A Molecular Dynamics Study of Structure and Short-time Dynamics of Water in Kaolinite

The method of molecular dynamics was used to investigate the structure and short-time dynamics of the interlayer water molecules in kaolinite structures with 8.5 and 10 Å clay layer spacings. Two types of adsorbed water molecules were identified, that are characterized by different orientations with respect to the surface of the clay sheets. The first type of the molecules is adsorbed on the surface of the silicate clay sheet, and oriented with its molecular HH vector parallel to the surface and with molecular dipole inclined by 30° to the surface normal. For the second type of molecules the HH vectors and the molecular dipoles are perpendicular to the surface and the surface normal, respectively. The increase of the interlayer spacing and of the number of water molecules in the interlamellar space leads to the formation of an intermediate layer. Molecules of this layer form weak hydrogen bonds with hydroxyls of the octahedral clay sheet and strongly interact with the molecules adsorbed on the surface of the silicate sheet. The dynamics of the water molecules in the interlamellar space are analyzed in terms of the molecular center-of-mass motion, the orientational relaxation times, and the diffusion coefficient of the molecules. The results show significant decrease of the diffusion coefficient and increase of the relaxation times relative to those in bulk water.