Force Field for Mica-Type Silicates and Dynamics of Octadecylammonium Chains Grafted to Montmorillonite

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Abstract

Layered silicates are widely used in nanotechnology and composite materials. We describe a force field for phyllosilicates (mica, montmorillonite, and pyrophyllite) on the basis of physically justified atomic charges, van der Waals parameters, vibrational constants, and distributions of charge defects in agreement with solid state 29Si NMR data. Unit cell parameters deviate only ∼0.5% relative to experimental X-ray measurements and surface (respectively cleavage) energies deviate less than 10% from experimental data, including the partition between Coulomb and van der Waals contributions. Reproduction of surface energies facilitates quantitative simulations of hybrid interfaces with water, organics, and biomolecules for which accurate force fields are available. Parameters are consistent with the force fields PCFF (polymer consistent force field), CVFF (consistent valence force field), CHARMM (chemistry at Harvard macromolecular mechanics), and GROMACS (Groningen machine for chemical simulations). As an example of interest, we investigate the structure and dynamics of octadecylammonium montmorillonite (“C18”−montmorillonite, cation exchange capacity = 91 mmol/100 g) by molecular dynamics simulation. The surfactant chains assemble essentially as a bilayer with minimal interpenetration within the gallery while the ammonium headgroups are hydrogen-bonded to cavities in the montmorillonite surface. In contrast to quaternary ammonium ions, no rearrangements on the surface have been observed (cavity crossing barrier >5 kcal/mol). The alkyl chains are in a liquidlike state with approximately 30% gauche conformations, in agreement with previous Fourier-transform infrared and solid-state NMR measurements. Computed X-ray diffraction patterns of sodium and C18−montmorillonite agree very well with X-ray patterns from experiment, and the computational model can assist in the assignment of complex reflections.

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