Calculation of contact angle via Young-Dupré equation with molecular dynamic simulation: Kaolinite as an example

To explore the surface wettability, the contact angle was usually experimentally measured by sessile drop method, which is very sensible to the experimental conditions. Molecular Dynamics (MD) simulation method has been applied to study nanodroplets, but the contact angles were usually obtained via curve fittings of the droplets, which is subjective and the errors inevitable. In this work, Young-Dupré equation was applied to calculate the contact angle with the surface tension coefficient of water and the adhesion work of water/surface, which were achieved via MD simulations results. The wettability of layered kaolinite was studied as an example. The computed contact angle of water on the Al-O octahedral surface of kaolinite was 0°, and those on the Si-O tetrahedral surface were roughly 92° ~ 104°, which agreed well with the results of curve fitting method, about 0° for the Al-O octahedral surface and 106° ± 9° on the Si-O tetrahedral surface. The dipole moment distribution, dipole angle distribution of water molecules, as well as the evolution of hydrogen bond number during the whole dripping process of water droplet on kaolinite surfaces were analyzed to reveal the mechanism of hydrophobic/hydrophilic of different surfaces of kaolinite.