Molecular dynamics simulation of cationic and anionic clays containing amino acids

The detailed arrangements of guest species in the interlayers of a cationic and an anionic clay have been determined using molecular dynamics computer simulations. The guest species studied were the amino acids (S)-phenylalanine and (S)-tyrosine, chosen for comparison because both anionic and cationic forms are possible. A modified version of the Dreiding force field was used for all models. Our simulations provide for the first time a clear picture of the different arrangements in the interlayer of the two clay systems. Previous work using powder X-ray diffraction had allowed only possible arrangements to be proposed based on purely geometric considerations. In the anionic clay case, the interlayer anions assume a bilayer-like arrangement with their longest molecular axis oriented in an approximately perpendicular direction with respect to the hydroxide layers. In the montmorillonite system, the interlayer cations prefer a monolayer arrangement with their longest molecular axis parallel to the layers. We believe the difference to be a direct consequence of the different layer charge densities of the clays. In all cases, the water molecules are generally excluded from the hydrophobic midplane formed by the phenyl rings of the counterions. Given the poorly crystalline nature of such intercalates, computer simulations of the type reported here are likely to provide the only means of knowing such details of the interlayer arrangements.