Microscopic adhesion between flotation collector molecules and coal surfaces

The hydrocarbon oil collectors play an important role in separating low-quality coal while there are few reports on the microscopic interaction between collector molecules and coal surface. In this study, the attachment/detachment forces and interfacial adsorption behavior between collectors and coals were analyzed by Atomic Force Microscopy Single Molecule Force Spectrum, DLVO/EDLVO theories, and Density Functional Theory calculation. On hydrophilic surface, the RCH3 probe always shows repulsion with the hydrophobic surface, while the RCOOH probe has a weak jump in attachment with the hydrophobic surface. The detachment force generated by the RCOOH probe is greater than that generated by the RCH3 probe, which indicates that the polar carboxyl collector adheres firmly to the hydrophilic surface. The DLVO theory can fit the force curves of the hydrophilic system. The van der Waals force is the gravitational effect, and electrostatic repulsion dominates the interaction process. On the hydrophobic surface, it is found that both RCH3 and RCOOH probes have repulsion first and then jump in attachment with the hydrophobic surface, and the detachment forces generated by the RCH3 probe is greater than that generated by the RCOOH probe during the distance process, indicating that the non-polar alkyl collector adheres firmly to the hydrophobic surface. The EDLVO theory can fit the force curves of the hydrophobic system. The electrostatic force is always a repulsive force, and the hydrophobic force is always an attractive force. The attenuation length is the core parameter that determines the hydrophobic force. The attenuation length between RCH3 and hydrophobic surface is significantly larger than that between RCOOH and hydrophobic surface. Low-rank coal molecules have electrostatic potential extre-mum sites, so that the non-polar alkyl collector can only be adsorbed on the aromatic ring structure of low-rank coal through hydrophobic hydrocarbon chain. The polar carboxyl collector has positive/negative electrostatic potential extreme points on the hydrophilic head group, which can be adsorbed on the negative electrostatic potential extreme points of low-rank coal molecule through weak interactions such as hydrogen bonds. The polar carboxyl collector can cover the oxygen-containing group to achieve hydrophobic effect. The surface hydrophobicity of high-rank coal leads to the easy adsorption of non-polar alkyl collector on its surface, while the hydrophilic groups on the polar carboxyl collector have negative electrostatic potential extremes, which makes it difficult to interact with the surface of high-rank coal. The results clarify the adhesion forces of typical collector molecules on the surface of coal at micro-nano scales, which can provide theoretical reference for the selection and design of flotation collectors. © 2022 China Coal Society. All rights reserved.