A Computer Modeling Study of the Competitive Adsorption of Water

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A Computer Modeling Study of the Competitive Adsorption of Water and Organic Surfactants at Surfaces of the Mineral Scheelite Timothy G. Cooper†,‡ and Nora H. de Leeuw*,†,§ Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom, Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, United Kingdom, and School of Crystallography, Birkbeck College London, Malet Street, London WC1E 7HX, United Kingdom Received January 22, 2004. In Final Form: February 25, 2004 Atomistic computer simulation techniques were employed to investigate the interaction of a selection of organic surfactant molecules with a range of scheelite surfaces. The adsorbates coordinate mainly to the surfaces through interaction between their oxygen (or nitrogen) atoms to surface calcium ions, followed by hydrogen-bonded interactions to surface oxygen ions. Bridging between two surface calcium ions is the preferred mode of adsorption, but a bidentate interaction by two adsorbate oxygen ions to the same surface calcium ion is also a stable configuration and multiple interactions between surfaces and adsorbate molecules lead to the largest adsorption energies. All adsorbates containing carbonyl and hydroxy groups interact strongly with the surfaces, releasing energies between approximately 80 and 170 kJ mol-1, but methylamine containing only the -NH2 functional group adsorbs to the surfaces to a much lesser extent (55-86 kJ mol-1). Both hydroxymethanamide and hydroxyethanal adsorb to some surfaces in an eclipsed conformation, which is a requisite for these functional groups. Sorption of the organic material by replacement of preadsorbed water at different surface features is calculated to be mainly exothermic for methanoic acid, but less so for the hydroxymethanamide and hydroxyethanal molecules, whereas methylamine would not replace preadsorbed water at the scheelite surfaces. The efficacy of the surfactant molecules is hence calculated to be carboxylic acids > alkyl hydroxamates > hydroxyaldehydes > alkylamines. The results from this study suggest that computer simulations may provide a route to the identification or even design of particular organic surfactants for use in mineral separation processes.

1. Introduction Scheelite CaWO4 is a commercially important material with interesting structural and luminescence properties.1-3 It is used in an extensive range of applications, for example, in the ceramics industry, in catalysts,4 as lasers,5,6 and even as a support in photographic emulsions.7 In addition, scheelite is the main mineral ore for the extraction of tungsten metal,8,9 one application of which is as filaments in light bulbs because of its very high melting point. In the environment, scheelite is often found in granite pegmatites, calc-silicate gneisses, and in high-temperature hydrothermal veins associated with granite rocks. It is commonly associated with other salt-type minerals, such as calcite and fluorite, from which it then needs to be * Address correspondence to this author. E-mail: n.h.deleeuw@ ucl.ac.uk. † University College London. ‡ University of Reading. § Birkbeck College London. (1) Wang, B. G.; Shi, E. W.; Zhong, W. Z.; Yin, Z. W. J. Inorg. Mater. 1998, 13, 648. (2) Koepke, C.; Wojtowicz, A. J.; Lempicki, A. J. Lumin. 1993, 54, 345. (3) Sinelnikov, B. M.; Sokolenko, E. V.; Zvekov, V. Y. Inorg. Mater. 1996, 32, 999. (4) Kim, D. S.; Ostromecki, M.; Wachs, I. E. J. Mol. Catal. A-Chem. 1996, 106, 93. (5) Faure, N.; Borel, C.; Couchaud, M.; Bassett, G.; Templier, R.; Wyon, C. Appl. Phys. B 1996, 63, 593. (6) Kaminski, A.; Eichler, H. J.; Ueda, K.; Klassen, N. V.; Redkin, B. S.; Li, L. E.; Findeisen, J.; Jaque, D.; Garcia-Sole, J.; Fernandez, J.; Balda, R. Appl. Opt. 1999, 38, 4533. (7) Guo, X. M.; Wang, S. Imaging Sci. J. 1999, 47, 29. (8) Gurmen, S.; Timur, S.; Arslan, C.; Duman, I. Scand. J. Metall. 2002, 31, 221. (9) Welham, N. J. J. Mater. Res. 1999, 14, 619.

separated. Because of its high density and coarse liberation, scheelite is often concentrated by gravity separation methods, but with the increasing need to process lowgrade dissemination ores, mineral separation techniques such as froth flotation are increasingly used as a complement or substitute for gravity separation. Flotation is a surface-chemistry based process for the separation of fine solids that takes advantage of the difference in wettability at the solid particle surfaces. The separation process is based on the attachment of mineral particles to the liquid/ gas interface whereby the conditions of equilibrium are based on a comparison of the magnitudes of the initial and final free energies for attachment of a bubble to a particle in an isolated system.10 Originally used in the mining industry for the concentration of mineral ores,11,12 the depletion of mineral ores and environmental concern have made it an important separation process in many fields, from mineral processing13 to water purification and the treatment of industrial wastewater14 and sewage15 and hydrometallurgy.16,17 In view of its significance, it is important to understand the (10) Singh, B. P. Langmuir 1994, 10, 510. (11) Zouboulis, A. I.; Kydros, K. A.; Matis, K. A. Sep. Sci. Technol. 1992, 27, 2143. (12) Loewenberg, M.; Davis, R. H. Chem. Eng. Sci. 1994, 49, 3923. (13) Moolman, D. W.; Aldrich, C.; Van Deventer, J. S. J. Chem. Eng. Sci. 1995, 50, 3501. (14) Cabezon, L. M.; Caballero, M.; Perez-Bustamante, J. A. Sep. Sci. Technol. 1994, 29, 1491. (15) Kydros, K. A.; Gallios, G. P.; Matis, K. A. Sep. Sci. Technol. 1994, 29, 2263. (16) Zouboulis, A. I.; Zamboulis, D.; Matis, K. A. Sep. Sci. Technol. 1991, 26, 355. (17) Lazaridis, N. K.; Matis, K. A.; Stalidis, G. A.; Mavros, P. Sep. Sci. Technol. 1992, 27, 1743.

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Computer Modeling Study of Competitive Adsorption

underlying processes which influence the efficiency of the flotation process. Most mineral surfaces are hydrophilic in nature and selective hydrophobization of the mineral of interest can be used as a way of removing it from the undesirable material. To this end, surfactant molecules (collectors) containing long hydrophobic chains are adsorbed to the mineral,18,19 either via physisorption or ionic bonding, which gives the particles the required hydrophobic surface. The resulting hydrophobic particles are then removed by the dispersion of bubbles through the solution, typically air in dissolved air flotation (DAF). The desired mineral particles, concentrated as a layer of scum on the top of the solution, are removed from the undesired material (gangue) by skimming off the froth layer. For a successful flotation scheme, it is thus necessary to have a collector molecule that is selective for the material of interest, forming strong interactions with the mineral surfaces. At present, the surfactants used in many flotation processes generally do not provide efficient separation of many calcium-bearing minerals, such as CaCO3, CaF2, and CaWO3, which are often found together in mineral ore deposits. The aim of this paper is to report a comprehensive computational investigation of the modes of adsorption and strength of interaction of a selection of model collector molecules with the major surfaces of scheelite. Earlier studies employing a variety of computer modeling techniques have been successful in elucidating important aspects of surfactant interactions with a variety of minerals,20-26 and we have employed established computational methods21 to study at the atomic level the adsorption of organic acids and aldehydes, including two nitrogen-containing molecules. Apart from relative concentrations27 and the occurrence of cross-linking between adsorbed collector molecules,20 the interactions between collector molecules and the mineral surface are mainly electrostatic in nature28 and we have therefore concentrated in this study on the adsorption of collector molecules with different polar functional groups. We have studied methanoic acid as a model for a range of longer chain carboxylic acids, such as oleic acid, and two forms of the simplest alkyl hydroxamate, hydroxymethanamide, which are used extensively in industrial flotation schemes.29,30 In addition to carboxylic acids and alkyl hydroxamates, which are currently used in flotation processes, we have investigated two alternative collector molecules, namely, methylamine and hydroxyethanal, to compare their activity with that of existing collector molecules and also because inclusion of these surfactants will give us the opportunity to study the effect of the different parts of the functional groups separately. Finally, as mineral separa(18) Miller, J. D.; Yalamanchili, M. R. Langmuir 1992, 8, 1464. (19) Armstrong, D. W.; Zhou, E. Y.; Chen, S.; Le, K.; Tang, Y. Anal. Chem. 1994, 66, 4278. (20) Arad, D.; Kaftory, M.; Zolotoy, A. B.; Finkelstein, N. P.; Weissman, A. Langmuir 1993, 9, 1446. (21) de Leeuw, N. H.; Parker, S. C.; Rao, K. H. Langmuir 1998, 14, 5900. (22) Gordeijev, J.; Hirva, P. Surf. Sci. 1999, 440, 321. (23) Pradip; Rai, B.; Rao, T. K. Langmuir 2002, 18, 932. (24) Mielczarski, E.; Mielczarski, J. A.; Cases, J. M.; Rai, B.; Pradip, Colloids Surf., A 2002, 205, 73. (25) Pradip; Rai, B. Colloids Surf., A 2002, 205, 139. (26) Hirva, P.; Tikka, H.-K. Langmuir 2002, 18, 5002. (27) Zhao, Y.; Zouboulis, A. I.; Matis, K. A. Sep. Sci. Technol. 1996, 31, 769. (28) Hancer, M.; Celik, S. Sep. Sci. Technol. 1993, 28, 1703. (29) Rao, K. H.; Cases, J. M.; de Donato, P.; Forssberg, K. S. E. J. Colloid Interface Sci. 1991, 145, 314. (30) Rao, K. H.; Cases, J. M.; Forssberg, K. S. E. J. Colloid Interface Sci. 1991, 145, 330.

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tion occurs under aqueous conditions, we have taken into account the presence of solvent in our simulations and calculated the competitive interactions of the different surfactants and preadsorbed water. 2. Methodology The structures and energies of the scheelite/adsorbate systems were modeled using atomistic simulation techniques. These methods are based on the Born model of solids,31 which assumes that the ions in the crystal interact via long-range electrostatic forces and short-range 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,32 in which each polarizable ion, in our case the oxygen ions of the scheelite crystal and water molecule, 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 tungstate anion. We have employed the METADISE code33 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,34 as well as lowest energy configurations of the adsorbate/solid interface. 2.1. Interatomic Potential Model. We used a combination of potential models for a description of the interactions of the various atoms in the systems, namely, the scheelite potential model developed by ourselves,35 the cvff force field for the organic adsorbates,36 and the water potential model by de Leeuw and Parker.37 The parameters for the interactions between water and organic adsorbates with the scheelite surfaces were derived following the approach by Schro¨der et al.,38 which has been shown to be successful on several occasions, for example, for interactions of water and methanoic acid with calcite and fluorite.21,39 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. For example, we found in a combined DFT and interatomic potential study of the adsorption of water in the scheelite material that both techniques were in good agreement as to the mode of adsorption and the hydration energies, and the experimental morphology of the scheelite crystal was accurately reproduced, indicating that the relative stabilities of the surfaces are calculated correctly.35 In (31) Born, M.; Huang, K. Dynamical Theory of Crystal Lattices; Oxford University Press: Oxford, 1954. (32) Dick, B. G.; Overhauser, A. W. Phys. Rev. 1958, 112, 90. (33) Watson, G. W.; Kelsey, E. T.; de Leeuw, N. H.; Harris, D. J.; Parker, S. C. J. Chem. Soc., Faraday Trans. 1996, 92, 433. (34) de Leeuw, N. H.; Watson, G. W.; Parker, S. C. J. Phys. Chem. 1995, 99, 17219. (35) Cooper, T. G.; de Leeuw, N. H. Surf. Sci. 2003, 531, 159. (36) Osguthorpe, P.; Osguthorpe, D.; Hagler, A. Proteins: Struct., Funct., Genet. 1988, 14, 31. (37) de Leeuw, N. H.; Parker, S. C. Phys. Rev. B 1998, 58, 13901. (38) Schro¨der, K. P.; Sauer, J.; Leslie, M.; Catlow, C. R. A. Chem. Phys. Lett. 1992, 188, 320. (39) de Leeuw, N. H.; Cooper, T. G. J. Mater. Chem. 2003, 13, 93.

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addition, simulations of water adsorption at surfaces of calcium carbonate, a similar mineral to scheelite, predicted geometries for the adsorbed water molecules, which were later confirmed by experimental surface science techniques.40,41 Furthermore, in addition to numerous computational studies of the sorption of a range of organic molecules in complex oxide systems such as zeolites, for example,42-44 we have shown in earlier work45 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 calciumaluminum salt when grown in the presence of organic acids.46 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.21,39 On the basis of 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 scheelite surfaces. 2.2. Surface and Adsorption Energies. 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 stoichiometric units, and A is the surface area. A low positive value for γ indicates a stable surface. As the mineral particles are present in an aqueous environment, we modeled the hydrated surfaces with a full monolayer of water, which is here defined as the maximum number of water molecules which can be adsorbed at the surface in a single layer before formation of a second layer. The water molecules were added sequentially, identifying the energetically most favorable location and configuration at each step, and a full geometry optimization was performed after addition of each water molecule. Addition of more water molecules was stopped once the adsorption energy became endothermic or at the onset of a second layer of water. In a previous molecular dynamics study of water adsorption at MgO surfaces, it was shown that there is a clear difference between the monolayer of water adsorbed at the mineral surface and the bulk water, the liquid structure of which was disturbed by the adsorbed water molecules, leading to a gap of low water density between the two types of water.37 Only the adsorbed water monolayer affected the structure and energy of the underlying surface and in this study we therefore only consider an adsorbed monolayer. In our investigation of the sorption of the organic molecules, we adsorbed one molecule per surface simulation cell at a variety of different surface sites to identify the energeti(40) de Leeuw, N. H.; Parker, S. C. J. Phys. Chem. B 1998 102, 2914. (41) Fenter, P.; Geissbu¨hler, P.; DiMasi, E.; Srajer, G.; Sorensen, L. B.; Sturchio, N. C. Geochim. Cosmochim. Acta 2000, 64, 1221. (42) Titiloye, J. O.; Parker, S. C.; Stone, F. S.; Catlow, C. R. A. J. Phys. Chem. 1991, 95, 4038. (43) Catlow, C. R. A.; Freeman, C. M.; Vessal, B.; Tomlinson, S. M.; Leslie, M. J. Chem. Soc., Faraday Trans. 1991, 87, 1947. (44) Sastre, G.; Catlow, C. R. A.; Corma, A. J. Phys. Chem. B 2002, 106, 956. (45) Cooper, T. G.; de Leeuw, N. H. Mol. Simul. 2002, 28, 539. (46) Fogg, A. M.; Freij, A. J.; Oliveira, A.; Rohl, A. L.; Ogden, M. I.; Parkinson, G. M. J. Cryst. Growth 2002, 234, 255.

cally most favorable location. In each case, the surface simulation cells were supercells of sufficient size (105.3194.6 Å2) to ensure the absence of any computational artifacts because of the periodic boundary conditions parallel to the surface and to accommodate the adsorbate molecules, without constraints as to the geometry of the adsorbed molecule or interference between the adsorbate and its images in the periodically repeated surface cell. 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 self-energy of the free adsorbate molecule, calculated using the same simulation parameters (methanoic acid, -1913.7 kJ mol-1; hydroxymethanamide, -2554.1 kJ mol-1; hydroxyethanal, -3347.8 kJ mol-1; methylamine, -1438.3 kJ mol-1). 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 self-energy of a water molecule in the liquid phase, calculated at -921.4 kJ mol-1 for the potential model used in this work, which value is the sum of the energy of the gaseous water molecule and the condensation energy of water, which was calculated at -43 kJ mol-1 in molecular dynamics simulations,37 in excellent agreement with experiment (-44 kJ mol-1).47 Although Eadsorbate for the organic molecules should include a term for their energy of solvation, these energies are not known experimentally for all of the adsorbates considered. However, as they are likely to be very similar to methanoic acid, for hydroxymethanamide and hydroxyethanal, which contain very similar functional groups, this would add only a constant term to their self-energies, whereas the solvation energy of the nonpolar methylamine molecule is expected to be very small. In addition, adsorption of the organic molecules to the surface still allows for substantial interaction with surrounding water molecules and hence the adsorbed molecule can retain its solvation shell to a large extent; as this solvation shell is not included in the calculations, it should be deducted again from the bulk solvation energy. As such, the necessary correction to the self-energy term will be small (about 15 kJ mol-1 if we base our estimate on approximately a third of the bulk solvation energy of methanoic acid) and using the energy of the free molecule will give sufficient insight into the relative adsorption trends for the various molecules, which is the topic of this paper. As a solvation shell does not form for the water molecules in the water layer, but interactions between them are explicitly included in the calculation, we do need to include the energy of condensation of bulk water in the self-energy term of the water molecule. 2.3. Mineral Surfaces. Scheelite has a tetragonal crystal structure with space group I1/a with a ) b ) 5.2429 Å, c ) 11.3737 Å, and R ) β ) γ ) 90°.48 The calcium ions in the structure are eightfold coordinated to one oxygen each of eight surrounding tungstate groups at Ca-O distances of 2.44 or 2.48 Å. The tungsten atoms are (47) Lide, D. R. Handbook of Chemistry and Physics, 81st ed.; CRC Press: Boca Raton, FL, 2000. (48) Hazen, R. M.; Finger, L. W.; Mariathasan, J. W. E. J. Phys. Chem. Solids 1985, 46, 253.


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Figure 1. Geometry optimized structures of the hydrated scheelite surfaces (a) the calcium-terminated and (b) the tungstateterminated {101} planes, (c) the calcium-terminated and (d) the tungstate-terminated {103} planes, (e) the {111} and (f) the {001} surfaces, showing surface vacancies and adsorbed water molecules. (Ca ) gray, H ) white, O ) black, WO4 shown as tetrahedra).

tetrahedrally coordinated to four oxygen atoms at 1.78 Å, where the tetrahedral angles are slightly distorted with angles of 107.4° and 113.8°. On geometry optimization, the structure contracted somewhat to a ) b ) 5.1370 Å, c ) 11.2844 Å, but the a/c parameter at 2.19 is in good agreement with the experimental value (2.17). The structural parameters such as Ca-O distances at 2.43 and 2.46 Å and W-O bond lengths at 1.69 Å as well as the O-W-O angles at 106.6° and 115.4° all adequately reproduce the experimental values.35 Scheelite is a sedimentary material, which grows in an aqueous environment, where the exposed surfaces will be hydrated. However, when the mineral particles are crushed in the first stages of the flotation process, the minerals will mainly cleave along surfaces that have large interplanar spacings and few interplanar bonds, usually low-index surfaces with low surface energies under dry conditions. Hence, the mineral surfaces that we may expect to find in the aqueous slurry will be both the surfaces expressed in the crystal morphology of the mineral and cleavage planes because of the crushing of the mineral particles, all of which will be suitably hydrated. The {101} and to a lesser extent the {001} and {111} surfaces of scheelite are cleavage planes,49 while the {101} and {103} surfaces are expressed in the experimental crystal mor(49) Allaby, A.; Allaby, M. A Dictionary of Earth Sciences; Oxford University Press: Oxford, 1999.

phology,50 as well as being the dominant planes in the calculated thermodynamic morphology of the hydrated crystal.35 In this work, we have therefore considered the sorption of organic molecules at the {001}, {101}, {111}, and {103} surfaces, shown in Figure 1. Previous ab initio calculations of the bulk material have shown that the tungstate groups in the crystal act as polyanions,51 and these groups were hence kept intact when we created the relevant surfaces. Both the {101} and {103} surfaces can be terminated by either calcium ions or tungstate groups and they thus have a dipole perpendicular to the surface plane. Dipolar surfaces are not stable and their energies will diverge with increasing crystal size,52 leading to surface defects and reconstruction. We reconstructed the dipolar surfaces to form stable nondipolar planes, which was achieved by introducing surface vacancies in the topmost calcium or tungstate layers. We thus obtain a reconstructed calcium- and tungstate-terminated plane for each surface (shown in Figure 1), which all contain a range of different surface sites, that is, surface and subsurface calcium ions or tungstate groups, surface vacancies, and hence complementary low-coordinated adsorption sites. The {001} and {111} planes are nondipolar surfaces and, unlike the reconstructed dipolar {101} (50) Horvath, L.; Gault, R. A. Mineral. Record 1990 21, 284. (51) de Leeuw, N. H.; Cooper, T. G. Phys. Chem. Chem. Phys. 2003, 5, 433. (52) Bertaut, F. C. R. Acad. Sci., Paris, 1958, 246, 3447.

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Table 1. Calculated Surface and Average Hydration Energies of the Geometry Optimized Dry and Hydrated Scheelite Surfaces surface energies γ (Jm-2) and average hydration energies Ehydr (kJ mol-1) surface

γdry

γhydrated

Ehydr

{101} Ca {101} WO4 {103} Ca {103} WO4 {001} {111}

0.79 1.19 1.16 1.26 0.64 0.88

0.20 0.09 0.14 0.58 0.36 0.23

-52 -76 -79 -34 -22 -61

and {103} surfaces, they do not contain surface vacancies. The {100} surface is completely planar whereas the {111} surface is slightly corrugated in nature. These two surfaces hence offer different surface sites for adsorption from the morphologically dominant {101} and {103} planes, such as step edges, and it is thus of interest to include them in our study even apart from the fact that they are experimental cleavage planes and hence may be present in the mixture of crushed mineral particles. The surface and hydration energies of the relaxed scheelite surfaces are listed in Table 1. It is clear from the decrease in surface energies that all surfaces are stabilized significantly by hydration, which is an exothermic process at the different planes. The water molecules interact with the surface by attaching by their oxygen atoms to surface calcium ions at distances ranging from 2.3 to 2.6 Å, while coordinating by their hydrogens to surface oxygen atoms at 1.9 to 2.8 Å. In addition, the water molecules form an extensive network of intermolecular hydrogen-bonded interactions within the water layer. The average adsorption energies per water molecule in Table 1 show that hydration of the {001} surface releases the least energy, only 22 kJ mol-1, because the dry {001} surface is already a stable plane with a low surface energy, containing tungstate groups in their bulk coordination and calcium ions in sixfold coordination, which is a common coordination number for calcium (e.g., the solvated ion is coordinated to six water molecules). Conversely, the calcium terminated {103} surface releases the largest energy upon hydration, an average of 79 kJ mol-1. When creating this surface, the calcium ions lose four bonds to oxygen atoms, leaving fourfold coordinated calcium ions in the plane, and one oxygen atom of the surface tungstate groups is also undercoordinated leaving dangling bonds. This surface is as a result highly reactive and upon hydration, each calcium ion coordinates to two water molecules, hence increasing its coordination number to six, whereas the undercoordinated oxygen atom forms short hydrogen bonds to two water molecules. The other surfaces have structures intermediate between the stable {001} and highly reactive {103} Ca surface and they release on average 40-60 kJ mol-1, with coverages varying between 5 and 10 water molecules/nm2, depending on the structure of the surface. The dipolar planes generally accommodate more water molecules as the surface vacancies supply more adsorption sites per unit area, especially where the vacancies would have contained the large tungstate groups. 3. Results We considered the adsorption of four different organic molecules to the various mineral surfaces, namely, methanoic acid HCOOH, hydroxymethanamide HC(dO)NHOH, methylamine H3CNH2, and hydroxyethanal HC(dO)CH2OH. This selection of adsorbates 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 hydroxymethanamide and hydroxyethanal, 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. Although the free molecule is energetically more stable in a staggered conformation, we have also 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. In addition, when a long hydrocarbon chain is attached to the molecule, as it would be in a surfactant, the eclipsed conformer would be the energetically preferred configuration of the functional group. We also found during our calculations that often the staggered conformer would itself rotate during the energy minimization to form the eclipsed form at the surface. However, 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. Dissociation of the adsorbate molecules was not considered in this work. Preliminary investigations have shown that the use of fully associated molecules in the simulations give good agreement between calculated adsorption trends and experimentally found behavior and, in any case, the presence of the proton in the adsorbate molecule does not prevent the relevant oxygen atom from interacting with the mineral surface species, as is described in the next sections.21 3.1. Adsorption Structures. A complete description of the modes of adsorption, adsorbate/substrate configurations, and interatomic interactions of the different adsorbate molecules to the various surfaces is given in ref 53, which is, however, outside the scope of this paper. The most relevant adsorption features for each surfactant are given below and in Table 2. Generally, the major adsorption interactions are electrostatic with the adsorbate molecules preferentially coordinating by their negative atoms, oxygen or nitrogen, to surface calcium atoms. In addition, hydrogen bonding between most of the available hydrogen atoms and surface oxygen atoms also plays an important role. Often, when coordination to surface calcium atoms is similar between two surfaces, it is the presence or absence of multiple hydrogen-bonded interactions which determines the magnitude of the adsorption energies. Geometric and steric factors can of course make a difference to the exothermicity of the adsorption process, where the spacing between surface adsorption sites (usually calcium ions) and between the adsorbate atoms forming the interactions to the surface (oxygen/nitrogen and to a lesser extent hydrogens) should match to obtain an optimum adsorbed configuration leading to the largest adsorption energies. 3.1.1. Methanoic Acid HCOOH. In general, the most favorable mode of adsorption for methanoic acid is by bridging between two surface calcium ions, coordinating to each by one of the molecule’s oxygen atoms. This type of configuration is seen on the {001} and {111} surfaces, where in addition the carbonyl oxygen forms multiple (53) Cooper, T. G. Ph.D. Thesis, University of Reading, U.K., 2003.


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Table 2. Interatomic Distances between the Surfaces and Adsorbate Molecules for the Energetically Preferred Adsorption Configurations surface-adsorbate interatomic distances for the energetically preferred configurations/Å surfactant HCOOH

HC(dO)NHOH

H3CNH2

HC(dO)CH2OH

surface

Ca-Od

{101}Ca {101}WO4 {103}Ca {103}WO4 {001} {111} {101}Ca {101}WO4 {103}Ca {103}WO4 {001} {111} {101}Ca {101}WO4 {103}Ca {103}WO4 {001} {111} {101}Ca {101}WO4 {103}Ca {103}WO4 {001} {111}

2.21 2.18 2.18 2.27, 2.81 2.26 2.43, 2.44 2.20 2.43, 2.78 2.14 2.25, 2.53 2.15 2.40, 2.63

Ca-OH

Ca-N

Osurf-HO 2.51-2.55 2.41, 2.67 2.63-2.75 2.52, 2.59 2.60, 2.61 2.54 ∼2.43 2.50, 2.54 2.39 2.61 2.60 2.56, 2.67

2.60 2.78 2.75 2.37 2.89 2.25 2.35 2.81 2.29 2.27 2.25 2.25 2.33 2.30

2.29 2.17 2.12 2.23 2.22 2.43, 2.47

2.28 2.36 2.45 2.41 2.37 2.61

interactions to two calcium ions, with the hydroxy oxygen coordinating to one of these calciums as well. Figure 2a shows the {111} surface, which is slightly corrugated, where the methanoic acid adsorbs across the step bridging between two calcium ions. However, sometimes the spacing between surface calcium ions is too large for the methanoic acid to bridge between two calcium ions, for example, the {101} and {103} surfaces where the surface calcium ions are 5.13 Å apart. The methanoic acid molecules then interact with only one surface calcium ion through their carbonyl oxygen atom. On the tungstate-terminated surfaces, the adsorbates still interact preferentially with the calcium ions that are accessible underneath the tungstate vacancies, which were created for the removal of the dipole. In all cases, the configuration of the adsorbed methanoic acid is such as to encourage further hydrogen bonding to oxygen atoms of surface tungstate groups, for example, the {101}WO4 surface shown in Figure 2b. 3.1.2. Hydroxymethanamide HC(dO)NHOH. Although as mentioned above, the free hydroxymethanamide molecule is more stable in the staggered than in the eclipsed conformation by 29 kJ mol-1, often the energetically most favorable mode of adsorption for the adsorbate molecule is as the eclipsed conformer. The reason for this preference at many surfaces of adsorption of hydroxymethanamide molecules in an eclipsed conformation is due to the geometry of the surface, which determines the number and strength of interactions between the adsorbates and surface species. For example, on the {101}WO4 surface, the eclipsed conformer interacts through both its oxygen atoms with one surface calcium atom in a bidentate fashion, in addition to weakly coordinating to another surface calcium through its carbonyl oxygen atom (Figure 3a). However, the structure of this surface does not allow the staggered conformer to coordinate by bridging two surface calcium atoms, and it can only coordinate via its carbonyl oxygen atom to one surface calcium ion (Figure 3b), which is a much weaker overall interaction between surface and adsorbate than for the eclipsed conformer. Conversely, on the calcium-terminated {103}Ca surface, the energies released upon adsorption of both eclipsed

2.48 2.51 2.55 2.47 2.42

Osurf-HC

Osurf-HN

2.70, 2.76 2.62 2.74-2.90 2.66, 2.72 2.60 ∼2.56 2.42, 2.55 2.41 2.49, 2.56 2.54 2.58 2.60-2.89 2.58, 2.66 2.60, 2.70 2.62-2.97 2.61, 2.73 2.60, 2.67 2.40, 2.75 2.48-2.73 2.54 2.40, 2.63 2.58 2.42, 2.69-2.83

2.71 2.47 2.85 2.48-2.70 2.88 2.87, 2.90 2.84 2.61-2.68 2.79, 2.89

Figure 2. Plan views of (a) the {111} surface, with adsorbed methanoic acid molecule bridging between two surface calcium ions on a slight step, and (b) the {101} WO4 surface with the methanoic acid molecule coordinated only via its carbonyl oxygen atoms to one surface calcium ion. Key: Ca ) pale green, W ) blue, O (tungstate) ) red, O (methanoic acid) ) dark blue, C ) gray, and H ) white.

and staggered conformers are very similar, although slightly more favorable for the staggered conformation. Again, the eclipsed conformer adsorbs to one surface

3990


Figure 3. Hydroxymethanamide adsorbed to the scheelite {101}WO4 surface, (a) with the eclipsed conformer adsorbing to calcium atoms between rows of surface-terminating tungstate groups and (b) with the staggered conformer coordinating only via its carbonyl oxygen atom. Key: Ca ) pale green, W ) blue, O (tungstate) ) red, C ) gray, O (adsorbate) ) dark blue, N ) purple, and H ) white.

calcium atom in a bidentate fashion through both its oxygen atoms (Figure 4a), but now the staggered confomer adsorbs by bridging between two surface calcium atoms, making its adsorption the more favorable on this surface (Figure 4b). So, in general, if the eclipsed conformer can adsorb in a bidentate fashion, preferably with further coordination to another calcium atom, adsorption of the eclipsed conformer is more favorable than the staggered conformer. On the other hand, if the staggered conformer can bridge between two surface calcium ions, then adsorption in the staggered conformation may be preferred. However, the two conformers are not rigid molecules and free rotation is allowed along the C-N bond. As such, the final configuration of the adsorbed molecule is often intermediate between the two extreme conformer structures and in some instances, the final configuration is obtained from either starting conformer.54 3.1.3. Methylamine H3CNH2. Methylamine is the only adsorbate in this series where the nitrogen atom interacts with surface calcium ions. In hydroxymethanamide, the nitrogen is shielded by the oxygen and hydrogen atoms and is not readily accessible for interaction with the surface and only the amide hydrogen atoms interact with surface oxygen atoms. However, in methylamine, the nitrogen atoms are not shielded by big functional groups and as the adsorbate molecule’s negative species, it interacts with surface calcium atoms. In all cases, there are also extensive interactions between surface oxygen atoms and the adsorbate’s hydrogen atoms, always with hydrogen atoms of the methyl group and in most cases also with hydrogens (54) de Leeuw, N. H.; Cooper, T. G. Cryst. Growth Des. 2004, 4, 123.

Cooper and de Leeuw

Figure 4. Adsorption of hydroxymethanamide at the {103}Ca surface, showing (a) the eclipsed conformer adsorbed in a bidentate fashion and (b) the staggered conformer adsorbed by bridging between two surface calcium atoms. Key: Ca ) pale green, W ) blue, O (tungstate) ) red, C ) gray, O (hydroxamic acid) ) dark blue, N ) purple, and H ) white.

of the amine group, for instance, on the {111} surface shown in Figure 5, where the methylamine molecule is adsorbed across a depression in the surface. 3.1.4. Hydroxyethanal HC(dO)CH2OH. The modes of adsorption of hydroxyethanal are similar to those of hydroxymethanamide. The structure of the adsorbate is similar, with a carbon replacing the nitrogen of hydroxymethanamide. Due to the presence of the extra carbon atom between the two oxygen groups compared to methanoic acid, the staggered conformer of hydroxyethanal interacts with all surfaces by bridging via its two oxygen atoms between two surface calcium atoms, shown in Figure 6 for the tungstate-terminated {103}WO4 surface. Like hydroxymethanamide, the eclipsed conformer, on the other hand, usually adsorbs in a bidentate fashion to the surface, coordinating by both oxygen atoms to a single surface calcium, shown in Figure 7 for the calcium-terminated {103}Ca surface, where this adsorption configuration is energetically more favorable than adsorption by the staggered conformer. Again, we find that sometimes adsorption of hydroxyethanal in an eclipsed conformation is energetically preferred over a staggered conformation, for example, on the {103}Ca (Figure 7) and {001} surfaces, even though the free molecule is 24 kJ mol-1 more stable in the staggered conformation than as an eclipsed conformer. In all cases, hydrogen-bonded interactions between the adsorbates’ hydrogen atoms and surface oxygens play an important role in further stabilizing the adsorption of the hydroxyethanal molecules.


Langmuir, Vol. 20, No. 10, 2004 3991

Figure 5. Methylamine adsorbed to the slight step on the {111} surface, (a) space-filled and (b) showing the interatomic distances between adsorbate molecule and surface species. Key: Ca ) pale green, W ) blue, O (tungstate) ) red, C ) gray, N ) purple, and H ) white.

Figure 6. The staggered conformer of hydroxyethanal adsorbed to the {103}WO4 surface, (a) a plan view showing bridging of the adsorbate between two surface calcium ions and (b) the interatomic distances between the adsorbate and surface species. Key: Ca ) pale green, W ) blue, O (tungstate) ) red, C ) gray, O (hydroxyethanal) ) dark blue, and H ) white.

3.2. Adsorption Energies. The energies released upon adsorption of the various surfactants to the series of mineral surfaces, calculated according to eq 2, are listed in Table 3. Of the different surfaces, adsorption of all surfactants is most favorable at the {103}WO4 surface.

Figure 7. The eclipsed conformer of hydroxyethanal adsorbed to the {103}Ca surface, (a) a plan view showing the bidentate coordination by the adsorbate’s oxygen ions to a surface calcium ion and (b) the interatomic distances between the adsorbate and surface species. Key: Ca ) pale green, W ) blue, O (tungstate) ) red, C ) gray, O (hydroxyethanal) ) dark blue, and H ) white.

The reason for this preference is due to the relative instability of this surface compared to the other planes, as shown by its relatively high surface energies when either dry or hydrated. This relative instability makes the {103}WO4 surface more reactive than the other surfaces which is exemplified by the larger energies released upon adsorption at this plane. In addition, on

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Cooper and de Leeuw

Table 3. Calculated Adsorption Energies of Organic Adsorbates at Unhydrated Scheelite Surfaces adsorption energies (kJ mol-1) hydroxymethanamide

hydroxyethanal

surface

methanoic acid

methylamine

eclipsed

staggered

eclipsed

staggered

{101} Ca {101} WO4 {103} Ca {103} WO4 {001} {111}

-102 -98 -131 -138 -93 -104

-65 -65 -82 -86 -55 -68

-97 -129 -136 -173 -105 -164

-104 -119 -139 -150 -98 -165

-97 -92 -138 -150 -77 -92

-109 -98 -116 -158 -69 -111

those surfaces where the surface and adsorbate geometries allow the formation of multiple interactions to a range of surface species, as is, for example, the case for methanoic acid on both calcium- and tungstate-terminated {103} planes, the adsorption energies at 131-138 kJ mol-1 are significantly larger than for the other surfaces (93-104 kJ mol-1). Whereas we might expect that the adsorption energies on the {103}Ca and {101}Ca surfaces would be similar, it is clear that in all cases adsorption at the {103}Ca plane releases more energy than at the {101}Ca plane even though the interatomic spacing between surface calcium atoms is the same for both surfaces (5.13 Å). However, the calcium atoms terminating the {101}Ca surface are surrounded by more oxygen atoms of surface tungstate groups than the calcium atoms terminating the {103}Ca surface. As a result, there are more repulsive interactions between surface oxygen atoms and oxygen (or nitrogen) atoms of the adsorbate molecules on the {101}Ca surface, leading to weaker coordinations between adsorbate and surface species than on the {103}Ca surface. Our calculations thus indicate that the strength of adsorption of the different adsorbates to the surfaces depends first upon the interactions, both number and strength, of adsorbate oxygen (or nitrogen) atoms to surface calcium ions, followed by hydrogen-bonded interactions to surface oxygen atoms, but that repulsive interactions between adsorbate oxygens and surface oxygens also play a role. Adsorption of methylamine is least favored of the four adsorbates and the reason for this weak interaction with the scheelite surfaces is twofold. First, only the -NH2 functional group interacts with the polar surfaces 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 because of the steric hindrance by the hydrogen atoms of the -NH2 group which makes the nitrogen atom less accessible than the oxygen atoms in carbonyl or hydroxy groups. Second, methylamine only has the one functional group, unlike hydroxymethanamide and hydroxyethanal, 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 surface calcium atoms. Comparison of the hydroxymethanamide and hydroxyethanal 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. In general, the energies released by adsorption of hydroxymethanamide and hydroxyethanal are the largest of the four adsorbates studied,

with those for hydroxymethanamide often somewhat larger than for the hydroxyethanal, if we compare the most favorable energies for each adsorbate at a particular surface (eclipsed or staggered). The presence of the nitrogen atom thus enhances adsorption to the scheelite surfaces, both because it is more polar and also less sterically hindered than the equivalent carbon atom in hydroxyethanal, which leads to closer interactions between the nitrogen atom and the surface atoms. Comparison of adsorption of hydroxyethanal 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 3 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 hydroxyethanal. The adsorption energies of methanoic acid and hydroxyethanal are very similar and it thus appears that an adsorbate with a simple carboxylic acid functional group will be equally effective as a surfactant as a hydroxy aldehyde, although we have 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 because of the greater flexibility of the longer chain. The increase in energies released at some surfaces, notably the {103}WO4 plane, when the extra carbon atom is introduced in the adsorbate, shows that the increased flexibility of the hydroxyethanal, coupled with its ability to bridge between calcium atoms that are farther apart on the surface, can have a distinct effect on the strength of interaction with the surfaces. 4. Discussion The adsorption energies listed in Table 3 give insight into the strength of adsorption of the adsorbates to the pure mineral surfaces. However, mineral separation takes place in an aqueous environment 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.54 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 in the adsorbed monolayer and, more importantly, that the


Langmuir, Vol. 20, No. 10, 2004 3993

Table 4. Number of Preadsorbed Water Molecules Replaced by Organic Adsorbate Molecules number of replaced water molecules hydroxymethanamide

hydroxyethanal

surface

methanoic acid

methylamine

eclipsed

staggered

eclipsed

staggered

{101} Ca {101} WO4 {103} Ca {103} WO4 {001} {111}

2 3 2 2 2 2

2 3 2 3 2 2

2 3 2 3 2 2

2 2 2 3 2 3

2 2 3 3 2 2

2 2 3 3 2 2

Table 5. Calculated Net 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) hydroxymethanamide

hydroxyethanal

surface

methanoic acid

methylamine

eclipsed

staggered

eclipsed

staggered

{101} Ca {101} WO4 {103} Ca {103} WO4 {001} {111}

+2 +39 -44 -13 -49 -38

+58 +69 +37 +19 -11 +71

+8 +46 +116 -50 -61 -51

+1 +13 +113 -45 -54 +32

-52 +14 +170 -42 -33 +61

-64 +8 +192 -50 -25 -21

relative energies for the different sites do not change significantly when water is included explicitly in the calculations, as the interactions between adsorbate and surrounding water molecules are very similar at all the different surfaces.53 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 taken into account separately by a careful quantitative comparison with the hydrated rather than anhydrous surfaces.54 In most cases, the organic adsorbate will replace two or more water molecules, which will increase the entropy of the system hence leading to variation between the calculated enthalpies of the different surface/adsorbate systems. However, as can be seen from Table 4 listing the number of water molecules replaced by the adsorbates at each surface, usually the number of water molecules replaced at each surface is the same for the different surfactants and the relative strengths of adsorption of the various adsorbates will therefore not be expected to vary significantly with entropy, especially as this effect is likely to be small compared to the enthalpies concerned. A detailed description of the comparison between the sorption of the organic adsorbates and the preadsorbed water is given in ref 53. A summary is shown in Tables 4 and 5, which, respectively, list the number of replaced water molecules and the net adsorption energies of the adsorbates with respect to the preadsorbed water molecules at the same surface sites. Because of the structural variations between the surfactant molecules, or indeed between conformers, they generally adsorb at different surface features on the mineral planes and hence do not replace the same water molecules at these surfaces, leading to variations in the net adsorption energies (Table 5) that are different from the variations in the raw adsorption energies of Table 3, as the latter are due only to the surfactant/surface interactions. For example, methanoic acid replaces three water molecules at the tungstate-terminated {101} surface with a combined hydration energy of -137 kJ mol-1, whereas the sum of the hydration energies of the water molecules replaced by methylamine is -134 kJ mol-1. Even when the adsorbates are structurally similar, as hydroxymethanamide and hydroxyethanal clearly are, they still adsorb to the surfaces in different manners. The two conformers of hydroxyethanal adsorb in similar positions and as a result replace

the same two water molecules at the tungstate-terminated {101} surface, with combined hydration energies of -106 kJ mol-1, but the water molecules replaced by the hydroxymethanamide conformers at the same surface have combined hydration energies of -175 kJ mol-1 and -132 kJ mol-1 for the eclipsed and the staggered forms, respectively. So, we see that the position of the different adsorbates at the surface is not only important in determining their own strength of adsorption to the surfaces, but it also determines the energy needed to replace the preadsorbed water from the adsorption site. Apart from variations in the net adsorption energies, compared to Table 3, caused by the replacement of different water molecules by the surfactants at each particular surface, taking into account the hydration of the surfaces also leads to variations in the net energies for each adsorbate from one surface to the other. For example, the dry {001} surface is a stable plane with a low surface energy and hydration of this surface hence only releases an average energy of 22 kJ mol-1 (Table 1). On the other hand, the calcium-terminated {103} surface is highly reactive toward water, releasing an average hydration energy of 79 kJ mol-1, as its surface species are all significantly undercoordinated compared to the bulk material and hydration increases the coordination number of the surface calcium ions from four to six and terminates the oxygen dangling bonds. If we compare the adsorption energies for methanoic acid at these two surfaces in Tables 3 and 5, we note that whereas the raw adsorption energies of methanoic acid at these surfaces differ by a significant amount, 38 kJ mol-1 (Table 3), once we have taken into account the strength of adsorption of the replaced water, the difference in net adsorption energies for methanoic acid at these two surfaces is reduced to only 5 kJ mol-1. Although methanoic acid is clearly adsorbed more strongly to the Ca{103} surface than to the {001} surface, the energetic cost incurred in replacing the equally strongly adsorbed water leads to a net adsorption process which is only moderately exothermic and comparable to the less reactive {001} surface. However, this effect can also work in the opposite direction. If we compare the adsorption of eclipsed hydroxymethanamide at the same two surfaces, the difference in raw adsorption energies (Table 3) is similar at 31 kJ mol-1 to the case of methanoic acid, but now the effect of the dissimilar hydration behavior of the two surfaces is to increase the difference in net adsorption

3994


energies to 177 kJ mol-1, hence making adsorption at the Ca{103} surface energetically far less favorable and even endothermic. Although the trend for the {001} surface for these two molecules and indeed for all adsorbates is similar, some adsorbates on the Ca{103} surface disrupt the very stable hydrogen-bonded network of adsorbed water molecules to a much larger extent than others, leading to large differences in net adsorption energies compared to the raw energies in Table 3. In general, despite the significant adsorption energies of the four adsorbates to the dry scheelite surfaces (Table 3), once we compare them with the hydrated surfaces, the energies released upon replacement of the water by the surfactants are either fairly small or positive, indicating that surfactant adsorption is generally not favored at the hydrated surfaces (Table 5). Methylamine in particular is unable to replace preadsorbed water at the surfaces and the single nitrogen group is clearly not sufficient to attach strongly enough to the mineral surfaces. Methanoic acid is capable of competing with the water molecules for surface adsorption sites, at least on the major {103} surfaces and the two minor {001} and {111} cleavage planes. The reason for its successful adsorption is that on these surfaces, the methanoic acid molecules only replace two water molecules each, rather than three on the {101} surface where it is not competitive compared to water. As the major surface-adsorbate interactions are between adsorbate oxygen atoms and surface calcium ions, the Ca-O interactions between the two oxygen atoms of the methanoic acid molecule are similar to the Ca-O interactions of two water molecules. However, in addition to Ca-O interactions, methanoic acid forms more hydrogen-bonded interactions to the surfaces than the water molecules, leading to an exothermic reaction when water is replaced by methanoic acid on the {103}, {001}, and {111} surfaces. Generally, adsorption of hydroxymethanamide is more favorable in the eclipsed form, which is significant as in flotation processes a long hydrocarbon chain would be attached to the carbon atom, in effect forcing the hydroxymethanamide to adopt the eclipsed configuration. Competitive adsorption to the two major surfaces of scheelite, the {103}Ca and {101}WO4 surfaces (Table 5), is an endothermic process, although it is favorable at the minor {001} and {111} surfaces. The {101}WO4 and {103}Ca surfaces dominate the hydrated crystal morphology and as such competitive adsorption of hydroxymethanamide would be unfavorable. However, in flotation processes the mineral particles get crushed and we can therefore expect cleavage planes, such as the {001} and {111} surfaces, to become more important. As such, hydroxymethanamide may be a suitable surfactant, although methanoic acid, which does attach to the major {103}Ca surface, should be a better flotation reagent. Like hydroxymethanamide, hydroxyethanal does not preferentially adsorb at the {101}WO4 and {103}Ca surfaces either. It does attach to the {101}Ca plane, but adsorption of the eclipsed conformer to the {111} surface is endothermic. The staggered conformer does adsorb to

Cooper and de Leeuw

the {111} surface, but if a hydrocarbon chain were attached to the second carbon atom, the presence of staggered conformers would become rare. As such, this surfactant appears to be a less effective flotation reagent than the hydroxymethanamide it resembles. 5. Conclusions We have executed a series of computer simulations to investigate the adsorption of four organic molecules with different functional groups at a range of scheelite surfaces. Our simulations show that 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 surface calcium ions. Adsorption of methylamine to the surfaces is not particularly exothermic and competition with preadsorbed water is unlikely to lead to a high coverage by methylamine at the surface and we therefore do not consider that organic molecules containing only the amine functional group will be efficient flotation reagents. Carboxylic acids, on the other hand, should be good surfactants for mineral separation as, on the basis of these thermodynamic arguments, they should be effective in replacing water at the major surfaces, hence rendering the particle hydrophobic. To a lesser extent, surfactants with the hydroxyethanal and the hydroxymethanamide functional groups should also be able to float scheelite mineral particles. Although the nitrogen atom does not itself interact readily with the surfaces, its presence in the methanamide adsorbate still enhances its sorptive properties compared to the ethanal, mainly, through steric effects of the -NH rather than the -CH2 residue. This computational study of the interaction of a range of related organic adsorbates with scheelite surface 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 mineral separation. Future work will include investigating the effect of temperature through molecular dynamics simulations and a comparative study of different minerals, including carbonates, tungstates, fluorides, and phosphates, to asses the capability of each surfactant molecule to separate particular materials out of a mixture of these coexisting minerals. Acknowledgment. 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. T.G.C. thanks the Chemistry Department of the University of Reading for financial support and NHdL thanks EPSRC for an Advanced Research Fellowship. Supporting Information Available: Interatomic potential parameters. This material is available free of charge via the Internet at http://pubs.acs.org. LA049796W

A Computer Modeling Study of the Competitive Adsorption of Water

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