Polymorph Control of Calcium Carbonate Using Insoluble Layered

Article pubs.acs.org/crystal

Polymorph Control of Calcium Carbonate Using Insoluble Layered Double Hydroxide Arpita Sarkar, Kingshuk Dutta, and Samiran Mahapatra* Unilever R & D Bangalore, 64 Main Road, Whitefield, Bangalore-560066, India ABSTRACT: This paper demonstrates crystallization of calcium carbonate in the presence of insoluble Mg−Al layered double hydroxides (LDH). The metastable aragonite phase is preferentially stabilized over the thermodynamically more stable calcite phase in the presence of this insoluble inorganic matrix at ambient temperature. Such aragonite particles possess an unconventional “rosette-like” morphology. In the intermediate stages of crystallization, another metastable phase, vaterite, having a “flaky-floret-like” morphology, is initially formed, and an unusual transformation of ‘vaterite to aragonite’ is observed. The kinetics of such a transformation is systematically studied and captured through their crystal characterization. The presence of Mg-rich layers and the surface hydroxyl groups in LDH is postulated as one of the probable factors for such phase selectivity. The role of “soluble” Mg2+ ions in affecting polymorph selectivity is ruled out based on the very low concentration of Mg2+ ions leaching out of LDH. This is the first report of the use of Mg−Al LDH as an “insoluble inorganic material” to promote polymorph selectivity in calcium carbonate. This can also have potential significance in biomineralization.

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serves as skeletal support.5 Although these metastable phases are difficult to synthesize under ambient laboratory conditions, in the biological systems, aragonite, for example, is naturally crystallized in the presence of various proteins and other biopolymers.6 Polymorph control is guided by the complex interplay between thermodynamic and kinetic parameters. Stabilization of metastable phases such as vaterite and aragonite has been elucidated in several reports via the incorporation of suitable soluble or insoluble organic or inorganic additives. For example, organic additives such as dopamine, poly(styrene sulfonate), poly(glutamic acid), and block copolymers are known to stabilize the vaterite phase, and aragonite is formed in the presence of poly(vinyl alcohol) and alginate.7 Among various soluble inorganic additives, Li+, Mg2+, Sr2+, Cu2+, Cd2+, and Cr6+ ions have also been used to control the polymorph selectivity of CaCO3.8 In addition to soluble additives, organic insoluble matrices, such as eggshell membranes, polymer brushes, and self-assembled monolayers (SAMs), have been investigated to mimic the process of biomineralization.9 In contrast, very few reports deal with the influence of “insoluble inorganic materials” on phase selectivity and morphology control of CaCO3. Teng and co-workers reported the effect of solid-state oxides and silicates on CaCO3 crystallization, analogous to the effects imparted by the −OH and −COOH in organic molecules.10 In another recent report, Kuroda and co-workers have described stabilization of vaterite via an

INTRODUCTION Polymorphism of crystalline substances has long been considered as a key area of research. Such ability of a chemical system to adopt multiple crystal structures has tremendous technological importance in terms of its differential photochemical, thermodynamic, and optical properties based on the solid-state structures. Accordingly, studies on polymorphic control of crystalline materials are both fascinating and significantly relevant in pharmaceutical, food, paint, agrochemical, and various other fields.1 Calcium carbonate (CaCO3), being one of the most important naturally occurring polymorphic biominerals, serves a pivotal role in understanding not only the mechanism of biomineralization, but it also helps to replicate the key fabrication strategies and structural features found in marine organisms into new materials design. In particular, there still exists a long-standing challenge of predicting and controlling the occurrence of polymorphism.2 CaCO3 occurs in three anhydrous crystalline polymorphic forms, namely calcite, aragonite, and vaterite.3 Calcite is the most stable phase under ambient conditions, aragonite usually forms at elevated temperature, and vaterite is particularly the most unstable phases.4 This, in turn, leads to the application of more stringent synthetic conditions, such as elevated temperature and pressure, and use of nonaqueous solvents and additives, to facilitate the selective precipitation of the desired metastable phase. In the context of biomineralization, selection of polymorphs is one of the key issues as material properties change from one form to the other. For example, metastable phases of CaCO3, such as aragonite and vaterite, are more frequent in biological systems, such as in mollusks or in coccolithophores, where it © 2012 American Chemical Society

Received: September 18, 2012 Revised: October 30, 2012 Published: December 3, 2012 204

dx.doi.org/10.1021/cg301368v | Cryst. Growth Des. 2013, 13, 204−211

Crystal Growth & Design

Article

intermediate ACC phase on insoluble mesoporous silica material.11 Herein, we report a facile method for polymorph control of CaCO3 mediated by insoluble MgAl-layered double hydroxide (MgAl-LDH). LDHs are flat 2-D anionic clays consisting of octahedral layers of positively charged mixed metal hydroxides [(MII,MIII)(OH)x] with interlayer galleries occupied by An− and H2O molecules that bind the layers together.12 The chemical composition of LDH is described by the general formula [MII1−xMIIIx(OH)2]x+(An−)x/n·mH2O (MII = divalent metals such as Zn, Mg, Fe, CO, Ni; MIII = trivalent metals such as Al, Cr, Ga; An− = anions such as CO32−, SO42‑, NO3−, Cl−). LDH finds various applications in biology and catalysis owing to the high surface area and effective adsorption capability of reactant molecules onto the active surface-directed catalytic centers. In the present study we have observed that LDH triggers the formation of aragonite over calcite at ambient temperature. In the intermediate stages of crystallization, vaterite is formed which gradually transforms to aragonite in the presence of LDH. The aragonite sample is quite stable and does not undergo further transformation to calcite. To the best of our knowledge, this is the first report that describes the influence of insoluble LDH particles on the polymorph selectivity of CaCO3.

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Figure 1. Powder X-ray diffraction pattern and SEM images (inset) of Mg−Al LDH particles.

the literature.14 For samples containing all three polymorphscalcite, vaterite, and aragonitethe following formula was used:

XV =

EXPERIMENTAL SECTION

7.691(IV110) IC104

+ 7.691(IV110) + 3.157(IA 221)

XC + XV + XA = 1

Materials. CaCl2·2H2O (AR grade), Mg(NO3)2·6H2O (AR grade), and Al(NO3)3·9H2O (AR grade) were obtained from Merck (India) and used as received. Na2CO3 (anhydrous, AR grade) and (NH4)2CO3 (anhydrous, AR grade) were also procured from Merck (India). Aqueous solutions of CaCl2, Mg(NO3)2, Al(NO3)3, and Na2CO3 were made using distilled water. Distilled water used in all reactions had the following characteristics: pH 7.1, electrical conductance 2 × 10−6 S cm−1, total dissolved salt pHpzc. Thus, the LDH particles under the present crystallization conditions (pH of growth medium ∼ 9− 9.5) carry a negative or near-zero surface charge. We believe that the negatively charged surface of LDH is responsible for promoting the formation of the metastable Vaterite phase with a “flaky-floret” like morphology in the initial stages of crystallization. As time progresses, this metastable vaterite phase gradually dissolves and recrystallizes as “rosette-like” aragonite particles. Considering the solubility products of the different polymorphs (vaterite, 1.22 × 10−8 M2; aragonite, 4.61 × 10−9 M2; calcite, 3.31 × 10−9 M2 at 25 °C, respectively),3a the phase transformation of vaterite to the aragonite phase is a thermodynamically feasible process. However, such polymorph transformation is uncommon. In accordance with the results discussed, the insoluble inorganic matrix with interactive surface functional groups plays

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-8039831078. Fax: +91-80-2845-3086. Notes

The authors declare no competing financial interest.

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REFERENCES

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