Molecular Dynamics Simulation of Hydration and Swelling of Mixed

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Molecular Dynamics Simulation of Hydration and Swelling of Mixed-Layer Clays Mahsa Rahromostaqim, and Muhammad Sahimi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03693 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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The Journal of Physical Chemistry

Molecular Dynamics Simulation of Hydration and Swelling of MixedLayer Clays Mahsa Rahromostaqim and Muhammad Sahimi∗ Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-1211, Unired States ABSTRACT: Swelling of clay minerals is important to a broad class problems in science and engineering. While the problem has been extensively studied experimentally, past molecular modeling of the phenomenon was focused on pure clays of one type or another. In practice, however, there is a diverse class of mixed-layer clays (MLCs) in sedimentary rock with intermixed stacking sequence of two or more types of distinct layers within a single crystal. In fact, more than 60 percent of sedimentary rocks in the U.S. contain various types of MLCs. We present the results, to our knowledge, of the first molecular dynamics simulation of hydration energetics and swelling of illite-montmorillonite (I-MMT) MLCs, the most common type of mixed clays. The swelling is studied as a function of the water concentration with four combinations of interlayer cations, namely, Na+ and K.+ The hydration energies, the radial distribution functions, and the density profiles in the interlayer region are computed. For regular Na-MMT with layer charge concentrated in the octahedral sheet, weak cation-surface interaction results in fully hydrated ions and significant swelling. In the asymmetric interlayer of the MLC, however, the illite sheet with stronger interaction of surface and ions causes adsorption of the cations deep in the ditrigonal cavities of the siloxane surface. Given that the hydration enthalpy of K+ is smaller than that of Na,+ its hydration shell is quite unstable compared with that of Na.+ Therefore, swelling is inhibited as the ratio K+ /Na+ increases. The results demonstrate the significant differences between the hydration and swelling properties of pure clays and the mixed ones, which have important implications in practice, particularly for sequestration of CO2 in sedimentary rock. 1. INTRODUCTON Swelling of clay minerals is of prime importance to many important fields of science and engineering, including geotechnical and petroleum engineering, and environmental and soil sciences. Some studies have claimed1−5 that the size and charge of cations in the interlayer region of clay ACS Paragon Plus Environment

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minerals play the main role in the swelling, while others have argued that the amount and spatial location of the negative charges are the prime factors in determining the extent of hydration and, hence, the swelling. Most of the past studies, both experimental and computational, have, however, focused on pure clays of one type or another. In reality, there is a wide and diverse class of mixed-layer clays (MLCs) in sedimentary rock that contain an intermixed stacking sequence of two or more types of distinct layers within a single crystal. Such clays are characterized by a few factors, including the type and number of the layers, as well as the order of their sequence. In fact, more than 60 percent of sedimentary rock in the Unired States contains6 various type of MLCs. One important factor for the abundance of MLCs is that, under a solid-state transformation pure clay layers are converted into MLCs. For example, in smectite illitization the interlayer smectites are replaced by illites, resulting in gradual change of the crystal size and shape.7 One example of MLCs is bentonite, a mixture of montmorillonite (MMT) and beidellite, which is a mixed layer illite-smectite clay. Potasium-rich bentonites are dominated by illites, which have been proposed for use in addressing the problem of waste disposal.8 Their porosity is reduced by hydration, whereas the tendency of the sodium-saturated smectites to swell is the main reason for the observed instability in drilling operations, which may lead to the collapse of the wellborne. Another motivation for understanding swelling of MLCs is global warming. Sequestration of CO2 , the main culprit in global warming, in rock formations is being studied as an effective method of removing the gas from the environment. Understanding the properies of MLCs swelling in the presence of CO2 will lead to improved sequestration of CO2 . The most abundant and interesting MLCs are illite-montmorillonite (I-MMT) clays.9 . While there are numerous possible combinations of the mixed layers based on their order, such as regular, segregated regular, and seemingly stochastic, it has been reported9 that large percentages of I-MMT clays - 40 percent and higher - are randomly interstratified. In addition to temperature and pressure, several other factors influence swelling of such clays, including1,10−18 the layer’s charges and their spatial distribution, their magnitude, and the ratio and type of the interlayer cations. In particular, the structures of both illite and MMT consist of an octahedral sheet sandwiched between two tetrahedral sheets, the so-called T-O-T pattern. The main difference between the structures of the two clays is in the location where the substitutions occur. The majority of the negative charges in illite are distributed in the tetrahedral sheet ACS Paragon Plus Environment

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where substitution of Si4+ by Al3+ occurs, in contrast with MMT in which the negative charges are dominant in the octahedral sheet where Al3+ is substituted by Mg.2+ Generally speaking, however, few substitutions occur in the tetrahedral and octahedral sheets of the two types of clays. To balance the negative charges in illite, potasium ions, K+ , are distributed in the interlayer space. K+ do not have strong tendency to hydrate and, thus, they lock the illite layers to each other. In contrast, MMT is made electrically neutral by Na+ , Mg2+ or Ca2+ , which are usually hydrated to a certain degree and, therefore, the clay swells easily. The mixed spatial distribution of Na+ and K+ in the interlayer region of the mixed I-MMT is, therefore, important to balancing the layers’ negative charges. In addition, the concentration of the cations in the interlayer region and that of the water molecules, and the interactions between them and the negatively-charged layers are the important factors in swelling of MLCs. Molecular dynamics (MD) simulation is a powerful tool for studying swelling of MLCs. In particular, it can be used to address whether the size and charges of interlayer’s cations play the main role in setting their spacing, or it is the spatial distribution of the negative charges that determines the extent of the hydration and swelling. Such a combination of both sets of variables in MLCs has never been considered. Past molecular studies, both MD and Monte Carlo simulations, focused on the effect of the spatial distribution of the layers’ charges; considered separate pairs of clay layers with different charge densities and distributions, and compared the results for the two. For example, McGoldrick et al.19 studied separately the interactions of water with MMT and beidellite, which is very similar to illite. Using only one layer charge, Boek et al.5 examined interlayer expansion for cations with different hydration properties. Teich-McGoldrick et al.19 studied the interactions of water with MMT and beidellite, which is very similar to illite. They reported that at low water concentrations the basal the spacing of beidellite is larger than or equal to that of MMT, whereas at higher water content, it is the basal spacing of the MMT that is larger. A study by Liu et al.12 indicated that in Cs-smectites layer charge location has significant effect on the mobility of the interlayer counterions and their binding to the surface, although the difference in the swelling of the three different layer charge distributions was reported to be negligible. Another study20 focused on how shifting the negative charges from the octahedral to tetrahedral layers affects binding of the counterions to the surface and, thus, swelling of clays. Sun et al.21 reported that with increasing octahedral charge fraction, the swelling pressure increases, while Skipper et al.22 claimed that ACS Paragon Plus Environment

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increasing the tetrahedral layer charge results in the inner sphere surface complexes (ISSC) of Na. Other previous studies10,23−25 also reported that since K+ only perturbs the location of the water molecules, but is unable to hydrate, it prevents the clay from swelling, implying that the existence of K+ in the MMT interlayer and the ratio R of K+ and the overall number of cations contribute significantly to controlling the swelling, with 0.3 < R < 0.56 strongly inhibiting swelling.26 Several previous studies of interaction of clays’ layers with the ions focused on smectites, rather than illites, and assumed that the clay is rigid. Very few studies27−30 focused on fully flexible clays. For example, Lammers et al.30 used the force field flexible CLAYFF investigated adsorption of cesium and sodium on illite surface and reported that, due to penetration of ions into the ditrigonal cavities, the ISSC with cation density peak considerably closer to the clay surface. Using extensive MD simulations, we study in this paper swelling of mixed I-MMT clays for a range of water concentration. We also study the same in MMT-MMT whose interlayer contains Na+ , and use the results as a reference to compare with the results for the MCLs. To our knowledge, our MD study of swelling of MLCs is the first of its kind. The rest of this paper is organized as follows. In the next section we describe the details of the MD simulations. The results are presented and discussed in Section 3, while the paper is summarized in the last section. 2. MOLECULAR DYNAMICS SIMULATION The MD simulations were carried out using the LAMMPS package. We simulated MLCs of sodium-saturated MMT and potasium-illite whose chemical compositions are, respectively, Na0.75 [Si7.75 Al0.25 ](Al3.5 Mg0.5 )O20 (OH)4 , and K0.9 [(Al1.9 Mg0.1 )(Si3.2 Al0.8 )O10 (OH)2 ]. The negative charges in the interlayer regions were generated by symmetrically selecting substitutions over the clays’ sheets. Half of the K+ cations in illites - 1.8 ions per unit cell - were in the illite-illite (I-I) interlayer, while the remaining half was equally distributed between the I-MMT interlayers. Figure 1 presents a snapshot of the equilibrated supercell, with the top and bottom layers being illite, surrounding the MMT layers, together with the cations. The clay layers were symmetric with respect to the central line in the middle between the two MMT sheets. As more water molecules were inserted in the interlayers, the supercell was allowed to freely expand along the z axis. We constructed a supercell of 80 unit cells that contained two layACS Paragon Plus Environment

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ers each of MMT and illite. Each layer contained 20 unit cells, and in order to minimize the differences between the dimensions of illite and MMT sheets along the x and y directions (see Figure 1), they were organized as 5 × 4 unit cells. The size of each unit cell was 5 × 9 ˚ A2 and 5 × 8.8 ˚ A2 for MMT and illite, respectively. Thus, under dry conditions the dimensions of the ˚2 , and 26 × 35 A ˚2 for illite. individual sheets or layers of MMT were approximately 26 × 36 A Periodic boundary conditions were imposed in all directions. The force field utilized to represent the interatomic interactions betwen the atoms in the clays was CLAYFF,31 based on which we generated the equilibrium molecular structures of both types of clay. The flexible SPC model was used to model the interlayer water molecules, but not the hydroxyl group of the clay layers (since CLAYFF already contains the necessary parameters for them). According to CLAYFF the total interaction energy E of the material is given by, 

E=

X

k1 (rij − r0 )2 +

bonds

X

k2 (θijk − θ0 )2 +

angle

X ij

σij ij  rij

!12

σij −2 rij

!6  +

e2 X qi qj . (1) 4πε0 i,j rij

Here, k1 and k2 are force constants, θijk is the angle between bonds ij and jk, e is the electron’s charge, qi is the partial charge of atom i, ε0 is the dielectric permittivity of vacuum, and r0 and θ0 are the equilibrium values of the corresponding quantities. i and σi are the usual LennardJones (LJ) energy and size parameters, and the Lorentz-Berthelot mixing rules were used for the pairs ij. The simulations were initially carried out in the (N P T ) ensemble at T = 348 K and P = 130 bar, which are close to the conditions for geological cap rock formation.32 A time step of 0.01 fs was used for 100 ps, after which it was increased to 1 fs for 5 ns after which equilibrium was reached. An additional 5 ns second of simulation was used for the production step. Simulations longer than 10 ns were also carried out, but the differece between their results and those obained with shorter simulations were negligible. The Nosé-Hoover thermostat and barostat were used with the coupling constants of 1000 and 100, respectively. After some preliminary simulations the long-range interactions were truncated for r > 13 ˚ A. The electrostatic interactions were computed using the particle-mesh Ewald summation with a precision of 10−4 . To calculate the density profiles and the radial distribution functions, after equilibrium was reached via simulation in the (N P T ) ensemble, we switched to the canonical (N V T ) ensemble and continued the simulation for another 5 ns. ACS Paragon Plus Environment

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A main goal of our study is gaining insight into the effect of asymmetric interlayer structure, as well as that of mixed counterions, K+ and Na+ , on swelling of I-MMT MLCs. Note that the illite has more negative charges in the tetrahedral sheets, whereas MMT contains more of such charges in the octahedral layer. Three series of simulations were carried out in which the counterions ratio in the interlayer was varried. In one series, the interlayer of I-MMT contained both K+ and Na+ , whereas in the other two series of simulations the MLC contained only Na+ or K+ to compensate for the negative charges of illite and MMT. This allowed us to monitor the effect of the spatial distribution of the layer’s charge location and that of the two types of ions on the swelling. To understand the effect of water concentration, the number of water molecules in the interlayer region was systemically increased from 1 to 10 molecules per unit cell, and for each case an independent simulation was carried out, which was enough to capture one-layer (1W) and two-layer (2W) hydration of the clays. All the simulations were initiated with inserting the cations in the mid-plane of each interlayer, while the initial spatial distribution of the water molecules was random. We computed the density profiles along the z axis and averaged them over the interlayer regions over a 5 ns interval in the (N V T ) ensemble. 3. RESULTS AND DISCUSSION We have computed several important properties of the clays that we are studying. In what follows we present and describe the results and discuss their implications. 3.1 Hydration Energies. We computed the change ∆U (N ) in the potential energy U (N ) of the system as a function of the water content, ∆U (N ) = [U (N ) − U (0)]/N , where N is the number of water molecules, and U (0) is the potential energy of the dry clay. The results, which represent the hydration energies, are presented in Figure 2. In this and the following figures the estimated numerical uncertainty in the results, obtained by averaging over five realizations, is smaller than the size of the symbols used in the figures. Throughout the first stage of hydration, the hydration energy of clays with only Na+ decreases significantly below the internal energy of bulk flexible SPC water, -41.5 kJ/mol. Due to the strong surface-ion interaction in I-MMT with only Na+ , the minimum energy is relatively constant for a wide range of water content, when compared with MMT-MMT that respond more rapidly to the added water. At the same time, broad energy minimum in the case of Na+ with the I-MMT surface is due to the higher number ACS Paragon Plus Environment

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of cations and, hence, the charge magnitude required to charge compensate the illitic surface leads to increased water content in the interlayer. This indicates that it is energetically more favorable to add water to clays with tetrahedral charge sites than to those with octahedral ones. The lack of change in the minimum hydration energy of I-MMT with Na+ becomes clearer by considering the density profiles, which were presented in the main text of the paper. In the cases with K+ , however, ∆U (N ) decreases less rapidly. Moreover, in clays with only K+ in the interlayer regions the limited interaction of the cations with water results in reduced energy change, which is expected. Thus, in the presence of K+ the water molecules should be repelled by the clay. As soon as the initial energy barrier is crossed, the hydration energy of clays with K+ also decrease, but to values above that of clayes with Na+ . A comparison of the four cases in Figure 2 reveals that it is energetically more favorable for water molecules to invade the anhydrous Na+ clay. In addition, the more Na+ are in the interlayer region, the more the clays swell. On the other hand, keeping the interlayer cations the same, the more charges in the tetrahedral layer, the more stable and energetically favorable is the material. Thus, in clays with the same structures, the more Na+ are in the interlayer region, the more energetically favorable is for the system to swell. Note, however, that the number of Na+ in I-MMT is 25, compared with 14 in MMT-MMT, implying that although there are more cations in the former case with increased hydration as water molecules are added, MMTMMT responds more sensitively to them due to the weaker surface-ion interactions. After the minimum energy is reached in each case, the energies rise as more water is added, up to close to the internal energy of bulk water. 3.2 Swelling. The basal spacing d is defined as the sum of the clay sheet’s thickness and the interlayer distance between that sheet and its neighboring one. For I-MMT MCL all the d values were calculated based on MMT’s thickness. d was computed as a function of the number of water molecules per unit cell for I-MMT, MMT-MMT, and I-I for 10 separate cases. In the case of MMT-MMT only Na+ was inserted in the interlayer region, whereas three cases of I-MMT, with K+ plus Na+ , with K+ only, and with Na+ only in the interlayer region were also studied. In the I-I interlayer only K+ were inserted in all the cases, which resulted in the layers being “locked” together, with its basal spacing d remaining fairly constant, ≈ 10 ˚ A, in all the cases. As mentioned earlier, the cation content of MMT-MMT interlayer is identical in all the ACS Paragon Plus Environment

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simulations, but different from that of I-MMT interlayer in which we inserted, respectively, 18 and 7 K+ and Na,+ symmetrically distributed in y − z plane (see Figure 1), in order to offset the negative charges of illite and MMT. The dependence of the absolute basal spacing d on the number of water molecules for MMT-MMT, I-MMT with 72% K+ and 28% Na+ ), I-MMT with all K+ substituted by Na+ , and I-MMT with all Na+ substituted by K+ is shown in Figure 3. The overall swelling behavior of MMT-MMT clay agrees well with the existing literature on both simulation and experimental studies.1,33,34 Water adsorption in clays occurs in two steps. First, some adsorption occurs to form the hydration shell for the counterions that results in the layer’ expansion, whereas in the second step the increase in the water content fills up the remaining volume of the interlayer without much swelling. For cations with lower hydration enthalpy, however, due to the lower extent of hydration and the disturbed hydration shell, swelling happens more monotonically without distinguishable steps. The basal spacing d for the dry (0W), 1W and 2W states of the various clays are presented in Table 1. In the dry 0W state I-MMT with only K+ has the largest d value, which is due to the larger ionic radius35 of K,+ 1.52 ˚ A, than Na,+ 1.2 ˚ A. Table 1. Dependence of the basal spacing d (in ˚ A) on the type of interlayer cations in various hydration states. 0W (dry) 1W 2W MMT-MMT (with Na+ only)

9

12

15

I-MMT (with 72% K+ and 28% Na+ )

10

12

15

I-MMT (with Na+ only)

9

12

14

I-MMT (with K+ only)

10

12

15

Comparing the panels of Figure 3 reveals that MMT-MMT swells most in all the cases, and that its interlayer space manifests distinct 1W and 2W hydration states and the transition to them. Note also that there is a wider and clearer range for the 1W state than for the 2W. The variations of d for I-MMT are, however, significantly different from those of MMT-MMT with both K+ and Na+ . In addition, for all the water contents, swelling of I-MMT is smaller than MMT-MMT. Due to the two-step hydration of clays, the expansion due to formation of hydration shell around the cations is smaller for both I-MMT cases, including the case with K.+ Figure 3 also indicates that, compared with the other cases, I-MMT with only Na+ has an ACS Paragon Plus Environment

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intermediate level of expansion, and that, compared with MMT-MMT, the flattened 1W state for I-MMT with Na+ is postponed to higher water contents. This is due to the interaction between Na+ and the more negatively charged tetrahedral layer on the illite side. We will return to this shortly. Swelling in I-MTM with 72% K+ and 28% Na+ and one with only K+ happens more smoothly in the interlayer with a smaller change of slope in the transition from the 0W to 1W state. The two MLCs that contain K+ have the lowest extent of expansion. The weak transition between the hydration states in the presence of K+ is due to weaker attraction between K+ and the water molecules. Denis et al.26 reported that in mixed-ion MMT, K+ fractions of over 33% strongly control swelling. In the present study the response of the I-MMT interlayer to both cases containing K+ is very similar, which is presumably because the K+ fraction is relatively high in both cases. To better appreciate the differences between the basal spacing of the four types of clays, both pure and mixed that we have studied, Figure 4 compares the relative basal spacings d as a function of the water content. To compute the relative basal spacing, we subtracted its value from that in the dry state. Figure 4 reveals that MMT-MMT has the largest amount of swelling in all the cases, and that its interlayer space manifests distinct 1W and 2W hydration states, as well as the transition to them. Note also that there is a wider and clearer range for the 1W state than the 2W. The variations of the basal spacing for I-MMT are, however, significantly different from that of MMT-MMT with both K+ and Na+ . In addition, for all the water contents, swelling of I-MMT is smaller than that of MMT-MMT. According to the two-step hydration of clays that was described earlier, the expansion due to formation of hydration shell around the cations is smaller for both I-MMT clays, including the case with K.+ Figure 4 also indicates that I-MMT with only Na+ has an intermediate level of expansion when compared with the other cases, and that, compared with MMT-MMT, the flattened 1W state for this MLC occurs at higher water contents, which is due to the interaction between Na+ and the more negatively charged tetrahedral layer on the illite side. We will return to this shortly. 3.3 Molecular Structure of the Interlayer Region. To gain deeper insight into the interactions of K+ and Na+ with the clay surface, the distributions of the cations and water in the interlayer regions in the z direction, perpendicular to the sheet, were calculated. The results are shown in Figure 5. Our computed 1W and 2W distributions for MMT-MMT agree well ACS Paragon Plus Environment

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with the literature.1,19,36−40 We should keep in mind, however, that in this type of molecular modeling some inner sphere complexes are formed between Na+ and the clay surface, which are due to the distribution of the negative charges on the tetrahedral layer, as well as the overestimated surface-ion interaction for the surface oxygens incorporated in the CLAYFF. The resulting swelling is slightly smaller than the experimental data that, for the basal spacing of the 1W state are in the range 12.3 − 12.6 ˚ A and for the 2W state in the range 14.9 − 15.6 ˚ A for MMT-MMT.1,36,37,40 As Figure 5 indicates, asymmetry of the atomic distributions in I-MMT occurs due to two different clay layers facing each other. Due to the low hydration energy of K,+ independent of the water content, the cations bind strongly to the illite surface. In fact, they are adsorbed onto the surface as the inner sphere surface complexes (ISSCs), and are locked inside the surface’s hexagonal cavities. A previous study of K+ -MMT by Boek et al5 indicated that adding water to the interlayer results in K+ leaving the hexagonal sites and moving to the sites above the SiO4 tetrahedral. Boek et al. also noted that, compared with Na+ and Li,+ K+ is reluctant to fully hydrate and attach itself to the surface; see also Boek41 and Chang et al.42 In the present case there are two main reasons for K+ being tied to the illite surface. One is exposure to two different clay surfaces, with illite being more negatively charged on the outer layer, which attracts immediately all the positively charged ions toward the surface. The second reason is the concentrated tetrahedral charge of illite that, compared with MMT and its more negatively-charged octahedral sheet, makes the ionic bonding stronger. Interpretation of the results should be done carefully in the light of the complex behavior of Na. Consider I-MMT. The most diverse surface complexes are formed in the 2W of I-MMT with Na.+ There are two and one type of the ISSCs formed, respectively, on the illite and MMT sides of the interlayer region. Moreover, one type of outer sphere surface complex (OSSC) is also formed in that region. Thus, with only Na+ on the MMT side, the ISSCs exist in both the 1W and 2W states. With K+ in the interlayer on the MMT side of I-MMT, however, the single low-density peak of Na+ indicates that the ISSC forms at higher water contents. With mixed cations in the interlayer, only one type of ISSC and no OSSC of Na+ is formed. The OSSC in I-MMT with Na+ explains the difference in the basal spacings of the three I-MMT studied, discussed earlier. When only Na+ is present, some of them near the illite surface hydrate as the water content increases, giving rise to formation of the OSSCs similar to that in MMT-MMT ACS Paragon Plus Environment

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that contribute to more swelling. Figure 5 indicates that the locations of the first density peak of both Na+ and K+ ISSCs are much closer to the illite surface than expected. In the case of the ISSC it is common for the location of the first peak to be the sum of the ionic radii of the cation and surface Oxygen - (1.26 + 1.16 = 2.42) ˚ A for rO−Na and (1.26 + 1.52 = 2.78) ˚ A for rO−K . Figure 5 indicates, however, that the location is around 0.8 ˚ A for rO−Na and 1.5 ˚ A for rO−K . The larger radius for K+ is consistent with the ionic radius of K+ that is larger than that of Na.+ The large difference in the locations of the density peaks illustrates the strong adsorption of the cations deep in the ditrigonal cavities of siloxane surface. The comparably smaller penetration of K+ into the illite surface implies that the ditrigonal cavities are better fits for Na+ than K.+ Similar behavior was reported by Lammers et al.30 In their study, however, the distance from the surface for Na+ was slightly larger than what we report here, which is likely due to the difference in the chemical composition of illite that has more negative charges at its surface. Moreover, the asymmetry of the interlayer structure with the higher charge density on the illite side contributes to its dominant role in the adsorption of the ions. The two mechanisms cause the stronger attraction of the ions to the ditrigonal cavities of the tetrahedral layer. Note that previous MD simulation of Na-smectite produced a sodium density peak that was considerably farther from the surface. Bourg et al.,43 for example, reported that in a rigid clay the Na-ISSC is at 2.6 ˚ A from the surface. The reason for the difference is30 the substitutes that are distributed more in the tetrahedral layer of illite, as well as the flexible clay structure. Deep penetration of the ions into the hexagonal cavities is due to an unphysical relaxation of tetrahedral layer, expanding the ditrigonal cavities to larger hexagonal one. The OSSCs are formed when the ions are at distances larger than 3 ˚ A from the clay surface.30 They are present in I-MMT only in its 2W state with Na+ , while they do not form in any MLCs with only K.+ This confirms our results presented above, and implies that K+ is primarily adsorbed as the ISSC, while Na hydrates at higher water contents. The gap in the energy density of the ISSCs and OSSCs also indicates the energy barrier of ion exchange at the clay surface. The density profile of water oxygen is also affected by the counterions. On the MMT side at lower water contents, hydrogen atoms of water bond to the oxygen on the clay’s surface and form the 1W layer. As the water content increases, the 2W state is formed, so that hydrogen ACS Paragon Plus Environment

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preserves its distance from the surface. On the illite side, however, when K+ is in the interlayer region, it only perturbs the water network. Therefore, it contributes to making the surface hydrophobic, which is the reason for not having a distinct water oxygen peak near the illite surface. Nevertheless, in terms of the water density distribution, the 2W state in the Na-I-MMT is analogous to that of MMT-MMT. 3.4 Radial Distribution Functions. Figure 6 presents the calculated radial distribution functions (RDF) g(r) of MMT-MMT and I-MMT with both Na+ and K+ , as well as with only Na,+ while Figure 7 shows the results with K.+ In all cases the locations rOW −K and rOW −Na of the peaks corresponding to hydration are, respectively, 2.78 ˚ A and 2.4 ˚ A, which are in agreement with the previous studies. As the water content increases, the height of the peak in g(r) decreases in all cases. A comparison between Figures 6 and 7 indicates that the peak corresponding to OW − K hydration is smaller than that of OW − Na, which is due to the difference in the tendency of Na+ and K+ to hydrate. Figures 6 and 7 also show the RDFs of the counterions and the surface-bridging oxygen OS . In MMT-MMT with Na+ with the negative charges concentrated in the octahedral sheet, the hydration shell of Na+ is more stable than that of K+ , as the hydration energy of the former is higher; see Figure 2. Therefore, as the water content increases, the cations move toward the center of the interlayer region and hydrate. Figure 6(b) confirms this as it indicates that the height of the RDF peak decreases considerably in the 2W bilayer region. In the case of I-MMT with Na+ , however, due to the strong adsorption of the cations onto the ditrigonal cavities, changing the water content in the interlayer region does not change considerably the location of the ions near the surface. The same behavior is also manifested by I-MMT with K+ in the interlayer region; see Figure 7. Comparison of the results for I-MMT with 72% K+ and 28% Na+ with those for I-MMT with Na+ indicates that the latter swells more than the former. According to Figures 6(c) and 6(e), when K+ is in the interlayer region, Na+ ions are surrounded more by water, while fewer Na+ are coordinated near the surface. It should, however, be noted that in the cases with Na only more swelling is occurred because there is nearly 3.6 times more Na+ in the interlayer, compared with the cases with mixed cations. With regard to Figures 6(d) and 6(f), the sharper Na-OS peak in the case with only Na is caused by two factors: First, as mentioned earlier, there are two types of the ISSC in I-MMT with Na.+ Second, there are more Na+ in the interlayer. ACS Paragon Plus Environment

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Figure 7 indicates that the RDFs of the MLCs with only K+ and a mixture of K+ and Na+ are quite similar, confirming the results for the basal spacing d and the density profiles, presented in Figures 3-5. Due to the lower hydration enthalpy of K+ , however, the peaks at rOS −K , which are near the illite surface, are sharper and more distinct. In the case of I-MMT, the very slight change in the RDF of OS− counterion confirms their dominant distribution of near the illite surface. Otherwise, the dynamic behavior of the cations near the MMT surface would have resulted in substantial changes in the RDFs with increasing the water content. In contrast with Na+ , K+ is more strongly bonded to the surface oxygens. Figure 7 also confirms that when the ratio K+ /Na+ is larger than a certain limit, K+ will be the dominant counterion in the interlayer, results in similar swelling, density profile and RDF in the cases of I-MMT with 72% K+ and 28% Na+ . 4. SUMMARY We presented the results of what we believe to be the first MD simulations of hydration energetics and swelling of mixed-layer clays, consisting of montmorillonite and illite sheets. The effect of two important cations, Na+ and K,+ on the hydration energies and swelling of the MLCs was studied in detail. Very significant differences were demonstrated between such properties of pure clays, such as montmorillonites and illites, and the mixed structures of the same two clays studied in this paper. As such clays constitute the majority of natural clays, it is imperative to understand their properties, particularly in the presence of CO2 , which is highly relevant to the problem of its sequestration in natural porous formations. Work in this direction is in progress. The results will be reported in the near future.

AUTHOR INFORMATION Corresponding Author ∗

E-mail: [email protected]. Phone: (213) 740-2064.

ORCID Muhammad Sahimi: 0000-0002-8009-542X Notes The authors declare no competing interest. ACKNOWLEDGMENTS ACS Paragon Plus Environment

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This work was supported as part of the Center for Geologic Storage of CO2 , an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC0C12504. The calculations were carried out using 12 nodes (each with 16 processors) of the computer network at the University of Southern California HighPerformance Computing Center. REFERENCES (1) Mooney, R.; Keenan, A.; Wood, L. Adsorption of Water Vapor by Montmorillonite. II. Effect of Exchangeable Ions and Lattice Swelling as Measured by X-Ray Diffraction. J. Am. Chem. Soc. 1952, 74, 1371-1374. (2) Brown, G.; Brindley, G.W. Crystal Structures of Clay Minerals and Their X-Ray Identification. Mineralogical Society, London, 1980. (3) Keren, R.; Shainberg, I. Water Vapor Isotherms and Heat of Immersion of Na/CaMontmorillonite Systems - I: Homoionic Clay. Clays Clay Miner. 1975, 23, 193-200. (4) Suquet, H.; De La Calle, C.; Pezerat, H. Swelling and Structural Organization of Saponite. Clays Clay Miner. 1975, 23 p. 1-9. (5) Boek, E.; Coveney, P.; Skipper, N. Monte Carlo Molecular Modeling Studies of Hydrated Li-, Na-, and K-Smectites: Understanding the Role of Potassium as a Clay Swelling Inhibitor. J. Am. Chem. Soc. 1995, 117, 12608-12617. (6) Weaver, C.E. The Distribution and Identification of Mixed-Layer Clays in Sedimentary Rocks. Publication 57, Shell Development Company, Exploration and Production Division, Ilouston, Texas (1955). (7) Altaner, S.P.; Ylagan, R.F. Comparison of Structural Models of Mixed-Layer Illite/Smectite and Reaction Mechanisms of Smectite Illitization. Clays Clay Miner. 1997, 45, 517-533. (8) Bibi, I.; Icenhower, J.; Niazi, N.K.; Naz, T.; Shahid, N.; Bashir, S. Clay Minerals: Structure, Chemistry and Significance in Contaminated Environments and Geological CO2 Sequestration. In, Environmental Materials and Waste: Resource Recovery and Pollution Prevention, Prasad, M.N.V.; Shih, K. (eds.), Elsevier, Boston, 2016, pp. 543-567. ACS Paragon Plus Environment

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(9) Reynolds, R.C.; Hower, J. The Nature of Interlayering in Mixed-Layer Illite-Montmorillonites. Clays Clay Miner. 1970, 18, 25-36. (10) Young, D.A.; Smith, D.E. Simulations of Clay Mineral Swelling and Hydration: Dependence upon Interlayer Ion Size and Charge. J. Phys. Chem. B 2000, 104, 9163-9170. (11) Slade, P.; Quirk, J. The Limited Crystalline Swelling of Smectites in CaCl2 , MgCl2 , and LaCl3 Solutions. J. Colloid Interface Sci. 1991, 144, 18-26. (12) Liu, X.; Lu, X.; Wang, R.; Zhou, H. Effects of Layer-Charge Distribution on the Thermodynamic and Microscopic Properties of Cs-Smectite. Geochimica Cosmochimica Acta 2008, 72, 1837-1847. (13) Sato, T.; Watanabe, T.; Otsuka, R. Effects of Layer Charge, Charge Location, and Energy Change on Expansion Properties of Dioctahedral Smectites. Clays Clay Miner. 1992, 40, 103-113. (14) Viani, B.E.; Low, P.F.; Roth, C.B. Direct Measurement of the Relation Between Interlayer Force and Interlayer Distance in the Swelling of Montmorillonite. J. Colloid Interface Sci. 1983, 96, 229-244. (15) Zhang, F.; Zhang, Z.Z.; Low, P.F.; Roth, C.B. The Effect of Temperature on the Swelling of Montmorillonite. Clay Minerals 1993, 28, 25-31. (16) Laird, D.A.; Shang, C.; Thompson, M.L. Hysteresis in Crystalline Swelling of Smectites. J. Colloid Interface Sci. 1995, 171, 240-245. (17) Cases, J.; Bérend, I.; Francois, M.; Uriot, J.P.; Michot, L.J.; Thomas, F. Mechanism of Adsorption and Desorption of Water Vapor by Homoionic Montmorillonite: 3. The Mg2+ , Ca2+ , Sr2+ and Ba3+ Exchanged Forms. Clays Clay Miner. 1997, 45, 8-22. (18) Cancela, G.D.; Huertas, F.J.; Taboada, E.R.; Sánchez-Rasero, F.; Laguna, A.H. Adsorption of Water Vapor by Homoionic Montmorillonites. Heats of Adsorption and Desorption. J. Colloid Interface Science 1997, 185, 343-354.

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(19) Teich-McGoldrick, S.L.; Greathouse, J.A.; Jové-Colón., C.F.; Cygan, R.T. Swelling Properties of Montmorillonite and Beidellite Clay Minerals from Molecular Simulation: Comparison of Temperature, Interlayer Cation, and Charge Location Effects. J. Phys. Chem. C 2015, 119, 20880-20891. (20) Chávez-Páez, M.; DePablo, L.; DePablo, J. Monte Carlo Simulations of Ca-Montmorillonite Hydrates. J. Chem. Phys. 2001, 114 10948-10953. (21) Sun, L.; Ling, C.Y.; Lavikainen, L.P.; Hirvi, J.T.; Kasa, S.; Pakkanen, T.A. Influence of Layer Charge and Charge Location on the Swelling Pressure of Dioctahedral Smectites. Chem. Phys. 2016, 473, 40-45. (22) Skipper, N.T.; Sposito, G.; Chang, F.-R.C. Monte Carlo Simulation of Interlayer Molecular Structure in Swelling Clay Minerals. 2. Monolayer Hydrates. Clays Clay Miner. 1995, 43, 294-303. (23) Degreve, L.; Vechi, S.M.; Junior, C.Q. The Hydration Structure of the Na+ and K+ Ions and the Selectivity of Their Ionic Channels. Biochimica Biophys. Acta (BBA)Bioenergetics 1996, 1274, 149-156. (24) Hensen, E.; Tambach, T.J.; Bliek, A.; Smit, B. Adsorption Isotherms of Water in Li-, Naand K-Montmorillonite by Molecular Simulation. J. Chem. Phys. 2001, 115, 3322-3329. (25) Ferrage, E.; et al. Hydration Properties and Interlayer Organization of Water and Ions in Synthetic Na-Smectite with Tetrahedral Layer Charge. Part 2. Toward a Precise Coupling Between Molecular Simulations and Diffraction Data. J. Phys. Chem. C 2011, 115, 1867-1881. (26) Denis, J.; Keall, M.J.; Hall, P.L.; Meeten G.H. Influence of Potassium Concentration on the Swelling and Compaction of Mixed (Na, K) Ion-Exchanged Montmorillonite. Clay Minerals 1991, 26, 255-268. (27) Martins, D.M.; Molinari, M.; Goncalves, M.A,; Mirao, J.P.; Parker, S.C. Toward Modeling Clay Mineral Nanoparticles: The Edge Surfaces of Pyrophyllite and Their Interaction with Water. J. Phys. Chem. C 2014, 118, 27308-27317. ACS Paragon Plus Environment

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(28) Newton, A.G.; Sposito, G. Molecular Dynamics Simulations of Pyrophyllite Edge Surfaces: Structure, Surface Energies, and Solvent Accessibility. Clays Clay Miner. 2015, 63, 277-289. (29) Newton, A.G.; Kwon, K.D.; Cheong, D.-K. Edge Structure of Montmorillonite from Atomistic Simulations. Minerals 2016, 6, 25-40. (30) Lammers, L.N.; Bourg, I.C.; Okumura, M.; Kolluri, K.; Sposito, G.; Machida, M. Molecular Dynamics Simulations of Cesium Adsorption on Illite Nanoparticles. J. Colloid interface Sci. 2017, 490, 608-620. (31) Cygan, R.T.; Liang, J.-J.; Kalinichev, A.G. Molecular Models of Hydroxide, Oxyhydroxide, and Clay Phases and the Development of a General Force Field. J. Phys. Chem. B 2004, 108, 1255-1266. (32) Romanov, V.N. Evidence of Irreversible CO2 Intercalation in Montmorillonite. Int. J. Greenhouse Gas Cont. 2013, 14, 220-226. (33) Meleshyn, A.; Bunnenberg, C. Swelling of Na/Mg-Montmorillonites and Hydration of Interlayer Cations: A Monte Carlo Study. J. Chem. Phys. 2005, 123, 074706. (34) Marry, V.; Turq, P.; Cartailler, T.; Levesque, D. Microscopic Simulation of Structure and Dynamics of Water and Counterions in a Monohydrated Montmorillonite. J. Chem. Phys. 2002, 117, 3454-3463. (35) Burgess, J. Ions in Solution: Basic Principles of Chemical Interactions. Woodhead Publishing, Cambridge, 1999. (36) Bérend, I.; Cases, J.-M.; Francois, M.; Uriot, J.P.; Michot, L.; Masion, A.; Thomas, F. Mechanism of Adsorption and Desorption of Water Vapor by Homoionic Montmorillonites: 2. The Li+ , Na+ , K+ , Rb+ , and Cs+ -Exchanged Forms. Clays Clay Miner. 1995, 43, 324-336. (37) Chiou, C.T.; Rutherford, D.W. Effects of Exchanged Cation and Layer Charge on the Sorption of Water and EGME Vapors on Montmorillonite Clays. Clays Clay Miner. 1997, 45, 867-880. ACS Paragon Plus Environment

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(38) Rutherford, D.W.; Chiou, C.T.; Eberl, D.D. Effects of Exchanged Cation on the Microporosity of Montmorillonite. Clays Clay Miner. 1997, 45, 534-543. (39) Calvet, R. Hydratation de la Montmorillonite et Diffusion des Cations Compensateurs. II. Etude de la Diffusion des Cations Compensateurs Dans la Montmorillonite. Annales Agronomiques 1973; http://agris.fao.org/agris-search/search.do?recordID=US201303274995 (40) Cases, J.; Berend, I.; Besson, G.; Francois, M.; Uriot, J.P.; Thomas, F.; Poirier, J.E. Mechanism of Adsorption and Desorption of Water Vapor by Homoionic Montmorillonite. 1. The Sodium-Exchanged Form. Langmuir 1992, 8, 2730-2739. (41) Boek, E.S. Molecular Dynamics Simulations of Interlayer Structure and Mobility in Hydrated Li-, Na-and K-Montmorillonite Clays. Mol. Phys. 2014, 112, 1472-1483. (42) Chang, F.-R.C.; Skipper, N.T.; Sposito, G. Monte Carlo and Molecular Dynamics Simulations of Electrical Double-Layer Structure in Potassium-Montmorillonite Hydrates. Langmuir 1998, 14, 1201-1207. (43) Bourg, I.C.; Sposito, G. Molecular Dynamics Simulations of the Electrical Double Layer on Smectite Surfaces Contacting Concentrated Mixed Electrolyte (NaCl-CaCl2 ) Solutions. J. Colloid Interface Sci. 2011, 360 701-715.


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Figure 1: The supercell structure of mixed-layer clay. Top and bottom layers are illite, while the two middle layers are MMT with the interlayer cations.


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Figure 2: Hydration energy as a function of the water content for MMT-MMT (blue), I-MMT with Na+ only (yellow), I-MMT ACS with 72% Plus K+ Environment and 28% Na+ (black), and I-MMT with K+ Paragon 20 (red).

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Figure 3: Basal spacing d of (a) MMT-MMT; (b) I-MMT with Na+ ; (c) I-MMT with 72% K+ + and 28% Na+ , and (d) I-MMT with . ACS K Paragon Plus Environment 21


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Figure 4: Comparison of the relative basal spacings as a function of the water content for MMTMMT (blue), I-MMT with Na+ only (yellow), I-MMT with 72% K+ and 28% Na+ (black), and ACS Paragon Plus Environment 22 I-MMT with K+ only (red).

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Figure 5: Atomic density profile for the 1W and 2W states. (a,b) MMT-MMT; (c,d) I-MMT with Na+ ; (e,f) I-MMT with 72% K+ and 28% Na+ , and (g,h) I-MMT with K+ , for waterhydrogen (blue), water-Oxygen (red), Na+ (yellow), and K+ (grey).


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Figure 6: Radial distribution functions for Na-OW (left) and Na-OS (right) for (a,b) MMTMMT; (c,d) I-MMT with 72% K+ and 28% Na+ and (e,f) I-MMT with Na+ .


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Figure 7: Radial distribution function of K-OW (left) and K-OS (right) for I-MMT with 72% K+ and 28% Na+ (a and b), and for I-MMT with K+ only (c and d).


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Molecular Dynamics Simulation of Hydration and Swelling of Mixed

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