Effect of palmitic acid on the wettability of kaolinite and metakaolinite: Insights from microscopic experiments and reactive molecular dynamics simulation

To investigate the mechanisms by which soil organic matter, acting as hydrophobic impurity layers, decreases the hydrophilicity of clay mineral surfaces, and the reasons behind severe soil water repellency following forest fires, this study selected kaolinite, metakaolinite, and palmitic acid (PA) as subjects for research. Combining microscopic experiments with reactive molecular dynamics (RMD) simulations, the results revealed that kaolinite exhibited a stronger adsorption capacity for PA compared to metakaolinite. The hydrophilicity of PA-modified kaolinite (metakaolinite) was significantly reduced, primarily because the particle size of the PA-modified kaolinite (metakaolinite) samples increased considerably. Additionally, hydrophobic groups were predominantly located on the surfaces of these modified particles, resulting in a substantial decrease in the H-bond number formed between water and the PA-modified kaolinite (metakaolinite). The hydrophilicity of PA-modified kaolinite was stronger than that of PA-modified metakaolinite, attributed to the greater exposure of hydrophobic groups on the PA-modified metakaolinite surfaces. Contact angle measurements showed that when PA dosage was ≥ 5 × 10 -6 mol/g, its influence on the surface wettability of kaolinite became negligible. For metakaolinite, a similar effect was observed when PA dosage was ≥ 10 × 10 -6 mol/g. RMD simulations also indicated that when the PA dosage was ≥ 1 layer, it has minimal effect on the wettability of PA-modified kaolinite and metakaolinite octahedra.