Systematic exploration of quaternary ammonium salt adsorption on oxidized coal based on experiments and MD simulations

To explore the hydrophobic modification mechanism of quaternary ammonium salts on oxidized coal surface, the surface hydration of oxidized coal and the adsorptions of different quaternary ammonium salts on oxidized coal surface were investigated in this study, involved a series of experiments and MD simulations. The results of adsorption, contact angles, and agglomeration observation show that quaternary ammonium salts can adsorb on the oxidized coal surface to improve its surface hydrophobicity. Furthermore, the adsorption quantity of quaternary ammonium salts on oxidized coal surface and their ability to improve surface hydrophobicity increase when the carbon chain length increases. However, quaternary ammonium salts cannot effectively promote the formation of the hydrophobic agglomeration of oxidized coal particles. MD simulation results show that water molecules can undergo strong hydration on oxidized coal surfaces by forming hydrogen bonds with the oxygen-containing functional groups. The OTAC(+) cations mainly accumulate on oxidized coal surfaces through the dispersive force occurring between the non-polar carbon chains of OTAC(+) and non-polar aromatic rings of oxidized coal, and there is a strong self-agglomeration occurs between OTAC(+) cations. The dispersion force between non-polar carbon chains and coal molecules dominates the adsorption of quaternary ammonium salt on oxidized coal surface, which is a very weak force and difficult to ensure the stable adsorption of quaternary ammonium cations on oxidized coal surface in aqueous solution.