J. Phys. Chem. C 2007, 111, 11259-11266
11259
Kaolinite:Dimethylsulfoxide IntercalatesA Theoretical Study Andrea Michalkova´ *,†,‡ and Daniel Tunega†,§,⊥ Institute of Inorganic Chemistry, SloVak Academy of Sciences, Du´ braVska´ cesta 9, 842 36 BratislaVa, SloVakia, Computational Center for Molecular Structure and Interactions, Department of Chemistry, Jackson State UniVersity, 1400 Lynch Street, P.O. Box 17910, Jackson, Mississippi 39217, Austrian Research Centers Seibersdorf, A-2444 Seibersdorf, Austria, and Institute for Theoretical Chemistry, UniVersity of Vienna, Wa¨hringerstrasse 17, A-1090, Austria ReceiVed: January 22, 2007; In Final Form: May 10, 2007
Three models of the kaolinite:dimethylsulfoxide intercalate with 25, 50, and 100% saturation of dimethylsulfoxide (DMSO) have been investigated by means of density functional theory (DFT, fully periodic approach). The analysis of interatomic distances was used to characterize interactions between DMSO molecules and kaolinite layers. It was found that the most dominant forces in the formation of stable intercalates are the hydrogen bonds formed between the sulfonyl oxygen atom and surface hydroxyl groups. Weak hydrogen bonds, formed between the methyl groups and oxygen atoms of both adjacent layers, play an important role in the final localization and stabilization of the DMSO molecule in the interlayer space. Very good agreement was obtained between calculated (13.0 and 15.2 kJ mol-1) and experimental (13.0 and 16.5 kJ mol-1) energetic barriers for the free rotations of both methyl groups of the intercalated DMSO molecule. The calculation of the reaction energy of the intercalation process has shown a relatively high energetic gain despite two endothermic processes are involved in the intercalation processsthe interlayer separation of the kaolinite structure and the separation of the DMSO molecules from liquid phase.
Introduction Kaolinite (Al2Si2O5(OH)4) is a layered aluminosilicate mineral belonging to the class of dioctahedral 1:1 clay minerals, and its structure is well-known.1-5 Individual kaolinite layers consist of an octahedral aluminum hydroxide sheet and a tetrahedral silicate sheet interconnected via the plane of apical oxygen atoms. One side (octahedral) of the kaolinite layer is covered with the surface hydroxyl group while the opposite side (tetrahedral) is formed from basal oxygen atoms of the tetrahedral sheet. The structural model of the single kaolinite layer is presented in Figure 1. Kaolinite layers remain together via the hydrogen bonds formed between the surface hydroxyl groups and basal oxygen atoms. Small, usually polar chemical species can relatively easily penetrate into the interlayer space. It can lead to the formation of perspective materials with new rheological, structural, and surface properties.6,7 The insertion of molecules, known as intercalation, produces an expansion of the interlayer space and is accompanied by the changing of hydrogen bond configuration in this space. Original hydrogen bonds between layers are broken and new ones are formed between guest molecules and both surfaces if intercalated molecules have a proton donor or proton acceptor functional group like -COOH, -COH, -OH, CdO, -NH2, or dNH. The surface hydroxyl groups can also act as proton donors or proton acceptors, as observed recently in theoretical studies of the interactions of water and acetic acid with the surface hydroxyl groups of the single kaolinite layer.8-10 * Corresponding author. E-mail:
[email protected]. † Slovak Academy of Sciences. ‡ Jackson State University. § Austrian Research Centers Seibersdorf. ⊥ University of Vienna.
Recent years represent increasing interest in hybrid materials formed through the intercalation of small organic species and polymers between the sheets of layered silicates minerals. New materials can show novel properties such as tremendous improvement in mechanical properties due to an enhanced strength, higher gas barrier behavior, better thermal resistance, and flame retardancy of intercalated polymers.11-13 In order to expand the interlayer space and lower the barrier for an intercalation of nonpolar polymers, the silicate layers are often treated with an expansion precursorsa solvent comprised of small polar molecules, e.g., DMSO.14-16 The process of the intercalation of DMSO into kaolinite was often in the center of various experimental investigations. The first structural studies showed that the kaolinite structure expands from 7.2 to 11.2 Å after intercalation.17,18 Later structural and several solid-state NMR studies documented that the expansion of the interlamellar space is also accompanied with a three-dimensional structural ordering.19-21 It was supposed that this ordering is related to the locking of the DMSO molecule above the hole in the octahedral sheet by means of the hydrogen bonds formed between the sulfonyl oxygen atom and surface hydroxyl groups. An additional coordination of two methyl groups by the basal oxygen atoms of the tetrahedral surface from the adjacent layer was also found. Dynamic NMR studies of the molecular motion of DMSO have shown that one methyl group is keyed into the ditrigonal hole of the tetrahedral sheet and the S-C bond of the second methyl group is parallel to the sheet.22-24 Significant changes in the infrared and Raman bands assigned either to kaolinite or the DMSO molecule have confirmed breaking of the hydrogen bonds between kaolinite layers and the formation of new ones between the guest DMSO molecule and surface hydroxyl groups.25-28 Kinetics, thermal stability, and thermal
10.1021/jp070555h CCC: $37.00 © 2007 American Chemical Society Published on Web 07/10/2007
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Figure 1. Structure of the isolated kaolinite layer.
decomposition of kaolinite:DMSO intercalates were studied using the thermal methods.29-33 There are only several theoretical studies of the intercalated phyllosilicate systems based on a quantum-chemical approach published to date. The cluster model approach and the B3LYP/ 3-21G* quantum-chemical method were applied in the study of intercalates of dickite and kaolinite with formamide, Nmethylformamide, and DMSO.34 Ab initio molecular dynamic simulation was used in the study of interactions between trichloroethene and the clay mineral nontronite.35 Structural aspects of interactions of D- and L-peptides with nontronite were studied by the DFT approach and the periodic models.36 The mechanism of copolymerization of methanal and ethylendiamine in the montmorillonite interlayer space was investigated using the DFT calculations.37 This work follows our previous theoretical investigation of kaolinite intercalates,34 where the cluster model approach was used. This approach allows the use of standard quantumchemical programs for molecules and offers a variety of methods and basis sets and easiness in the calculations of various properties. However, this approach has also some disadvantages. First, the cluster represents a limited number of atoms around the active site. Second, formation of the cluster as a cutoff from the periodic structure of the solid creates spurious electronic states at the border of the cluster, which must be handled in some way to avoid artifacts in the subsequent calculation.38 Finally, long range interactions are missing in the cluster approach which can, for example, produce a systematic error of calculated properties. Usually, several clusters have to be tested to see the dependence of studied properties on the cluster size. However, the cluster can quickly reach a size limit if some demanding quantum-chemical methods are used. Therefore, the theoretical method with implemented periodic boundary conditions seems to be often more effective than the cluster approach. In the present work periodic models of kaolinite:DMSO intercalates with different concentrations of DMSO in the interlayer space of kaolinite have been studied by means of a periodic DFT approach. Energetic and structural descriptions of the kaolinite:DMSO models are presented. The nature of
interactions between kaolinite layers and DMSO molecules is explained, and formed hydrogen bonds are analyzed. Finally, the decomposition of the intercalation energy is proposed. Computational and Structural Details Static periodic ab initio calculations have been used to obtain the relaxed structures and corresponding electronic energies for all studied systems. All calculations have been performed using the Vienna ab initio simulation package (VASP)39,40 based on an iterative solution of the Kohn-Sham equations of the density functional theory41 based on minimization of the norm of the residual vector to each eigenstate and an efficient charge density mixing. The localized density approximation (LDA) parametrized according to Perdew-Zunger42 and the generalized gradient approximation (GGA) parametrized according to Perdew-Wang43 were used for the description of the electron exchange-correlation interaction. Projector-augmented wave (PAW) atomic pseudopotentials44,45 in combination with a plane-wave basis set were applied in all calculations. The PAW method combines the advantage of the plane-wave basis set with the accuracy of the all-electron schemes. The Brillouin-zone sampling was restricted to the Γ point since the computational unit cells were sufficiently large. In all structural relaxations, the plane-wave cutoff energy was 400 eV, implying a high-precision quality of the calculation. The optimization of atomic positions was performed using a conjugate-gradient algorithm with a stopping criterion of 10-5 eV for the total energy. The experimental structure (P1 symmetry, a ) 5.187 Å, b ) 8.964 Å, c ) 11.834 Å, R ) 91.53°, β ) 108.59°, γ ) 89.92°) of the kaolinite:DMSO intercalate20 was used as the basis for our structural models. X-ray and neutron powder diffraction methods used in the referred experimental work showed two DMSO molecules in the unit cell. We used a doubled-lattice parameter a, since we want to study several concentrations of DMSO in the interlayer space. It offers a higher variability with the number of the DMSO molecules in the computational cell. Three models were prepared. The first one, labeled K:DMSO4, corresponds to the experimental structure with a maximum
Kaolinite:Dimethylsulfoxide Intercalate
Figure 2. Two views of the three studied models: K:DMSO4 (A), K:DMSO2 (B), and K:DMSO1 (C). The unit cell is drawn as a dashed line.
number (4) of DMSO molecules in the computational unit cell (the A model in Figure 2). In the second model, K:DMSO2, only 50% saturation is used. It means that 2 DMSO molecules are localized in the computational cell as can be seen in the B model in Figure 2. Only one DMSO molecule (25% saturation) is presented in the computational cell of the last structure, K:DMSO1 (the C model in Figure 2). Two regimes of the geometry optimizations were applied for all models. First, the full relaxation of all atomic positions was performed at the constant unit cell parameters. In the second step, the unit cell parameters were also relaxed together with the atomic positions. In order to evaluate the intercalation energy for each model, the total electronic energies of both subsystems (mineral and DMSO) were calculated. For this purpose, both subsystems of each model were relaxed in the same computational cells as the parent model. Since the interlayer space of the intercalated kaolinite structure is expanded, the total electronic energy of the original, unintercalated mineral kaolinite was also calculated to estimate the energy necessary for the expansion. The experimental structure of the kaolinite mineral derived by Neder et al.5 was used for this purpose. Results and Discussion Structural Relaxations. Figure 2 represents a global view on all three studied models. It is clear that in the A model (K: DMSO4) all possible places located nearly below the ditrigonal
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11261 hole in the tetrahedral sheet of the upper layer are occupied by the DMSO molecules. In the B model, 50% of these positions are empty, and the DMSO molecules are organized in a chain. In the C model, only 1/4 of all possible positions are occupied, and each place with the DMSO molecule is surrounded by six empty places. In the first two models (A and B), the mutual interactions of the long-range character (a dipole-dipole type) among the DMSO molecules can be expected since the distance between centers of two neighboring ditrigonal holes is about 5.2 Å. However, it seems that these interactions have only a small effect on the geometry of the intercalated molecules if models A and B are compared to the C model. This is documented in Table 1 where all important geometry parameters of the DMSO molecules in all three models are collected (averaged values are given for the A and B models). Table 1 also contains the DMSO structural parameters calculated by a molecular cluster approach34 and taken from two experimental works.20,21 The experimental data were obtained for the fully saturated kaolinite intercalate that corresponds to our A model (K:DMSO4). As one can see, the cluster model calculation leads to larger bond lengths, angles, and dihedral angles of the DMSO molecule (except the C1-S-C2 angle) in comparison to the periodic model A, which is structurally closest to the cluster model (no neighboring DMSO molecules). The observed difference is mainly due to different DFT approaches (B3LYP functional and SVP basis set in the case of the cluster calculation,34 and PW91 functional and the plane wave basis set in this work). Similar differences between the same approaches were observed in the study of the water molecule interactions on the kaolinite surfaces.9 Comparison of the structural parameters of the intercalated DMSO molecule showed a good agreement between calculated (K:DMSO4 model) and experimental values (see Table 1). Table 2 contains results of the optimized unit cell parameters for all three studied systems. Presented numbers are compared to the experimental values,21 which were obtained for the fully saturated kaolinite intercalate (it corresponds to the A model (K:DMSO4)). Very good agreement has been found between optimized (K:DMSO4 model) and experimental unit cell parameters, and only minimal differences are observed between the system with a half occupation of the interlayer space (K:DMSO2 model). The largest change (small shrinking) is observed for the c parameter of the system with the 25% occupation (K:DMSO1 model). In this case it means a consequent reduction of the computational cell volume to 990.7 Å, but it is still only about a 5% reduction of the experimental unit cell volume. From the molecular symmetry, two methyl groups in free DMSO molecule are equivalent. This equivalency is lost in the intercalated DMSO molecule. One methyl group is keyed in the tetrahedral sheet while the C-S bond of the second group is nearly parallel to the surfaces of the kaolinite layers. XRD measurements have revealed21 that the C1-S distance of the keyed CH3 group is shorter than the second one (C2-S). Our calculated C-S distances for all three studied models (Table 1) are in agreement with experimental observations. Generally, both calculated C-S distances are shorter than in the free DMSO molecule (Table 1). Also, measured splitting of the C-S stretching vibrations21,26-29 and splitting of 13C MAS NMR signal21-24 confirm the difference between both methyl groups. Interactions. The authors of experimental works20-29 have concluded that the DMSO molecule is aligned in the interlamellar space by two types of interactions with the adjacent kaolinite layerss(a) the interaction of the sulfonyl oxygen atom
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TABLE 1: Structural Parameters of the DMSO Moleculea parameterb
free DMSO
K:DMSO1 (C model)
K:DMSO2 (B model)
K:DMSO4 (A model)
ref 34c
expt 1d
expt 2e
d(SdO) d(C1-S) d(C2-S) d(C1-H) f d(C2-H) f R(C1-SdO) R(C2-SdO) R(C1-S-C2) γ(C1C2SO)
1.502 1.822 1.822 1.096 1.096 106.0 106.0 97.3 110.0
1.549 1.777 1.800 1.092 1.095 102.4 104.9 100.9 106.1
1.541 1.787 1.804 1.092 1.094 105.4 105.5 98.1 108.6
1.537 1.786 1.795 1.093 1.095 105.6 105.7 97.5 108.6
1.566 1.813 1.824 1.101 1.094 106.1 106.3 97.4 109.4
1.53 1.80 1.80 1.09 1.09 102 102 98.5 107
1.55(5) 1.75(6) 1.81(6) 109(2) 105(2) 100(3) -
a Loading of the kaolinite sample in both experiments corresponds to the A model (full saturation by DMSO). b Bond lengths in angstroms; angles in degrees. c Cluster model, B3LYP/SVP method. d Ref 20, standard deviations were not given. e Ref 21, standard deviations are in parentheses. f Averaged values.
TABLE 2: Calculated and Experimental Unit Cell Parameters and Unit Cell Volumesa system
a/Å
b/Å
c/Å
R/deg
β/deg
γ/deg
volume/Å3
K:DMSO1 (C) K:DMSO2 (B) K:DMSO4 (A) K:DMSOb,c kaolinitec,d
10.368 10.397 10.444 10.394(4) 10.308(9)
8.960 8.977 8.986 8.960(6) 8.942(4)
11.253 11.774 11.761 11.866(7) 7.401(10)
91.55 91.33 91.73 91.63(2) 91.69(9)
108.54 108.82 108.39 108.74(2) 104.61(5)
89.97 90.07 89.85 89.91(2) 89.92(4)
990.7 1039.9 1046.9 1046.04(6) 659.8(1)
a Loading of the kaolinite sample in both experiments corresponds to the A model (full saturation by DMSO). b Experimental, ref 20. c Originally, parameter a is half of the parameter a used in our calculations. d Unexpanded kaolinite, parameters taken from ref 5.
with the surface hydroxyl groups of the octahedral sheet and (b) the interaction of one methyl group with the tetrahedral sheet of the opposite layer. A very detailed picture of the localization and contacts of the DMSO molecule is given in Figure 3 (only the C model). The sulfonyl oxygen atom of the DMSO molecule plays the most dominant role in the stabilization and the localization of the molecule in the interlayer space. The SdO group forms several hydrogen bonds with the surface hydroxyl groups of the octahedral sheet. The surface hydroxyl groups are very flexible, and they are able to behave as proton donors or acceptors in the hydrogen bonds with incoming polar species to the octahedral surface.8-10 Table 3 contains distances and angles of formed hydrogen bonds, X-H‚‚‚Y, in all studied models. For the hydrogen bond classification we used a distance cutoff limit of 3.2 Å for H‚‚‚Y and an angular cutoff of >90° for the X-H‚‚‚Y angle, as is usually proposed in the hydrogen bonds studies (e.g., refs 46-50). The sulfonyl oxygen atom contacts three surface OH groups around the vacant position in the octahedral sheet. Owing to the flexibility of the OH groups, the hydrogen bonds are effectively arranged having lengths between 1.66 and 1.92 Å (HB1-HB3 in Table 3) and a colinearity of more than 170°. Similarly, three hydrogen bonds with the surface hydroxyl groups were found for the carbonyl oxygen atom of acetic acid and for the water oxygen atom.8,9 The participation of the sulfonyl oxygen atom in the formation of the hydrogen bonds produces an elongation of the SdO bond of about 0.04 Å compared to the free DMSO molecule (Table 1). In the case of the fully saturated C model, all surface hydroxyl groups are involved in the hydrogen bonds with the sulfonyl oxygen atoms since each hole in the octahedral sheet is surrounded by three surface hydroxyl groups. Detailed analysis of the interatomic distances for all hydrogen atoms of both methyl groups was performed for all three optimized models. All three H atoms of the first methyl group, which is close to the basal oxygen atoms of the upper layer, form very weak hydrogen bonds with these oxygen atoms. This methyl group is “keyed” in the ditrigonal hole of the tetrahedral sheet. It is illustrated in Figure 3 for the K:DMSO1 model. One of these H atoms (upper) is located above the center of the
ditrigonal hole. This H atom is in optimal contact with three of six oxygen atoms surrounding the ditrigonal hole. These weak contacts can be classified as very weak hydrogen bonds of C-H‚ ‚‚O type (HB7, HB8, and HB9 in Figure 3). The second H atom is in contact with another two basal oxygen atoms forming the HB10 and HB11 hydrogen bonds (Figure 3), and the last hydrogen atom is closest to another oxygen atom of the ditrigonal hole (HB12 in Figure 3). Thus, all six basal oxygen atoms around the ditrigonal hole are in some way involved in weak hydrogen bonds with the keyed -CH3 group. The analysis of the interatomic distances for the second -CH3 group, labeled as unkeyed, showed that two of its hydrogen atoms also enter to the formation of weak hydrogen bonds with the oxygen atoms from both the upper and bottom kaolinite layers (HB4, HB5, and HB6 in Figure 3 and Table 3). One H atom of this methyl group contacts two of the basal oxygen atoms of the tetrahedral sheet (HB4 and HB5). The second H atom interacts with the oxygen atom of one surface hydroxyl group of the octahedral sheet of the second layer, forming the HB6 hydrogen bond. This hydrogen bond is formed due to the flexibility of the surface hydroxyl groups. The third hydrogen atom of the second methyl group is too far from the oxygen atoms of both layer surfaces to form a hydrogen bond. Formed C-H‚‚‚O hydrogen bonds are characterized as weak with O‚‚ ‚O distances in the 2.57-3.17 Å interval and contribute to the final localization of the DMSO molecule in the interlayer space. The existence of these hydrogen bonds represents a certain energetic barrier for a free rotation of the whole molecule around the axis perpendicular to the surface planes and also for the rotation of both -CH3 groups. The activation energies, Ea, for both methyl groups (13.0 kJ mol-1 for the first, keyed, and 16.5 kJ mol-1 for the second, unkeyed) were estimated on the base 13C MAS NMR studies over the temperature range 170-330 K.23 We have performed the calculation of the rotational barrier for both methyl groups of the DMSO molecule in the K:DMSO1 model. All points were obtained as a single-point calculation with the rotational step of 10°. The first point (0°) represented the optimized structure. Figure 4 shows calculated energetic curves for both methyl groups. Both curves have three clear minima and maxima as is expected due to the symmetry of the
Kaolinite:Dimethylsulfoxide Intercalate
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11263
Figure 4. Calculated energetic curves for the rotation of both methyl groups of the DMSO molecule for the K:DMSO1 model. Energies are relative to the optimized K:DMSO1 structure.
Figure 5. Calculated energetic curve for the rotation of the whole DMSO molecule around the axis perpendicular to the kaolinite layer for the K:DMSO1 model. Energies are relative to the optimized K:DMSO1 structure.
TABLE 3: X-Y Distances (in Å) and Angles (in degrees) of X-H‚‚‚Y Hydrogen Bonds for All Three K:DMSO Modelsa
Figure 3. Different hydrogen bonds formed between the DMSO molecule and the surfaces of the kaolinite layers in the K:DMSO1 model (three views). The unit cell is drawn as a dashed line.
methyl group. Obtained energetic barriers are 13.0 kJ mol-1 and 15.2 kJ mol-1 for the keyed and unkeyed methyl group, respectively. This result is in very good agreement with the experimentally derived values. Because both of our calculated activation energies are in some way approximative (the rotated -CH3 group is rigid and no thermal contributions are included in the calculations), they can be directly compared to energies from the experiment23 since the experimental values were derived according to the Arrhenius relation, in which activation energy is temperature independent. The following step in the calculations was to simulate a movement of the DMSO molecule in the interlayer space, particularly some rotation and/or translation, respectively. The aim of these calculations was to estimate the energetic barriers for such types of movements. We emphasize that, similar to the case of the calculations of the rotational barriers of the -CH3 groups, all calculations are performed without the inclusion of zero-point energy, and thermal corrections and presented values
hydrogen bond
K:DMSO1
K:DMSO2
K:DMSO4
HB1b HB2b HB3b HB4 HB5 HB6 HB7 HB8 HB9 HB10 HB11 HB12
1.659 (175.7) 1.692 (175.0) 1.810 (171.9) 2.567 2.926 2.568 2.407 2.855 2.501 2.649 2.721 2.771
1.792 (175.3) 1.780 (176.0) 1.920 (178.9) 2.966 3.174 2.862 2.384 2.895 2.817 2.727 2.952 2.819
1.831 (174.1) 1.753 (175.4) 1.920 (178.5) 2.795 2.592 2.708 2.425 2.792 2.675 2.746 2.852 3.020
a The labeling of hydrogen bonds corresponds to Figure 3. b Values of the X-H‚‚‚Y angles are in parentheses.
can be considered as some upper limits for the calculated barriers. Figure 5 shows the calculated energy curve for the rotation of the DMSO molecule starting from the optimized structure of the K:DMSO1 model. The molecule was rotated around the axis, which is perpendicular to the kaolinite layer and passes through the oxygen atom of the sulfonyl group. We assume that during such rotation this oxygen atom is bound more strongly to the surface OH groups via three hydrogen bonds than both methyl groups which are bound only via weak hydrogen bonds. In each point of the curve in Figure 5, the geometry optimization was performed. The curve has two local minima, which are higher than the found global minimum of about 5-7 kJ mol-1. The energetic barrier of the molecular rotation estimated from the calculation is about 23 kJ mol-1, and it is higher than that for the rotation of the methyl groups. The calculated barrier is relatively high and hinders free molecular rotation in the interlayer space. This is in agreement
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Figure 6. Calculated energetic profiles for the translational movement of the DMSO molecule along the two unit cell vectors a and b for the K:DMSO1 model. Energies are relative to the optimized K:DMSO1 structure.
TABLE 4: Calculated Intercalation Energies (in kJ mol-1)a model
∆E1b
∆E1/ DMSO
K:DMSO1 K:DMSO2 K:DMSO4
-93.3 -170.5 -289.4
-93.3 -85.3 -72.4
∆E2c
∆E2/ DMSO
-13.2 -43.6 -92.2
-13.2 -21.8 -23.1
a E(K:DMSOx) opt is the energy of the optimized intercalate model. E(Ke)opt and E(Ks)opt are energies of the optimized expanded (Ke) and i experimental (Ks) structures of kaolinite. E(DMSOx) opt is the energy of the DMSO clusters in the geometry and the position as in the optimized intercalate models after optimization. E(DMSO1)opt is the energy of the optimized isolated DMSO molecule. b ∆E1 ) E(K: DMSOx)opt - (E(Ke)opt + xE(DMSO1)opt). c ∆E2 ) E(K:DMSOx)opt i (E(Ks)opt + E(DMSOx) opt .
with the 13C NMR measurements23 where no exchange between two methyl groups was observed in the temperature range of 170-330 K. Figure 6 shows the energetic profiles calculated for the translational movement of the DMSO molecule along two unit cell vectors, a and b. The calculations were performed with the fixed cell, and in each point the atomic positions were relaxed at the fixed corresponding fractional coordinate (x and y, respectively, where x is parallel to a and y is parallel to b) of the sulfur atom. Energies are relative to the energy of the minimum found in the geometry relaxation of the K:DMSO1 model. One can see that the translation of the DMSO molecule along the b vector is energetically less favorable than along the a vector. It means that the intercalation will proceed preferentially in the a direction. The minima and maxima on both curves in Figure 6 are dominantly related to breaking and forming of the hydrogen bonds between the sulfonyl oxygen atom and the surface hydroxyl groups. The minima are approximately located close to the center of the ditrigonal holes in the tetrahedral and octahedral sheets as is visualized in Figure 1. Calculated interaction energies for all three models with fixed cell parameters are presented in Table 4. In the first reaction scheme, the interaction energy, ∆E1, is calculated as the difference between the energy of the intercalate (E(K:DMSOx)opt) on the side of reaction products and the sum of the energies of the expanded kaolinite structure (E(Ke)opt) and the isolated DMSO molecule (E(DMSO1)opt) multiplied by the corresponding number of the DMSO molecules (x ) 1, 2, or 4)
on the side of reactants. In all cases, both corresponding subsystems were optimized in the same computational cell. The interaction energy calculated in this way does not cover the energy that is necessary for the expansion of the kaolinite layers or the energy of the interactions among DMSO molecules in the liquid phase since, in practice, the intercalation process is a reaction between solid (kaolinite) and liquid (DMSO) phases. It means that during the intercalation two partial endothermic reactions have to proceed: the expansion of the kaolinite structure and certain extraction of DMSO molecules from the liquid phase. Only the DMSO molecules insertion into the expanded interlayer space is an exothermic process where the main energetic gain comes from the hydrogen bond formation between the sulfonyl oxygen atom and the surface hydroxyl groups and the minor contribution comes from the weak hydrogen bonds formed between both methyl groups and the basal oxygen atoms. Because of these assumptions, the calculated interaction energies (∆E1 in Table 4) are relatively large (in absolute value). For illustration, the standard vaporization enthalpy of DMSO is 52.7 kJ mol-1.51 The third column in Table 4 represents the ∆E1 energy related to the one DMSO molecule (the energy is divided by the corresponding number of DMSO molecules in the computational cell). It seems that according to the ∆E1/DMSO energies that the relative stability slightly decreases from K:DMSO1 to K:DMSO4. However, this is given by the reaction scheme for the calculation of the ∆E1 energy. Two factors affect such calculated interaction energy: (a) the relaxation energy of the DMSO molecule resulting from the geometrical changes between the geometry of the optimized isolated molecule and the geometry of this molecule in the intercalate, and (b) a contribution coming from the mutual interactions of the DMSO molecules in the intercalate since, on the reaction side, there are only isolated DMSO molecules. Table 5 presents computed interaction energies among the DMSO molecules and documents the strong effect of the geometrical relaxation of the DMSO molecule. ∆E3 is the interaction energy computed as the difference between the single-point energy of the DMSOx cluster in the geometry and the position taken from the optimized intercalate model (E(DMi ) and the energy of the optimized isolated DMSO. A SOx) sp substantial change of the DMSO interaction energy (∆E4 in Table 5) is obtained if the energy difference is calculated with i ). the energy of the optimized DMSOx clusters (E(DMSOx) opt
Kaolinite:Dimethylsulfoxide Intercalate
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11265
TABLE 5: Calculated Interaction Energies (in kJ mol-1)a b
c
model
∆E3
∆E4
K:DMSO1 K:DMSO2 K:DMSO4
7.2 12.0 19.1
0.0 -23.4 -29.2
i a E(DMSOx) sp is the single-point energy of the DMSOx clusters in the geometry and the position as in the optimized intercalate i models. E(DMSOx) opt is the energy of the same DMSO clusters after optimization. b ∆E3 ) E(DMSOx)isp - xE(DMSO1)opt. c ∆E4 ) E(DMi SOx) opt - xE(DMSO1)opt.
Technically, the DMSOx clusters were fully optimized, and the starting positions of the DMSO molecules in the clusters were taken from the corresponding optimized kaolinite intercalates. Despite the enormous gain of the relaxation energy that is obtained, the DMSO molecules do not dramatically change their mutual positions in the final optimized DMSOx clusters. On the other hand, the contribution from the interaction energy coming from the interactions among the DMSO molecules in the structure and their position in the intercalate is small. The interaction energy of -1.2 kJ mol-1 was obtained for the DMSO2 cluster and -5.7 kJ mol-1 for the DMSO4 cluster, respectively, from the single-point calculations. Such small contributions result from not-effectively-oriented DMSO molecules in the K:DMSO2 and K:DMSO4 models. The interaction between the DMSO molecules is mainly of a dipole-dipole nature since DMSO is a polar molecule and in both models the dipoles of the DMSO molecules are parallel and have the same orientation, which is not an effective arrangement of the interacting dipoles. The computed interaction energy for the K:DMSO1 system can be compared to the energy obtained using the cluster approach (molecular fragment of solid) and B3LYP/ 3-21G* level of the quantum-chemical theory34 since both energies were computed according to the same reaction scheme. The B3LYP/3-21G* energy of -40.3 kJ mol-1 is substantially smaller (in absolute value) than the energy of -93.3 kJ mol-1 in this work. This difference has at least two reasons: (a) different DFT functionals (B3LYP versus PW91) and a different basis set (small, localized 3-21G* atomic basis set versus nonlocalized plane waves) were used and (b) important longrange contributions to the interaction energy in the cluster approach were missing. Since the previously described reaction scheme has some disadvantages, we decided to estimate intercalation energies according to a different reaction scheme, which could better represent the whole process of intercalation. In this second scheme, the interaction energy, ∆E2, is related to the unexpanded kaolinite structure (E(Ks)opt) so that the energy of the layer separation of the kaolinite structure is included. For this purpose the geometry optimization of the unexpanded kaolinite model was performed on the experimental structure reported by Neder et al.5 (see Table 2). The energy difference between the unexpanded and expanded kaolinite structure is of 80.3 kJ mol-1. This represents the energy necessary for the interlayer separation. The interaction energy, ∆E2, is also related to the clusters of the DMSO molecules rather than to the isolated DMSO molecule. These optimized DMSO clusters are the same as in the calculation of the ∆E4 energies. These interaction energies are collected in Table 5 and have already been discussed in previous paragraph. It means that in this second reaction scheme the interaction energy among the DMSO molecules is also included on the side of reactants. It is the energy that has to be overcome if we want to place the DMSO molecules from these molecular clusters into the interlayer space. This can be considered as a very simple estimation of the energy
necessary for extracting the DMSO molecule from the liquid phase during the real intercalation process; however, these molecular clusters do not really represent liquid DMSO. The calculated reaction energies, ∆E2, are collected in Table 4. It is clear that these values are much lower than corresponding ∆E1 energies (in absolute value). For the K:DMSO4 model (this model corresponds to the fully saturated, experimentally prepared K:DMSO intercalate), the intercalation energy of -92.2 kJ mol-1 was calculated. One-half of this energy (-46.1 kJ mol-1) can be related to the experimental unit cell and a corresponding chemical formula of the kaolinite:DMSO intercalate. Table 4 contains also the ∆E4 energy related to one DMSO molecule. One can see that the ∆E4 energies for the K:DMSO2 and K:DMSO4 models are similar. Unfortunately, we did not find any relevant experimental thermochemical data for the K:DMSO intercalation process to compare with our theoretical investigations. We have to note that such comparison would be only approximative because the calculated energies are not corrected to the zero-point vibrational energies and thermal corrections. If such corrections would be included, even smaller intercalation energies can be expected. It should be mentioned that at the finite temperatures the possibility of some positional and orientational disorder of DMSO at lower concentrations in the interlayer space (the B and C models) can represent another source of uncertainty in comparing the calculated energetic and structural results with the experimental data. On the other hand, the process of thermal decomposition of the K:DMSO complex was investigated several times by means of thermochemical methods,29-33 and the enthalpy of the thermal decomposition of 58.1 kJ mol-1 was obtained using the thermogravimetric measurements.29 It can be mentioned that the DMSO molecules in the thermal decomposition are lost without decomposition of the clay or DMSO. Conclusions Static structural relaxation of three models of the kaolinite: DMSO intercalate (25, 50, and 100% DMSO saturation) has been performed using the DFT approach and applying the periodic boundary conditions for all models. Good agreement was found for the experimental and computed (model K:DMSO4) structural parameters. Analysis of the interatomic distances between atoms of the guest molecule and atoms of the kaolinite layers showed that the dominant forces for a localization of the DMSO molecule in the interlamellar space are three hydrogen bonds formed between the sulfonyl oxygen atom and the surface hydroxyl groups of the octahedral sheet. In agreement with conclusions from experiments, the final arrangement of the DMSO molecule is driven by the locking of one methyl group in the ditrigonal hole of the tetrahedral sheet. It was found that also the second methyl group, considered as free, forms weak hydrogen bonds not only with the basal oxygen atoms of the tetrahedral sheet but with one oxygen atom of the octahedral surface hydroxyl group as well. The analysis of the interatomic distances has shown differences in the nearest environment of both methyl groups which is in agreement with the experimental observations in the infrared/Raman and NMR spectra. Very good agreement was obtained between the calculated and experimental energetic barriers for a free rotation of both methyl groups of the intercalated DMSO molecule. The simulation of the rotational and translational movement of the whole DMSO molecule in the interlayer space has revealed that the DMSO molecules are relatively firmly fixed in their positions, mainly due to the hydrogen bonds formed between the sulfonyl oxygen atoms and the surface OH groups of the kaolinite layer.
11266 J. Phys. Chem. C, Vol. 111, No. 30, 2007 Adding the energy necessary for the expansion of the kaolinite structure and the energy of interactions among the DMSO molecules into the reaction scheme significantly decreases the energetic gain of the whole intercalation process. Both processes (the interlayer expansion and the separation of DMSO molecules) are endothermic processes with respect to the suggested reaction scheme. However, there is still a significant gain of the intercalation energy, and the whole intercalation process is exothermic. Our best estimation of the energy of intercalation is of -46.1 kJ mol-1. This strong energy gain is mainly due to the formation of several relatively strong hydrogen bonds between the sulfonyl group and the surface OH groups of kaolinite. It can be concluded that the whole intercalation process is driven thermodynamically. Acknowledgment. This work was supported by the Austrian Science Fund (Project P15051-CHE) and by the Slovak Grand Agency (Project VEGA 1/7008/20). We are grateful for the technical support from and computer time at the computational resources of the Computer Center of the Slovak Academy of Sciences and at the Linux-PC cluster Schro¨dinger III of the Computer Center of the University of Vienna. References and Notes (1) Adams, J. M. Clays Clay Miner. 1983, 31, 352. (2) Young, R. A.; Hewat, A. W. Clays Clay Miner. 1988, 36, 255. (3) Bish, D. L.; von Dreele, R. B. Clays Clay Miner. 1989, 37, 289. (4) Bish, D. L. Clays Clay Miner. 1993, 41, 738. (5) Neder, R. B.; Burghammer, M.; Grasl, T.; Schulz, H.; Bram, A.; Fiedler, S. Clays Clay Miner. 1999, 47, 487. (6) Theng, B. K. G. Clays Clay Miner. 1982, 30, 1. (7) Lagaly, G. Philos. Trans. R. Soc. London 1984, A311, 315. (8) Tunega, D.; Haberhauer, G.; Gerzabek, M. H.; Lischka, H. Langmuir 2002, 18, 139. (9) Tunega, D.; Benco, L.; Haberhauer, G.; Gerzabek, M. H.; Lischka, H. J. Phys. Chem. B 2002, 106, 11515. (10) Tunega, D.; Gerzabek, M. H.; Lischka, H. J. Phys. Chem. B 2004, 108, 5930. (11) Okada, A.; Usuki, A. Mater. Sci. Eng. C 1995, 3, 109. (12) LeBaron, P. C.; Wang, Z.; Pinnavaia, T. J. Appl. Clay Sci. 1999, 15, 11. (13) Okada, A.; Usuki, A. Macromol. Mater. Eng. 2006, 291, 1449. (14) Gardolinski, J. E.; Carrera, L. C. M.; Canta˜o, M. P.; Wypych, F. J. Mater. Sci. 2000, 35, 3113. (15) Vempati, R. K.; Mollah, M. Y. A.; Reddy, G. R.; Cocke, D. L.; Lauer, H. V., Jr. J. Mater. Sci. 1996, 31, 1255. (16) Letaief, S.; Detellier, Ch. J. Mater. Chem. 2005, 15, 4734. (17) Jackson, M. L.; Abdelkader, F. H. Clays Clay Miner. 1978, 26, 81.
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