Research on the influence of hardness-indicating ions on the wetting characteristics of coal dust by surfactant fluids

Traditional evaluations of surfactant fluid wettability on coal dust often overlook high-salinity water environments. Thus, it is essential to systematically investigate the impact of hardness-indicating ions (Ca2+/Mg2+) in surfactant solutions on coal dust wettability. This study explores the influence mechanisms of ion type and concentration on coal–water interfacial behavior modified by various surfactants through macroscopic wettability experiments (surface tension, contact angle, spreading work, coal dust settling, viscosity, and foaming) and microscopic molecular theoretical calculations (quantum chemistry and molecular dynamics). Macroscopic results show that under optimal wettability concentrations, surface tensions of YL (anionic) and YG (cationic) series rise at different rates with increasing hardness, while viscosities exhibit U-shaped and arch-shaped trends, and foaming capacities decline stepwise. FL (nonionic) series surface tensions follow a decline-then-rise pattern, with minima of 29.1 mN/m (Ca2+) and 28.9 mN/m (Mg2+), and hardness thresholds align with YL and YG at Ca-500 and Mg-700. FL viscosity and foaming remain stable. The best hard-water tolerance and wettability are observed in 0.2% YL-1, 0.25% YG-1, 0.2% FL-1, and 0.2% FL-3, while 0.2% YL-1 and 0.25% YG-1 excel in atomization. Microscopic calculations reveal high sensitivity of YL-1 and YG-1 to Ca2+/Mg2+, whereas FL-1 and FL-3 show limited wettability changes due to ethoxy groups. The differing effects of Ca2+ and Mg2+ stem from variations in their charge-to-ionic radius ratios. These findings provide theoretical insights for optimizing dust suppressant performance.