Morphological control of calcium carbonate crystals by polyvinylpyrrolidone and sodium dodecyl benzene sulfonate

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1 Colloids and Surfaces A: Physicochem. Eng. Aspects 251 (2004) Morphological control of calcium carbonate by polyvinylpyrrolidone and sodium dodecyl benzene sulfonate Qiang Shen a,, Hao Wei b, Ying Zhao b, Du-Jin Wang b,, Li-Qiang Zheng a, Duan-Fu Xu b a Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong University, Ji nan , P.R. China b State Key Laboratory of Polymer Physics and Chemistry, Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences, Beijing , P.R. China Received 8 March 2004; accepted 21 August 2004 Abstract The crystallization of calcium carbonate was conducted by the reaction of sodium carbonate with calcium chloride in the presence of polyvinylpyrrolidone (PVP), sodium dodecylbenzene sulfonate (SDBS), or the mixture of PVP and SDBS, respectively. The morphology and polymorphism of these CaCO 3 were characterized with scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). The results showed that the organic additives and their self-assemblies turn out to be important factors to affect the crystallization habit of CaCO 3. In particular, the controlling effect on the morphology depends upon not only the total concentration of PVP and SDBS, but also the relative concentration ratio of PVP to SDBS Elsevier B.V. All rights reserved. Keywords: Crystallization; Biomimetic mineralization; Calcium carbonate; Morphology; Polymorph 1. Introduction Surfactant aggregates, such as monolayers, foams, micelles, vesicles, emulsions, and lyotropic liquid are interesting templates for the deposition of inorganic compounds [1 5]. These template molecules act as nucleators for inorganic substances and induce oriented nucleation of complementary crystal face. These control crystallization processes by addition of organic templates is of great interests not only for the understanding of the formation of natural biomaterials (e.g., molluscous shells, bones, teeth, and many others), but also for the formation of new hybrid materials from commonly available cheap educts. However, the structural re- Corresponding author. Co-corresponding author. addresses: qshen@sdu.edu.cn (Q. Shen), djwang@iccas.ac.cn (D.-J. Wang) /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.colsurfa lationship between organic matrixes and the inorganic minerals is too ambiguous to be widely used for the biomimetic syntheses of functional composite materials. Calcium carbonate is one of the most abundant minerals in nature as structural materials, which has three crystalline polymorphs (calcite, aragonite, metastable vaterite) and one unstable amorphous phase in synthetic systems [6,7]. The biomimetic synthesis of CaCO 3 with specific size, morphology, structure, brightness, oil adsorption, or the other has received much attention owning to its wide application in such industrial fields as paper, rubber, plastics, paint, etc. [8,9]. In this work, an anionic surfactant of sodium dodecyl benzene sulfonate (SDBS) and an inert polymer of poly(vinylpyrrolidone) (PVP) were used singly or combinatedly as organic additives in the crystallization process of CaCO 3. The crystallization habit (crystal shape and polymorph) of calcium carbonate and the morphological control of these organic additives on the synthesized CaCO 3 were discussed. 转载

2 88 Q. Shen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 251 (2004) Experimental 2.1. Chemicals and solution preparation All chemicals, CaCl 2,Na 2 CO 3, SDBS, and PVP (molecular weight 30,000) are of A.R. grade and used without further purification. Doubly deionized water was used to prepare aqueous CaCl 2 and Na 2 CO 3 solutions just before each experiment. For a reactive crystallization system before mixing, the PVP was added into both of the aqueous CaCl 2 and Na 2 CO 3 solutions, while the SDBS was only added into Na 2 CO 3 solution CaCO 3 precipitation experiments CaCO 3 were precipitated by means of pouring the aqueous CaCl 2 solution (0.1 M, 100 ml) into 300-mL beaker containing the aqueous Na 2 CO 3 solution (0.1 M, 100 ml). The system was continuously stirred at a constant rate of 200 rpm by a Teflon-coated magnetic stirring bar to minimize the heterogeneous nucleation at the glass walls, which was kept under the saturated CaCO 3 solution for 10 h to allow further crystal growth by Ostwald ripening and to see the morphological effects of organic additives. During the precipitation and incubation process, the reactive systems were kept at 26 ± 0.2 C in a thermostatic bath and at a constant ph value ( 7.0). Filtered through cellulose nitrated/acctate membrane filters (0.2 m) and rinsed three times with doubly deionized water, the CaCO 3 were dried in vacuous desiccator cabinet at room temperature for at least 24 h and used for further measurements. The advantage of this method is to eliminate all other affecting factors than the additives Crystal characterization All samples were Au-coated prior to examination by a Hitachi S-4300 scanning electron microscopy (SEM), fitted with a field emission source and operating at an accelerating voltage of 15 kv. The X-ray diffraction (XRD) patterns were collected on a Rigaku D/max-2400 powder X-ray diffractometer with Cu K radiation (40 kv, 120 ma). The step of 0.02 per 25 s and the 2θ ranging from 20 to 60 were selected to analyze the crystal structure and orientation. 3. Results and discussion 3.1. Influence of additives on the morphology of CaCO 3 In the absence of additives, stirring of the medium in a period of 10-h incubation caused the formation of polydispersity of CaCO 3 crystalline particles [10] (Fig. 1A). Under the same condition the addition of 100 g/l PVP led to the formation of rhombohedral (Fig. 1B). Although the obtained have different morphology, however, it is difficult to correlate the activity of PVP in crystallization process with its random-coil conformations. The concentration of surfactants ( mol/l) used here is higher than the critical micellarization concentration (CMC) of SDBS in pure water ( mol/l), and the micellar phases induced the formation of spherical particles (Fig. 1C). Actually, when the SDBS concentration is lower than its CMC value, only rhombohedral particles was obtained in the precipitation system. In aqueous solutions, the presence of PVP and anionic Fig. 1. SEM images of CaCO 3 precipitated from aqueous solution in absence of additives (A) and presence of 100 g/l PVP (B), mol/l SDBS (C) and 100 g/l PVP mol/l SDBS (D), respectively.

3 Q. Shen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 251 (2004) surfactants can form complex at the critical aggregate concentration (CAC, CAC < CMC) [11], which indicates that the PVP/SDBS complex forms in the aqueous system of 100 g/l PVP mol/l SDBS. The CaCO 3 precipitated from the aqueous PVP/SDBS system acquired schistose structures with irregular edges (Fig. 1D) XRD characterization of the synthesized CaCO 3 Fig. 2 shows the typical X-ray diffraction patterns of synthetic CaCO 3. As for the crystal habits of CaCO 3 polymorphs, the results of Fig. 2 have good accordance with that of Fig. 1. Fig. 2A exhibits the characteristic reflection of samples shown in Fig. 1A or B, which corresponds to calcite (d-spacings (Å): 3.87, 3.04, 2.84, 2.50, 2.29, 2.10, 1.93, 1.91, 1.88, 1.63, 1.60; corresponding to hkl: 0 1 2, 1 0 4, 0 0 6, 1 1 0, 1 1 3, 2 0 2, 0 2 4, 0 1 8, 1 1 6, 2 1 1, 1 2 2, respectively). Fig. 2B shows the typical reflection for vaterite (dspacings (Å): 4.24, 3.58, 3.30, 2.73, 2.32, 2.12, 2.07, 1.85, 1.82, 1.65; corresponding to hkl: 004, 110, 112, 114, 2 1 1, 0 0 8, 3 0 0, 3 0 4, 1 1 8, 2 2 4, respectively) corresponding to the spherical shown in Fig. 1C. The precipitated from the aqueous system of 100 g/l PVP mol/l SDBS (Fig. 1D) showed the XRD pattern for the coexistence of calcite and vaterite (Fig. 2C), although no crystalline particles of rhombohedral calcite and spherical vaterite were observed Relationship between the self-organized structures of additives and the morphologies of the resulted CaCO 3 The different crystalline morphologies of CaCO 3, together with the different structural properties of additives, suggest different crystallization habit of calcium carbonate, which was schematically shown in Fig. 3. At ambient conditions, vaterite was initially the only solid phase precipitated from aqueous systems, followed by the dissolution of vaterite and at mean time the crystallization of calcite [12 15]. However, either the dissolution of vaterite or the crystallization step of Fig. 3. Schematic depiction for the morphological effect of surfactant micelle, polymer coil, and polymer/surfactant complex on the CaCO 3. calcite could be the rate-controlled process according to the experimental conditions [16 19]. In despite of controlling steps of the transformation rate from vaterire to calcite, the presence of SDBS molecules ( mol/l) was proved to play an important role on the crystallization habit, preventing the transformation from vaterite to calcite [14,15,17,18] and only resulting spherical vaterite, due to the strong electrostatic interaction between calcium ion and the polar-head of SDBS molecule (Fig. 3A). The inert polyamide polymer of PVP interacts with many inorganic materials, such as metallic salt [20], metal oxide [21], and kaolinite [22], showing unusual colloidal and complexing properties. So, as an effective stabilizer of colloidal particles in the precipitation process, 100 g/l PVP showed the morphological effectiveness and favored the formation of rhombohedral calcite (Fig. 3B). Despite the oriented nucleation effectiveness of organic additives [23], the structural properties of PVP/SDBS complex, especially the hydrophobic interactions between uncharged polymer and anionic surfactant molecules, were enhanced by the 0.1 mol/l NaCl resulted from precipitation reaction of CaCl 2 and Na 2 CO 3 [11]. Because the resulted NaCl has no influence on the crystallization habit of CaCO 3, due to its lower supersaturation than that of calcium carbonate [12 19], the obtained schistose CaCO 3 from the precipitation system containing 100 g/l PVP mol/l SDBS was attributed to the presence of the self-organized structure of PVP/SDBS complex (Fig. 3C) Morphologies of the CaCO 3 precipitated at different ratio between PVP and SDBS Fig. 2. Powder X-ray spectra of the crystalline CaCO 3 precipitated from 100 g/l PVP (A), mol/l SDBS (B), and 100 g/l PVP mol/l SDBS (C), respectively. Due to the hydrophobic association of the SDBS molecules and the intermolecular interactions of PVP to SDBS, the aggregation state of PVP/SDBS complexes depend on the concentration ratio between PVP and SDBS. The crystalline CaCO 3 controlled by the different aggregation state of PVP/SDBS complexes was shown in Fig. 4. Although the sampled CaCO 3 exhibited the same XRD characteristics for metastable vaterite or thermodynamically stable calcite, their morphologies were

4 90 Q. Shen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 251 (2004) Fig. 4. SEM images of the crystalline CaCO 3 precipitated from the aqueous systems of 100 g/l PVP mol/l SDBS (A), 50 g/l PVP mol/l SDBS (B), 1 g/l PVP mol/l SDBS (C), and 1 g/l PVP mol/l SDBS (D), respectively. quite different from that of the CaCO 3 shown in Fig. 1D. At low SDBS concentrations, single SDBS molecule starts to be bound to a hydrophobic site of PVP molecules, and the increase of surfactant concentration causes several SDBS molecules to be bound to each hydrophobic site until the formation of SDBS beads in PVP chains. Consequently, the bead-bead electrostatic repulsion will occur, so each polymer molecule can accommodate only a limited number of beads. Then, new amphiphilic molecules added will go into the solution again until the normal SDBS micelles are formed. When SDBS concentration is decreased, the structural rigidity of the PVP (100 g/l)/sdbs ( mol/l) complex becomes weaker. The random-coil of PVP molecules with semi-beads of SDBS molecules in their hydrocarbon chains (100 g/l PVP mol/l SDBS) will induce the formation of rhombohedral calcite with porous structures (Fig. 4A), while in presence of 50 g/l PVP mol/l SDBS, the grown CaCO 3 acquire another morphology of rhombohedral calcite (Fig. 4B). In the precipitation system of 1 g/l PVP mol/l SDBS, the superstructure of the PVP/SDBS complexes are quantitatively few or none, meaning that the SDBS micelles mainly dominated the crystallization process of spherical vaterite (Fig. 4C). The coexistence of the PVP/SDBS complexes and SDBS micelles induced the formation of ellipsoidal calcite, schistose calcite, and spherical vaterite in the precipitation system of 1 g/l PVP mol/l SDBS (Fig. 4D) Morphology of the spherical CaCO 3 The comparison of Fig. 1C and Fig. 4C also shows different morphologies of spherical vaterites, implying the synergistic effect of PVP and SDBS on the morphological control of CaCO 3. The spherical CaCO 3 prepared from different precipitation systems were selected and shown in Fig. 5. The vaterite sphere obtained from mol/l SDBS micellar system shows smooth surface (Fig. 5A); while the spherical calcite, the aggregate of nano-sized rhombohedrons, sampled from the precipitation solution of 1 g/l PVP exhibits relatively rough surface (Fig. 5B). The surface of the vaterite sphere obtained from the crystallization system containing 1 g/l PVP mol/l SDBS shows moderate roughness (Fig. 5C). Fig. 5. SEM images of spherical CaCO 3 aggregates precipitated from the aqueous systems of mol/l SDBS (A), 1 g/l PVP (B), and 1 g/l PVP mol/l SDBS (C), respectively.

5 Q. Shen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 251 (2004) It has been shown in our previous work [19] that the mesoscale CaCO 3 sphere is composed of nano-sized particles, and the addition of PVP into CaCO 3 precipitation system increases the transformation rate from vaterite to calcite. So far as the spherical CaCO 3 is concerned, the addition of PVP molecules into SDBS micellar system caused the surface of spherical vaterite to be more rough, due to the adsorption of PVP molecules onto the nano-sized particles. Although the crystallization of calcium carbonate in the presence of 1 g/l PVP occasionally resulted in spherical aggregate of calcite, the rough surface of spherical calcite (Fig. 5B) compared to the relatively smooth surface of the spherical vaterite (Fig. 5A or C) supports the conclusion that PVP molecules can not only be absorbed at the crystal surface, but also play the role of accelerator for the transformation from vaterite to calcite. 4. Conclusions In summary, we have investigated the crystallization of calcium carbonate in the presence of PVP, SDBS, or the mixture of PVP and SDBS. Spherically shaped CaCO 3 vaterites were prepared from the SDBS micellar solution, while rhombohedral calcites were obtained from the aqueous system of 100 g/l PVP. The mixture of PVP and SDBS resulted in schistose calcite, ellipsoidal calcite, spherical vaterite, and/or the other, depending both upon the total concentration of organic additives and on the concentration ratio of PVP to SDBS. The synergistic effect of PVP molecules and SDBS in the reactive crystallization systems controls the crystallization habit of calcium carbonate by affecting the morphology and polymorph of the investigated. Acknowledgements This work was partially supported by the Excellent Young Scientist Fund from Shandong Province (02BS115), and the Foundation from Ministry of Science and Technology of China (2003CCA02900) and from the State Key Laboratory of Polymer Physics and Chemistry. References [1] J. Lahiri, G. Xu, D.M. Dabbs, N. Yao, I.A. Aksay, J.T. Groves, J. Am. Chem. Soc. 119 (1997) [2] D. Walsh, S. Mann, Nature 377 (1995) 320. [3] D. Walsh, B. Lebeau, S. Mann, Adv. Mater. 11 (1999) 324. [4] J.J.J. Donners, B.R. Heywood, E.W. Meijer, R.J.M. Nolte, C. Roman, A.P.H.J. Schenning, N.A.J.M. Sommerdijk, Chem. Commun. (2000) [5] P.V. Braun, P. Osenar, V. Tohver, S.B. Kennedy, S.I. Stupp, J. Am. Chem. Soc. 121 (1999) [6] K. Naka, D.-K. Keum, Y. Tanaka, Y. Chujo, Chem. Commun. (2000) [7] E. Loste, F.C. Meldrum, Chem. Commun. (2001) 901. [8] E. Dalas, P. Klepetsanis, P.G. Koutsoukos, Langmuir 15 (1999) [9] Y. Wen, L. Xiang, Y. Jin, Mater. Lett. 57 (2003) [10] H. Tsuno, H. Kagi, T. Akagi, Bull. Chem. Soc. Jpn. 74 (2001) 479. [11] P. Purcell, R.K. Thomas, J. Penfold, A.M. Howe, Colloids Surf A 94 (1995) 125. [12] D. Kralj, L. Brecevic, J. Kontrec, J. Cryst. Growth 177 (1997) 248. [13] N. Spanos, P.G. Koutsoukos, J. Cryst. Growth 191 (1998) 783. [14] A. Katsifaras, N. Spanos, J. Cryst. Growth 204 (1999) 183. [15] F. Manoli, E. Dalas, J. Cryst. Growth 218 (2000) 359. [16] A. Katsifaras, N. Spanos, J. Cryst. Growth 204 (1999) 183. [17] F. Manoli, E. Dalas, J. Cryst. Growth 217 (2000) 416. [18] P. Malkaj, E. Dalas, J. Cryst. Growth 242 (2002) 405. [19] H. Wei, Q. Shen, Y. Zhao, D.-J. Wang, D.-F. Xu, J. Cryst. Growth 250 (2003) 516. [20] M. Liu, X. Yan, H. Liu, W. Yu, Reactive Functional Polym. 44 (2000) 55. [21] M. Zheng, M. Gu, Y. Jin, H. Wang, P. Zu, P. Tao, J. He, Mater. Sci. Eng. B 87 (2001) 197. [22] K.A. Carrado, L. Xu, Microporous Mesoporous Mater 27 (1999) 87. [23] J.J.J.M. Donners, R.J.M. Nolte, N.A.J.M. Sommerdijk, J. Am. Chem. Soc. 124 (2002) 9700.