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SIMULATIONS OF INTERLAYER METHANOL IN Ca- AND Na-SATURATED MONTMORILLONITES USING MOLECULAR DYNAMICS

¹ Laboratory of Chemometrics & BioInformatics, University of Orléans, B.P. 6759, 45067 Orléans Cedex 2, France
² Dipartimento di Scienze Ambientali Agrarie e Biotecnologiche Agroalimentari, Università di Sassari, V. le Italia 39, I-07100 Sassari, Italy
³ Dipartimento di Chimica, Università di Sassari, Via Vienna 2, I-07100 Sassari, Italy
⁴ Istituto di Chimica Agraria, Università di Bologna, V. le Berti-Pichat 10, I-40127 Bologna, Italy

E-mail of corresponding author: marco.pintore{at}univ-orleans.fr

Molecular dynamics computer simulations were used to study methanol molecules confined between the layers of 2:1 phyllosilicates. The model systems are based on natural Ca- and Na-rich montmorillonites. Data from the literature and determined by fitting the calculated layer spacing to experimental values were employed to obtain interactions between the charged 2:1 layers and the solvent molecules. The montmorillonite surface atoms were held rigid and the methyl group in the methanol molecule was represented by a soft Lennard-Jones sphere. Electrostatic interactions were determined by the Ewald sum method, whereas the van der Waals interactions were described by a Lennard-Jones potential. Comparison of our results with diffraction data indicates a good reproduction of the layer spacing. After the initial solvent layer forms, additional solvent layers form only after previous layers are complete. Each Ca²⁺ and Na⁺ ion in the monolayer has four and two methanol molecules, respectively, in the first solvation shell, whereas the solvation shell in the multilayer contains six and four methanol molecules, respectively. This agrees well with experimental data.

Key Words: Methanol • Molecular Dynamics • Montmorillonite