Transport-Mediated Control of Particles of Calcium Carbonate

DOI: 10.1021/cg900009j

Transport-Mediated Control of Particles of Calcium Carbonate Ranjith Krishna Pai, Kjell Jansson, and Niklas Hedin*

2009, Vol. 9 4581–4583

Materials Chemistry Research Group, Department of Physical, Inorganic and Structural Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden Received January 5, 2009; Revised Manuscript Received September 23, 2009

ABSTRACT: Micrometer-sized particles of calcium carbonate were formed by adding NaHCO3 (aq) to a buffered aqueous solution of CaCl2 (aq) and polyelectrolytes. Particle morphology and crystal polymorphology were tuned by varying the stirring rate. Vigorous stirring led to the formation of micrometer-sized spherical particles of nanocrystalline vaterite; slow stirring formed rhombohedral nanostructured particles of calcite.

*To whom correspondence should be addressed. E-mail: niklas.hedin@ inorg.su.se.

particles contained 3 wt % of the polymer, which corresponds to about 6-7 vol %. When slow stirring (50 rpm) was employed during the mixing, rhombohedral particles formed (Figure 1). The SEM image in Figure 1 (top left) shows a uniform particle distribution of rhombohedra with a characteristic length of about 10 μm. The SEM image in Figure 1 (top right) shows surface texture on a rhombohedra. The internal structure of the rhombohedra was imaged by SEM after cross-sectional polishing using an Ar-beam polisher. The obtained surface texture is in agreement with previous studies.15 One may speculate that this texture is related to chemical “etching,” as previous studies have reported that similar features become more noticeable after lengthy treatment in reactive environments.16-18 No internal porosity was observed (Figure 1, bottom left). The random pattern of light and dark areas in a transmission electron microscopy (TEM) image (Figure 1, bottom right) reveals that the rhombohedra are not single crystals. The nanosized features of 5-10 nm are most visible near the interfaces of the crushed particles. Please note that these interfaces are not identical with those in the Figure 1: bottom, left. X-ray diffraction (XRD) experiments confirm that the rhombohedra are calcite (see the Supporting Information for XRD diffractograms and additional TEM images). The electron microscopy data reveal that the rhombohedra consist of small (5-10 nm) nanocrystallites of calcite and large volumes of ordered calcite. The occurrence of two different arrangements of calcite inside the “cubelike” particles could explain the fact that the XRD lines are narrow for this sample. Similar calcite particles have been prepared in previous studies under a wide range of conditions, and are well explained in terms of the concept of mesocrystals.2 Spherical particles formed when the mixing involved rapid stirring (1000 rpm) during the double decomposition reaction. All the other conditions (i.e., concentrations, temperature, mixing time, and aging time) were identical among the different experiments. The SEM image in Figure 2 (top left) shows spherical particles (average diameter, about 10 μm) with a uniform distribution. The texture on the surface of the spherical particles (Figure 2, top left) is less pronounced than that on the rhombohedra (cf. Figure 1). The TEM images in Figure 2 (bottom) show features typical of vaterite particles synthesized at slightly elevated pH conditions in the presence of additives.3,4,19 The interfaces of crushed particles are smooth and covered with small particles (∼30 nm; Figure 2, bottom left). Dark-field TEM images (using the information from the diffractogram marked by a circle in the inset in Figure 2, bottom right) show streaks that indicate an ordered arrangement of crystals less than 30 nm in size. XRD experiments confirmed that the spheres are largely vaterite (see Supporting Information).

r 2009 American Chemical Society

Published on Web 09/30/2009

In living organisms, calcium carbonate is the most common mineral, and it plays, for example, a significant role in the mollusc shell.1 These particles are structured on the nanoscale and higher, providing skeletal support to the organism. Recent advances in attempts to mimic these carbonates have raised the possibility of the environmentally friendly production of carbonates with internal structures on many scales.2 In this mimicking, polymers or surfactants are used as structure-directing agents. Such organic additives determine the shape of calcium carbonate crystals as well as the polymorphs of calcium carbonate.3,4 Calcium carbonate may nucleate via traditional mechanisms in solutions or via nontraditional mechanisms. Particles of calcium carbonate have been shown to form by the aggregation of small nanoparticles into larger particles structured on many length scales;2 such particles are expected to yield new properties. In the present work, we highlight the importance of mass transport during the synthesis of calcium carbonate at high concentrations of reactants and high temperatures in the presence of a copolymer. In particular, we show that mass transport controls the formation of calcite versus vaterite in these solutions. Calcite is the most stable polymorph of calcium carbonate.5 The polymorphs aragonite, vaterite, and amorphous calcium carbonate are energetically disfavored; however, the differences in free energy are relatively small among the different forms. Certain molecules or ions (e.g., anionic polyelectrolytes) stabilize the otherwise unstable polymorph of vaterite.6-9 In addition to the various specific effects of additives, one would expect colloidal particles of calcium carbonate to be stabilized in the presence of adsorbed polyelectrolytes.10 Aqueous gels containing polyacrylamide or related polyelectrolytes swell at high temperatures, related to the increased solubility of the acrylamide moiety in water at high temperatures.11,12 Hence, a similar enhanced swelling and increased viscosity is expected for the polymer employed in this study, poly(acrylamide-co-sodium-2-acrylamido-2-methylpropane-sulfonate) [Poly(AA-NaAMPS)]. Further characterization of the polymer can be found in a recent study. In water, this polymer forms very strong gels at high temperatures.13 The calcium carbonate particles shown in Figures 1 and 2 formed via a double decomposition14 reaction by mixing two aqueous solutions at a temperature of 60 °C. A sodium bicarbonate solution was added to a solution of CaCl2, Poly(AANaAMPS), and Tris (see Supporting Information for experimental details). The mixed solutions were stirred for 30 min; the mixture turned opaque almost instantly. Thereafter, the dispersions were aged for 24 h under stagnant conditions. The resulting

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Figure 1. Top: SEM images of rhombohedra formed by a double decomposition reaction under slow stirring. Bottom left: crosssectional SEM image; bottom right: TEM image showing the nanostructured nature of the particles. All particles are calcite with a small amount of polymer included.

Figure 2. Top: SEM images of spherical particles formed by a double decomposition reaction under rapid stirring during mixing. A small amount of polymer was included. Bottom left: TEM micrograph of a crushed particle of vaterite. Bottom right: darkfield TEM image of vaterite (the inset shows an electron diffractogram; the size and the position of used DF-mode aperture is marked).

Vaterite typically converts to calcite in aqueous solutions in the absence of special stabilizing conditions. The vaterite particles shown in Figure 2 were stable after 7 months of storage in an ambient atmosphere. Such prolonged stability, also observed previously in similar systems,20 and is not surprising in the form of a dry powder. A moderate stirring rate (300 rpm) during the double decomposition reaction resulted in a mixture of spherical vaterite particles and rhombohedral calcite particles. In combination with the findings obtained at low and high stirring rates, this result corroborates the transport-mediated control of the formation of these nanostructured calcite and vaterite particles. In control experiments performed without added polyelectrolyte (but with all other conditions being identical), calcite particles formed via the double decomposition reaction at all stirring rates. To explain the control of mass transport on the polymorphology of the nanostructured particles, we separately discuss the

Pai et al. nucleation and growth (aggregation) of the particles. A priori, it is possible that the convective streams in the reaction vessel influence both nucleation and aggregation. In a related study, a very similar polymer has been shown to promote the initial formation of amorphous calcium carbonate.14 This amorphous preamble appears to transform to vaterite, in case of rapid stirring. The guiding principles of Ostwald step rule21 may be valid here. This rule states that high-energy phases form before the stable phases during crystallization. The rule is only a surmisal, but often supported experimentally. In the present study, both the rhombohedra and spheres contain nanostructures, indicating that nanoparticles of some type (not necessarily vaterite or calcite) formed at some stage during the decomposition reaction. The aggregation of these nanoparticles into larger structures is consistent with findings reported in related systems.2 Convection is inherently a macroscopic phenomenon; consequently, its effects are significantly reduced at small length scales. Hence, we assume that the formation of nanoparticles proceeded in a similar manner in all the double decomposition reactions, regardless of stirring rate. The stirring rate affected both the polymorphology and particle shape of the obtained calcium carbonate particles. Having disregarded the possibility of a convection control of the nucleation/precipitation of calcium carbonate nanoparticles, we propose transport-mediated control of particle growth or nanoparticle aggregation. In particular, we identify a mixing time of 30 min as the window of control in the investigated systems. The aging step of 24 h was identical for all reactions. The following discussion relates the observed findings to the manner in which the thermodynamics of the polymorphs of calcium carbonate change with particle size. The discussion is presented within the framework of local equilibrium and contains hypothetical statements; however, despite these limitations it helps to rationalize the finding of transport-mediated control of calcium carbonate polymorphology. The aggregation of small particles into compact aggregates generally results in a reduction in the number of solvent molecules in contact with the solid-liquid interface. The free energy is typically reduced, provided that solid-solid interactions are more favorable than solid-liquid interactions. This reduced contact with the solvent can be reformulated to the concept of reduced curvature: the curvature of the interface between the solid and liquid phases is reduced, thereby reducing the dissolution tendency according to the Gibbs-Thomson effect (Kelvin equation). The concentration (cs) at the crystal surface is related to the equilibrium concentration (c0) as cs = c0[1 þ ΓK].22 Here, Γ depends on the surface tension and K is the curvature. The directionalities of the surface free energies23 make the equation above qualitative in practice. Calcium carbonates of different types form in solution depending on the growth conditions;2 an amorphous form commonly precipitates during the reaction. In a recent study, one of us showed that an amorphous phase of calcium carbonate forms on double decomposition using a closely related polyelectrolyte.14 Very probably, such a phase forms transiently in this system, albeit transiently, during the formation of both vaterite and calcite in the present study. An amorphous form has been used previously in the micropatterning of single crystals.24,25 A low stirring rate (50 rpm) employed during the double decomposition reaction enhances the formation of rhombohedral calcite particles. Small particles of vaterite are more soluble than small particles of calcite.20 Particles of amorphous calcium carbonate form and transform to calcite. Calcium carbonate particles have a strong tendency to aggregate, due to the significant van der Waals forces (the Hamaker constants are large for calcium carbonates). The eventual aggregates of vaterite are unable to attain a size sufficient to prevent dissolution, and the calcite particles aggregate slowly.

Communication A high stirring rate (1000 rpm) during the mixing period of the decomposition reaction results in the formation of spherical vaterite particles. Please note that the enthalpy difference between calcite and vaterite is only about 3.5 kJ/ mol (1.4 kT).21 Small changes in the surface energy or effects of the polyelectrolyte may lead to a lower activation energy for vaterite, along the tendencies of Ostwald step rule. Apparently, the rapid stirring prohibits (or delays) the growth of the somewhat more stable polymorph (calcite). An intermediate stirring rate during the mixing period of the double decomposition reaction leads to the formation of a mixture of spherical and rhombohedral particles. In this case, it appears that collisions are sufficiently frequent to enable the formation of large aggregates of vaterite that are not susceptible to dissolution; however, the collisions are not frequent enough to prevent the simultaneous, slow growth of calcite particles. The effects of kinetics on the formation of calcium carbonate have been studied previously in simple systems;26-29 however, in the present study we demonstrate the importance of mass transport during the formation of particles of calcium carbonate in dispersions containing polyelectrolytes. We rationalize the formation of nanostructured vaterite or calcite in terms of GibbsThomson effects and the very similar enthalpies of formation for calcium carbonates. Acknowledgment. We thank the Carl Trygger Foundation; the Swedish Science Council (VR), and the CODIRECT center for financial support. Professor Lennart Bergstr€ om and Dr. Denis Gebauer are thanked for stimulating discussions on the subject. An anonymous scientist is thanked for helpful suggestions. Supporting Information Available: Information on the synthesis, experimental details, XRD diffractograms, and TEM images. This information is available free of charge via the Internet at http:// pubs.acs.org.

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