Nucleation of Salt Crystals in Clay Minerals: Molecular Dynamics

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Letter

Nucleation of Salt Crystals in Clay Minerals: Molecular Dynamics Simulation Hassan Dashtian, Haimeng Wang, and Muhammad Sahimi J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01306 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on June 26, 2017

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The Journal of Physical Chemistry Letters

Nucleation of Salt Crystals in Clay Minerals: Molecular Dynamics Simulation Hassan Dashtian, Haimeng Wang, Muhammad Sahimi Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-1211, USA ABSTRACT Nucleation of salt crystals in confined media occurs in many processes of high importance, such as injection of CO2 in geological formations for its sequestration. In particular, salt precipitation in clays, a main component of sedimentary rock, is an important phenomenon. The crystals precipitate on the pores’ surface, modify the pore space morphology, and reduce its flow and transport properties. Despite numerous efforts to understand the mechanisms of nucleation of salt crystals in confined media, the effect of the clay’s chemistry on the growth, distribution and properties of the crystals is not well understood. We report the results of extensive molecular dynamics simulation of nucleation and growth of NaCl crystals in a clay pore using molecular models of two types of clay minerals, the Na-montmorillonite and kaolinite. Clear evidence is presented for the nucleation of the salt crystals that indicates that the molecular structure of clay minerals affects their spatial distribution, although the nucleation mechanism is the same in both types of clays.

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Graphical Abstract Kaolinite

Nucleated NaCl

Na-MMT

Nucleated NaCl


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Nucleation and growth of salt crystals from supersaturated solutions by evaporation of water is a common phenomenon in many natural systems, both on the surface1,2 of Earth and in porous geological formations.3 For example, salt weathering damages porous rock and the soil profile.4 While geologic sequestration of CO2 is a promising way of mitigating global warming, salt precipitation, both near and far from the injection wells, hampers flow and transport of CO2 in the porous formations, as it leads to pressure build up and reduction in the permeability and porosity, and ultimately the capacity of the formations for holding CO2 . Deep insight into crystal nucleation and growth in the pores, and more generally in confined media, is critical to controlling such processes and avoiding salt weathering. The problem is more severe when the geological formation contains significant amount of clay minerals. Surface chemistry at the clay-brine interface plays a fundamental role in a wide range of phenomena that occur in geological formations, including solute transport and salt precipitation. Clay minerals consist of parallel tetrahedral and octahedral structures, and are further divided into subgroups that are distinguished based on the type of the isomorphic cation substitution that they contain. Kaolinite, illite, vermiculite, and smectite are the four major clay groups.5 Clay particles are phyllosilicates that consist of numerous combinations of bonded tetrahedral and octahedral sheets stacked together. Silicon-oxygen tetrahedrons, SiO4− 4 are linked together by sharing the basal oxygen atoms and form a tetrahedral sheet, referred to as the T layer. The layers do not exist by themselves. Instead, they are always bonded to octahedral sheets, the O layers that are formed from aluminum-oxygen octahedrons, AlO5− 4 , with shared apical and basal oxygen. Hydrogen bonding bridges the T and/or O sheets. Thus, one may classify clay minerals into subgroups that are distinguished based on the way the T and O sheets are stacked together. The compositions of the T and O layers define the overall charge of the clay layers, which can be zero or negative. In the latter case the charge is balanced by introducing such cations as K+ and Na+ into the interlayer space (clay pore) between the clay particles. The 1:1 clay particles, such as kaolinite, are formed by binding of one T and one O sheet. Their structure does not contain additional cations and, thus, they are electrically neutral. The 2:1 clay particles are formed by bonding of two T and one O sheets. Due to isomorphic substitution in their structure, they have a net negative charge on their surfaces. Such clays, ACS Paragon Plus Environment

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including the well-known montmorillonites (MMT), also have high surface area and adjustable spacing between their layers that contribute to their swelling and/or shrinkage and reactions. The differences in the chemical compositions and structures of clay minerals determine they diverse behavior under similar conditions. For example, clay-water interaction is completely different for kaolinite and the MMT. Whereas kaolinite exhibits little or no swelling on hydration, the MMTs swell considerably when water is added to their pore space. They also differ in the degree of isomorphous replacements in their structure, as well as in the amount and nature of their associated exchangeable cations. Clay minerals, water and sodium chloride, the most abundant salt on Earth, co-exist everywhere. Although recent theoretical and experimental studies6,7 have provided valuable insights into the mechanisms of nucleation and growth of NaCl crystals out of brine in the presence of clay minerals, many questions remain unresolved. Given the significant role that salt nucleation on clay particles plays in nature, gaining deep understanding of the phenomenon is of great importance. Experimental studies always face difficulties associated with the small size of the clay particles and their mixing with other minerals. On the other hand, fundamental, molecular-scale understanding of clays and their properties can be obtained by molecular dynamics (MD) simulation, if accurate force fields (FFs) are available. The utility of MD simulation is manifested by the fact that water near the surface - the bound water - and its properties are completely different from those under bulk conditions, leading to the so-called dual water model. Macroscopic theories cannot differentiate between the two types of water and, hence, are incapable of describing accurately the process that occur there. It is, of course, imperative to include the interactions of all the atoms and molecules in any model of clays, if accurate MD simulation is to be carried out.8 This requires very accurate FFs. The clay force field, CLAYFF,9 which was developed based on the metal-oxygen interactions connected with hydrated phases, is one such accurate FF for MD simulation that we utilize in the present Letter. In this Letter we report on extensive MD simulation of evaporation of brine in clay minerals, and nucleation of salt crystals. We demonstrate that Na+ and Cl− ions prefer to be absorbed, and then precipitate, on specific sites of clay surfaces, hence affecting the spatial ordering of the nucleated crystals. Macroscopic crystals may form from the initial nuclei with only a few atoms. The growth of the nuclei affects the solution’s thermodynamics. Although capturing ACS Paragon Plus Environment

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such mechanisms by experiment is difficult, MD studies provide clear indications for them. In addition, MD simulations allow one to study pure systems, whereas experimental systems may contain impurities. As the model clay particles we use kaolinite and the Na-MMTs, for molecular modeling of which we have extensive experience.10−12 The Na-MMT model is based on a triclinic pyrophillite structure,13 which has been used widely in clay-related studies. The chemical structure of the MMT is characterized by random isomorphic substitution of Al by Mg atoms. In the present study the chemical composition of the MMT unit cell was set to be13 Na3 [Si31 Al1 ][Al14 Mg2 ]O80 (OH)16 . ˚3 . The unit cell was replicated six times The size of the unit cell was 20.92 × 18.12 × 12.50 A along the x and y axes and twice along the z axis (see Figure 1), in order to obtain a super cell with a size of 125.52 × 108.72 × 25.00 ˚ A3 . The same approach was used to construct a model kaolinite,5,14 for which the chemical formula is Al2 Si2 O5 (OH)4 and the size of its super cell was 123.68 × 107.30 × 14.78 ˚ A3 . The structures of the MMT and kaolinite are shown in Figure 1. Note the differences in the thickness of two types of clay. Clay minerals swell and/or shrink when they come into contact with water, either pure or saline, which may influence the nucleation and growth of salt crystals. Here, we focus on the effect of the clay types on the nucleation of salt crystals. As mentioned earlier, CLAYFF was used for generating the molecular structures of both clays. The FF contains a single bond-stretching parameter for the structural O-H groups. As a smectite particle, the MMTs hold a net negative structural charge, due to isomorphic substitution by elements of lower valences in the tetrahedral and octahedral sheets.15 The charge deficiency caused by the isomorphic substitution of Mg is compensated by inserting four Na+ ions in the structure. According to CLAYFF the total energy E of the material is given by,9,15 

E=

X bonds

k1 (rij − r0 )2 +

X angle

k2 (θijk − θ0 )2 +

X ij

σij ij  rij

!12

σij −2 rij

!6  +

e2 X qi qj . (1) 4πε0 i,j rij

Here, k1 and k2 are two 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 Lennard-Jones (LJ) energy and size parameters, and the Lorentz-Berthelot mixing rules, √ σij = (σi + σj )/2 and ij = i j were used for the pairs ij. Sodium chloride was used to ACS Paragon Plus Environment

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represent salt, with its molecular structure and parameters given by CLAYFF. After some preliminary simulations, the cutoff distance for the LJ interactions was set at 12 ˚ A. Water was represented by the SPC/E model. The electrostatic interactions were computed using the particle mesh Ewald summation method. The simulations were carried out in the (N P T ) ensemble at 1 atmosphere and 298 K. Pressure and temperature were held constant using, respectively, the Andersen16 and NoséHoover algorithms.17,18 The masslike damping parameter for the Andersen barostat was 100 amu. For the Nosé-Hoover thermostat the parameter that we used to scale the fictitious mass was 1. The time step was always 0.5 fs. In the Supplementary Information all the parameters of the FF are given. We consider a slit pore that consists of two parallel kaolinite or MMT sheets. In both cases 16128 water molecules were used in the simulation cell. The number of NaCl molecules were 1576 in the Na-MMT to 1584 in koalininte, which is equivalent to 30.9 NaCl weight percent in the solution, representing the saturation limit of salt in water at 298 K. This concentration is slightly above the crystallization limit for NaCl at 298 K and 1 atm under which the previous experiments had been carried out. The salt concentration was set at a relatively high value because it facilitates nucleation of salt crystals during the MD simulations. Note that depending on the precise values of the molecular parameters of water molecules and the interaction parameters, the crystallization condition slightly varies, as reported in some MD simulation studies.19,20 Alejandre and Hansen21 and Mendoza and Alejandre22 investigated the sensitivity of the solubility limit of NaCl in water to the NaCl model parameters. The MD simulations were carried out using the LAMMPS package.23 The initial spatial distribution of the water molecules and the salt ions was obtained by minimizing the total energy of the system that consisted of the water molecules, the salt ions and the clay pore and plates. The system was then equillibrated for 2 ns, followed by a production run of up to 70 ns. Water molecules were removed from the simulation cell at a very slow rate24 to simulate evaporation, as the nucleation and crystallization occur at higher salt concentrations than the initial value. Each time a water molecule was removed, the system was allowed to reach equilibrium. Our simulations indicated that, so long as the evaporation rate is small, which is the case in our simulations, it has a minor effect on the nucleation mechanism and growth of salt crystals. We used three slightly different rate of evaporation, namely, 168, 161 ACS Paragon Plus Environment

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and 155 water molecules/ns, which are very small. Of course, if evaporation is rapid, then non-equilibrium effects become important, but this limit is beyond the scope of our present work. Figure 2 shows the top view of the nucleation and growth of NaCl crystals in the interlayer space of kaolinite and Na-MMT, as they evolve with time. For clarity the water molecules and clay sheets are not shown. In the case of Na-MMT, there is no distinct order among the ions at early times, less than 3 ns. After about 5 ns, the local ion density around some particular sites increases, but there is still no distinguishable order amongst Na+ and Cl− . In addition, by this time some water molecules have left the interlayer region. After about 7 ns, some initial small nuclei of crystals have formed, but they are still not stable. After about 10 ns larger crystals have formed and the central part of the system appears to be stable. At the same time, the outer surface of the nuclei indicates some sort of disorder, indicating that the structure there is not stable yet, but the inner part includes parallel arrangements of Cl− and Na+ . The final stage is marked by stable crystals filling some portions of the interlayer space of the clay, on the one hand, and some empty regions with essentially zero ion density, on the other hand. Note that since the Na-MMT has extra Na+ in its structure, they are attached to the clay or the salt crystal lattice. Qualitatively, the same sort of phenomena is seen in the kaolinite system, except that the time scales and the spatial distributions of the crystals are different; we will return to the kaolinite case shortly. Our simulations indicate that crystallization in both the MMT and kaolinite follow qualitatively the well-known two step mechanism.25−28 First, fluctuations in the concentration form several regions with local ion concentrations higher than the rest, followed by a second step that includes nucleation and development of spatial ordering of the ions. The salt concentration fluctuations in the interlayer space lead to spatial inhomogeneity of brine, with nucleation of ions expected to be in the regions with higher concentrations. The regions that are formed in the first step are unstable, so that only a few of them will eventually produce stable crystals. One may be tempted to describe the crystal nucleation based on the classical nucleation theory (CNT) that consist of a single step. The CNT has, however, been challenged in recent experimental29−32 and simulation28 studies. According to the CNT crystallization involves a single free energy barrier, and the initial state of nucleation is a simplified version of the final, stable crystal structure. Recent studies have shown, however, that the phase diagrams of sysACS Paragon Plus Environment

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tems experiencing nucleation and crystallization involve a phase transition region between two stable disordered states with distinct concentration densities, implying that before formation of stable crystal, the system transitions to an intermediate state in which the ions form disordered aggregates.26 Such observations are usually described based on the Ostwald rule,33,34 according to which the free energy of the initial emerging nucleation in a solution is closer to that of the solution, rather than that of the salt crystals, and that the initial nuclei eventually represent the final stable crystals after going through the stages. Chakraborty and Patey28 used large-scale MD simulation and showed that nucleation and crystallization of NaCl solution also follows the two-step mechanism. As Figure 2 indicates, our simulation too indicates that, initially, small size aggregates or clusters of ions form, and their number increases with time. Initially, the solution tends to form several small nuclei because35 it is entropically more favorable for such systems as the NaCl solution plus the clay minerals. Then, the small clusters combine gradually and form larger aggregates that are referred to as the critical nuclei. The Gibbs free energy of a LJ system approaches its maximum when the the first critical nucleus appears in the system.35 For kaolinite, the birth time of the crystal nucleation is longer than the MMT clay. The birth time and location of the first nucleation site have important effect on the subsequent crystal growth.36 Moreover, as Figure 2 indicates, the structure of the two systems are different. One obvious reason is the extra Na+ in the MMT system that increase the chances of Cl− and Na+ interactions, on the one hand, and can act as seeds to initialize nucleation, on the other hand. In fact, seeding the solution with a crystal cluster has been used in several MD simulations of nucleation and crystallization,37−41 including those by Espinosa et al.42 and Zimmermann et al.36 for nucleation and crystallization of NaCl solution. In Figure 3 we show the dynamic evolution of the radial distribution function (RDF) g(r). They represent averaged distributions, with the average taken over all the ions in the interlayer space. The initial RDFs for kaolinite and Na-MMT are, of course, similar, because there is no cluster in the two systems and the salt concentrations in the two are very close. As the water leaves the system and nucleation proceeds, the RDF peaks become more evident. This is due to the fact that the range of ion-ion correlation increases as salt crystals grow, and a larger lattice of Cl− and Na+ is formed. For example, after 30 ns sharper peaks are evident that correspond to the small patches of ordered ions. At still longer times the long-ranged structural ACS Paragon Plus Environment

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peaks become even sharper. Similar RDFs of NaCl systems, computed by MD simuation, have been reported elsewhere.43,44 The difference in the shape of the final RDFs for kaolinite and Na-MMT arises from the distinct distribution of the nucleation centers in the two systems. The spatial arrangement of the ions and water in the interlayer space is an important factor in nucleation and growth of the salt crystals. The ions and water molecules bind to the surface of the clay and form two major sorption sites in the interlayer space between the two clay sheets. The area (site) close to the surface of clay is called the inner sphere (at a distance of 2.5-3.0 ˚ A from the surface), while the one farther away from the clay surface is called the outer sphere (at distances of about 2.0-2.5 ˚ A beyond those of the inner spheres). In the former the electrostatic forces hold the ions and water molecules attached to the clay surface. The water molecules surround the ions in the interlayer space and form hydration shells. This prevents direct contact of ions in the outer sphere with the clay. As Figure 1 indicates, the interlayer space of kaolinite contains two different surfaces. The Na+ ions migrate close to the AlO6 octahedral surface and are paired with Cl− , leading to adsorption of Na+ on the octahedral basal surface. The number of such ions are, however, small when compared with the total number of ions in the interlayer space. In addition, Na+ forms the outer sphere adsorption complexes on the tetrahedral SiO4 surface. As disussed by Vasconcelos,45 , the mobility of Na+ parallel to the clay surface is higher than the other ions, such as Cs2+ or Pb2+ .44 In addition, octahedral surface (AlO6 ) attracts Cl− ions and forms inner sphere complexes. This is clear in the narrow peak of the Cl− density profile, shown in Figure 4(a). That side of kaolinite is slightly positively charged and, hence, attracts Cl− ions. However, the opposite tetrahedral SiO4 surface has a small negative charge, because Si-bridging oxygen atoms relax outward slightly with respect to the Si atoms. Therefore, the surface repels Cl− ions. On the other hand, as Figure 1 indicates, the interlayer space of the Na-MMT clay includes two surfaces, but with identical structures. Each layer is formed by two tatrahedral (T) silicate sheets surrounding one octahedral (O) aluminum sheet, giving rise to the aforementioned T-OT structure. Therefore, both surfaces in the interlayer space are of the tetrahedral (SiO4 ) type. As Figure 4(b) shows, the density profiles of the species in the interlayer are symmetrical. As discussed earlier, the SiO4 surface repels the Cl− ion. Hence, they form peaks at the midplane of the interlayer. Similar to the kaolinite tetrahedral surface, Na+ ions form the outer sphere ACS Paragon Plus Environment

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adsorption complexes near each clay particle surface that includes two maxima. It is due to such diverse arrangements and adsorption of ions in kaolinite and the Na-MMT interlayer space that distinct nucleation and crystallization patterns emerge in the two types of clay particles. Indeed, as discussed earlier, the initial step of nucleation is the emergence of high ion concentration region. The interlayer space of kaolinite contains one region in which the total concentration of Na+ and Cl− exceeds significantly that of the bulk, whereas the Na-MMT has two of such regions with highest ion concentration. The density profiles of Na+ and Cl− together, shown in Figure 4, clearly indicate this. Another way of looking at the evolution of the system is through the time-dependence of the mean-square displacements (MSDs) of the ions, as nucleation of the crystals is approached. They were calculated and averaged over all the ions with respect to a reference position. In Figure 5 we show the resulting MSDs for both clay types. In both cases the MSDs increase over time, albeit with different rates for two two types of clay, but they approach saturation, hence indicating that the ions eventually stop moving since nucleation has commenced and salt crystals have begun to form. The rate of approach to the saturation limit is higher in the Na-MMT case, indicating that the spatial ordering of the nucleation in kaolinite under the conditions that we study is not as stable as that in the Na-MMT. As discussed earlier, the competition for the adsorption sites in kaolinite and the tendency of ions to form nucleation centers are responsible for the phenomenon. Summarizing, we carried out extensive MD simulation to study nucleation and growth of salt crystals in the nanopore space of clay minerals. Two types of clay minerals with distinct structures were studied, namely, kaolinite and Na-montmorillonite. We provided clear evidence that crystal nucleation occurs in the interlayer space of the clay particles that is initially filled by oversaturated brine, and that it follows a two-step mechanism. The rate of nucleation in the Na-MMT particles is higher than in kaolinite, which is due to isomorphic substitution in the structures of the Na-MMT clay particles pointed out earlier. AUTHOR INFORMATION Corresponding Author †

E-mail: [email protected]. Phone: (213) 740-2064 ACS Paragon Plus Environment

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ACKNOWLEDGMENTS 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 of the University of Southern California High-Performance Computing Center, with each node having 16 processors. Notes The authors declare no competing financial interest.


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(22) Mendoza, F. N.; Alejandre, J. The Role of Ion-Water Interactions in the Solubility of Ionic Solutions. J. Mol. Liq. 2012, 185, 50-55. (23) Plimpton, S.J. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comp. Phys. 1995, 117, 1-19. (24) Mucha, M.; Jungwirth, P. Salt Crystallization from an Evaporating Aqueous Solution by Molecular Dynamics Simulations, J. Phys. Chem. B 2003, 107, 8271-8274. (25) An, S.; Li, J.; Li, Y.; Li, S.; Wang, Q.; Liu, B. Two-Step Crystal Growth Mechanism During Crystallization of an Undercooled Ni50 Al50 Alloy. Sci. Rep. 2016, 6, 31062. (26) Lupi, L.; Peters, B.; Molinero, V. Pre-ordering of Interfacial Water in the Pathway of Heterogeneous Ice Nucleation does not Lead to a Two-Step Crystallization Mechanism. J. Chem. Phys. 2016, 145, 211910. (27) Vekilov, P. G. Two-Step Mechanism for the Nucleation of Crystals from Solution. J. Cryst. Growth 2005, 275, 65-76. (28) Chakraborty, D., Patey, G. N. How Crystals Nucleate and Grow in Aqueous NaCl Solution. J. Phys. Chem. Lett. 2013, 4, 573-578 (29) Billinge, S. J. L. How Do Your Crystals Grow? Nature Phys. 2009, 5, 13-14 (30) Lutsko, J. F.; and Nicolis, G. Theoretical Evidence for a Dense Fluid Precursor to Crystallization. Phys. Rev. Lett. 2006, 96, 046102. (31) Chung, S. Y.; Kim, Y. M; Kim, J. G.; Kim, Y. J. Multiphase Transformation and Ostwald’s Rule of Stages during Crystallization of a Metal Phosphate. Nat. Phys. 2009, 5, 68-73. (32) Zhang, T. H.; Liu, X. Y. Multistep Crystal Nucleation: A Kinetic Study Based on Colloidal Crystallization. J. Phys. Chem. B 2007, 111, 14001-14005. (33) Ostwald, W. Studien uber die Bildung und Umwandlung Fester Korper Z. Phys. Chem. 1897, 22, 289-330.


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(34) Threlfall, T. Structural and Thermodynamic Explanations of Ostwald’s Rule. Org. Process Res. Dev. 2003, 7, 1017-1027. (35) ten Wolde, P. R.; Ruiz-Montero, M. J.; Frenkel, D. Numerical Evidence for BCC Ordering at the Surface of a Critical FCC nucleus. Phys. Rev. Lett. 1995, 75, 2714. (36) Zimmermann, N. E. R. ; Vorselaars, B.; Quigley, D.; Peters, B. Nucleation of NaCl from Aqueous Solution: Critical Sizes, Ion-Attachment Kinetics, and Rates. J. Am. Chem. Soc. 2015, 137, 13352. (37) Bai, X. M.; Li, M. Calculation of Solid-Liquid Interfacial Free Energy: A Classical Nucleation Theory Based Approach. J. Chem. Phys. 2006, 124(12), 124707. (38) Knott, B. C.; Molinero, V.; Doherty, M. F.; Peters, B. Homogeneous Nucleation of Methane Hydrates: Unrealistic under Realistic Conditions. J. Am. Chem. Soc. 2012, 134, 19544-19547. (39) Pereyra, R. G.; Szleifer, I.; Carignano, M. A. Temperature Dependence of Ice Critical Nucleus Size. J. Chem. Phys. 2011, 135, 034508. (40) Sanz, E.; Vega, C.; Espinosa, J. R.; Caballero-Bernal, R.; Abascal, J. L. F.; Valeriani, C. Homogeneous Ice Nucleation at Moderate Supercooling from Molecular Simulation. J. Am. Chem. Soc. 2013, 135, 15008-15017. (41) Espinosa, J. R.; Vega, C.; Valeriani, C.; Sanz, E. Seeding Approach to Crystal Nucleation. J. Chem. Phys. 2016, 144, 034501. (42) Espinosa, J.R.; Vega, C.; Valeriani, C.; Sanz, E. The Crystal-Fluid Interfacial Free Energy and Nucleation Rate of NaCl from Different Simulation Methods. J. Chem. Phys. 2015, 142, 194709. (43) Galamba N.; de Castro, C. N.; Ely, J. F. Shear Viscosity of Molten Alkali Halides from Equilibrium and Non-equilibrium Molecular-Dynamics Simulations. J. Chem. Phys. 2005, 120, 224501.


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Captions Figure 1. (a) and (c) show the kaolinite molecular structure, while (b) and (d) depict that of Na-MMT clays. Water molecules in (a) and (b) are shown by sticks. Colors represent oxygen (red), hydrogen (white), Si (yellow), Mg [green, only in (b)], Al [pink in (a) and (b)] and [green in (c) and (d)], Na (purple), and Cl (green; very small). Figure 2. Top view of the positions of Na+ (pink) and Cl− (yellow) within the interlayer space of kaolinite and Na-MMT. Time increases from (a) to (d). (a) Early positions (after 0.1 ns) of the ions after the system reached equilibrium. (b) The initial spatial ordering of the ions after 7 ns for kaolinite and 6 ns for Na-MMT. (c) and (d) show the nucleation steps and spatial ordering after 30 and 70 ns for, respectively, kaolinite and Na-MMT. Colors show Na+ (pink) and Cl− (yellow). Figure 3. Evolution of the radial distribution function (RDF), indicating nucleation and spatial ordering of the ions in the interlayer space of kaolinite (a)-(d) and Na-MMT (e)-(h). (a) and (e) show the early-stage (< 1 ns) RDF of the ions. Figure 4. Initial density profiles of the salt ions and oxygen in water along the z axis (perpendicular to the clay planes) for salt concentration of 30.9% weiht. z < 6 represents locations inside the lower clay sheet. Figure 5. Evolution with time of the mean-square displacements (MSD) of the ions in the interlayer space.


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(a)

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(b)

Z

(c)

(d)

X

Y

Figure 1: (a) and (c) show the kaolinite molecular structure, while (b) and (d) depict that of Na-MMT clays. Water molecules in (a) and (b) are shown by sticks. Colors represent oxygen (red), hydrogen (white), Si (yellow), Mg [green, only in (b)], Al [pink in (a) and (b)] and [green in (c) and (d)], Na (purple), and Cl (green; very small).


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(a)

(b)

(c)

(d)

(a)

(b)

(c)

(d)

Kaolinite

Na-MMT

Figure 2: Top view of the positions of Na+ (pink) and Cl− (yellow) within the interlayer space of kaolinite and Na-MMT. Time increases from (a) to (d). (a) Early positions (after 0.1 ns) of the ions after the system reached equilibrium. (b) The initial spatial ordering of the ions after 7 ns for kaolinite and 6 ns for Na-MMT. (c) and (d) show the nucleation steps and spatial ordering after 30 and 70 ns for, respectively, kaolinite and Na-MMT. Colors show Na+ (pink) and Cl− (yellow).


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0.5

0.5 (a)

0.5 (b)

0.5 (c)

(d)

0.3

0.3

0.3

0.3 g(r)

0.4

g(r)

0.4

g(r)

0.4

g(r)

0.4

0.2

0.2

0.2

0.2

0.1

0.1

0.1

0.1

0

5

0

10 15 r (nm)

(e)

5

0

10 15 r (nm)

5

0

10 15 r (nm)

(g)

(f)

0.6

0.5

0.5

0.5

0.5

0.4

0.4

0.4

0.4

g(r)

0.6

g(r)

0.6

0.3

0.3

0.3

0.2

0.2

0.2

0.2

0.1

0.1

0.1

0.1

0

5

10 15 r (nm)

0

5

0

10 15 r (nm)

5

10 15 r (nm)

(h)

0.6

g(r)

g(r)

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0.3

5

10 15 r (nm)

0

5

10 15 r (nm)

Figure 3: Evolution of the radial distribution function (RDF), indicating nucleation and spatial ordering of the ions in the interlayer space of kaolinite (a)-(d) and Na-MMT (e)-(h). (a) and (e) show the early-stage (< 1 ns) RDF of the ions.


20

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0.06 Kaolinite

−

Cl

Density (A−3)

0.05

Na+ O

0.04

−

+

Cl + Na 0.03 0.02 0.01 0

2

4

6

8 10 Z (A)

12

14

16

0.08 Na−MMT

Cl− +

Na O

0.06 Density (A−3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


−

+

4

6

Cl + Na 0.04

0.02

0

2

8

10 Z (A)

12

14

16

18

Figure 4: Initial density profiles of the salt ions and oxygen in water along the z axis (perpendicular to the clay planes) for salt concentration of 30.9% weiht. z < 6 represents locations inside the lower clay sheet.


21


9 8 Kaolinite

7 6 MSD (A)

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5 4 3 Na−MMT 2 1 0 0

10

20

30

40

50

60

70

Time (ns)

Figure 5: EEvolution with time of the mean-square displacements (MSD) of the ions in the interlayer space.


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Nucleation of Salt Crystals in Clay Minerals: Molecular Dynamics

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