Article pubs.acs.org/JPCC
Controlling Clay Swelling−Shrinkage with Inorganic Nanoparticles: A Molecular Dynamics Study Lucas S. de Lara,†,‡ Vagner A. Rigo,†,§ and Caetano R. Miranda*,†,∥ †
Centro de Ciências Naturais e Humanas (CCNH), Universidade Federal do ABC (UFABC), Santo André, SP, Brazil Departamento de Física, Universidade Estadual de Ponta Grossa (UEPG), Ponta Grossa, PR, Brazil § Universidade Tecnológica Federal do Paraná (UTFPR), Cornélio Procópio, PR, Brazil ∥ Instituto de Física, Universidade de São Paulo (IF-USP), São Paulo, SP, Brazil ‡
S Supporting Information *
ABSTRACT: The control of clay swelling can be achieved by using nanoparticles. This paper describes the use of atomistic molecular dynamics simulations to study the interaction of SiO2 nanoparticles (NPs) with Na-montmorillonite (MMT) clay platelets. The NPs@MMT interfaces were simulated by taking into account different aqueous solutions (NaCl and CaCl2) and three different coverages for NPs (hydroxylated, PEGlyated, and sulfonated). The formation of electric double layers (EDL) was observed on the NP and MMT surfaces. The free energies as a function of the NP−platelet distance were determined for each interface, while global minima near MMT surfaces and local minima in the middle path were observed. The presence of NPs in the local (dispersed) or global energy minima (adsorbed) leads to a broadening and compressing of the EDLs, respectively. Accordingly, a mechanism for this swelling−shrinkage can be proposed, based on changes in the EDLs caused by NPs. Because of overlap between the EDLs, for the adsorbed NP the ion accumulation on the MMT surface increases, resulting in an attractive potential and compression of the clay. This MMT swelling−shrinkage transition leads to interplatelet distance changes of ∼20%, which is consistent with the results of previous studies. These indicate an effective way to use NPs to tune clay swelling inhibition.
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INTRODUCTION Clay minerals are found in many natural systems and provide valuable compounds for a wide range of applications in sectors such as cosmetics, pharmaceuticals, manufactured goods, and oil and gas,1−5 in addition to use in drug delivery.6,7 Many of these applications come from the unique properties of clays, such as their capacity to absorb and exchange ions, which is important for water purification and catalytic processes.5 Clays can swell depending on their structure, composition, and the aqueous environment.8 In particular, swelling is a key factor for the oil and gas industry, as clays are found in elements such as sedimentary crust and drilling fluids.9 In the drilling process, the unwanted swelling of sedimentary clays can cause severe damage in wells.4,9 This is a critical issue, as drilling fluids (muds) comprise brine dispersions of clays. On the other hand, the swelling of sediments can be helpful for well blockage by preventing blowouts.9 While drilling, good control of clay swelling is fundamental to keeping the mud within the desired density and avoiding either sedimentary fracture or a blowout. Na-montmorillonite (MMT) is a common type of clay9 and features a layered structure characterized by a basal spacing between the platelets. It is well-known that the phenomenon of MMT swelling is due to the migration of water and ions to the region between the platelets.9−12 This process can substantially © XXXX American Chemical Society
increase the basal spacing and is determined by variations in the interaction energy between the platelets, which itself is primarily affected by changes in the interplatelets aqueous solution.13−15 Under controlled conditions, swelling can occur through two distinct processes: crystalline and osmotic. Crystalline swelling is characterized by a much lower MMT swelling (compared to osmotic) and is favored under high cation concentrations and when divalent cations are present in the aqueous solution between the platelets.4 Osmotic swelling is favorable at lower salt concentrations and in the presence of monovalent cations, principally Na+. At the molecular level, crystalline swelling occurs through the formation of hydrogen bonds between clay and hydrated ions. Osmotic swelling occurs primarily due to electrostatic interactions between formed EDLs in the interplatelet region. Interestingly, a transition between these two states takes place as a function of salt concentration,4,12 which in fact defines a critical salt concentration (CSC). From a theoretical standpoint, swelling can be described as a competition between the attractive van der Waals forces and the electrostatic repulsion Received: May 26, 2017 Revised: August 30, 2017 Published: August 30, 2017 A
DOI: 10.1021/acs.jpcc.7b05130 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
poly(ethylene glycol) Si(OH)3-(CH2-CH2-O)2-H are used to graft the nanoparticles. These groups correspond to hydrophobic and hydrophilic coverages, and the resulting grafted NPs are named as NP-SA and NP-PEG, respectively.22 Following a computational procedure described by Rigo et al.,23 the graft densities with the lowest energies are considered for each functionalized NP. The concentration of NPs within water corresponds to 8.0, 9.0, and 11.0 wt % for NP-H, NP-SA, and NP-PEG, respectively. The CHARMM-based interatomic potential developed by Cruz-Chu et al.24 is used to describe the interactions in the SiO 2 nanoparticles, including bond stretching and angle bending parameters.25,26 The molecular dynamics simulations are first equilibrated in NVE over 1.0 ps, followed by 10.0 ps in NVT, and a NPT run along 4.0 ns at ambient conditions (1 atm and 300 K) and mass rate of MH2O/Mclay = 3.2. The production runs at NVT are simulated over 30 ns, at the equilibrated distance along [001] direction. Temperature and pressure are controlled by the Nosé−Hoover thermostat27,28 and the Andersen barostat,29 respectively. The reciprocal space particle-particle particle-mesh (PPPM) method is used for long-range electrostatic interactions30 and fully periodic boundary conditions, and the Lorentz−Berthelot mixing rules31 are applied. All molecular dynamics simulations are conducted by using the Large Atomic/Molecular Massively Parallel Simulator (LAMMPS) package.30 To fully address the properties according to the position of the NPs, different conditions are evaluated: (i) NPs are initially placed at the middle point between platelets, (ii) NPs are placed near the platelet surface (adsorbed), and (iii) starting from (i), the NPs are moved perpendicularly toward the platelet surface. For condition iii, the NPs are displaced with a constant velocity of 2.0 Å/ns between platelets, allowing for possible rotations. This is a converged applied velocity value that allows an NP full relaxation within the brine along the dynamics. Simulations without NPs (pristine MMT-brine) are carried out for each salt concentration as a reference basis. To quantify both swelling and shrinkage with the inclusion of NPs, we propose a relative swelling−shrinkage index (SSI) which can be defined as SSI = D/D0, where D and D0 are the distances between platelets, with and without the NP, respectively. The forces acting on each atom are monitored during the simulations, and the resulting force on the nanoparticles (FNP) is averaged with steps of 50 ps through 30.0 ns simulations. The free energy (FE) profiles along the platelets could be obtained as a function of the distance by using the FNP values. This allows us to calculate the potential of mean force (PMF) of each NP@MMT-brine system:
between overcharged platelets, according to the DLVO theory.11,12,16 As a refinement of the CSC description, Mohan et al.12 introduced a Born-type repulsion between the platelets to describe the non-DLVO interactions. On the basis of these results, it is clear that the formation of the EDLs and their intensity within platelets is a key factor for controlling the driving mechanisms of swelling−shrinkage. More recently, a number of studies have been conducted in order to develop methods for the control of clay swelling.4,6,15,17 One result of these efforts has been the improved use of nanoparticles.4 In their recent paper, Pham and Nguyen4 report the use of poly(ethylene glycol) (PEG)coated silica nanoparticles (NPs) to avoid MMT swelling, using NaCl and KCl aqueous solutions. These experimental results cover electrolyte concentrations between 0 and 6 wt % (representative of brine present in hydrocarbon reservoirs). The authors showed that even at a low nanoparticle concentration, the MMT swelling is reduced by a factor of approximately 2 compared to results without NP dispersion within NaCl and KCl solutions at 1 wt %. This is an important finding for enhanced oil recovery (EOR) applications and drug delivery applications, where swelling is unwanted.6 Together, these results clearly show that a precise molecular mechanism to control swelling is highly desirable; however, a fundamental understanding of the process behind nanoparticle-controlled swelling is still lacking. This study employs fully atomistic molecular dynamics simulations in order to examine the interaction of silica nanoparticles within MMT containing aqueous solutions with different concentrations of NaCl and CaCl2. Additionally, the hydrophilicity of the nanoparticles was addressed by taking into account both hydrophobic and hydrophilic functional groups. The formed EDLs, atomic, charge density, and free energy profiles between platelets were determined for each system. Our findings suggest that NPs represent a potential means for controlling the swelling−shrinkage behavior of clays and shed light on the molecular mechanisms that underlie the clay swelling inhibition processes involving NPs.
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METHODS On the basis of the literature, 18 we adopt the Namontmorillonite (MMT) unit cell, Na3(Si31Al)(Al14Mg2)O80(OH)16·16H2O, with sizes of 10.360 and 17.960 Å along the [100] and [010] directions, respectively. The basal spacing (d) is along [001]. The computational cell is modeled by ten and six unit cells along the [100] and [010] directions, respectively, where platelets present Mg and Al substitutions and the Na and H2O molecules from MMT are dispersed between platelets. The description of MMT interactions is carried out by means of the CLAYFF force field.18 Four different brine solutions are used between the platelets. These comprise a total of 32 000 water molecules and randomly inserted salt ions for NaCl/CaCl2 concentrations of 8.0/2.0, 7.2/1.8, 5.6/1.4, and 4.0/1.0 wt %. Water is described by the SPCE/FH potential,19 and van der Waals and Coulomb interactions are included for ions. The potential parameters for monovalent and divalent ions are obtained from the literature.19,20 The atomistic models of hydroxylated silica nanoparticles (NP-H) are generated using a Monte Carlo scheme proposed by Miranda et al.21 The resulting NP-H shows a spherical shape with a diameter of 3 nm. In addition to NP-H, functional groups composed of sulfonic acid Si(OH)3-(CH2)3-SO3H and
FE = PMF =
∫z
z
⟨FNP⟩ dz + constant 0
(1)
The constant is chosen in order to ensure that the potential goes to zero at infinity. To analyze the ion distribution between platelets, the atomic density, ρ(z), and the charge density profile, μ(z), are obtained along the direction perpendicular to the MMT−brine interface (z-axis). As in previous publications,32−34 μ(z) is determined by μ(z) =
F |σs|
∫0
z
[c+(z) − c −(z)] dz
(2)
where F is the Faraday constant and σs = 0.5 e/nm2 is the local surface charge density. The charge distribution along the z B
DOI: 10.1021/acs.jpcc.7b05130 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C direction is given by c+,−(z) = Nq+,−(z)AΔz, wherein A and Δz are the area and thickness of the slabs, respectively, N is the number of slabs, and q+,−(z) is the number of the positive/ negative charges within the slab. For charge distribution around NPs (radial charge distribution), μ(r), charge integrals are evaluated along spherical shells in a radial direction (dr) from the NP center of mass.
(0.5 nm). Conversely, when NPs are adsorbed on MMT, a contraction of nearly 8% (0.5 nm) occurs compared to systems without NPs. These results correspond to a relative swelling− shrinkage index (SSI) of 1.08 and 0.91, respectively. These results are consistent with experimental findings,4 where the authors reported a reduction in MMT swelling after the inclusion of 3 wt % of NPs in solution. Figure 2 shows that salt concentration also plays a secondary role in the variation of D values compared to that generated by the NP dispersion. Although the presence of NPs results in a significant change of D, the distinct functional groups lead to marginal D variations. The analysis of the swelling−shrinkage behavior of NP@ MMT-brine systems starts with the atomic density profiles, ρ(z). Figures 3a−c present these diagrams for pristine MMT-
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RESULTS Density Profiles. Figure 1a gives a snapshot of the NP-H@ MMT-brine system for the API brine solution (8.0/2.0 wt % of
Figure 1. (a) Snapshot of the atomistic configuration for NP-H@ MMT-brine system, for an 8.0/2.0 wt % aqueous solution of NaCl/ CaCl2, where water molecules were suppressed for better visualization. The NP-H appear adsorbed on MMT, and this system is representative of the typical atomistic configuration of the systems studied. Snapshots of atomistic details of NP-H (b), NP-AS (c), and NP-PEG (d) are also presented.
Figure 3. Density profiles for (a) pristine MMT-brine and NP-H@ MMT-brine systems with (b) centered and (c) MMT-adsorbed nanoparticle. The profiles (d), (e), and (f) shows a zoom of (a), (b), and (c) density profiles, respectively. The aqueous solution shows a concentration of 8.0/2.0 wt % of NaCl/CaCl2. In (b) and (c), the NPH is located close to the MMT layer with large z perpendicular to the surface. For better visualization, the MMT density profile (dotted brown line) has been reduced by 100 times in (a) and (d).
NaCl/CaCl2). The basal spacing (d) and the distance between platelets (D) are indicated, and the NP-H is shown adsorbed onto the MMT surface. This configuration is representative of the systems studied here, where a total of four different salt concentrations (4.0/1.0, 5.6/1.4, 7.2/1.8, and 8.0/2.0 wt % of NaCl/CaCl2) and three NP functionalizations are used. Details of NP-H, NP-SA, and NP-PEG appear in Figures 1b−d. Figure 2 shows the converged distance between platelets (D) for each system studied. Compared with the pristine systems, those with dispersed NPs increase the D by approximately 8%
brine and NP-H@MMT-brine, for a solution with 8.0/2.0 wt % of NaCl/CaCl2. As a general observation, the formation of charged layers of ions close to MMT occurs for each system studied, where the asymmetries in ion distributions are due to the ions substitution in the MMT platelets. Compared to the pristine MMT-brine (Figure 3a), the sequence of charged layers is maintained for systems with the presence of NP-H (Figures 3b,c). However, when the NP-H is adsorbed on MMT (Figure 3c), there is an increase of ρ(z) peaks related to Cl anions compared to MMT-dispersed NP-H (Figure 3b). This effect is more pronounced on the NP-adsorbed surface side (large z, in Figure 3c). Interestingly, the accumulation of Ca ions on MMT is slightly reduced when the NP-H is adsorbed. Figures 3d−f show a zoom of ρ(z) for pristine and for NP-H at dispersed and adsorbed on MMT, respectively. The detailed view of pristine ρ(z) (Figure 3d) clearly has a first minimal layer of Na ions close to the MMT surface. The same behavior is seen for dispersed (Figure 3e) and MMT-adsorbed NP-H
Figure 2. Distance between MMT layers for each system evaluated. The wt % on the x-axis represents the NaCl/CaCl2 concentration. C
DOI: 10.1021/acs.jpcc.7b05130 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C (Figure 3f). This small positive layer is overcharged by Cl anions for all systems. These Na layers appear primarily due to the negatively charged MMT platelets (due to Mg substitutions of Al ions). However, the preferential location of the NP within the MMT-brine modifies the ion density surrounding the MMT layers. As a result, the system with dispersed NP-H (Figure 3e) shows broader ρ(z) peaks compared with the adsorbed NP-H system (Figure 3f). The ρ(z) diagrams in Figures 3 are representative of results obtained for other salt concentrations and nanoparticles. Other coverages (NP-SA and NP-PEG) show ρ(z) results with similar trends, which appear in Figure S1 of the Supporting Information. Figure 4 shows the charge density profile, μ(z), for pristine and NP-H within MMT-brine, for an aqueous solution with
Figure 5. Radial charge density profiles surrounding the NP-H. The radial distance starts from the NP center of mass, while the curves represent the interplatelet NaCl/CaCl2 salt concentration in wt %.
charged layer, where its precise location varies depending on salt concentration; i.e., a higher concentration provides more compact charged layers. From the μ(r) and μ(z) results (Figures 4 and 5), it is expected that the EDLs surrounding the MMT and NPs interact with each other and also with the MMT itself, suggesting that the positive layer of ions accumulated on MMT and the negative layer of ions on NPH mean that adsorbed NPs would be energy favorable. Furthermore, the broader EDLs for lower salt concentrations on NP-H (Figure 5) show that this system has a stronger interaction with the MMT charged layers. In order to quantify the relative stability of each system, the free energy profiles were computed for each system, as discussed in the following paragraphs. Free Energy Profiles. To address the energy of nanoparticles within MMT-brine layers, the free energy profile of evaluated systems is obtained through the PMF calculations (eq 1). Figures 6a−c present the free energy profile along the z-axis for NP-H, NP-SA, and NP-PEG within MMT-brine, respectively. Interestingly, all systems show local minima at
Figure 4. Charge density profiles for pristine MMT-brine, NP-H centered, and adsorbed on MMT. To facilitate the comparison between different systems, all curves are shifted to start from the same point (1 nm from MMT). In all cases the brine is composed of 8.0/2.0 wt % of NaCl/CaCl2.
8.0/2.0 wt % of NaCl/CaCl2. Results show the most representative variations and the EDLs formation up to 1.6 nm from the MMT. The oscillations are broader further from the MMT surface with similar periodicity, as observed experimentally (∼0.25 nm).35 In keeping with ρ(z) results, the peaks and troughs in μ(z) for dispersed NP-H are broader than the adsorbed NP-H. The tail of the second negative trough varies by 1.55, 1.45, and 1.39 nm for dispersed NP-H, pristine, and adsorbed NP-H, respectively. As previously discussed in the literature,32 a broad layer of charged ions surrounding the surfaces increases the Coulomb repulsion, consistent with the DLVO theory.16 Accordingly, systems with dispersed NPs have shown a greater molecular swelling behavior compared to those of MMT-adsorbed NPs. This can be understood as a simple electrostatic interaction between the EDLs. The μ(z) results for NP-SA and NP-PEG within MMT-brine present the same trends and are included as Figure S2. Our findings show that the swelling−shrinkage behavior depends on the NP location within platelets, namely, (i) when NPs are adsorbed on MMT the interplatelet distance reduces, while (ii) dispersed NPs increase the interplatelet distance. These data also suggest that the position of NPs within platelets can induce the compactness of EDLs (shrinkage) compared with the spreading observed by dispersed NPs (swelling). Figure 5 shows the radial charge distribution, μ(r), surrounding the NP-H center of mass. Although the entire nanoparticle is charge neutral, some charge accumulation on NP-H took place, in line with previous results for overall charge neutral NPs within a salt solution.32 The overall results show that a first negatively charged shell appears around the NP-H, located at approximately 1.8 nm from the center of mass for all salt concentrations. This first layer is enclosed by a positively
Figure 6. Free energy profiles for (a) NP-H, (b) NP-SA, and (c) NPPEG within MMT in different concentrations of salt solution in wt %. In each diagram z = 4.0 nm is the central position between the MMT platelets. D
DOI: 10.1021/acs.jpcc.7b05130 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C the middle path between the platelets (z ∼ 4 nm in Figures 6a− c). Two other minima appear closer to each MMT layer. All cases present an energy barrier between the central minima and those close to MMT. These results are explained by the obtained EDLs around MMT (Figure 4) and NP (Figure 5): the higher energy barrier on NP-H occurs for the broader EDL, at 4.0/1.0 wt % for the NaCl/CaCl2 salt concentration. Other relevant information comes from the free energy diagrams; i.e., there is a reduction in the free energy consistent with the salt concentration and the nanoparticle coverage. The lower free energy values obtained for NP-SA (Figure 6b) and NP-PEG (Figure 6c), compared to NP-H (Figure 6a), are in agreement with the reduction in free energy associated with the increase in the molecular weight of the molecules.36 Moreover, the results show that these variations also affect the energy barriers. As the energy barriers are lower than the thermal energy under ambient conditions, it is expected that NPs can migrate to the global minima after longer dynamics. This situation was verified for all systems after simulations along 30 ns, confirming that the central minima is a metastable position at 300 K and 1 atm. Thermodynamically, the nanoparticles can jump to and from the central and local minima, and the free energy data from Table S1 can be used to obtain the related Boltzmann probabilities. The data return that the probability to nanoparticles jump from the local (dispersed) to the global (adsorbed) energy minima at equilibrium is approximately 38 times lower than the probability to jump from the global to the local minima, for all evaluated nanoparticle coverages and solutions. These show that nanoparticles stay mostly adsorbed on clay platelets, and clay swelling is unfavorable (shrinkage is expected for nanoparticles dispersed within platelets). This is in line with experiments of Pham and Nguyen,4 who observed a swelling reduction after including nanoparticles on clay solution. These results also shed light on the origin of the experimental observed great affinity between clay particles and nanoparticles in an aqueous suspension.37 Table S1 in the Supporting Information summarizes the free energy values for each NP and brine for the first and second minima and maxima and central minima in the free energy profiles of Figures 6a−c. The free energy profiles present some asymmetries concerning the center of the platelets. This effect comes from the interaction between the clay and nanoparticle EDLs, primarily the Cl− layer on platelets (Figure 3) and on nanoparticles (Figure 5). Table S2 presents the asymmetry between the free energy peaks and the charge within the first shell around nanoparticle, showing that a large Cl− layer surrounding the nanoparticle correspond to a bigger asymmetry of the free energy profiles.
global), separated by an energy barrier, where the lower salt concentration provided the higher energy barriers. The presence of NPs in each of these minima resulted in swelling−shrinkage behavior, which is explained in terms of the identified EDLs around the platelets. This is the first time that the swelling−shrinkage behavior of MMT has been fully explained in terms of the EDL effects driven by the insertion of NPs at different positions between platelets, namely, the NPs charged layers affect the ELDs close to the MMT surface, which weakens and narrows those EDLs and thereby produces the MMT shrinkage. On the other hand, the charges surrounding dispersed NPs results in broader ELDs on the MMT surface, leading to strong Coulomb repulsion and swelling of MMT. As a result, we found silica nanoparticles to be an effective means to control swelling−shrinkage transitions of clays.
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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.jpcc.7b05130. Components of the density and charge density profiles for NP-SA@MMT-brine and NP-PEG@MMT-brine systems, for an aqueous solution composed of 8.0/2.0 wt % of NaCl/CaCl2, as well as detailed data regarding the free energies for each system studied (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel +55 11 3091-7009; e-mail
[email protected] (C.R.M.). ORCID
Vagner A. Rigo: 0000-0002-4526-8938 Caetano R. Miranda: 0000-0002-8008-4907 Present Address
V.A.R.: King’s College London, Strand, London, United Kingdom. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial support of the Advanced Energy Consortium (AEC) and the Brazilian agencies CAPES, FAPESP, and CNPq. The calculations have been partially performed at CENAPADSP, UFABC, SDumont, and UTFPRCP supercomputer facilities.
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REFERENCES
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CONCLUSIONS This study adopted fully atomistic molecular dynamics simulations in order to examine the interaction between SiO2 nanoparticles (NPs) and MMT clay, where the interplatelet solutions were composed of aqueous solutions of NaCl and CaCl2. This interaction was the basis of the overall aim of the study, namely, to explain the molecular mechanisms underlying the inhibition of clay swelling by using NPs, as reported in previous studies.4 The occurrence of electric double layers (EDLs) in the aqueous salt solutions was monitored by means of the atomic and charge density profile between platelets, while the stability of NPs within MMT platelets was obtained through free energy profiles. Our results showed a two energy minima as a function of the NP−platelet distance (local and E
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DOI: 10.1021/acs.jpcc.7b05130 J. Phys. Chem. C XXXX, XXX, XXX−XXX
