- Email: [email protected]
A DFT study on clay–cation–water interaction in montmorillonite and beidellite
Computational Materials Science 14 (1999) 119±124
A DFT study on clay±cation±water interaction in montmorillonite and beidellite A. Chatterjee a
a,b,*
, T. Iwasaki a, T. Ebina a, A Miyamoto
b
Inorganic Materials Section, Tohoku National Industrial Research Institute, AIST, 4-2-1 Nigatake, Miyagino-ku, Sendai 983, Japan b Department of Materials Chemistry, Graduate School of Engineering, Tohoku University, Aoba-ku, Sendai 980-77, Japan
Abstract This study is the ®rst attempt to generate a realistic model for the clay±cation±water system. We use a molecular description of the solvent and clay sheet. We have chosen two clay materials from 2:1 dioctahedral smectites (1) montmorillonite and (2) beidellite to monitor the eect of negative charge on the location of interlayer cation (Na ), as the negative charge gets introduced in the clay system from octahedral Al substitution and tetrahedral Si substitution as the case may be (1) and (2), respectively. We use Grand Canonical Monte Carlo (GCMC) simulation to locate the interlayer cation and to calculate the number of interlayer water molecules surrounding the cation (Na ) for both the clay materials. The results show that each Na cation is surrounded by ®ve water molecules. The minimum energy conformer obtained from GCMC calculations has been used to generate the cluster model for Local Density Functional (LDF) calculations. The results show the Na cation moves towards the negative center of the clay cluster. It is also observed that Na cation gets more stabilized in montmorillonite in comparison to beidellite. Ó 1999 Elsevier Science B.V. All rights reserved. Keywords: Clay±cation±water interaction; Montmorillonite; Beidellite; GCMC; LDF
1. Introduction Clays are lamellar aluminosilicates showing a large variety of physicochemical properties: swelling, adsorption, ion exchange, surface acidity etc. Their uses are widespread: gelling, decoloring [1], water pollutant elimination [2], radioactive waste repository [3], cracking [4] and heterogeneous catalysis [5]. Montmorillonite and beidellite are member of 2:1 layer silicate family which share the common feature that two tetrahedral sheets sandwich a sheet of octahedrally coordinated
* Corresponding author. Fax: +81 22 236 6839; e-mail: [email protected]
metal ion. Substitution of a divalent metal ion for octahedral Al in montmorillonite and substitution of a trivalent metal ion for tetrahedral Si in beidellite results in a net negative charge and the interaction with positive ions (exchangeable cation) to form an interlayer hydrated phase. There are many experimental investigations of the clay± substrate interactions performed by various techniques e.g. XRD [6], IR [7], EPR [8] and NMR [9]. Many authors also proposed theoretical models to reproduce the solvent±ion±support interaction system [10]. They either neglect the structure of solvent assuming it as continuum, or the support structure assuming it as a smooth surface. Many MC simulations have been directed to understand the swelling of clays or to study clay±water
0927-0256/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 5 6 ( 9 8 ) 0 0 0 8 3 - 4
120
A. Chatterjee et al. / Computational Materials Science 14 (1999) 119±124
interface [11,12]. A reasonable and complete modeling of all the observed data requires a molecular description of the interacting species. Computer simulation can aord a tool for viable solution to this problem. In our earlier study we rationalized the structure property correlation in montmorillonite clays [13] and in a recent study we derived the location of water molecules inside the interlayer of montmorillonite by LDF study [14]. The present communication is a ®rst attempt to design a model for clay±cation±water interaction. It results from a compromise between the complexity of the model, the satisfactory accuracy of the calculations and obviously the computation time. Here we restrict our calculation to clay±cation±water system to predict the location of interlayer cation Na over the surface of clay and the number of water molecules surrounding the interlayer cation at the interface. GCMC simulation was used to study the number of water molecules surrounding Na cation inside the interlayer. Then LDF calculations were performed on the cluster model generated from the minimum energy conformer of GCMC calculation to study the eect of both octahedral and tetrahedral substitution on the orientation of Na cation and water molecules at both the clay surface models.
between accuracy and computational eciency and was used for all calculations presented here. The self consistent ®eld (SCF) tolerance was set to 10ÿ4 and the gradient convergence to 10ÿ3 a.u/ Bohr. The static visualization of molecular con®gurations and the optimized molecular geometries were made with the InsightII code of MSI on a Silicongraphics Indigo2 workstation. Montmorillonite and beidellite structure model has been generated from the well de®ned crystal structure of dioctahedral pyrophyllite having formula Si8 Al4 O20 (OH)4 has been discussed elsewhere in detail [13]. The cluster model chosen to study the clay±cation±water interaction was with formula Al2 Si6 O24 H18 . The cluster model is shown in Fig. 1 along with Na , the bridging oxygens are labeled which has been used to measure the interatomic distance between Na and the bridging oxygens. All the terminal oxygens are capped with hydrogens. One octahedral Al was substituted by divalent Mg to mimic montmorillonite and one tetrahedral Si was substituted by a trivalent Al to mimic beidellite as shown in Fig. 1. To reproduce
2. Method and model GCMC calculation has been performed using SORPTION module of MSI. This is a constant PVT canonical ensemble, however, the number of guest particles are variable. This has been adjusted during the simulation so as to preserve the stipulated values. GCMC simulation can thus capture the equilibrium between a solid sorbent phase and a liquid or gas phase and provides a simulation result in terms of particle population density in the sorbent at the de®ned condition. The force ®eld used in this calculation is cv-aug force ®eld of MSI. LDF calculations were performed using the Dmol code of MSI, version 3.00. The geometry optimization calculations were carried out using a double numerical with polarization (DNP) basis set [15] with ®ne mesh grid and frozen core electrons. The ®ne grid gives a reasonable compromise
Fig. 1. The cluster model used for LDF study along with Na cation. The bridging oxygens are labelled. For montmorillonite one of the octahedral Al will be replaced by Mg and for beidellite one of the tetrahedral Si will be replaced by Al for generating the respective cluster models.
A. Chatterjee et al. / Computational Materials Science 14 (1999) 119±124
the symmetry of the cavity of silicates, the clay fragment contains a hexagonal cavity of silicates plus two inner octahedral aluminum. We ignore the second layer of silica tetrahedral sheets. The calculation consists ®ve water molecules surrounding Na and the clay framework; the framework was kept constant through out the calculation and the rest was optimized. 3. Results and discussion Initially GCMC calculations have been performed on both the clay materials to obtain the minimum energy conformer for Na and the number of surrounding water molecules at the interlayer position. With the minimum energy conformers LDF calculation has been performed to study the eect of both octahedral and tetrahedral substitution on the location of Na on the respective clay surfaces. 3.1. GCMC simulation results The model of clay used in these simulations contains six unit cells. Two clay sheets facing each between the surother with a distance of 2.6 A faces. Each clay sheet contains one Mg2 at the octahedral position for montmorillonite and one Al3 at the tetrahedral position for beidellite. The interlayer space contains three sodium per layer. All the potentials used in this calculation were obtained from cv-aug force ®eld. The potential is cut to zero when the radial distance between two interacting centers becomes larger than half of the length of the cell [16]. To avoid adding water molecule in a forbidden position, we introduce between two oxygens of minimal distances of 1.4 A between the oxygen of water molecules and 1.6 A water and Na cation. We start the simulation with dry clay with Na cation placed between the interlayer and stop the GC simulation when the number of water molecules are constant. This gives the initial con®guration of water molecules in equilibrium with the temperature of the system (298 K). The radial distribution function of water molecules around Na with respect to the number of surrounding water molecules shows a ®rst nar-
121
row hydration peak, followed by a secondary broad peak and the distribution decreases gradually to 0.5. The ®rst shell contains 5.12 0.02 water molecules per Na cation. This result is in close agreement with that obtained by Delville et al. [12]. As the calculation has been performed with three Na cations per layer and the simulation consists of six unit cells in comparison to 16 Na in 24 unit cells in Delville [12], the results reproduce the qualitative trend, but the absolute energy values having a little meaning. The radial distribution function of water molecules surrounding the interlayer cation shows two intriguing properties. First, the secondary peak is broader than the ®rst one, and the second, the radial distribution function shows a plateau at 0.5. The broadening of the secondary peak may be due to the loose packing of the second hydration layer of water surrounding the cation and also may be due to the average of results independently of the position of the cation. The solvation of the condensed countercations may dier from the solvation of the countercations far from the clay surface. The decrease of the function is due to the rigidity of the interlayer spacing which reduces the number of water molecules organized around the counterions. 3.2. LDF calculation results GCMC calculation results give us the number of water molecules surrounding the interlayer cation Na . Now to study the eect of dierent charge centers located in dierent clay material namely octahedral in montmorillonite and tetrahedral in beidellite, LDF calculations have been performed on the respective cluster models with ®ve water molecules and a interlayer cation. Here we study the situation at the surface of the individual clay materials. The adsorption energy of Na on individual clay clusters has been calculated by the dierence of energy between the adsorption complex and the individual energies of the adsorbates and the framework. The results of the adsorption energies are given in Table 1. At ®rst we performed the LDF calculation on AlMgSi6 O24 H18 cluster mimicking montmorillonite. At the starting con®guration the Na cation is at a
122
A. Chatterjee et al. / Computational Materials Science 14 (1999) 119±124
Table 1 The optimized distance between Na+ cation and bridging oxygens as labelled in Fig. 1, the average vertical distance between the Na+ cation plane and surface oxygen plane, the net charge on respective layers along with adsorption energy of Na+ cation in montmorillonite and beidelite cluster models Properties
Montmorillonite AlMgSi6 O24 H18
Beidellite Al2 Si5 AlO24 H18
Distance between Na+ and bridging oxygens (in A)
Na-O14 1.57 Na-O16 1.57 Na-O2 4.04 Na-O21 3.16 Na-O11 4.08 Na-O13 3.05 0.77 ÿ1.17 ÿ12.76
Na-O14 2.08 Na-O16 2.45 Na-O2 5.89 Na-O21 4.64 Na-O11 5.80 Na-O13 4.23 2.98 ÿ0.82 ÿ8.23
Average vertical distance between Na+ plane and the surface oxygen plane (in A) Net charge on octahedral and tetrahedral layer, respectively Adsorption energy of Na+ (in Kcal/mol)
from both Mg and Al. The clay distance of 4.37 A cluster was kept ®xed and the rest was optimized. It is observed that after optimization the Na cation moves towards the Mg2 center of mon tmorillonite and the distance changes to 3.93 A. Fig. 2 shows the optimized con®guration of Na and water molecules. Na cation is located above the hexagonal cavity just over the Mg2 octahedral center. The individual distances between Na and
the bridging oxygens were calculated and shown in Table 1. It clearly shows that the Na cation moves towards the negative center of the octahedral layer of montmorillonite. Now the average vertical distance between the Na plane and the It is oxygen plane was calculated and it is 0.77 A. observed that four water molecules are at the same plane with Na cation, but the 5th water molecule is pinched betweeen the cation and the clay
Fig. 2. The optimized location of Na cation and surrounding ®ve water molecules over the surface of the montmorillonite layer. Mg occupies one of the octahedral Al position.
A. Chatterjee et al. / Computational Materials Science 14 (1999) 119±124
surface. The calculation is repeated for the cluster model Al2 Si5 AlO24 H18 which resembles beidellite. The results are given in Table 1. It is observed that the water orientation changes a little bit but mainly the Na cation moves towards Al3 at the tetrahedral site con®rming the dependence of layer charge on the location of interlayer cation. Here the starting distance between Na and Al3 was which changes to 2.15 A. The average ver3.2 A tical distance between Na plane and the oxygen Now comparing the adsorption plane is 2.98 A. energy values for Na over both montmorillonite and beidellite as included in Table 1 shows that the Na adsorption over montmorillonite is more stable than that of beidellite. The optimized orientation of Na and water molecules over beidellite cluster model was shown in Fig. 3. Now to explain the dierence in adsorption energy and the distance of Na from the oxygen plane Mulliken Population results obtained from LDF study were analysed. It is observed that the net charge of the octahedral layer for montmorillonite is more negative in comparison to the net charge of tetrahedral layer of beidellite as shown in
123
Table 1. This result in a stronger attractive force between the octahedral ÿve charge center Mg2 of montmorillonite towards Na compared to that between tetrahedral ÿve charge center Al3 of beidellite towards Na . Now these results clearly indicates that the location of Na cation over the clay surface is governed by the presence of ÿve charge center in the respective clay materials. Now in the real situation the eect is a cumulative type from both sides of the layers as Na cation is located in the interlayer. Our proposition in terms of stability of Na cation over the respective clay surface is to be followed by a periodic density functional calculation to predict the absolute orientation of the interlayer cation for the studied clay materials which is an aim of our future study. 4. Conclusion This is the ®rst LDF study on clay±cation±water interaction model. The GCMC results show that the interlayer cation Na is surrounded by ®ve water molecules and the cation is located near
Fig. 3. The optimized location of Na cation and surrounding ®ve water molecules over the surface of the beidellite layer. Al occupies one of the tetrahedral Si position.
124
A. Chatterjee et al. / Computational Materials Science 14 (1999) 119±124
to the surface. The LDF study rationalize the dependence of octahedral and tetrahedral charge on the location of Na cation. It is observed that in case of montmorillonite the distance between Na cation plane and oxygen is less than that in case of beidellite. The adsorption energy for Na also proposes a stable adsorption complex in montmorillonite in comparison to beidellite. This results explains the adsorption of Na over clay surfaces, so the cumulative eect of the layer facing this surface is neglected. The optimistic results enable us to think for a periodic density functional calculation to propose the exact location of Na cation at the interlayers. References [1] S. Herjavec, L. Maric, Z. Mlinaric, V. Krauthakar, D. Razoumovic, Jugosl. Vinograd. Vinar. 21 (1987) 19.
[2] C. Ray, R. Ramsey, Environ. Prog. 6 (1987) 145. [3] L.H. Baetsle, P. Henrion, M. Put, Proceedings of the International Topical Meeting, Colombus OH, 1985, p. 313. [4] L.B. Ryland, M.W. Tamle, J.N. Wilson, Catalysis, Ch. 1, vol. 7, Rheinhold, New York, 1960. [5] T.J. Pinnavia, Science 220 (1983) 365. [6] C.H. Pons, F. Rousseaux, D. Tchoubar, Clay. Miner. 16 (1981) 23. [7] G. Sposito, R. Prost, Chem. Rev. 82 (1982) 553. [8] S.A. Boyd, M.M. Mortland, Nature 316 (1985) 532. [9] J. Grandjean, P. Laszlo, Clays Clay Miner. 37 (1989) 403. [10] G. Aloisi, M.L. Foresti, R. Guidelli, P. Barnes, J. Chem. Phys. 91 (1989) 5592. [11] A. Delville, S. Sokolowski, J. Phys. Chem. 97 (1993) 6261. [12] A. Delville, J. Phys. Chem. 99 (1995) 2033. [13] T. Ebina, T. Iwasaki, A. Chatterjee, K. Katagiri, G.D. Stucky, J. Phys. Chem. B 101 (1997) 1125. [14] A. Chatterjee, T. Iwasaki, T. Ebina, H. Hayashi, Appl. Surf. Sci. 121/122 (1997) 167±170. [15] B. Delly, J. Chem. Phys. 94 (1991) 7245. [16] C. Pangali, M. Rao, B.J. Berne, J. Mol. Phys. 40 (1980) 661.
A DFT study on clay±cation±water interaction in montmorillonite and beidellite A. Chatterjee a
a,b,*
, T. Iwasaki a, T. Ebina a, A Miyamoto
b
Inorganic Materials Section, Tohoku National Industrial Research Institute, AIST, 4-2-1 Nigatake, Miyagino-ku, Sendai 983, Japan b Department of Materials Chemistry, Graduate School of Engineering, Tohoku University, Aoba-ku, Sendai 980-77, Japan
Abstract This study is the ®rst attempt to generate a realistic model for the clay±cation±water system. We use a molecular description of the solvent and clay sheet. We have chosen two clay materials from 2:1 dioctahedral smectites (1) montmorillonite and (2) beidellite to monitor the eect of negative charge on the location of interlayer cation (Na ), as the negative charge gets introduced in the clay system from octahedral Al substitution and tetrahedral Si substitution as the case may be (1) and (2), respectively. We use Grand Canonical Monte Carlo (GCMC) simulation to locate the interlayer cation and to calculate the number of interlayer water molecules surrounding the cation (Na ) for both the clay materials. The results show that each Na cation is surrounded by ®ve water molecules. The minimum energy conformer obtained from GCMC calculations has been used to generate the cluster model for Local Density Functional (LDF) calculations. The results show the Na cation moves towards the negative center of the clay cluster. It is also observed that Na cation gets more stabilized in montmorillonite in comparison to beidellite. Ó 1999 Elsevier Science B.V. All rights reserved. Keywords: Clay±cation±water interaction; Montmorillonite; Beidellite; GCMC; LDF
1. Introduction Clays are lamellar aluminosilicates showing a large variety of physicochemical properties: swelling, adsorption, ion exchange, surface acidity etc. Their uses are widespread: gelling, decoloring [1], water pollutant elimination [2], radioactive waste repository [3], cracking [4] and heterogeneous catalysis [5]. Montmorillonite and beidellite are member of 2:1 layer silicate family which share the common feature that two tetrahedral sheets sandwich a sheet of octahedrally coordinated
* Corresponding author. Fax: +81 22 236 6839; e-mail: [email protected]
metal ion. Substitution of a divalent metal ion for octahedral Al in montmorillonite and substitution of a trivalent metal ion for tetrahedral Si in beidellite results in a net negative charge and the interaction with positive ions (exchangeable cation) to form an interlayer hydrated phase. There are many experimental investigations of the clay± substrate interactions performed by various techniques e.g. XRD [6], IR [7], EPR [8] and NMR [9]. Many authors also proposed theoretical models to reproduce the solvent±ion±support interaction system [10]. They either neglect the structure of solvent assuming it as continuum, or the support structure assuming it as a smooth surface. Many MC simulations have been directed to understand the swelling of clays or to study clay±water
0927-0256/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 5 6 ( 9 8 ) 0 0 0 8 3 - 4
120
A. Chatterjee et al. / Computational Materials Science 14 (1999) 119±124
interface [11,12]. A reasonable and complete modeling of all the observed data requires a molecular description of the interacting species. Computer simulation can aord a tool for viable solution to this problem. In our earlier study we rationalized the structure property correlation in montmorillonite clays [13] and in a recent study we derived the location of water molecules inside the interlayer of montmorillonite by LDF study [14]. The present communication is a ®rst attempt to design a model for clay±cation±water interaction. It results from a compromise between the complexity of the model, the satisfactory accuracy of the calculations and obviously the computation time. Here we restrict our calculation to clay±cation±water system to predict the location of interlayer cation Na over the surface of clay and the number of water molecules surrounding the interlayer cation at the interface. GCMC simulation was used to study the number of water molecules surrounding Na cation inside the interlayer. Then LDF calculations were performed on the cluster model generated from the minimum energy conformer of GCMC calculation to study the eect of both octahedral and tetrahedral substitution on the orientation of Na cation and water molecules at both the clay surface models.
between accuracy and computational eciency and was used for all calculations presented here. The self consistent ®eld (SCF) tolerance was set to 10ÿ4 and the gradient convergence to 10ÿ3 a.u/ Bohr. The static visualization of molecular con®gurations and the optimized molecular geometries were made with the InsightII code of MSI on a Silicongraphics Indigo2 workstation. Montmorillonite and beidellite structure model has been generated from the well de®ned crystal structure of dioctahedral pyrophyllite having formula Si8 Al4 O20 (OH)4 has been discussed elsewhere in detail [13]. The cluster model chosen to study the clay±cation±water interaction was with formula Al2 Si6 O24 H18 . The cluster model is shown in Fig. 1 along with Na , the bridging oxygens are labeled which has been used to measure the interatomic distance between Na and the bridging oxygens. All the terminal oxygens are capped with hydrogens. One octahedral Al was substituted by divalent Mg to mimic montmorillonite and one tetrahedral Si was substituted by a trivalent Al to mimic beidellite as shown in Fig. 1. To reproduce
2. Method and model GCMC calculation has been performed using SORPTION module of MSI. This is a constant PVT canonical ensemble, however, the number of guest particles are variable. This has been adjusted during the simulation so as to preserve the stipulated values. GCMC simulation can thus capture the equilibrium between a solid sorbent phase and a liquid or gas phase and provides a simulation result in terms of particle population density in the sorbent at the de®ned condition. The force ®eld used in this calculation is cv-aug force ®eld of MSI. LDF calculations were performed using the Dmol code of MSI, version 3.00. The geometry optimization calculations were carried out using a double numerical with polarization (DNP) basis set [15] with ®ne mesh grid and frozen core electrons. The ®ne grid gives a reasonable compromise
Fig. 1. The cluster model used for LDF study along with Na cation. The bridging oxygens are labelled. For montmorillonite one of the octahedral Al will be replaced by Mg and for beidellite one of the tetrahedral Si will be replaced by Al for generating the respective cluster models.
A. Chatterjee et al. / Computational Materials Science 14 (1999) 119±124
the symmetry of the cavity of silicates, the clay fragment contains a hexagonal cavity of silicates plus two inner octahedral aluminum. We ignore the second layer of silica tetrahedral sheets. The calculation consists ®ve water molecules surrounding Na and the clay framework; the framework was kept constant through out the calculation and the rest was optimized. 3. Results and discussion Initially GCMC calculations have been performed on both the clay materials to obtain the minimum energy conformer for Na and the number of surrounding water molecules at the interlayer position. With the minimum energy conformers LDF calculation has been performed to study the eect of both octahedral and tetrahedral substitution on the location of Na on the respective clay surfaces. 3.1. GCMC simulation results The model of clay used in these simulations contains six unit cells. Two clay sheets facing each between the surother with a distance of 2.6 A faces. Each clay sheet contains one Mg2 at the octahedral position for montmorillonite and one Al3 at the tetrahedral position for beidellite. The interlayer space contains three sodium per layer. All the potentials used in this calculation were obtained from cv-aug force ®eld. The potential is cut to zero when the radial distance between two interacting centers becomes larger than half of the length of the cell [16]. To avoid adding water molecule in a forbidden position, we introduce between two oxygens of minimal distances of 1.4 A between the oxygen of water molecules and 1.6 A water and Na cation. We start the simulation with dry clay with Na cation placed between the interlayer and stop the GC simulation when the number of water molecules are constant. This gives the initial con®guration of water molecules in equilibrium with the temperature of the system (298 K). The radial distribution function of water molecules around Na with respect to the number of surrounding water molecules shows a ®rst nar-
121
row hydration peak, followed by a secondary broad peak and the distribution decreases gradually to 0.5. The ®rst shell contains 5.12 0.02 water molecules per Na cation. This result is in close agreement with that obtained by Delville et al. [12]. As the calculation has been performed with three Na cations per layer and the simulation consists of six unit cells in comparison to 16 Na in 24 unit cells in Delville [12], the results reproduce the qualitative trend, but the absolute energy values having a little meaning. The radial distribution function of water molecules surrounding the interlayer cation shows two intriguing properties. First, the secondary peak is broader than the ®rst one, and the second, the radial distribution function shows a plateau at 0.5. The broadening of the secondary peak may be due to the loose packing of the second hydration layer of water surrounding the cation and also may be due to the average of results independently of the position of the cation. The solvation of the condensed countercations may dier from the solvation of the countercations far from the clay surface. The decrease of the function is due to the rigidity of the interlayer spacing which reduces the number of water molecules organized around the counterions. 3.2. LDF calculation results GCMC calculation results give us the number of water molecules surrounding the interlayer cation Na . Now to study the eect of dierent charge centers located in dierent clay material namely octahedral in montmorillonite and tetrahedral in beidellite, LDF calculations have been performed on the respective cluster models with ®ve water molecules and a interlayer cation. Here we study the situation at the surface of the individual clay materials. The adsorption energy of Na on individual clay clusters has been calculated by the dierence of energy between the adsorption complex and the individual energies of the adsorbates and the framework. The results of the adsorption energies are given in Table 1. At ®rst we performed the LDF calculation on AlMgSi6 O24 H18 cluster mimicking montmorillonite. At the starting con®guration the Na cation is at a
122
A. Chatterjee et al. / Computational Materials Science 14 (1999) 119±124
Table 1 The optimized distance between Na+ cation and bridging oxygens as labelled in Fig. 1, the average vertical distance between the Na+ cation plane and surface oxygen plane, the net charge on respective layers along with adsorption energy of Na+ cation in montmorillonite and beidelite cluster models Properties
Montmorillonite AlMgSi6 O24 H18
Beidellite Al2 Si5 AlO24 H18
Distance between Na+ and bridging oxygens (in A)
Na-O14 1.57 Na-O16 1.57 Na-O2 4.04 Na-O21 3.16 Na-O11 4.08 Na-O13 3.05 0.77 ÿ1.17 ÿ12.76
Na-O14 2.08 Na-O16 2.45 Na-O2 5.89 Na-O21 4.64 Na-O11 5.80 Na-O13 4.23 2.98 ÿ0.82 ÿ8.23
Average vertical distance between Na+ plane and the surface oxygen plane (in A) Net charge on octahedral and tetrahedral layer, respectively Adsorption energy of Na+ (in Kcal/mol)
from both Mg and Al. The clay distance of 4.37 A cluster was kept ®xed and the rest was optimized. It is observed that after optimization the Na cation moves towards the Mg2 center of mon tmorillonite and the distance changes to 3.93 A. Fig. 2 shows the optimized con®guration of Na and water molecules. Na cation is located above the hexagonal cavity just over the Mg2 octahedral center. The individual distances between Na and
the bridging oxygens were calculated and shown in Table 1. It clearly shows that the Na cation moves towards the negative center of the octahedral layer of montmorillonite. Now the average vertical distance between the Na plane and the It is oxygen plane was calculated and it is 0.77 A. observed that four water molecules are at the same plane with Na cation, but the 5th water molecule is pinched betweeen the cation and the clay
Fig. 2. The optimized location of Na cation and surrounding ®ve water molecules over the surface of the montmorillonite layer. Mg occupies one of the octahedral Al position.
A. Chatterjee et al. / Computational Materials Science 14 (1999) 119±124
surface. The calculation is repeated for the cluster model Al2 Si5 AlO24 H18 which resembles beidellite. The results are given in Table 1. It is observed that the water orientation changes a little bit but mainly the Na cation moves towards Al3 at the tetrahedral site con®rming the dependence of layer charge on the location of interlayer cation. Here the starting distance between Na and Al3 was which changes to 2.15 A. The average ver3.2 A tical distance between Na plane and the oxygen Now comparing the adsorption plane is 2.98 A. energy values for Na over both montmorillonite and beidellite as included in Table 1 shows that the Na adsorption over montmorillonite is more stable than that of beidellite. The optimized orientation of Na and water molecules over beidellite cluster model was shown in Fig. 3. Now to explain the dierence in adsorption energy and the distance of Na from the oxygen plane Mulliken Population results obtained from LDF study were analysed. It is observed that the net charge of the octahedral layer for montmorillonite is more negative in comparison to the net charge of tetrahedral layer of beidellite as shown in
123
Table 1. This result in a stronger attractive force between the octahedral ÿve charge center Mg2 of montmorillonite towards Na compared to that between tetrahedral ÿve charge center Al3 of beidellite towards Na . Now these results clearly indicates that the location of Na cation over the clay surface is governed by the presence of ÿve charge center in the respective clay materials. Now in the real situation the eect is a cumulative type from both sides of the layers as Na cation is located in the interlayer. Our proposition in terms of stability of Na cation over the respective clay surface is to be followed by a periodic density functional calculation to predict the absolute orientation of the interlayer cation for the studied clay materials which is an aim of our future study. 4. Conclusion This is the ®rst LDF study on clay±cation±water interaction model. The GCMC results show that the interlayer cation Na is surrounded by ®ve water molecules and the cation is located near
Fig. 3. The optimized location of Na cation and surrounding ®ve water molecules over the surface of the beidellite layer. Al occupies one of the tetrahedral Si position.
124
A. Chatterjee et al. / Computational Materials Science 14 (1999) 119±124
to the surface. The LDF study rationalize the dependence of octahedral and tetrahedral charge on the location of Na cation. It is observed that in case of montmorillonite the distance between Na cation plane and oxygen is less than that in case of beidellite. The adsorption energy for Na also proposes a stable adsorption complex in montmorillonite in comparison to beidellite. This results explains the adsorption of Na over clay surfaces, so the cumulative eect of the layer facing this surface is neglected. The optimistic results enable us to think for a periodic density functional calculation to propose the exact location of Na cation at the interlayers. References [1] S. Herjavec, L. Maric, Z. Mlinaric, V. Krauthakar, D. Razoumovic, Jugosl. Vinograd. Vinar. 21 (1987) 19.
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