Intercalation of Sulfonate into Layered Double Hydroxide: Comparison

Thus, the final Mg:Al ratio in the LDH layer is 2:1. ... Discover in Material Studio (MS) 4.3 was employed to study organo−LDH intercalations throug...
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J. Phys. Chem. C 2009, 113, 559–566

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Intercalation of Sulfonate into Layered Double Hydroxide: Comparison of Simulation with Experiment H. Zhang,*,†,‡ Z. P. Xu,‡ G. Q. Lu,‡ and S. C. Smith*,†,‡ Centre for Computational Molecular Science and ARC Centre for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology, The UniVersity of Queensland, Queensland 4072, Brisbane, Australia ReceiVed: August 18, 2008; ReVised Manuscript ReceiVed: NoVember 03, 2008

We perform computational modeling studies to explore the properties of functionalized Mg-Al layered double hydroxides (LDHs). Using molecular dynamics (MD) simulations we study the intercalation of C8H17SO3sulfonate into a Mg:Al 2:1 LDH system for which the experimental data have recently been reported (J. Phys. Chem. C 2007, 111, 4021). An ab initio force field (condensed-phase optimized molecular potentials for atomistic simulation studies, COMPASS) is used for the MD simulations of the hybrid organic-inorganic system. Quantum mechanical density functional theory is also employed in order to establish structural and spectroscopic benchmarks for the sulfonate as a means of testing the force field. The interlayer structure, arrangement, and orientation of the intercalated species are examined and contrasted with the geometry of the isolated sulfonate. The self-diffusion coefficients of both the interlayer sulfonate and water are estimated to be 2.05 × 10-7 and 3.07 × 10-7 cm2/s at 300 K on the basis of 500 ps MD simulations. Computed powder X-ray diffraction patterns are in good accord with experiment. Computed infrared spectra are comparable with experiment in terms of line positions, while line intensities show room for improvement. 1. Introduction Layered double hydroxides (LDHs) are anionic clay materials1 that have received much attention due to their applications in the fields of heterogeneous catalysis,2,3 heat stabilizers,4 molecular sieves or ion exchangers,5 biosensors, and halogen scavengers.6 The general formula for LDHs is 2+ n2+ M3+ and M3+ are divalent M 1-x x (OH)2(Ax/n ) · mH2O, where M and trivalent metallic cations, respectively, and A is an anion of valence n. The most studied class of LDHs is Mg6Al2(OH)16CO3 · 4H2O, a popular pharmaceutical antacid talcid for ulcers. LDH structure is closely related to brucite: Mg(OH)2. In a brucite layer each Mg2+ ion is octahedrally surrounded by six OH- ions and the different octahedrons share edges to form an infinite two-dimensional layer. Partial replacements of Mg2+ ions by Al3+ give the “brucite-like” layers a permanent positive charge, which is balanced by negatively charged anions located in the interlayer region. This gallery also contains water molecules hydrogen bonded to layer OH and/or to the interlayer of anions. Through electrostatic interactions and hydrogen bonds the layers may be stabilized in a crystalline form. The charge density in the hydroxide layer, represented by x in the general formula, is normally in the range 0.2-0.33. The anions in the interlayer gallery are generally exchangeable, and indeed, anion exchange is the most widely used intercalation method. Many different kinds of anions have been successfully intercalated into LDH, including almost all of the common inorganic anions. Many organic and biomolecular anions, including carboxylated sulfates,7 benzoate,8 sulfonate,9,10 amino acids, and peptides,11 as well as nucleotide phosphates * To whom correspondence should be addressed. Fax: 61-7-3346-3992. E-mail: [email protected]; [email protected]. † Centre for Computational Molecular Science. ‡ ARC Centre for Functional Nanomaterials.

and DNA chains,12 can be intercalated within the interlayers of LDHs. This unique property of intercalating organic and biomolecular molecules makes LDH an extremely promising delivery carrier for drug delivery and gene therapy applications.13,14 Their importance in areas such as oil-field technology and medicine has also greatly increased in the last two decades. For applications such as in the synthesis of polymer nanocomposites, hydrophilic LDH has to be made hydrophobic for compatibility with polymers in some cases. Thus, pretreatment of LDH using organic anion species is desirable, often involving insertion of organic molecular anions with a long hydrophobic tail into the layers. This treatment, on one hand, offers a gentle way of expanding the interlayer space to allow insertion of large molecules and, on the other hand, makes the LDH materials more compatible with organic polymers. Very recently, LDH-surfactant hybrids with varying hydrocarbon tails and different functionalities have been synthesized and characterized experimentally, which include sulfate, sulfonate, carboxylate, and phosphate.9,10 Also, recent experimental15 and computational studies16 demonstrate that if LDH is loaded with suitable organic intercalates the enhanced swelling can lead to exfoliation of LDH layers in a solution environment, which in turn makes insertion of large bio- or organic molecules possible. In general, LDHs are polycrystalline materials, and precise experimental location of interlayer anions is extremely difficult. Only rarely can sufficiently large crystals for full structural determination by conventional single-crystal X-ray diffraction be obtained. Powder X-ray diffraction (PXRD) gives some indication of the bulk structure of the material, but in general, LDH intercalates are characterized by the absence of significant long-range order. Reflections tend to be broad, and structure solution from powder X-ray data has been achieved only for small inorganic guests. Interlayer arrangements may be postulated from PXRD patterns but are frequently little more than educated guesses based solely on the molecular dimensions of

10.1021/jp807411x CCC: $40.75  2009 American Chemical Society Published on Web 12/18/2008

560 J. Phys. Chem. C, Vol. 113, No. 2, 2009 the guest. Moreover, the interlayer arrangements depend strongly on the interlayer water content of the LDH as well as the anion size. Other techniques such as FTIR have been used to characterize LDHs, but these give only limited information about the arrangement of the guests within the interlayer region. More recently, computational atomistic simulation methods have been employed to study the arrangement of the guest ions inside LDH systems,17-21 which are likely to provide, at the moment, the only means of knowing the details of the interlayer arrangements given the poorly crystalline nature of the intercalates. Use of computer modeling has thus greatly augmented our present understanding of the orientation and organization of the interlamellar guest molecules. In this regard, molecular dynamics (MD) provides a powerful technique to probe the structure as well as the dynamic properties at a molecular level and offers a direct connection between local structural details and experimental measurements. To date several groups have performed molecular dynamics (MD) simulations of interlayer arrangements and energetics for LDHs containing organic or biomolecular anions such as terephthalate,22 cinnamate,18 carboxylic acids,23 amino acid,17 and DNA.24 Experimentally observed parameters, such as the interlayer spacing, can be simulated successfully. In addition, close contacts and preferred orientations of the intercalates have been analyzed through the simulations. In these simulations different force fields such as the modified Dreiding force field,25 CLAYFF force field,26 and CVFF force field27 (or hybrid force field) have been used and different hydration states modeled. While the reliability of the simulation outcomes largely depends upon the accuracy of the force field employed, the simulations can provide significantly increased molecular-scale insights into the structural and energetic origins of the interactions of organic molecules with LDH compounds. Combined with experimental results, these computer simulations can provide a clearer and more detailed picture of the different arrangements in the interlayer of the clay systems beyond pure geometric considerations (see, e.g., refs 8 and 19). In the present work, molecular dynamics simulations are used to probe the interlayer properties of model MgAl-LDHs containing C8H17SO3- sulfonate intercalates. The ab initio general force field (COMPASS)28 is used for all MD simulations. The calculations allow comparison with the recent experimental powder X-ray diffraction and IR vibrational spectroscopy results and also provide plausible insights into the structural and dynamical properties of the interlayer in this intercalated clay system. 2. Simulation Methods In this section we outline the techniques used to simulate the LDH-sulfonate system. For the C8H17SO3- chain sulfonate anion we employed both a classical force field and quantum mechanical density functional theory to perform geometry optimization and compute vibrational frequencies and intensities. For the hybrid LDH-sulfonate system the general ab initio force field (COMPASS) is used for all the molecular dynamics simulations. COMPASS enables simultaneous predictions of structural, conformational, vibrational, and thermophysical properties for a broad range of molecules in isolation and condensed phases. It is also the first high-quality general force field that consolidates parameters for organic and inorganic materials previously found in different force fields. 2.1. Model Construction. The initial LDH model used in this study was built from the crystallographic structure data obtained from refinement by means of the Rietveld procedure.29

Zhang et al. The atomic coordinates are constructed from the previously reported crystal structure of hydrotalcite, Mg4Al2(OH)12CO3 · 3H2O. The unit cell of the original host structure is trilayer, the space group is R3jm, and the rhombohedral lattice parameters are a ) 3.0460 Å, c ) 22.772 Å, R ) 90°, β ) 90°, and γ ) 120°. The initial interlayer carbonate anions CO32- and water molecules were then removed, and a supercell of 6a × 6a × 2c was set up with lattice parameters 6a ) 6b ) 18.2761 Å and 2c ) 45.5444 Å. After that we removed some middle layers such that a two-layer repeat was built with an interlayer spacing of 20.4 Å, which is close to the experimental interlayer distance for the LDH-sulfonate (C8H17SO3-) system.9 Each hydroxide layer contains 12 Al3+ and 24 Mg2+ ions with the latter being arranged in such a way that they are not located in adjacent octahedrals. Thus, the final Mg:Al ratio in the LDH layer is 2:1. Since the charge of each layer is +12e, 12 optimized chain sulfonate anions C8H17SO3- (see below for the optimization procedures) were introduced into the LDH interlayer with the carbon chain aligned parallel to the c axis with six S atoms in the SO3- group placed close to the Al atoms in the upper and lower LDH layers, respectively. The simulation model thus consists of 2 host layers and 2 guest layers. Twelve water molecules in each layer were also added in between the SO3groups in sulfonates and the LDH layers. 2.2. Characterization and Validation for the Sulfonate Anion. First, we optimized the C8H17SO3- chain sulfonate anions using the Discover Minimizer module in Material Studio (MS) version 4.3.30 The COMPASS force field was employed to perform the optimization, and the quasi Newton procedure was employed for minimization. The total number of iterations used for the convergence is 132 for the isolated sulfonate anion. We then performed vibrational analysis using the Hessian generated at the optimized geometry and associated structure. To cross check the validity of the COMPASS force field we also performed geometry optimization for the same anions C8H17SO3- using quantum mechanical density functional theory (DFT) with the local (LDA) potentials PWC31 and Double Numerical plus d-functions (DND) for the basis, which provides reasonable accuracy at modest computational cost. The DFT calculations are implemented in Dmol3 of MS version 4.3.30 Using the optimized structure a single-point energy calculation was performed, which also yields the Hessian and associated harmonic vibrational frequencies. For this single-point calculation the gradient-corrected (GGA) potential RPBE32 was employed for more accurate results. Comparison of the vibrational frequencies and IR intensities computed from the classical force field calculations and from the DFT calculations is then made for the gas-phase sulfonate anion C8H17SO3-, allowing some conclusions to be drawn as to the strengths and weaknesses of the force field (see below). 2.3. Molecular Dynamics Techniques. Discover in Material Studio (MS) 4.3 was employed to study organo-LDH intercalations through molecular dynamics simulations. Prior to the MD simulations energy minimization was performed to reduce strain. Two sets of minimizations and subsequent MD simulations were performed. First, the LDH framework in the supercell was fixed, while minimizations and subsequent MD simulation were performed for the intercalated species and water. Following this the full organic-inorganic hybrid system was relaxed for a second minimization and ensuing MD simulation. The total nonbonded potential interaction energy of the simulated system consists of long-range Coulombic interactions between partial atomic charges and van der Waals interactions, computed using the Ewald summation technique. The cut off distance is set to

Intercalation of Sulfonate into LDH

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Figure 1. Optimized structures for C8H17SO3- computed using the classical force field method (a) and using quantum mechanical density functional theory (b). One angle between the hydrocarbon chain and the SO3- group is highlighted, which has the most noticeable change after intercalation into LDH.

9.5 Å. All atoms including the LDH, water, and sulfonates are assigned with the COMPASS force field, excepting only the charges for the LDH part which were modified according to CLAYFF force field of Cygan et al.26 since this force field is more accurate for LDH materials specifically. Previous studies have shown that the layer charge can have a major influence on the anion packing mode in the interlayer.18 According to CLAYFF, we set the partial charge for Al to 1.575e, Mg to 1.05e, O to -0.95e, and H to 0.425e. These charges for the LDH are then scaled appropriately for the present case of a 2:1 Mg/Al ratio since the original partial charges in CLAYFF are for 3:1 Mg/Al LDH. Molecular dynamics simulations were performed for a constant-volume/constant-temperature ensemble (NVT) at room temperature T ) 298 K. A time step of 1.0 fs was used, and the total simulation time was 500 ps. Temperature was maintained using an Andersen thermostat with a collision ratio of 1.0. Periodic boundary conditions were applied in three dimensions so that the simulation cell is effectively repeated infinitely in each direction. Analysis reveals that the equilibrium values for the crystallographic parameters and thermodynamic parameters were generally reached within the first 30 ps. In order to calculate the angular distributions a 200 ps MD simulation was separately performed to monitor a specific “tilt” angle between the carbon chain and the SO3- head of the sulfonate, as indicated in Figure 1. The objective was to study whether the geometry of the sulfonate would change significantly upon intercalation into the LDH layers in comparison with the values computed above with both quantum and classical methods for the isolated sulfonate as well as the experimentally suggested value in the gas phase. The procedure for vibrational frequency analysis for the hybrid system is similar to the one performed for the sulfonate, as discussed above. The mean square displacement (MSD), self-diffusion coefficient, and concentration profiles, etc., were calculated using the analysis part in the Discover module in MS 4.3 using the trajectory files generated from 500 ps fully relaxed MD simulations. For the powder X-ray diffraction pattern calculations we employed the REFLEX module in Discover in MS 4.3.30 In our calculations the diffractometer range 2θ was set from 2° to 40° with a step size of 0.05°, and the radiation wavelength was set to 1.54180 Å, which matches with the experimental set up.9 3. Results 3.1. Simulation Results. The optimized structures for the gas-phase sulfonate (C8H17SO3-) are shown in Figure 1. Figure

Figure 2. Minimized structure (a) and final structure (b) after 500 ps MD simulations from fixed LDH layer simulations. During the simulations only the intercalated sulfonate anions and water molecules are allowed to relax.

1a is from the classical force field calculations with the force field COMPASS, whereas structure in Figure 1b is from the quantum DFT calculations. Comparisons of the structures from the two methods indicate that the COMPASS force field is reasonably accurate at a very low cost of computer time. In this figure one angle is specifically labeled since we will monitor the change of this angle after intercalation of the sulfonate into LDH layers. This angle is calculated to be 144.361° from the classical force field method and 143.773° from the quantum mechanical method. Both agree quite well and also are very close to the experimental suggested value of 145°.9 This angle has a noticeable change after intercalation into LDHs (see below for more details), and other geometry parameters such as bond lengths or angles only have minor changes. Next, we present the results from the two sets of minimizations and molecular dynamics simulations performed for fixed LDH layers and for the fully relaxed system using the COMPASS force field. Figure 2 shows the minimized structure from the fixed LDH layer simulations (a) and the final structure after 500 ps MD simulations (b). In the first stage of the simulations the intercalated organic ions as well as water molecules are allowed to relax but the LDH layers have been fixed. From this figure

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Figure 4. Calculated MSD for sulfonate from 500 ps of fully relaxed MD simulations at 300 K.

Figure 3. Same as in Figure 2 except that the full LDH hybrid system is allowed to relax.

we can see that the long axis of the guest molecules is tilted but not uniformly and ideally at the experimentally suggested angle of 55°.9 The sulfonate anions are anchored to the host layers via the SO3- groups, mostly through the electrostatic interactions. The stacking arrangement, especially the orientation of the carbon chains in sulfonates, is somewhat more random than the suggested anion packing modes.9 For such organic hydrocarbon chains several packing modes in the interlayer have been proposed by different researchers.33-35 In general, the hydrocarbon chains are supposed to stand vertically or tilt at an angle with respect to the hydroxide layer to form monolayer, bilayer, or antiparallel packing structures. For the LDH-C8H17SO3- system the experimentalists have suggested the antiparallel (more precisely, interpenetrating bilayer) model to best describe the chain packing modes. Our simulations support such an antiparallel packing mode for the hybrid system, albeit with some degree of disorder as could be expected on a statistical basis. The water molecules are generally hydrogen bonded to the sulfonate oxygen or oxygen in LDH hydroxide layers. In the second stage of simulations the LDH layers as well as intercalates (sulfonates and water molecules) have been fully relaxed. In Figure 3 we show the minimized structure (a) and final structure after 500 ps MD simulations (b) from the fully relaxed simulations. It can be seen that the guest anions are tilted more randomly, but they are still anchored to the host layers via the SO3- groups, thus adopting the antiparallel packing modes in general. The MD simulations indicate that there is a lower degree of ordering arrangement for sulfonates in the interlayer, which is different from the experimental suggestion of perfect highly ordering arrangement. Of course, the experimental suggestion is based on the assumption that the geometry and hence the molecular dimensions of the guest molecules are unaffected upon intercalations. Since the COMPASS force field is a general ab initio force field (i.e., not specifically designed for LDH system) the precise degree of deformation of the layers that is observed may vary somewhat from the true situation, but nevertheless the observation is indicative and in accord with findings for other intercalated LDH systems (e.g., ref 24). In all cases, the water molecules are generally located in between the sulfonate oxygen layers and LDH hydroxide layers and excluded from the hydrophobic regions formed by the sulfonate chains.

Figure 5. Same as Figure 4 except for H2O.

In the following we analyze the mean-square displacement (MSD) of the intercalates, e.g., sulfonate and water, and report the self-diffusion coefficients (D), respectively. Also, we analyze the angular distribution for the labeled angle in sulfonates after intercalation into LDH and the atomic density profiles for selected atoms in the hybrid system. These analyses are based on the trajectories from the fully relaxed MD simulations. In Figure 4 we show the calculated MSD for intercalated sulfonate from 500 ps of fully relaxed MD simulations at 300 K, and similarly, in Figure 5 we plot the calculated MSD for water. The linear nature of the MSDs suggests that, for the purpose of calculating D for water and sulfonate in these models, 500 ps of molecular dynamics simulations is satisfactory, although longer simulations would be more desirable for more accurate simulations. The sulfonate self-diffusion coefficient D was calculated from the gradient of the simulated MSD with time, which is roughly 2.05 × 10-7 cm2/s at 300 K. For water, the estimated self-diffusion coefficient D from the gradient of the simulated MSD is 3.07 × 10-7 cm2/s at 300 K. This value for D is in between the simulated water self-diffusion coefficient of 4.4 × 10-7 cm2/s at 300 K for Mg3Al (terephthalate) LDH (with 64 water molecules) and 1.1 × 10-7 cm2/s at 300 K for Mg2Al (terephthalate) LDH (with 44 water molecules).22,36,37 As would be expected for water constrained between the LDH hydroxide layer and the sulfonates, the simulated water selfdiffusion coefficients are much lower in all cases than the values obtained from simulations of bulk water (1.88 × 10-5 cm2/s at 300 K using the Dreiding force field or TIP3P water parameters)38 or the experimental value obtained for the bulk water (2.3 × 10-5 cm2/s at 298 K).39 This is due to the geometry constraint for water in the LDH layers; thus, water molecules are not as mobile as in the bulk case. In Figure 6 we show the calculated angle distribution for the labeled angle in sulfonates from 200 ps of fully relaxed MD simulations for the LDH-sulfonate hybrid system at 300 K.

Intercalation of Sulfonate into LDH

Figure 6. Calculated angular distribution for the labeled angle in sulfonate from 200 ps of fully relaxed MD simulations for the LDH-sulfonate system at 300 K. The maximum angle appears at 160.00°, which is increased from the gas-phase value of 143.773° (quantum calculation) or 144.361° (classical calculation).

Figure 7. Atomic density profiles from 500 ps of fully relaxed MD simulations for the LDH-sulfonate system at 300 K. The red solid line represents S atoms in the SO3- group, the blue long dashed line represents oxygen atoms of water, the black medium dashed line represents oxygen atoms of the SO3- head, and the purple short dashed line represents hydroxide O atoms in the LDH layers.

The maximum angle appears at 160.00° which is increased from the gas-phase value of 143.77° (quantum calculation) or that of 144.36° (classical calculation) in sulfonate. This angle experienced the most significant change in comparison with the changes in bond length and other angle parameters in the sulfonate after intercalation. An examination of the structure shows that the changes in the labeled angles originate from the electronic interaction between the negatively charged SO3groups in sulfonate and the positively charged metal hydroxide sheets of the anionic clay in order to allow the SO3- groups to anchor to the LDH layers. This phenomenon has been observed for other molecules intercalated into LDH such as the antiinflammatory drugs Diclofenac and Indomethacin.19 This is an interesting observation, which indicates that upon intercalation the geometry of the sulfonates might have changed. Traditionally the interlayer arrangement is inferred from the interlayer separation and the geometrical considerations assuming that the guest molecule is rigid and has the same geometry as that outside the layers. Thus, the experimental analysis regarding the anion packing modes which still assumes the gas-phase geometries of sulfonates might underestimate the flexibility of the sulfonates upon intercalation. In Figure 7 we plot the atomic density profiles orthogonal to LDH layers from 500 ps of fully relaxed MD simulations for the LDH-sulfonate hybrid system at 300 K. In this figure the red solid line represents S atoms in the SO3- group, the blue long dashed line represents oxygen atoms of water, the black medium dashed line represents oxygen atoms of the SO3- head,

J. Phys. Chem. C, Vol. 113, No. 2, 2009 563 and the purple short dashed line represents oxygen atom in the LDH hydroxide layers. The results indicate that although there are deformations for the LDH layer structures the hybrid system still has a well-defined structure with oxygen atoms of OH in the LDH layer closest to the Mg/Al sheet (the z distance is zero for the first Mg/Al sheet in LDH layer), followed by oxygen atoms of water as well as oxygen atoms of the SO3- head, and then the S atoms of the SO3- group in sulfonate. The z distance of the oxygen atoms in water and in the SO3- groups are actually very close and can be regarded at the same layer in general. This confirms that on average the distribution of water molecules is highly structured as they are generally located in between the LDH OH layers and the S atom layer in SO3- groups, and hydrogen bonds between water and LDH layer are formed at room temperature. 3.2. Comparisons with Experiments. The calculated powder X-ray diffraction (XRD) pattern of the optimized structures and the experimental diffraction pattern of the intercalated structure are shown in Figure 8. In this figure the solid line represents the calculated XRD pattern, whereas the dotted line represents the experimental XRD pattern.9 In Figure 8a the calculated XRD pattern is based on the structure from fixed LDH layer simulations, and in Figure 8b the calculated XRD pattern is based on the structure from the fully relaxed simulations. The figure exhibits characteristic features of a layered structure, e.g., quite pronounced basal reflections (0, 0, 3n) due to the strong preferred orientation and weak broaden nonbasal reflections indicating some stacking disorder. The latter cannot be solved by the experimental diffraction method only, and it is more suitable to use molecular modeling. The measured value of basal spacing was dexp ) 20.61 Å and the calculated one from the XRD pattern in Figure 8b is 20. 65 Å. Both agree very well. As pointed out in refs 8 and 40 the intensity difference in the XRD pattern between theory and experiment might be due to the crystallites morphology and roughness of the surface of the experimental sample. In the calculation we assume the periodic structure in three dimensions, while the actual LDH particles are platelet-like sheets. The roughness of the surface has a stronger effect on diffraction at a lower angle than in a higher angle. Comparison of Figure 8a and 8b indicates there is a lower degree of ordering for the fully relaxed structure than for the fixed LDH layer structure, as evidenced by, e.g., the larger weak peak in between the first two strongest peaks in the calculated XRD pattern. Next we compare the calculated and experimental infrared (IR) spectra from 400 to 4000 cm-1 in Figure 9. In Figure 9a the calculated IR spectrum is based on the minimized structure from fixed LDH layer calculations, in Figure 9b the calculated IR spectrum is based on the minimized structure from the fully relaxed calculations, and in Figure 9c the experimental FTIR spectrum is shown.9 Also shown in Figure 9d is the calculated IR spectrum based on the final structure after 500 ps MD simulations for the fully relaxed LDH-sulfonate system. Comparing Figure 9b with 9d it can be seen that they are in general agreement, and thus thermally averaged spectra from a distribution of representative structures sampled during the MD simulations are not expected to change significantly. We note that the general shapes of the spectra are different in Figure 9a and Figure 9b, the differences being associated more with the IR intensities than the frequencies. In principle, the computed IR spectra from the fully relaxed structure should agree better with experimental IR spectra than from the fixed LDH structure. While Figure 9b does in fact mirror the experimental IR spectrum in terms of the broadening and spreading of peaks

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Figure 8. Comparison of the PXRD pattern for the LDH-sulfonate system. The solid line represents the calculated XRD pattern, whereas the dotted line represents the experimental XRD pattern.9 (a) Calculated XRD pattern is based on the structure from fixed LDH layer simulations, and (b) calculated XRD pattern is based on the structure from the fully relaxed simulations. The peak at 28° in the experimental XRD pattern is due to the plane (111) of the internal calibrant.

Figure 9. Comparison of the FTIR spectra for the LDH-sulfonate hybrid. (a) Calculated IR spectrum is based on the minimized structure for which the LDH layers have been fixed and (b) calculated IR spectrum is based on the minimized structure for the fully relaxed LDH-sulfonate system. (c) Experimental FTIR spectrim reported in ref 9 is shown. (d) Calculated IR spectrum is based on the final structure after 500 ps MD simulations for the fully relaxed LDH-sulfonate system.

Figure 10. Calculated IR spectra for one straight-chain sulfonate only. (a) Calculated IR spectrum is based on the classical minimized structure with COMPASS force field as shown in Figure 1a, and (b) calculated IR spectrum is based on the quantum mechanical minimized structure as shown in Figure 1b.

from Figure 9a, it is at the same time apparent that intensities are not well reproduced. Also, shown in Figure 10 are the gasphase sulfonate IR spectra calculated from both quantum and classical methods, which provide some background spectroscopy

information for the sulfonate anion before intercalation into the LDH layer. The IR spectra in Figure 9 show the characteristic vibrations expected for the hydroxide layer, the interlayer water, and the inserted sulfonates. First, experimental FT-IR spectros-

Intercalation of Sulfonate into LDH copy shows a strong broad band peaking around 3470 cm-1 which is due to the OH stretching vibrations. Such a band is shown on the calculated IR spectroscopy in Figure 9a and Figure 9b for LDH-sulfonate system but is not shown in the pure sulfonate IR spectroscopy from both quantum and classical calculations (see Figure 10). Second, LDH-sulfonate system shows a band from 2800 to 3000 cm-1 from both theory and experiment, which is due to the C-H stretching vibrational pattern in the sulfonate. Such a band is also shown in the gasphase sulfonate IR spectroscopy from both quantum and classical calculations. Third, an experimental band peaking at around 1630 cm-1, which corresponds to the HOH bend of water, is also shown on the calculated IR spectroscopy in Figure 9a and Figure 9b for the LDH-sulfonate system but is not shown in the gas-phase sulfonate IR spectroscopy. Other characteristic experimental bands such as SO3 antisymmetrical (1091 cm-1) and symmetrical stretching (1051 cm-1) vibrations as well as metal-oxygen vibrations at 680 cm-1 are also broadly reproduced from simulations. We noticed, however, that the intensity of the calculated IR spectroscopy does not agree well with the experimental FT-IR spectroscopy, which might be due to the fact that the general ab initio COMPASS force field is not specifically designed for LDH hybrid system, and further improvements for this force field are needed in order to fully reproduce the experimental IR spectroscopy. Comparisons of the vibrational frequencies of the gas-phase sulfonate from both quantum and classical methods (see Figure 10) also indicate that the general ab initio force field COMPASS is quite accurate to compute the frequencies but not the intensity. Due to its general nature it might not be as accurate as the CLAYFF force field to describe the LDH part, which is specifically designed for the clay minerals such as hydroxides. 4. Conclusions A combination of molecular modeling and experiment offers the potential to elaborate both structural and dynamical details of organic-inorganic nanostructured materials such as the LDH-sulfonate hybrid system examined in this work. Molecular modeling reveals the arrangement of the guest molecules, layer stacking, and spatial distribution of water molecules in the interlayer gallery of the host structure. The present simulations indicate that the long axis of the guest sulfonate molecules is tilted (but not uniformly and ideally at the experimentally suggested angle), anchored to the host layers via the SO3- group, thus adopting an antiparallel packing mode. Water molecules are arranged in the planes adjacent to the host layers together with SO3- groups, and alkyl chains serve as a hydrophobic region. Results of the modeling have been compared with experimental findings such as the X-ray diffractions patterns and IR spectroscopy. Good agreement is found between the calculated and measured basal spacing as well as the PXRD patterns. General agreement for IR spectroscopy in terms of the location of broad vibrational bands has been obtained, but intensities are generally not well predicted. It is apparent that further improvement for the COMPASS force field, or possibly using a combination of force fields for the hybrid system, is likely to be needed in order to predict such quantities more accurately. A combined force field might be, e.g., CLAYFF for the LDH part and CVFF or AMBER for the organic part. Since the parameters of these force fields have been more extensively calibrated for the separate inorganic and biological systems, such an approach might yield better results, but this is yet to be explored and verified one way or the other. At present, we note that this is the first work to employ the COMPASS force field

J. Phys. Chem. C, Vol. 113, No. 2, 2009 565 for LDH hybrid systems, demonstrating both strengths and weaknesses through comparison with experimental data as well as through theory versus theory comparisons with the results of first principal electronic structure theory where the latter is feasible. Despite some apparent shortcomings, the results of the study demonstrate that MD simulations can provide detailed structural and dynamical information, allowing one to draw plausible conclusions in the interpretation of experiment. Close cooperation of modeling and experiment is likely to be a necessity for successful elaboration of the structure and dynamics in these complex yet technologically important hybrid systems. Acknowledgment. We are grateful to the Australian Research Council and The University of Queensland for supporting this work. We also acknowledge generous grants of high-performance computer time from both The University of Queensland (namely, the Computational Molecular Science cluster computing facility within our centre) and the Australian Partnership for Advanced Computing (APAC) National Facility. References and Notes (1) Allmann, R.; Jepsen, H. P. Neues Jahrbuch Miner., Monatsh. (12) 1969, 544. (2) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173. (3) Tichit, D.; Coq, B. Cattech 2003, 7, 206. (4) VanDerVen, L.; VanGemert, M. L. M.; Batenburg, L. F.; Keern, J. J.; Gielgens, L. H.; Koster, T. P. M.; Fischer, H. R. Appl. Clay Sci. 2000, 17, 25. (5) Miyata, S. Clays Clay Miner. 1983, 31, 305. (6) Choy, J. H.; Kwak, S. Y.; Park, J. S.; Jeong, Y. J. J. Mater. Chem. 2001, 11, 1671. (7) Krishnamoorti, R.; Vaia, R. A.; Giannelis, E. P. Chem. Mater. 1996, 8, 1728. (8) Kova´rˇ, P.; Mela´nova´, K.; Zima, V.; Benesˇ, L.; Cˇapkova´, P. J. Colloid Interface Sci. 2008, 319, 19. (9) Xu, Z. P.; Braterman, P. S. J. Phys. Chem. C 2007, 111, 4021. (10) Costa, F. R.; Leuteritz, A.; Wagenknecht, U.; Jehnichen, D.; Ha¨uβler, L.; Heinrich, G. Appl. Clay Sci. 2008, 38, 153. (11) Nakayama, H.; Wada, N.; Tsuhako, M. Int. J. Pharm. 2004, 269, 469. (12) Choy, J. H.; Kwak, S. Y.; Park, J. S.; Jeong, Y. J.; Portier, J. J. Am. Chem. Soc. 1999, 121, 1399. (13) Xu, Z. P.; Zeng, Q. H.; Lu, G. Q.; Yu, A. B. Chem. Eng. Sci. 2006, 61, 1027. (14) Xu, Z. P.; Niebert, M.; Porazik, K.; Walker, T. L.; Cooper, H. M.; Middelberg, A. P. J.; Gray, P. P.; Bartlett, P. F.; Lu, G. Q. M. J. Controlled Release 2008; doi:10.1016/j.jconrel.2008.05.021. (15) Hibino, T.; Kobayashi, W. J. Mater. Chem. 2005, 15, 653. (16) Kumar, P. P.; Kalinichev, A. G.; Kirkpatrick, R. J. J. Phys. Chem. B 2006, 110, 3841. (17) Newman, S. P.; Cristina, T. D.; Coveney, P. V. Langmuir 2002, 18, 2933. (18) Greenwell, H. C.; Jones, W.; Newman, S. P.; Coveney, P. V. J. Mol. Struct. 2003, 647, 75. (19) Mohanambe, L.; Vasudevan, S. J. Phys. Chem. B 2005, 109, 15651. (20) Kim, N.; Kim, Y.; Tsotsis, T. T.; Sahimi, M. J. Chem. Phys. 2005, 122, 214713. (21) Wang, J.; Kalinichev, A. G.; Amonette, J. E.; Kirkpatrick, R. J. Am. Mineral. 2003, 88, 398. (22) Newman, S. P.; Williams, S. J.; Coveney, P. V.; Jones, W. J. Phys. Chem. B 1998, 102, 6710. (23) Kumar, P. P.; Kalinichev, A. G.; Kirkpatrick, R. J. J. Phys. Chem. C 2007, 111, 13517. (24) Thyveetil, M.-A.; Coveney, P. V.; Greenwell, H. C.; Suter, J. L. J. Am. Chem. Soc. 2008, 130, 4742. (25) Mayo, S. L.; Olafson, B. D.; Goddard, W. A. J. Phys. Chem. 1990, 94, 8897. (26) Cygan, R. T.; Liang, J.-J.; Kalinichev, A. G. J. Phys. Chem. B 2004, 108, 1255. (27) Kitson, D. H.; Hagler, A. T. Biochemistry 1988, 27, 5246. (28) Sun, H. J. Phys. Chem. B 1998, 102, 7338. (29) Belloto, M.; Rebours, B.; Clause, O.; Lynch, J.; Bazin, R.; Elkaim, E. J. Phys. Chem. 1996, 100, 8527. (30) Materials Studio Release Notes, Release 4.3; Accelrys Software Inc.: San Diego, 2008.

566 J. Phys. Chem. C, Vol. 113, No. 2, 2009 (31) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244. (32) Hammer, B.; Hansen, L. B.; Norskov, J. K. Phys. ReV. B 1999, 59, 7413. (33) Meyn, M.; Beneke, K.; Lagaly, G. Inorg. Chem. 1990, 29, 5201. (34) Carlino, S. Solid State Ionics 1997, 98, 73. (35) Xu, Z. P.; Braterman, P. S. J. Mater. Chem. 2003, 13, 268. (36) Aicken, M.; Bell, I. S.; Coveney, P. V.; Jones, W. AdV. Mater. 1997, 9, 496.

Zhang et al. (37) Bell, I. S.; Kooli, F.; Jones, W.; Coveney, P. V. Mater. Res. Soc. Proc. 1996, 453, 73. (38) Boek, E. S.; Coveney, P. V.; Williams, S. J.; Bains, A. Mol. Simul. 1996, 18, 145. (39) Mills, R. J. Phys. Chem. 1973, 77, 685. (40) Kova´rˇ, P.; Pospı´sˇil, M.; Nocchetti, M.; Cˇapkova´, P.; Mela´nova´, K. J. Mol. Model. 2007, 13, 937.

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