Nonstoichiometric Polyelectrolyte Complexes Versus Polyanions as

Article pubs.acs.org/crystal

Nonstoichiometric Polyelectrolyte Complexes Versus Polyanions as Templates on CaCO3‑Based Composite Synthesis Marcela Mihai* “Petru Poni” Institute of Macromolecular Chemistry of Romanian Academy, Grigore Ghica Voda Alley 41A, 700487 Iasi, Romania

Simona Schwarz and Frank Simon Leibniz-Institut für Polymerforschung, Dresden e. V., Hohe Strasse 6, 01069 Dresden, Germany ABSTRACT: The characteristics of calcium carbonate microparticles formed in supersaturated aqueous solutions, in the presence of some strong/weak anionic polymers or with nonstoichiometric polyelectrolyte complexes with negative charges in excess (NPEC-n), have been investigated under different relative inorganic/polymer ratios, tuned by initial solution supersaturation and polymer concentration. For this purpose two polyanions which contain the carboxylic and sulfonic ionic groups, poly(2-acrylamido-2methylpropanesulfonic acid-co-acrylic acid) and chondroitin-4-sulfate, were used. NPEC-n dispersions prepared with the same polyanions and chitosan as a polycation were also used as templates for calcium carbonate crystallization. Scanning electron microscopy was used to provide the particle morphologies and flow particle image analysis to evidence their mean sizes and sphericities. The polymer presence in the composite particles was evidenced by X-ray photoelectron spectroscopy, particle charge densities, and zeta potentials. The microparticle charge densities and their zeta-potential values suggested the uniform embedment of NPEC-n in the composites, irrespective of molar ratio, size, and charge density of NPEC-n.

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INTRODUCTION Calcium carbonate is a common biomineral with wide applications, such as filler in pigment, paper, rubber, and plastic industries.1,2 Due to its biocompatible and biodegradable nature, a strong interest for pharmaceutical and biomedical applications has emerged in recent years.3 A main interest in CaCO3 crystallization characteristics aims to facilitate the understanding of biological control of biomineralization. Most research in calcium carbonate microparticle synthesis has been done using the method of mixing solutions containing calcium and carbonate ions.4−6 During the fast-reactive crystallization process, calcium ions and carbonate groups combine in amorphous CaCO3, the most instable solid-state phase.7 Then, the initially formed CaCO3 is transformed, within a few minutes, to a mixture of crystalline CaCO3. The transformed carbonates are vaterite and calcite at low temperatures (14−30 °C) and aragonite at high temperatures (60−80 °C).8 The stability of these CaCO3 crystalline polymorphs decreases in the following order: vaterite < aragonite < calcite.9 Actually, the stability and lifetime of these polymorphic species is strongly influenced by their own solubility in aqueous solution. The spontaneous precipitation of calcium carbonate from aqueous supersaturated solutions depends also on the presence of additives, which may act as nucleation centers. The crystallization mechanism can be altered by specific interactions with polar functional groups such as −COOH, −PO3H, and −SO3H.10,11 Therefore, the use of polymeric templates induces the formation of new hybrid © XXXX American Chemical Society

materials with specific structure, advanced properties, and functions.4−6,12−19 Our previous studies showed that polyanion presence during the CaCO3 polymorph crystallization influence the ratio between the calcite/vaterite polymorph in composite, the particle sizes and shapes, the composites sorption capacity, and their pH stability. Thus, 0.05 wt % of poly(2-acrylamido-2methylpropanesulfonic acid-co-acrylic acid (PSA) conduct to the formation of spherical particles, of about 8−10 μm in diameter, where vaterite is the main polymorph, even if an increase of calcite content comparative with bare CaCO3 particles has been noticed.15 Composite particles prepared with a polymer−drug conjugate, based on poly(N-vinylpyrrolidone-co-maleic anhydride) and 2-amino-5-(4-methoxyphenyl)-1,3,4-oxadiazole, were stable up to the polymer isoelectric point, located at pH = 3.4, irrespective of carbonate content in composite particles.17 The sorption capacity of composite materials, tested with methylene blue, increased with the increase of polymer content in the composites, suggesting that the sorption process takes place mainly by electrostatic interactions. A previous study showed that the spontaneous precipitation of vaterite from an aqueous solution in the presence of chondroitin sulfates resulted in a reduction of the crystal growth rate by 23−65%.20 Chondroitin sulfate influences the Received: April 9, 2013 Revised: June 4, 2013

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dx.doi.org/10.1021/cg400520m | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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and CaCl2 aqueous solution, with Na2CO3 and CaCl2 having equal concentrations, were rapidly mixed; the pH of the mixtures was adjusted to 8.5. The mixtures were stirred for 1 min on a magnetic stirrer, at room temperature, and then the dispersions were kept under static conditions for 60 min. The obtained microparticles were separated by filtration, intensively washed with water, and finally washed with acetone and dried in the oven at 40 °C, for 1.5 h. The produced microparticles were kept in hermetically closed tubes, at room temperature. The concentration of the Na2CO3 and CaCl2 in the crystallization medium was varied between 0.05 and 0.3 mol L−1 and the polymer concentration varied between 0 and 0.5 wt %. NPEC-n Preparation. Aqueous solution of 5 × 10−4 mol L−1 CS was prepared in 1 vol % acetic acid solution. Aqueous solutions of 5 × 10−3 mol L−1 CSA and PSA were prepared by dissolution of appropriate amounts in water. Dispersions of NPEC-n were prepared at room temperature by mixing the solutions of oppositely charged polyelectrolytes in appropriate proportions. The amount of polyanion was kept constant within a complex series, while the amount of CS has been varied according to the desired mixing molar ratio, n+/n−. The CS solution was added dropwise to the polyanion solution, under magnetic stirring. After mixing, the formed dispersions were stirred 60 min and were characterized and used in composite formation after 24 h. Preparation of CaCO3/NPEC-n Composite Microparticles. The formation of CaCO3/NPEC-n composites was carried out in glass beakers at 25 °C. In 40 mL NPEC-n dispersion was solved 0.2 mol L−1 Na2CO3, and then an equal volume of 0.2 mol L−1 CaCl2 aqueous solution was added; the pH of the mixtures was adjusted to 8.5. The mixtures were stirred for 1 min on a magnetic stirrer, at room temperature, and then the dispersions were kept under static conditions for 60 min. The obtained microparticles were separated by filtration, intensively washed with water, and finally washed with acetone and dried in an oven at 40 °C for 1.5 h. The resultant microparticles were kept in hermetically closed tubes, at room temperature. Characterization of Composite Microparticles. The particle shapes and surfaces were examined by using a FEI Phenom Desktop scanning electron microscope, in high vacuum mode. To avoid electrostatic charging, the particles were sputtered with gold. The particle size, size distribution, and the sphericity of the carbonate particles have been evaluated using the Sysmex Dynamic Flow Particle Image Analyzer 2100. To obtain correct values, the Sysmex FPIA 2100 image analyzer was checked before starting the main experiment by using certified size standards. Electrokinetic potential of carbonate samples was measured by means of ZetaSizer Nano ZS (Malvern, U.K.), operating at the 633 nm wavelength. The instrument measured the electrophoretic mobility of the particles. From the electrophoretic mobility, the apparent zetapotential values (ζapp) were calculated by Smoluchowski’s equation. The presented values are the average values of at least three independent measurements performed on 0.5 mg/mL aqueous dispersions, for each sample. The concentrations of the charged groups in the examined solutions and microparticle dispersions were determined by titration using the particle charge detector Mütek PCD 03 (BTG Instruments GmbH, Herrsching, Germany). The charge density was calculated from the amount of standard solutions [poly(sodium ethylenesulfonate) or poly(diallyldimethyl ammoniumchloride), with a concentration of 10−3 mol L−1], needed to reach the zero value of the streaming potential. All measurements were run at room temperature. The qualitative and quantitative elemental surface compositions of the carbonate and carbonate/polymer microparticles were carried out by means of an Axis Ultra X-ray photoelectron spectrometer (Kratos Analytical, Manchester, U.K.). The spectrometer was equipped with a monochromatic Al Kα (hν = 1486.6 eV) X-ray source of 300 W at 15 kV. The kinetic energy of the photoelectrons was determined with a hemispheric analyzer set to pass energy of 160 eV for wide-scan spectra and 20 eV for high-resolution spectra. During all measurements, electrostatic charging of the sample was overcompensated by means of a low-energy electron source working in combination with a

particle size distribution of the vaterite crystals formed and stabilizing this mineral phase, preventing the transformation to calcite. Micron-sized calcium carbonate/chondroitin sulfate particles, synthesized through a reaction between sodium carbonate and calcium nitrate tetrahydrate solutions suspended with chondroitin sulfate macromolecules, were used as support for core−shell particles.21 The polymeric matrix allowed the subsequent selective control of drug loading and release, as biodegradable devices, which have advantages in reducing systemic side effects and increasing drug efficacy. Even if there are numerous studies on polyanion control of CaCO3 growth, to our knowledge no studies concerning the use of nonstoichiometric polyelectrolyte complexes (NPEC) as templates for controlling CaCO3 crystal growth have been reported up to now. Polyelectrolyte complexes (PECs) are obtained by mixing aqueous solutions of oppositely charged polymers.22 PECs are of interest due to their facile preparation and responsiveness to environmental stimuli. Moreover, using water as a solvent, PECs are attractive for biomedical applications. At relatively low concentrations and when one of the components is taken in excess, PEC formation can lead to stable colloidal dispersions.23−28 The characteristics of the polyelectrolyte components (molecular weight, nature of ionic groups, charge density, and architecture) and the solvent (ionic strength and pH) determine the internal structure of the particles. In this context, the aim of this study was to follow the CaCO3/polymer composite microparticles formation using even some strong/weak polyanions or NPEC with negative charges in excess (NPEC-n). For this purpose, we used two polyanions which contain carboxylic and sulfonic ionic groups and a different polymeric backbone, poly(2-acrylamido-2methylpropanesulfonic acid-co-acrylic acid) (PSA), which is a synthetic, flexible polymer, and chondroitin-4-sulfate (CSA), a sulfated glycosaminoglycan with a semirigid macromolecular chain. Some NPEC-n dispersions, prepared with the same polyanions and chitosan (CS) as polycation, were also used as templates for calcium carbonate crystallization. Scanning electron microscopy (SEM) was used to provide the particle morphologies and flow particle image analysis (FPIA) to evidence their mean sizes and sphericities. The polymer presence in the composite particles was evidenced by X-ray photoelectron spectroscopy (XPS), particle charge densities, and zeta potentials.

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EXPERIMENTAL SECTION

Materials. CaCl2·2H2O and Na2CO3 from Sigma-Aldrich were used as received. The copolymer PSA, which contains 55 mol % 2acrylamido-2-methylpropanesulfonic acid and 45 mol % acrylic acid, was synthesized and purified according to ref 29. The intrinsic viscosity of the PSA copolymer, determined in 1 mol L−1 NaCl at 25 °C, was [η] = 0.72 dL/g. CSA from bovine trachea was purchased from Sigma and used as received. The viscometric molar mass of CSA, Mv = 20320 g/mol, has been determined according to the method proposed by Wasteson.30 The intrinsic viscosity of the CSA, determined in 0.2 mol L−1 NaCl at 25 °C, was [η] = 0.41 dL/g. For these experimental conditions, the value of constant K was 5.0 × 10−6 and α = 1.14. CS with low molar mass, from Fluka, was used without further purification. The CS degree of deacetylation, of about 85%, was determined by 1H NMR.31 Preparation of CaCO3/Polymer Composite Microparticles. The formation of CaCO3/polymer composites was carried out in glass beakers at 25 °C. For composite particles, a polymer aqueous solution was first prepared, and then the Na2CO3 was solved into. Equal volumes of as-prepared solutions (Na2CO3 with or without polymer) B



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Figure 1. SEM images of CaCO3/polyanion particles prepared with constant polymer concentration (0.2 wt %) and different initial concentration of calcium and carbonate ions. The scale bar is 10 μm.

Figure 2. Mean particle sizes (closed symbols) and sphericities (open symbols) for CaCO3 (square), CaCO3/PSA (triangles), and CaCO3/CSA (circles), as a function of initial concentration of calcium and carbonate ions. The values are the mean of five independent experiments that deviated from 0−7%.

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magnetic immersion lens. Later, all recorded peaks were shifted by the same amount, which was necessary to set the C 1s peak to 285.00 eV for saturated hydrocarbons.32 Quantitative elemental compositions were determined from peak areas using experimentally determined sensitivity factors and the spectrometer transmission function. Spectrum background was subtracted according to Shirley.33 The high-resolution spectra were deconvoluted by means of a computer routine (Kratos Analytical, Manchester, U.K.). Free parameters of component peaks were their binding energy (BE), height, full-width at half-maximum, and the Gaussian−Lorentzian ratio. The maximum information depth of the XPS method is about 8 nm for the C 1s region.34 The characterization of the samples by X-ray diffraction was carried out using a D8 Advance Bruker AXS device. The X-rays were generated using a Cu Kα source with an emission current of 36 mA and a voltage of 30 kV. Scans were collected over the 2θ = 20−60° range, using a step size of 0.01° and a count time of 0.5 s/step. The semiquantitative analysis was performed with an EVA soft from DiffracPlus package and an ICDD-PDF2 database, based on the patterns’ relative heights. The criteria used to compare the simulated and the measured scan is the R/R0 ratio, where R is the weighted reliability and R0 represents the inevitable discrepancy due to the statistics of the X-ray diffraction (noise modeled by Poisson’s law). For an ideal fit, the R/R0 value is 1.

RESULTS AND DISCUSSION Influence of Polymers Structure and Inorganic/ Organic Ratio on CaCO3/Polymer Composites. In the following, the CaCO3/polyanion composite samples were coded CxPy, where x is the molar concentration of Na2CO3 and CaCl2 aqueous solutions, P is CSA or PSA, and y is the polymer concentration in the initial mixture, in wt %. The influence of polyanion backbone structure on the composite characteristics has been followed as a function of the inorganic/ organic ratio, keeping one of the parameters constant: polymer concentration or inorganic concentration. Thus, first series were prepared using 0.2 wt % polymer and different calcium and carbonate concentration in the starting solution (from 0.05 mol L−1 to 0.3 mol L−1). The morphology of the as-prepared particles has been evidenced by SEM (Figure 1) and the particle mean sizes and sphericities by FPIA (Figure 2). As Figures 1 and 2 show, the inorganic content has an important role on the particle sizes and morphologies. Thus, at very low inorganic content (0.05 and 0.1 mol L−1), the formation of a mixture of particles with irregular shapes and a large size distribution has been obtained. The further increase C



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Figure 3. Particle charge densities, CD (closed symbols), and apparent zeta-potential values, ζapp (open symbols), for bare CaCO3 (square), CaCO3/ PSA (triangles), and CaCO3/CSA (circles), as a function of initial concentration of calcium and carbonate ions. The values are the means of three independent experiments that deviated from 0−7%.

Figure 4. SEM images of CaCO3/polyanion particles prepared with constant inorganic concentration (0.2 mol L−1) and different initial concentration of polymers. The scale bar is 10 μm.

Figure 3 summarizes complementary electrokinetic measurements carried out on the composite particles, to determine the particles charge density (CD), investigated with the PCD particle charge detector, and the apparent zeta-potential values (ζapp), determined by Zetananosizer. Compared to bare CaCO3 particles, the presence of polymers during the particle preparation increases the particle charge density (CD) (closed symbols in Figure 3), irrespective of the concentration of the inorganic ions and the structure of the polyanions. This behavior is an evidence of the presence of polymer in composite particles. Increasing the inorganic concentration, less negative CD values were obtained due to the decrease of polymer content into the composites, irrespective of used polymer. For the same inorganic concentration, the more negatively charged particles were obtained in the presence of PSA, probably due to a better polyanion distribution in composite material, as a consequence of its flexible backbone. The presence of PSA and CSA in CaCO3 microparticles has also been confirmed by the zetapotential values (Figure 3, open symbols). The negative zetapotential values determined for the differently prepared samples followed the same trend as CD and can be explained by the fact

of calcium and carbonate concentrations in the initial solutions leads to the formation of bigger and more spherical-shaped particles, for both polymer-based composites. Calcite usually crystallize as monocrystalline cubic well-faceted particles, whereas vaterite particles are polycrystalline, exhibit a spherical shape, and are built up by 25−35 nm crystallites.35 Therefore, taking into account the microparticle shapes observed in the images included in Figure 1, we can assume that PSA favors the formation of vaterite polymorph, at an inorganic concentration higher than 0.1 mol L−1. The presence of cubic and spherical particles in CSA-based composites suggest the formation of both calcite and vaterite polymorphs, irrespective of inorganic concentration. The particle sizes significantly increased with the inorganic concentration increase from 0.05 mol L−1 to 0.3 mol L−1, irrespective of the polyanion structure. However, as shown in Figure 2 (closed symbols), the polymer presence leads to the formation of slightly smaller particles compared with bare CaCO3 particles prepared with the same inorganic concentration. At the same time, the particle sphericities (empty symbols in Figure 2) increased with the inorganic concentration up to 0.15 mol L−1 and remain almost constant at higher concentrations, irrespective of the polymer presence and its chain structure. D



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Figure 5. Mean particle sizes (closed symbols) and sphericities (open symbols) for CaCO3/PSA (triangles) and CaCO3/CSA (circles), as a function of the initial concentration of polymers. The values are the mean of five independent experiments that deviated from 0−7%.

Figure 6. Particle charge densities, CD (closed symbols), and apparent zeta-potential values, ζapp, (open symbols) for CaCO3/PSA (triangles) and CaCO3/CSA (circles), as a function of initial concentration of polymers. The values are the mean of three independent experiments that deviated from 0−7%.

that the macromolecules have been intertwined in the carbonate particles. The second series of samples were prepared keeping constant the inorganic concentration at 0.2 mol L−1, and the inorganic/organic ratio was varied by the initial polymer concentration in the mixtures. Figure 4 shows the SEM images of microparticles prepared with different concentrations of PSA (first line) and CSA (second line) and the same inorganic concentration (0.2 M). The results on particle mean diameters and sphericities, determined by FPIA, are graphically represented in Figure 5. The polymer concentration and its structure have an important role in particle surface morphologies (Figure 4) and mean sizes (Figure 5). Thus, the size of PSA-based particles decreased with the polymer concentration increase, on the investigated concentration range (0.1−0.5 wt %). Moreover,

compared to the typical cauliflower shape of bare CaCO3 particles prepared with the same initial inorganic concentration,17 a low amount of PSA (0.1 and 0.2 wt %) induced the formation of smoother particles, with very small grain size. With the further increase of the initial polymer concentration up to 0.5 wt %, some agglomerated features are evident on the particle surfaces, with the simultaneous decrease of the composite particle sizes, as was also observed by FPIA (Figure 5). The features on the surface are probably given by the formation of a denser cross-linked network with the increase of calcium content into the composite particles. A different surface pattern has been observed when CSA was used as a template in particle formations. Thus, a small amount of CSA (0.1 wt %) conducts to an increase of particle sizes compared with bare CaCO3 particles, followed by the size decrease with the polymer content increase up to 0.2 and 0.3 wt E



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Calcium can be considered as a label for the inorganic phase, and therefore, changes in the relative content of Ca 2p can be explained by the distribution of the organic phase in the inorganic phase. The use of polymers to control the crystallization process should influence the [Ca]:[O]|spec ratios. Even if the majority of oxygen is constituent of the inorganic phase, the polymers also contribute to the total oxygen content of the sample. Therefore, the decrease of the [Ca]:[O]|spec ratios for CSA and PSA-based samples (Table 1) suggest an increase of oxygen content and sustain the polymer presence in the composite microparticle. The increase of the initial amount of PSA during the crystallization process lowered the [Ca]: [O]|spec ratio. These findings are in good agreement with results of SEM images for the sample with 0.4 wt % PSA (Figure 4), which show some irregularities on the particle surfaces. Furthermore, electrokinetic measurements verified a stronger negative character of surfaces of these composite particles (Figure 6). In the case of the polymer CSA, the initially observed decrease of the [Ca]:[O]|spec ratio was not changed after adding the higher amount of CSA (0.4 wt %). Obviously, the additional amount of the added CSA polymer is preferably incorporated into the bulk phase of the composite material. As mentioned above, calcium can be considered as a label element for the inorganic phase, while the majority of nitrogen is introduced by the polymers PSA and CSA. Hence, as compared to bare CaCO3 (sample C0.2), the increase of the relative amount of nitrogen ([N]:[C]|spec) is evidence for the presence of the polymers on the composite particle surfaces. The sulfate groups of the CSA polymer and the sulfonate group of the PSA polymer should also introduce sulfur in the composite particle surface. In accordance with the stoichiometries of the polymer molecule, the [N]:[S] ratio should be [N]: [S] = 1. However, for all composite particle surfaces, smaller amounts of sulfur were observed. It is assumed that as a first step of crystal formation, when calcium ions were added to the polymers solution, the high affinity of the sulfate and sulfonate groups from polymers to calcium ions fixes the calcium ions irreversibly in their immediate environment. The further growth of the CaCO3 crystals is taking place around these nucleation centers. Thus, the sulfate and sulfonate groups appear masked by calcium, while the amide groups are just surrounded by the calcium carbonate crystals. Moreover, the polymer chain flexibility can further influence their arrangements into the composites and on their surface. Therefore, the chain structures of CSA and PSA were 3D optimized (using

%. With a further increase in the polymer concentration up to 0.4 and 0.5 wt %, the increase of composite particle sizes has been observed. This behavior can be explained by the polymer conformation in aqueous solution, given by its semirigid macromolecular chain, which probably converts to a rigid crosslinked network, which serves as a template for CaCO3 crystal growth. In this respect, electrokinetic measurements were performed (Figure 6). For all composite materials, similar curve shapes were obtained for the electrokinetic properties, such as charge densities and apparent zeta-potential values, which were determined in dependence on the initial concentration of polymers. Corresponding to the findings discussed above, the polymer presence increased the negative values of charge densities and apparent zeta-potential values, comparative with bare CaCO3 particles, irrespective of polymer structure. The highest negative character was observed when PSA was used. These findings are also in agreement with the previous assumption of the PSA network features on the surface of the composite CaCO3-based particles (Figure 4). In order to study the presence of the polymers on the surface of the composite materials, XPS spectra were recorded. The maximum information depth of the XPS method is not more than 8 nm.28 However, a spherically shaped surface, such as a particle surface, is characterized by an angular distribution of the surface normal. That means the XPS spectrometer records photoelectrons from different depths of the particle surface because the takeoff angle, which is defined as the angle between the surface normal and the electron-optical axis of the spectrometer, corresponds to the angular distribution of the surface normal. In this manner, the XPS method is more surface-sensitive for small particles. The atomic ratios of composite sample surfaces, which contain 0.2 and 0.4 wt % of both polymer and 0.2 mol L−1 calcium and carbonate initial concentration, are summarized in Table 1. Table 1. XPS Atomic Composition of Samples Which Contain 0.2 and 0.4 wt % of Both Polymer-Based Composites [Ca]:[O]|spec [N]:[C]|spec [S]:[C]|spec [O]:[C]|spec [Ca]:[C]|spec

C0.2

C0.2PSA0.2

C0.2PSA0.4

C0.2CSA0.2

C0.2CSA0.4

0.297 0.007 − 0.839 0.249

0.272 0.033 0.021 0.902 0.246

0.240 0.026 0.012 0.541 0.130

0.273 0.027 traces 0.611 0.167

0.276 0.046 traces 0.911 0.251

Figure 7. Schematic representation of the chain structures of CSA and PSA 3D optimized (using ACD/Lab 6.0) and their interaction with Ca2+ ions. F



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Figure 8. High-resolution C 1s and N 1s element spectra of (a) bare CaCO3, (b) C0.2PSA0.4, and (c) C0.2CSA0.4 composite particles. The chemical structures of the polymers PSA and CSA are shown to visualize the assignments of the component peaks to the differently bonded atoms.

ACD/Lab 6.0), and their interaction with Ca2+ ions is schematically represented in Figure 7. As Figure 7 shows, intra- and interchain ionic cross-links in CaCO3/PSA composites can occur: interchains, between −SO3− groups of different PSA molecules, and intrachains, between −SO3− and −COO− or two −COO− groups on the same polymeric chain. Thus, the CaCO3/PSA composites may have on the surface some free −SO3− groups, which can be detected by XPS. On CaCO3/CSA composites, interchain cross-links between the −SO3− groups of CSA and the intrachain between −SO3− and −COO− groups on the polymer chain are expected. Thus, the −SO3− groups are expected to be located inside the particles, whereas on the surface is evident N in amide groups. As can be seen in Table 1, the surface of the bare CaCO3 sample (C0.2) contains a clear excess of carbon (stoichiometric ratio is [Ca]:[C] = 1). The reason for the excess of carbon is the surface free energy of the polar components on the CaCO3 surface. In order to minimize its total free energy, the particles spontaneously adsorb hydrocarbon impurities, which are also known as adventitious carbon.36 In order to identify the polymers and evaluate their contributions to the C 1s and N 1s peaks, high-resolution element spectra were recorded. Figure 8 shows the high-resolution C 1s and N 1s spectra of bare CaCO3 and the polymer-modified CaCO3 samples, C0.2PSA0.4 and C0.2CSA0.4. As mentioned above, the shape of the C 1s spectrum of bare CaCO 3 is characterized by carbon atoms, which are constituents of the carbonate group and carbon atoms showing the presence of surface contaminations. In Figure 8a, the carbonate group is identified by component peak CO3 at 289.8 eV. Four additional component peaks (A, B, C, and E) were necessary to fit the C 1s spectrum. Component peak A (at 285.00 eV) shows saturated hydrocarbons (CxHy). Carbonyl carbon atoms of carbonic ester groups (OC−O−C) were found at 288.25 eV (component peak E). The corresponding alcohol-sided carbon atoms of the ester groups (OC−O−C) contributed to the component peak C (at 286.58 eV). The intensity of component peak C is higher than the intensity of component peak E. Obviously, other C−O species, such as alcohol (C−OH) and/or ether (C−O−C) groups are also present on the sample surface. Carbon atoms in the α position to the strong electronegative carbonyl carbon atoms (C−

COOC) appear as component peak B. The intensity of this component peak is slightly higher than the intensity of component peak E because C−N bonds also contributed to component peak B. The C 1s spectra of samples prepared in the presence of PSA and CSA show similar shapes to the bare CaCO3 (Figure 8, panels b and c). The carbonate groups (CO32−) of the inorganic CaCO3 phase were clearly identified as component peaks CO3 (the binding energies for these component peaks were not different to the binding energy determined for the CO3 component peak of the bare CaCO3 sample). Component peaks A at 285.00 eV arose from photoelectrons that escaped from saturated hydrocarbons (CxHy). The C 1s spectrum of the CaCO3/PSA composite sample (Figure 8b) shows two component peaks D and F, which were assigned to the carbonyl carbon atoms of the polymer’s amide groups (ODC−NH−C) and the carbonyl carbon atoms of the carbonic acid groups (OFC−OH) and their corresponding carboxylates (OFC−Oθ ↔ θO−FCO). Beside the carbon atoms in the α position to the strongly electronegative carbonyl carbon atoms amine-sided C−N bonds of the amide groups and C−S bonds also contributed to component peak B. The N 1s spectrum of PSA-based composite samples showed, with component peak K (at 399.77 eV), nitrogen atoms of the polymer’s amide groups (OC−NH−C). In the C 1s spectrum of the CaCO3/CSA composite sample (Figure 8c), the polymer phase was also identified by the presence of the two component peaks D and F. However, here carbon atoms of the hemiacetal groups of the polysaccharide sequences (O−C−O) also contributed to component peak D. The intensive component peak C shows C−O(H,C) bonds. Surprisingly, it was necessary to introduce a second component peak L to fit the N 1s spectrum of the CaCO3/CSA composite sample (Figure 8c). Its binding energy of 400.64 eV seems to be too low to show fully protonated amino groups. However, it could be assumed that some of the nitrogen atoms of the amide groups are involved in strong interactions with the sulfonic acid groups. Sulfonic acids and their monoesters are strong acids, which are characterized by the nearly complete dissociation in aqueous media. Due to the presence of the free electron pair of the nitrogen atom, a hydronium ion, which is formed during the dissociation reaction of the sulfonic monoester group, can be bonded to the nitrogen. The corresponding decrease of the G



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Table 2. Some Characteristics of NPEC-n Dispersions: Ratio between Positive and Negative Charges (n+/n−), Charge Densities (CD), Zeta-Potential Values (ζapp), Hydrodynamic Diameter (Dh), and Size Distributions (PI)a NPEC-n

n+/n−

PSA/CS

0.2 0.4 0.2 0.4

CSA/CS a

ζappb (mV)

CDb (meq/g) −74.9 −49.3 −80.4 −55.7

± ± ± ±

−26.1 −21.5 −30.2 −22.9

0.9 0.6 0.8 0.7

± ± ± ±

Dh (nm)

0.5 0.4 0.5 0.6

172.1 184.8 309.9 323.7

± ± ± ±

2.5 3.6 5.6 6.2

PI 0.066 0.079 0.168 0.251

± ± ± ±

0.004 0.011 0.009 0.013

The values are the mean of three independent experiments that deviated from 0−7%. bDetermined at pH = 8.

Table 3. Electrokinetic Results [Charge Densities (CD) and Zeta-Potential Values (ζapp)] and FPIA Mean Diameters (d̅) and Sphericities (Sm) for NPEC-n-Based Composites, with Initial Inorganic Concentration of 0.2 mol L−1a sample C0.2N-PSA0.2 C0.2N-PSA0.4 C0.2N-CSA0.2 C0.2N-CSA0.4 a

CD (meq/g) −12.2 −11.7 −12.9 −11.5

± ± ± ±

0.8 0.5 0.7 0.6

ζapp (mV) −11.6 −11.4 −12.4 −12.3

± ± ± ±

0.5 0.8 0.7 0.8

d̅ (nm) 10.75 10.85 11.40 11.45

± ± ± ±

1.51 1.28 1.46 1.49

Sm 0.983 0.987 0.986 0.987

± ± ± ±

0.011 0.009 0.011 0.012

The values are the mean of three independent experiments that deviated from 0−7%.

Figure 9. SEM images of CaCO3/NPEC particles prepared with constant inorganic concentration (0.2 mol L−1) and different NPEC molar ratio between charges. The scale bar is 10 μm.

increase of the polyanion amount requested for the charge compensation and the formation of higher aggregates.24 The polydispersity index (PI) followed almost the same trend as did the particle sizes, and for the same ratio between charges, the higher values of PI were obtained when CSA was used as polyanion (Table 2). The NPEC-n dispersions were further used as templates in the composite particle formation with CaCO3, when the initial inorganic concentration was 0.2 mol L−1. The CaCO3/NPEC-n composite samples were coded CxN−Py, where x is the molar concentration of Na2CO3 and CaCl2 aqueous solutions (0.2 mol L−1), N−P is the nonstoichiometric complex based on polymer P (CSA or PSA), and y is the molar ratio of NPEC dispersion. Table 3 shows the electrokinetic results (CD and ζapp), FPIA mean diameters, and sphericities for the as-prepared composites. Compared with the starting NPEC-n dispersions, the charge densities of the particles and their zeta potential values sharply decreased after composite formation (Table 3) close to bare

electron density at the nitrogen atom shifts the apparent binding energy of its 1s electrons to slightly higher values. CaCO3/NPEC-n Composites. The NPEC-n dispersions used in this study are different in the chemical structure of the polyanions and the ratios between the negative and positive charges (n+/n−). Therefore, different charge densities (CD), zeta-potential values (ζapp), particle sizes (Dh) and size distribution (PI) are expected (Table 2). For the same molar ratio between charges, higher values of hydrodynamic particle sizes, charge density, and ζapp were obtained when CSA was used as a polyanion (Table 2). As both polyanions have the same kind of ionic groups in almost the same ratio on the polymeric macromolecular chain (50:50), the difference in particles size and surface charge (CD and ζapp), with the variation of ratio between charges (n+/n−), can be ascribed to the polymeric backbone structure: PSA with a linear and rather flexible chain, whereas the CSA polysaccharide chain is semirigid. The increase of the chain rigidity had two unfavorable consequences on the complex formation: the H



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polyanions chain flexibility differences induced the formation of NPEC nanoparticles with larger hydrodynamic diameters when CSA was used compared with those prepared with PSA (Table 2), due to the rigidity of the polyanion in excess. Therefore, the crystallite sizes and the ratio between polymorphs are not influenced by the polymer concentration or the presence of NPECs or their molar ratio between charges. The atomic compositions of samples prepared with NPEC dispersions with n+/n− = 0.4 and both polymer-based composites are summarized in Table 5.

CaCO3 particles values,17 with all investigated parameters being slightly influenced by the n+/n− ratios. This can be considered as a hint of the good NPEC-n embedment into the composites. However, compared to the particles prepared with the same polyanions and the same CaCO3 content, higher particle sizes were obtained when NPEC-n were used (Figure 5, Table 3), irrespective of the polyanion used in NPEC preparation. We assume that the use of complex nanoparticles as templates may restrict the CaCO3 growth, even if the polyanion in excess can act as a ligand for NPEC and CaCO3 nanocrystals. The morphology of CaCO3/NPEC composite particles observed by SEM is shown in Figure 9. As Figure 9 shows, the particle surface morphologies are very similar, irrespective of polyanion structures and NPEC molar ratios and differs from that of particles prepared with polyanions as the templates (Figure 1). These findings are in agreement with the values of parameters listed in Table 3, when almost the same values were obtained for the investigated parameters for all NPEC tested as templates in CaCO3 composite synthesis. To determine the polymorph content on the CaCO3 particles obtained in the presence of NPECs aqueous dispersions, comparative with composites prepared in the presence of polyanions, X-ray diffractograms were registered. The XRD semiquantitative analysis results listed in Table 4 were obtained with an EVA soft from DiffracPlus and an ICDD-PDF2 database, based on the patterns’ relative heights.

Table 5. XPS Determined Atomic Composition of Samples with NPEC Dispersions with n+/n− = 0.4 of Both PolymerBased Composites [N]:[C]|spec [O]:[C]|spec [S]:[C]|spec [Ca]:[C]|spec

crystallite size (Scherrer) (Å)

sample

R/R0

calcite

vaterite

calcite

vaterite

C0.2PSA0.2 C0.2PSA0.4 C0.2N-PSA0.2 C0.2N-PSA0.4 C0.2CSA0.2 C0.2CSA0.4 C0.2N-CSA0.2 C0.2N-CSA0.4

1.06 1.08 1.07 1.05 1.05 1.06 1.04 1.08

6.5 7.5 12.4 12.1 22.3 22.7 22.1 23.6

93.5 92.5 87.6 87.9 77.7 77.3 77.9 76.4

348.1 284.3 776.9 721.3 708.3 729.5 703.1 721.9

148.3 142.4 220.8 209.8 215.2 211.0 230.6 225.0

C0.2N-CSA0.4

0.025 0.519 0.009 0.124

0.024 0.472 traces 0.112

As Table 5 shows, the relative Ca 2p atomic concentration values are lower than those obtained for samples prepared with polyanions (Table 1). Obviously, the polymer content on the surface of the particles is increased. On the other hand, the low values of charge densities and zeta-potential values demonstrate that the sample surfaces can be considered largely electrically neutralized. Figure 10 shows the high-resolution C 1s and N 1s spectra of bare CaCO3 and the NPEC-based composite particle CaCO3 samples, C0.2N-PSA0.4 and C0.2N-CSA0.4.

Table 4. XRD Quantitative Analysis of Some Composite Samples, Performed with an ICDD-PDF2 Database polymorph ratio (%)

C0.2N-PSA0.4

As shown in Table 4, the polymorph ratio in the investigated composite particles is strongly influenced by the nature of the polyanion and less by the ratio between the organic and inorganic parts and the use of NPECs with the polyanion in excess. Thus, when the synthetic polyanion was used, PSA, the calcite polymorph growth was almost inhibited (just 6.5−7.5% calcite in PSA-based samples), irrespective of the polymer content into the composites, whereas microparticles with CSA contain almost 22% calcite. With the use of NPECs based on PSA, a slight increase of the calcite polymorph has been noticed (up to ∼12%), irrespective of their molar ratio between charges. The calcite crystallite size increased significantly when PSA-based NPECs were used (Table 4), with the highest value of about 77 nm being obtained when the molar ratio between charges was lower (C0.2N-PSA0.2). The higher calcite crystallite size obtained when CSA was involved in microparticle preparation could be explained by the lower macromolecular chain flexibility, compared with PSA, which probably induced the formation of an ionic cross-linked network with the larger apertures and allows the growth of larger crystals. The

Figure 10. High-resolution C 1s and N 1s element spectra of bare (a) CaCO3, (b) C0.2N-PSA0.4, and (c) C0.2N-CSA0.4 composite particles.

As can be seen in Figure 10, the addition of chitosan as a polycation did not significantly affect the shapes of the C 1s and N 1s spectra, with the high-resolution C 1s and N 1s spectra very similar to the spectra shown in Figure 7. The origins of the different component peaks can be explained as mentioned above. Compared to the unmodified CaCO3 sample, the C 1s spectra of both composite samples clearly showed that the intensities of the component peaks CO3 are clearly decreased. From these findings it can be concluded that the surfaces of the inorganic phase also contain polymeric material. Chitosan, which was used as a polycation to form polyelectrolyte complexes, carries primary amino groups. Component peaks I



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L on the N 1s element spectra show that these amino groups are partly protonated. Nitrogen atoms of nonprotonated amino and amide groups contributed to component peaks K.

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CONCLUSIONS Composite calcium carbonate microparticles were prepared from supersaturated inorganic aqueous solutions in the presence of some strong/weak anionic polymers (PSA and CSA) or NPEC dispersions with anionic charges in excess. The influence of the polyanion structure and relative inorganic/ polymer ratio tuned by initial solution supersaturation, polymer concentration, and NPEC molar ratio were thoroughly investigated. SEM was used to visualize the particles’ shape and morphology and FPIA to evidence their mean sizes and sphericities. The polymer presence on the composite particle surfaces was evidenced by XPS and by the determination of the particles’ charge densities as well as zeta-potential values. Increasing the inorganic concentration from 0.05 mol L−1 to 0.3 mol L−1, the particles sizes significantly increased, irrespective of the polyanion structure. For the same inorganic concentration, more negatively charged particles were obtained in the presence of PSA, probably due to a better polyanion distribution in the composite material as a consequence of its flexible backbone. When 0.4 wt % PSA was used, an increased amount of polyelectrolyte molecules increased the polymer content on the particles surfaces, and therefore, there was a lower content in the inorganic phase (decrease of the relative concentration of calcium expressed as [Ca]:[C]|spec). By contrast, when 0.4 wt % CSA was used, the relative calcium concentration increased compared with that of sample C0.2CSA0.2, with the polymer being more uniform embedded into the composite and sustained by SEM and electrokinetic measurements. The use of NPEC-n as templates in the composite formation strongly decreases the particle charge densities and their zeta-potential values compared with starting NPEC dispersions, suggesting their uniform embedding into the composites. The NPEC-n initial dispersion characteristics, such as molar ratio, particle size, and charge density, slightly affect the CaCO3/NPEC composite particle surface morphologies and electrokinetic parameters, almost the same values being obtained for the investigated parameters, for all NPECs tested as templates in the CaCO3 composite synthesis.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +40.2322217454. Fax: +40.232211299. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of Project ID_313/2011 is gratefully acknowledged.

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REFERENCES

(1) Raiteri, P.; Gale, J. D.; Quigley, D.; Mark Rodger, P. J. Phys. Chem. C 2010, 114, 5997−6010. J


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