Molecular Dynamics Simulation on Structure of Water Molecules in a Kaolinite-Water System

Using Hendricks's model as initial structure, a molecular dynamics simulation without crystallographic restrictions was used to investigate the kaolinite-water systemwith the CLAYFF force field. Results showthat interlayer water molecules have three types: type I is similar to Costanzo's“hole water”molecule and its HH vector (a vector from one hydrogen atompointing to another hydrogen atomin water molecule) is parallel to the (001) plane while its C2-axis is slightly sloped and is nearly normal to the (001) plane; type II is similar to“associated water”where one O—H bond points to its neighboring layer tetrahedral oxygen and its hydrogen forms hydrogen bonds with oxygen while the other O—Hbond is approximately parallel to the (001) plane; type III watermolecule approximates a vertical form where one hydrogen forms a bond with tetrahedral oxygen and the other forms a hydrogen bond with a hydroxyl oxygen in the opposite clay layer. The concentration curve of the hydroxyl hydrogen of kaolinite along the normal line of the (001) plane shows that a portion of the hydroxyls change their orientation to be nearly parallel to the (001) plane. The corresponding oxygen atoms in the hydroxyl can therefore be exposed and forma hydrogen bond with interlayer water. Furthermore, some oxygen atoms of Type II water molecule deviate from their average position in the interlayer space and are closer to the tetrahedral layer and this result is consistent with Costanzo's result and may be the reason for the weak (002) peak observed in X-ray diffraction.