Molecular simulation of the regulation mechanism of surfactant on microscopic chemical wetting of coal

Abstract

To explore the influence mechanism of surfactants on the wettability of coal, elemental analysis, Fourier-transform infrared spectroscopy, 13C nuclear magnetic resonance spectroscopy, and X-ray photoelectron spectroscopy were applied to characterize the chemical structure of coal before and after modification with solutions having different concentrations of sodium dodecyl sulfate. Macromolecular models of four different coal samples before and after modification were constructed, and the mechanism of the change in the chemical structure of the coal before and after modification on its wettability was then studied with molecular dynamics (MD) simulations. The molecular formulas of the four coal samples were determined to be C160H114O34N2S, C203H144O44N3S, C196H137O42N3S, and C242H171O56N3S. Aromatic carbon accounts for a relatively high proportion of these formulas, mainly in the form of benzene rings and naphthalene. The ratios of aromatic bridge carbons to pericarbons are 0.3, 0.23, 0.21, and 0.22, respectively. Nitrogen (N) and sulfur (S) are present as pyrrole nitrogen and thiol thiophenol, respectively. MD simulations show that compared with the original coal sample, the thickness of the absorption layer between the modified coal and water molecules increases, and the interaction energy is enhanced. The maximum electrostatic potential of the surface of coal molecules in the four systems (−0.07826, −0.07915, −0.08511, and −0.10711 a.u.) is concentrated around the oxygenated functional groups. After the modification, the distribution of oxygenated functional groups is more extensive, and the content of water molecules adsorbed on the surface of the coal increases because of static electricity, which promotes the wettability of the coal. This research reveals the microscopic wetting mechanism of coal dust at the molecular level and provides a research path that offers important guidance for the reduction of coal dust that is generated during mining.