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Molecular Simulations of Carbon Dioxide and Water: Cation Solvation Louise J

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Molecular Simulations of Carbon Dioxide and Water: Cation Solvation Louise J Article pubs.acs.org/est Molecular Simulations of Carbon Dioxide and Water: Cation Solvation Louise J. Criscenti* and Randall T. Cygan Geochemistry Department, P.O. Box 5800 MS 0754, Sandia National Laboratories, Albuquerque, New Mexico 87185-0754, United States ABSTRACT: Proposed carbon dioxide sequestration scenarios in sedimentary reservoirs require investigation into the interactions between supercritical carbon dioxide, brines, and the mineral phases found in the basin and overlying caprock. Molecular simulations can help to understand the partitioning of metal cations between aqueous solutions and supercritical carbon dioxide where limited experimental data exist. In this effort, we used classical molecular dynamics simulations to compare the solvation of alkali and alkaline-earth metal cations in fl water and liquid CO2 at 300 K by combining a exible simple point charge fl fi model for water and an accurate exible force eld for CO2. Solvation energies for these cations are larger in water than in carbon dioxide, suggesting that they will partition preferentially into water. In both aqueous and CO2 solutions, the solvation energies decrease with cation size and increase with cation charge. However, changes in solvation energy with ionic radii are smaller in CO2 than in water suggesting that the partitioning of cations into CO2 will increase with ion size. Simulations of the interface between + + aqueous solution and supercritical CO2 support this suggestion in that some large cations (e.g., Cs and K ) partition into the CO2 phase, often with a partial solvation sphere of water molecules. ■ INTRODUCTION Because of limited experimental data on metal partitioning To develop accurate regional scale models to evaluate the fate between supercritical CO2 and brine solutions, it is valuable to of supercritical CO in various CO sequestration scenarios, it is consider the use of computational chemistry methods to 2 2 explore this subject. Metal partitioning into the supercritical important to assess the exchange of solutes across the 1−7 CO phase may influence chemical reactions such as the supercritical CO −water interface. Experimental studies of 2 2 dissolution of primary phases or precipitation of secondary the solubility of CO in water and simple aqueous solutions 2 minerals, and physical properties such as both the density and have been used to determine the range of CO concentrations 2 viscosity of the CO phase, and the contact angles formed trapped in idealized reservoir fluids in regional scale multiphase 2 between supercritical CO , water, and mineral phases. Super- models. However, the interaction of supercritical CO with the 2 2 critical CO has been considered as a potential fluid for complex brine compositions found in aquifers have typically 2 extracting metal ions from liquid phases. The ions cannot been ignored in such hydrological simulations, most likely readily be extracted in supercritical CO , but the use of metal− because of the lack of data (solubility, density, partition 2 organic complexes greatly increases metal partitioning into the coefficients, etc.) for these interactions at the temperatures and 15 supercritical phase. This suggests that metal speciation in the pressures expected in geological reservoirs. Recent experimental 8−12 brine may also impact species partitioning between the two and simulation studies of the geochemistry of reservoir fluids. fluids and mineral interactions have provided new insights on fi 13 The research presented here provides a rst look at the supercritical CO2 behavior in geological media. Rempel et al. solvation of alkali and alkaline-earth metal cations in CO fluids present experimental data for the fractionation of Na, Fe, Cu, 2 − and the likelihood that these cations will partition between and Zn between brine and CO2 for pressures of 6.6 16 MPa at brines and supercritical CO2 in sedimentary basin environ- 340 K. The data exhibit an increase in Na concentrations with ments. An overall goal of our research is to use molecular increasing CO2 density and suggest that Fe, Cu, and Zn could simulations to model the structure and behavior of injected be transferred to adjacent aquifer systems. However, this supercritical CO in subsurface reservoirs and how it eventually experimental approach is recently developed and difficult to 2 implement. Therefore, geochemical modeling of reactions Special Issue: Carbon Sequestration among reservoir rocks, brine, and supercritical CO2 focus on investigating the impact of high PCO2 on reactions between Received: April 23, 2012 saline brines and minerals under reservoir conditions, but, as a Revised: July 6, 2012 first approximation, neglect the solubility of water, salts, or Accepted: July 10, 2012 metals in the supercritical phase.14 Published: July 10, 2012 © 2012 American Chemical Society 87 dx.doi.org/10.1021/es301608c | Environ. Sci. Technol. 2013, 47, 87−94 Environmental Science & Technology Article interacts with resident aqueous fluids and minerals over time. In Molecular Dynamics Simulations. Classical molecular the present study we use molecular dynamics to investigate the dynamics (MD) was used to evaluate the structure and − structure of the CO2 brine interface and how electrolytes, thermodynamics of the solvation of alkali and alkaline earth fi − speci cally alkali and alkaline earth cations, are partitioned cations by CO2, and to examine the structure of the water 22 between aqueous and CO2 phases. We report the application of CO2 interfaces. The Forcite software was used to evaluate the fl fi an accurate and exible CO2 force eld, compare cation electrostatic and short-range interactions for each atomic fi solvation in water and CO2 systems, and examine the solubility con guration and time step. MD simulations were performed of CO2 in water. using a canonical NVT thermodynamic ensemble, maintaining a fixed cell volume for a fixed number of atoms. Temperature was ■ COMPUTATIONAL METHODS controlled using the Nose−Hoover23 method. One million Carbon Dioxide Force Field. For the molecular time steps of 1 fs were used to obtain 1 ns of simulation time; fl fi simulations of this study, we use the fully exible CO2 force atomic con gurations were saved every 1000 time steps (1 ps) field developed by Cygan et al.16 The three-point force field for efficient data storage and trajectory analysis. MD 17 − − uses the refined Van der Waals parameters of Zhu et al. that simulations of systems involving water CO2 and brine CO2 were derived from the original model of Harris and Young.18 interfaces were completed for 0.5 ns of simulation time using The force field also allows for full intramolecular bond stretch the same frequency of molecular configuration storage. and angle bend and thus correctly predicts the vibrational Cation Solvation in Liquid Carbon Dioxide. MD spectra of CO2. The CO2 potential is compatible with most simulations of alkali and alkaline earth cation solvation in 19 point-charge force fields including Clayff and other related liquid CO2 were performed using the NVT thermodynamic force fields for the simulation of geochemical systems. The ensemble with 512 CO2 molecules in a simulation box (V = 3 3 interatomic potentials should be appropriate for simulating 48.23 nm ; ρ = 0.77 g/cm at 8 MPa) at 300 K for comparison complex CO2 systems involving multiple phases, interfaces, and with cation solvation in liquid water which is typically the addition of dissolved cations in both water and CO phases. determined at ambient temperature and pressure (300 K, 0.1 2 24,25 In the Cygan et al.16 force field, the total potential energy of MPa). A simulation was performed for 1 ns to equilibrate the molecular system is described by Coulombic and van der the liquid CO2 in a cubic periodic cell with a cell length of 36.4 Waals contributions representing nonbonded energies and Å. Subsequently, each cation was placed in the equilibrated bond-stretch and angle-bend terms representing the intra- model CO2 liquid. A similar set of simulations was completed molecular energies. The long-range Coulombic or electrostatic for the same number of CO2 molecules at supercritical energy is given by eq 1, where q and q are the partial charges of conditions (350 K, 20 MPa) using a cubic periodic cell with i j 3 ρ 3 the atoms and rij is the distance between the atoms. a cell length of 39.2 Å (V = 60.24 nm ; = 0.62 g/cm ). For qq both conditions, each solvation model was initially geometry- ij optimized for 500 iterations prior to 1 ns of MD simulation. ECoul = rij (1) Charge balance was established by using a charge-neutralizing background.20,26 The solvation energy for a particular cation The short-range van der Waals energy is given by was estimated by subtracting the potential energy of the ⎡ 12 6⎤ equilibrated liquid CO2 simulation cell from the potential ⎛ σσ⎞ ⎛ ⎞ ⎢⎜ ij ⎟ ⎜ ij ⎟ ⎥ energy of the equilibrated simulation cell containing liquid CO2 EVDW=−4ε ij⎢⎜ ⎟ ⎜ ⎟ ⎥ − (or supercritical CO2) and the cation [i.e., PE(CO2 + cation) ⎣⎝ rrij ⎠ ⎝ ij ⎠ ⎦ (2) PE(CO2 cell)]. The van der Waals parameters used for the + + + + 2+ ε σ alkali (Li ,Na,K, and Cs ) and alkaline earth cations (Mg , where ij and ij are optimized for intermolecular interactions 2+ 2+ 2+ using values determined by Zhu et al.17 The interaction Ca ,Sr ,Ba ) are provided in Table 1. Four cations of each parameters between unlike atoms are calculated according to σ Table 1. Force Field Parameters for Cations the arithmetic mean rule for the distance parameter, ij, and the 20,21 ε a ε σ geometric mean rule for the energy parameter, ij. cation i (kJ/mol) i (Å) Harmonic potentials are used for the bond stretch and angle Li+ 40 0.0766 1.3723 bend terms. Na+ 25 0.5447 2.3500 + 41 1 2 K 0.4187 3.3340 Ekrr=−() + 42 Stretch2 S ij o (3) Cs 0.4187 3.8310 Mg2+ 40 3.6634 1.6444 1 2+ 41 Ek=−()θθ2 Ca 0.4187 2.8720 Bend2 B ijk o (4) Sr2+ 40 0.4187 3.4620 Ba2+ 40 0.1968 3.8166 Both expressions describe the increase in the potential energy aSmith and Dang,25 Koneshan et al.,41 Smith and Dang,42 Åqvist40.
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