Water Adsorption on Kaolinite Basal and Edge Surfaces

HH Zhang and J Wu and C Zhang and Y Dong, LANGMUIR, 39, 7539-7547 (2023).

DOI: 10.1021/acs.langmuir.2c03282

The sequence of water adsorptionis significant to understand themechanism of clay-water interactions on clay mineral surfaces.Kaolinite is a typical non-expansive phyllosilicate clay, and itswater adsorption is generally recognized to occur on the basal surfacesof aluminum-silicate particles, whereas edge surface adsorptionis prevalently overlooked due to its complexity despite its potentiallarge surface area available for adsorption. In this study, we usedmolecular dynamics and metadynamics simulation to quantitatively assessthe free energy of water adsorption, viz., matric potential, on kaolinitefor 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. Theresults show that edge surfaces exhibit adsorption sites that aremore active with the lowest matric potential of -1.86 GPa,lower than that of basal surfaces (-0.92 GPa), due to protonationand deprotonation processes of the dangling oxygen. The adsorptionisotherm from 0.2% of relative humidity (RH) was measured and analyzedusing an augmented Brunauer-Emmet-Teller model to separatethe edge and basal surface adsorption, further verifying that edgesurface adsorption may prevail in kaolinite and occur earlier thanbase surface adsorption in RH less than 5%.

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