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 structur es with 8.5 and 10 Angstrom clay layer spacings. Two types of adsorbed wate r molecules were identified, that are characterized bq different orientatio ns with respect to the surface of the clay sheets. The first type of the mo lecules is adsorbed on the surface of the silicate clay sheet, and oriented with its molecular HH vector parallel to the surface and with molecular di pole inclined by 30 degrees to the surface normal. For the second type of m olecules the HH vectors and the molecular dipoles are perpendicular to the surface and the surface normal, respectively. The increase of the interlaye r spacing and of the number of water molecules in the interlamellar space l ends to the formation of an intermediate layer, Molecules of this layer for m weak hydrogen bonds with hydroxyls of the octahedral clay sheet and stron gly interact with the molecules adsorbed on the surface of the silicate she et, The dynamics of the water molecules in the interlamellar space are anal yzed in terms of the molecular center-of-mass motion, the orientational rel axation 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.