New insights into the role of interfacial components in oil-in-water nanoemulsion collector for low rank coal flotation

Low rank coal (LRC) presents flotation challenges due to its high oxygen content, large specific surface area, and poor hydrophobicity. In this study, we design various oil-in-water nanoemulsion collectors to enhance the LRC flotation efficiency and selectivity, with different interfacial charge components based on nonionic surfactant (fatty alcohol polyoxyethylene ether, AEO-7), anionic surfactant (sodium dodecyl sulfate, SDS), cationic surfactant (dodecyltrimethylammonium bromide, DTAB), and their typical hybrids. The microscopic characteristics and dynamic stability of nanoemulsions were systematically evaluated using particle size analysis, zeta potential measurements, and multiple light scattering methods. In addition, the LRC flotation performance and relative fundamental principles were identified through flotation dynamic tests, bubble–particle wrap angle measurements, particle–bubble adhesion/detachment force analyzation, and nanoemulsion-particle adhesion force tests. Results indicated that the AEO-7:SDS (8:2) nanoemulsion exhibited superior stability, small droplet size, and high negative zeta potential due to synergistic interfacial film formation that combined electrostatic repulsion and steric hindrance, inhibiting droplet aggregation and Ostwald ripening. In contrast, the AEO-7:DTAB (8:2) nanoemulsion showed the best flotation selectivity, achieving 83.76% clean coal yield with 5.11% ash content. This performance enhancement was attributed to the highest bubble–particle wrap angle (258.8°), strongest adhesion force (66.1 µN), and improved particle interactions. Mechanistically, DTAB electrostatically adsorbed onto oxygen-containing groups on the LRC surface, while AEO-7 synergistically increased surface hydrophobicity and enhanced bubble–particle attachment. This study provides both theoretical insights and practical guidance for designing optimum nanoemulsion collectors with combined interfacial components for coal flotation applications.