Structure of montmorillonite intercalated with methylene blue. Molecular simulations and experiment

M. Pospíšil¹, R. Macháň¹, P. Malý¹, Z. Klika², P. Čapková^1,2, P. Horáková²,

M. Valášková²

¹Faculty of Mathematics and Physics, Charles University Prague, Ke Karlovu 3, 12116, CZ

²VŠB-Technical University Ostrava, 708 33 Ostrava-Poruba, CZ

pospisil@karlov.mff.cuni.cz

Structure analysis using combination of experimental (X-ray powder diffraction, infrared spectroscopy, fluorescence, etc.) and theoretical (molecular simulations) methods is powerful tool for solving disordered structures. In this work the structures of two different types of montmorillonites (MMT) intercalated with methylene blue cations are solved. Crystallochemical formula of Wyoming MMT is Na_0.41K_0.14Ca_0.07 (Al_3.01Mg_0.48Fe³⁺_0.49) [Si_7.81Al_0.17Ti_0.02]O₂₀(OH)₄ and Ca-Cheto MMT: Na_0.10K_0.04Ca_0.50 (Al_2.80Mg_1.00Fe³⁺_0.20) [Si_7.86Al_0.14]O₂₀(OH)₄. The fully intercalated samples of Wyoming and Cheto MMT with methylene blue cations were prepared repeated intercalation by methylene blue solutions. The concentration of methylene blue in solutions was determined by photometry method (UV-VIS spectrophotometer – Lambda 25, Perkin Elmer). X-ray powder diffraction (XRPD) measurements were carried out in INEL X-ray powder diffract meter. Fluorescence was excited using a He-Cd laser at 325 nm. Molecular simulations were carried out in Cerius² modelling environment [1] to describe the structure characterization obtained from XRPD. Initial model of montmorillonite: space group C2/m, the unit cell parameters: a = 5.208 Å and b = 9.020 Å [2]. To create the supercell of reasonable size for calculations, the structural formula determined from chemical analysis was slightly modified. That means for Wyoming MMT the supercell 3a x 2b x 1c with the layer composition (Al₁₈Mg₃Fe³⁺₃)(Si₄₇Al₁)O₁₂₀(OH)₂₄ was built with the total negative layer charge (-4) and for Cheto MMT the supercell 3a x 2b x 1c with the layer composition (Al₁₇Mg₆Fe³⁺₁)(Si₄₇Al₁)O₁₂₀(OH)₂₄ was built with the total negative layer charge (-7). The potential energy was described with Universal force field [3], charges were calculated by Charge equilibration [4] and minimization was done in Minimizer module with fix cell parameters: a, b, g. Quench molecular dynamics simulations in NVT ensemble was done for minimized models. Temperature was T = 300 K and kept constant using Berendsen thermostat [5], silicate layers were fixed during dynamic simulation.

The amount of MB intercalated into MMT increase with increasing MB concentration in the intercalation solution. Different amount of MB cations was placed in the interlayer space of MMT to built models for fully and partially exchanged MB-MMT. Optimized structures for the fully exchanged MB-Wyoming MMT (4 MB⁺ cations per supercell) led to the basal spacing 1.77 nm and for partially exchanged MB-Wyoming MMT (2 MB⁺ and 2 Na⁺ cations per supercell) exhibit the basal spacing in the range 1.51 – 1.53 nm. The arrangement of MB guests is monolayer with MB planes slightly tilted and the long axis almost parallel with the silicate layers. Modeling of MB-Cheto MMT led to the similar results. Models of fully exchanged MB-Cheto MMT (7 MB⁺ cations per supercell) led to the basal spacing 2.03 nm and where MB⁺ cations create bilayer arrangement of MB dimmers with the tilting angles higher than in case of MB-Wyoming MMT. Optimized structure of the partially exchanged MB-Cheto MMT (4 MB⁺ and 3 Na⁺ cations per supercell) exhibits the basal spacing 1.77 nm. Comparison of calculated basal spacing with the experimental values confirmed the conclusion, that the samples of Wyoming and Cheto MMT were saturated but not fully exchanged. According to molecular modelling the basal spacing calculated for the fully exchanged Cheto MMT was 2.03 nm, which means that the samples MB-Cheto MMT with experimentally measured basal spacing 1.71 nm and 1.86 nm were fully not fully exchanged.

Fluorescence measurements for MB-Wyoming MMT show very similar band profile covering a wide wavelength range for all investigated samples. In opposite of this the fluorescence intensity is very low nearly negligible for MB-Cheto. The high layer charge in Cheto MMT requires the high MB⁺ concentration in the interlayer. MB⁺ cations interact with negatively charged silicate layer, with neighbouring guests and also with the rest of exchangeable cations in the interlayer space. Consequently the charge transfer between MB⁺ cations and their environment is enhanced and the fluorescence intensity decreased.

Structure analysis using combination of diffraction data with molecular modelling revealed the differences in the interlayer arrangement of MB⁺ guests in Wyoming and Cheto MMT. Moreover fluorescence measurements showed the strong effect of the silicate layer charge on the spectroscopic behaviour of MB⁺ guests intercalated in MMT. Methylene blue exhibits the strong luminescence in Wyoming MMT and almost no luminescence in Cheto MMT.

1. Cerius² documentation, Molecular Simulations Inc. San Diego (June 2000), (CD-ROM).

2. S.I. Tsipursky and V.A. Drits, Clay Miner., 19, (1984), 177.

3. A.K. Rappé, C.J. Casewit, K.S. Colwell, W.A.III. Goddard, W.M. Skiff, J. Am. Chem. Soc., 114, (1992), 10024.

4. A.K. Rappé and W.A.III Goddard, J. Phys. Chem., 95, (1991), 3358.

5. H.J.C. Berendsen, J.P.M. Postma, W.F. van Gunsteren, A. DiNola, J.R. Haak, J. Chem. Phys., 81, (1984), 3684.

This research was supported by VZ MSMT 0021620835, 6198910016 and GA ČR 205/03/D111, 205/05/2548.