Journal of Colloid and Interface Science 454 (2015) 1–7

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Influence of interlayer cations on organic intercalation of montmorillonite Limei Wu, Libing Liao ⇑, Guocheng Lv ⇑ Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 30 January 2015 Accepted 12 April 2015 Available online 5 May 2015 Keywords: Montmorillonite Interlayer cations Organic intercalation Molecular simulation

a b s t r a c t The influence of the types of interlayer cations on organic intercalation of montmorillonite (Mt) was studied in this paper. The distribution of Na+, K+, Mg2+, Ca2+ and Fe3+ in montmorillonite interlayer, their interaction with structure layers and the effect of interlayer cations on the basal spacing of Mt, the amount of binding water for different interlayer cations and the binding force between them were investigated systematically. 1-Hexadecy1-3-methylimidazolium chloride monohydrate (C16mimCl) was intercalated into montmorillonites with different interlayer cations. The influence of interlayer cations on organic intercalation was investigated. Molecular dynamics (MD) modeling was used to speculate the interlayer microstructures of the organically intercalated Mt with different interlayer cations. These simulations help to predict the microstructure of organo-Mt and guide their relevant engineering applications. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction Montmorillonites (Mts) are clay minerals with regular layer structure composed of octahedral sheet between two tetrahedral

⇑ Corresponding authors. E-mail addresses: [email protected] (L. Liao), [email protected] (G. Lv). http://dx.doi.org/10.1016/j.jcis.2015.04.021 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.

sheets. Intercalating organic and inorganic species into layered host materials has drawn researchers’ increasing interests. It is an effective way to construct ordered inorganic–organic and inorganic–inorganic assemblies with unique microstructures and properties [1,2]. The Mts have attractive features, such as large surface area, swelling behavior, adsorption and ion exchange properties [3,4]. The Mts have been extensively used as the host materials

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and the other possible applications such as catalysis, separation, delivery and storage [5–11] because of their attractive properties and ability to accommodate various kinds of organic and inorganic guest species [12]. The intercalated nano-sized Mt particles (dimension of a single layer is 0.6–1 nm) are ideal dispersing additives for the synthesis of organic–inorganic hybrid nano-composites to enhance mechanical, thermal and chemical stabilities. The composites have been applied into the fields of plastic, paint, petroleum engineering, etc. [12–19]. Most of these Mt-based materials take advantages of their specific laminar structures in which a variety of ions, molecules and polymers can be adsorbed [20–24]. These properties can be affected directly by the types of cations in the interlayer of Mt. Interlayer hydrated cations are mainly stabilized by electrostatic interaction, and they could be exchanged by other cations in solution. The species of interlayer hydrated cations would affect interlayer spacing, surface area, adsorption, swelling, dispersion, and crystal stability and other properties of Mt. The common cations in the interlayer of Mt are Li+, Na+, K+, Cs+, Mg2+, Ca2+, Fe3+, etc. Certain difference in properties can be expected in Mt with different interlayer cations. Recently, utilization of computer simulation on study of clay material like Mt has attracted more and more interest due to its possibility in providing micro-view on structure and properties and layed ground for experimental work. Cations in the interlayer of Mt, such as Li+, Na+ and K+, are hydrated, so the spacing of Mt is actually a function of water content. With different types of interlayer cations, their distribution, hydration and diffusion properties in the interlayer of Mt are different [25–27]. For example, swelling property of Mt depends on the size and charge of the interlayer cations. The behaviors of Li+, Na+ and K+ in the interlayer of Mt are significantly different, especially for their interaction with the sheet. Li+ and Na+ hydrate more easily than K+, so they can be separated from the silicon oxygen tetrahedral surface of Mt. On the other hand, K+ is close to Mt’s layer sheet and bound to the surface of silicon oxygen tetrahedra. So far, most of the studies on Mt have been focused on the hydration of interlayer cations, interlayer structure and swelling property, etc. Few have been done to reveal the relationship between organic intercalation and the type of interlayer cations. Molecular modeling recently has been recognized as an efficient method for understanding the interlayer microstructure of clay minerals [28–33]. In molecular dynamic simulation, energy of the system is an important parameter to represent the stability of montmorillonite with different interlayer cations and their difficulty in ion exchange. A higher total energy of interaction means a better distribution in the interlayer space and more combining water. However, lower electrostatic energy will leading to weaker interaction with structure sheet, more easily exchanged by other cations. In montmorillonite, the energy involved is mainly non-bonded energy, including mostly Van der Waals energy and some other energie like coulomb’s force, angle torsion interaction and bond stretching. During the molecular dynamic simulation, except for slightly increase in energy caused by structure distortion, all the other bonding energy and non-bonding energy decreased in varying degrees, especially for bonding energy and Van der Waals energy. In the present paper, we investigated in details the nature of the exchangeable interlayer cations (Na+, K+, Mg2+, Ca2+ and Fe3+) in montmorillonite and its influence on organic intercalation. Properties of montmorillonite with different interlayer cations were studied systematically. The content of binding water molecules for interlayer cations and the force between them, the distribution of cations in the interlayer and the interaction between interlayer cations and structure layers were investigated from a microcosmic view.

2. Materials and methods 2.1. Materials The montmorillonite(Mt) used was obtained from the Clay Mineral Repositories in Purdue University (West Lafayette, IN) without further purification. It has a chemical formula of (Ca0.12 Na0.32 K0.05)[Al3.01 Fe(III)0.41 Mg0.54][Si7.98 Al0.02]O20(OH)4, a Cation exchange capacity (CEC) of 85 ± 3 mmolc/100 g [34], a layer charge of 0.32 eq/mol per (Si,Al)4O10 [35], an external surface area (ESA) of 23 m2/g [36], and a mean particle size of 3.2 lm with a d25 to d75 in the range of 3–10 lm. 2.2. Methods The homoionic Mt samples were generated from exchanging the interlayer cations of the Mt with Na+, K+, Ca2+, Mg2+, and Fe3+ to produce, Na-Mt, K-Mt, Mg-Mt, Ca-Mt, and Fe-Mt. Weigh a certain amount of Mt, mix them with water to from slurries. Weigh NaCl, KCl, CaCl2, MgCl2, and FeCl3H2O at 3 times of Mt CEC and dissolve them in water. Mix the slurry and the solution separately, and then stir the mixture at 60 °C, hooding for 30 min. The supernatant was rejected and the clay mineral was washed repeatedly until the chloride level was not measurable. The initial 1-Hexadecy1–3-methylimidazolium chloride monohydrate (C16mimCl) concentrations varied from 10 to 5000 mg/L for the adsorption isotherm study. The mass of Mt used was 0.2 g while the volume of solution used was 25 mL for all studies except the kinetic study. The solid and solution were combined in each 50 ml centrifuge tube and shaken for 2 h at 150 rpm and room temperature for all studies except the kinetic study. After the mixtures were centrifuged at 10,000 rpm for 20 min, the supernatants were filtered through 0.22 lm syringe filters before being analyzed for equilibrium C16mimCl concentrations. The equilibrium C16mimCl concentrations were analyzed by a UV–Vis spectrophotometer (Model T6 New Century 1650, made by General Instrument, Inc. LLT, Beijing China) at the wavelength of 210 nm, corresponding to its maximal absorbance. Calibrations were made using standards of 10, 20, 30, 40, 50, and 60 mg/L with a regression coefficient of 0.9998. The amount of adsorbed C16mimCl was calculated from the difference between the initial and final concentrations. The exchangeable cations desorbed were determined by atomic absorption on a Perkin Elmer AAnalyst-100 Atomic Absorption Spectrometer (Waltha, MA, USA). In the ammonium acetate method, Na+, K+, Ca2+, Mg2+ and Fe3+ ions were exchanged by small ammonium cations (NH+4) which is more intimate to the Mt. It used a solution of ammonium acetate in excess to assure that all cations were exchanged. The dispersion of the Mt in 1 M ammonium acetate solution was shaken overnight. It was filtered in a Buchner funnel. If the filtrate was not clear, it was passed again through the filter. After that, the Mt was washed four times with 25 mL ammonium acetate. Then, the Mt was washed with eight separate aliquots of ethanol to remove the excess cations. The calibration was made using seven standard solutions with concentrations from 0.2 to 3.0 mg/L for K+, Na+, and Mg2+, 1.0 –25.0 mg/L for Ca2+, and 0.5–20.0 mg/L for Fe3+. The detection wavelengths were 766.5, 589.0, 285.2, 422.7, 248.3 nm with a detection limit of 0.01, 0.006, 0.06, 0.04, and 0.06 mg/L for K+, Na+, Mg2+, Ca2+, and Fe3+, respectively. The relative standard deviation is 0.2%. Powder XRD analyses were performed on a Rigaku D/max-IIIa diffractometer (Tokyo, Japan) with a Ni-filtered Cu Ka radiation at 30 kV and 20 mA. Orientated samples were scanned from 1° to 10° at 2°/min with a scanning step of 0.01°. Powder samples were packed in horizontally held trays. The changes in the XRD peak

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positions reflected the hydrated size of the metal cations in the interlayer of Mt. Thermo gravimetric analyses were carried out on TGA Q-500 (TA Instruments, New Castle, USA) from room temperature to 800 °C, at a heating rate of 10 °C/min under a nitrogen flow of 60 mL/min. TG curves were used to determine the percentage of weight loss. Differential scanning calorimetry (DSC) was performed using a differential scanning calorimeter (TA Instruments Q100) fitted with a cooling system using liquid nitrogen. It was calibrated with an indium standard. Samples of 6 mg of Mt were accurately weighed into aluminum pans, sealed and then heated from 30 to 800 °C at 10 °C/min under a nitrogen flow of 60 mL/min.

d=14.95

Intensity

Ca-Mt

3.1. Properties of Mt with different cations The chemical and physical characteristics of clay minerals with different interlayer cations can be investigated by a large variety of techniques, including techniques measuring macroscopic properties (such as the bulk volume and water content) and microscopic properties (such as interlayer spacing, the specific interactions between clay layers and any intercalated species). d001 value of Na-Mt, K-Mt, Mg-Mt, Ca-Mt, Fe-Mt are 12.66 Å, 10.46 Å, 14.72 Å, 14.95 Å and 15.27 Å, shown in Fig. 1. The sharp symmetric peaks indicate there is only one type of interlayer cation in each Mt sample. The differences of d001 values are mainly caused by the different properties of interlayer cations. The radius of Na+, K+, Mg2+, Ca2+, Fe3+ are 1.04 Å, 1.29 Å, 0.70 Å, 0.95 Å, 0.64 Å respectively. Since all of these different types of Mt are prepared with exactly the same kind of Mt by ion exchange, their layer structure and CEC are the same. Therefore all of the Mt have the same layer thickness, which is about 9.6 Å. Based on the d001 value calculated from the XRD result, interlayer space of Na-Mt, K-Mt, Mg-Mt, Ca-Mt, Fe-Mt are 3.06 Å, 0.86 Å, 5.12 Å, 5.35 Å, 5.67 Å. Apparently the differences in five Mts basal spacing are significant, so the five cations ability in combining water is different. A reasonable explanation is that their ability in combining water is different.

Mg-Mt K-Mt

d=10.46

Na-Mt 4

6

8

10

2θ e Fig. 1. X-ray diffraction patterns of montmorillonites with different interlayer caions (d/Å).

After cation exchange, only one kind of cations is in the gallery of Mt (Fig. 2a). However, as organic intercalation is not a complete exchange, some previous cations still remain in the gallery, for these five cations, The exchange cation amount of Ca2+ is 0.53 mmol/g, while that of Fe3+ is only 0.2 mmol/g (Fig. 2b). Ca2+ has the most exchange amount, then the Mg2+, and Fe3+ has the least. In temperature range of 50–200 °C, the weight loss of Mt is due to the removal of the adsorbed surface and interlayer water [37]. In this temperature range, the weight loss for Na-Mt, K-Mt, Mg-Mt, Ca-Mt and Fe-Mt is 4.0%, 2.0%, 9.5%, 9.8% and 11.2% respectively (Fig. 3a). Clearly, K-Mt has the least surface absorbed and interlayer water, while Fe-Mt has the highest. In temperature range of 500–700 °C, the five Mts lost structural water. According to TG result, the smaller weight loss in the case of K-Mt sample gives an indication that less water is bound to K+; while the lower temperature the weight loss occurs at indicate that this interaction is also weaker than that with the other cations. The differences of interlayer water content for different Mt show the differences in the ability of combining water for different interlayer cations. The hydration state influences the potential barrier associated to the cation hopping process significantly [38]. In other words, amount of interlayer bonding water affect the diffusion of the interlayer cations and force of structure layer. Noteworthy, this behavior is quite comparable to the one reported onzeolites, with Xn+-exchangeable cations [39]. To describe the evolution of H2O, it is important to distinguish the water molecules which are adsorbed in the cation vicinity from those which locate more distant from it. Such discrimination can be made since the forces

Amount of cations (mmol/g)

3. Results and discussion

d=14.72

d=12.66

2.3. Computation details Molecular simulation was performed under the module ‘CASTEP’ of Materials Studio 6.0 software to investigate the sorption sites of Na+, K+, Mg2+, Ca2+and Fe3+ on Mt. The primitive unit cell of Mt was optimized with the generalized gradient approximation (GGA) for the exchange–correlation potential (PW91) which is appropriate for the relatively weak interactions present in the models studied. The resulting primitive unit cell was characterized by the parameters a = 15.540 Å, b = 17.940 Å, c = 12.56 Å, and a = c = 90°, b = 99°. Based on the primitive unit cell, a series of (3  2  1) supercells were built. In order to investigate the ion exchange mechanism of Mt, simulated annealing algorithm was used to perform canonical Monte Carlo (MC) simulation with cations as adsorbate on the layer of Mt. The number of cycles is 3 and the steps of one cycle is 106 a representative part of the interface devoid of any arbitrary boundary effects. Based on the structure of the preferential adsorption model of cations in the layer of Mt predicted by MC calculation, GGA-PW91 was used to optimize the structure again and to predict the interaction energy between cations and Mt layer to a greater accuracy. All of the GGA-PW91 calculations were performed using a double numerical plus polarization function (DNP) as basis set and DFT-D correction. For all calculations, the heavy atoms of Mt were frozen, whereas the hydrogen of Mt, cationic molecules was full relaxed.

d=15.27

Fe-Mt

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

a

a

a

Na+

a b

K+

Ca2+

b b

Na-Mt

K-Mt

a b

Fe3+

Mg2+

b Ca-Mt

Mg-Mt

Fe-Mt

Fig. 2. Content of Mt interlayer cations (a), desorption of cations accompanying C16mimCl adsorption on Mt (b).

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200

102 Na-Mt K-Mt Mg-Mt Ca-Mt Fe-Mt

2.0 4.0

5.0

9.5 9.8 11.2

94 92 90

5.0

Weight (%)

96

180

1.9

88

4.0

86 84 82

Intercalate amount mg/g

98

(a) 5.0

100

160

K-Mt Na-Mt Ca-Mt

140 120 100

Mg-Mt Fe-Mt

80 60 40 20

100

200

300

400

500

600

700

800

Temperature

Heat (m v/mg)

(b)

0

0

1000

2000

3000

4000

5000

Equilibrium concentration of C16mimCl

6000

mg/L

Fe-Mt

Fig. 4. Amount of C16mimCl intercalated into montmorillonites with different interlayer caions.

Ca-Mt

intercalation amount for Fe-Mt. The divalent cations have higher intercalation amount than monovalent cations. The driving force for Mt intercalation with C16mim+ is cationic exchange, so it is much easier for Mt with weaker force between interlayer cations and structure layers to be intercalated with C16mim+. Based on the experimental results, it can be deduced that divalent cations have weaker interaction force for the structure layer than monovalent cations. While for Fe3+, it forms Fe(OH)3 through hydrolysis, which affects the amount of intercalation significantly. The amount of C16mim+ intercalated into Ca2+-Mt is more than Mg2+-Mt, which can be explained by a stronger force between Mg2+ and structure layers [40]. The low adsorption capacity of Mt for C16mim+ at the layer edge can be attributed to its positive charge and the possible occurrence of steric repulsive interactions between surface adsorbed C16mim+ and the C16mim+ in solution. [28]. C16mim+ intercalation expanded the structure of Mt, which is readily measured by XRD (Fig. 5). Basal spacings of Na-Mt, K-Mt, Mg-Mt, Ca-Mt and Fe-Mt after being intercalated by C16mimCl are 17.64 Å, 14.22 Å, 20.58 Å, 21.02 Å and 15.31 Å respectively. The differences of the basal spacings between the different organo-Mts are due to the different intercalation amount and arrangement of the surfactants in the interlayer region. To sum it up, the difficulty in organic intercalation and the amount of organic agent intercalated are affected by the types of interlayer cations of Mt, because of the differences in the diffusion behavior of the interlayer cations and their interaction with

Mg-Mt K-Mt

Na-Mt

100 200 300 400 500 600 700 800

Temperature Fig. 3. TG (a) and DTA (b) curves of montmorillonites with different interlayer caions.

acting in the hydration process, which are due to ionicity and dispersive forces respectively, are significantly different. [38] The DTA curves show a similar trend in the range of 50–150 °C (Fig. 3b). The endothermic valley is due to removal of interlayer and surface adsorbed water. The temperatures of the endothermic valley for different Mts are different. For example, the endothermic valley of K-Mt in this temperature region is at 60 °C, which is the lowest, while that of Mg-Mt and Fe-Mt are the highest (around 105 °C). The temperature of the endothermic valley can indicate the force between cations and water molecules, which are in the order for the five Mts: Fe-Mt > Mg-Mt > Ca-Mt > Na-Mt > K-Mt. As less interlayer water means smaller interlayer spacing, a stronger electrostatic attraction for the interlayer cation and more difficult in ion exchange of Mt would be anticipated. Therefore, the amount of interlayer water playes an important role in facilitating the cation exchange of Mt.

15.31 Fe-Mt 21.02 Ca-Mt

3.2. Organic intercalation of Mt with different interlayer cations

Intensity

The amounts of C16mim+ intercalated are different for Mts with different interlayer cations (Fig. 4). Initial concentration of C16mim+ in solution also has a profound influence on the intercalation. At initial concentration of 500 mg/L, intercalation amounts of C16mim+ intercalated five Mt have slight difference; while the intercalation amount differs greatly above the concentration of 1000 mg/L. However, as the initial concentration increased, the intercalation amounts varied markedly. At the initial concentration of 5000 mg/L, the intercalation amounts for Na-Mt, K-Mt, Mg-Mt, Ca-Mt and Fe-Mt are 146 mg/g, 92 mg/g, 167 mg/g, 184 mg/g and 63 mg/g, respectively. C16mim+ has the highest intercalation amount for Ca-Mt, which is followed by Mg-Mt, and the smallest

20.58 Mg-Mt 17.64 Na-Mt 14.22 K-Mt 3

4

5

6

7

8

9

10

2θ (°) Fig. 5. X-ray diffraction patterns of C16mimCl intercalated montmorillonites with different interlayer caions (d/Å).

L. Wu et al. / Journal of Colloid and Interface Science 454 (2015) 1–7

structure layers. This will be further discussed in the computer molecular simulation section. 3.3. Simulations In order to get a better understanding of thebove phenomena, computer molecular simulation is used to investigate the interlayer micro structure and the interlayer interaction for these Mts and organo-Mts at atomic scale. Based on force-field, equilibrium molecular dynamics simulations were performed. The interactions between structure layers and interlayer cations were calculated by combining the parameters of noncovalent energy terms. The combined force field has been proven to be able to maintain the structures of both clay mineral and interlayer cations, and guarantees full interactions among them [41–43]. The simulated system had its total energy as the combination of Coulombic interaction, van der Waals interaction, and bonded interaction:

Etotal ¼ EVDW þ ECoulombic þ Ebond

stretch

þ Eangle

bend

þ Etorsion þ Eimproper

The first two terms of the total energy contributed to the Lennard–Jones potential (12–6 potential) and Coulombic potential energy terms, and the sum of them represented the noncovalent

interaction that is universal for any two atoms. For bonded interactions, the energy terms that compensate bond stretch, angle bend, torsional, and improper movements were considered. All the bonded terms were calculated based on CVFF, except for the OH bond and angles in the clay framework which were calculated from CLAYFF. We simulate the interlayer cations’ interaction with Mt platelets and the bound water. Interlayer spacing of X-Mt is set according to XRD results (Fig. 1), interlayer cation select bound water automatically. After calculation equilibrium, optimal conformational relationship and interaction mode of interlayer cation and bound water could be obtained (Fig. 5). Na+(Fig. 6a), K+(Fig. 6b), Mg2+(Fig. 6c), Ca2+(Fig. 6d) and Fe3+(Fig. 6e) are coordinated by 2 (monolayer), 1(monolayer), 4 (bilayer), 4 (bilayer) and 6 (triple layer) water molecules with oxygen atom in the water molecule closer to the central cation. It turns out that Fe3+ has more bound water than the other four cations, while K+ has the least. This corroborates the earlier TG-DTA results. Other than the amount of bound water, the interaction between cation and water is also an important factor to be considered. The shortest distances between Na+, K+, Mg2+, Ca2+, Fe3+ and the oxygen in the bound water are 2.31, 2.56, 2.07, 2.22 and 2.12 Å. These distances are short enough to form weak chemical bonds. The cations would exist in the interlayer in the form of Xn+mH2O. As shown in Table 1, the binding

(a) 2.31

5

(d)

2.37

2.25 2.03

2.22 2.98

(b) 2.56

(e)

0.85

2.12 2.22

2.25 2.25 3.08

(c) 2.07 2.16 2.76

Fig. 6. Molecular dynamic simulation of montmorillonites with different interlayer caions. Na-Mt (a), K-Mt (b), Mg-Mt(c), Ca-Mt (d) and Fe-Mt (e). For all species, H = white, Al = pink, O = red, Si = yellow, Na, K = purple, Ca = green, Mg = blue, and Fe = gray. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 1 Interaction energies of montmorillonites with different interlayer caions.

Na-Mt K-Mt Mg-Mt Ca-Mt Fe-Mt

d001 (Å)

Total energy 4E (eV)

Energy of Water and intercalation cation 4E (eV)

17.64 14.22 20.58 21.02 15.31

855 963 731 719 562

15 2 80 58 60

energies for Na+, K+, Mg2+, Ca2+, Fe3+ with water are 15, 2, 80, 58 and 60 eV respectively. Since the force between Mg2+ and water is the strongest, the loss of the bound water for Mg2+-Mt occurred at the highest temperature. K+ has the weakest force with water, so K-Mt loses the bound water at a lower temperature. This is also consistent with DTA result. The amount of bound water around cations and the interaction force between them affected the diffusion ability of these cations. During the organic intercalation, the organic ions go into the interlayer of Mt by ion exchange. It would inevitably involve the diffusing out of interlayer cations and diffusing in of the organic ion. If diffusion of the interlayer cation is fast, the organic intercalation process will take place much easier and the intercalation amount is higher. According to the above calculations, it would be much easier for Mg-Mt and Fe-Mt to be intercalated and with higher intercalation amount and the opposite for K-Mt. However, the experiment result shows that both K-Mt and Fe-Mt have the least intercalation mount. As mentioned before, while for Fe3+, it forms Fe(OH)3 through hydrolysis, which affects the amount of intercalation significantly. The interaction between interlayer cations and structure layers also affects organic intercalation According to the simulation (Fig. 6), interlayer cations locate above the octahedron where Al is replaced by Mg. This indicates that these octahedral attract interlayer cations the strongest. So electrostatic attraction between interlayer cations and structure layers dominates the distribution of interlayer cations. The minimum vertical distances from Na+, K+, Mg2+, Ca2+ and Fe3+ to the structure layer are 2.03, 0.85, 2.76, 2.98 and 3.08 Å respectively. A shorter distance means a larger electrostatic force. Therefore the electrostatic force between K+ and the structure layer is the largest, while that between Fe3+ and the structure layer is the smallest. The total forces, including electrostatic force, van der waals and so on, between Na+, K+, Mg2+, Ca2+, Fe3+ and structure layers of Mt are calculated which are 855, 963, 731, 719 and 563 eV respectively. It helps to explain why K-Mt has the smallest organic intercalation amount (interaction between K+ and structure layers is the strongest) and the organic intercalation amount for Ca-Mt is larger than that for Mg-Mt (interaction between Mg2+ and structure layers is stronger than that for Ca2+). For Fe3+ forms Fe(OH)3 through hydrolysis during the ion exchange which makes the situation too complex, Fe3+-Mt is not discussed here. As shown in the study of similar systems conducted by Xu et al. [44], which involves the presence of water molecules, the electrostatic contribution plays dominant role in the interaction between montmorillonite and C16mim+. Note that the partial charges utilized by Xu et al. [44] were taken from extended Born model developed by Heinz and Suter [45]. Although the effects of solvent molecules could improve the quantitative description of the systems studied, the proportions between energy values will be unaffected by the chosen model simplification. 4. Conclusions A series of experiments and molecular dynamics simulations proved that the interlayer cation played a critical role in

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