Coal-Rank-Dependent Interfacial Regulation of Coal Wettability by SiO ₂ Nanofluids

Coal wettability is strongly governed by surface chemistry and morphology associated with coal rank, which influence water interaction, coal seam water injection, and dust suppression performance. Water-based SiO 2 nanofluids have shown potential for regulating coal wettability; however, their coal-rank-dependent interfacial effects remain insufficiently understood. In this study, SiO 2 nanofluids with different concentrations were used to modify lignite, bituminous coal, and anthracite, and their effects on coal wettability were investigated by integrating macroscopic wetting behavior with surface chemical and morphological evolution. Surface tension measurements, contact angle tests, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) were employed as multiscale interfacial characterization methods. The results show that SiO 2 nanofluid-induced wettability enhancement exhibits a pronounced dependence on both nanofluid concentration and coal rank, with coal wettability continuously improving as SiO 2 concentration increases. Among the investigated coal samples, bituminous coal shows the strongest response, with the absolute enhancement rate (AER) reaching 48.6% at 1.0 wt %, followed by anthracite and lignite. FTIR and SEM analyses further indicate that the increased Si–O–Si contribution and hydroxyl-related polar sites, together with nanoparticle deposition and surface roughening, promote interfacial hydrophilization of coal surfaces. These findings reveal a coal-rank-dependent interfacial regulation mechanism of coal wettability by SiO 2 nanofluids and provide guidance for the design of rank-specific nanofluid-based wetting systems.