Binding of Ethanol on Calcite: The Role of the OH Bond and Its

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Binding of Ethanol on Calcite: The Role of the OH Bond and Its Relevance to Biomineralization K. K. Sand,*,† M. Yang,† E. Makovicky,‡ D. J. Cooke,§ T. Hassenkam,† K. Bechgaard,† and S. L. S. Stipp† † Nano-Science Center, Department of Chemistry, University of Copenhagen, Denmark, ‡Department of Geography and Geology, University of Copenhagen, Denmark, and §Department of Chemical and Biological Sciences, University of Huddersfield, U.K.

Received March 22, 2010. Revised Manuscript Received May 31, 2010 The interaction of OH-containing compounds with calcite, CaCO3, such as is required for the processes that control biomineralization, has been investigated in a low-water solution. We used ethanol (EtOH) as a simple, model, OHcontaining organic compound, and observed the strength of its adsorption on calcite relative to OH from water and the consequences of the differences in interaction on crystal growth and dissolution. A combination of atomic force microscopy (AFM) and molecular dynamics (MD) simulations showed that EtOH attachment on calcite is stronger than HOH binding and that the first adsorbed layer of ethanol is highly ordered. The strong ordering of the ethanol molecules has important implications for mineral growth and dissolution because it produces a hydrophobic layer. Ethanol ordering is disturbed along steps and at defect sites, providing a bridge from the bulk solution to the surface. The strong influence of calcite in structuring ethanol extends further into the liquid than expected from electrical doublelayer theory. This suggests that in fluids where water activity is low, such as in biological systems optimized for biomineralization, organic molecules can control ion transport to and from the mineral surface, confining it to specific locations, thus providing the organism with control for biomineral morphology.

Introduction Understanding how simple organic compounds bind to mineral surfaces is a step toward understanding biomineralization in general, and offers the key to biomimetic materials design. Biomineralization by, for example, coccolithophorids is a sophisticated process that takes place inside the cell in vesicles or cisternae.1 The mechanisms for simple ion transport in aqueous systems and adsorption on mineral surfaces are well-known, but inside a cell, where the ratio of organic molecules to water is high and the interacting entities are complex, we have much to learn. It is well-known that organisms, such as oysters, wood lice, and algae, use organic molecules to control calcite, CaCO3, growth. Many crystal growth modifying compounds have been identified, and many studies have explored their interaction with calcite.2,3 These organic molecules are often complex and contain a variety of functional groups, including OH and COOH. Some efforts have been made to isolate and investigate exactly how functional groups influence calcite growth.4 A recent molecular dynamics (MD) study established that neutral trisaccharides bind to calcite through OH and the relative strength of the bond depends on the OH position and the structure of the sugar ring.5 Another MD simulation demonstrated that the OH of ethanol has a stronger affinity for the calcite {10.4} surface than water. Ethanol from a *To whom correspondence should be addressed. E-mail: [email protected]. Address: Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK 2100. (1) Young, J. R.; Davis, S. A.; Bown, P. R.; Mann, S. J. Struct. Biol. 1999, 126, 195–215. (2) Marsh, M. E.; Ridall, A. L.; Azadi, P.; Duke, P. J. J. Struct. Biol. 2002, 139, 39–45. (3) Henriksen, K.; Stipp, S. L. S.; Young, J. R.; Marsh, M. E. Am. Mineral. 2004, 89, 1709–1716. (4) Sommerdijk, N. A. J. M.; With, G. D. Chem. Rev. 2008, 108, 4499–4550. (5) Yang, M. J.; Stipp, S. L. S.; Harding, J. Cryst. Growth Des. 2008, 8, 4066– 4074.

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50:50 water and ethanol mix is preferentially attracted to calcite, and, in contact with 100% ethanol, the water initially adsorbed on calcite was displaced. These predictions were confirmed by AFM experiments: calcite surface activity was significantly modified when ethanol was present in aqueous solutions.6 Ethanol’s strong affinity for calcite was unexpected considering the dipolar nature of water and the ionic structure of calcite. Ethanol serves as the most fundamental, linear organic molecule where a fatty acid end is separated from an OH end, that is, in the molecular formula CH3CH2-OH (Figure 1a); methanol, CH2-OH, is a spherical entity. Comparing ethanol’s behavior with that of water, the most basic OH containing molecule, H-OH, would provide fundamental understanding about the driving force for attachment and perhaps shed light on the activity of more complex organic compounds that depend on OH for bonding. The purpose of this work was to learn more about how biologically produced organic molecules attach to mineral surfaces by elucidating how OH-calcite binding influences adsorption, crystal growth, and dissolution in simple low-water systems. This study was designed to explore the behavior on calcite step edges, in contact with ethanol-water solutions, and to extend the work on terraces by Cooke et al.6 The calcite {10.4} plane is the face always produced by cleavage, and it is commonly expressed by crystal growth surfaces because it is the thermodynamically most stable (Figure 1b). To model steps, we constructed planes, {31.8} and {31.16}, that represent vicinal faces to the rhombohedral cleavage plane, where one molecular layer thick steps (∼3.A˚) link bands of {10.4} planes. The steps formed by the {31.8} plane make an angle of 78 to the {10.4} terraces above and below the step; these are the acute step surfaces. Plane {31.16} makes steps (6) Cooke, D. J.; Gray, R. J.; Sand, K. K.; Stipp, S. L. S.; Elliott, J. A. Langmuir, accepted.

Published on Web 09/02/2010

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Sand et al. 22 nN/nm. For both instruments, the force between the tip and the sample was varied to control the force exerted by the tip on the surface. Scan angle, scan rate, and set-point were systematically varied. For the purpose of reproducibility, multiple sites on each sample were measured and measurements were repeated on a number of samples. Features on the surfaces were examined using width, height, and roughness measurements calculated with the AFM software (Nanoscope 6.13R1 and the MFP3D package). After cleavage in air, the tiny particles of calcite that always adhere to the surfaces were mechanically removed with a jet of 99.999% N2. Previous work shows that not all particles are removed, however, and those remaining dissolve easily, whether in the water adsorbed from humidity in air or in solution.8 A jet of nitrogen was also used to mechanically sweep away the adhering liquid films to avoid evaporation and precipitation of dissolved material before scanning in air.

Set up for the Molecular Dynamics (MD) Simulations.

Figure 1. (a) Atomic structure of an ethanol molecule and (b) perpendicular view onto the {10.4} surface. The thickness of the slice is equivalent to one molecular layer, ∼3 A˚. The {10.4} set of faces form acute (78) and obtuse (102) angles at their intersection. (c) A cross-section across part b, parallel to any of the cleavage edges, illustrating the angular relationships at the intersection of the {10.4} planes.

with an angle of 102 to the terraces above and below, so this vicinal surface is composed of obtuse steps. The bands of {10.4} planes between consecutive acute steps are 3 atomic rows wide, whereas those between the obtuse steps are 4 rows wide. We explored the behavior of these terraces and steps, combining the results of molecular dynamics modeling with observations from experiments using AFM, where we examined calcite surfaces, in situ, under solutions of ethanol and water.

Experimental Details Materials. Single crystals of optical quality Iceland spar calcite (purchased from Ward’s Natural Science, USA) were cleaved in air, under water, or under 96% ethanol using the method developed by Stipp and Hochella.7 For the experiments in water, the crystals were cleaved in air, and fresh, ultrapure, deionized (DI) water (Milli-Q; resistivity >18 MΩcm; Millipore Corporation) was added to the AFM fluid cell. Previous work has shown that surfaces, cleaved in air, immediately satisfy dangling bonds with water from the air, so there is no difference in surface composition whether the sample is cleaved under water or in air.7 Crystals intended for the ethanol experiments were imaged immediately after cleavage under ethanol or stored in a Teflon beaker with an airtight screw lid to avoid evaporation of the ethanol or concomitant entry of water vapor. Surface analysis of samples using X-ray photoelectron spectroscopy, XPS, surprisingly revealed that Zn was adsorbed on calcite exposed to commercially produced ethanol from glass bottles and an abundant hydrocarbon contamination resulted from ethanol delivered in plastic bottles. Therefore, we redistilled the ethanol through a 50 cm Raschig column and stored it in sealed glass bottles. Atomic Force Microscopy (AFM). For scanning in air and liquids, we used a Digital Instruments Multimode IIIa AFM running in contact mode and an Asylum Research MFP-3D AFM in both contact and tapping mode. We used sharpened tips of Si and Si3N4 with spring constants between 5 pN/nm and (7) Stipp, S. L.; Hochella, M. F. Geochim. Cosmochim. Acta 1991, 55, 1723– 1736.

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MD simulations were performed for the calcite cleavage surface, the terrace, {10.4}, the acute, {31.8}, and the obtuse, {31.16}, stepped viscinal faces, when solvated with 100% ethanol. Modeling the acute- and obtuse-stepped surfaces was performed to compare with the behavior observed in the AFM experiments. The calcite surfaces were constructed with Material Studio 5.0,9 and the ethanol molecule was produced using the LEap program in Amber 9.0.10 The surfaces used in our calculations were nonpolar, without a net dipole moment, and they had been relaxed before the water or ethanol was added. Hence there were no contributions from interacting dipole moments in the computational setup. The force field used in our study was a hybrid between Amber and Pavese et al.11 potentials. The interaction potentials used for CaCO3 were those derived by Pavese for modeling a range of properties for calcite and aragonite crystals. The ethanol potential was constructed with the Antechamber program in AMBER 9.010 and the flexible TIP3P potential was used for water.12 The mixing terms for van der Waals (VDW) interactions between water/ethanol and calcite were generated with the method proposed by Freeman et al.,13 and the VDW parameters are listed in Table 3 in the Supporting Information. The starting configurations were generated using the PACMOL code.14 This code packs molecules into a cubic box, avoiding atomic overlaps that would result in an excessively large potential energy. Two kinds of MD simulations were performed using DL_POLY 2.20:15 (i) calcite surfaces covered with water molecules and (ii) calcite surfaces covered with ethanol molecules. Each surface was solvated with either 150 ethanol or 600 water molecules and the simulations were carried out using a time step of 1 fs for a total time of 8 ns. The first 4 ns of the simulation was used for equilibration and the 4 ns trajectory after that was used for data analysis. A three-dimensional periodic boundary condition was applied in the simulations, and in each model there was a vacuum gap with a thickness of approximately 100 A˚ to avoid interactions between the system and its images. Standard Ewald summation was used for calculating electrostatic interactions with a precision of 1.0  10-5. (8) Stipp, S. L. S.; Gutmannsbauer, W.; Lehmann, T. Am. Mineral. 1996, 81, 1–8. (9) Accelrys, MS Materials Visualizer, release 5.0. Accelrys Software, Inc.: San Diego, CA, 2009. (10) Case, D. A.; Darden, T. A.; Cheatham, T. E., III; Simmerling, C. L.; Wang, J.; Duke, R. E.; Luo, R.; Merz, K. M.; Pearlman, D. A.; Crowley, M.; Walker, R. C.; Zhang, W.; Wang, B.; Hayik, S.; Roitberg, A.; Seabra, G.; Wong, K. F.; Paesani, F.; Wu, X.; Brozell, S.; Tsui, V.; Gohlke, H.; Yang, L.; Tan, C.; Mongan, J.; Hornak, V.; Cui, G.; Beroza, P.; Mathews, D. H.; Schafmeister, C.; Ross, W. S.; Kollman, P. A. AMBER 9; University of California: San Francisco, CA, 2006. (11) Pavese, A.; Catti, M.; Parker, S. C.; Wall, A. Phys. Chem. Miner. 1996, 23, 89–93. (12) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. J. Chem. Phys. 1983, 79, 926–935. (13) Freeman, C. L.; Harding, J. H.; Cooke, D. J.; Elliott, J. A.; Lardge, J. S.; Duffy, D. M. J. Phys. Chem. C 2007, 111, 11943–11951. (14) Martinez, J. M.; Martinez, L. J. Comput. Chem. 2003, 24, 819–825. (15) Smith, W.; Forester, T. R. J. Mol. Graphics. 1996, 14, 136–141.

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Figure 2. AFM images of calcite behavior in air (a-c) and surfaces cleaved under 96% ethanol and stored there, before exposure to air during imaging (d-f). (a) Imaged 10 min after cleavage in air; the terraces are 3 or 6 A˚ high, corresponding to one or two molecular layers of calcite. (b) After exposure to deionized water for 15 min; the rhombic etch pits reflect the cleavage directions. Defects in the crystal structure, such as vacancies or trace substitutions of other ions, probably serve as nucleation sites for dissolution to begin. (c) Image 24 h after cleavage and exposure to air: Recrystallization has decorated the surface with islands that are several molecular layers high. (d) Calcite freshly cleaved and stored in ethanol for 8 days and then exposed to air for ∼1.5 h; recrystallized material forms bands along step edges and isolated small islands on terraces. (e) A similar sample stored in ethanol for 12 days, then in air for 5 h: the small, round islands (white dots) that formed during exposure to ethanol serve as nucleation sites for the more dendritic growth in air. (f) A different site on the same surface as image d after 25 h in air. Many small islands have formed, but they do not grow or coalesce, in contrast with the ethanol-free surface in air image c.

In addition, both molecular mechanics calculations using DL_POLY and density functional theory (DFT) calculations with CPMD 3.13.216 were made to obtain the adsorption energy of water and ethanol on calcite {10.4}. We first put a water or ethanol molecule on the surface. After geometric optimization, potential energy calculations of the following systems were implemented: (i) single water or ethanol molecule; (ii) calcite surface only; (iii) calcite surface with water or ethanol. In CPMD calculations, the energy cut off for the plane-wave basis was 150.0 Ry, and the Ceperley-Alder (CA) exchange correlation function in the local density approximation (LDA) was used. (16) CPMD; http://www.cpmd.org/. Copyright IBM Corp 1990-2008; copyright MPI f€ur Festkorperforschung Stuttgart, 1997-2001.

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Figure 3. AFM images of calcite cleaved under ethanol and also stored and imaged, in situ in ethanol. (a) Exposed for 8 days: terrace islands and step edge decorations. (b) A larger scan area than image a after several scans with increased tip force. (c) Freshly cleaved calcite exposed to ethanol for 40 min. The striations parallel to the scan direction (x direction) are caused by the tip moving molecules or particles along the surface. The insert is a zoom collected 30 min later. By then, the particles were firmly attached. (d) A surface exposed to ethanol for 25 min; step edge decorations and terrace islands are already present. (e) A surface cleaved and stored in ethanol for 2 days. (f) A surface cleaved in air and exposed to DI water for 15 min and then to ethanol for 45 min. Step edges on etch pits are decorationed and there are abundant small terrace islands. Although exposed to water first, the surface resembles the samples exposed to ethanol only.

Results AFM Images. Reference images for calcite exposed to air and water are shown in Figure 2. Figure 2a depicts the characteristic terraces of a freshly cleaved calcite crystal. During exposure to ultrapure deionized water (DI), etch pits develop with faces defined by the rhombohedral crystal structure (Figure 2b). On dry surfaces, not in contact with liquid water, humidity from air produces a solution, so the surface rearranges. Islands that are one to several calcite monolayers high form on otherwise atomically flat terraces. Island height is consistent over several square micrometers. Depending on temperature and humidity, it can take from a few hours to days for the surface to recrystallize extensively (Figure 2c). More details about this phenomenon can be found in Stipp et al.,17 Stipp,18 and Bohr et al.19 (17) Stipp, S. L. S. Langmuir 1996, 12, 1884–1891.

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Figure 4. Side view of the three calcite surfaces with ethanol (left) and water (right): calcium, green; oxygen, red; carbon, gray; and hydrogen, white; (a and b) {10.4}; (c and d) {31.8}, and (e and f) {31.16}. Ethanol is structured on calcite with the CH3 ends forming a hydrophobic layer. The next layer of ethanol is more random, but the fatty ends of the EtOH molecules are oriented toward the first layer.

AFM of calcite imaged in 96% ethanol-water showed that small decorations grew along all of the step edges and terraces (white spots in Figure 3a). The development of the decorations is not a simple function of time. We observed that other parameters had an influence, such as the ratio of calcite surface area to the amount of ethanol, temperature, calcite surface defect density, and forces acting between the tip and sample. Especially the force of the tip on the surface proved to have a profound effect. Moderate to high tip forces (∼300 nN/V) usually removed the islands and step edge decorations from the scanned area, whereas a low force (0.4-3 nN/V) using a cantilever with a low spring constant (an Olympus BL-RC-150VB biolever series, 26.8 pN/nm) allowed imaging of the onset of step edge decoration even earlier than with a normal tip and gentle forces (10-100 nN/V). The first two images of Figure 3 demonstrate the effect of sweeping with a normal AFM tip. Figure 3a shows a typical surface after exposure to ethanol for 9 days and Figure 3b shows the same area after increasing tip force. The area outlined by the white box had been scanned 6 times with an increased force of ∼300 nN/V to sweep (18) Stipp, S. L. S. Geochim. Cosmochim. Acta 1999, 63, 3121–3131. (19) Bohr, J.; Wogelius, R. A.; Morris, P. M.; Stipp, S. L. S. Geochim. Cosmochim. Acta, in press.

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Figure 5. Atomic distances measured from modeling snapshots: (a) ethanol-calcite interface; (b) water-calcite interface. Color codes as in Figure 1.

the surface clean. Then the forces were decreased, the scan area was enlarged, and Figure 3b was captured. Figure 3c illustrates the growth of step edge decorations recorded with light forces by a biolever. The streaks in the x direction are traces of the tip moving loose material. As time passed and the material became more Langmuir 2010, 26(19), 15239–15247

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Figure 6. Density profiles of the oxygen from water (red) and ethanol (green) interaction with calcite along the direction perpendicular to the three modeled surfaces. The adsorbed molecule distance from the surface at the steps is represented with a negative value, which is a consequence of step morphology, that is, the surface is not flat.

firmly attached to the surface, there were fewer loose particles to cause streaks and the decorations became clear (insert in Figure 3c). The decorations appeared on all step edges, they had no preference for either obtuse or acute sides and we did not observe a change in size with ethanol exposure time. Figure 3d was recorded after 25 min of ethanol exposure, whereas Figure 3e was recorded after 2 days. The islands and step edge decorations had similar width and height, but height varied, ranging from a few A˚ to several nm. No etch pits were observed on any surface and only slight step dissolution was occasionally visible from a swept area, such as in Figure 3b. Exposing the calcite surface to water for a few minutes before immersing it in ethanol resulted in surfaces similar to those with only ethanol, as would be expected if adsorbed ethanol replaces the initially adsorbed water, but etch pits formed during the brief water exposure and the density of terrace decorations was higher (Figure 3f). The decorations remained after the ethanol evaporated (Figure 2d-f). As with clean calcite exposed to the humiLangmuir 2010, 26(19), 15239–15247

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Figure 7. The radial distribution function (RDF) for atom pairs of Ca (calcite)-O (water/ethanol). The graphs show the local density of calcium ions around oxygen atoms from ethanol. The Ca-O bond is tighter for ethanol than for water.

dity in air, after ethanol evaporated, the surface recrystallized, but patterns were different than those for calcite that had only been exposed to air (Figure 2a-c). The initial point decorations on terraces, observed soon after the sample was removed from ethanol, serve as nucleation sites for island growth, but extensive island growth and coalescence, as observed for the air only sample, was absent. Instead, many small islands formed (Figure 2e,f ). The overall behavior for surfaces exposed to 96% ethanol was the same as for samples made with 100% pure ethanol.6 Molecular Dynamics. The modeling results provide information about the structural arrangement for ethanol and water molecules in close association with the three different calcite faces: a flat face (terrace), {10.4}; a viscinal face containing acute steps, typified by {31.8}; and a face with obtuse steps, {31.16}. First, the MD simulations determined the atomic arrangement of the three surfaces. At the termination of the bulk solid, the solid surface atoms relax, yielding bond distances in the top layers of Ca and CO3 that are different from the bulk below. This restructuring is further enhanced by the adsorbed water and ethanol. This can be seen by comparing the structure in the base of the drawings with that at the surface in Figure 4. Restructuring was especially DOI: 10.1021/la101136j

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Figure 9. Projection of the first ethanol layer onto the {10.4} calcite surface. Insert shows the layer to scale with van der Waal radii. The unit mesh dimension (surface unit cell), rectangle A, based on Ca from calcite (Ca is green, C is yellow and O is gray) matches the unit mesh dimension based on hydrogen (white balls, rectangle B) from the OH groups of ethanol (gray skeleton). This indicates regular ethanol ordering at the termination of bulk calcite. At the level of the ethanol’s oxygen (red) the match still holds, but at the level of the carbon chain, rectangles C and D, the match becomes less definite. The OH groups are symmetric along glide planes defined by carbon from the CO3 and by Ca in calcite (dashed line).

Figure 8. The RDF for atom pairs of O(calcite)-H(water/ethanol). The H from ethanol is much closer to the O from the topmost CO3 group than the H from water. This means that the alcohol OH has a different orientation than the water OH, consistent with its stronger attachment. The second peak for ethanol shows that the H interacts with two O from each CO3. H bonds to O corresponding to the first peak and merely resides close to the O atom for the second peak.

evident for the stepped surfaces. Second, the simulations showed how the various atomic arrangements of the three surfaces produced different geometric arrangements in the adsorbed water and ethanol molecules. From the three sets of simulation snapshots, we visualized the atomic arrangement of the adsorbed layer on the terraces and at the steps between them. The MD simulations showed that at the near surface, each ethanol hydroxyl is oriented toward the surface with its O pointing toward a calcium ion and its H binding to an oxygen from a carbonate (e.g., Figure 4a and Figure 5). The CH3 ends of the ethanol point away from the surface, making a hydrophobic termination (CH2-CH3) for this first adsorbed layer. We have called the rest bulk ethanol, even though molecular orientation, especially in the second layer out from the calcite surface, is not random. Evidence of structuring is still apparent even after three equivalent ethanol layers out from the surface into the fluid (Figure 6). The CH3 end of the first layer in the bulk ethanol points toward the fatty ends of the adsorbed layer. This arrange15244 DOI: 10.1021/la101136j

ment of oppositely oriented CH3 terminations produces a clear gap between the first adsorbed layer and the bulk ethanol. The density profiles perpendicular to the {10.4} surface for ethanol illustrate the well-ordered first adsorbed layer and the less-ordered bulk ethanol (Figure 6). The three first peaks representing ethanol closest to the surface are large and narrow. The first peak at ∼0.8 A˚ corresponds to OH, the second at ∼2.2 A˚ corresponds to CH2 and the last at ∼3 A˚ corresponds to the CH3 group. The first peak of the bulk ethanol, at approximately 6.5 A˚, is separated from the adsorbed ethanol layer by a gap of ca. 3.9-5.7 A˚. Simulation snapshots of water at the {10.4} surface show that oxygen points toward calcium and hydrogen orients toward the carbonate ions (Figure 4b and Figure 5b). In this way, clusters of oppositely oriented water molecules form in the first adsorbed layer above the Ca and CO3 sites. The density profile for O in water (Figure 6) has two peaks, at ∼1 and 2.2 A˚, representing the two orientations the water molecule can have. Between the layer of structured water and the broad peaks that represent bulk water (>3.5 A˚) is a narrow depression. The MD simulations on stepped surfaces showed that the water molecules keep an alternating configuration in the terrace region but they have no particular structure above the steps (Figure 4c,e). In the terrace region for ethanol, the orientation of OH, and hence the gap between the first adsorbed layer and the bulk, followed the pattern observed for the {10.4} surface, but the pattern was disturbed at the step edges. The step edges create disordered areas, allowing surface adsorbed ethanol to form a bridge through the gap to the bulk ethanol; this bridging zone extends along the full length of the steps, destroying the uniform hydrophobic layer that parallels the calcite surface (Figure 4d,f). This bridging is illustrated in the density profiles. The complete density gap observed for the {10.4} surface was present only as a depression for the two Langmuir 2010, 26(19), 15239–15247

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Figure 10. The MD simulations have demonstrated some local domains with different ethanol structuring. The pseudohexagonal pattern at the lower right corner shows the orientation of oxygen for the first layer OH from ethanol. OH forms a consistent layer over the calcite surface except in a domain (shaded gray) where the ordering pattern is locally shifted because of double ethanol attachment on some carbonates (black ellipses), forcing OH orientation to change in the domain. In the main structure, the carbon chains point up and away from the top O(calcite) to which the H(ethanol) is bonded (gray arrows), so adjacent molecules alternate in orientation in the same way that the carbonate groups of the surface alternate in the calcite layer below. Inside the domain, the alternating orientation is disturbed along [010] in the surface mesh, inverting the carbon chains (red arrows), forcing ethanol along [010] into a different hydrogen position. H is attached to the top O of the CO3 group and outside the domain it also resides closer to the horizontal O of the carbonate group, whereas in the domain, H resides over the deepest oxygen atom.

stepped surfaces (Figure 6b,c). The density profiles also showed small and broad peaks for the first adsorbed layer, indicating a nonuniform orientation of the ethanol molecules at the steps. This is particularly apparent on the surface with obtuse step edges. The local density, g(r), of species of atoms (A) around another species of atoms (B) can be calculated using the radial distribution function (RDF), and the mean bond distances can be determined: Æ gðrÞ ¼

NA P NB P i∈A i∈B

δðrij - rÞæ

4πr2

ð1Þ

where r represents a distance; rij is the distance between atoms i and j; NA represents the total number of atoms that belong to species A; NB is the total number of atoms that belong to species B; δ is the delta function (δ = 1 if rij = r; δ = 0 if rij 6¼ r); and the angled brackets denote the time average. At the {10.4} face, the radial distribution functions for CaO(ethanol) and CO3-H(ethanol) show stronger surface-solvent interactions than calcite-water. This is illustrated in Figures 7a and 8a where the bond distances between Ca-O(ethanol) and CO3-H(ethanol) (2.45 and 1.47 A˚) are shorter than for water (2.5 and 1.8 A˚). On the {10.4} surface, the peak of g(r) for Ca-O(ethanol) is about 20% higher than that for Ca-O(water) (Figure 7a), implying that more calcium ions interacted closely with oxygen when the surface was solvated with ethanol instead of water. On the stepped surfaces, the Ca-O bond was only marginally stronger for ethanol (Figure 7b,c). The equal height and width of the peaks showed that a stepped calcite surface could interact with almost as many molecules of water as ethanol. The RDF between O(calcite) and H(ethanol/water) (Figure 8) showed little difference between the three surfaces, indicating that more ethanol than water interacts with calcite at close range. The first peak for calcite solvated with water was at 1.8 A˚, whereas with ethanol, it was at 1.5 A˚, and the first ethanol peak was about 30% Langmuir 2010, 26(19), 15239–15247

higher than the first peak of water. This means that ethanol was attached more strongly through O-H bonding and that calcite bonding to O in both molecules was nearly equal. These differences in attachment orientation motivated a detailed investigation of the first adsorbed ethanol layer on the {10.4} face. Figure 9 shows a projection of a surface produced spontaneously from the modeling. The unit mesh (surface unit cell) of CaCO3 (rectangle A) that can be defined by Ca atoms can be copied to the distribution of the O and H atoms in the ethanol OH group (rectangle B), and the periodicity is apparent. In copying it further to CH2 (rectangle C) or CH3 (rectangle D) in the ethanol framework, the periodicity becomes approximate. The position of the OH(ethanol) can be represented in the crystal point group pg because it is symmetrical along a glide plane along [010] that is defined by C from CO3 of the surface, and the O from the OH groups is arranged in a pseudohexagonal pattern (Figure 10). The order, however, can be disrupted locally, when two ethanol molecules attach to one CO3-group (black ellipses) generating a local domain with different adsorption density (domain of double attachment). Outside this domain, the carbon chains point up and away from the top O(calcite) to which the H(ethanol) is bonded (gray arrows), giving an alternating orientation of the neighbor molecules. In the domain of double attachment, the alternation is disturbed along [010] resulting in an inversion of the carbon chains (red arrows). This forces the ethanol along [010] to have a different hydrogen position. The hydrogen is attached to the top O of the CO3 as usual, but instead of residing close to the horizontal O of the CO3, the hydrogen is also near the deepest O atom of the carbonate group. The adsorption energies of water and ethanol on the {10.4} calcite surface from molecular mechanics and DFT calculations are listed in Table 1. Water has an adsorption energy of -68.7 kJ/mol (MM) or -71.6 kJ/mol (DFT), while ethanol has an adsorption energy of -91.3 kJ/mol (MM) or -86.9 kJ/mol (DFT). Both MM and DFT results show that ethanol has DOI: 10.1021/la101136j

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Table 1. Adsorption Energy of Water and Ethanol on the {10.4} Calcite Surface

MM with DLPOLY DFT with CPMD

water (kJ/mol)

ethanol (kJ/mol)

-68.7 -71.6

-91.3 -86.9

significantly more negative adsorption energy, which means that it has stronger binding with the surface.

Discussion 6

Cooke et al. demonstrated that ethanol is firmly bonded, very ordered, and stabilizes the {10.4} calcite surface more than water. In a mixed solution of ethanol and water, Cooke and colleagues pointed out that the gap on the {10.4} surface, seen in the ethanol density profile, defines the minimal ion exchange between the first adsorbed layer and the rest of the solution. The hydrophobic layer produced by the CH3 ends of the ethanol layer restricts the possibility for ion transport from or to the surface, explaining why calcite surface growth and dissolution in ethanol are considerably diminished at the {10.4} face, compared to behavior in water. Density profiles from this study illustrate a decrease in the ordering for both ethanol and water on stepped surfaces. For water, this is most pronounced on the surface with obtuse steps, where bulk water over the steps is merely a continuation of the first adsorbed layer (Figure 4f; Figure 6c). Hence water molecules are readily transported in and out of this first layer. For ethanol, the clear gap seen at the {10.4} face between the first adsorbed ethanol layer and the bulk is, at the steps, reduced to a wide depression (with full width at half-maximum, approximately 1.7 A˚; Figure 6). This and the molecular bridges observed between the first adsorbed layer and the bulk ethanol (Figure 4c and e), translate to a lower barrier for the mobility of ions and molecules from the bulk to the surface at the steps. We argue that the disruption in the arrangement of ethanol molecules at steps eases mass transport, thus calcite precipitation and dissolution along the step edges, explaining the step edge decorations observed in the AFM images. The AFM images also showed decorations forming sporadically on terraces. Using the same line of reasoning, the gap in the density profiles on the {10.4} face would prevent precipitation and dissolution on the terraces. The calcite surface designed for MD simulations is perfect, but the natural crystals used for the AFM experiments always contain small quantities of other ions, as well as dislocations.20 It is well known that etch pits, such as those we see in Figure 2b, nucleate at defects. These can be sites of trace element substitution or locations where atomic structure defects occur. Defects are likely to cause an increase in the density of double attachment domains in the first adsorbed ethanol layer, such as seen in Figure 10, and hence, disturb the molecular ordering. This would facilitate ion transport and precipitation. The variable heights of the step edge and terrace decorations suggest that the ion transport path is variable from site to site. The composition of the precipitates along the step edges and on the terraces is not known but there is a limited set of possibilities, namely, some compound containing Ca, CO3, and EtOH. We argue that the clusters are dominated by CaCO3. This is based on the AFM observations, where, when imaging the ethanol exposed calcite surfaces in air, the edge and terrace decorations persisted and served as centers for further growth (Figure 2e). Exposing a calcite surface to water before ethanol resulted in an increased (20) Harstad, A. O.; Stipp, S. L. S. Geochim. Cosmochim. Acta 2007, 71, 56–70.

15246 DOI: 10.1021/la101136j

density of decorations. In water, etch pits formed (Figure 3f) allowing the solution to dissolve calcite. Adding ethanol promotes loss of water, favoring CaCO3-containing precipitates, and explains the increased density of islands on the terraces for the water-exposed sample. Ethanol dissociates into (CH3CH2O-), and H2Oþ -CH2-CH3.21 The anion may bind to Ca2þ, resulting in Ca(EtO)2 (calcium-diethoxide). Alternatively, by reaction with CO2, ethoxide ions may produce CH3-CH2-O-CO2- which can precipitate as Ca (EtOCOO)2 (calcium-di(ethylcarbonate)).22 Thus, we could expect these species at the calcite-ethanol ( water interface. Whether the precipitates are CaCO3 or complexes of alkyloxy carbonates is not known. Ca(EtOCOO)2 and Ca(EtO)2 have hydrophobic ends, and on the basis of the modeling studies, hydrophobic entities are not likely to be able to enter the first adsorbed layer. However, the Ca ions bound in the calcite surface could readily attract EtOCOO- and a cluster of these molecules would be observable with AFM. Several techniques (FTIR, pickup by templating, and atomic force spectroscopy using force mapping) were used to try to determine the composition of these nanoscale clusters. We could find no evidence to suggest that they were anything other than calcite. The classical crystallization models describe the importance of steps for growth, based on the number of bonds formed during the attachment process. Our simulations showed that the influence of steps extends much further from the surface, into the bulk liquid. The complex structure of the liquid above the steps facilitates transport from the bulk fluid to the surface by means of networks of hydrogen bonds and dipole-dipole interactions. According to our simulations, such transport can be largely excluded from terrace areas that are protected by the hydrophobic terminations of the ethanol molecules. This extension of the EDL (electrical double layer) theory gives us a new means for understanding adsorption, growth, and dissolution at the molecular level, and opens a window into understanding the processes organisms use for biomineralization.

Conclusions The H-bonding for EtOH to calcite is stronger than for HOH to calcite, contrary to expectations for solvent behavior. Also, once ethanol attaches to calcite, it does not completely evaporate, so it continues to control surface behavior during exposure to air. In ethanol, the surface is influenced by the solution and the solution is influenced by the surface, as expected from descriptions of the electrical double layer. However, this work shows that structuring extends out into the fluid much further than previously predicted by theory, especially at steps. The algal cell produces organic compounds to control mineral growth and dissolution in a cell environment where the relative concentration of complex molecules to water is high. Our understanding of mineral-solution interaction is generally based on behavior in aqueous systems, but this is not sufficient for describing biomineralization inside a cell where organic compounds dominate. This work demonstrates that in a solution where water activity is low, the OH entity, even on an extremely simple organic molecule, produces a structured, hydrophobic layer, that limits water access to the mineral surface. This structuring is disrupted at the steps and defects, providing a bridge for mass transport between bulk solution and the solid, allowing the organism to control addition or removal of material at preferred sites. (21) Fonrodona, G.; Rafols, C. R.; Bosch, E.; Roses, M. Anal. Chim. Acta 1996, 335, 291–302. (22) Kurov, V. I.; Tsurkan, T. S. Zh. Obshchei Khim. 1967, 37, 1208–1211.

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Sand et al.

Acknowledgment. We sincerely thank Keld West, Christian Schack Pedersen, Joanna Nissenbaum, and the NanoGeoScience group for help and discussion about this work. Nico Bovet is thanked for XPS work. Funding was provided by the Nano-Chalk Venture, from the Danish National Advanced Technology Foundation (HTF), Mærsk Oil and Gas A/S and the University of Copenhagen. The NanoGeoScience Laboratory was initially established through a grant from the

Langmuir 2010, 26(19), 15239–15247

Article

Danish Natural Sciences Research Council. Computer time was provided by the Danish Center for Scientific Computing (DCSC). Supporting Information Available: Force field and atom types and descriptions; expressions for force field potentials. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la101136j

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