Article pubs.acs.org/Langmuir
Molecular Simulation Study of Hydrated Na-Rectorite Jinhong Zhou,†,‡ Edo S. Boek,§ Jianxi Zhu,*,‡ Xiancai Lu,*,† Michiel Sprik,∥ and Hongping He‡ †
State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China ‡ Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China § Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom ∥ Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, United Kingdom ABSTRACT: The swelling behavior of clay minerals is an important issue in industrial processes and environmental applications. Mixed-layer clay minerals containing a smectite fraction, such as rectorite, are neglected even though they could swell and exist in nature widely. The hydration of rectorite has not been well comprehended even though they are meaningful to mineralogy and industry. This study combines molecular dynamics (MD) and Monte Carlo (MC) simulations to disclose the swelling behavior of rectorite and compare with montmorillonite. From grand canonical Monte Carlo (GCMC) and MD simulations, we obtain swelling curves and swelling free-energy curves of rectorite with a relative humidity of 100%. With the comparisons of swelling free-energy minima, we find that the bilayer hydrate of Na-rectorite is more thermodynamically stable than the monolayer hydrate, which is similar to Na-montmorillonite. However, the interlayer sodium ions in rectorite show an asymmetrical distribution quite different from the symmetrical distribution in montmorillonite. Because of unequal layer charges between the smectite part and illite part of retorite, sodium ions prefer to distribute close to the illite part surface.
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small-angle neutron scattering (SANS),10 and Fourier transform infrared (FTIR) spectroscopy,11−13 have been employed to investigate clay swelling, only limited microscopic information was obtained. However, computational simulation methods can provide not only atomistic structures of hydrated montmorillonite but also thermodynamic parameters. In the last decades, many studies on smectite swelling using molecular simulations were carried out.14−20 Boek and his colleagues acquired accurate intramoleculer swelling curves of Li+, Na+, and K+-montmorillonite by using Monte Carlo (MC) methods to understand the role of K+ as a swelling inhibitor.21,22 Liu and Lu proposed a thermodynamic explanation of the inhibition of K+ in montmorillonite using molecular dynamics (MD) simulations.23 Brice et al. used MD simulations to study the effects of disordered isomorphic substitutions on the swelling properties and structures of smectites with new clay models. Chang combined MC and MD simulations to investigate Namontmorillonite and calculated the mobility of interlayer species.24 Ferrage et al. and Dazas et al. both combined molecular simulations and XRD data to analyze the hydrate structures of smectite.25−27 Carrier et al. used MD simulations to obtain the elastic properties of swelling montmorillonite and determine the effects caused by different interlayer hydrates.28
INTRODUCTION During many industrial processes, such as petroleum recovery and environmental material developments, swelling clay minerals are an important component determining the material performance.1 In nature, the smectite group is the most common class of swelling clay minerals, including montmorillonite, hectorite, and saponite. Not only pure smectite but also clay minerals containing a fraction of the smectite part may swell, such as illite/smectite and chlorite/smectite interlayered clays. Rectorite is a kind of regularly mixed illite/smectite interlayered clay consisting of illite sheets (nonexpandable) and smectite sheets (expandable) with the order of R1, i.e., along the z axis in an ABABAB sequence.2,3 The alternate combination of the illite part (high charge) and the smectite part (low charge) results in special layer charge characteristics and interlayer properties. As is well known, the hydration and swelling behavior of montmorillonite have been comprehensively studied with many methods including experiments and computational simulations. It is found that exchangeable interlayer cations of smectite determine the swelling process. Systematic research on the hydration and dehydration mechanisms of montmorillonite with different interlayer cations (Na+, Li+, Ca2+, K+, Mg2+, Rb+, Sr2+, ...) was carried out by Cases and his colleagues.4−6 Although novel experimental techniques, such as in situ X-ray diffraction (XRD), inductively coupled plasma-atomic emission microscopy (ICP-AES),7 thermogravimetric analysis (TGA),8,9 © XXXX American Chemical Society
Received: October 1, 2014 Revised: December 5, 2014
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Figure 1. Water contents in equilibrium at different fixed basal spacings of montmorillonite (a) and rectorite (b).
Figure 2. Changes in pressure along the z axis of montmorillonite (a) and rectorite (b) as the basal spacing increases.
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Although ab initio molecular dynamics simulation can offer more refined information about clay swelling,29,30 classical molecular simulations are still suitable for clay swelling studies due to their low computational expense and the development of reliable force fields.31,32 In particular, the grand canonical Monte Carlo (GCMC) method can simulate the adsorption behavior so that it has become popular in clay swelling studies.16,33−35 Compared to smectite, rectorite attracted much less attention in swelling studies. However, illite/smectite mix-layered clays are widely abundant in nature. For example, rectorite and other mixed clays are common in reservoirs of conventional hydrocarbons or unconventional petroleum.36,37 Their swelling properties are always important in oil recovery operations.1 In many cases, the hydration and swelling of mixed clays in soil and quaternary sediments are crucial to the construction of highways, buildings, bridges, dams, and so on. Several experimental studies showed the hydration and dehydration properties of rectorite with different cations.38,39 With the rapid development of rectorite-based composites, industrial applications of rectorite are increasing.40−44 The complicated structure of rectorite makes it more difficult to obtain exact interlayer information by experiments than that of montmorillonite. In this study, we combined MC and MD simulations to investigate the swelling behavior of rectorite and discussed the similarities and differences with montmorillonite.
MODELS AND METHODS
Clay Models. The chemical formula of the model of rectorite in this study is KNa0.5(Mg0.5Al7.5)(AlSi15)O40(OH)8.45 The cation exchange capacity (CEC) of this model is 32.3 mmol/100 gclay. In this model, every unit cell contains 1.5e charge, including 1 tetrahedral charge and 0.5 octahedral charge. Tetrahedral substitutions (Al for Si) are all in the illite part, and octahedral substitutions (Mg for Al) are in the smectite part. Interlayer cations for the illite part are K+ ions, and for the smectite part, they are Na+ ions. This model has been used in our previous study on organic-intercalated rectorite.45 The Wyoming model is used for montmorillonite as a reference system, which has been studied in great detail. The chemical formula of this model is Na0.75(Si7.75Al0.25)(Al3.5Mg0.5)O20(OH)4. Its CEC is 102 mmol/100 gclay, and the layer charge is 0.75e per unit cell with both tetrahedral substitutions (Al for Si) and octahedral substitutions (Mg for Al). The isomorphic substitutions in the clay sheets obey Loewenstein’s rule, i.e., two substitution sites cannot be adjacent. The simulated clay models both consist of two clay platelets of 16 unit cells: 4 in the x dimension, 2 in the y dimension, and 2 in the z dimension. The basal surface area of both clays is 21.12 × 18.28 Å2. The initial basal spacing of dry rectorite is about 20 Å; for montmorillonite this is about 10 Å. Simulation Details. Both MC and MD simulation methods are employed to study the hydrated clay under ambient conditions (1 atm, 298 K). The force field for clay and water is CLAYFF.31 MC simulations are carried out using the TOWHEE package,46 and MD simulations, using the LAMMPS package.47 B
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Figure 3. Swelling free-energy curves of montmorillonite (a) and rectorite (b) under ambient conditions.
Figure 4. Density profiles for the monolayer hydrates of montmorillonite (a) and rectorite (b). For clarity, the density distributions of Na have been doubly enlarged. First, we conduct GCMC simulations48 in the constant (μVT ensemble) for different fixed basal spacings of the clay to obtain the adsorbed amounts of water. The chemical potential (μ) of liquid water is −43.095 kJ/mol49 under ambient conditions (relative humidity, RH = 100%). The electrostatic interaction is treated using the Ewald summation. Every system is run for 20 000 000 steps to reach the adsorption equilibrium, which was verified by checking the number of adsorbed water molecules as a function of time. Then, the final configurations from GCMC are used as initial configurations in the following MD simulations (NVT ensemble), which are all run for 2 ns with a time step of 1 fs. The electrostatic interaction in the MD simulation is also treated using the Ewald summation. In the clay phase, the pressure has only one component normal to the clay surface (z axis). We derived the z-axis pressure and interlayer dynamics information from the last 1 ns MD trajectory. The swelling free energy (F) of a system at a fixed basal spacing has been calculated from integrating the average z-axis pressure via eq 134 ΔF = − LxLy
∫z
z 0
[P(z′) − P app] dz′
Figure 5. Snapshots of equilibrated monolayer hydrates of montmorillonite (a) and rectorite (b). O = Red, H = white, Al = pink, Na = purple, Si = yellow, Mg = green.
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increasing basal spacing for both montmorillonite and rectorite. Both of the curves show a clear plateau at around 40 water molecules in the monolayer hydrate region (from zmont = 12 Å, zrec = 22.25 Å). The shape of the montmorillonite adsorption curve is consistent with previous molecular simulation studies.21,50 In both clays, the number of Na+ ions is constant, six for montmorillonite and four for rectorite. Although the Na+ number is different, the first plateau for both appears at a similar water content (40). This indicates that the formation of
where Lx and Ly are the dimensions of the simulated box along the x and y axes. P(z′) is the pressure corresponding to the basal spacing z′. z0 is the reference basal spacing. Papp is the external pressure applied to the clay surface.
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RESULTS AND DISCUSSION Swelling Curves. We calculate the water contents at different fixed basal spacings by using GCMC simulations (Figure 1). It is found that the water content increases with C
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Figure 6. Density profiles of bilayer hydrates of montmorillonite (a) and rectorite (b). For clarity, the density distributions of Na have been doubly enlarged.
montmorillonite and rectorite.5,38 It is obvious that the first minimum is higher than the second one in both montmorillonite and rectorite, which means that the bilayer hydrate is thermodynamically more stable than the monolayer for both montmorillonite and rectorite. This result is consistent with the experimental results for Na-montmorillonite.5 These two stable state minima are separated by a maximum, which leads to energy barriers for swelling (the difference between the first minimum and maximum). When clay swells from the monolayer hydrate to the bilayer hydrate, it needs to overcome the energy barrier.33 In Figure 3, we can find that the swelling energy barrier of montmorillonite is smaller than that of rectorite. Thus, montmorillonite swells more easily to a bilayer hydrate than rectorite. Interlayer Structures. The density profiles (Figure 4) for the first local energy minimum shows that the interlayer water molecules occur in the middle of the interlayer spaces in both clays and show a sharp peak in water density distribution, i.e., a clear monolayer hydrate structure. As for the second energy minimum (Figure 6), the water distribution has two sharp peaks in both systems, suggesting that a bilayer hydrate structure has formed. The snapshots of the microstructures clearly show monolayer and bilayer hydrate structures (Figures 5 and 7). The observation of local minima corresponding to specific hydrate structures (monolayer and bilayer) is further supported by water density profiles with one peak and twin peaks. Although the swelling curve and water distribution of rectorite are similar to those of montmorillonite, the distribution of sodium ions is clearly different. From the monolayer density profiles (Figure 4), the distribution of Na in montmorillonite exhibits three peaks that are symmetrical, but in rectorite, Na+ ions prefer to distribute close to the surface of the illite part and only show one peak in the distribution curve appears. This can be attributed to the stronger interaction between Na+ ions and the illite sheets due to the higher charge of the illite part compared to that of the smectite part. The Na+ density profile for the bilayer montmorillonite also shows three peaks and maintains a symmetrical distribution, but the middle peak is much higher than that in the monolayer hydrate. It is suggested that Na+ ions tend to escape from the clay surfaces as the water content increases. However, in the bilayer hydrate structure of rectorite, only two peaks appear in the density profile of Na+ ions. One peak close to the illite part is similar to that of the monolayer hydrate but is getting a little
Figure 7. Snapshots of equilibrated bilayer hydrates of montmorillonite (a) and rectorite (b).
the monolayer hydrate is mainly determined by the water content. However, only the basal spacings of the hydrate structures corresponding to energy minima could be thermodynamically stable. Pressure and Swelling Free Energy. To determine stable hydrate structures of montmorillonite and rectorite, we calculate their swelling free-energy curves (Figure 3) from zpressure (pressure along the z axis) curves (Figure 2). Compared to the z-pressure and swelling free-energy curves of Na-montmorillonite from Smith,51 our results (Figures 2(a) and 3(a)) show similar trends, which validates our simulations and lends credibility to our results for rectorite. The pressures normal to the clay surfaces vary with the interlayer spacings in both montmorillonite and rectorite (Figure 2). Swelling freeenergy curves are calculated from z-pressure curves via eq 1 (Figure 3). The local minimum of the swelling free energy indicates that the corresponding hydration structure is stable. Figure 3(a) shows two local minima for montmorillonite at 12 and 15 Å, while two local minima occur for rectorite at 22.25 and 25 Å (Figure 3(b)). According to the equilibrated configurations, the first minimum is a monolayer hydrate structure, and the second one is a bilayer hydrate structure. The results agree well with the experimental spacing of montmorillonite hydrates (normally a monolayer in the 12−12.5 Å range, with a bilayer at about 15.5 Å)5 and rectorite hydrates (monolayer at about 22.5 Å, bilayer at about 25 Å).38 Both of the obtained basal spacings of the monolayer and bilayer agree well with previous experimental studies within 0.5 Å for both D
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Langmuir weaker. In rectorite, the interactions between Na+ and the illite part of the clay are stronger than that between Na+ and the smectite part. Therefore, part of the Na+ ions are attracted to the positions near the illite sheets by electrostatic interactions while the other ions move to the middle of the interlayer space. Thus, the density distribution of Na+ ions in rectorite is asymmetrical for both monolayer and bilayer hydrates.
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CONCLUSIONS We find that rectorite has similar swelling behavior to montmorillonite. With water content increasing, monolayer and bilayer hydrate structures are sequentially formed in the interlayer spaces of both montmorillonite and rectorite. As the calculated swelling free energies show, in ambient condition with RH = 100% the bilayer hydrate is more stable than the monolayer hydrate, but montmorillonite swells more easily from monolayer to bilayer hydrate than rectorite due to a lower energy barrier. In the equilibrated configurations, interlayer Na+ ions present a symmetrical distribution in montmorillonite in both monolayer and bilayer hydrates. However, the Na+ ions in rectorite show an asymmetrical distribution different from that in montmorillonite, which can be attributed to the stronger electrostatic interactions of the illite sheet compared to those of the smectite sheet.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by grants from the Knowledge Innovation Program of the Chinese Academy of Sciences (KZCX2-EW-QN101), the National Science Foundation of China (nos. 40972034, 41002013, and 41425009), and the Natural Science Foundation of Jiangsu Province (BK2010008). We are grateful to the High Performance Computing Center of Nanjing University for doing the calculations in this article on its IBM Blade cluster system.
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