Amphiphilic nitrogen-doped carbon dots derived from bituminous coal: Enhanced oil-water interfacial activity and nanomaterial dispersibility

Efficient and clean utilization of coal resources is critical to achieving carbon emission reduction and sustainable energy development. In this study, a series of amphiphilic nitrogen-doped coal-based carbon dots (C₁₂ ∼ C₁₈-NCDs) were synthesized from bituminous coal through a green oxidation-depolymerization process mediated by formic acid and hydrogen peroxide, followed by hydrothermal nitrogen doping, alkylation, and sulfonation. The resulting NCDs exhibited tunable amphiphilicity, as confirmed by comprehensive structural and interfacial characterizations. Increasing the alkyl chain length from C12 to C18 significantly reduced the critical micelle concentration (from 802 to 121 mg/L at 25 °C) and lowered the surface tension at CMC (from 28.13 to 25.31 mN/m). Notably, C₁₈-NCDs achieved ultra-low oil-water interfacial tension (0.00119 mN/m) under 10 wt% NaCl and exhibited excellent wettability on hydrophobic paraffin surfaces (contact angle: 44.63°). Furthermore, the amphiphilic NCDs demonstrated outstanding dispersibility for multi-walled carbon nanotubes (MWCNTs), effectively suppressing aggregation due to the synergistic effects of π–π stacking interactions, hydrophobic anchoring from alkyl chains, and electrostatic repulsion induced by sulfonate groups. This work presents a scalable and sustainable approach for converting coal into advanced carbon nanomaterials with multifunctional properties, showing promising potential for applications in petroleum interfacial engineering, fuel emulsification, and nanomaterial dispersion.