Study on the effect of surfactant ethoxy content on the wettability adjustment of bituminous coal interface

Coal dust significantly limits the production capacity, fine processing, and efficient utilization of coal resources, and also presents substantial health risks and safety hazards. The modification of wetting and adsorption at the anthracite interface by aqueous solutions of non-ionic surfactants is essential for the wet treatment of fine hydrophobic coal dust. In this study, ethoxylated non-ionic surfactants, octylphenol polyoxyethylene ethers ((C2H4O)nC18H30O,n = 5.10,15), were used to conduct interfacial wetting experiments, surface adsorption tests, and molecular simulation calculations. The properties of surfactant solutions, dynamic contact angle variations, and interfacial adsorption kinetics were analyzed. At the macroscopic level, the analysis revealed differences in the physical adsorption of non-ionic surfactants with varying ethoxy content on the coal interface. Analysis of vector diffusion coefficients, molecular spatiotemporal changes, system density distribution, interfacial dielectric constants, contact surface area, spatial parameters, interface curvature, interaction energy, and hydrogen bond differences obtained from molecular dynamics simulations revealed the mechanism of modified interfacial wettability transition at the microscopic level. The results indicate that the modification of the bituminous coal interface by ethoxy-based nonionic surfactants enhances interfacial wettability. This improvement occurs through two primary mechanisms: first, the physical adsorption of surfactants onto the bituminous coal interface alters the molecular conformation of the surfactant, reducing the roughness of the modified interface and facilitating the spreading and wetting of water molecules on a smoother surface. Second, the adsorption coverage of the bituminous coal interface introduces a large number of ethoxy polar hydrophilic groups, which occupy the hydrophobic sites of the coal surface. This increases the adsorption strength and binding energy between water molecules and the modified interface, thereby enhancing interfacial polarity and promoting improved wettability. Furthermore, as the ethoxy content in octylphenol polyoxyethylene ether increases, the hydrophilicity and polarity of the surfactant-modified interface are enhanced. This is due to the increased number of potential hydrogen bonding sites, leading to a greater number and strength of hydrogen bonds. Additionally, a higher ethoxy content results in a larger molecular radius of gyration and greater conformational flexibility, contributing to a smoother modified interface. This structural modification further facilitates the spreading and wetting of water molecules at the interface. The study reveals differences in the physical adsorption of ethoxylated non-ionic surfactants on the coal interface and the mechanism underlying the transition in wettability of the modified interface, thereby providing a theoretical basis for selecting ethoxylated surfactants for the wettability modification of coal.