Effect of Layer Charge on CO2 and H2O ... - ACS Publications

Oct 14, 2016 - under T = 323 K and P = 90 bar geologic sequestration conditions has been further investigated. This effect includes the charge amount ...
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Effect of Layer Charge on CO2 and H2O Intercalations in Swelling Clays Qi Rao and Yongsheng Leng* Department of Mechanical and Aerospace Engineering, The George Washington University, Washington, D.C. 20052, United States ABSTRACT: The effect of layer charge on the intercalation of supercritical carbon dioxide (scCO2)−H2O mixture in Na-montmorillonite clay interlayers under T = 323 K and P = 90 bar geologic sequestration conditions has been further investigated. This effect includes the charge amount and its location (within either octahedral or tetrahedral layers due to isomorphic substitutions). Two clay models with different layer charges are used in this study. Simulation results show that the increase of charge amount shifts the monolayer-to-bilayer (1W-to-2W) hydration transition toward the lower relative humidity (RH), increasing water sorption at the expense of reducing the overall sorption amount of CO2 in the clay interlayer. However, the combination of the influence of charge amount and charge location leads to insignificant changes in equilibrium basal spacings of the high- and low-charge clays. Molecular dynamics simulations show that the CO2 dimers, which are frequently seen in low-charge clay interlayers, vanish in high-charge clay interlayers even at low RH of 30%.

1. INTRODUCTION Intercalation of carbon dioxide (CO2) and water in swelling clay interlayers has been an important and active research in CO2 geologic sequestration.1−15 Recent experimental studies1−5 unambiguously showed that the expansion of smectite clay minerals, such as montmorillonite clays, depends on their hydration state. In the submonolayer hydration state (2W), exposure to wet CO2 with undersaturated water concentration results in the contraction of clay interlayers.1,2,8 These different experimental observations could be unified by the dependence of CO2 sorption on the relative humidity (RH). Loring and Schaef et al.7,8 recently investigated the effect of RH on the intercalation of CO2 in Na-montmorillonite interlayers. They found that as the RH increases, the sorbed water concentration increases almost continuously while the clay basal spacing increases in a stepwise fashion. They also found that the sorbed CO2 concentration increases initially but then decreases at high RH. Recent experimental studies also showed that different types of interlayer ions have remarkable influence on the sorption amount of CO2.1,5 Obviously, understanding the interaction between swelling clays and CO2 has important implications in CO2 geologic sequestration16−23 and CO2 enhanced oil and gas recovery.24−26 Given the fact that a variety of charge deficits exist in either tetrahedral or octahedral sheets in 2:1 smectite clays (such as montmorillonite with more octahedral charge deficit and beidellite with more tetrahedral charge deficit),27 there is a fundamental question concerning the effect of layer charge on the intercalation of CO2−H2O mixture in clay interlayers. For the clay expansion in water, several experimental27−34 and theoretical34−36 studies were carried out in the past two © XXXX American Chemical Society

decades. Of these investigations, a few studies need mentioning: Sato et al.27 examined several natural smectites and suggested that the interlayer expansion is attributed to the combined effects of the charge location and amount. The equilibrium basal spacings are usually larger when the layer charge is located in octahedral sites than in tetrahedral sites. However, this change in basal spacing is not significant. Michot et al.31 found that for Na-saponites with only tetrahedral charges available clay swelling can occur at lower values of RH as the layer charge increases. The increase of the layer charge in tetrahedral sites also leads to an increase in intercalated water content. More recent simulation work of the effect of layer charge on the diffusion and conductivity in montmorillonite interlayers has been carried out by Greathouse et al.37 We recently investigated the RH effect on the intercalation of CO2 and H2O in Na-montmorillonite clay interlayers under typical CO2 geologic sequestration conditions.15 We showed that with the increasing of RH, the clay basal spacing increases in a stepwise fashion. The hydration of CO2 molecules is changed from the partial hydration in 1W state to the full hydration in 2W state, and CO2 dimers are frequently seen in both 1W and 2W hydration states. The sorption amount of water compares reasonably well with the in-situ infrared (IR) spectroscopy experimental data measured by Loring et al.,7 but the sorption amount of CO2 is larger than the interlayer amount measured by IR spectroscopy.7 We anticipate that one of the important factors leading to this discrepancy is the layer charge difference between our clay model used in the simulation and the samples used in IR spectroscopy. In this Received: June 22, 2016 Revised: October 4, 2016

A

DOI: 10.1021/acs.langmuir.6b02326 Langmuir XXXX, XXX, XXX−XXX

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Table 1. Chemical Potentials of H2O and CO2 in CO2-Rich Phase under Different RH Conditions15 T, P, and RH 323 323 323 323 323 323 323 323

2. COMPUTATIONAL MODELING We follow the same protocol in our previous simulation study to compute the possible equilibrium basal spacing and structural properties of CO2−H2O mixture in clay interlayers. The detailed molecular models and simulation method are given in our previous publications.15 Essentially, we use the computational packages Towhee38 and LAMMPS39 to perform grand canonical Monte Carlo (GCMC) and constant-NVT molecular dynamics (MD) simulations. We consider two sodium-saturated montmorillonite clay models with different layer charges. The first clay model is an Arizona-type montmorillonite (SAz-1) with the chemical formula Na1.0[Si8.0](Al3.0Mg1.0)O20(OH)4. This clay model only has isomorphic substitutions in the octahedral site and was used in previous simulation studies.40−43 The second clay model is a Wyoming-type montmorillonite (SWy-1) with the chemical formula Na0.75[Si7.75Al0.25](Al3.5Mg0.5)O20(OH)4. This clay model has isomorphic substitutions in both octahedral and tetrahedral sites and is also used in previous simulation studies.13,15,44 In the following discussion, the two clay models corresponding to unit cell (uc) layer charges of −1.0 e/uc and −0.75 e/uc are denoted as the “high-charge” and “low-charge” clays, respectively. The Na-montmorillonite clay models contain eight clay unit cells in the lateral direction, forming a clay patch of 20.77 × 18.03 Å2. In the normal direction, we adopt a two-interlayer clay model. Periodic boundary conditions are applied in three directions. The montmorillonite and water are simulated by CLAYFF45 force field and SPC water model,46 respectively. CO2 is represented by the elementary physical model (EPM2) incorporating the flexible bond stretching and angle bending terms.14 Further, intramolecular bond stretch and angle bending terms are also considered in the SPC water model47 to ensure full flexibility of these molecules and hydroxide components on clay surface. A cutoff distance of 9.0 Å is used for the dispersive interactions, while the long-range electrostatic interactions are treated by the Ewald summation method.48 The temperature and pressure (T/P) of 323 K and 90 bar are considered in the simulation. This situation has been studied previously in experiments6,7 to investigate CO2−clay hydrate interactions under typical CO2 geologic sequestration condition. The GCMC simulations are performed at each clay interlayer spacing with different RH for at least 80 million moves, including insertion and deletion, as well as translation and rotation of molecules. The equilibrium configurations at different RH are determined through GCMC and are used as input for the subsequent NVT MD simulations in LAMMPS to study the structural and dynamic properties of interlayer species. The chemical potentials for water and CO2 at T/P = 323 K/ 90 bar with different RH values in CO2-rich phase are reported in our previous study.15 These chemical potentials are given in Table 1 and are used in the GCMC simulations to study the clay swelling in CO2−H2O mixtures. Using the chemical potentials for water and CO2 at 100% RH in Table 1, we performed GCMC simulation in CO2-rich phase and verified that the CO2 mole fraction (0.9977 ± 0.0005) is very close to the experimental value (0.9959 ± 0.0005).49

K, K, K, K, K, K, K, K,

90 90 90 90 90 90 90 90

bar, bar, bar, bar, bar, bar, bar, bar,

30% 40% 50% 60% 70% 80% 90% 100%

μH2O (kJ/mol)

μCO2a (kJ/mol)

−49.50 −48.72 −48.12 −47.60 −47.24 −46.80 −46.51 −46.21

− − − −33.10 ± 0.31 − − − −

± ± ± ± ± ± ± ±

0.02 0.04 0.05 0.06 0.11 0.11 0.05 0.05

a

The chemical potentials of CO2 at different RH conditions are the same.

We calculate the normal pressure (Pz) variation versus basal spacing distance according to the formulation48 N

Pz =

NkT 1 + ⟨∑ rizf ⟩ V 3V i = 1 iz

(1)

where N is the number of particles, k is the Boltzmann constant, T is the temperature, V is the volume, and riz and f iz are the z-direction components of the coordinate and force on particle i, respectively. In order to accurately determine the equilibrium basal spacing and provide a physical basis of this quantity, we further calculate the free energy change as a function of basal spacing distance. This is obtained by integrating the normal pressure Pz with respect to the basal spacing (sz′):40,50 ΔF = −A

∫s

sz

0 z

(Pz(sz′) − Pext) dsz′

(2)

s0z

Here, A is the lateral area of the clay surface, is the reference basal spacing (selected as the natural clay spacing at the dehydrated state), and Pext is the external pressure determined by the geologic sequestration condition (i.e., Pext = 90 bar).

3. RESULTS AND DISCUSSION 3.1. Effect of Layer Charge on the Equilibrium Basal Spacing and Sorption of CO2 and H 2 O in Clay Interlayers. 3.1.1. Stable Interlayer Spacing. Figure 1 shows the variations of the normal pressures (Pz) as a function of basal spacing for the high-charge and low-charge clay models at three RH values: RH = 30% (low), RH = 60% (intermediate), and RH = 100% (saturation). The very likely stable interlayer spacings at each RH value correspond to the intersections between the normal pressure curve with negative slopes and the horizontal external pressure line. From Figure 1, the stable basal spacings for both clay models are approximately around 11.5−12 Å and 15−16 Å, corresponding to the 1W and 2W hydration states, respectively. The high- and low-charge clay models yield roughly the same equilibrium basal spacings. Figure 1 shows that in the basal spacing 10.5−11.5 Å and 14− 15 Å the pressure in the high-charge clay interlayer is somewhat above that in the low-charge model. Since the normal pressure Pz represents the repulsive hydration pressure in clay interlayer, this difference in Pz indicates that high-charge clay has a stronger tendency to swell than the low-charge clay. We can also determine the stable basal spacings from the free energy curves as shown in Figure 2. Here, the equilibrium basal spacing should correspond to the free energy minima. We find that at low RH (RH = 30%) both high-charge and low-charge B

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the 2W hydration state should prefer to stay at 15.5−16 Å basal spacing at 100% RH in the CO2−H2O mixture. Figure 2 also shows that both high-charge and low-charge clays at the 1W hydration state have roughly the same basal spacings. In Table 2 we compare in detail the 1W and 2W Table 2. Equilibrium Basal Spacings and Sorbed Water and CO2 Concentrations for the High- and Low-Charge NaMontmorillonites at 323 K and 90 bar with Different RH Valuesa cH2O (mmol/g)

cCO2 (mmol/g)

low charge

high charge

low charge

high charge

low charge

11.9 12.0 12.0 12.0 15.7 15.7 16.0 15.9

4.99 5.21 5.32 13.08 13.63 13.86 14.48 13.94

3.65 4.00 4.09 4.35 11.18 11.30 12.78 12.86

0.17 0.13 0.09 0.09 0.08 0.05 0.05 0.03

0.70 0.70 0.61 0.52 0.70 0.70 0.35 0.26

equilibrium basal spacing (Å)

Figure 1. Variations of normal pressure (Pz) as a function of basal spacing for the high- and low-charge Na-montmorillonite clay at T/P = 323 K/90 bar and at three RH values. The horizontal line of 90 bar pressure is very close to the zero pressure line.

RH (%)

high charge

30 40 50 60 70 80 90 100

11.8 11.9 11.9 15.5 15.7 15.7 16.0 15.7

(12.5) (12.6) (12.7) (13.8) (15.0) (15.5) (15.7) (18.4)

(10.5) (11.2) (11.8) (12.5) (15.2) (15.7) (15.8) (23.3)

a

Data in parentheses are experimental results for SAz-1 and SWy-1 samples in water under ambient condition.27

equilibrium basal spacings for the two clay models in CO2− H2O mixtures. Experimental results of SAz-1 and SWy-1 samples in pure water under ambient condition are shown in parentheses, which show that the 1W equilibrium basal spacing of the high-charge SAz-1 is larger than that of the low-charge SWy-1.27 We attribute this difference to the different layer charge amounts between our simulation models and experiments, to the different T/P conditions (323 K/90 bar versus ambient condition) and, more importantly, to the role of intercalated CO2 molecules in clay hydration in our present study. The layer charge amount and location in clay deserve further discussion. For the high-charge clay, the electrostatic attraction between interlayer cations and clay surfaces is enhanced due to the increase of layer charge, resulting in the decrease of basal spacing. However, since the isomorphic substitutions only take place in octahedral sites, the strong attraction between interlayer cations and clay surfaces is compensated by the increased distance between the opposite charges. These two opposite effects result in no significant changes in equilibrium basal spacing. Overall, at a constant RH the free energy of the high-charge clay is always lower than that of the low-charge clay (Figure 2), indicating that the stability of CO2−clay hydrates increases as the layer charge increases. A similar trend of clay swelling in water is also found in previous experimental study by Ferrage et al.32 The reason is that high-charge clays contain more interlayer cations, resulting in more water molecules bound to hydrated metal ions. This increase of bound water molecules decreases the interlayer hydration energy and thus increases the stability of clay hydrate. In Figure 3a, we plot the equilibrium basal spacings versus different RH values for both high- and low-charge clay models. In-situ X-ray diffraction (XRD) experimental results measured by Loring et al.7 for Na-SWy-2 clay sample (a type of lowcharge clay similar to Na-SWy-1) are also shown in the figure for comparison. While the basal spacing curve of the low-charge clay model has a small shift relative to the experimental curve during the 2W-to-1W transition, the one corresponding to the

Figure 2. Variations of free energy (ΔF) as a function of basal spacing for the high- and low-charge Na-montmorillonite clay at T/P = 323 K/ 90 bar and at three RH values. The dashed lines show possible 1W minima of basal spacings.

clay interlayers prefer to stay in the 1W hydration spacing at 11.8−11.9 Å. At intermediate RH (RH = 60%) the free energy minimum of the high-charge clay shifts to the 2W hydration state with a new equilibrium basal spacing of 15.5 Å, while the low-charge clay still prefers to stay in the 1W hydration state at 12 Å. This early 1W-to-2W swelling of the high-charge clay is consistent with experimental results observed by Sato et al.27 for Na-saturated smectites and Michot et al.31 for Na-saponites (with only tetrahedral charges available). These experiments showed that clay swelling can occur at lower values of RH as the layer charge increases. In particular, Sato et al.27 found that the SAz-1 sample whose layer charge is −1.14 e/uc swells at lower RH (20−30%) than SWy-1 whose layer charge is −0.68 e/uc.27 Finally, at RH = 100% the free energy minima for both clay models are all below the 1W minima. Although there are no well-defined minima corresponding to the 2W state, the pressure curves in Figure 1 provide an estimation which is around 15.5−16 Å. Thus, we assume that the clay swelling at C

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Figure 3. Variations of (a) equilibrium basal spacing, (b) sorbed H2O concentration, and (c) sorbed CO2 concentration as a function of the relative humidity for the high- and low-charge Na-montmorillonite clay at T/P = 323 K/90 bar. The XRD and IR experimental data obtained by Loring et al.7 are also shown in the figure for comparison.

Figure 4. Sorbed (a) H2O and (b) CO2 concentrations in the high- and low-charge clay interlayers as a function of basal spacing at T/P = 323 K/90 bar and at three RH values.

experimental results by Loring et al.7 are also shown in the figures for comparison. Table 2 clearly shows that the highcharge clay contains more water than the low-charge clay in both 1W and 2W states. Water concentration in the highcharge clay also has a large shift toward low RH values (Figure 3b). Compared with the IR experimental results,7 water concentrations in both high- and low-charge clays exhibit stepwise increase with RH, while the experimental curve shows

high-charge clay model has an even larger shift toward the lower RH during this transition. 3.1.2. Sorption of Water and CO2. To find the effect of layer charge on intercalated water and CO2 contents in Namontmorillonites, we further calculate sorbed water and CO2 concentrations (defined as the mole number of water or CO2 per unit weight of clay) for the two clay models, which are shown in Table 2 and Figure 3b,c. In-situ IR spectroscopy D

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Langmuir an almost continuous increase with RH and “smears out” the low-charge water concentration curve. This difference between simulation and experimental results was discussed in our previous publication,15 which can be explained by the spacefilling and hydration heterogeneity models.7 In contrast to the increase of water content in the highcharge clay, Table 2 shows that CO2 concentration in the highcharge clay is dramatically decreased, which is also clearly shown in Figure 3c, in which the high-charge CO 2 concentration curve is even below the IR experimental curve.7 Since the CO2 concentration curve of the low-charge clay is well above the experimental data (Figure 3c), this scenario suggest that perhaps the clay samples used in the insitu IR spectroscopy experiment7 is a combination of lowcharge and high-charge clays with different stoichiometry. As we have discussed previously, the fact that high-charge clays containing more water and less CO2 as the RH increases is associated with the increased number of interlayer cations and the strong hydration energy of sodium ions with polar water molecules. This enhanced hydration process reduces the chance of intercalations of nonpolar CO2 molecules. During the calculations of normal pressure and free energy change versus clay basal spacing (Figures 1 and 2), we also calculate the related water and CO2 concentrations for the high-charge and low-charge clays at three RH values (RH = 30, 60, and 100%). Note that on each concentration curve only one data point corresponds to the equilibrium concentration of sorbed H2O or CO2, as shown in Table 2. Our aim of calculating H2O and CO2 concentrations over a large range of basal spacing is to elaborate the overall trend of the dependence of these sorbed concentrations on RH. For this reason, in Figure 4a,b we show the variations of the sorbed H2O and CO2 concentrations in the clay interlayer as a function of basal spacing at three RH values. Water molecules begin to enter the clay interlayer at basal spacing of 10.5 Å, while CO2 molecules cannot enter the interlayer until the basal spacing reaches 11.5 Å. As expected, water concentrations in the high-charge clay are always greater than those in the low-charge clay. Moreover, the dependence of water concentration on RH is more pronounced in the low-charge clay than in the high-charge clay. This is also true for the variation of CO2 concentration. For example, in the low-charge clay the CO2 concentration can increase dramatically as the RH is decreased from 100% to 30%, while in the high-charge clay the CO2 concentration is barely changed at RH = 100 and 60%. At RH = 30% it has only a limited increase versus basal spacing. 3.2. Layer Charge Effect on the Structural and Diffusion Properties of Interlayer Species. Molecular dynamics (MD) simulations are further carried out to study the layer charge effect on the structure and dynamics of interlayer species. Since the CO2 concentration in the highcharge clay can reach an appreciable amount only at low RHs, therefore we only focus on the CO2−H2O mixture at RH = 30% for the high- and low-charge clays. Note that in this case both clays are in the 1W hydration state. For the statistic purpose, the GCMC equilibrium configurations of CO2−H2O mixtures in both clay models are duplicated in the lateral (x−y) directions to guarantee a sufficient number of CO2 molecules in the system. MD simulations usually take 16 ns with a sampling interval of 0.1 ps in the production runs to obtain the related structural and dynamic properties. Figure 5 shows the density profiles of CO2, water and Na+ in the interlayer region at RH = 30%. One-layer

Figure 5. Normal density distributions of CO2 (carbon atom in CO2), H2O (water oxygen Ow), and sodium ions in the high- and low-charge clay interlayers at T/P = 323 K/90 bar and at three RH values. The origin and vertical dashed lines (black for the high-charge and red for the low-charge) correspond to the clay surface oxygen planes.

central peaks of H2O (1W) in both low- and high-charge clays are dominant. CO2 concentrations in the two clay models exhibit single-peak distributions, with the one in the low-charge clay showing much higher peak than that in the high-charge clay. The position of the CO2 peak almost coincides with the water density peak between two clay surfaces, indicating that CO2 molecules are coordinated by water molecules in the midplane. The Na+ density distribution exhibits more symmetric distribution in the high-charge clay than in the low-charge clay. The two symmetric peaks in the high-charge clay interlayer are located between the clay surfaces and the water layer. This is largely due to the more uniform electrostatic interactions between interlayer cations and octahedral negative charges in the high-charge clay. The less symmetric Na+ distribution in the low-charge clay interlayer is attributed to the stronger electrostatic interactions between interlayer cations and tetrahedral negative charges in the clay surface. Figure 6 shows the radial distribution functions (RDFs) of CO2−water oxygen (Ow) and Na+−water oxygen (Ow) in both high- and low-charge clays at RH = 30%. The coordination numbers of water oxygen atoms around CO2 molecules or Na+ ions are calculated by integrating the RDFs to the first minima. For CO2, the coordination numbers are 5.6 and 3.7 in the highand low-charge clay interlayers, respectively. Obviously, the high-charge clay containing more water molecules in the 1W layer provides a higher coordination number for CO2. However, these coordination numbers are still much less than that of CO2 in the bulk water (18.7)15 (based on the bulk RDF curve of CO2 shown in Figure 6). The surface oxygen atoms in clays provide additional coordination environment for CO2 molecules (not shown in the figure). The coordination numbers of water molecules around interlayer Na+ ions in two clay models are essentially the same (3.6). This is not surprising because the strong hydration energy between Na+ and water molecules enable interlayer cations readily hydrated, independent of layer charge. Note that in the 1W hydration state Na+ ions are coordinated by water molecules and clay surface oxygen atoms, yielding a total hydration number around six.15 Figure 7 shows typical snapshots of the CO2−H2O mixtures in both high- and low-charge clays at 30% RH. It is clearly seen E

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Figure 6. Radial distribution functions (RDFs) of (a) CO2−Ow and (b) Na+−Ow interaction pairs in the high- and low-charge clay interlayers at T/ P/RH = 323 K/90 bar/30%.

Figure 7. Snapshots of CO2−H2O mixtures at T/P/RH = 323 K/90 bar/30% in (a) the high-charge clay interlayer (equilibrium basal spacing of 11.8 Å) and (b) the low-charge clay interlayer (equilibrium basal spacing of 11.9 Å). The left and right panels show the side and top views of the molecular configurations. The arrow connects the top view with the specific interlayer in the side view. Color scheme: oxygen = red, hydrogen = white, carbon = brown, silicon = blue, aluminum = cyan, magnesium = orange, and sodium = yellow.

T-shaped geometry, with the former taking a high probability of occurrence. To further provide molecular insights into the hydration structure of CO2−H2O−ion complexes in both high- and lowcharge clay interlayers, in Figure 8, we plot the two-dimensional (2D) density distributions of CO2, Na+, and H2O molecules

that CO2 molecules distribute sparsely in the high-charge clay interlayer (Figure 7a). In the low-charge clay interlayer more CO2 molecules exist in the form of aggregates of CO2 dimers (Figure 7b). These dimers, as we found in our previous study,15 can be either in the slipped parallel geometry or in the distorted F

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Figure 8. In-plane 2D density distributions of CO2 (left), sodium ions (middle), and water molecules (right) under T/P/RH = 323 K/90 bar/30% in (a) the high-charge clay interlayer and (b) the low-charge clay interlayer. Isomorphic substitutions on octahedral sites (Al by Mg) are shown by white dots, while isomorphic substitutions on tetrahedral sites (Si by Al) are shown by black dots.

projected onto the lateral x−y plane at RH = 30%. In the highcharge clay interlayer (Figure 8a), CO2 molecules distribute sparsely in some localized regions, while Na+ ions and water molecules distribute more evenly than CO2 in the whole interlayer region. Since there are only octahedral substitutions distributed in the high-charge clay (the white dots in the figure), the figure clearly shows that these Na+ and water molecules are largely associated with the locations of these isomorphic substitutions, while CO2 molecules try to stay away from these locations. Considering the interlayer Na+ ions are situated between the 1W water layer and the clay surface oxygen atoms (Figure 5), the almost overlap of the Na+ and H2O distributions indicates that interlayer cations are fully hydrated by water molecules and clay surface oxygen atoms.15 Figure 8b shows the 2D density distributions of different species in the low-charge clay interlayer, which has both octahedral substitutions (white dots) and tetrahedral substitutions (black dots). These density distributions are more localized with higher density values. In particular, Na+ ions are more strongly associated with the surface tetrahedral charges than the inner octahedral charges, leading to most of the water molecules bound in the same regions. At the same time, the density distribution of CO2 stays largely in a mutually excluded region of Na+ and H2O (similar to the case of high-charge clays shown in Figure 8a). Moreover, the mutual complementary distributions of H2O and Na+ which are located in the same region show that Na+ ions are fully hydrated by water molecules, as was discussed in our previous study.15

The 2D self-diffusion coefficients of CO2, water, and sodium ions are calculated for both high- and low-charge clays at 30% RH. The detailed method is presented in our previous work.51 The results are tabulated in Table 3. Experimental data for pure Table 3. Diffusion Coefficients of CO2, Water, and Sodium Ions in High- and Low-Charge Na-Montmorillonite Clay Interlayers under T/P/RH = 323 K/90 bar/30% RH (%) 30 30 43

equilibrium basal spacing (Å)

DH2O (× 10−9 m2/s)

DCO2 (× 10−9 m2/s)

DNa+ (× 10−9 m2/s)

11.8 (high charge) 11.9 (low charge) hectoriteH2O, 1Wa

0.354 ± 0.001

0.388 ± 0.001

0.157 ± 0.001

0.665 ± 0.003

0.131 ± 0.001

0.018 ± 0.001

0.53 ± 0.05

a

NSE experiments at T/P = 323 K/1 bar without CO2 by Marry et al.52

water in clay from quasi-elastic neutron scattering (QENS) experiments by neutron spin echo (NSE) are also included in Table 3 for comparison. Overall, the layer charge in clays does not have a significant effect on the diffusion of water and CO2. They are comparable to each other in both clay models and are also comparable to the diffusion coefficient of water in the NSE experiment. The diffusion coefficient of Na+ ions in the highcharge clay is about 1 order of magnitude larger than that in the low-charge clay. We attribute this to the octahedral charge deficit in the high-charge clay that leads to a relatively weak G

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electrostatic interaction between interlayer cations and octahedral negative charges.

REFERENCES

(1) Schaef, H. T.; Ilton, E. S.; Qafoku, O.; Martin, P. F.; Felmy, A. R.; Rosso, K. M. In situ XRD Study of Ca2+ Saturated Montmorillonite (STX-1) Exposed to Anhydrous and Wet Supercritical Carbon Dioxide. Int. J. Greenhouse Gas Control 2012, 6, 220−229. (2) Ilton, E. S.; Schaef, H. T.; Qafoku, O.; Rosso, K. M.; Felmy, A. R. In Situ X-Ray Diffraction Study of Na+ Saturated Montmorillonite Exposed to Variably Wet Super Critical CO2. Environ. Sci. Technol. 2012, 46, 4241−4248. (3) Rother, G.; et al. CO2 Sorption to Subsingle Hydration Layer Montmorillonite Clay Studied by Excess Sorption and Neutron Diffraction Measurements. Environ. Sci. Technol. 2013, 47, 205−211. (4) Giesting, P.; Guggenheim, S.; van Groos, A. F. K.; Busch, A. Interaction of Carbon Dioxide with Na-Exchanged Montmorillonite at Pressures to 640 bar: Implications for CO2 Sequestration. Int. J. Greenhouse Gas Control 2012, 8, 73−81. (5) Giesting, P.; Guggenheim, S.; van Groos, A. F. K.; Busch, A. XRay Diffraction Study of K- and Ca-Exchanged Montmorillonites in CO2 Atmospheres. Environ. Sci. Technol. 2012, 46, 5623−5630. (6) Loring, J. S.; et al. In Situ Molecular Spectroscopic Evidence for CO2 Intercalation into Montmorillonite in Supercritical Carbon Dioxide. Langmuir 2012, 28, 7125−7128. (7) Loring, J. S.; Ilton, E. S.; Chen, J.; Thompson, C. J.; Martin, P. F.; Benezeth, P.; Rosso, K. M.; Felmy, A. R.; Schaef, H. T. In Situ Study of CO2 and H2O Partitioning between Na-Montmorillonite and Variably Wet Supercritical Carbon Dioxide. Langmuir 2014, 30, 6120−6128. (8) Schaef, H. T.; et al. Competitive Sorption of CO2 and H2O in 2:1 Layer Phyllosilicates. Geochim. Cosmochim. Acta 2015, 161, 248−257. (9) Cole, D. R.; Chialvo, A. A.; Rother, G.; Vlcek, L.; Cummings, P. T. Supercritical Fluid Behavior at Nanoscale Interfaces: Implications for CO2 Sequestration in Geologic Formations. Philos. Mag. 2010, 90, 2339−2363. (10) Botan, A.; Rotenberg, B.; Marry, V.; Turq, P.; Noetinger, B. Carbon Dioxide in Montmorillonite Clay Hydrates: Thermodynamics, Structure, and Transport from Molecular Simulation. J. Phys. Chem. C 2010, 114, 14962−14969. (11) Myshakin, E. M.; Saidi, W. A.; Romanov, V. N.; Cygan, R. T.; Jordan, K. D. Molecular Dynamics Simulations of Carbon Dioxide Intercalation in Hydrated Na-Montmorillonite. J. Phys. Chem. C 2013, 117, 11028−11039. (12) Makaremi, M.; Jordan, K. D.; Guthrie, G. D.; Myshakin, E. M. Multiphase Monte Carlo and Molecular Dynamics Simulations of Water and CO2 Intercalation in Montmorillonite and Beidellite. J. Phys. Chem. C 2015, 119, 15112−15124. (13) Sena, M. M.; Morrow, C. P.; Kirkpatrick, R. J.; Krishnan, M. Supercritical Carbon Dioxide at Smectite Mineral−Water Interfaces: Molecular Dynamics and Adaptive Biasing Force Investigation of CO2/H2O Mixtures Nanoconfined in Na-Montmorillonite. Chem. Mater. 2015, 27, 6946−6959. (14) Cygan, R. T.; Romanov, V. N.; Myshakin, E. M. Molecular Simulation of Carbon Dioxide Capture by Montmorillonite Using an Accurate and Flexible Force Field. J. Phys. Chem. C 2012, 116, 13079− 13091. (15) Rao, Q.; Leng, Y. S. Molecular Understanding of CO2 and H2O in a Montmorillonite Clay Interlayer under CO2 Geological Sequestration Conditions. J. Phys. Chem. C 2016, 120, 2642−2654. (16) Bickle, M. J. Geological Carbon Storage. Nat. Geosci. 2009, 2, 815−818. (17) Haszeldine, R. S. Carbon Capture and Storage: How Green Can Black Be? Science 2009, 325, 1647−1652. (18) Orr, F. M. Onshore Geologic Storage of CO2. Science 2009, 325, 1656−1658. (19) Schrag, D. P. Storage of Carbon Dioxide in Offshore Sediments. Science 2009, 325, 1658−1659. (20) Bachu, S.; Adams, J. J. Sequestration of CO2 in Geological Media in Response to Climate Change: Capacity of Deep Saline Aquifers to Sequester CO2 in Solution. Energy Convers. Manage. 2003, 44, 3151−3175.

4. CONCLUSION AND FURTHER DISCUSSION The effect of layer charge on the structure and dynamics of CO2−H2O mixture in clay interlayers under typical CO2 geologic sequestration conditions has been further investigated through computational molecular simulation studies. Simulation results show that while CO2 intercalation in swelling clay minerals is strongly influenced by RH, the more important finding is that the layer charge has a significant effect on this process. Our simulation results show that both high RH and high layer charge facilitate water molecules entering clay interlayers and decrease the CO2 intercalation. For the two clay models studied here, the 1W and 2W equilibrium basal spacings seem not quite dependent on layer charge due to possible counteracting effect of the layer charge amount and charge location. However, the 1W-to-2W transition occurs at a lower RH in the high-charge clay than in the low-charge clay. MD simulations show that in highcharge clay interlayers CO2 molecules are sparsely distributed, and the number of CO2 dimers is negligible even at low RH = 30%. In the low-charge clay interlayers, CO2 dimers are frequently observed due to sizable amount of CO2 intercalated, leading to a reduction of CO2 coordination number. No significant changes in diffusion of CO2 and H2O are observed in the two clay interlayers. However, the diffusion coefficient of interlayer Na+ ions is increased by roughly 1 order of magnitude in the high-charge clay. This increase is explained by the isomorphic substitutions in octahedral sheets of highcharge clays, resulting in weak electrostatic interaction between clay surfaces and sodium ions. On the basis of the present simulation work, we point out that clay samples with different stoichiometry may result in the discrepancies between our simulation and IR experimental results.7 In other words, simulations suggest that perhaps the clay samples used in experimental measurements might contain a mix of high- and low-charge clay particles. On the other hand, we are aware that our current usage of CLAYFF force field parameters does not include the three-body energy function for the surface hydroxyl group-metal angle bend term.45 This energy term has been suggested in the original CLAYFF45 as an optional feature to enhance better description of metal sorption on hydrated clay surfaces and the vibrational behavior of surface hydroxyl groups, but with more computational cost (time step should be less than 0.5 fs). We anticipate that including this hydroxyl−metal angle bend term in the current clay model may result in better consistency with experimental results.

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AUTHOR INFORMATION

Corresponding Author

*Tel 202-994-5964; e-mail [email protected] (Y.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the American Chemical Society Petroleum Research Fund (ACS PRF) and the National Energy Research Scientific Computing Center (NERSC). H

DOI: 10.1021/acs.langmuir.6b02326 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.6b02326 Langmuir XXXX, XXX, XXX−XXX