A Computer Modeling Study of the Inhibiting Effect of Organic

A Computer Modeling Study of the Inhibiting Effect of Organic Adsorbates on Calcite Crystal Growth Nora H. de

Leeuw*,†,‡

and Timothy G.

Cooper†,§

Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK, School of Crystallography, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK, and Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK Received June 16, 2003;

CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 1 123-133

Revised Manuscript Received July 18, 2003

ABSTRACT: Computer modeling techniques were employed to investigate the adsorption of a selection of organic molecules to a series of monatomic growth steps of the major calcium carbonate polymorph calcite. Incorporation of the organic material by replacement of preadsorbed water at calcite {101 h 4} surface features is calculated to be considerably exothermic for organic molecules containing carbonyl and hydroxy functional groups with adsorption energies at the growth steps ranging from about -30 to -140 kJ mol-1. The calculated energies suggest that carboxylic acids, hydroxy aldehydes, or amides will be effective growth inhibitors through their strong adsorption to the growth steps, usually binding across the step to the terrace below, thereby blocking these sites to further attachment by calcium carbonate. Organic molecules containing only the amine functional group do not adsorb very strongly to the growth steps and thus will not be particularly effective in inhibiting crystal growth. On the stoichiometric steps, which are found in abundance on the experimental surface, hydroxy ethanal preferentially blocks the faster growing obtuse steps, which will slow calcite growth significantly. On the steps terminated by either calcium or carbonate groups, the adsorbates are found to attach preferentially to either the acute or the obtuse steps, which may lead to asymmetric growth and surface morphology. The results from this study suggest that computer simulations may provide a route to the identification or even design of particular organic additives for specific tailored crystal growth. Introduction Calcite is one of the most abundant minerals in the environment and of fundamental importance in many fields, both inorganic and biological. It is a building block of shells and skeletons1 and is used as a carbon isotope counter in marine carbonates, with a view to assessing the relationship between the CO2-induced greenhouse effect and climate.2 Furthermore, calcium carbonate is important in ion exchange, due to its strong surface interactions with heavy metals in the environment,3,4 in energy storage where the products of its endothermic decomposition into CaO and CO2 can be stored and subsequently reacted exothermically to rerelease the energy,5 and in industrial water treatment.6 Hence, calcite has been the subject of extensive and varied research. One area of research, which has attracted much attention, is crystal growth and dissolution, e.g., refs 7-10. As the concentration of calcium carbonate in many natural waters exceeds the saturation level, the precipitation of calcite in industrial boilers, transportation pipes, and desalination plants is of concern,11 and it is therefore important to learn how crystal growth and dissolution are affected and can be modified. The {101 h 4} surface is by far the most stable plane of calcite and dominates the observed morphology.12-14 Hence, it has been the subject of many investigations, * To whom correspondence should be addressed. Nora H. de Leeuw, School of Crystallography, Birkbeck College, Malet Street, London WC1E 7HX. Tel: +44 207 679 7465. Fax: +44 207 631 6803. E-mail: [email protected]. † University College London. ‡ Birkbeck College London. § University of Reading.

both in ultrahigh vacuum such as the SEM study by Goni et al.,15 in air,16 and under aqueous conditions such as the AFM investigations by Ohnesorge and Binnig17 and Liang et al.18 However, no experimental surface is truly planar, and there are always defects present like steps and kinks. Indeed, calcite growth and dissolution are found to occur through steps19 and spiral dislocations,20 often in monolayers from the step as observed by Liang et al. in their AFM study of the calcite {101 h 4} plane under aqueous conditions18 and by Stipp et al., who used SFM to study the same surface in air over some days and found the steps to spread one layer at a time.16 In addition, side reactions, such as the oxidation of pyrite and ammonia, often affect the rate of CaCO3 dissolution.21 Recent models of step dissolution have included a terrace-ledge-kink model, successfully describing the initial stages of pit growth on the {101 h 4} surface,22,23 a kinetic Monte Carlo model which reproduces experimental pit-growth behavior,24 and molecular dynamics simulations of calcite dissolution from experimentally observed step edges.25 Many impurity ions are found to affect crystal growth, for example, magnesium which inhibits calcite nucleation,26-29 and hence studies of crystal growth have often concentrated on the effect of incorporating divalent foreign cations into the crystal,30-33 often at the growth steps.34-36 Moreover, even leaving aside biological situations, where calcium carbonates in, for example, skeletons and shells, nucleate and grow on an organic matrix,37,38 the precipation and growth of inorganic calcium carbonates are also affected significantly by dissolved organic material,39,40 to a considerable degree through its preferential adsorption to specific surfaces or surface features.41,42 As phosphate-containing com-

10.1021/cg0341003 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/14/2003

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pounds have been shown to have growth inhibiting properties,6,43 many experimental, e.g., refs 44 and 45, and computational studies, e.g., refs 46-48, have concentrated on the effect of phosphates and phosphonates on calcite nucleation and growth. However, although organic acids, such as fulvic and citric acid, have been shown to affect calcite crystal growth/dissolution,42,49 apart from the work by Orme and co-workers on the adhesion of aspartic acid,50 few computational investigations have been carried out on molecules with this type of functional group. The aim of this paper is to report a comprehensive investigation of the modes of adsorption and strength of interaction of a selection of organic molecules with calcite surface features, which may affect crystal growth through their blocking of growth sites. We have employed computer modeling methods to study at the atomic level the adsorption of organic acids and aldehydes, including two nitrogen-containing molecules, to known growth sites at the calcite surface. As crystal growth occurs under aqueous conditions, we have taken into account the presence of solvent in our simulations, and calculated the competitive interactions of the different adsorbates and preadsorbed water. Computational Methods The structures and energies of the calcite/adsorbate systems were modeled using atomistic simulation techniques. These methods are based on the Born model of solids,51 which assumes that the ions in the crystal interact via long-range electrostatic forces and shortrange forces, including both the repulsions, and the van der Waals attractions between neighboring electron charge clouds, which are described by simple analytical functions. The electronic polarizability of the ions is included via the shell model of Dick and Overhauser,52 in which each polarizable ion, in our case, the oxygen ion of the calcite crystal, is represented by a core and a massless shell, connected by a spring. The polarizability of the model ion is then determined by the spring constant and the charges of the core and shell. When necessary, angle-dependent forces are included to allow directionality of bonding as, for example, in the model of the covalent carbonate anion developed by Pavese et al.,53 which is used in this work. We have employed the METADISE code54 to investigate the surface systems by energy minimization, which is achieved by adjusting the atoms in the system until the net forces on each atom are zero. Energy minimization simulations will yield adsorption energies, which have previously been shown to give good agreement with experimental surface sampling techniques, such as temperature programmed desorption, e.g., ref 55, as well as lowest energy configurations of the adsorbate/solid interface. We used a combination of three potential models for a description of the interactions of the various atoms in the systems, namely, by Pavese et al. for the calcium carbonate crystal,53 the cvff force field for the organic adsorbates,56 and the water potential model by de Leeuw and Parker.57 The parameters for the interactions between water and organic adsorbates with the calcite surfaces were derived following the approach by Schro¨der et al.,58 which has been shown to be successful on a number of occasions, for example, for interactions

de Leeuw and Cooper

of water and methanoic acid with calcite and fluorite.59,60 Generally, good agreement is found for substrate/ adsorbate structures and energies between interatomic potential calculations with parameters obtained in this way and ab initio techniques such as those based on the density functional theory (DFT). For example, we found in a combined DFT and interatomic potential study of the adsorption of water molecules at the CaF2 (111) surface that both techniques were in good agreement as to the mode of adsorption, optimum coverage, and hydration energies, which were within 8 kJ mol-1 of each other, which hence gives us a measure of the uncertainty for the adsorption energies in this work.60 In addition, when using the potential parameters derived here, the experimental morphology of the calcite crystal is calculated correctly, indicating that the relative stabilities of the surfaces are accurately reproduced, and the calculated cleavage energy of the calcite crystal of 0.33 J m-2 is in adequate agreement with the experimentally found value of 0.23 J m-2.61 Furthermore, we have shown in earlier work62 that the effect of adsorption of methanoic acid to calcium carbonate surfaces is to stabilize the surfaces to the same extent, leading to a calculated spherical morphology of the crystal, which finding was confirmed to some extent by the work of Fogg et al., who obtained spherical crystals of a calcium-aluminum salt when grown in the presence of organic acids.63 When investigating the competitive adsorption of water and methanoic acid at calcite and fluorite surfaces, we successfully calculated the relative strengths of adsorption of the two adsorbates compared to experiment,60,64 and based upon these preliminary studies we are now confident that the potential model is reliable for the quantitative comparison of the adsorption behavior of these organic molecules to calcite surfaces. The stability of the surfaces with or without adsorbed species is determined by their surface energy, which experimentally is the energy required to cleave the bulk crystal exposing the surface, given by

γ ) (Esurf - Ebulk)/A

(1)

where Esurf is the energy of the simulation cell containing the surface, Ebulk is the energy of an equivalent number of bulk CaCO3 units, and A is the surface area. A low positive value for γ indicates a stable surface. The strength of interaction of the surface with the adsorbate is shown by the adsorption energy, calculated according to eq 2:

Eads ) Esystem - (Esurf + Eadsorbate)

(2)

where Esystem is the energy of the surface with adsorbate, Esurf is the energy of the surface as above, and Eadsorbate is the energy of the free adsorbate molecule. When we consider hydration of the surfaces, we will call the adsorption energy of the water the hydration energy to facilitate comparison of the relative strengths of adsorption of water and the organic adsorbates. The hydration energy is calculated in the same manner as eq 2, where the Eadsorbate is now the energy of a water molecule in the liquid phase, calculated at -921.4 kJ mol-1 for the potential model used in this work.

Organic Adsorbates Effect on Calcite Crystal Growth

Crystal Growth & Design, Vol. 4, No. 1, 2004 125

Figure 1. Geometry optimized structures of the six stepped calcite {101 h 4} surfaces with (a) the acute and (b) obtuse stoichiometric steps; (c) the acute and (d) the obtuse calcium-terminated reconstructed dipolar steps; and (e) the acute and (f) obtuse carbonateterminated reconstructed dipolar steps, showing the edge vacancies on the calcium- and carbonate-terminated steps (Ca ) gray, C ) white, O ) black).

Results Calcite has a rhombohedral crystal structure with space group R3 h c and a cleavage structure not unlike h 4} cleavage plane is distorted rock salt.18 As the {101 by far the most stable surface of calcite, both theoretically and experimentally, it dominates the morphology, usually to the exclusion of other surfaces, under both ultrahigh vacuum (UHV) and aqueous conditions. Stepped surfaces are important in crystal growth and dissolution, as surfaces grow/dissolve in a spiral with migrating steps assuming individual heights of 1-2 monolayers,6,20 and in this study we have concentrated on a variety of steps on the {101 h 4} surface. Two of the steps are the experimentally observed growth steps, also seen as the edges of etch pits on the calcite surface, which contain stoichiometric step edges of alternating calcium ions and carbonate groups, where the carbonate groups lie at either an acute (78°) or an obtuse angle (102°) with respect to the {101h 4} terrace below the step, which we hence call the acute and obtuse steps (Figure 1, panels a and b, respectively). We have also included four different stepped surfaces, which all contain dipolar steps, i.e., terminated by either calcium ions or carbonate groups. The orientations of the carbonate groups in these steps are also either at an acute or an obtuse angle with respect to the underlying terrace, and hence we call them the acute and obtuse calcium steps (Figure 1c,d), and the acute and obtuse carbonate steps (Figure 1e,f). Dipolar surfaces are not stable, and their energies will diverge with increasing crystal size,65 leading to surface defects and reconstruction. We reconstructed the step edges to form stable stepped surfaces, which was achieved by creating crenellated step edges with alternating calcium or

Figure 2. Directions of the steps with respect to the calcite {101 h 4} surface, where the experimentally found acute and obtuse growth steps, labeled 1 and 2, respectively, are shown as the sides of an etch pit, and the reconstructed dipolar steps are shown only for the calcium-terminated edge, labeled 3 and 4 for the acute and obtuse step edges, respectively (Ca ) gray, C ) white, O ) black).

carbonate species and vacancies. The reconstructed dipolar steps modeled in this work are now no longer dipolar, and they are likely to be present on the experimental surface in the curves of the growth spirals observed on the calcite {101 h 4} plane. The directions of the steps with respect to the {101 h 4} surface are outlined in Figure 2. The directions of the experimentally observed acute and obtuse steps, shown in Figure 1a,b, are the same, as the difference in their edge structure only depends on which side the lower terrace is. They are at opposite sides of etch pits, an example of which is outlined in Figure 2. Similarly, the acute or obtuse

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Figure 3. Plan view of the planar calcite {101h 4} surface with adsorbed water layer in a regular herringbone pattern (Ca ) green, C ) gray, Ocalcite ) red, Owater ) dark green, H ) white). Table 1. Calculated Surface and Hydration Energies of the Geometry Optimized Dry and Hydrated Planar and Stepped Calcite {101 h 4} Surfaces surface energies γ (Jm-2) and average hydration energies Ehydr (kJ mol-1) {101 h 4} surface

γdry

γhydrated

Ehydr terrace sites

Ehydr step sites

planar acute obtuse acute-Ca obtuse-Ca acute-CO3 obtuse-CO3

0.59 0.77 0.71 0.78 0.74 0.75 0.71

0.33 0.47 0.41 0.46 0.38 0.43 0.37

-30 -31 -28 -31 -29 -28 -29

-30 -31 -31 -40 -28 -34

nature of the dipolar steps is only due to the side of the lower terrace. The surface energies of the anhydrous and hydrated stepped surfaces are listed in Table 1. Although the planar surface is clearly the most stable, both before and after hydration, the stepped surfaces are similar in stability, in agreement with the fact that the experimental surface contains many stepped features. The stepped surfaces show some rumpling of the calcium lattice and rotation of the carbonate groups near the step edges, especially on the obtuse-Ca step and acuteCO3 step, respectively (Figure 1d,e). Hydration has a stabilizing effect on both planar and stepped surfaces, but all are stabilized to a similar extent. On the planar calcite surface, the water molecules adsorb in a regular herringbone pattern (Figure 3). The water molecules are calculated to adsorb to the surface at a Ca-Owater distance of 2.4 Å,66 which was later confirmed by highresolution X-ray reflectivity experiments by Fenter and co-workers.67 In addition, previous simulations of the hydrated calcite surface using the present potential model, showed symmetry-breaking tilting of surface carbonate groups, resulting in a difference in surface oxygen height of approximately ∆h ) 0.05 Å.68 Although Liang and co-workers had previously identified this height difference in their AFM study,18 they found a height difference of ∆h ) 0.35 Å, clearly not in quantitave agreement with our subsequent calculations. However, they suggested that the effect may have been exacerbated by the AFM tip, which was confirmed by the later work of Fenter et al., who found a tilting of only ∆h ) 0.04 Å, obviously in excellent agreement with our simulations, and we may thus be confident that our

Figure 4. (a) Methanoic acid HCOOH, (b) eclipsed and (c) staggered hydroxy methanamide HC(dO)NHOH, (d) methylamine H3CNH2 and (e) eclipsed and (f) staggered hydroxyethanal HC(dO)CH2OH. (C ) gray, N ) purple, O ) dark blue, H ) white).

computational methods accurately reproduce the experimental calcite-water interface. Although the water molecules interact with surface oxygen atoms through their hydrogen atoms (at 1.89 and 1.97 Å), they do not interact significantly with each other and the introduction of the steps has little effect on the adsorption pattern on the terraces away from the steps, which can also be seen from the very similar average hydration energies for the various surfaces (Table 1). We observe some clustering of the water molecules around the step edges and in the vacancies on the reconstructed dipolar steps, i.e., the acute- and obtuse-Ca and acute- and obtuse-CO3 edges. These vacancies form attractive sites for adsorption of the water molecules with multiple interactions to the surface and hydrogen-bonding between the clustered water molecules. This increased reactivity is particularly clear on the obtuse-Ca and obtuse-CO3 steps, which are thus stabilized more by hydration than the other stepped surfaces. The dipolar surfaces have very similar surface energies to the stoichiometric steps, which together with their likely presence in spiral growth features indicates that we need to include them in our calculations. In addition, these six stepped surface are a good representation of the much larger number of possible step orientations, as they include all salient surface features, namely, stoichiometric or reconstructed dipolar steps; edges terminated by calcium ions, carbonate groups, or both; vacancies in the step and hence corner sites; and acute or obtuse angles with respect to the terrace. As calcite growth often occurs at kink sites in the nondipolar steps, inclusion of the reconstructed dipolar steps is particularly relevant, as through their crenellation (Figure 2) these step edges provide kink sites, which are similar to experimentally found kink sites in the nondipolar steps and hence give us insight into the processes occurring at these kinks. We considered the adsorption of four different organic molecules (Figure 4) to the various calcite surfaces, namely, methanoic acid HCOOH, hydroxy methanamide HC(dO)NHOH, methylamine H3CNH2, and hydroxy ethanal HC(dO)CH2OH. This selection of adsorbates



Table 2. Calculated Adsorption Energies of Organic Adsorbates at Dry Planar and Stepped Calcite {101 h 4} Surfaces adsorption energies (kJ mol-1) hydroxy methanamide

hydroxy ethanal

surface

methanoic acid

methylamine

eclipsed

staggered

eclipsed

staggered


-84 -121 -114 -135 -122 -112 -106

-56 -73 -84 -81 -79 -69 -66

-80 -99 -123 -176 -190 -160 -205

-107 -128 -156 -146 -178 -135 -122

-93 -102 -129 -140 -138 -139 -120

-59 -102 -117 -137 -114 -121 -105

will give us the opportunity to study separately the interactions of the carboxylic acid and amine functional groups as well as the effect of separation of the dO and OH groups and the replacement of a carbon by a nitrogen atom on the strength of interaction with the surface. In addition, for hydroxy methanamide and hydroxy ethanal we have considered two extreme conformers, one where the two oxygen atoms are eclipsed and the other in a staggered conformation with the oxygen atoms as far away from each other as possible. We have included the eclipsed conformer in our adsorption calculations as in this conformation the two oxygen atoms may form bridging interactions with the surface, which may well be more favorable than a single interaction with the staggered conformer. We found during our calculations, however, that often the staggered conformer would itself rotate during the energy minimization to form the eclipsed form at the surface, but including both conformers in our simulations ensured that we did not overlook an energetically more stable mode of adsorption for these molecules. In the same vein, we calculated a large number of initial starting positions and orientations for all adsorbates to the different surface sites to ensure that we identified the lowest energy substrate/adsorbate structure rather than a local minimum. The adsorption energies of the four adsorbates to the series of {101 h 4} surfaces, calculated according to eq 2, are listed in Table 2. However, calcite crystal growth takes place in aqueous solution and the mineral surfaces will be hydrated, as is clear from the exothermic hydration energies in Table 1. To attach to the mineral surfaces, the surfactant molecules will therefore have to displace this preadsorbed water. We thus need to compare the adsorption of the surfactants and the adsorption of water at the same sites if we are to assess the competitive adsorption of the two species for the same surface adsorption sites.59 From this comparison, we can obtain a difference in energy for the purely hydrated surface and the surface with the adsorbed surfactant, although this calculation will not include the enhanced stability arising from any interactions between the surfactant molecules and surrounding water molecules. However, previous calculations of coadsorbed methanoic acid and water have shown that generally there is no significant interaction between the coadsorbed surfactant and surrounding water and, more importantly, that the relative energies for the different sites do not change significantly when water is included explicitly in the calculations.60 The adsorption trends for the various organic adsorbates at the different adsorption sites will thus be valid, whether water is included explicitly in the calculations or whether it is

Figure 5. (a) Side and (b) plan view of the geometry optimized configuration of methanoic acid adsorbed on the {101 h 4} surface, with the methanoic acid bridging two surface calcium atoms, strongly coordinated by its doubly bonded oxygen atom, but more weakly via its hydroxy oxygen atom (Ca ) pale green, C ) gray, Ocarbonate ) red, Ometh ) dark blue, H ) white, distances in Å).

taken into account separately by a careful quantitative comparison with the hydrated rather than anhydrous surfaces. Methanoic Acid HCOOH. The adsorption of methanoic acid to the planar calcite {101 h 4} surface is the least energetically favorable process of all the surfaces considered (-84 kJ mol-1). The methanoic acid molecules coordinate bridging two surface calcium atoms, strongly via their carbonyl oxygen atoms and weakly via their hydroxyl oxygen atoms (Figure 5). Because the hydroxy oxygen atom is only weakly coordinated, the methanoic acid would only replace one water molecule if adsorbed to the surface. This water molecule has a hydration energy of -33 kJ mol-1 and by comparing the adsorption energies of methanoic acid and water we see that the adsorption of methanoic acid on the surface is more exothermic (by -51 kJ mol-1), compared to the purely hydrated surface.

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Figure 6. Top view of the obtuse-Ca step with adsorbedmethanoic acid, where the hydroxyl hydrogen atom is situated in the calcium vacancy and the remaining portion of the methanoic acid approximately takes the lattice position of a carbonate group (Ca ) pale green, C ) gray, Ocarbonate ) red, Omethanoic ) dark blue, and H ) white).

On the two growth steps of calcite, the acute and obtuse steps, the methanoic acid can only coordinate to the surface via its carbonyl oxygen atom. On the obtuse step it coordinates to the step edge, while on the acute step it adsorbs on the terrace below the step. Because the methanoic acid molecules only coordinate to one calcium atom, they would only replace one water molecule on both the acute and obtuse steps, where these water molecules have hydration energies of -50 and -41 kJ mol-1 for the acute and obtuse steps, respectively. The methanoic acid molecules adsorb to the steps with energies of -121 kJ mol-1 for the acute step, and -114 kJ mol-1 for the obtuse step, so the adsorption of methanoic acid is energetically more favorable than adsorption of water at the same site for both steps, by -71 kJ mol-1 for the acute step and by -73 kJ mol-1 for the obtuse step. Adsorption to the two calcium-terminated steps occurs to the step-terminating calcium atom. On both steps the methanoic acid molecules coordinate via their carbonyl oxygen atom to the step-terminating calcium atom, while on the obtuse-Ca step the methanoic acid molecule also coordinates via its hydroxy oxygen atom to a calcium atom on the terrace below (Figure 6). In both cases, the hydroxy hydrogen atom is approximately situated in the calcium vacancy on the step edge, with the remaining portion of the molecule approximately filling the lattice position a carbonate group would occupy if added to the step. Adsorption of methanoic acid to the calcium-terminated steps releases the largest energies, -122 kJ mol-1 for the obtuse-Ca step and -135 kJ mol-1 for the acute-Ca step. On the obtuse-Ca stepped surface, methanoic acid would replace only one water molecule if adsorbed instead of water, with a hydration energy of -33 kJ mol-1, while on the acuteCa stepped surface it would replace two, which have combined hydration energies of -102 kJ mol-1. As a result, the adsorption to both calcium terminated steps is preferred over water at the same sites, by -89 kJ mol-1 for the obtuse-Ca step and by -33 kJ mol-1 for the acute-Ca step.

Figure 7. Plan views of the planar {101 h 4} surface with (a) the eclipsed conformer and (b) the staggered conformer of hydroxy methanamide (Ca ) green, C ) gray, Ocarbonate ) red, Omethanamide ) dark blue, N ) purple, H ) white).

Methanoic acid adsorbs to the two carbonate-terminated steps by bridging between one calcium atom in the gap on the step edge and one on the terrace below and the methanoic acid molecules would as a consequence replace two water molecules on the hydrated steps. The adsorption energies for the methanoic acid to the dry steps are -106 kJ mol-1 for the obtuse-CO3 step and - 112 kJ mol-1 for the acute-CO3 step, whereas the two replaced water molecules release hydration energies of -34 and -28 kJ mol-1 for the obtuse step, and -39 and -49 kJ mol-1 for the acute step. Comparing the adsorption of methanoic acid and water on the two carbonate terminated steps hence shows that adsorption of methanoic acid in the place of water is again favorable for both steps, with an increase in released energy of -44 kJ mol-1 for the obtuse-CO3 step and -30 kJ mol-1 for the acute-CO3 step. Hydroxy Methanamide HC(dO)NHOH. Hydroxy methanamide in the eclipsed form coordinates to the planar {101 h 4} surface via both its oxygen atoms to the same surface calcium atom (Figure 7a), in what we shall term a “bidentate” fashion, releasing an adsorption energy of 80 kJ mol-1. Because the eclipsed conformer is only coordinated to one calcium atom it would replace only one water molecule on the purely hydrated surface, with a hydration energy of -33 kJ mol-1. The staggered conformer adsorbs with an energy of -107 kJ mol-1 and bridges two surface calcium atoms (Figure 7b). As a


result, the staggered molecule would replace two water molecules on the hydrated surface, with hydration energies of -29 and -33 kJ mol-1. Comparing the adsorption of hydroxy methanamide to that of water, we see that the replacement of water by both conformers is exothermic to a similar extent, by 47 and 45 kJ mol-1 for the staggered and eclipsed forms, respectively. Both forms adsorb to the acute step on the step edge, where they would replace two water molecules from the hydrated step. The eclipsed form adsorbs with an energy of -99 kJ mol-1 and takes the positions of water molecules with hydration energies of -28 and -19 kJ mol-1, while the staggered form adsorbs with an energy of -128 kJ mol-1 and takes the positions of water molecules with hydration energies of -50 and -28 kJ mol-1. As a result, the adsorption of methanoic acid is energetically favorable in both forms compared to the hydrated surface, by -52 kJ mol-1 for the eclipsed and -50 kJ mol-1 for the staggered form. On the obtuse step, the methanamide molecules coordinate with energies of -127 and -133 kJ mol-1 for the eclipsed and staggered forms, respectively, again replacing two water molecules each. The eclipsed form would replace water molecules with combined hydration energies of -63 kJ mol-1, while the staggered molecule would replace water molecules with combined hydration energies of -82 kJ mol-1, again resulting in an increase in released energy when adsorbed to the obtuse step (-64 and -51 kJ mol-1 for the eclipsed and staggered forms, respectively). On the obtuse-Ca step, both forms of methanamide adsorb to the calcium atom terminating the step. Both would replace the same three water molecules with combined hydration energies of -127 kJ mol-1. Adsorption of methanamide releases energies of 190 and 178 kJ mol-1 for the eclipsed and staggered forms, respectively, so even though the water adsorption is considerably exothermic around the step-terminating calcium atoms, the adsorption of hydroxamic acid to the step still results in an increased energy release, by -63 kJ mol-1 for the eclipsed and -51 kJ mol-1 for the staggered conformation. The two conformers adsorb at different sites on the obtuse-CO3 stepped surface. The staggered form coordinates to the calcium atom directly below the gap in the step, where it takes the positions of two water molecules, one on the terrace (-34 kJ mol-1) and one coordinated to a calcium atom in the gap in the step (-27 kJ mol-1). The eclipsed molecule coordinates in a bidentate fashion to a calcium atom in the gap in the step, where it would also take the position of two water molecules with combined hydration energies of -65 kJ mol-1. The adsorption of the eclipsed form to the dry obtuse-CO3 step has the most energetically favorable adsorption energy of all the surfaces considered, -205 kJ mol-1, while adsorption of the staggered molecule is considerably less energetically favorable, releasing only 122 kJ mol-1. Compared to water, the adsorption of the eclipsed molecule is thus the most energetically favorable of all the surfaces, with an increase in exothemicity of 140 kJ mol-1, while the adsorption of the staggered form is also favorable, but not by nearly as much (-61 kJ mol-1). Both forms adsorb to the acute-Ca stepped surface on the terrace below the step, although they both also


Figure 8. Geometry optimized configuration of methylamine adsorbed to the acute step edge (Ca ) green, C ) gray, Ocarbonate ) red, N ) purple, H ) white).

coordinate to a step-terminating calcium atom. The eclipsed molecule adsorbs to the dry step with an energy of -176 kJ mol-1 while adsorption of the staggered form releases 146 kJ mol-1, and they would both replace two water molecules on the hydrated surface, one on the step-terminating calcium atom (-50 kJ mol-1) and one on the terrace below (-52 kJ mol-1). So the adsorption of methanamide to the acute-Ca step is again preferred over water for both forms, by -74 kJ mol-1 for the eclipsed and -44 kJ mol-1 for the staggered forms. On the acute-CO3 step both types of methanamide coordinate to a calcium atom in the gap in the step edge created when removing the dipole. They both attach in such a way that they would take the positions of two adsorbed water molecules on the hydrated surface, one on the step edge (-39 kJ mol-1) and one on the terrace below (-45 kJ mol-1). The methanamide molecules themselves adsorb with energies of -160 kJ mol-1 for the eclipsed and -135 kJ mol-1 for the staggered form and comparing these adsorption energies, we see that the adsorption of the eclipsed conformer is preferred by 76 kJ mol-1 over water and the adsorption of the staggered conformer by 51 kJ mol-1. Methylamine H3CNH2. Methylamine adsorbs to the calcite {101 h 4} surface releasing 56 kJ mol-1, which is the lowest adsorption energy of all the surfaces considered. It would replace only one water molecule if adsorbed to the surface instead of water, with a hydration energy of -33 kJ mol-1 and the adsorption of methylamine to the {101 h 4} surface is hence favorable by -23 kJ mol-1. On the acute step, methylamine coordinates to a calcium atom on the step edge (Figure 8) releasing 73 kJ mol-1. Adsorbed methylamine would replace two water molecules, one at the base of the step and one on the step edge calcium atom, with combined hydration energies of -62 kJ mol-1. Adsorption of methylamine is thus only slightly more favorable than adsorbed water at the same site, by -11 kJ mol-1. Although methylamine adsorbs to the obtuse step directly onto the step edge, releasing 80 kJ mol-1, it would only replace one water molecule with a hydration energy of -41 kJ mol-1, so the adsorption of methylamine at the obtuse step edge (-39 kJ mol-1) is much more favorable than at the acute step edge. On the obtuse-Ca surface, methylamine adsorbs to the step-terminating calcium atom with an adsorption

130


Figure 9. Geometry optimized configuration of hydroxy ethanal adsorbed to the calcite acute step, with the hydroxyethanal molecule bridging the step edge and the terrace below (Ca ) green, C ) gray, Ocarbonate ) red, N ) purple, H ) white).

energy of -79 kJ mol-1, where it would replace two water molecules with combined hydration energies of -65 kJ mol-1. The adsorption of methylamine is hence more favorable than water at the same site, by -14 kJ mol-1. On the obtuse-CO3 surface the methylamine molecule adsorbs to a calcium atom just back from the step edge, releasing 66 kJ mol-1. It would again replace only one water molecule compared to the hydrated surface, with a hydration energy of -27 kJ mol-1, so the adsorption of methylamine is again energetically more favorable than the adsorption of water at the same site (-39 kJ mol-1). On the acute-Ca step, methylamine again coordinates to the step-terminating calcium atom, releasing 81 kJ mol-1, where it would replace three water molecules with a combined hydration energy of -85 kJ mol-1. The adsorption of methylamine to the acute-Ca step is thus less exothermic (by 4 kJ mol-1) than hydration of the step, and it would therefore not replace water at this position. On the acute-CO3 step, however, the methylamine coordinates to a calcium atom in the gap in the step, releasing 69 kJ mol-1, which is 16 kJ mol-1 more than the two water molecules it replaces, which have a combined hydration energy of -53 kJ mol-1. Hydroxy-ethanal HC(dO)CH2OH. Both forms of hydroxyethanal adsorb to the calcite {101 h 4} surface in a similar fashion, with adsorption energies of -93 and -77 kJ mol-1 for the eclipsed and staggered forms, respectively. Both molecules bridge two surface calcium atoms, and therefore would replace two water molecules with hydration energies of -29 and -33 kJ mol-1. The adsorption of both hydroxyethanal forms on the planar surface are thus preferred over water, by 31 and 15 kJ mol-1 for the eclipsed and staggered conformers, respectively. Adsorption of hydroxyethanal to the acute step results in the same final configuration for both forms, with hydroxyethanal bridging the step edge and the terrace below (Figure 9), releasing an adsorption energy of -102 kJ mol-1. Consequently, the hydroxyethanal molecules would take the positions of two water molecules, one on the step edge (-28 kJ mol-1), and one on the terrace directly below the step (-50 kJ mol-1). So the adsorption of hydroxyethanal to the acute step is favorable for both forms by 24 kJ mol-1 compared to the adsorption of water at the same position.

de Leeuw and Cooper

On the obtuse step, both forms of hydroxyethanal also coordinate bridging the step and the terrace below, where again they would take the positions of the same two water molecules, with a combined hydration energy of -54 kJ mol-1. Adsorption energies for the eclipsed and staggered conformers are -129 and -117 kJ mol-1, respectively, and the adsorption of hydroxy ethanal instead of water is hence energetically favorable for both forms, by 75 and 63 kJ mol-1, respectively. Both forms of hydroxyethanal adsorb to the obtuseCa surface by bridging between the step-terminating calcium and a calcium atom on the terrace below, the eclipsed form with an energy of -138 kJ mol-1 and the staggered molecule with and energy of -114 kJ mol-1. They would both replace two water molecules, the same one on the step-edge (-33 kJ mol-1), but as they coordinate to different calcium atoms on the terrace below they would therefore replace different water molecules. The eclipsed form replaces a second water molecule with a hydration energy of -75 kJ mol-1, while the water molecule replaced by the staggered conformer has a hydration energy of -25 kJ mol-1. The net result is that the adsorption of both forms is favorable on the obtuse-Ca step, by -30 kJ mol-1 for the eclipsed hydroxyethanal and -53 kJ mol-1 for the staggered form. On the obtuse-CO3 stepped surface, both forms again adsorb by bridging between a calcium atom on the step edge, this time in the gap, and one on the terrace below the step. The eclipsed molecule is situated in the gap and its adsorption releases 120 kJ mol-1, while the staggered conformer effectively spans the gap and adsorbs with an adsorption energy of -105 kJ mol-1. Both would replace two water molecules if adsorbed instead of water, the same water molecule on the terrace below the step (-34 kJ mol-1), but different water molecules on the step, with hydration energies of -27 kJ mol-1 for the one replaced by the eclipsed form and -37 kJ mol-1 for the water molecule replaced by the staggered conformer. Adsorption of hydroxyethanal is again energetically favorable compared to the adsorption of water, for the eclipsed conformer by -59 kJ mol-1 and the staggered molecule by -34 kJ mol-1. Similarly to the obtuse-Ca step, the hydroxy ethanal molecules both adsorb to the acute-Ca stepped surface by bridging the step-terminating calcium atom and a calcium atom on the terrace below the step, the eclipsed form releasing 140 kJ mol-1 and the staggered conformer 137 kJ mol-1. Whereas the eclipsed conformer replaces two water molecules, one on the step edge and one on the terrace directly below with combined hydration energies of -69 kJ mol-1, the staggered molecule takes the place of three water molecules, two on the step-terminating calcium atom and one on the terrace directly below with combined hydration energies of -52 kJ mol-1. Once again, the adsorption of both forms of hydroxyethanal is energetically favorable compared to the adsorption of water, by 71 kJ mol-1 for the eclipsed form of hydroxyethanal and 85 kJ mol-1 for the staggered conformer. On the acute-CO3 stepped surface, however, the hydroxyethanal molecules adsorb in two different ways. The eclipsed conformer adsorbs by bridging a calcium atom in the gap in the step edge and the calcium atom directly below the gap (Figure 10)


Figure 10. Geometry optimized configuration of the eclipsed conformer of hydroxy ethanal adsorbed to the acute-CO3 step, bridging between calcium atoms in the gap in the step and on the terrace below (Ca ) green, C ) gray, Ocarbonate ) red, Oethanal ) dark blue, N ) purple, H ) white).

similar to the obtuse-CO3 step, but the staggered molecule adsorbs by bridging two calcium atoms, which are both located in the gap in the step edge. Adsorption of the eclipsed form is more exothermic, releasing an energy of 139 kJ mol-1, and it would replace two water molecules with combined hydration energies of -52 kJ mol-1. The staggered conformer adsorbs with an energy of -121 kJ mol-1, and it would also replace two water molecules with a combined hydration energy of -84 kJ mol-1. Hence, adsorption of the eclipsed hydroxyethanal molecule to the acute-CO3 step (-87 kJ mol-1) is energetically significantly more favorable than the adsorption of the staggered form (-37 kJ mol-1) due to their different modes of adsorption to the step and the different water molecules they hence replace. Discussion If the organic adsorbates are to be effective inhibitors of calcite crystal growth, it is necessary that they attach strongly to the calcite surface in such a way as to prevent the further addition of calcium carbonate to the growing steps. They therefore first need to be successful in replacing preadsorbed water at the surface, and, second, they need to adsorb at the actual growth sites for the inhibition to be most efficient. For example, although adsorption on the {101 h 4} terrace away from the step edges may eventually stop step growth, it is more effective if the inhibitors are strongly bound to the step edge itself, making it inaccessible to calcium ions and carbonate groups in the surrounding environment. Considering the adsorption sites for the four organic adsorbates, we see that, apart from methylamine, they all interact more strongly with the stepped planes than with the terraces on the planar surface (Table 2), especially with the dipolar steps. Methylamine, on the other hand, does not adsorb to any of the surfaces very strongly and its preference for the growth steps rather than the terrace is not significant, and as such methylamine does not appear to be a very efficient calcite growth inhibitor. The reason for the weak interaction of methylamine with the calcite surfaces is 2-fold. First, only the -NH2 functional group interacts with the polar calcite surface, and these interactions are not as strong as those formed by the oxygen ions of the more polar


functional groups of the other adsorbates, partly due to the steric hindrance by the hydrogen atoms of the -NH2 group which make the nitrogen atom less accessible than the oxygen atoms in carbonyl or hydroxy groups. Second, methylamine only has the one functional group, unlike hydroxy methanamide and hydroxy ethanal, and as such cannot form multiple interactions to the surface by bridging between different surface species. Although methanoic acid also has the one carboxylic acid functional group, it consists of two oxygen atoms, which can both form strong interactions to the terrace and steps. Comparison of the hydroxy methanamide and hydroxy ethanal adsorbates, both containing a carbonyl (dO) group on the first carbon position linked to a -HNOH and -H2COH group, respectively, gives insight into the relative strengths of adsorption of alkyl or hydroxy amide functional groups. As the carbonyl functional groups are the same for the two adsorbates, the differences in strengths and modes of adsorption between the two adsorbates will be due to the nitrogen versus carbon atom at the second position. One difference we see from Table 2 is that hydroxy ethanal preferably adsorbs in the eclipsed position where both its oxygen atoms can interact with surface species. The case for hydroxy methanamide is less clear-cut, with the eclipsed conformation preferred at the dipolar steps, where it adsorbs in a bidentate fashion to calcium ions in or near the step edge vacancies, whereas the staggered conformation is more favorable at the planar and stoichiometric growth steps. The adsorption energies are generally somewhat larger for hydroxy methanamide, if we compare the most favorable energies for each adsorbate at a particular surface (eclipsed or staggered) and the presence of the nitrogen atom thus enhances adsorption to the calcite surface, both because it is more polar and also less sterically hindered than the equivalent carbon atom in hydroxy ethanal, which leads to closer interactions between the nitrogen atom and the surface atoms. Comparison of adsorption of hydroxy ethanal with methanoic acid gives us an indication of the importance of the separation of the dO and -OH groups on the strength of adsorption. The data in Table 2 show, however, that it makes little difference to the adsorption energies whether the dO and -OH groups are attached to the same carbon to form the carboxylic functional group of methanoic acid or whether they are separated over two carbon atoms as in hydroxy ethanal. Adsorption to the planar surface is more favorable for the eclipsed hydroxy ethanal molecule compared to methanoic acid, but less so for the staggered molecule. Otherwise, the adsorption energies between methanoic acid and the staggered form of hydroxy-ethanal are very similar, apart from the acute stoichiometric step, where the staggered molecule rotates to adsorb in an eclipsed configuration. The eclipsed conformer, forming bridging interactions across the step edges, binds more strongly than methanoic acid to the stepped surfaces, but only by 27 kJ mol-1 at best (on the acute-CO3 step). As the planar and stoichiometric steps will be the major surface features found on the experimental calcite surface, it thus appears that an adsorbate with a simple carboxylic acid functional group will be equally effective as a growth inhibitor as a hydroxy aldehyde, although we

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de Leeuw and Cooper

Table 3. Calculated Adsorption Energies of Competitive Adsorption of the Organic Adsorbates Compared to the Adsorption of Water at the Same Surface Adsorption Sites adsorption energies (kJ mol-1) hydroxy methanamide

hydroxy ethanal

surface

methanoic acid

methylamine

eclipsed

staggered

eclipsed

staggered


-51 -71 -73 -33 -89 -30 -44

-23 -11 -39 +4 -14 -16 -39

-47 -52 -64 -74 -63 -76 -140

-45 -50 -51 -44 -51 -51 -61

-31 -24 -75 -71 -30 -87 -59

-15 -24 -63 -85 -53 -37 -34

have of course not considered separation of the carbonyl and hydroxy groups further apart than two carbon atoms. It may well be that separation over a longer hydrocarbon chain may lead to more favorable interactions with the surface, due to the greater flexibility of the longer chain. The competitive adsorption between water and organic adsorbates is now evaluated for all the surfaces studied in Table 3, where the net adsorption energies for the organic adsorbates are listed with respect to preadsorbed water molecules at the same surface sites. As could be expected from the low adsorption energies for methylamine in Table 2, replacement of the preadsorbed water by this organic adsorbate is energetically not very favorable and even endothermic at the acuteCa step. On the major planar and stoichiometric stepped surfaces, adsorption is not very exothermic either, and this adsorbate is hence not a particularly good growth inhibitor. The other three adsorbates do appear to be good growth inhibitors; they attach strongly to the growth steps and the replacement of water by these organic molecules is considerably exothermic at most surface sites. Hydroxy methanamide and hydroxy ethanal interact particularly strongly with the reconstructed dipolar steps, but they also interact favorably with the stoichiometric step edges and these adsorbates should thus be good calcite growth inhibitors. Methanoic acid in general interacts most strongly with the stoichiometric steps, where it forms bidentate interactions to the low-coordinated edge calcium atoms. Due to its structure, it only forms bridging interactions between step edge and terrace on the two calcium-terminated steps, but only on the obtuse-Ca step edge is this adsorption particularly exothermic. As such, methanoic acid will be a good calcite crystal growth inhibitor, when the surface mainly contains regular edge pits and growth edges, which are experimentally found to be formed of the stoichiometric step edges. However, when the surface is less regular, hydroxy methanamide and hydroxy ethanal will be more efficient at inhibiting calcite growth. In addition, the preference of some of the adsorbates for specific growth sites, such as methanoic acid and hydroxy methanamide for the obtuse-Ca and obtuse-CO3 steps, respectively, and hydroxy ethanal for both acute-Ca and acute-CO3 steps, would lead to stronger growth inhibition in particular directions, leading to an asymmetric morphology of the growing calcite terrace as was, for example, shown by Orme et al., who observed this phenomenon when they added amino acids to growing calcite surfaces.50 The basic amino acid structure is not unlike the hydroxy methanamide molecule (without of course the -OH group) and

our calculations are certainly in qualitative agreement with their findings. Conclusions We have executed a series of atomistic simulations to investigate the adsorption of four organic molecules with different functional groups at a range of calcite surface features. Our simulations show that all adsorbates adsorb preferentially at the step edges, especially the steps terminated by single Ca ions or CO3 groups and containing edge vacancies. The strongest attachments between adsorbates and surfaces are found where the organic molecules are capable of forming multiple interactions with surface species, particularly if they can bridge between two calcium ions from the edge to the terrace below. Adsorption of methylamine to the surfaces is not particularly exothermic, and competition with preadsorbed water is unlikely to lead to a high coverage of methylamine at the surface and we therefore do not consider that organic molecules containing only the amine functional group will be efficient growth inhibitors. Carboxylic acids, on the other hand, should be good growth inhibitors as, based on these thermodynamic arguments, they should be effective in replacing water at the experimental growth steps, hence blocking these sites to the addition of further calcium carbonate material. Separating the carboxylic acid functional group into dO and -OH parts spread over two carbon atoms enhances the interaction with the reconstructed dipolar steps as the molecule is now better capable of forming bridging interactions between the step and the terrace. In addition, as a result of the larger spacing between dO and -OH groups, the adsorption energies at the obtuse and acute stoichiometric steps have diverged from about -70 kJ mol-1 for methanoic acid to -24 and -75 kJ mol-1 for hydroxy ethanal at the acute and obtuse step, respectively, which should lead to preferential inhibition of growth at the obtuse step. As the stoichiometric obtuse step is found to both dissolve and grow more rapidly than the acute step,23,25 growth inhibition of the obtuse step would have a significant effect on the growth of the calcite crystal as a whole. Finally, the preferential adsorption at particular surface features, such as shown by methanoic acid and hydroxy methanamide at the obtuse-Ca and obtuse-CO3 steps, respectively, and by hydroxy ethanal to the acute calcium- and carbonate-terminated steps, will lead to an asymmetric surface morphology of the growth islands on the calcite surface. This computational study of the interaction of a range of related organic adsorbates with calcite surface growth


features suggests that computer simulations may in future come to play a significant role in the identification or even design of particular organic additives for specific tailored crystal growth. We hope that future experimental research may confirm our present findings. Perhaps the EXAFS spectroscopic techniques successfully employed by Reeder and co-workers to elucidate metal ion adsorption complexes at calcite surfaces69 could be employed to study certain aspects of these molecular binding phenomena. Acknowledgment. N.H.dL. thanks EPSRC for an Advanced Research Fellowship and T.G.C. thanks the Chemistry Department of the University of Reading for financial support. We thank EPSRC for Grant No. GR/ N65172/01, NERC for Grant No. NER/T/S/2001/00855 and the Royal Society for Grant No. 22292. References (1) Beruto, D.; Giordani, M. J. Chem. Soc., Faraday Trans. 1993, 89, 2457. (2) Romanek, C. S.; Grossman, E. L.; Morse, J. W. Geochim. Cosmochim. Acta 1992, 56, 419. (3) Stipp, S. L.; Hochella, M. F. Geochim. Cosmochim. Acta 1991, 55, 1723. (4) Park, N.-S.; Kim, M.-W.; Langford, S. C.; Dickinson, J. T. Langmuir 1996, 12, 4599. (5) Chakraborty, D.; Bhatia, S. K. Ind. Eng. Chem. Res. 1996, 35, 1995. (6) Dove, P. M.; Hochella, M. F. Geochim. Cosmochim. Acta 1993, 57, 705. (7) Dreybolt, W.; Eisenlohr, L.; Madry, B.; Ringer, S. Geochim. Cosmochim. Acta 1997, 61, 3897. (8) Liu, Z.; Dreybolt, W. Geochim. Cosmochim. Acta 1997, 61, 2879. (9) Reeder, R. J.; Valley, J. W.; Graham, C. M.; Eiler, J. M. Geochim. Cosmochim. Acta 1997, 61, 5057. (10) Zuddas, P.; Mucci, A. Geochim. Cosmochim. Acta 1998, 62, 757. (11) Chakraborty, D.; Agarwal, V. K.; Bhatia, S. K.; Belare, J. Ind. Eng. Chem. Res. 1994, 33, 2187. (12) Blanchard, D. L.; Baer, D. R. Surf. Sci. 1992, 276, 27. (13) MacInnes, I. N.; Brantley, S. L. Geochim. Cosmochim. Acta 1992, 56, 1113. (14) Didymus, J. M.; Oliver, P. M.; Mann, S.; De Vries, A. L.; Hauschka, P. V.; Westbroek, P. J. Chem. Soc., Faraday Trans. 1993, 89, 2891. (15) Goni, S.; Sobrado, L.; Hernandez, M. S. Solid State Ionics 1993, 63, 786. (16) Stipp, S. L.; Gutmannsbauer, W.; Lehrmann, T. Am. Mineral. 1996, 81, 1. (17) Ohnesorge, P.; Binnig, G. Science 1993, 260, 1451. (18) Liang, Y.; Lea, A. S.; Baer, D. R.; Engelhard, M. H. Surf. Sci. 1996, 351, 172. (19) Gratz, A. J.; Hillner, P. E.; Hansma, P. K. Geochim. Cosmochim. Acta 1993, 57, 491. (20) Hillner, P. E.; Manne, S.; Hansma, P. K.; Gratz, A. J. Faraday Discuss. 1993, 95, 191. (21) Wilson, T. R. S.; Thomson, J. Geochim. Cosmochim. Acta 1998, 62, 2087. (22) Liang, Y.; Baer, D. R.; McCoy, J. M.; Amonette, J. E.; LaFemina, J. P. Geochim. Cosmochim. Acta 1996, 60, 4883. (23) Liang, Y.; Baer, D. R. Surf. Sci. 1997, 373, 275. (24) McCoy, J. M.; LaFemina, J. P. Surf. Sci. 1997, 373, 288. (25) de Leeuw, N. H.; Parker, S. C.; Harding, J. H. Phys. Rev. B 1999, 60, 13792. (26) Bischoff, J. L. J. Geophys. Res. 1968, 73, 3315. Bischoff, J. L.; Fyfe, W. S. Am J. Sci. 1968, 266, 65. (27) Compton, R. G.; Brown, C. A. J. Colloid Interface Sci. 1994, 165, 445. (28) de Leeuw, N. H.; Harding, J. A.; Parker, S. C. Mol. Simul. 2002, 28, 573. (29) de Leeuw, N. H. Am. Mineral. 2002, 87, 679. (30) Nassralla-Aboukais, N.; Boughriet, A.; Fischer, J. C.; Wartel, M.; Langelin, H. R.; Aboukais, A. J. Chem. Soc. Faraday Trans. 1996, 92, 3211.

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CG0341003

A Computer Modeling Study of the Inhibiting Effect of Organic

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