Article pubs.acs.org/JPCC
Surface Tension Alteration on Calcite, Induced by Ion Substitution H. Sakuma,*,†,‡,§ M. P. Andersson,† K. Bechgaard,† and S. L. S. Stipp† †
Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark Department of Earth and Planetary Sciences, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
‡
S Supporting Information *
ABSTRACT: The interaction of water and organic molecules with mineral surfaces controls many processes in nature and industry. The thermodynamic property, surface tension, is usually determined from the contact angle between phases, but how does one understand the concept of surface tension at the nanoscale, where particles are smaller than the smallest droplet? We investigated the energy required to exchange Mg2+ and SO42− from aqueous solution into calcite {10.4} surfaces using density functional theory. Mg2+ substitution for Ca2+ is favored but only when SO42− is also present and MgSO4 incorporates preferentially as ion pairs at solution−calcite interfaces. Mg2+ incorporation weakens organic molecule adhesion while strengthening water adsorption so Mg2+ substitution renders calcite more water wet. When Mg2+ replaces 10% of surface Ca2+, the contact angle changes dramatically, by 40 to 70°, converting a hydrophobic surface to a mixed wet surface or rendering a mixed wet surface hydrophilic. This increase in water wettability decreases affinity for organic compounds. An important outcome is that we can now explain why oil recovery from carbonate reservoirs is enhanced when both Mg2+ and SO42− are present in the pore water. Incorporation of MgSO4 into calcite, which is energetically favored, decreases surface tension and releases polar oil compounds.
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INTRODUCTION The interaction of water and organic molecules with mineral surfaces controls a range of processes in nature and in industry such as ion exchange,1 contaminant migration,2,3 enhanced oil recovery,4 biomineralization,5−8 flocculation,9,10 and dispersion of nanoparticles.11 The affinity of a solid surface for a liquid, understood as the thermodynamic property surface tension, is most commonly determined macroscopically, by measuring the contact angle between phases, such as that of a liquid droplet on a solid surface in air.12 In a macroscopic system, this works well, but how do we understand the concept of surface tension at the nanoscale, in systems where the solid surface is smaller than the smallest droplet that can be made? How do we understand surface tension from the fundamental properties of single molecules? As a step toward answering these questions, we used density functional theory to simulate a mineral surface, pure and with substituted ions, in contact with water, to explore how some model organic compounds interact with the mineral, and then we used our results to estimate a change in contact angle, providing a link between molecular-scale properties and macroscopic observable behavior. We chose calcite, CaCO3, because it is an important mineral both commercially and in nature. It constitutes ∼4% of the Earth’s crust and is present in nearly all geological systems, where it serves as a buffer for pH and concentration of dissolved calcium.13 It is the biomineral preferred by many organisms, including some species of marine algae, and thus plays an important role in global carbon and oxygen cycles.14 © 2014 American Chemical Society
Calcite is the main constituent of chalk and limestone, which serve as hosts for oil reservoirs and drinking water aquifers, and it is an important material in industry, such as cement and optical devices, as pigment, filler, and sealant in paint, plastic, pharmaceuticals, and paper. The relative affinity of the mineral surface for water versus organic molecules controls how much oil can be extracted from a reservoir, how clean the drinking water can become after remediation of a contaminated site, and how strong paper or cement can be made. If a surface has a higher affinity for organic molecules than for water, then the capillary pressure of the organic matter is high, and water is unable to replace it.12 We chose to investigate the {10.4} surface of calcite because this set of faces is the most thermodynamically stable15,16 and thus is often manifested in natural samples and in industrial products unless an additive is used to enhance the stability of other faces. For the model organic compounds, we chose benzene as the simplest aromatic hydrocarbon and the acetate/ acetic acid pair as the simplest model anion/fatty acid because carboxyl is a common functional group in a host of organic compounds. Recently, considerable effort has been focused on understanding the wettability of mineral surfaces because of its importance in enhancing oil recovery17,18 and the affinity of Received: November 13, 2013 Revised: January 21, 2014 Published: January 24, 2014 3078
dx.doi.org/10.1021/jp411151u | J. Phys. Chem. C 2014, 118, 3078−3087
The Journal of Physical Chemistry C
Article
nobilis),41,42 foraminifera, and the coccoliths that cover some species of algae.20,21,43 The careful experimental data presented by the Austad group37,44−48 motivated us to use a theoretical approach to explore this possibility. Changes in zeta potential for a calcite surface, in the presence of either cations or anions, can be explained by adsorption of outer sphere complexes of the potential determining ions,16 but Zhang and colleagues showed that the presence of either the cations or the anions alone did not change wettability. Their results suggest that adsorption as inner sphere complexes, exchange of surface ions, or absorption/exchange in the internal calcite crystal might be a necessary criterion for wettability change. Our aim was to elucidate the fundamental mechanism. In this paper, we report on our investigations of the effects on wettability for a calcite {10.4} surface where Ca2+ and CO32− ions were sequentially replaced with Mg2+ and SO42−, in the bulk solid and at the surface. We used electronic structure calculations based on density functional theory (DFT) to compare with the previously published experimental results. Wettability of the mineral surface was determined from the balance of the interfacial tension between the calcite surface, water, and simple organic compounds that represent some model functional groups that are common in industrial processes, groundwater contamination, oil, and a range of biological systems.
biomineral surfaces for proteins, polysaccharides, and inorganic elements.6,19−23 In the example of fine-grained carbonate rocks, which can be oil wet or mixed wet,24 it is difficult to push oil out by water flooding, consistent with the Young−Laplace equation. Capillary pressure is large where pore size is small and/or mineral surfaces are hydrophobic.12 Research over many years on mineral separation by floatation25,26 and recent work on enhancing oil recovery have shown that mineral surface behavior is modified by organic molecule attachment,27−30 and there are indications that changing the composition of the solution could render mineral surfaces more water wet.31,32 In biomineralization, the morphology and crystal growth kinetics of calcite can be controlled by a monolayer of organic molecules as a template and addition of Mg2+ in an aqueous solution.23,33 It is important to mention that the macroscopic wettability of chalk results from a sum of interactions that take place at the nanoscale. The wettability of chalk is heterogeneous at the molecular scale, where the change from hydrophilic and hydrophobic areas is over only a few nanometers.34 The result is a mixed wet surface. Absolute contact angles can be determined using properly parametrized force field molecular dynamics,35,36 and this could be done on a model calcite surface; however, this would be meaningless and simply impossible on a natural material. Examples are the surfaces on the particles in chalk or in soil, where the chemical composition of the hydrophobic areas is unknown.17,34 All of the contributions from all of the surfaces exposed to the fluid must be considered, which is unfeasible at the moment. We use the fact that in chalk recrystallized, rhombohedrally shaped calcite particles are found,34 which shows that some calcite {10.4} surfaces exposed to fluid are present. If the wettability of these areas of pure calcite changes, then the macroscopic contact angle would also change as a result. We have chosen to focus on simple organic molecule interaction with calcite in aqueous solutions containing Mg2+ and SO42− to investigate how surface free energy changes with substitution of Ca2+ and CO32−. This system is convenient because Zhang and colleagues37 presented careful experimental evidence that the interaction of Ca2+, Mg2+, and SO42− with calcite in chalk can make the surfaces more water wet, releasing more organic compounds by spontaneous imbibition, especially at elevated temperatures (70 °C).37 They listed several key points: (i) Ca2+, Mg2+, and SO42−, which are potential determining ions on calcite,37−39 are adsorbed on the mineral surface; (ii) the curious observation that neither Ca2+ nor Mg2+ alone increases spontaneous water imbibition; SO42− must also be present, but SO42− is inactive, unless either Ca2+ or Mg2+ is present; (iii) adsorption of all three ions increased as temperature increased. These observations could also be relevant for the behavior of organic compounds in biological systems. Ca2+, Mg2+, CO32−, and SO42− are major ions in seawater, are present in the body fluids of organisms, and have been shown to serve as a switch for turning on and off the behavior of polysaccharides that are essential for coccolith development.22 We hypothesize that something as simple as ion exchange could control the organic−mineral interactions that are responsible for the intricate and elegant formation of delicate echinoderm (sea urchin) tests,40 mollusk shells (e.g., Atrina rigida and Pinna
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COMPUTATIONAL DETAILS The DFT calculations were performed using the code Quantum Espresso.49 The interaction of core−valence electrons was described using the projector augmented-wave (PAW) method.50 The electronic wave functions were expanded by plane waves with the cutoff energy of 50 Ry. The exchange and correlation functional was expressed by the generalized gradient approximation (GGA) proposed by Perdew et al. (PBE).51 Semiempirical dispersion force corrections52,53 were used in the calculations to take into account van der Waals forces responsible for interactions between nonpolar organic molecules. This is essential for comparing molecules such as benzene and water. The validity of the pseudopotentials and the dispersion corrections were confirmed by the reproducibility of the lattice constants and bulk moduli of calcite, CaCO3, dolomite, MgCa(CO3)2, anhydrite, CaSO4, and the lattice constants of the molecular crystal of acetic acid. The calculated lattice constants that we derived for calcite in a rhombohedral crystal structure were a = 6.356 Å (−0.3%) and α = 46.715° (+1.3%). The numbers in the parentheses are discrepancies between our data and those from standard X-ray diffraction experimental data.54 The VESTA program55 was used for drawing the structures of molecules and calcite surfaces. We investigated the {10.4} surface, named with the hexagonal coordinates, because it is more commonly used. This same surface is also named {211} using rhombohedral coordinates. The simulation supercell for the slab geometry was orthorhombic, and the sides were x = 8.115, y = 10.0795, and z = 30.687 Å, corresponding to a 1 × 2 surface unit cell. The {10.4} plane was placed perpendicular to the z axis of the supercell and is characterized by an atomically smooth layer composed of Ca2+ and CO32− ions. Four layers were included in the supercell so there were 80 atoms in the initial, pure calcite. In all geometry optimizations, the positions of the atoms in the lowest layers were fixed, to represent the bulk structure. Periodic boundary conditions were applied in all 3079
dx.doi.org/10.1021/jp411151u | J. Phys. Chem. C 2014, 118, 3078−3087
The Journal of Physical Chemistry C
Article
cases. A dipole correction56 was added in the direction normal to the {10.4} plane to avoid an artificial electric field generated by the periodic boundary conditions, when polar molecules are adsorbed. Because the initial configuration is important for getting to the minimum energy configuration, we carried out classical molecular dynamics simulations to find a reasonable initial configuration for water molecules in proximity to the calcite surface. The computational method is described in the Supporting Information. To be sure that we eventually reached the overall minimum energy configuration, a number of possible initial configurations were tested for a system with the same number of atoms in the cell. In total, we tested more than 150 configurations. The adsorption energy for water molecules is presented in two different ways and was determined from the following equations. The average adsorption energy,57Eave, is Eave =
calculated using the COSMO-RS method.58 We used DMol3 to generate the COSMO surfaces (which are periodic for the calcite slabs) and to calculate the corresponding vacuum energy. We performed geometry optimizations of the solvated surface and the surface in vacuum using the PBE functional, the DNP basis set, and medium quality for convergence criteria and cutoffs (change in energy
