Understanding adsorption of amine surfactants on the solvated quartz (101) surface by a jointed Dreiding-ClayFF force field

Highlights: • A jointed Dreiding-ClayFF force field is used to study adsorption mechanism of amine surfactants on quartz. • The solvated quartz and surfactant systems are built in a range of pH values. • Two amine surfactants, DDA and LAO, have different adsorption properties in various pH environments. • The simulation results are closely consistent with flotation tests. In this study, ClayFF and Dreiding force fields are jointed to model the adsorption of amine surfactants or their hydrolysate species on the solvated quartz surface under a wide range of pH values. As two case studies, cationic surfactant DDA (dodecylamine) and amphoteric surfactant LAO (N,N-Dimethylaminopropyldodecylamide oxide) are respectively selected as typical surfactants in molecular dynamic simulations. The simulation results demonstrate that DDA and LAO have excellent adsorption under natural conditions, but they have different adsorption mechanisms owing to their different polar groups. The polar group (-RN⁺H₃) of hydrolysis DDA can penetrate the first hydration layer and perpendicularly adsorb on the quartz surface via strong electrostatic attraction and hydrogen bonds. Compared with DDA, the hydrolysis LAO has a larger polar group consisting of R₂N⁺→OH and -C(=O)NH– and its polar group lay horizontally around the first hydration layer rather than penetrate it. Besides, the adsorption of amine surfactants on quartz is inferred as multilayer based on the partial density distributions (PDD) analysis results. Subsequently, we determined the adsorption amounts, adsorption configurations and interaction energy that agreed with the quartz recovery in flotation test with a wide range of pH values.