Interlayer Structure and Dynamics of HDTMA+-Intercalated Rectorite

May 16, 2012 - Interlayer Structure and Dynamics of HDTMA+-Intercalated Rectorite with and without Water: A Molecular Dynamics Study. Zhou Jinhong†â...
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Interlayer Structure and Dynamics of HDTMA+-Intercalated Rectorite with and without Water: A Molecular Dynamics Study Zhou Jinhong,†,‡ Lu Xiancai,*,§ Zhu Jianxi,*,† Liu Xiandong,§ Wei Jingming,† Zhou Qing,†,‡ Yuan Peng,† and He Hongping† †

Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, China § State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China ABSTRACT: Rectorite is a special kind of clay mineral, consisting of illite layer and smectite layer in a regular order. In this study, we employ classical molecular dynamics simulations to study the microscopic interlayer properties of HDTMA+ (hexadecyl trimethyl ammonium)-intercalated rectorites with and without water at different HDTMA+ loading levels. The simulation results show that as the loading level increases, monolayer, bilayer, transition, and trilayer configurations of HDTMA+ occur in succession. The different layer charge characteristics between illite sheet and smectite sheet lead to special interlayer distributions of HDTMA+. In the systems without water, the headgroups of HDTMA+ and Na+ are controlled by six-membered rings of clay surface. With water addition, water molecules show the highest mobility among the interlayer species. However, HDTMA headgroups are still controlled by clay surfaces mainly with low mobility. At the same time, most of Na+ cations escape from surfaces because of the attractions from water. But water has little influence on the mobility of Na+. Water can decrease the mobility of alkyl chains because water fills up the empty region in the organic phases. In the sample with loading level exceeding 1 CEC, anions (Cl− in this study) present very poor mobility due to the electrostatic attractions with headgroups. applied in many fields like organo-intercalated smectites due to their unique structures. There have been a great variety of studies focusing on organoclays, especially alkylammonium-intercalated smectites. Based on X-ray diffraction (XRD) analysis, the basal spacings could be obtained, and the results indicate that both the electrostatics of phyllosilicates and the length of the alkyl chain are the major factors influencing interlayer arrangements.10,13,14 Several ideal arrangement models of alkylammonium ions have been proposed based on basal spacing measurements and the length of the carbon chain: (a) monolayer, which is always formed when short-chain alkylammonium ions intercalated; (b) bilayer, as the carbon chain becomes longer; (c) pseudotrilayer, which is usually formed within highly charged phyllosilicates, e.g., mica;15 (d) paraffin structure, which is formed by the alkylammonium ions with two or more carbon hydrocarbon chains. Fourier transform infrared (FTIR) spectroscopy16−24 and nuclear magnetic resonance (NMR) spectroscopy18−20,25−27 have

1. INTRODUCTION Decades ago, people started to pay attention to the crystal structures and identification of illite−smectite mixed-layer clay minerals because of their significance in geology,1 e.g., used as geothermometers.2 Rectorite is a kind of regular mixed-layer clay mineral. It always consists of illite part and smectite part with the order of R1; i.e., it has a strict periodicity along the z axis as an ABABAB... sequence.3,4 Generally, rectorite has a smectite part as a swelling component while the illite part is nonexpandable.3 Especially, the alternate combination of illite part (high-charge) and smectite part (low-charge) results in special layer charge characteristics and interlayer properties. Recently, organicsintercalated rectorites have attracted more and more attention;5−10 e.g., Li et al. investigate the effects of chain lengths and loading levels of alkylammonium on interlayer configurations of intercalated rectorite.10 Organics-intercalated phyllosilicates (organoclays) are important in many environmental and industrial applications, e.g., used to adsorb organic pollutants, as fire retardants, and as gas and solvent barrier devices.11,12 As a kind of oraganoclays, organo-intercalated rectorites can also be © 2012 American Chemical Society

Received: January 8, 2012 Published: May 16, 2012 13071

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Figure 1. Rectorite model. Na = purple, K = light blue, Mg = green, Al = pink, Si = gray, H = white, O = red. The upper is the illite part, and the lower is smectite part.

Table 1. Denotations, Interlayer Ion Contents, and Basal Spacings of Simulated Pure and Intercalated Rectorites

shown the cases that disordered (mixture of trans and gauche) and ordered (all trans) conformations can coexist in organic phases. Numerous molecular simulation studies have been carried out to investigate alkylammonium−clay systems.11,15,28−36 These studies confirmed the previously proposed interlayer structures and also provided detailed dynamics of interlayer species.37 Indeed, molecular simulation is a very powerful tool to complement experiments, especially in the aspect of obtaining thermodynamics and dynamics information at a microscopic level.38−42 In this study, we carried out systematic molecular dynamics (MD) simulations with the CLAYFF−CVFF force field31,32 on alkylammonium-intercalated rectorites. From the simulations, we derived the basal spacings for intercalated rectorites with and without water. We analyzed the interlayer structures and dynamics properties of interlayer species in detail. The layering behaviors of alkyl chains, interlayer bonding, and the dynamics have been uncovered at the atomic level, and the effects of water are also discussed.

clay models

denotation

ions per interlayer

basal spacing (Å)

dry pure rectorite 0.5 CEC without water 0.5 CEC with water

Rec Rec0.5

16 Na+ 8 HDTMA+ + 8 Na+

19.5 24.5

Rec0.5w

25.1

Rec1

8 HDTMA+ + 8 Na+ + 40 H2O 16 HDTMA+

Rec1w

16 HDTMA+ + 90 H2O

28.4

1 CEC without water 1 CEC with water

27.3

it is assumed that only the interlayer occupied by Na+ could be expanded in our simulations; i.e., Na+ can be replaced by HDTMA+, but K+ cannot. As the initial configurations, the interlayer species (i.e., HDTMA+, H2O, Na+, K+, and Cl− for clays with high HDTMA+ loading) are placed randomly in the interlayer regions. For the cases containing water, water just exists in the interlayer of the smectite part. The water content is set at about 5% of the total mass of the smecite part and HDTMA+, based on previous TGA (thermal gravity analysis) measurements of organoclays.46 Water molecules are inserted together with the ions randomly. For the cases of 0.5 CEC HDTMA+-intercalated smectites, TGA data are lacking, and we estimate the number of water molecules based on the coordination number of Na+ cations (generally 5 water molecules for 1 Na+ with water saturation according to ab initio molecular dynamics simulations47−49). In the 2 CEC case, for charge balance, we inserted Cl− ions. 2.2. Simulation Details. The structural units of rectorite are also tetrahedral layer and octahedral layer as smectites, so it could be described by CLAYFF. In our simulations, the CLAYFF−CVFF force field is applied to describe the interatomic interactions. The validity of this combined force field has been proven.31,32 In the simulation, a 10.0 Å cutoff is used for the short-range interactions. The electrostatic interaction is treated using the Ewald summation,50 and the number of k-space vectors is determined to reach a precision of 1.0 × 10−4 in conjunction with the pairwise calculation within a cutoff of 10.0 Å.

2. METHODS 2.1. Intercalated Rectorite Model. The chemical formula of the model rectorite is KNa 0.5 (Mg 0.5 Al 7.5 )(AlSi 15 )O40(OH)8.43 This model has 1.5e per unit cell, consisting of 1 tetrahedral charge and 0.5 octahedral charges. The CEC (cation exchange capacity) of this model is 32.3 mmol/100 g. In this model, the illite part has tetrahedral substitutions (Al for Si), and the smectite part has octahedral substitutions (Mg for Al). The isomorphic substitutions in clay sheets obey Loewenstein’s rule; i.e., two substitution sites cannot be adjacent. The cations in the interlayer space for the illite part are K+ ions and for the smectite part are Na+ ions. The simulation cell consists of one clay platelet of 32 unit cells: 8 in the x dimension, 4 in the y dimension, and 1 in the z (Figure.1). The area of basal surface is 41.29 × 35.83 Å2, and the basal spacing of pure rectorite is 19.47 Å. Initially, the basal spacing is set to be 70 Å, which is large enough to contain HDTMA+ (hexadecyl trimethyl ammonium) ions. The simulation cell is set as a 3D periodic box. The models of HDTMA+ and water are first optimized. The geometry optimization has been performed using the LAMMPS package44 with the CVFF45 by taking 1.0 × 10−4 as the energy change tolerance. The SPC model is used for water. In view of the above-mentioned basic characteristic of rectorite (only the smectite part as a swelling component),

Clayff:

Etotal = Ecoul + E VDW + E bondstretch + Eanglebend (1)

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Figure 2. Density distributions of intercalated rectorites. (a), (c), and (e) are systems without water. (b) and (d) are systems with water. In (c), the intensity of the density profiles of ammonium N is increased by a factor of 2 for the purpose of clarity. Zero point of the horizontal axis denotes the center of the z axis of each system. The yellow solid line denotes the smectite part surface, and the green one denotes the illite part.

D0,ij and R0,ij are empirical parameters derived from the fitting of the model to observed structural and physical property data.

CVFF: Etotal = Ecoul + E VDW + E bondstretch + Eanglebend + Etorsion Ecoul =

e2 4πε0

∑ i≠j

E bondstretchij = k1(rij − r0)2

(2)

qiqj rij

k1 is a force constant, and r0 represents the equilibrium bond length.

(3)

Eanglebendijk = k 2(θijk − θ0)2

The partial charges qi and qj are derived from quantum mechanics calculations, e is the charge of the electron, and ε0 is the dielectric permittivity of vacuum. E VDW =

⎡⎛ ⎞12 ⎛ ⎞6 ⎤ ⎢⎜ R 0, ij ⎟ − 2⎜ R 0, ij ⎟ ⎥ D ∑ 0, ij⎢⎜ ⎟ ⎜ r ⎟⎥ r ⎝ ij ⎠ ⎦ i≠j ⎣⎝ ij ⎠

(5)

(6)

k2 is a force constant, and θ0 represents the equilibrium bond angle. Etorsion = Kϕ × [1 + cos(nϕ − ϕ0)]

(7)

All MD simulations are carried out using the LAMMPS package.44 To obtain the swelling curve, we performed NPT

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Figure 3. Snapshots of intercalated rectorites. In each snapshot, the stick layers at the top and the bottom represent the clay sheets. The top one is the illite part, and the bottom is the smectite part. For the atoms in the interlayers, Na = purple, K = light blue, N = blue, C = gray, H = white, O = red, Cl = gray−green. (a) and (b) monolayer; (c) transition; (d) bilayer; (e) trilayer.

(300 K, 1 atm) simulations. Each simulation was performed first for 1500 ps, and then a production stage of 500 ps simulation is performed for recording the basal spacing results. The basal spacing of rectorite refers to the thickness of the smectite layer and the illite layer. Then, to derive the interlayer structural and dynamic properties, a further 2000 ps NVT (300 K) simulation is performed, and an interval of 50 fs is used for recording trajectories. A time step of 1.0 fs is used for all simulations. In each run of the simulation, all atoms are allowed to move.

and trilayer configurations (see section 3.2). Our results coincide with the measured basal spacings of HDTMAintercalated rectorites (25.2 Å)24 and OTMA (octyltrimethylammonium)-intercalated rectorites (24.5 Å)10 with monolayer configurations. By comparing the systems with and without water (i.e., 0.5 and 1 CEC in Table 1), it can be seen that the adsorption of water can also slightly enlarge the basal spacings. 3.2. Density Distribution of Interlayer Species. All interlayer species present clear peaks on the density distribution profiles (Figure 2). In the Rec0.5 system (Figure 2a), ammonium N presents a clear single peak and C presents a single peak only with a slight split, suggesting the formation of monolayer (Figure 3a). Because of the attraction of the clay sheets, Na+ ions are close to clay surfaces. The difference in layer charge amount between illite and smectite parts causes the single peak of N to be closer to the illite surface in cases both with and without water (Figure 2a and b). In the Rec2 system (Figure 2e), C presents three well-defined peaks, indicating a trilayer structure (Figure 3e). Chlorine ions prefer to stay in the middle of the interlayer region. However, in the Rec1 system (Figure 2c), one can see that C presents three

3. RESULTS AND DISCUSSION 3.1. Basal Spacing. Table 1 lists the basal spacings of simulated systems. The experimental data of dry rectorites are lacking. By considering that rectorite is a regular illite−smectite complex, one can estimate its basal spacing by dry illite (9.98 Å (3)) plus dry smectites (about 9.6 Å (51)), which gives about 19.6 Å. Our simulation result, 19.5 Å, agrees well with that value. As the contents of HDTMA increase from 0.5 to 2 CEC via 1 CEC, the basal spacings increase from 24.5 Å to 27.3 Å, and to 34.5 Å. These values correspond to monolayer, transition, 13074

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Figure 4. Radial distribution functions: (a) surface oxygen atoms of the clay sheets (SO) around HDTMA N in dry systems; (b) surface oxygen atoms of the clay sheets (SO) around HDTMA N in systems with water; (c) water oxygen atoms (WO) around HDTMA N; (d) chlorine ions around HDTMA N in the Rec2 system; (e) water oxygen atoms (WO) around Na ions in the Rec0.5w system.

between Na+ ions and six-membered rings. In Rec1w, the altitude difference of the two N peaks gets lower than that in Rec1, suggesting more ammonium N atoms distribute close to the smectite part. The alkyl chain C in Rec1w presents two peaks, suggesting the formation of bilayer. Therefore, addition of water not only affects the distribution of Na, but also those of ammonium N and alkyl chain C in bilayer configurations (Figure 3d). 3.3. Interlayer Bonding Behavior. From RDFs (radial distribution functions) and CNs (coordination numbers) in Figure 4, we can investigate the bonding behaviors of interlayer species. The CNs of HDTMA N-SO in both Rec0.5 and Rec0.5w are higher than the others (Figure 4a and 4b), as a result of monolayer configuration described above. The CNs of both Rec0.5 and Rec0.5w are about 12, which is exactly the number of oxygen of two six-membered rings. In monolayer configuration, the distance between HDTMA N and sixmembered rings of the illite part is slightly less than that between HDTMA N and six-membered rings of smectite part as shown in Figure 2a and b. When bilayer, transition, or trilayer forms, the number of the oxygen around the HDTMA

poorly separated peaks. Compared with the snapshot (Figure 3e) of Rec2, the carbon chains in Rec1 (Figure 3c) do not form a clear trilayer structure. Therefore, we assign this configuration to be “transition structure” (Figure 3c) as suggested in the previous study.10 As shown in Rec1 (Figure 2c), the two peaks of N are dissymmetrical, which is different from the symmetrical distribution of ammonium N in organointercalated smectites.32,35 A much higher charge amount of the illite part than the semctite part is responsible for this phenomenon. In the systems containing water, one can see similar layering behaviors of alkyl chains to those in the dry systems. But there are still some differences after water is added. The profiles of water in these two systems (Rec0.5w and Rec1w) are similar. The dissymmetric distribution of water molecules is clearly different from the case in organo-intercalated smectites.32 Water is closer to the illite part due to the higher charge amount of the illite part. For Rec0.5w (Figure 2b), Na prefers to distribute close to the illite part. One peak of Na (X = 2.2 Å) means the correlation between water molecules and Na+ ions, and the other peak (X = 3.8 Å) indicates the correlation 13075

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the RDFs of Rec1 and Rec1w proves that the addition of water weakens the interaction between SO and HDTMA N as remarked above. Figure 4d illustrates the curves of RDF and CN for chlorine ions around HDTMA N in Rec2. The sharp peak in the RDF curve shows a strong correlation between chlorine ions and HDTMA N due to electrostatic interactions. In the dry system, Na+ ions prefer to hover above sixmembered rings (Figure 5a). With water added, most of the Na+ ions escape from clay surfaces (Figure 5b) because of the strong attractions from water molecules. However, the content of water in Rec0.5w is not enough to make all Na+ ions escape, so a few Na+ ions still stay near six-membered rings, which corresponds to the small peak of the density distribution profile of Na in Rec0.5w (Figure 2b). Figure 5c shows that water molecules tend to distribute near the illite surface, consistent with the density distribution profile of water (Figure 2d). 3.4. Dynamics of Interlayer Species. The MSDs (meansquared displacement, Figure 6) of alkyl carbon, nitrogen, sodium, chlorine, and water oxygen are derived to describe the mobility of alkyl chains, headgroups, sodium ions, chlorine ions, and water molecules, respectively. The MSD of nitrogen in each case is as low as less than 4.0 Å2 over the simulation period. The density distribution profiles and RDFs described above show that there are strong interactions between headgroups and six-membered rings, and each nitrogen is closely related to one six-membered ring on one clay sheet surface. The area of six-membered ring is about 20 Å2. Therefore, the MSDs indicate that the headgroup is hard to escape from the attraction of six-membered rings. The MSDs of nitrogen are also very low with water intercalation (Figure 6b and d). In both Rec0.5 and Rec0.5w, the MSDs of Na+ cations are very low, indicating limited motions. That is because Na+ stays close to the surfaces and interacts directly with surface oxygen of clay sheet in both systems, as discussed above in section 3.3. The MSDs of water are higher than those of the other species, which indicates water has the highest mobility in the interlayer space. As above, the addition of water makes Na+ cations take part in filling up the monolayer in Rec0.5w (Figures 5b and 7b). Also water affects the mobility and distribution of HDTMA+. In Rec2, the MSD of chlorine ions is only a bit higher than that of nitrogen. From Figure 2e, Figure 3e, and Figure 4d, one can observe that chlorine ions distribute around the headgroups of HDTMA+ in the middle of the trilayer structure due to electrostatic attractions. Therefore, Cl− ions also have very low mobility. In dry systems, the alkyl chains present higher MSDs than the other species. But, the MSDs of alkyl chains are relatively low in wet systems (Figure 6a vs Figure 6b and Figure 6c vs Figure 6d). As shown in density distribution profiles (Figure 2a, 2b) and snapshots (Figure 3a, 3b), alkyl chains form monolayers in Rec0.5 and Rec0.5w. In Rec0.5, the HDTMA+s are not enough to fully fill the monolayer (Figure 7a), which leaves much space for the alkyl chains to move freely. When water molecules are added, the space is filled up (Figure 7b), which reduces the mobility of alkyl chains. In Rec1 and Rec1w, the same reason results in the decrease of the mobility of alkyl chains.

Figure 5. (a) Snapshot for Na ions in Rec0.5. (b) Snapshot for Na ions in Rec0.5w. (c) Snapshot for HDTMA N in Rec1w. The top surface is the illite part, and the bottom is the smectite part. For clarity, some atoms are removed. The green dash line in (c) denotes the middle plane of interlayer.

N at the first peak is about 6 or less because HDTMA N is just near one clay surface and farther away from the other one; i.e., each HDTMA N is related to just one six-membered ring at most (Figure 5c). Figure 4c illustrates the curves of RDFs and CNs for water oxygen atoms (WO) around HDTMA N in Rec1w and Rec0.5w. These RDFs both show one sharp peak, suggesting a strong correlation between WO and HDTMA N (pictured in the snapshot in Figure 5c). The comparison of 13076

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Figure 6. MSDs of interlayer species in the intercalated rectorites.

Figure 7. Top views of interlayer species: (a) Rec0.5 and (b) Rec0.5w. The clay frameworks are removed for clarity. Na = purple, N = blue, C = gray, H = white, O = red.

rations of HDTMA+. The different layer charge charateristics between the illite part and the smectite part lead to dissymmetric distributions of HDTMA+ headgroups, Na+, and water, which are different from the observations for alkylammonium intercalated smectites.

4. CONCLUSION Using MD simulations, this study reveals interlayer structures of HDTMA+-intercalated rectorites and dynamic properties of interlayer species. As HDTMA+ loading level increases, we observe monolayer, bilayer, transition, and trilayer configu13077

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In dry systems, HDTMA headgroups and Na+ cations stay close to the clay surfaces because of the electrostatic interactions with six-membered rings. They almost do not present obvious mobility, whereas alkyl chains show a little higher mobility. After water is added, water molecules show the highest mobility among the interlayer species. HDTMA headgroups still stay close to clay surfaces while most of the Na+ cations escape from surfaces due to the attractions from water. However, water has no influence on the mobility of HDTMA headgroups and Na+. Moreover, water decreases the mobility of alkyl chains because water fills up the empty region in the organic phases. In addition, in the sample whose loading level exceeds 1 CEC, anions (Cl− in this study) stay far from the clay surfaces, and they present very poor mobility due to the electrostatic attractions with headgroups.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.X.). E-mail: [email protected] (Z.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the grant of the Knowledge Innovation Program of the Chinese Academy of Sciences (KZCX2-EW-QN101), National Science Foundation of China (Nos.40972034, 40973029, and 41002013), the NSFCGuangdong Union Project (U0933003), and Joint Research Fund (KLMM20110205) of Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. We are grateful to the High Performance Computing Center of Nanjing University for doing the calculations in this paper on its IBM Blade cluster system. This is contribution No. IS-1496 from GIGCAS.

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dx.doi.org/10.1021/jp303856n | J. Phys. Chem. C 2012, 116, 13071−13078