Surfactant-nanoparticle hybrids boosted oil in water emulsions: Interface synergistic mechanisms and coal flotation application

The synergistic stabilization mechanisms of surfactant-nanoparticle hybrids in oil-in-water emulsions and their innovative application in enhancing low-rank coal flotation were systematically investigated. Two types of emulsions were initially prepared using a high-shear energy input method: Fatty alcohol polyoxyethylene ether-7 (AEO-7) and AEO-7/nanoparticles (NPs) stabilized emulsions. Multiple light scattering analysis was utilized to investigate their spatiotemporal evolution, revealing that emulsion destabilization arises from gravitational aqueous phase separation in the bottom, creaming in the middle layer, and droplet flocculation/coalescence at the top. Comparative analysis of AEO-7 emulsions and AEO-7/NPs emulsions under equivalent concentrations demonstrated that NPs synergize with AEO-7 to hinder spontaneous droplet coalescence, thereby reducing droplet size variation and enhancing emulsion stability. Notably, this synergistic stabilization mechanism exhibited dependence on surfactant concentration, functioning optimally below the critical micelle concentration (CMC) of 0.4 wt. %, while excessive concentrations induce competitive adsorption. Further investigation into NPs concentration effects revealed that higher NPs concentrations (up to 4.0 wt. %) corresponded to smaller backscattering fluctuations, lower Turbiscan Stability Index values, and improved dynamic stability. However, the minimum droplet diameter of 7.06 μm was achieved at 0.2 wt. % NPs, beyond which droplet size gradually increased, likely due to the formation of complex oil-in-water emulsion structures. NPs elevated surface tension and interfacial tension through competitive adsorption, but compensated for this by forming rigid barriers that hindered droplet migration and coalescence. The optimum surfactant/NPs formulation (0.2 wt. % AEO-7 + 0.2 wt. % NPs) was applied to low-rank coal flotation, demonstrating enhanced combustible recovery, accelerated flotation kinetics, and improved selectivity. This was attributed to the strengthening of coal-emulsion interactions and coal-bubble attachment through AEO-7/NPs emulsion. These findings advance colloidal science by revealing surfactant-NPs synergisms in emulsion stabilization and provide potential strategies for mineral processing.