring around 1414 cm-1 was characteristic of CaCO3. All the samples with the delayed addition of PAA showed two absorption bands at 877 and 746 cm-1, indicating the vaterite polymorph. The products obtained (36) Joint Committee on Powder Diffraction Standards - International Center for Diffraction Data, file no. 13-192 (Vaterite). (37) Joint Committee on Powder Diffraction Standards - International Center for Diffraction Data, file no. 47-1743 (Calcite). (38) Chakrabarty, D.; Mahapatra, S. J. Mater. Chem. 1999, 9, 2953. (39) Wang, L.; Sondi, I.; Matijevic´, E. J. Colloid Interface Sci. 1999, 218, 545.
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Table 1. Formation of Crystalline CaCO3 in the Presence of the Sodium Salt of PAA at Different Feed Ratios of the Sodium Salt of the Carboxylate Group to Calcium Ions at 30 °C for 1 Day, Poly(acrylic acid) Mw ) 5000 run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
[Ca2+], mM
[-COONa]/ [Ca2+]
addition time, min
2.75
0.62
0 1 3 20 60 0 1 3 20 60 0 1 3 20 60
3.3
0.52
5.5
0.31
polymorphisma
shape of particlesb
vaterite particle size,c µm
vaterite vaterite vaterite vaterite
spherical spherical spherical + irregular spherical + irregular
2.9 ( 0.6 2.9 ( 0.6 3.3 ( 0.8 3.2 ( 1.0
vaterite vaterite vaterite vaterite
spherical spherical spherical + irregular spherical + irregular
2.9 ( 0.8 3.7 ( 0.9 3.5 ( 1.0 3.7 ( 1.0
vaterite vaterite vaterite vaterite calcite calcite
spherical spherical + irregular spherical + irregular spherical + irregular rhombhedral rhombhedral
3.4 ( 1.2 3.8 ( 1.0 3.9 ( 0.8 3.7 ( 0.9
0f 0f
2.75 5.5
yield,d %
adsorbed amount of PAA on particles,e %
0 52.1 72.1 74.2 70 0 62.1 76.3 80.1 81.8 0 84.8 85.6 85.7 87.3 79.7 94.9
1.9 1.8 1.5 1.0 1.7 1.4 1.4 1.3 1.5 1.1 1.4 1.4
a Polymorphism was characterized by XRD. b The shape of the particles was observed by SEM. c The size of the particles was measured by SEM. The yield was calculated by the final crystal weights compared with the theoretical weights of CaCO3 from injected calcium reagents. e The adsorbed amount of PAA was measured by TGA (heating rate: 10 °C/min under an air atmosphere). f The precipitation of CaCO3 was carried out without the addition of PAA.
d
Table 2. Formation of Crystalline CaCO3 in the Initial Presence of the Sodium Salt of PAA at 30 °C for 1 Day, Addition Time: 0 min, Poly(acrylic acid): Mw ) 5000 run
[Ca2+] mM
[-COONa]/ [Ca2+]
addition time, min
1 2 3 4 5
2.75 3.3 5.5 5.5 5.5
0.62 0.52 0.31 0.1 0.05
0 0 0 0 0
6
5.5
0.01
0
polymorphisma NA NA NA NA vaterite + calcite (14.1%) vaterite + calcite (18.7%)
shape of particlesb
irregular spherical + rhombohedral
yield,c %
adsorbed amount of PAA on particles,d %
0 0 0 0 63.6
3.8
68.1
0.9
a Polymorphism was characterized by XRD. b The shape of the particles was observed by SEM. c The yield was calculated by the final crystal weights compared with the theoretical weights of CaCO3 from injected calcium reagents. d The adsorbed amount of PAA was measured by TGA (heating rate: 10 °C/min under an air atmosphere).
in the absence of PAA were calcite according to the bands at 877 and 713 cm-1. The morphologies of the obtained crystals were observed by SEM (Figure 1 and Figure s3, Supporting Information). The average particle sizes of the spherical vaterite particles were around the range of 3 to 4 µm for all samples. The particle sizes increased 0.4 to 0.6 µm from the earlier addition time (1 or 3 min) to the later addition time (20 or 60 min) and also increased 0.5 to 0.7 µm upon increasing the concentration of calcium ions from 2.75 to 5.5 mM. It is interesting that the vaterite particles had a spherical shape with smooth surfaces (Figure 1a and b) for the early addition time (1 or 3 min), whereas the vaterite particles showed a spherical shape with rough surfaces and some irregularly shaped particles (Figure 1c and d) for the late addition time. The contents of PAA adsorbed on the obtained CaCO3 crystals were measured by TGA (Figure s4, Supporting Information). The pure CaCO3 has a theoretical weight loss of 44 wt %39 above 750 °C, which is due to decomposition to CO2 and CaO. The adsorbed amount of PAA on the vaterite particles was calculated by the weight loss at 800 °C after subtracting the weight lost by CaCO3. The adsorbed amounts of PAA on the vaterite particles were in the range of 1 to 2 wt % for all of the samples. At the lower calcium concentration (2.75 mM), the adsorbed value of PAA decreased from 1.9 to 1.0 wt % when increasing the addition time of PAA from 1 to 60 min, respectively, and it showed a
smaller difference for the different addition time with the higher calcium concentration (5.5 mM). It was found that the amount of PAA adsorbed on the vaterite particles decreased and the average particle sizes increased with increasing the delay in the addition time. The smaller particles had the higher amount of PAA, while the larger particles had the lower value. Furthermore, the difference in the amount of PAA adsorbed on the modified vaterite decreased with increasing calcium concentration. For the addition time of 0 min, different concentrations of calcium reagents were injected into an aqueous solution of the sodium salt of PAA. The results are summarized in Table 2. The initial presence of PAA acted as an inhibitor of crystal formation when the ratio of [-COO-]/[Ca2+] was higher than 0.1. The obtained CaCO3 crystal with the ratio of [-COO-]/[Ca2+] ) 0.05 was a mixture of calcite and vaterite identified by XRD analysis. The fraction of calcite was 14.1% as determined by Rao’s equation.40 The SEM image of the product of run 5 in Table 2 (Figure 2a) showed smaller particles with irregularly shaped crystals compared with the products obtained in the absence of PAA as shown in Figure 1e. This is because crystal growth was inhibited by PAA, and the PAA content is not high enough to inhibit the nucleation of CaCO3. The TGA result showed that the amount of PAA adsorbed on the obtained CaCO3 crystal was 3.8 wt %, which was higher than the amounts adsorbed (40) Rao, M. S. Bull. Chem. Soc. Jpn. 1973, 46, 1414.
Formation of Stable Vaterite with Poly(acrylic acid)
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Figure 1. SEM images of the CaCO3 products of (a) run 2, (c) run 5, and (e) run 17 with higher-magnification images of the products of (b) run 2 and (d) run 5 in Table 1.
on the crystals obtained by the delayed addition of PAA. When the ratio of [-COO-]/[Ca2+] was 0.01 for the addition time of 0 min, the inhibition effect of PAA in the crystal growth was not as apparent. The SEM image (Figure 2b) showed that the obtained product of run 6 in Table 2 was larger particles of spherical vaterite and rhombohedral calcite than that of run 5 in Table 2. Vaterite is the thermodynamically most unstable form of the three crystal phases of CaCO3. Without any additives, it is well known that the vaterite phase transforms into stable calcite via a solvent-mediated process.41,42 The complete phase transformation into the thermodynamically most stable calcite usually occurs (41) Lo´pez-Macipe, A.; Go´mez-Morales, J.; Rodrı´guez-Clemente, R. J. Cryst. Growth 1996, 166, 1015. (42) Kralj, D.; Brecˇevic´, L.; Kontrec, J. J. Cryst. Growth 1997, 177, 248.
easily and irreversibly within 3 days when vaterite particles are in contact with water.18,26,43 We checked the phase transformation of the vaterite particles from the delayed addition method in an aqueous solution for longer incubation at 30 °C. The results are summarized in Table 3. When the samples were continuously kept in the reaction mixtures for 5 days, all of the crystals (runs 1, 6, 10) obtained from the conditions of the ratio of [-COO-]/ [Ca2+] ) 0.1, 0.31, 0.62 and at the early addition time (1 or 3 min) were vaterite as identified by XRD analysis. These results indicate that the vaterite surfaces were stabilized by the carboxylate group of PAA in the aqueous solutions to prevent the phase transformation to calcite. Under the condition of the (43) Naka, K.; Tanaka, Y.; Chujo, Y. Langmuir 2002, 18, 3655.
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Figure 2. SEM images of the CaCO3 products of (a) run 5 and (b) run 6 in Table 2. Table 3. Ability to Stabilize the Surface of Vaterite with PAA by Increasing the Stirring Time of the Mixture Solution run 1 2 3 4d 5d 6 7 8 9d 10
[Ca2+], mM
[-COONa]/ [Ca2+]
addition time, min
incubation time, days
2.75
0.62 0.62 0.62 0.62 0.62 0.31 0.31 0.31
3 3 60 3 60 1 1 1
5 30 5 5 5 5 10 14
0.31
1
5
1
5
5.5
0.1
a
polymorphisma vaterite vaterite vaterite vaterite vaterite vaterite vaterite vaterite + calcite (41.8%) vaterite + calcite (64.1%) vaterite
yield,b %
adsorbed amount of PAA on particlesc %
71.8 64.2 67.3 64.2 55.5 86.5 84.4 65.2
1.6 0.9 1.1 1.3 0.9 1.8 1.5 0.8
71.7
0.5
92.3
1.6
b
Polymorphism was characterized by XRD. The yield was calculated by the final crystal weights compared with the theoretical weights of CaCO3 from injected calcium reagents. c The adsorbed amount of PAA was measured by TGA (heating rate: 10 °C/min under an air atmosphere). d After incubation in the mixture solution for 1 day, the product was filtered and washed with water and then incubated in pure water.
higher ratio of [-COO-]/[Ca2+] ) 0.62, no transformation of the vaterite product into the thermodynamically more stable form was observed even after incubation in the aqueous solution for more than 30 days (run 2). Under the condition of the lower ratio of [-COO-]/[Ca2+] ) 0.31, transformation to calcite was observed after the sample was kept in the aqueous solution for more than 14 days (run 8). The stability of the isolated vaterite particles was evaluated in a freshwater. The crystal products incubated in aqueous solution for 1 day were filtered and washed with water three times, and then the isolated crystals were put into freshwater for further incubation for 5 days at 30 °C. The crystal products (runs 4 and 5) obtained from the calcium concentration at 2.75 mM were vaterite, whereas the crystal product (run 9) obtained from the calcium concentration at 5.5 mM was a mixture of vaterite and calcite (64.1%). Current results indicate that the vaterite particles modified with PAA exhibited a strong stabilizing effect to prevent the phase transformation.
Discussion The precipitation of CaCO3 in the initial presence and by the delayed addition of PAA was investigated. The initial presence of PAA inhibited the nucleation and growth of crystallization, whereas the vaterite particles were formed by delaying the addition time of the same amount of the sodium salt of PAA. For the earlier addition time, PAA was effectively adsorbed on the surface of the vaterite particles, and the crystalline dissolution and recrystallization could have been inhibited with PAA. The irregularly shaped particles and rough surface under the later addition condition might be caused by some surface of crystallines
dissolved in aqueous solution, and recrystallization occurred to form smaller crystals during the crystal-forming process before PAA was added. Vaterite is thermodynamically the most unstable phase among the three crystal systems of CaCO3 and transforms into calcite when in contact with an aqueous environment for a long time. The phase completely transforms into calcite within 3 days. According to the results of long-term stability in an aqueous solution even after the vaterite particles were isolated and washed with water, the vaterite surfaces might be stabilized by PAA. The binding strength between PAA and the surfaces of the vaterite particles was strong enough to prevent the phase transformation of vaterite to calcite. Furthermore, the TGA results also showed that the adsorbed PAA was hardly desorbed when the vaterite particles were washed with water. The adsorbed amounts of PAA on the vaterite particles obtained from the ratio of [-COO-]/ [Ca2+] ) 0.62 before and after washing with water showed no remarkable difference as shown in Tables 1 and 3. After washing with water, the adsorbed amounts of PAA of 1.8 and 1.0 wt % for the samples of runs 3 and 5 in Table 1 were 1.3 and 0.9 wt %, respectively (Table 3). Thus, the isolated vaterite particles were stable even in contact with pure water for 5 days. However, the adsorbed amount of PAA on the vaterite particles obtained from the ratio of [-COO-]/[Ca2+] ) 0.31 decreased from 1.5 to 0.5 wt % for an addition time of 1 min after washing with water for another 5 days of incubation. The final crystal product became a mixture of vaterite and calcite. From the present results, we should emphasize that the stable vaterite particles were formed by a simple method by using a common additive. Such stable vaterite particles could be applied as pigments or fillers or could
Formation of Stable Vaterite with Poly(acrylic acid)
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Figure 3. SEM images in the interior of cracked open vaterite particles of (a) run 2 and (b) run 3 in Table 1.
Figure 4. SEM images in the interior of cracked open CaCO3 particles of (a) run 5 and (b) run 6 in Table 2.
be further modified with the carboxylate group on the surface of CaCO3 to make functional fillers and could also be used in the construction of new composite materials processed under aqueous conditions and assembled on the nanoscale. An interesting feature of crystalline CaCO3 evaluated in this study was that it was found to exhibit a different formation mechanism of CaCO3 particles by the delayed addition of PAA, compared with that by the initial presence of PAA. The SEM images of the interior of cracked vaterite particles obtained by the delayed addition of PAA (runs 2 and 3 of Table 1) are shown in Figure 3. Characteristic radiating features were observed in the crystals, which were formed by a spherulitic growth mechanism.23 However, the SEM image of the CaCO3 particles obtained in the initial presence of PAA (run 5 of Table 2) showed that they consist of aggregated nanosized primary particles (Figure 4a), which is a well-established concept of the vaterite formation mechanism in the literature.18-21,44 Co¨lfen and Antonietti studied the precipitation of CaCO3 by the double jet method in the presence of double-hydrophilic block copolymers and showed that spherical vaterite particles were formed by the aggregation mechanism of nanosized crystallites.19 Zhang et al.44 reported the biomimetic assembly of CaCO3 nanoparticles in the presence of a long-chain polypeptide. It was found that 15-25 nm vaterite nanoparticles can be assembled into highly spherical aggregates through a stepwise process and that the “soft” poly(aspartate) chains binding onto the nanoparticle surface resulted in the soft nature of the nanoparticle assembly. This reference suggested that the vaterite particles formed by nanoaggregates are induced in the initial presence of polymer additives, consistence with our results. The CaCO3 particle-formation mechanism obtained by spherulitic growth or nanoaggregation may acquire further support by checking the TGA analysis. According to the comparison of the (44) Zhang, Z.; Gao, D.; Zhao, H.; Xie, C.; Guan, G.; Wang, D.; Yu, S.-H. J. Phys. Chem. B 2006, 110, 8613.
adsorbed amounts of PAA on the CaCO3 particles measured by TGA, the adsorbed amounts of PAA on the CaCO3 particles formed in the initial presence of PAA were higher than those in the delayed addition at the same calcium concentration of 5.5 mM, even though the ratio of PAA was much lower under the former condition. Furthermore, no remarkable differences in the adsorbed amounts of PAA on the CaCO3 particles were observed at the same calcium concentration with different addition times. Thus, we assumed that PAA was mostly adsorbed on the surface of the final vaterite particles with an average particle size of around 3 to 4 µm in the delayed addition method. However, PAA was adsorbed on the surface of the nanocrystals, followed by nanoaggregation in the initial presence of PAA. Therefore, the adsorbed amounts of PAA in the later particles were higher than those in the former particles. A conclusion from the CaCO3 particle-formation mechanism with polymer additive PAA is proposed by following Scheme 1. The initial presence of PAA acts as an inhibitor to prevent crystal nucleation and growth. Nanosized amorphous particles might be preformed in the solution, and they seem to be stabilized with PAA to inhibit the nucleation of crystalline products. Evidence of the formation of such nanosized particles at an initial stage in crystallization was reported in the literature.28 Precipitates were formed only when the ratio of [-COO-]/[Ca2+] was as low as 0.05, in which the amount of PAA was not enough to cover all of the surfaces of the nanosized particles to prevent the nucleation and growth of the crystals. The amorphous precursors were transformed to vaterite nanosized particles as a result of the insufficient inhibiting effect of PAA, and the final crystal particles were formed by the nanoaggrgation mechanism. When the ratio of [-COO-]/[Ca2+] was 0.01 for the addition time of 0 min, the radiating features were observed in the SEM image (Figure 4b). However, the vaterite products obtained by the delayed addition of PAA were formed by spherulitic growth. The initially formed amorphous precursors might grow and transform to larger vaterite
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Figure 5. SEM images of the CaCO3 products immediately isolated when PAA was added after incubation for (a) a few seconds, (b) 1 min, (c) 3 min, and (d) 20 min; [Ca2+] ) 2.75 mM, and the ratio of [-COO-]/[Ca2+] ) 0.62. The inset shows the images in the interiors of cracked open vaterite particles. Scheme 1. Schematic Depiction of the Formation of CaCO3 Spherulites and the Transformation from the Amorphous Phase to Vaterite
particles before the addition of PAA because of the higher local concentration of the calcium reagents by the double jet method. That is, the crystal growth rate of the CaCO3 is much higher in
the environment before the addition of PAA than in the environment in the presence of PAA. Such larger particles might be difficult to aggregate as a result of the lower surface energy
Formation of Stable Vaterite with Poly(acrylic acid)
compared with the higher surface energy of nanosized crystallites. The size of the vaterite particles might increase upon increasing the addition time.26 The sodium salt of PAA quickly adsorbed on the surface of the vaterite particles and inhibited their further growth. Irregularly shaped particles and rough surfaces in the later addition might be caused by dissolution and recrystallization of the resulting vaterite particles before adding PAA. Further support for the above argument may be confirmed by the SEM observation of the following experiments. The crystalline products were immediately isolated after PAA was added to stabilize and inhibit further nucleation and growth of the crystals. The delayed addition times of the sodium salt of PAA after the calcium reagents were injected were a few seconds and 1, 3, and 20 min. The SEM images showed that samples isolated after the addition of PAA for 1, 3, and 20 min were spherical vaterite particles (Figure 5). It was found that the particle sizes of the vaterite particles increase when delaying the addition time of PAA. Radiating features were observed in these cracked particles. Although a trace amount of precipitate was obtained when the sodium salt of PAA was added after a few seconds, the SEM image of the sample shows small spherical particles that were vaterite by FTIR analysis. These results suggested that the nucleation and growth of crystallization of CaCO3 might be conducted through spherulitic growth under our crystallization condition before PAA was added to the aqueous solution. The radiating features in the cracked particles were also observed in the precipitates obtained from the initial presence of a small amount of PAA with the ratio of [-COO-]/[Ca2+] ) 0.01 as shown in Figure 4b. In the delayed addition method, the sodium salt of PAA was then adsorbed on the surface of the vaterite particles and prevented dissolution and phase transformation. Although Andreassen described the formation of spherical vaterite crystals by the spherulitic growth mechanism,23 the
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obtained vaterite particles were transformed to calcite after incubation in water for 20 h. However, the present spherical vaterite particles prepared by the delayed addition of PAA were stable in aqueous solution for more than 30 days. The present simple delayed addition method provided for the stabilization of the crystalline products; even PAA acts as an inhibitor for the crystallization and growth of CaCO3. The polymer additive was used as a strong stabilizer on an unstable form of CaCO3 particles by the delayed addition method.
Conclusions We examined the crystallization of CaCO3 by changing the addition time of the sodium salt of PAA to the calcium carbonate solution from 0 to 60 min. The current results suggest that the stable vaterite particles were successfully made by the delayed addition of PAA from 1 to 60 min. These vaterite particles were stable in aqueous solution for more than 30 days as a result of the strong binding between the carboxylate group of PAA and the Ca2+ ion to prevent phase transform into the most thermodynamically stable calcite where it normally occurs in aqueous solution within 3 days. Furthermore, PAA acts as an inhibitor for the precipitation of CaCO3 in the initial presence of the calcium carbonate solution, and a mixture of vaterite and calcite was formed by the nanoaggregation mechanism when the ratio of [-COO-]/[Ca2+] was 0.05. To change the addition time of PAA, we found that the vaterite particles were formed by the spherulitic growth mechanism. Supporting Information Available: XRD, FTIR, and TGA analyses of CaCO3 particles and SEM images of CaCO3 particles. This material is available free of charge via the Internet at http://pubs.acs.org. LA060874K
