Modification of kaolinite/water interfacial hydration by OTAC and PAMT: Insights from AFM force spectroscopy and molecular dynamics

Understanding how hydrophobizing reagents modify the kaolinite/water interfacial environment is important for controlling kaolinite aggregation and solid-liquid separation in coal-slime systems. In this study, a combined experimental-simulation approach was employed to investigate the kaolinite-water-reagent system by integrating wettability, FTIR, XPS, zeta potential, particle-size analysis, and AFM force spectroscopy with molecular dynamics simulations. The addition of OTAC and PAMT significantly enhanced kaolinite hydrophobicity, as evidenced by increased contact angles and decreased surface free energy. Reagent adsorption also promoted particle aggregation, accompanied by a zeta-potential shift from −23 to −14 mV for OTAC and −4 mV for PAMT, and a reduction in AFM pull-off forces by 32 and 75 nN, respectively. Spectroscopic analyses confirmed organic reagent adsorption and indicated changes in the local chemical environment of hydroxylated and oxygen-containing surface groups, but they do not independently prove complete surface coverage or direct occupation of all hydration-active sites. OTAC mainly forms an alkyl-rich interfacial layer through the association of its quaternary ammonium headgroup with the kaolinite surface, whereas PAMT forms a polymeric adsorption layer involving multipoint attachment and bridging interactions. MD simulations further suggest reagent-induced restructuring of interfacial water, as reflected by changes in water-density profiles and weakened water-kaolinite interaction energies. Overall, the combined results support an inferred mechanism in which OTAC and PAMT adsorption modify hydration-active surface environments, reduce solid-water affinity, and promote kaolinite aggregation. PAMT exhibits stronger interfacial modification ability than OTAC because of its polymer-chain architecture and bridging capability. This AFM-MD coupled approach links reagent adsorption, interfacial water restructuring, and macroscopic aggregation responses, providing guidance for reagent design in coal-slime treatment.