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Formation of Calcium Carbonate in Liquid Crystalline Phases Per Kjellin,*,† Martin Andersson,† and Anders E. C. Palmqvist†,‡ Department of Applied Surface Chemistry and Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96, Go¨ teborg, Sweden Received April 25, 2003 Calcium carbonate was synthesized according to a generic three-phase synthesis route in the presence of self-assembling liquid crystalline phases, and a comparison was made between the products formed in hexagonal and reverse hexagonal liquid crystalline phases. The surfactants used to form the liquid crystalline phases were the three different triblock copolymers Pluronic P105 (hexagonal) and P104 or P123 (reverse hexagonal), consisting of poly (propylene glycol) as the middle block and poly (ethylene glycol) as the end blocks. An aqueous solution of CaCl2 was mixed with the Pluronics and organic solvents to make up the liquid crystalline phases, which subsequently were exposed to CO2 (g), resulting in the formation of CaCO3. The specific surface areas of the CaCO3 products obtained varied from 5 to 226 m2/ g, depending on the initial concentration of CaCl2. Small-angle X-ray scattering showed that the reactant salt and the CaCO3 crystals were situated in the water domains of the liquid crystalline phases and that these remained throughout the course of the experiment. Transmission electron microscopy studies in combination with powder X-ray diffraction revealed that CaCO3 with high specific surface areas consisted of fibrils with the vaterite structure and formed at low CaCl2 concentrations. Samples of lower surface areas consisted of flakes with the calcite structure and formed predominantly at higher CaCl2 concentrations. Nitrogen adsorption showed a narrow pore size distribution for some of the materials prepared in the reverse hexagonal phases. Poly(acrylic acid) added to a hexagonal phase increased the specific surface area of the CaCO3 substantially, whereas when added to a reverse hexagonal phase no increase in surface area was found. Powder X-ray diffraction showed that higher specific surface areas obtained with poly(acrylic acid) were associated with the formation of a larger fraction of amorphous CaCO3.
Introduction Calcium carbonate attracts a lot of interest because of its various uses. In industry, it is extensively utilized in applications such as paper coatings,1 sulfur dioxide removal,2 and plastics.3 It is also a common building block in nature. Clams, oysters, corals, and other marine species build up their shells with calcium carbonate, and the research field of biomineralization aims to understand how this crystal growth process works. Calcium carbonate also occurs as unwanted deposits, so-called scaling, in water-intensive industries. Most of the research on the growth of calcium carbonate focuses on the possibility to control the crystal structure with additions of small amounts of organic or inorganic substances.4 This control can be utilized to prevent growth, as in the antiscaling case, or to promote it, as in the biomineralization case. Numerous studies have shown that it is possible to use a liquid crystalline phase as a template for the growth of inorganic materials, such as noble metals and oxides.5 Mobil Oil scientists were the first to present ordered mesoporous materials made from surfactant self-assembly.6 They made silica and alumino silicate materials (MCM) with uniformly sized pores between 1.6 and 10 nm and with high specific surface areas (>1000 m2/g). There has ever since been an increasing interest in * Address correspondence to this author. † Department of Applied Surface Chemistry. ‡ Competence Centre for Catalysis. (1) Inoue, M.; Lepoutre, P. J. Adhes. Sci. Technol. 1992, 6, 851. (2) Wei, S. H.; Mahuli, S. K.; Agnihotri, R.; Fan, L. S. Ind. Eng. Chem. Res. 1997, 36, 2141. (3) Zuiderduin, W. C. J.; Westzaan, C.; Huetink, J.; Gaymans, R. Polymer 2003, 44, 261. (4) Naka, K.; Chujo, Y. Chem. Mater. 2001, 13, 3245. (5) Go¨ltner, C. G.; Henke, S.; Weissenberger, M. C.; Antonietti, M. Angew. Chem., Int. Ed. 1998, 37, 613. (6) Kresge, C.; Leonowicz, M.; Roth, W.; Vartuli, J.; Beck, J. Nature 1992, 710.
synthesizing new ordered mesoporous materials with other elemental compositions. The synthesis of silica has been most thoroughly studied, but many other materials have also been synthesized recently.7,8 Such materials are of considerable interest as catalysts, molecular sieves, adsorbents, electrodes, low-k dielectrics, optical waveguides, and so forth. The surfactant templating techniques can also be utilized to synthesize nanoparticles. The most widely used method for this is the microemulsion method, where small water droplets surrounded by a monolayer of surfactants are used as microreactors for the formation of small inorganic or organic particles.9 In such a system, one can take advantage of the high dynamics between the water droplets, which makes it possible to prepare nonsoluble materials by mixing separate microemulsions containing different water soluble salts. When using a liquid crystalline phase as template for materials preparation, other techniques have to be used, since it is not possible to rapidly mix two different liquid crystalline phases because of their high viscosity. Several studies have examined the effect of polymers on the calcium carbonate crystal growth. Most of these concern systems with a low polymer concentration,10,11 while other works deal with synthesis in microemulsions12-14 and crystallization of CaCO3 in the form of thin films.15,16 In the present study, a generic three-phase (7) Schu¨th, F. Chem. Mater. 2001, 13, 3184 (8) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (9) Lo´pez-Quintela, M. A. Curr. Opin. Colloid Interface Sci. 2003, 8, 137. (10) Co¨lfen, H.; Qi, L. Chem. Eur. J. 2001, 7, 106. (11) Lo´pez-Macipe, A.; Go´mez-Morales, J.; Rodrı´guez-Clemente, R. J. Cryst. Growth 1996, 166, 1015. (12) Kandori, K.; Kon-No, K.; Kitahara, A. J. Colloid Interface Sci. 1988, 122, 78. (13) Walsh, D.; Mann, S. Nature 1995, 377, 320. (14) Walsh, D.; Lebeau, B.; Mann, S. Adv. Mater. 1999, 11. (15) Hosoda, N.; Kato, T. Chem. Mater. 2001, 13, 688.
10.1021/la034703g CCC: $25.00 © 2003 American Chemical Society Published on Web 10/02/2003
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synthesis route, involving liquid, solid, and gas phase, has been applied for the formation of CaCO3 in liquid crystalline phases. The concept utilizes a water soluble salt (CaCl2) mixed with surfactant and solvent to form the desired liquid crystalline phase and subsequently exposed to a water soluble gas (CO2) to form the desired compound (CaCO3). The gas diffuses into the water domains of the liquid crystal and the reaction/crystallization takes place, while preserving the structure of the liquid crystalline phase throughout the experiment. Materials and Methods The triblock copolymers Pluronic P104, P105, and P123 which are poly(ethylene oxide)-block-poly(propylene oxide)-blockpoly(ethylene oxide) with the compositions (EO)27(PO)61(EO)27, (EO)37(PO)58(EO)37, and (EO)20(PO)70(EO)20, respectively, were kindly donated by BASF corporation, Mount Olive, New Jersey. Calcium chloride hexahydrate and poly(acrylic acid) (MW ) 2000 g/mol) were purchased from Aldrich and used as received. Ammonia (25 wt %, p.a.) was purchased from Merck, Germany. CO2 gas (99.90%) was purchased from AGA Gas AB, Sweden. The water used was double distilled. The synthesis of CaCO3 was effected by mixing a CaCl2 aqueous solution with surfactant and solvent to form the desired liquid crystalline phase. The aqueous solution used in all the experiments consisted of CaCl2 and ammonia solution of varied concentrations, but with a fixed Ca/NH3 molar ratio of 1:1. The ammonia was added to the solution to serve as a buffer. In some of the experiments, poly(acrylic acid) was also added to the aqueous solution. One hexagonal and two reverse hexagonal liquid crystalline phases were studied. The hexagonal phase consisted of 55 wt % P105 and 45 wt % aqueous solution. The first reverse hexagonal phase consisted of 50 wt % P123, 35 wt % butyl acetate, and 15 wt % aqueous solution. The second reverse hexagonal phase consisted of 58 wt % P104, 30 wt % p-Xylene, and 12 wt % aqueous solution. The mixture was left to equilibrate and found to be a homogeneous liquid crystal after a few hours. The mixture was placed in a steel autoclave and subjected to CO2 gas at a pressure of 0.15 MPa, upon which CaCO3 formed. The reaction was allowed to proceed for 72 h, and the resulting opaque gel was washed with ethanol and water to remove the surfactants and remaining salts, and then dried. All steps of the synthesis were carried out at room temperature. Small-angle X-ray scattering (SAXS) experiments were carried out on the liquid crystalline samples, with and without the presence of the salt and before and after exposure to CO2. The measurements were done using a Kratky compact small-angle system equipped with a position-sensitive wire detector (OED 50M from MBraun, Graz) containing 1024 channels of width 53.0 µm. Cu KR radiation of wavelength 1.542 Å was provided by a Seifert ID 3000 X-ray generator operating at 50 kV and 40 mA. The dried samples were analyzed with the following methods. N2-adsorption measurements were done with a Micromeritics ASAP 2010 instrument at 77 K. Before the measurements, the samples were exposed to vacuum treatment for 2 h at 200 °C to remove any remaining moisture. The specific surface areas were obtained by the BET method and the pore size distributions were calculated from the N2-desorption isotherm using the BJH method.17 Transmission electron microscopy (TEM) studies were performed on samples deposited on a holey carbon-covered copper grid with a Philips CM 120 Bio TWIN cryo-electron microscope operating at an accelerating voltage of 120 kV. X-ray powder diffraction analyses were done using a Siemens D5000 X-ray diffractometer and a Cu KR radiation of wavelength 1.542 Å in the 2θ range of 20-60°.
Results and Discussion Two different types of liquid crystalline phases have been investigated as templates for the formation of CaCO3 (16) Kuther, J.; Nelles, G.; Seshadri, R.; Schaub, M.; Butt, H. J.; Tremel, W. Chem. Eur. J. 1998, 4, 1834. (17) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373.
Figure 1. SAXS patterns of liquid crystalline structures showing (a) the hexagonal phase with no salt present, with 20 wt % CaCl2 added and before reaction with CO2, and with 20 wt % CaCl2 added and after CO2 treatment, and (b) the reverse hexagonal phase with no salt present, with 20 wt % CaCl2 added and before reaction with CO2, and with 20 wt % CaCl2 added and after CO2 treatment.
in this study. In the hexagonal phase the surfactants form cylinders, which are surrounded by water, whereas in the reversed hexagonal phase the water domains are in the form of cylinders, separated by a layer of surfactant molecules and the oil phase. Pluronic P105 and water was used for the making of the hexagonal liquid crystalline phase, whereas for the reverse hexagonal phase, two different compositions were used containing P104, water, and p-Xylene, respectively P123, water, and butyl acetate. Mixtures of Pluronics, water, and solvents have been studied by Alexandridis et al., and phase diagrams are available in the literature for the presently studied systems.18-20 A. Calcium Carbonate Formed in a Hexagonal Phase. One of the most important parameters in the synthesis of mesoporous silica is the molar ratio of silica and surfactant.21 For this reason, the effects of the molar ratio of calcium ions and polymer on the formation of CaCO3 were studied. The calcium chloride concentration in the water phase of the hexagonal phase was varied from 0.5 to 20 wt %. From the SAXS analyses shown in Figure 1a, it was found that the structure of the liquid crystalline phase was retained after addition of the salt and also after the reaction with CO2 had taken place. It was seen that the lattice spacing of the liquid crystal first decreased and then increased with increasing CaCl2 concentration as summarized in Table 1. Figures 2a and (18) Alexandridis, P.; Zhou, D.; Khan, A. Langmuir 1996, 12, 2690. (19) Svensson, B.; Alexandridis, P.; Olsson, U. J. Phys. Chem. B 1998, 102, 7541. (20) Holmqvist, P.; Alexandridis, P.; Lindman, B. J. Phys. Chem. B 1998, 102, 1149. (21) Palmqvist, A. E. C. Curr. Opin. Colloid Interface Sci. 2003, 8, 145.
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Kjellin et al. Table 2. Summary of the Specific Surface Areas of the CaCO3 Products Formed in the Presence of Various Proportions of Poly(acrylic acid), with a CaCl2 Concentration of 7.5 wt %
Figure 2. TEM micrographs of the products from the syntheses of CaCO3 in hexagonal liquid crystals using a CaCl2 concentration of (a) 0.5 wt % and (b) 20 wt %, and in reverse hexagonal liquid crystals using a CaCl2 concentration of (c) 0.5 wt % and (d) 20 wt %. Table 1. Summary of the Lattice Spacing from SAXS Data of the Hexagonal and Reverse Hexagonal Liquid Crystals with Additions of Different Amounts of CaCl2 and after Formation of CaCO3 CaCl2 conc, wt %
hexagonal, unreacted, Å
hexagonal, reacted, Å
reverse hexagonal, unreacted, Å
reverse hexagonal, reacted, Å
0 1 10 20
137.4 133.4 146.1 150.9
135.3 143.8 146.1
141.6 135.3 150.9 153.4
133.4 141.6 153.4
2b show the TEM micrographs of the product from the syntheses using 0.5 and 20 wt % CaCl2, respectively. The low CaCl2 concentration resulted in fibrillar particles, whereas the high CaCl2 concentration mainly gave sheetshaped crystals. The X-ray powder diffraction analysis in Figure 3a showed that the CaCO3 formed at low CaCl2 concentrations was mainly vaterite. Higher CaCl2 concentrations gave instead more calcite, as Figure 3b shows. Figure 4 shows the corresponding specific surface areas of the CaCO3 product as a function of CaCl2 concentration. As can be seen from this figure, the surface area depends strongly on the CaCl2 concentration. The pore size distributions of the CaCO3 formed in the hexagonal phase can be seen in Figure 5a and were comparably wide for all CaCl2 concentrations studied, but centered around different mean values depending on the concentration. B. Calcium Carbonate Formed in a Reverse Hexagonal Phase. To evaluate the effect of the liquid crystalline structure on the crystallization of CaCO3, a similar series of experiments were performed in a reverse hexagonal liquid crystal. As for the crystallization in the hexagonal phase, the calcium chloride concentration was varied from 0.5 to 20 wt %. Also for the reverse hexagonal phase, the SAXS measurements in Figure 1b showed that the lattice spacing increased with an increased salt concentration and decreased somewhat after reaction with
weight proportion of PAA/CaCl2
specific surface area, m2/g
0 0.0125 0.125 0.25
19 21 57 128
CO2 as summarized in Table 1. The crystals formed with the reverse hexagonal phase exhibited the same two morphologies found in the hexagonal phase. The fibershaped crystals were obtained when the CaCl2 concentration was low, whereas the sheet-shaped crystals resulted from higher CaCl2 concentrations, as shown in Figure 2c and 2d, respectively. Also, the low concentration of CaCl2 resulted in vaterite and the high concentration in calcite as shown in Figure 3c and 3d. The specific surface areas obtained varied between 20 and 226 m2/g, as shown in Figure 4b, and the pore size distributions for the crystals formed in the reversed hexagonal phase are shown in Figure 5b. Apparently, the pore size distribution is narrowest for concentrations of CaCl2 between 2.5 and 10 wt %. The pore size for these concentrations of CaCl2 was centered around 3.8 nm. The fibrillar crystallites formed typically had diameters of 8-10 nm. The water domains in the reverse hexagonal phases created with P104 and P123 have a diameter of around 8 nm. A second reverse hexagonal liquid crystal phase was also used to investigate the effect on CaCO3 formation. The surfactant used to create this phase was P104, and the oil phase consisted of p-Xylene. No distinguishable difference was, however, found between P104 and P123 with respect to the formed crystals. Since the water domain in the hexagonal phase is continuous in three directions, the growth of calcium carbonate crystals should also be less hindered. The water domain of the reversed hexagonal phase is continuous in one direction (along the cylinder length), and the crystal growth will therefore be more restricted. This is likely the explanation for the narrower pore size distribution. C. Effect of Poly(acrylic acid) on the Formation of CaCO3 in Liquid Crystalline Phases. Poly(acrylic acid) has a well-documented effect on the growth of calcium carbonate crystals.15,22 It is known to decrease the size of the precipitated crystals and, if used in higher concentrations, promotes formation of amorphous calcium carbonate. Additions of poly(acrylic acid) to the hexagonal phase increased the surface area of the products quite dramatically, as summarized in Table 2. X-ray diffraction analyses of these crystals showed that the higher surface areas found were associated with an increasing amount of amorphous CaCO3, but no vaterite was present in the samples with high surface area. When poly(acrylic acid) was added to the reversed hexagonal phases, it had no effect on the surface area. The most likely explanation for this may be that poly(acrylic acid) adsorbs differently to the surfactant aggregates in the reverse and normal hexagonal phases due to their different curvatures. Thus, the concentration of poly(acrylic acid) in the water domains and their interaction with the calcium ions may be different. Conclusions The growth of calcium carbonate crystals within hexagonal and reversed hexagonal liquid crystalline phases has been investigated using a three-phase synthesis (22) Yang, Q.; Liu, Y.; Gu, A.; Ding, J.; Shen, Z. J. Colloid Interface Sci. 2001, 240, 608.
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Figure 3. X-ray diffractograms of the CaCO3 products formed in hexagonal liquid crystals using a CaCl2 concentration of (a) 0.5 wt % and (b) 20 wt %, and with reverse hexagonal liquid crystals using a CaCl2 concentration of (c) 0.5 wt % and (d) 20 wt %.
Figure 4. Specific surface areas of CaCO3 formed in (a) hexagonal and (b) reverse hexagonal liquid crystals using different concentrations of CaCl2.
procedure. The crystal structure, specific surface area, and pore size distribution of the products were much dependent on the CaCl2 concentration used. At low concentrations of CaCl2, the CaCO3 product formed showed high surface areas, predominantly of the vaterite structure with crystallites of fibrillar shape. At high concentrations, the CaCO3 formed was instead mainly calcite with crystallites having sheet morphologies and lower specific
surface areas. The lowest specific surface area obtained with the reversed hexagonal phase was around 20 m2/g, while for the hexagonal phase it was 5 m2/g for the same concentration of CaCl2. Calcium carbonate grown in the reversed hexagonal phases had a more narrow pore size distribution compared to the hexagonal phases. X-ray diffraction showed that samples of low surface areas consisted predominantly of calcite, while samples with
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Figure 5. Pore size distribution with a normalized pore volume of CaCO3 formed in (a) the hexagonal liquid crystal phase and (b) the reverse hexagonal liquid crystal phase.
high surface areas consisted of vaterite. In particular, we found the vaterite form of calcium carbonate to be amenable for templated growth, while the calcite polymorph proved less susceptible to templating. The fibrils formed in the reverse hexagonal phase had a diameter of approximately 8-10 nm, comparable to the water channels (8 nm) in this phase. The diameter of these fibrils remained the same, regardless of the specific surface area. The CaCl2 concentration seems to be determining the calcite/vaterite ratio and thus the specific surface area of the calcium carbonate samples formed. Addition of poly(acrylic acid) to the hexagonal phase evidently increased the specific surface area of CaCO3, but instead of forming vaterite, an increasing amount of amorphous calcium carbonate resulted. One explanation may be that poly(acrylic acid)
serves as growth sites for CaCO3, thereby affecting the crystal growth more and increasing the templating effect of the liquid crystal phase. Acknowledgment. The authors thank Katarina Flodstro¨m at Physical Chemistry 1, Lund, Sweden, for assistance with the SAXS measurements. P.K. thanks BIM Kemi AB and KK-stiftelsen for funding this research. Financial support from the Swedish Foundation for Strategic Research (SSF) through its Colloid and Interface Technology program is gratefully acknowledged by M.A. and A.E.C.P. LA034703G
