Spherical and Porous Particles of Calcium Carbonate Synthesized

Article pubs.acs.org/crystal

Spherical and Porous Particles of Calcium Carbonate Synthesized with Food Friendly Polymer Additives Mihret Abebe, Niklas Hedin,* and Zoltán Bacsik* Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden S Supporting Information *

ABSTRACT: Porous calcium carbonate particles were synthesized by adding solutions of Ca2+ to solutions of CO32− containing polymeric additives. Under optimized conditions welldefined aggregates of the anhydrous polymorph vaterite formed. A typical sample of these micrometer-sized aggregates had a pore volume of 0.1 cm3/g, a pore width of ∼10 nm, and a specific surface area of ∼25−30 m2/ g. Only one mixing order (calcium to carbonate) allowed the formation of vaterite, which was ascribed to the buffering capacity and relatively high pH of the CO32− solution. Rapid addition of the calcium chloride solution and rapid stirring promoted the formation of vaterite, due to the high supersaturation levels achieved. With xanthan gum, porous and micrometer-sized vaterite aggregates could be synthesized over a wide range of synthetic conditions. For the other food grade polymers, hydroxypropyl methylcellulose (HPMC), methylcellulose (MC), and sodium carboxyl methylcellulose, several intensive and extensive synthetic parameters had to be optimized to obtain pure vaterite and porous aggregates. HPMC and MC allowed well-defined spherical micrometer-sized particles to form. We expect that these spherical and porous particles of vaterite could be relevant to model studies as well as a controlled delivery of particularly large molecules.

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INTRODUCTION Porous calcium carbonates are highly promising for applications in catalysis1 and biomedicine.2 This promise and the intriguing mechanisms of biomineralization of calcium carbonate have inspired several recent research efforts devoted to the synthesis of porous calcium carbonates2−5 as well as using such calcium carbonates to induce porosity in solid polymers and proteins.6−11 In general, porous calcium carbonates consist of aggregates of small particles of calcium carbonate. Hence, their porosity is interparticle based and controlled by the size and arrangement of the underlying nanoparticles within the aggregates. The three anhydrous forms of calcium carbonate have similar free energies and often form according to Ostwald’s stepping rule. Accordingly, the calcium carbonate initially precipitates in the hydrous form of amorphous calcium carbonate (ACC) followed by a transformation to vaterite or aragonite before a transformation to the thermodynamically most stable form: calcite.12 ACCs usually have the highest specific surface areas among the porous calcium carbonates. ACCs with surface areas of ∼100−200 m2/g have been synthesized.13 The polymorph vaterite14−16 shows the highest specific surface areas of the anhydrous and crystalline polymorphs.17 Vaterite can be synthesized in numerous ways. Just to mention a few: it can be synthesized from a double diffusion setup with solutions of CaCl2 and K2CO3.18 Kralj et al. studied the precipitation of vaterite by mixing solutions of CaCl2 with solutions of Na2CO3 at temperatures of 10−45 °C.19 Vaterite can also be formed during bubbling of CO2 through a solution or a slurry of NH4OH/CaCl2.20 Spheroidal particles of © XXXX American Chemical Society

vaterite have been synthesized in microemulsion systems of sodium dodecyl sulfate, water, octane (and dodecanol).21 Various studies have used anionic polyelectrolytes to stabilize vaterite: Nagaraja et al. used poly(vinylsulfonic acid),22 Gower and Tirrell, and Falini et al. used poly-L-aspartate,23 and Naka et al. used a delayed addition of poly(acrylic acid) (PAA).24 Cölfen and Antonietti used different double hydrophilic block copolymers in their seminal study and could select in between calcite and vaterite.25 However, also other methods can be used to synthesize vaterite. Qi and Zhu precipitated vaterite in water− ethylene glycol mixtures under microwave irradiation.26 Further discussions of vaterite, its synthesis, and applications to drug release and personal care can be found in the recent review of Trushina et al.27 However, calcite can also be highly porous. Zhao et al. synthesized calcite with high specific surface areas (∼130 m2/g) with various degrees of crystallinity using a hydrothermal method and the addition of cetyltrimethylammonium bromide, sodium dodecyl sulfate, polyvinylpyrrolidone, and polyethylene glycol.28 Pores >10 nm can be introduced to calcite with the use of sacrificial organic templates.29−31 In a study of sintered calcium carbonates, Gebauer et al. showed that aragonite-rich calcium carbonates can exhibit high surface areas.13 It is worth noting that highly porous magnesium carbonate has been produced and characterized.32 Received: December 23, 2014 Revised: May 16, 2015

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DOI: 10.1021/cg501861t Cryst. Growth Des. XXXX, XXX, XXX−XXX

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of calcium carbonate for further studies on the uptake and release of molecules loaded in these particles. Since vaterite is particularly interesting for applications in drug delivery5,27 and foods, the first goal was the optimization of the fraction of vaterite. When only vaterite formed, the morphology and porosity were also taken into account for further setting of the reaction conditions.

The understanding of how particles of calcium carbonate form in supersaturated solutions has changed during the last few decades.33 This change has been coupled with the increased research efforts devoted to understanding the intriguing structures of calcium carbonates that form in living organisms34 in such large amounts that they are important for the overall CO2 cycle of the Earth.35 In classical descriptions, particles of calcium carbonates are formed from aqueous solutions first by nucleation and then through successive growth with the addition of ions one by one to the particles. However, these descriptions cannot explain the range of observations that relate to, for example, the rate of formation, and the often very intriguing shapes of crystalline calcium carbonates. The traditional model for crystal growth has been challenged,36−38 and it has been shown that calcite, aragonite, calcium phosphate, and iron oxide all formed via amorphous precursors in living systems.39,40 The insight that calcium carbonates and other biominerals form from disordered precursors is starting to be used to control the synthesis and processing of various materials with potential applications in catalysis, energy conversion, and biomedicine.41,42 Still, the mechanisms for the crystallization of calcium carbonates in aqueous solutions are not fully understood.43 Besides the importance of ACC, it has been shown that ACC may form via the aggregation of nanometer-sized clusters.33,37,44 We showed that proto-calciticACC and proto-vateritic-ACC can be synthesized without any additives and that they relate to calcite and vaterite, respectively.45 Because of the very close chemical potential for different polymorphs of calcium carbonate,46 it is possible to influence how ACC transforms into various polymorphs, not only with equilibrium conditions of the solution, but also with conditions being out of equilibrium.47 Besides the various specific effects of molecular additives and ions on the formation of calcium carbonates, as mentioned polymers have been shown to affect both the polymorphology and morphology of calcium carbonates when added to the reactive mixtures.48 Such polymeric additives can interact with the ions, the clusters of calcium and carbonate ions, particles of ACC, and the already formed crystals of anhydrous calcium carbonates. Carboxylated polyelectrolytes have been shown to interact strongly with calcium ions, not only through double layer interactions but also by forming complexes along the chain of the polyelectrolyte.49 In addition, negatively charged polyelectrolytes have been shown to interact with particles of ACC50 and the surfaces of crystalline particles of calcium carbonate.51 In the context of charge, note that the zeta potential of calcium carbonates can vary from positive to negative values for seemingly similar carbonates at ambient pH conditions, the reasons for which are still being studied.52 In the notion of Acosta, porous vaterite particles would relate to the chemical (bottom-up) procedures of producing nanoparticle based delivery systems.53 The porous vaterite particles are quite different than the commonly used emulsion based delivery systems. We expect the vaterite based system to overcome some of the drawbacks of the emulsion based system such as the decreased stability on freezing and chilling.54 Calcium is an important element and various delivery systems have been proposed that can enhance the absorption of calcium. Smith et al. showed that a calcium citrate-malate was at least as effective as calcium carbonates.55 The prospects of tuning the calcium absorption in humans by porous calcium carbonates have not yet been studied. Neither have such porous carbonates been used as delivery vehicles for flavors, etc. The motivation for this study was to synthesize the well-defined and porous particles

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EXPERIMENTAL SECTION

Materials. Calcium carbonate was synthesized using the following analytical grade chemicals: sodium carbonate (Sigma-Aldrich 99.7% [CAS number: 144-55-8]), calcium chloride hexahydrate (SigmaAldrich 99% [CAS number: 7774-34-7]), xanthan gum (XG) (SigmaAldrich [CAS number: 11138-66-2]), sodium carboxyl methylcellulose (NaCMC) (Sigma-Aldrich [CAS number: 9004-32-4]), hydroxypropyl methylcellulose (HPMC) [CAS number: 9004-65-3]), and methylcellulose (MC) (Sigma-Aldrich [CAS number 9004-62-0]). Millipore water was used throughout the syntheses. The water was degassed for 6 h with N2 to remove dissolved carbon dioxide. The polymers used in the study are illustrated in Figure 1.

Figure 1. Food grade polymers that are used as additives during the formation of calcium carbonate: (a) xanthan gum (inspired by Hassler and Doherty56), (b) sodium carboxyl methylcellulose, (c) hydroxypropyl methylcellulose, (d) methylcellulose (Ac-acetate, Glu-glucose, Glca-glucuronic acid, Man-mannose, Pyr-pyruvate). Synthesis. Aqueous solutions of calcium chloride hexahydrate (0.5, 1, 1.5, 2, and 3 mol/dm3) and sodium carbonate (0.5, 1, 1.5, and 2 mol/dm3) were prepared. A total of 50 mL of each solution of sodium carbonate was mixed with 100 mL of aqueous solutions of either 0.167, 0.25, 0.5, and 1 wt % of a polymer in a 250 mL flask (three-necked, round-bottom). Calcium carbonate was precipitated by pumping the calcium chloride solutions into the reaction flask. A lid was used for the flask, which has two drilled holes, allowing for mechanical stirring and to pump the calcium chloride solution. Different flow rates were used: 200 and 400 mL/h flow rates were ensured with a syringe pump (KD scientific syringe pump), and 500, 600, and 750 mL/h flow rates with a peristaltic pump (VWR peristaltic pump). Mechanical stirring was applied during the reaction with rates of 200, 500, 750, 1000, and 1500 rpm (IKA RW 20 stirrer motor, metal stirrer rod with centrifugal type, collapsible blade, blade size (H × L) 10 × 35 mm). A magnetic stirrer and a 4 mm stirrer rod were used for certain exploratory experiments. The temperature was controlled by a VWR digital temperature controller (model 1136-1D). The pH was measured by a Metrohm digital pH meter. Filter funnels with a paper and water suction vacuum, a centrifuge, a freeze-dryer, and an oven at 100 °C were used in the preparation of the dry solid samples. Table 1 shows the parameters investigated; nearly 300 different syntheses were conducted. The different conditions for the experiments were selected by excluding some parameters from the full parameter matrix. If a parameter adversely affected the product, it was excluded from further experiments. We selected the order of the experiments after executing some preliminary experiments. The order of the experiments is shown in Table 1 and discussed in the Results and Discussion section in detail. When ethanol B

DOI: 10.1021/cg501861t Cryst. Growth Des. XXXX, XXX, XXX−XXX

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radiation and a PIXcel detector at 10° < 2θ < 70° using a scanning speed of 0.05 s−1. The calcium carbonate was finely ground, dispersed in isopropanol, and spread uniformly on silicon plates. The fractions of the different polymorphs of calcium carbonates were determined by analyses of powder X-ray diffraction data throughout this study. More details about the analysis of the data are presented in the Supporting Information. N2 Adsorption and Desorption. Isotherms of N2 were recorded with a Micromeritics ASAP2020 device at the boiling point of N2 (−196 °C) and relative pressures of 0.01−0.99. Approximately 100−200 mg of calcium carbonate powders was used and degassed under a dynamic vacuum and a temperature of 120 °C for 8 h. Specific surface areas were evaluated in the model of Brunauer, Emmett, and Teller (BET) using the adsorbed amount of N2 at relative pressures of 0.05−0.16. The mesopore (2−50 nm) size distribution was derived in the Barrett−Joyner−Halenda (BJH) model employed at the adsorption branch of the isotherm. The total pore volume was determined from the uptake of N2 at a relative pressure of 0.99.

Table 1. Reaction Conditions and Parameters Studied reaction condition

parameters

temperature (°C) [Ca2+]/[CO32−] Ca2+ addition rate (mL/h) polymer concentration (wt %) stirring rate (rpm)

0, 4, 15, 24, 35, 45 3:1, 2:1, 1.5:1, 1:1, 1:2 200, 500, 750, 1000, 1250, 1500 0.25, 0.5, 1.0 200, 500, 750, 1000, 1250, 1500

was used in the mixtures it replaced a given volume of water when preparing the polymer solutions. Pure vaterite was synthesized without a polymer for comparison. The conditions for the synthesis of pure vaterite were room temperature, 1000 rpm stirring rate, and 200 mL/h addition rate of the Ca2+ solution. Stability Studies. The stability of the polymer containing vaterite product (prepared with MC and HPMC) was investigated. Approximately 300 mg of the vaterite products was placed in separate containers and contacted with 25 mL of pure water or 25 mL of a pH buffer (pH = 7.4). No stirring was used. The samples were removed from the solution after 3 h, 6 h, 9 h, 12 h, 24 h, 48 h (72 and 120 h), and one sample was prepared for each studied recrystallization time. For each sample, the pH was measured before decanting the samples. The samples were dried at 80 °C, before determining the polymorphology of the calcium carbonate with IR spectroscopy and X-ray diffraction. The pH buffer was prepared as a 0.5 mol/dm3 (tris(hydroxymethyl)aminomethane (TRIS) solution, and the pH value was adjusted by adding an aqueous HCl solution dropwise until the pH reached 7.4. Because of the TRIS, the pH was measured with a Metrohm pH meter equipped with a glass and a calomel reference electrode. Scanning Electron Microscopy (SEM). Images were recorded with a JEOL JSM-7000F microscope in its secondary electron imaging mode. Calcium carbonate powders were sprinkled on the sample holders that were covered with a carbon-based ink. X-ray Powder Diffraction (XRD). Diffractograms were recorded with a PANalytical X’Pert Pro diffractometer using a Cu Kα1 (λ = 1.5406 Å)

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RESULTS AND DISCUSSION Pure vaterite can be synthesized at certain reaction conditions without using any additive to calcium and carbonate ion sources; however, such vaterite agglomerate in a noncontrolled manner (see SEM images in Figure S1 in the Supporting Information). In order to produce spherical, well-separated particles of vaterite, it was necessary to use polymeric additives here. After an optimization procedure, micrometer-sized, spherical aggregates of nanoparticles of vaterite formed when an aqueous solution of calcium chloride solution was added to a sodium carbonate solution (water/ethanol) containing HPMC at room temperature. These spherical aggregates were relatively monodispersed, as displayed in Figure 2. The enlarged SEM image of Figure 2d shows nanoparticles on the spherical aggregates. Similar relatively monodispersed particles containing MC were

Figure 2. Scanning electron micrographs of pure vaterite particles synthesized by reacting an aqueous solution of calcium chloride with a solution of sodium carbonate (water/ethanol) and hydroxypropyl methylcellulose (a, b, c, d images are about the same sample, only the magnification is changed). C

DOI: 10.1021/cg501861t Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Crystal Growth & Design

Figure 3. Scanning electron micrographs of pure vaterite particles synthesized by reacting an aqueous solution of calcium chloride with a solution of sodium carbonate (water/ethanol) and methylcellulose (a, b, c, d images are about the same sample, only the magnification is changed).

synthesized using the optimal conditions for the HPMC containing syntheses. Such MC containing vaterite particles are displayed in Figure 3. The vaterite particles with HPMC contained about 3 wt % HPMC, as deduced by thermal analysis. Even if these particles are vaterite-polymer hybrids, we call them vaterite particles as the polymer content is low. The BET specific surface area of a typical sample (on the SEM images in Figure 2) was 27 m2/g with a specific pore volume of 0.1 cm3/g and an average pore width of 11 nm (BJH model on the adsorption branch). Adsorption and desorption isotherms, and a thermal gravimetric analysis plot are presented in Figures S2 and S3 in the Supporting Information. Micrometer-sized calcium carbonate aggregates of nanoparticles of vaterite also formed with XG or NaCMC as additives. For these two additives, the vaterite aggregates were rather nonuniform, as is visible from the SEM images in Figure 4. The XG additive gave rise to the most vaterite aggregates among the studied additives (Figure 4a,b) but was actually the best promoter of the polymorph vaterite; however, it did not stabilize spherical aggregates. As was mentioned in the Experimental Section, the detailed optimization of the syntheses was chosen after executing some preliminary experiments. From these preliminary experiments, it was revealed that the mixing order was very important for the fraction of vaterite that formed, that is, if the Ca2+ containing solution was added to the CO32− containing solution, or the opposite. In addition, it was observed that vaterite was favored approximately at room temperature, with a low Ca2+-to-CO32− ratio, at high addition rates of the Ca2+ containing solution, and at moderately high stirring rates. Effect of Temperature. The temperature significantly influenced which polymorphic forms of calcium carbonate formed when mixing Ca2+ containing solutions with the CO32−

containing ones; see Figure 5. At low temperatures basically only calcite formed, which was consistent with the study of Ogino et al.57 The largest fraction of vaterite was achieved at an intermediate temperature of 24 °C for all polymers studied. With NaCMC and HPMC polymers, aragonite was the main product at a temperature of >40 °C. As XG precipitated at a temperature of 30 °C, it was excluded from high temperature experiments. Effects of the Ca2+-to-CO32− Ratio. The fraction of vaterite decreased with the final Ca2+-to-CO32− ratio used during synthesis; see Figure 6 (top panel). For a Ca2+-to-CO32− ratio