Controlled Mineralization of Calcium Carbonate on the Surface of

Calcium carbonate (CaCO3),(1) as one of the most ubiquitous existing biominerals for hard tissues in seashells and coccolith,(2) has extensive applica...
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Controlled Mineralization of Calcium Carbonate on the Surface of Nonpolar Organic Fibers Jian Yang,†,‡ Yuhai Liu,† Tao Wen,† Xiaoxiao Wei,† Zhiyong Li,† Yuanli Cai,*,‡ Yunlan Su,*,† and Dujin Wang† †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China S Supporting Information *

ABSTRACT: Isotactic polypropylene (iPP) fiber, the surface of which is hydrophobic, can modulate the crystallization polymorphs of calcium carbonate (CaCO3) at the air/solution interface under mild conditions. The present results provide a novel perspective on controlling the crystallization of biominerals by an insoluble matrix, and they can shed new light on understanding the biomineralization process of CaCO3 as it occurs in nature.

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nucleation and growth, modulate the crystal shape and size, and control the organization of nanoscale building blocks into complex structures through molecular recognition under mild conditions.6 Many soluble organic additives such as surfactants,7 biomolecules,8 and some half-soluble organic matrices such as double hydrophilic block copolymers (DHBCs) have been implemented into in vitro synthesis.9 Such efforts mentioned above are mainly focused on the influence of organic functional groups, including the hydroxyl group, carboxyl group, amino group, phosphate, sulfates, and so on, and it is generally believed that the regulation of organic components on crystals is attributed to their adsorption onto crystals by the functional groups. The insoluble matrices, such as collagen and chitin, are biomacromolecules and form a threedimensional scaffold for facilitating the nucleation of calcium salt crystals. However, very little effort has been put forth to investigate the roles played by the insoluble matrix without any functional groups in the development of highly ordered hierarchical materials.10 From the traditional point of view, the primary function of an insoluble organic matrix is to subdivide the mineralization compartment into an organized network of microcompartments and thus to delimit the available room for crystal growth and/or to constrain the crystal packing arrangement to a certain extent. At the surface of an insoluble organic matrix, usually some macromolecular assembly may serve as a supramolecular template for oriented nucleation of a single crystal, and it has been proven that different polymorphs of CaCO3 can switch between each other by certain experiments in which a reconstructed matrix of

iominerals, such as bones, teeth, and shells, exhibit elaborate architectures and remarkable physical properties because of their complex hierarchical designs optimized over hundreds of millions of years. Calcium carbonate (CaCO3),1 as one of the most ubiquitous existing biominerals for hard tissues in seashells and coccolith,2 has extensive applications in various fields, such as drug delivery, bone implants, plains, plastics, rubbers, and paper.3 Calcium carbonate makes an attractive model biomineral for studies in the laboratory because its crystals are easily characterized, and the morphology of CaCO3 has been the subject to control in biomineralization processes. Calcium carbonate is also a material of considerable industrial interest, and the study of the factors that influence its formation has a long history. The underlying control over CaCO3 precipitation is moreover crucial to prevent the major industrial problem of scaling in fabrics and water pipes. Calcium carbonate generally exists in three anhydrous polymorphs, i.e., calcite, aragonite, and vaterite. Calcite is the most stable polymorph, while vaterite is the least stable one at room temperature and pressure. In mollusk shells, inorganic components are dominant with over 95% of the mass or volume. Those in the prismatic layer are [001] oriented calcite crystals, and those in the nacreous layer are c-axis-oriented aragonite tablets. Controlling the polymorphs, morphologies, and textures of CaCO3 during its crystallization process has been an interesting topic for better understanding the mineralization mechanism of such a fundamental inorganic crystal.4 In nature, organisms fabricate inorganic single crystals with occluded proteins and polysaccharides, resulting in biominerals with improved mechanical properties.5 During the fabrication process, organic components utilized on the basis of the solubility properties are believed to regulate the crystal © 2011 American Chemical Society

Received: September 13, 2011 Revised: November 15, 2011 Published: November 28, 2011 29

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purified mollusc shell macromolecules is employed.11 In the present work, therefore, in the presence of an insoluble matrix, isotactic polypropylene (iPP) fiber, we conducted calcium carbonate crystallization experiments in order to investigate whether a hydrophobically organic synthetic fiber can have an effect on the crystallization of CaCO3. As shown in Figure 1, different polymorphs of CaCO3 are formed in the solutions at different initial pH values. It is

Figure 2. FTIR spectra of CaCO3 crystals: (a) vaterite grown on the iPP fibers at the initial pH = 3.0; (b) vaterite and calcite grown at the interface of air/water at the initial pH = 3.0; (c) calcite and vaterite (predominating calcite) grown on the iPP fibers at the initial pH = 5.6; (d) calcite and vaterite (predominating calcite) grown at the interface of air/water at the initial pH = 5.6.

of vaterite on the iPP fibers at initial pH = 3.0 indicates that the nucleation of vaterite is favored (for kinetic reasons only) with respect to that of calcite and aragonite. It is generally recognized that the presence of the hydrogen chloride cannot affect the thermodynamically stable calcite crystal structure.12 Therefore, the stabilization of vaterite is attributed to the presence of iPP fiber. Since the iPP fiber can exert a strong influence on the preferential formation of different CaCO3 polymorphs, the feature of the iPP fiber itself is of vital importance. The FTIR spectrum of used iPP fibers shows good purity without any functional group (Figure S2 of the Supporting Information), and only very little amount of oxygen and other elements is detected by X-ray photoelectron spectroscopy in the fiber (Table S1), indicating the nonexistence of functional groups. Since the iPP fibers without any functional groups show the ability to discriminate the polymorphs of CaCO3 only in acid solution, it is speculated that the changed surface hydrophilicity of iPP fibers may play an important role in controlling the polymorphs of CaCO3 under an acidic environment. To confirm this speculation, the contact angle measurements have been performed for the iPP fibers soaked in solutions at different pH values (Figure 3), and the relative results are listed in Table 1. It is obvious that increasing pH values will lead to the decrease of hydrophilicity of iPP fibers, resulting in the transformation of fibers from hydrophilic (pH = 3.0, θm = 82.9°) to hydrophobic (pH = 5.6, θm = 111.0°). The calculation of adhesional work (Wsl) between the solution and the iPP fiber is given by the synthesis of Dupré’s formula and Young’s equation:13

Figure 1. Typical SEM images of (a, b) the dominating vaterite phase growing on the iPP fiber at initial pH = 3.0 and (c, d) the dominating calcite phase growing on the iPP fiber at initial pH = 5.6 at the air/ solution interface. The total reaction time was 24 h.

manifest that the crystal phase of CaCO3 on the iPP fibers can be perfectly controlled simply by tuning the initial pH value of the solution. Beautiful necklace-like vaterite emerges at the initial pH = 3.0 (Figure 1a and b), while both calcite and vaterite are obtained at the air/solution interface without iPP fibers at pH = 3.0 (Figure S1a of the Supporting Information). However, almost only (104) rhombohedra of calcite were observed on the iPP fibers when the initial pH is 5.6 (Figure 1c and d), and as a comparison, two kinds of polymorphs (calcite and vaterite) were obtained at the air/solution interface without iPP fibers (Figure S1b) at pH = 5.6. Therefore, in addition to the pH value, which can greatly affect the formation of different CaCO3 polymorphs, the existence of nonpolar iPP fibers on the solution surface also exerts an important influence on modulating the transformation of different crystal phases, and in the designed experimental time period (24 h), vaterite was inhibited to convert into stable calcite. To further confirm the crystal phases of CaCO3 grown at different pH values with or without iPP fibers, Fourier transform infrared (FTIR) measurements were carried out (Figure 2). As shown in Figure 2a, three characteristic peaks at 746, 877, and 1088 cm−1 indicate the emergence of vaterite, while the 712 cm−1 peak characteristic of calcite was hardly observed when the crystal was obtained at the initial pH = 3.0 on the iPP fibers (Figure 2a). When the iPP fibers were absent at the air/solution interface, two polymorphs of CaCO3 were observed at the initial pH = 3.0 (Figure 2b), which is quite different from the result when the iPP fibers were present at pH = 3.0. If the initial pH was set at 5.6, predominating calcite was formed on the iPP fibers (Figure 2c) and at the air/solution interface without iPP fibers (Figure 2d). Hence, FTIR results are in good agreement with SEM observations. The formation

Wsl = γl(1 + cos θ)

(1)

where γl is the surface tension of the solution and θ is the contact angle between solution and fiber. Table 1 and Figure 3 clearly show that the adhesional work (Wsl) decreases with increasing pH value from 3.0 to 5.6, and the acid-treated fibers become more hydrophilic with stronger polarity. In acid solution, the surface tension of the iPP fibers changed due to the adsorption of the acid. During the crystallization process, it seems that a slight increase of the polarity and surface tension of iPP fibers may sufficiently 30

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Figure 3. Shapes of the water droplets on the iPP fibers soaked in solutions of (a) pH = 3, (b) pH = 3.5, (c) pH = 4.0, and (d) pH = 5.6.

Table 1. Contact Angle (θ) and Adhesional Work of iPP Fiber (Wsl) Soaked in Solutions of Different pH Measured at 20 °Ca θ (deg)

pH 3.0 3.5 4.0 5.6 a

81.9 98.3 97.9 111.6

83.4 94.6 99.2 111.4

83.9 89.3 99.8 109.1

83.0 98.2 103.7 109.9

82.3 95.4 102.2 113.1

θm (deg)

σ

Wsl (mN/m)

82.9 95.2 100.6 111.0

0.72 3.28 2.10 1.40

82.0 66.4 59.6 46.8

(The surface tension of water at 20 °C is 73 mN/m.) θm is the mean value of five observations, and σ is the standard deviation.

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ASSOCIATED CONTENT S Supporting Information * Experimental procedures for the synthesis of CaCO3 crystals; SEM images of crystals (Figure S1, Figure S3, Figure S4); iPP fiber’s properties (Figure S2, Table S1); and contact angles of droplets on the iPP fibers soaked in the solutions of acetic acid (Table S2). This material is available free of charge via the Internet at http://pubs.acs.org.

stabilize the metastable crystal phase of CaCO3, preferentially inducing the formation of vaterite on the surface of iPP fibers. It is possible that the nucleation of vaterite is favored with respect to that of other calcium carbonates because the iPP/ vaterite interfacial energy is lower than that of iPP/calcite and iPP/aragonite in acid solution. Such a condition promotes the heterogeneous nucleation of vaterite on the iPP fibers, according to the classical nucleation theory.14 Therefore, vaterite is easy to be stabilized on the iPP fibers in acidic solution, not in neutral solution. As mentioned above, since the hydrophilicity of iPP fibers is originated from the acidic condition, the acid concentration should be intimately related to the preferential formation of different CaCO3 polymorphs. Hence, the time-dependent experiments on the crystallization process were carried out to examine the ultimately stable form of CaCO3 crystals (Figure S3 of the Supporting Information). As observed, after about 240 h, vaterite has totally transformed into calcite. It is suggested that the acid concentration decreases to a quite low level due to the formation of CaCO3 crystals, and thus, the hydrophilicity of iPP fibers could not be maintained any longer, which weakens the stabilization of vaterite by the iPP fiber. Under standard conditions, the vaterite phase nucleated and grown from pure aqueous solution is unstable and transforms to stable calcite, unless the stabilization occurs thanks to the variation of the surface tension in solution, due to the specific impurity adsorption. Considering that the preferential growth of vaterite on the iPP fibers is attributed to the decrease of the iPP/vaterite interfacial energy under acidic conditions, it is possible that, besides HCl, other acids can have a similar effect on the controlled crystallization of CaCO3. Therefore, acetic acid was chosen and similar experiments were conducted under the same conditions; the results were similar to those we predicted (Figure S4 and Table S2 of the Supporting Information). In conclusion, we have demonstrated for the first time that insoluble matrix−iPP fibers with no functional groups can discriminate polymorphs of CaCO3 in aqueous solution under mild conditions. The thermodynamic and kinetic regimes, which contribute to the polymorph control, can be manipulated in solution simply by tuning the pH value of the reaction system. The present study may further provide clues for a deeper understanding of the biomineralization process of CaCO3 as it occurs in nature.

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AUTHOR INFORMATION Corresponding Author *Y.S.: fax, +86-10-82612857; telephone, +86-10-82618533; email, [email protected]. Y.C.: fax, +86-512-65884419; telephone, +86-512-65884419; e-mail, [email protected].

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ACKNOWLEDGMENTS The financial support from the National Basic Research Program of China (2009CB930802) is gratefully acknowledged. J.Y. thanks professor Lei Jiang and Kan Li for the test of contact angles and thanks Dongsheng Fu for the preparation of the paper.

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