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Water Adsorption on Kaolinite Basal and Edge Surfaces
Haohao Zhang1, Jun Wu2, Chao Zhang3
1State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei Province 430071, China.
Langmuir : the ACS Journal of Surfaces and Colloids
|May 23, 2023
Summary
Water adsorption on kaolinite clay primarily occurs on edge surfaces, not basal surfaces, due to active sites. This finding impacts understanding clay-water interactions and soil hydrology.
Area of Science:
- Geochemistry
- Materials Science
- Soil Science
Background:
- Clay-water interactions are crucial for understanding clay mineral surface mechanisms.
- Kaolinite's water adsorption is typically attributed to basal surfaces, overlooking potentially significant edge surface adsorption.
- Edge surfaces offer large surface areas but are complex and often overlooked in adsorption studies.
Purpose of the Study:
- To quantitatively assess the free energy of water adsorption (matric potential) on different kaolinite surfaces.
- To investigate the role of edge surfaces in kaolinite's water adsorption mechanism.
- To compare water adsorption behavior between basal and edge surfaces of kaolinite.
Main Methods:
- Utilized molecular dynamics and metadynamics simulations to calculate the free energy of water adsorption.
- Quantitatively assessed matric potential on four types of kaolinite external surfaces: basal Si-O, basal Al-O, and deprotonated/protonated edge surfaces.
- Measured adsorption isotherms at 0.2% relative humidity (RH) and analyzed using an augmented Brunauer-Emmet-Teller model.
Main Results:
- Kaolinite edge surfaces exhibit more active adsorption sites with a lower matric potential (-1.86 GPa) compared to basal surfaces (-0.92 GPa).
- Protonation and deprotonation of dangling oxygen on edge surfaces enhance their adsorption activity.
- Experimental data verified that edge surface adsorption prevails and occurs earlier than basal surface adsorption at RH < 5%.
Conclusions:
- Kaolinite's edge surfaces play a dominant role in water adsorption, contrary to previous assumptions.
- The unique properties of edge surfaces, including deprotonation/protonation, significantly influence clay-water interactions.
- Understanding edge adsorption is critical for accurate modeling of water behavior in kaolinite-rich environments.
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