Water Adsorption on Kaolinite Basal and Edge Surfaces.

The sequence of water adsorption is significant to understand the mechanism of clay-water interactions on clay mineral surfaces. Kaolinite is a typical non-expansive phyllosilicate clay, and its water adsorption is generally recognized to occur on the basal surfaces of aluminum-silicate particles, whereas edge surface adsorption is prevalently overlooked due to its complexity despite its potential large surface area available for adsorption. In this study, we used molecular dynamics and metadynamics simulation to quantitatively assess the free energy of water adsorption, viz., matric potential, on kaolinite for four types of external surfaces, namely, a basal silicon-oxygen (Si-O) surface, a basal aluminum-oxygen (Al-O) surface, and edge surfaces with deprotonation and protonation. The results show that edge surfaces exhibit adsorption sites that are more active with the lowest matric potential of -1.86 GPa, lower than that of basal surfaces (-0.92 GPa), due to protonation and deprotonation processes of the dangling oxygen. The adsorption isotherm from 0.2% of relative humidity (RH) was measured and analyzed using an augmented Brunauer-Emmet-Teller model to separate the edge and basal surface adsorption, further verifying that edge surface adsorption may prevail in kaolinite and occur earlier than base surface adsorption in RH less than 5%.