Direct Observation of the Influence of Additives on Calcite Hydration

Article pubs.acs.org/crystal

Direct Observation of the Influence of Additives on Calcite Hydration by Frequency Modulation Atomic Force Microscopy Yuki Araki,*,† Katsuo Tsukamoto,† Ryosuke Takagi,‡ Tomoyuki Miyashita,‡ Noriaki Oyabu,§ Kei Kobayashi,§,⊥ and Hirofumi Yamada§ †

The Earth and Planetary Material Science, Graduate School of Science, Tohoku University, 6-3, Aoba, Aramaki, Aoba, Sendai, 980-8578, Japan ‡ Department of Genetic Engineering, Faculty of Biology-Oriented Science and Technology, Kinki University, 930 Nishimitani, Kinokawa, Wakayama 649-6493, Japan § Department of Electronic Science and Engineering, Kyoto University, Katsura, Nishikyo, Kyoto 615-8510, Japan ⊥ The Hakubi Center for Advanced Research, Kyoto University, Katsura, Nishikyo, Kyoto 615-8520, Japan ABSTRACT: The effects of inorganic and organic additives on the hydration structure on the crystal surface have been discussed in X-ray reflectivity studies and in molecular dynamics simulations. We now demonstrate their effects on the hydration structure by conducting in situ observations of the hydration structure at a growing calcite surface by frequency modulation atomic force microscopy (FM-AFM). We show the atomic scale change of the hydration structure on the calcite surface in a supersaturated solution of CaCO3 by the addition of magnesium ions and a hydrophilic polypeptide. The FM-AFM images of the hydration structure revealed that magnesium ions increase the number of hydration layers on the terrace of the calcite surface from two to four layers. On the other hand, the hydrophilic polypeptide was ineffective for the hydration of the calcite surface. When both the magnesium ions and the hydrophilic polypeptide were added to the CaCO3 solution, the number of hydration layers increased and the magnitude of the oscillation hydration force as well as the long-range electrostatic force became larger than in the case when they were individually added. This is a noteworthy effect on the hydration structure on the calcite surface by cooperation of the magnesium ions and the polypeptide.

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INTRODUCTION The influence of additives on the shape of hillocks of the calcite surface and the nucleation rate, such as the pinning of organic molecules and kink-blocking due to magnesium ions at the step front of calcite, has been extensively investigated.1−10 On the other hand, their effects on hydration in the vicinity of calcite surfaces remain unclear, although the effect of additives on the hydration of the calcite surface has been suggested. For example, magnesium ions incorporated into calcite surfaces11−13 have been considered to make the hydration at the calcite surface stronger. This is because magnesium ions more strongly hydrate than calcium ions in an aqueous solution due to the higher surface charge density of the magnesium ions.8,14 Also, recent experimental studies have been performed to investigate the effects of organic additives on the hydration of calcite surfaces.6,15,16 Elhadj et al. have shown that the step velocity of calcite increased in supersaturated CaCO3 solutions containing low peptide concentrations (10−6−10−4 M).15,16 This acceleration of the step velocity increased with the hydrophilicity of the organic peptides, which were proposed to change the hydration structure at the step front of the calcite. Furthermore, interferometry measurements of the calcite © 2014 American Chemical Society

growth rate revealed that the characteristic surface diffusion length of calcite terraces increased in the presence of L-aspartic acid.6 This observation may result from the capture of water molecules by L-aspartic acid on the terraces. Although studies have mentioned these additive effects on the hydration of calcite surfaces, none have reported any direct experimental evidence for these effects. The demonstration of the additive-induced changes in the hydration of a growing surface of calcite hinges on the observation of the interface between the calcite surface and solution. In addition, the hydration structure of the terrace and the step front should be distinguished, because adsorption of the additives depends on the surface topography of calcite. Surface X-ray reflectivity measurements have been utilized to visualize the hydration structure on the calcite surface.17−21 This method has succeeded in the detection of the hydration layers of the calcite−water interface with high vertical resolution and estimated the structure of water in the vicinity Received: June 18, 2014 Revised: October 29, 2014 Published: October 31, 2014 6254

dx.doi.org/10.1021/cg500891j | Cryst. Growth Des. 2014, 14, 6254−6260

Crystal Growth & Design

Article

Calcite Crystal. A calcite crystal (3 mm × 3 mm × 2 mm) was cleaved and fixed to the bottom of the open fluid cell so that the (101̅4) face became parallel to the scanning plane. A 100-μL droplet of growth solution was loaded onto the calcite surface. Preparation of Growth Solutions. The pure supersaturated CaCO3 solution was prepared by mixing the solutions of 0.1 M calcium chloride (CaCl2), 0.1 M sodium bicarbonate (NaHCO3), and 0.1 M sodium chloride (NaCl) dissolved in purified water (Millipore) at room temperature. The synthetic polypeptide (5.2 × 10−6 M) was dissolved in the supersaturated CaCO3 solution in a centrifuge tube, and the mixture was stirred for 1 min using ultrasonic vibrations. Magnesium ions were added by a 0.1 M MgCl2 solution so that the resulting concentrations of the calcium and magnesium ions equaled 0.01 and 0.05 M, respectively. The supersaturation σ is defined as

of the calcite surface in collaboration with molecular dynamics simulations.22,23 However, the X-ray reflectivity includes all information about the hydration structure of the terrace and the step front of the calcite surface. In order to reveal distinct and detailed structural differences in the hydration of the terrace and the step front of the calcite surface, visualization of the local hydration structure is required. The newly developed frequency modulation atomic force microscopy (FM-AFM) technique is expected to meet these requirements. FM-AFM has visualized the local hydration structure of the surfaces of several inorganic crystals24−30 and organic monolayers31−33 under static environments with atomic resolution. We demonstrate the influence of hydrophilic organic molecules and magnesium ions on the hydration structure of a calcite surface by in situ atomic-scale observation of the hydration structure of a growing calcite surface using FM-AFM.

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σ≡

⎛ aCa 2 +aCO2 − ⎞ Δμ 3 ⎟⎟ = ln⎜⎜ kT K sp ⎝ ⎠

(1)

where Δμ is the change in the chemical potential per molecule, k is the Boltzmann constant, T is the absolute temperature, and aCa2+ and aCO32− are the activities of the calcium and carbonate ions, respectively. The solubility product Ksp of calcite used was 10−8.48 at 25 °C.8 The supersaturation was adjusted to 0.8 for calcite at room temperature. The pH of each solution was adjusted to 8.1 ± 0.05 by adding a few drops of 0.5 M HCl and NaOH solutions immediately before the FMAFM imaging.

EXPERIMENTAL SECTION

FM-AFM. The FM-AFM instrument working in liquid was developed by modifying the optical beam displacement sensor of the commercial SPM-9600 microscope (Shimadzu Corp., Japan).34 The instrument was equipped with gold-coated, highly doped n-type Si cantilevers (Nanosensors, PPP-NCHAuD). The cantilevers displayed a typical eigenfrequency of 160 kHz in liquids and self-oscillated at their resonance frequency in the constant amplitude mode. Samples were placed in a 6-mm-deep open fluid cell with a diameter of 15 mm for imaging. The instrument was kept in an incubator (Mitsubishi Electric Engineering CN-40A) which was maintained at 295 ± 0.15 K during the experiments. The lateral thermal drift rate was reduced to be less than 1 nm/min. Topographic images were collected in the constant frequency shift mode with a peak-to-peak oscillation amplitude of about 1−2 nm. Two dimensional (2D) images of the interface between the calcite and solution were obtained by 2D force mapping.25 2D frequency shift maps at the solid−liquid interfaces were acquired by collecting the frequency shift vs. distance curves at 256 pixels allocated to a line segment on the surface. The frequency shift of the cantilever was recorded while the tip approached the sample surface. When the frequency shift signal reached a predetermined threshold value, the tip was immediately retracted to its original position.29 During this data acquisition, the cantilever peakto-peak oscillation amplitude was reduced to 0.2−0.6 nm to detect the oscillatory hydration force with a period comparable with the size of the water molecules. The WSxM software package (Nanotech Electronica) was used for image rendering and data processing.35 Observations were carried out in the pure, additive-free, supersaturated solution of calcium carbonate (CaCO3) and in the solutions which individually contained magnesium ions and the organic polypeptide. In addition, we prepared the CaCO3 solution which contained both the magnesium ions and the polypeptide to confirm their cooperative effect on the hydration structure of the calcite surface. Organic Polypeptide Additive. The synthetic polypeptide DFDRPDPYDPYDRFD (D: aspartic acid, F: phenylalanine, R: arginine, P: proline, Y: tyrosine), which consisted of 15 amino acid residues including six periodically interspaced aspartic acids, was selected as the organic additive to confirm the influence of aspartic acids on the hydration structure. This is part of the amino acid sequence of a protein, prismin, contained in pearl oyster shells which is effective for the polymorphism of the calcium carbonate crystal.36 Although the structure of the synthetic polypeptide molecule has not been determined, the length of the polypeptide is roughly estimated as 2−3 nm in its α-helix form, or as 5−6 nm if stretched. The polypeptide was purchased from Hokkaido System Science Corporation, Ltd., with a purity exceeding 90%. The 10% contamination is a majorly incomplete chain length peptide. The interaction of the contamination with the tip, the ions, and the calcite surfaces is considered to be equal or weaker than the synthetic polypeptide. The water solubility was 1 mg/mL.

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RESULTS Calcite Hydration in Pure CaCO3 Solution. Figure 1a shows an atomic-resolution FM-AFM image of the (1014̅ ) calcite surface in a CaCO3 solution in the absence of additives. The calcite surface exhibited the spiral growth steps and 2D nucleation sites (images not shown). The atomic-resolution image was taken on a flat terrace of 2D islands over a scan area of 5 nm × 2 nm. The atomic-scale features shown in Figure 1a were zigzag pattern. Under a UHV environment, the FM-AFM topographic images have reportedly given two representative patterns for the calcite (101̅4) face depending on the tip conditions: one lattice pattern for the calcium atom locations and one characteristic zigzag pattern for the oxygen atom locations.37 On the other hand, in aqueous solution, the interpretation of the topographic image has been controversial because of the tip hydration. Although it has been estimated that the hydrated tip detects water-mediated interaction with the sample surface, recent molecular dynamics simulation reported that the hydrated tip directly interacts with the sample surface by merging of the hydration layers of the tip and the sample surface when the tip is extremely close to the sample surface.38 It suggests that the topographic image in solution can be interpreted as well as the UHV environment when the tip is close to the sample surface by large amplitude.39 Our topographic images were taken with a large amplitude (1−2 nm) and large frequency shift (Δf). Hence, no oscillation due to hydrated water molecules was detected on the Δf-distance curves during the topographic imaging (data not shown). Therefore, we interpreted the patterns of the topographic images either formed by the direct interaction with the surface atoms as well as UHV environment, or by the interaction with the strongly bound hydrated water molecules on the calcite surface that follow the surface atomic structures, so that the zigzag pattern in Figure 1a is considered as the feature of the oxygen atoms. Figure 1b shows a 2D frequency shift map acquired along the [481̅] direction indicated by the dotted line in Figure 1a. The image was obtained in 3 s over the scan area 6255



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were formed in the CaCO3 solution in the absence of additives. The two hydration layers were always observed in several experiments regardless of the tip change which was suggested by the change of the topographic patterns. The three hydration layers were shown at the calcite−water interface by MD simulation.38 Also, previous FM-AFM measurement reported the three layered hydration structure at the calcite−KCl solution interface, which was similar to the simulated hydration structure in water.45 The distance of each hydration layer in KCl solution was consistent with the result of the MD simulation. In our FM-AFM image, the distance between the first layer and the second layer in the pure CaCO3 solution was 0.14 nm, and it was consistent with that in the KCl solution and the MD simulation in water. The bright areas in the force map seem to be water molecules, not the ions, because the concentration of the ions is extremely low. Also, the interval of the peaks of the force− distance curves (0.1−0.2 nm) was consistent with the size of a water molecule in comparison with the size of the hydrated calcium and magnesium ions (0.41−0.43 nm). Calcite Hydration in CaCO3 Solution in the Presence of the Synthetic Polypeptide. An atomic-resolution FMAFM image of the (1014̅ ) calcite surface representing the zigzag pattern of the oxygen atoms was acquired in a CaCO3 solution containing 5.2 × 10−6 M of the synthetic polypeptide (Figure 2a). In addition, the corresponding 2D frequency shift map was taken along the [481̅] direction of the calcite crystal in 2 s over a scan area of 5 nm × 2 nm (Figure 2b). This 2D hydration image was similar to the image obtained in the additive-free CaCO3 solution. The closest water molecules were located between the oxygen atoms.

Figure 1. FM-AFM images and force−distance curves of calcite surface and solid−liquid interface in pure CaCO3. (a) Atomic-scale image of the calcite surface. The red balls represent oxygen atoms of the (101̅4) surface of calcite. (b) 2D frequency shift map taken along the [481̅] direction of the calcite crystal (green dashed line, panel a). The darkest area at the bottom of the image is the calcite crystal region. Blue, white, and red balls represent calcium, carbon, and oxygen atoms, respectively. Bright areas marked by red dashed circles correspond to water molecules. (c) Averaged force profiles measured on the valley and the convex of the calcite surface. The starting point of the z distance indicates the position of the convex top in panel b. Peaks marked by arrows correspond to hydration layers.

of 5 nm in the lateral direction by 2 nm in the vertical direction. The observed contrast represented the frequency shift of the cantilever. The darkest area at the bottom of the 2D frequency shift map indicates the area without any data. The brightest area represents the area in which the frequency shift was close to the predetermined threshold value. Note that the interface between the darkest and brightest areas roughly represents the surface topography of the calcite. We assumed that the convex and the valley positions showed the location of the oxygen atoms and calcium atoms, respectively. Bright areas delimited by the red dashed circles originated from the repulsive forces that arose when the tip was brought close to the calcite surface. Although the relationship between the hydration force and the density of the water molecules was not straightforward,38,40,41 previous MD simulations42,43 and FM-AFM measurements29 have reported that the positive frequency shift is detected at the area of high density of the water molecules. Therefore, the bright areas corresponded to locations where the density of water molecules was higher than in the bulk solution. The frequency shifts were quantitatively converted to interaction forces exerted on the tip using Sader’s method.44 The positive and negative frequency shifts resulted from the repulsive and attractive forces, respectively. The site-specific force−distance curves for oxygen atoms and calcium atoms in the calcite surface are shown in Figure 1c. Overall, each force−distance curve showed a peak indicated by the arrows in Figure 1c, suggesting that two hydration layers

Figure 2. FM-AFM images and force−distance curves of the calcite surface and solid−liquid interface in a CaCO3 solution containing 5.2 × 10−6 M of the synthetic polypeptide. Descriptions of the symbols of a−c are the same as in Figure 1. 6256



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the additives. Figure 4a shows the lattice pattern of the calcium atoms in the calcite surface taken in a CaCO3 solution

Each site-specific force−distance curve showed a peak on the oxygen atoms and the calcium atoms (Figure 2c), indicating that the synthetic polypeptide did not affect the hydration structure of the calcite surface. The two hydration layers were constantly observed regardless of the change of the topographic pattern suggesting the tip change. Calcite Hydration in CaCO3 Solution in the Presence of Magnesium Ions. A 2D FM-AFM image of calcite representing the zigzag pattern of the oxygen atoms was obtained in a CaCO3 solution containing magnesium ions (Figure 3a). The corresponding 2D frequency shift map was

Figure 4. FM-AFM images and force−distance curves of the calcite surface and solid−liquid interface in a CaCO3 solution containing 5.2 × 10−6 M of the synthetic polypeptide and magnesium ions. Descriptions of the symbols of a−c are the same as in Figure 1.

containing magnesium ion and the synthetic polypeptide. The 2D frequency shift map was acquired in 12 s over a scan area of 4 nm × 2 nm on the terrace of the 2D island (Figure 4b). The closest packed water clusters marked by the red dashed circles were similar to those obtained in the CaCO3 solution containing 0.05 M magnesium ion. The closest water molecules were situated on the oxygen atoms. Overall, four hydration layers were recognized in the site-specific force−distance curves (Figure 4c). The force−distance curves included monotonic increase derived from the long-range electrostatic force in addition to the oscillatory hydration force. It suggests the increase of the electric double layer of the calcite surface in the coexistence of the magnesium ions and the synthetic polypeptide. Note that the peaks of the oscillatory hydration force were clearly shown in the force−distance curves regardless of the long-range electrostatic force. It indicates that the magnitude of the oscillatory hydration force was large. The closest packed patterns of water clusters were examined in CaCO3 solutions containing 2.6 × 10−5, 5.2 × 10−5, and 1.6 × 10−4 M of the synthetic polypeptide. The representative 2D frequency shift map and the force−distance curve acquired in the CaCO3 solution containing 1.6 × 10−4 M of the synthetic polypeptide are shown in Figure 5, panels a and b, respectively. These data indicate that the calcite hydration does not depend on the synthetic polypeptide concentration.

Figure 3. FM-AFM images and force−distance curves of the calcite surface and solid−liquid interface in a CaCO3 solution containing magnesium ions. Descriptions of the symbols of a−c are the same as in Figure 1.

taken in 3 s over a scan area of 5 nm × 2 nm (Figure 3b). Bright areas marked by the red dashed circles in Figure 3b showed the closest packed water clusters. The site-specific force−distance curves showed two peaks each on the oxygen atoms and the calcium atoms (Figure 3c), indicating four hydration layers formed in the presence of magnesium ions. The water molecules were located on the calcium atoms in the first and third layers and above the oxygen atoms in the second and fourth layers. The repulsive force exerted on the tip weakened when its distance from the calcite surface increased. This result suggested that magnesium ions promote the hydration of calcite surfaces into the multilayers. The four hydration layers were constantly observed regardless of the tip change. Calcite Hydration in CaCO3 Solution with the Coexistence of the Synthetic Polypeptide and Magnesium Ions. The 2D hydration structure of calcite in the presence of magnesium ions and 5.2 × 10−6 M of the synthetic polypeptide was examined to determine the multiple effects of 6257



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ions may contribute to the hydrogen bond between the hydrated water molecules in the closest hydration layer. As a result, the first hydration layer would be strongly structured in a plane in the vicinity of the calcite surface. The upper hydration layers would be easily structured based on the first layer. Although there is no current theoretical support of the effects of the ions in solution for the AFM measurement of the hydration structure, comparison of the theoretical and the experimental results is expected in the near future. Effects of the Synthetic Polypeptide on the Hydration Structure. Hydrophilic organic molecules have been proposed to capture water molecules and change the hydration structure of the calcite surface by adsorption on the step fronts15,16 or the terrace6 of calcite. The hydrophilicity of the synthetic polypeptide was 61.2 kJ/mol according to the Hopp and Woods hydrophilicity scale,47 which mostly amounted to the same value for the peptides used by Elhadj et al.15 Nevertheless, there was no change in the hydration structure on the terrace only in the presence of the synthetic polypeptide. It implies that organic molecules are individually ineffective for the hydration structure on the calcite surface even if these molecules are hydrophilic. Considering the magnitude of the oscillatory hydration force, the dehydration energy of the terrace in the CaCO3 solution containing the synthetic polypeptide alone would be as well as that in the pure CaCO3 solution. Therefore, it is estimated that the synthetic polypeptide would not decrease the energetic barrier of the ion capture by the calcite surface and the surface diffusion on the terrace. Effects of the Coexistence of the Synthetic Polypeptide and Magnesium Ions on the Hydration Structure. The fact that the long-range repulsive force of the first layer was larger under the coexistence of the synthetic polypeptide and magnesium ions in the CaCO3 solution was probably due to the electrostatic force between the magnesium ions and the tip. It indicates that the electric double layer at the calcite−solution interface was larger in the coexistence of the magnesium ions and the synthetic polypeptide. It is reasonable, because it has been confirmed that the range of the electric double layer increases with the concentration of the cation in the solution.48 On the other hand, the electric double layer did not change in the presence of the magnesium ions alone. Therefore, it is estimated that the concentrating of the magnesium ions in the vicinity of the calcite surface occurred in the coexistence of the

Figure 5. FM-AFM images and force−distance curves of the calcite surface and solid−liquid interface in a CaCO3 solution containing 1.6 × 10−4 M of the synthetic polypeptide and magnesium ions. Descriptions of the symbols of a−c are the same as in Figure 1.

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DISCUSSION Effects of Magnesium Ion on the Hydration Structure. The 2D hydration images showed that magnesium ions facilitated the formation of a multilayered hydration structure regardless of the presence of the synthetic polypeptide (Figures 3b, 4b, and 5a). It has been estimated that the magnesium ions having hydration shells stronger than those of the calcium ions are incorporated into the calcite surface and strongly bind the closest water molecules on the calcite surface.12−14,46 In accordance with this theory, outer hydration layers would be formed based on the strong first hydration layer. However, the measurements of the repulsive force, which reflects the hydration force, indicated that the hydration force of the first layer did not change only in the presence of magnesium ions. This result implies that the hydration force of the first layer is unrelated to the formation of the multilayered hydration structure. Although the mechanism governing this multilayered hydration is unknown, these data suggest that the magnesium

Figure 6. Schematic of the influence of the additives on the local hydration structure. The gradation of the background represents the strength of the electrostatic force. The balls indicate the hydrated water molecules, and the contrasts show the strength of the hydration force. The elongated balls represent the increase of the delocalization of the water molecules. 6258



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magnesium ions and the synthetic polypeptide. Concentrating of the magnesium ions seems to be involved by chelating with the synthetic polypeptide. The synthetic polypeptide may form a chelate with the magnesium ions due to the carboxyl group of its aspartic acid.49,50 Aspartic acid residues are periodically located in the sequence of the synthetic polypeptide so that two carboxyl groups can be ligands of a magnesium ion. The concentration of magnesium ions would become higher in the vicinity of the calcite surface than in the bulk solution due to the chelating, and as a result, the long-range electrostatic force between the concentrated magnesium ions and the tip was detected. Regardless of the increase of the electric double layer, the oscillatory hydration force was detected in the force−distance curves in the coexistence of the magnesium ions and the synthetic polypeptide. It suggests that the magnitude of the oscillatory hydration force was larger than that of the other solutions. This indicated that the binding energy between the water molecules and the atoms of the calcite surface increases. The water molecules may be captured by the concentrated the magnesium ions in the vicinity of the calcite surface. The local change of the hydration structure of calcite surface due to the additives is summarized in Figure 6. Figure 6 shows the change of the number of hydration layer, the electrostatic force, the hydration force, and the localization of the water molecules in the vicinity of calcite surface. When coexisting with the magnesium ions, the synthetic polypeptides may contribute to interactions between the water molecules and the atoms on the calcite surface, making the adsorption of the ion on the calcite surface and the surface diffusion more difficult than only in the presence of magnesium ions. As a result, the calcite growth rate would decrease according to the currently prevailing theory that claims that dehydration is a rate-determining process in solution growth.51−53 In order to examine the relation of the hydration structure and the growth rate of calcite, the hydration image at the step front is necessary. Investigations on the effect of the additives on the hydration of the step front by FM-AFM are currently underway.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The use of FM-AFM was enabled by the Kyoto-Advanced Nanotechnology Network. The authors acknowledge the Matsushige Laboratory, Kyoto University, for the technical support. Financial support was provided by a Grant-in-Aid of Research from Challenging Exploratory Research for K.T. and by a Research Fellowship from the Japan Society for the Promotion of Science for Y. A.

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REFERENCES

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CONCLUSIONS We demonstrated the atomic-level change of the local hydration structure on the calcite surface induced by magnesium ions and the synthetic polypeptide for the first time. We found that the four hydration layers consist of the closest packed water clusters in the vicinity of the calcite surface in the presence of magnesium ions. These magnesium ions affected the multilayering of hydration layers, but not the hydration force. Although hydrophilic organic molecules had previously been suggested to impact the calcite hydration, the synthetic polypeptide had no effect on either the number of the hydration layers or hydration force of the calcite terrace. On the other hand, cooperative effects between the synthetic polypeptide and the magnesium ions involved strengthening of the hydration force and an increase of the electric double layer. We showed that in situ observation of the atomic-scale hydration structure by FM-AFM is valid not only under the static environment but also on a growing surface. The clarification of the local hydration structure at the terrace and the step front on a growing surface enabled us to validate the importance of the hydration structure for solution growth. 6259



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Direct Observation of the Influence of Additives on Calcite Hydration

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