Computational Evaluation of MgâSalen Compounds as Subsurface

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Computational Evaluation of Mg−Salen Compounds as Subsurface Fluid Tracers: Molecular Dynamics Simulations in Toluene−Water Mixtures and Clay Mineral Nanopores Jeffery A. Greathouse,*,† Timothy J. Boyle,‡ and Richard A. Kemp‡,§ †

Geochemistry Department, Sandia National Laboratories, Post Office Box 5800, MS 0754, Albuquerque, New Mexico 87185-0754, United States ‡ Advanced Materials Laboratory, Sandia National Laboratories, 1001 University Boulevard Southeast, Albuquerque, New Mexico 87106, United States § Department of Chemistry and Chemical Biology, University of New Mexico, 300 Terrace Street Northeast, Albuquerque, New Mexico 87131, United States S Supporting Information *

ABSTRACT: Molecular tracers that can be selectively placed underground and uniquely identified at the surface using simple on-site spectroscopic methods would significantly enhance subsurface fluid monitoring capabilities. To ensure their widespread utility, the solubility of these tracers must be easily tuned to oil- or water-wet conditions as well as reducing or eliminating their propensity to adsorb onto subsurface rock and/or mineral phases. In this work, molecular dynamics simulations were used to investigate the relative solubilities and mineral surface adsorption properties of three candidate tracer compounds comprising Mg−salen derivatives of varying degrees of hydrophilic character. Simulations in water−toluene liquid mixtures indicate that the partitioning of each Mg−salen compound relative to the interface is strongly influenced by the degree of hydrophobicity of the compound. Simulations of these complexes in fluid-filled mineral nanopores containing neutral (kaolinite) and negatively charged (montmorillonite) mineral surfaces reveal that adsorption tendencies depend upon a variety of parameters, including tracer chemical properties, mineral surface type, and solvent type (water or toluene). Simulation snapshots and averaged density profiles reveal insight into the solvation and adsorption mechanisms that control the partitioning of these complexes in mixed liquid phases and nanopore environments. This work demonstrates the utility of molecular simulation in the design and screening of molecular tracers for use in subsurface applications.

1. INTRODUCTION The ability to track subsurface fluids to provide critical information for the design and implementation of associated industrial processes, such as fossil energy extraction,1,2 underground waste disposal,3 and geothermal energy production,4 is of great interest. One potential solution to this problem is the development of a library of tracer compounds with unique spectroscopic fingerprints that ultimately provide details about the geochemical environment encountered by injected fluids. Because these tracer compounds will be introduced underground, they must be compatible with both the injected solutions and any other fluids encountered in the specific subsurface environment. In addition, they must be able to withstand the harsh chemical and thermal environments encountered in these deep-underground (up to 5 km) wells. As injected or production fluids return to the surface, the detection of these tracer molecules will provide information about the movement of the fluid in specific zones or subzones. A series of metal−ligand complexes based on a strongly coordinating, multidentate organic ligand (i.e., salen ligands; Figure 1) would provide the necessary foundation for a library of tracer compounds by varying both the metal and ligand structure. Combinations of these metal−salen species would result in a unique vibrational spectroscopic [Fourier transform infrared (FTIR) or Raman] signature for each compound, and © XXXX American Chemical Society

the solubilities of these compounds in oil or aqueous liquids can be tuned through functionalization of the ligand. Recently, we synthesized a series of M−salen complexes based on the salen dianionic ligand and M = Mg2+ or Ca2+ ions, and these coordination compounds provided unambiguous spectroscopic detection using either visible or Raman spectroscopies.5 Preliminary data also indicated that the compounds could survive elevated temperature and pressure associated with deep underground wells as well as easily adjusting the solubilities by altering the substituents on the ligand. Because these compounds offer the potential to be functional tracer molecules, understanding how they interact with subsurface strata becomes an essential requirement for potential use and implementation. Herein, molecular simulations were used as an initial screening tool in determining relative solubilities of three Mg−salen complexes in toluene−water mixtures as well as adsorption trends in model subsurface environments. Specifically, molecular dynamics (MD) simulations were used to provide time-averaged structural and adsorption properties at both toluene−water and liquid−mineral interfaces. For the mineral phase considered here, layered clay minerals were Received: February 2, 2018 Revised: March 28, 2018

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DOI: 10.1021/acs.energyfuels.8b00435 Energy Fuels XXXX, XXX, XXX−XXX

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(hereafter referred to as a gibbsite surface) and a more hydrophobic siloxane surface.

2. MATERIALS AND METHODS 2.1. Force Field (FF) Parametrization. Three Mg−salen compounds of varying hydrophilic properties based on the H2−salen ligand [N,N′-ethylene bis(salicylideneimine)] and recently published structures5,24 were evaluated (Figure 1). Both the Mg−salen and Mg− salen−pyr derivatives possess the same salen ligand binding both O and N atoms to a single Mg metal center, forming a planar geometry. For Mg−salen−pyr (Figure 1b), two pyridine molecules bind to Mg through dative bonds, forming a slightly distorted octahedral geometry. In contrast, the metal center in Mg−salen (Figure 1c) adopts a square planar geometry with open coordination sites above and below the plane of the molecule. On the basis of these structures, the Mg−salen−pyr compound is expected to be more hydrophobic than Mg−salen as a result of the potential for coordination by water molecules or surface hydroxyl groups with the open coordination sites. The third Mg complex (Mg−pyrid2+ accompanied by Cl− ions in the MD simulations) based on a recently synthesized Pd−pyridiunium salt24 was also modeled to evaluate a charged species. This complex also has two open Mg sites and bears a positive charge; therefore, it is expected to be the most hydrophilic of the three complexes. During the simulations, all potential energies (fluid−fluid and fluid− mineral) included bonded (e.g., bond stretch and angle bend) and non-bonded terms. Non-bonded interactions were modeled with pairwise potentials, including electrostatic and van der Waals (Lennard−Jones 6−12) terms. FF parameters for all organic species (Mg−salen complexes and toluene solvent) were taken from the generalized AMBER force field (GAFF).25,26 Because no parameters are included for coordinated metal ions in GAFF or other general organic FFs, gas-phase quantum calculations were performed for each compound to obtain optimized geometries and atomic charges. Density functional theory (DFT) calculations were performed using the Gaussian09 code27 with the M062X functional28 and the cc-pVDZ basis set29,30 for all atoms. These methods have been used to derive FF parameters for other hybrid metal−organic compounds.31 DFToptimized structures of each complex are shown in Figure 1. After geometry optimization, partial atomic charges for all Mg−salen complexes were calculated using the CHELPG method32 with default radii for all atoms. There is some degree of covalency in the Mg− ligand bonds in all three compunds, as evidenced by the CHELPG charges calculated from the DFT-optimized structures (see the Supporting Information): +1.04 e, +1.05 e, and +1.2 e for Mg− salen−pyr, Mg−salen, and Mg−pyrid2+, respectively. Atomic charges for these compounds are given in the MD input files (Supporting Information). For classical MD simulations, intramolecular bond and angle terms involving Mg atoms were added to the potential energy expression for each Mg−salen complex, with equilibrium values taken from the DFToptimized structures. This method has been used to model similar metal−ligand species.31 This approach was used to keep the local geometry about Mg as close as possible to the DFT-optimized structure; hence, a large harmonic force constant (300 kcal mol−1) was used for Mg-containing bonds and L−Mg−L angles (L = O or N), and dihedral terms involving Mg atoms were not included. Force constants for Mg−O−C and Mg−N−C angle terms were taken from parameters derived for a metal−organic framework with the same planar coordination environment.31 A comparison of molecular geometries from DFT and GAFF optimization (Table S1 of the Supporting Information) reveals that our FF approach accurately reproduces the molecular structure of these complexes. Parameters for aqueous species (water, Na+, and Cl−) and clay minerals were taken from ClayFF,33 which includes the flexible simple point charge (SPC) water molecule34 and ion−water potentials from Smith and Dang.35 Lennard−Jones parameters for unlike atom pairs were generated using arithmetic mixing rules.36 All MD simulations were performed using the LAMMPS code.37 A time step of 0.5 fs was used for short-range interactions, and long-range electrostatics were

Figure 1. DFT-optimized structures and schematic representations of (a) salen ligand, (b) Mg−salen−pyr (pyridine ligands are shown as thin bonds), (c) Mg−salen, and (d) Mg−pyrid2+. Atoms are colored as follows: Mg, green; O, red; N, blue; C, gray; and H, white.

modeled as a result of their prevalence in shale reservoirs for oil and gas extraction.6 The computational screening was found to expedite the tracer design process, reducing the need for costly laboratory synthesis experiments and field tests. Molecular simulations have been widely used to evaluate aqueous interfaces of clay minerals, including both interlayer7 and external siloxane surfaces. 8−11 More recently, the interaction of organic species with clay mineral surfaces has been studied, including the adsorption of organic dye compounds12,13 and enhanced oil recovery applications involving representative crude oil components, such as nonpolar aromatics and saturates,14−18 asphaltenes,19,20 and carboxylate-containing resins.14,21 For computational simplicity, these studies usually focus on inert basal surfaces, but the adsorption of organic species at pH-dependent edge surfaces has also been reported.21,22 Most molecular simulation studies of clay−water interfaces use smectite clay minerals, such as montmorillonite, which bear a permanent negative charge as a result of isomorphic substitution. The basal siloxane surface of smectite clay minerals contains adsorbed extraframework cations, resulting in a hydrophilic surface. However, the siloxane surface in the neutral endmembers (e.g., pyrophyllite) is very hydrophobic and does not expand in the presence of water.17,23 To compare the adsorption properties of tracer compounds in a range of surface-wetting environments (water-wet and oil-wet), a variety of clays were evaluated, including a smectite clay (Na− montmorillonite) as well as a non-swelling clay (kaolinite), which consist of a hydrophilic aluminum hydroxide surface B


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Energy & Fuels evaluated every 1.0 fs using Ewald summation.38 The following discusses simulation methods for (i) toluene−water mixtures and (ii) mineral nanopores. 2.1.1. Toluene−Water Mixtures. Two-phase liquid models were constructed from 2137 water molecules and 362 toluene molecules, with each phase occupying a volume of ∼40 × 40 × 40 Å3. Each Mg− salen complex was simulated separately by placing the complex in either the water or toluene phase at either a low or high concentration (three or six complexes, respectively). Initially, atoms in the Mg−salen complex were fixed, and solvent molecules were simulated at a high temperature (1000 K) for 50 ps under constant volume conditions to remove the effect of the initial configuration, followed by cooling to 298 K over 100 ps. Next, all atoms were allowed to move during a 2.0 ns simulation at a constant pressure (0 atm), during which only the cell z parameter was allowed to change, while x and y were fixed at 40 Å. Finally, data were averaged over a 5.0 ns production simulation at a constant volume. Similar methods have been used to study the partitioning of solute at the interface of two immiscible liquids.39−41 A representative snapshot of a two-phase liquid simulation is shown in Figure 2.

nanopore Na + ions were placed at each external surface. Representative snapshots of each type of kaolinite nanopore are shown in Figure 3, and a snapshot of the Na−montmorillonite nanopore is shown in Figure 4.

Figure 3. Equilibrium snapshot from MD simulations of Mg−salen in (a) toluene-filled and (b) water-filled kaolinite nanopores. Toluene and water molecules are shown as gray and red lines, respectively. Atoms in Mg−salen are colored as in Figure 1, and kaolinite atoms are colored as follows: Si, orange; Al, magenta; O, red; and H, white.

Figure 2. Equilibrium snapshot from a MD simulation of Mg−salen in a toluene (gray)−water (red) mixture. Atoms in Mg−salen are colored as in Figure 1. 2.1.2. Mineral Nanopores. The MD simulation method for mineral nanopores is similar to the two-phase liquid simulations and based on previously developed methods to simulate clay nanopores9 and the binding of crude oil compounds in these nanopores.14,21 Atomic coordinates were taken from published crystal structures of kaolinite42 and pyrophyllite.43 Each unit cell was orthogonalized and expanded into 8 × 5 × 3 and 8 × 4 × 2 supercells for kaolinite and pyrophyllite, respectively. The pyrophyllite supercell was transformed into negatively charged montmorillonite by randomly replacing 24 octahedral Mg atoms per clay layer with Al atoms, while avoiding any Al−O−Al occurrences. The negative charge was balanced by 24 interlayer Na+ ions. The resulting unit cells for kaolinite and Na− montmorillonite were [Si 4 ](Al 4 )O 10 (OH) 8 and Na 0.75 [Si 8 ](Al3.25Mg0.75)O20(OH)4, respectively, where brackets and parentheses refer to cations in tetrahedral and octahedral sheets. Each bulk clay mineral was simulated for 1.0 ns at 298 K under constant pressure conditions. Average supercell dimensions were obtained for kaolinite (41.60 × 44.67 × 21.51 Å) and Na−montmorillonite (41.66 × 36.01 × 18.71 Å) and used to generate initial configurations for the nanopore simulations. The lateral (x and y) dimensions represent a clay surface area of at least 15 nm2 for each model. The thickness (z dimension) of each clay slab prevents any unwanted short-range interactions between fluid species near one surface and those at the opposite surface. As a result of both hydrophilic and hydrophobic surfaces of kaolinite,12,44 separate water-wet and toluene-wet kaolinite nanopores were modeled. The amount of solvent used to fill each nanopore corresponds to a thickness of ∼40 Å for consistency with the twophase liquid simulations. Kaolinite nanopores were created by separating adjacent basal surfaces and then filled with solvent molecules and Mg−salen complexes at either a high or low concentration (three or six complexes, respectively). Only water-filled nanopores were considered for Na−montmorillonite, and half of the

Figure 4. Equilibrium snapshot from a MD simulation of Mg−salen in a Na−montmorillonite nanopore. Water molecules and Na+ ions are shown as red lines and purple spheres, respectively. Atoms in Mg− salen and montmorillonite are colored as in previous figures.

Fluid species (solvent, Mg−salen complex, and Na+ and Cl− ions as needed) were simulated under constant volume conditions at 1000 K for 50 ps and then cooled to 298 K during a 100 ps simulation. After pinning three atoms in the mineral slab (Al or Si) to prevent slipping of mineral layers, all other atoms were simulated at 298 K for 100 ps at a constant volume followed by 1.0 ns at a constant pressure (0 bar) to equilibrate the nanopore fluid volume. During the constant pressure simulation, only the z component of the simulation cell was allowed to vary. Fluid atoms were then simulated at 1000 K for 200 ps (constant volume and clay atoms fixed), and configurations were saved every 10 ps. For each of the 20 initial configurations, the fluid was cooled to 298 K (constant volume) for 200 ps and all but three pinned atoms were simulated at 298 K for 3.0 ns (constant volume). Data from the final 2.0 ns of each simulation were used for analysis. Average properties from the nanopore simulations therefore represent 40 ns of production simulation with 20 different initial configurations. The composition C


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Energy & Fuels Table 1. Composition and Dimensions of Mineral Nanopore Models mineral solvent N_solvent x (Å) y (Å) z (Å)

Mg−salen

3 6 3 6 3 6

Mg−salen−pyr Mg−salen−pyr Mg−salen Mg−salen Mg−pyrid2+ Mg−pyrid2+

kaolinite water 2137 41.6 44.7 56.7 57.6 56.4 56.8 56.7 57.9

Na−montmorillonite toluene 362 41.6 44.7 58.0 59.4 58.9 59.0 59.0 60.4

water 2030 41.7 36.0 60.3 61.4 59.4 60.5 60.6 61.2

and cell parameter from each nanopore simulation are given in Table 1.

3. RESULTS 3.1. Structure of Mg−Salen Complexes. A comparison of geometric parameters from both DFT- and GAFF-optimized structures is given in Table S1 of the Supporting Information. A square-planar geometry is maintained for the tetradentate Mg− salen complexes throughout the calculations, whereas an octahedral coordination about the Mg atom in Mg−salen− pyr resulted from the bound pyridines (Figure 1b). It should be noted that the structure of Mg−salen−pyr agrees well with the X-ray crystal structure,5 which is a validation of the DFT methods for these complexes. Additionally, the GAFF parameters correctly predict the Mg-containing bond lengths and angles for all complexes (Table S1 of the Supporting Information). Some discrepancies can be seen in the dihedral angles, but this is not surprising because no Mg-containing dihedral angle terms were included in the FF parametrization. Overall, the GAFF parametrization for these complexes accurately reproduces the DFT-optimized geometries, which validates the parameters for use in MD simulations of Mg− salen complexes in mixed fluid phases as well as clay nanopores. 3.2. Toluene−Water Mixtures. The effect of Mg−salen molecular properties on solution and interfacial behavior was first investigated by comparing the partitioning of each complex in two-phase liquid mixtures consisting of water and toluene. Average atomic density profiles (Figure 5) indicate that toluene and water densities far from the liquid−liquid interface are close to their bulk liquid densities (1.00 and 0.87 g cm−3, respectively45), providing further validation of the interatomic potentials used for these species. The peak position of the Mg atoms in each complex relative to the toluene−water interface can be seen in Figure 5, indicating differences in the relative solubilities of each complex in toluene and water. A representative snapshot of Mg−salen complexes in this system (Figure 2) are predominately located at the toluene−water interface, regardless of their chemical properties. The results shown in Figure 5 were obtained from simulations in which the Mg−salen complexes were initially placed in the toluene phase. Similar interfacial structures were obtained when the complexes were initially placed in the water phase (see Figure S1 of the Supporting Information). Each complex contains some degree of hydrophilic and hydrophobic character; therefore, the interface is expected to be the preferred location of such complexes in an oil−water mixture. Calculating absolute solubilities for each complex is beyond the scope of this work, but comparing their relative solubilities in water and toluene can be deduced from the Mg peaks in

Figure 5. Atomic density profiles from MD simulations of Mg−salen complexes in toluene−water mixtures. Water (O atoms) and toluene (methyl C atoms) profiles were taken from a single simulation (Mg− pyrid2+) with densities scaled to g cm−3. Profiles for Mg in each Mg− salen complex and Cl were obtained from separate simulations at a high concentration (six Mg−salen complexes) and are scaled for ease of viewing. The Cl profile (dashed blue line) was taken from the Mg− pyrid2+ simulation. All density profiles were adjusted horizontally, so that z = 0 corresponds to the center of the toluene−water interface (equal toluene and water densities). Complete density profiles for each simulation are shown in Figure S1 of the Supporting Information.

Figure 5. The Mg peaks from the two salen complexes lie completely within the toluene phase, indicating their hydrophobic character. Surprisingly, only a slight hydrophilic shift was observed for Mg−salen compared to Mg−salen−pyr, even though the former readily binds water molecules at the Mg site (shown in more detail below) and the latter has a Mg atom that is fully coordinated by salen and pyridine ligands. In contrast, all Mg−pyrid2+ ions reside completely within the water phase. Chloride ions, present in the Mg−pyrid2+ simulations for charge balance, also have an unusually high density at the interface as a result of electrostatic interactions with Mg− pyrid2+. The lack of correspondence between Mg−pyrid2+ and Cl peaks indicates that Cl− ions are not directly coordinated to Mg−pyrid2+. Rather, Mg−pyrid2+ complexes preferentially bind water molecules at the Mg site (shown in more detail below). It was also noted that the most hydrophobic and hydrophilic complexes (Mg−salen−pyr and Mg−pyrid2+, respectively) are not completely bound to the interface. Each complex has nonzero density in the pure liquid phases (Figure 5 and Figure S1 of the Supporting Information). 3.3. Kaolinite Nanopores. Simulations of the Mg−salen complexes in fluid-filled clay nanopores allows the partitioning of complexes between adsorbed and liquid phases to be compared. Kaolinite presents two unique external basal surfaces in subsurface environments: a hydrophobic siloxane surface and D


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siloxane surface (92%), with almost no adsorption to the gibbsite surface. The pyridinium groups provide additional surface area for hydrophobic interactions with the siloxane surface, which is energetically preferred over fully toluenesolvated complexes. Unlike the toluene−water system, in which the density of aqueous Cl− ions mirrored that of Mg−pyrid2+, in a toluene-filled nanopore, most Cl− ions are found in the most hydrophilic environment possible (i.e., the gibbsite surface). The single peak in the Mg−salen density near the toluenewet gibbsite surface (Figure 6b) indicates a single type of surface complex, which is closer to the gibbsite surface than the fully coordinated Mg−salen−pyr compound (Figure 6a). As seen in Figure 7a, the predominant (inner sphere) surface

a hydrophilic gibbsite surface. Because it is possible that oil-rich liquid phases could be in contact with kaolinite surfaces, separate simulations were performed for kaolinite nanopores filled with either toluene or water. Atomic density profiles are shown in Figure 6 for each complex in toluene-filled kaolinite nanopores, and a represen-

Figure 7. Equilibrium snapshots from MD simulations of Mg−salen complexes adsorbed to kaolinite surfaces in (a) toluene-filled and (b) water-filled nanopores. Liquid molecules have been removed for clarity, except for water molecules coordinating Mg in panel b. Surface hydroxyl O and H atoms coordinating Mg in panel a are shown in balland-stick format.

complex for Mg−salen is characterized by the incorporation of a surface hydroxyl group in the first coordination shell of Mg. Thus, the gibbsite surface provides a hydrophilic anchor for Mg−salen under oil-wet conditions. A detailed analysis of the free energy of adsorption and residence times for this complex would require additional simulations, but we see from the atomic density profile (Figure 6b) that Mg−salen is also found in the toluene phase (46%) and even partially at the siloxane surface (3%). In a water-filled kaolinite nanopore, all three Mg−salen complexes were found to favor adsorption to the siloxane surface, with no interaction with the gibbsite surface (Figure 8). Although the first water O peak is closer to the siloxane surface than the Mg peaks, these complexes can interact directly with the surface (Figure 7b). A significant amount of Mg−salen−pyr is found in the water phase (Figure 8a), forming an organic-rich region in the nanopore. Both salen complexes have the potential to form dimers or other oligomeric complexes through strong hydrophobic interactions involving the organic rings. Salen dimers reside in the water phase and also at the gibbsite surface (Figure 7b). Despite a higher percentage of Mg−salen adsorbed at the kaolinite siloxane surface than Mg−salen−pyr (60 versus 34%, respectively), the water structure at this surface is almost identical for both systems. In contrast, the water structure at the siloxane surface in the Mg−pyrid2+ system (Figure 8c) is quite different from those seen in the salen simulations as a result of the different organic structures at the siloxane surface (two Mg peaks rather than a single peak). These same surface complexes are seen in the Na−montmorillonite nanopores and discussed

Figure 6. Atomic density profiles from MD simulations of (a) Mg− salen−pyr, (b) Mg−salen, and (c) Mg−pyrid2+ in toluene-filled kaolinite nanopores. Toluene densities (methyl H atoms, black) are reported as g cm−3. All other densities are scaled for ease of viewing. Mg atoms (green) in each Mg−salen complex are shown along with Cl− (blue) in panel c. For kaolinite, only the Si (orange) and Al (purple) densities are shown.

tative snapshot from one MD simulation is shown in Figure 3a. Toluene structures at the various kaolinite surfaces are similar despite the presence of different Mg−salen complexes with a range of hydrophobic properties. The toluene density is consistently higher at the siloxane surface (right side of panels a−c of Figure 6) as a result of strong hydrophobic interactions. Although the three Mg−salen complexes exhibit similar behavior in toluene−water mixtures, they display distinctly different adsorption properties at toluene-wet kaolinite surfaces. The most hydrophobic complex (Mg−salen−pyr; Figure 6a) is only weakly adsorbed on the two surfaces, with most species (81%) residing in the toluene phase. About half of the more hydrophilic Mg−salen complexes (Figure 6b) are strongly adsorbed to the hydrophilic gibbsite surface (52%), while the Mg−pyrid2+ complex is strongly adsorbed to the hydrophobic E


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Figure 8. Atomic density profiles from MD simulations of (a) Mg− salen−pyr, (b) Mg−salen, and (c) Mg−pyrid2+ in water-filled kaolinite nanopores. Water densities (O atoms, red) are reported as g cm−3. All other densities are scaled for ease of viewing. Mg atoms (green) in each complex are shown along with Cl atoms (blue) in panel c. For kaolinite, only the Si (orange) and Al (purple) densities are shown.

Figure 9. Atomic density profiles from MD simulations of (a) Mg− salen−pyr, (b) Mg−salen, and (c) Mg−pyrid2+ in water-filled Na− montmorillonite nanopores. Water densities (O atoms, red) are reported as g cm−3. All other densities are scaled for ease of viewing. Mg atoms (green) in each Mg−salen complex are shown along with Cl atoms (blue) in panel c. For Na−montmorillonite, only the Si (orange) and Na (purple) densities are shown.

in more detail below. It is noteworthy that a water-filled kaolinite nanopore can effectively separate Mg−pyrid2+ cations from Cl− anions without an imposed electric field (oppositely charged surfaces). The Mg peak farther from the surface indicates that the hydrophobic portion of the complex is closer to the surface, while the hydrophilic portion interacts with the water layer. For Mg−salen and Mg−pyrid2+, hydrophilic interactions with the water phase are facilitated by water O atoms that are directly bound to the Mg atoms. 3.4. Na−Montmorillonite Nanopores. In water-filled nanopores bounded by negatively charged montmorillonite surfaces, an electrical double layer is formed at each surface as a result of charge-balancing Na+ ions. This arrangement greatly reduces the adsorption tendencies of the two neutral salen complexes (panels a and b of Figure 9). While most of these neutral complexes were found to reside in the middle of the nanopore, some interact with the montmorillonite surface: 8% for Mg−salen−pyr and 24% for Mg−salen. In contrast, Mg− pyrid2+ is strongly adsorbed (86%), effectively replacing Na+ at the surface (Figure 9c). The first Na peak at each surface in Figure 9 corresponds to ∼25% Na+ adsorption in the neutral salen systems (panels a and b of Figure 9) but only 16% adsorption in the Mg−pyrid2+ system (Figure 9c). For all three complexes, the Mg peak positions relative to the first water

layer are very similar to the kaolinite siloxane surface (panels a and b of Figure 8). Although not evident from the density profiles, the adsorption of Mg−salen complexes on the montmorillonite surface creates hydrophobic regions that are free of water and Na+ ions. Magnesium density profiles near the siloxane surfaces in hydrated kaolinite and Na−montmorillonite nanopores (Figures 8 and 9) reveal similar adsorption mechanisms in the two nanopores. The single Mg peaks for the two neutral complexes and the outermost Mg peak for Mg−pyrid2+ correspond to a direct interaction of the organic ligand with the siloxane surface, with Mg directed toward the aqueous region. Examples of such surface complexes are seen in the rightmost surface complexes in Figures 7b and 10. In the Mg−pyrid2+ density profiles, the inner Mg peak is nearly coincident with the first water O peak, indicating direct interaction of both Mg and organic ligand with the siloxane surface (Figure 10).

4. DISCUSSION Trends from the nanopore simulations detailing the surface adsorption of the Mg−salen complexes in different solutions are shown in Figure 11. Simulations were performed at two different Mg−salen concentrations (three and six Mg−salen F


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results. Three trends are shown for (a) toluene-filled kaolinite, (b) water-filled kaolinite, and (c) water-filled Na−montmorillonite and discussed in order below. Solvent density profiles in kaolinite nanopores indicate that water and toluene interact quite differently at each surface (Figures 6 and 8), yet the overall adsorption of each Mg−salen complex is similar. However, the identify of the predominant surface for adsorption appears to be directed by the fluid present. Figure 11a shows that both neutral complexes primarily adsorb on the gibbsite surface when a hydrophobic solvent (toluene) is present, because this surface is the most hydrophilic region in the nanopore. In contrast, the Mg− pyrid2+ compounds almost exclusively bind to the siloxane surface under the same constraints. This result is consistent with recent experiments46,47 and FF modeling46−48 of bitumen fragments in kaolinite nanopores containing organic solvent, in which the hydrophilic bitumen compounds preferentially adsorb to the gibbsite surface via hydrogen bonding. In the water-filled kaolinite nanopore (Figure 11b), all of the salen complexes were found to adsorb solely on the hydrophobic siloxane surface. In contrast to the uncharged salen complexes, which form a single Mg layer at all surfaces, Mg−pyrid2+ forms two types of surface complexes with the planar Mg moiety parallel or perpendicular to the siloxane surface (Figure 10). Additionally, it was found that the Mg− pyrid2+ cations were completely separated from the Cl− anions. Recent experimental surface measurements on kaolinite samples indicate that the siloxane and gibbsite surfaces bear a slightly negative and positive charge, respectively.49 This suggests that surface polarity plays a role in the adsorption of these species on neutral kaolinite surfaces. The water-filled montmorillonite nanopore contains chargebalancing Na+ ions that form electrical double layers at each siloxane surface. The more polar surfaces are therefore less attractive to the neutral salen complexes but are strongly attractive to the Mg−pyrid2+ ions. The charge-balancing Cl− ions in this system demonstrate very little interaction with the Mg−pyrid2+ ions (Figures 6c, 8c, and 9c). Magnesium atoms in Mg−salen−pyr are fully coordinated by organic ligands, resulting in a more hydrophobic complex compared to the complexes with open Mg coordination sites. This complex also shows the lowest adsorption affinity to any of the surfaces and is the most likely to form organic-rich phases in aqueous fluids (Figures 8a and 9a). The more hydrophilic complexes (Mg−salen and Mg−pyrid2+) can bind water molecules or surface hydroxyl groups at the open coordination sites on the Mg metal centers. These two complexes show a tendency to form dimers in water phases, characterized by Mg···Mg distances of 3−4 Å seen in averaged radial distribution functions (Figures S2−S4 of the Supporting Information). However, these close Mg−Mg interactions are rarely seen, as evidenced by the low Mg−Mg coordination numbers (average number of close Mg−Mg interactions). A detailed analysis of water binding by Mg−salen and Mg− pyrid2+ (Figures S5 and S6 of the Supporting Information) indicates that each Mg atom binds at most one water molecule as a result of the formation of dimers (Figure 7b) blocking open coordination sites. The percent adsorption results (Figure 11) allow for a preliminary assessment of the viability of the Mg−salen complexes considered here as fluid tracers in subsurface energy applications. The neutral salen complexes show a minor tendency to adsorb on clay mineral surfaces under a variety

Figure 10. Equilibrium snapshot from a MD simulation of Mg− pyrid2+ complexes adsorbed to the montmorillonite surface. Water molecules have been removed for clarity, except those near Mg− pyrid2+ or Na+ ions. Hydrogen bonds between water molecules are shown as black dashed lines. Atoms are colored as in Figure 4.

Figure 11. Summary of adsorption of Mg−salen complexes on kaolinite and montmorillonite surfaces from MD simulations of Mg− salen complexes in (a) toluene-filled kaolinite nanopores, (b) waterfilled kaolinite nanopores, and (c) water-filled Na−montmorillonite nanopores. Results are shown for simulations of Mg−salen complexes at low and high concentrations (three and six molecules, respectively). Adsorption on kaolinite surfaces is differentiated by surface type (gibbsite and siloxane).

complexes) with a negligible effect on adsorption. The effects of ligand chemistry, nanopore fluid, surface hydrophobicity, and surface charge can all be seen from the percent adsorption G


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of conditions. It is likely that a portion of these complexes would remain mobile near oil-wet surfaces over the longer time scales (months) intended for such tracers. The Mg−pyrid2+ ion strongly adsorbs to siloxane surfaces under all conditions simulated, whether the attractive force involves electrostatic or hydrophobic interactions. Accompanying batch adsorption or column flow experiments would be needed to provide experimental confirmation of the predicted trends. The surface interactions could be short-lived, which would still allow these organic cations to migrate through aqueous pores in the subsurface.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b00435. Comparison of optimized geometries for M−L complexes (Table S1), atomic density profiles from MD simulations of toluene−water mixtures (Figure S1), Mg− Mg and Mg−Owater radial distribution functions and coordination numbers (Figures S2−S6), and full set of FF parameters for organic species in LAMMPS format (PDF)

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5. CONCLUSION MD simulations were performed on a series of Mg−salen complexes (Mg−salen−pyr, Mg−salen, and Mg−pyrid2+) in toluene−water liquid mixtures and clay nanopores containing toluene-wet or water-wet surfaces. The partitioning of these complexes in liquid and interfacial regions reveals important trends relevant to the feasibility of such compounds as fluid tracers in subsurface environments. Results from the toluene− water liquid simulations clearly demonstrate that the solubility of these complexes can be tuned by simply altering the functionality of the organic ligand. While all three complexes preferred the toluene−water interface, the relative position of each complex at the interface was controlled by the nature of the coordination/charge complex: Mg−salen−pyr demonstrated a preference for the toluene phase; Mg−salen favors the water phase; and Mg−pyrid2+ is even more hydrophilic. The hydrophobic (hydrophilic) nature of the Mg−salen complexes largely determined their propensity for adsorption on clay mineral surfaces. In toluene-wet kaolinite nanopores, surface adsorption decreased with increasing hydrophobicity of the Mg complex. The more hydrophilic Mg−salen adsorbed at the more hydrophilic gibbsite surface, but surprisingly Mg− pyrid2+ ions adsorbed strongly at the neutral siloxane surface. In water-wet kaolinite nanopores, adsorption of all three complexes occurred exclusively at the more hydrophobic siloxane surface. The presence of open Mg sites in Mg−salen resulted in enhanced adsorption on the siloxane surface through simultaneous hydrophobic interactions with the siloxane surface and hydrophilic interactions through water molecules coordinated directly to Mg atoms. Adsorption of the neutral salen complexes was greatly reduced in the water-wet Na−montmorillonite nanopores; the formation of an electrical double layer as a result of adsorbed Na+ ions at the siloxane surface effectively repels hydrophobic species. Adsorption of Mg−pyrid2+ was also reduced at the montmorillonite surface compared to kaolinite, although these organic cations were able to replace some Na+ ions at the interface. These simulations were performed using idealized liquid mixtures and mineral nanopores, but the identification of strong attractive forces between Mg−salen complexes and either liquid or mineral phases provides useful insight into the design and implementation of such complexes as chemical tracers for underground fluid monitoring. Molecular-scale interactions controlling the behavior of such tracers in subsurface environments can be predicted on the basis of these preliminary results.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jeffery A. Greathouse: 0000-0002-4247-3362 Timothy J. Boyle: 0000-0002-1251-5592 Richard A. Kemp: 0000-0002-2063-3812 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank K. Caras for writing Python scripts to automate the preparation of MD input files. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under Contract DE-NA0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

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