Wettability of kaolinite (001) surfaces — Molecular dynamic study

Wettability of two (001) basal kaolinite surfaces (tetrahedral and octahedral) was investigated by means of force-field molecular dynamics simulation. Water nanodroplets were deposited on the surfaces and microscopic contact angle was determined from a droplet profile. In case of the octahedral surface the nanodroplet was spread on the surface forming an incomplete monomolecular layer. Multiple, relatively strong hydrogen bonds of the water layer with the surface hydroxyl groups are responsible for the spreading. The contact angle for the octahedral surface is equal zero, thus this surface is strongly hydrophilic. In case of the tetrahedral surface water molecules of the droplet stayed together forming a deformed sphere on the surface. The computed microscopic contact angle is about 105° meaning that the tetrahedral surface is hydrophobic. In spite of weak interactions of the droplet with this surface the water in the vicinity of the surface is structured and ordered into a form of monomolecular water layers with altering density what reflects a polarization effect of the surface. The experimental values of contact angle for kaolinite are about 20° which indicates that the overall character of the kaolinite surface is hydrophilic and our calculations document that a key factor for this property is surface hydroxyl groups of the octahedral surface.