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Anionic surfactant sodium dodecyl benzene sulfonate (SDBS) at varying concentrations was selected to investigate the influence on the wettability of Zhaozhuang Coal by molecular dynamics simulation. Six groups of water/surfactant/coal systems with different concentrations were constructed. The influence of surfactant with different concentrations on the wettability of coal was concluded by analyzing various properties from the energetic behaviors to the dynamic characteristics. The results show that the interfacial tension decreases sharply and then rises slowly with the increase of SDBS surfactant concentration, obtaining that surfactants can obviously reduce the interfacial tension. The surfactant molecules could be detected at the water/coal interface through analyzing the system’s relative concentration distribution. In addition, the difference in the wettability of surfactants on coal surfaces is caused by the spatial distribution differences of alkyl chains and the benzene ring of the surfactant molecules. And the negative interaction energy between SDBS and the coal surface indicates that adsorption process is spontaneous. Furthermore, it is of great practical significance for improving the dust reduction effect and reducing the disaster of coal dust by exploring the effects of surfactant molecules on the wettability of coal. Keywords: Sodium dodecyl benzene sulfonate, Concentration, Wettability, Interfacial tension, Molecular dynamics simulation

Four coals samples at different ranks were analyzed by Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and solid-state13C nuclear magnetic resonance (NMR). The calculated coal molecular model was constructed according to the experimental data. The mode of evolution of four coal molecules with different metamorphic degrees was explored. The results indicate that the nanostructures of these four coal molecules mainly consist of aromatic structures, aliphatic structures and oxygen-containing functional groups. The coal metamorphic degree is the most important factor affecting the evolution of the coal molecular nanostructure. By increasing the coal rank, the aromatic carbon content and aromatic system increase, while the aliphatic carbon content and aliphatic system decrease, and the species and content of oxygen containing functional groups are also reduced. During the evolution of the molecular microcrystalline structure, the degree of vertical order of the aromatic structural unit, the flatness of the aromatic structural unit (La), the average crystallite stacking height (Lc), and the average number of crystallites in a stack (n) increase, while the interlayer distance between aromatic sheets (d002) decreases; the short-range ordering of the coal structure is mainly caused by changes in the orientational arrangement from intramolecular aromatic layers to intermolecular aromatic layers when low-rank coal molecules evolve to high rank coal molecules. The structural evolution mechanism of coal molecules of different ranks has been revealed through the analysis of the mode of evolution of the molecular structure the coal. This study enables us to better understand the nanostructure evolution mechanism of coal molecules at different ranks.

The adsorption mechanism of methane molecules and coal molecules at different ranks is of significant practical and theoretical importance in prevention gas disaster and methane storage. In this paper, four coal samples at different ranks were selected for experiments. The real coal molecules at different ranks were constructed. The relationship between the coal components and the gas adsorption capacity was analyzed. The adsorption mechanism was studied by performing combined Grand Canonical Monte Carlo(GCMC) and Density Functional Theory (DFT)simulation at temperatures of 298 K and at pressure range of 0–100 bar. The simulation results display a good agreement with the experimental results. In the process of methane adsorption simulation, methane molecules are preferentially adsorbed on the edge of the unit cell and then adsorbed inside the coal molecules. The methane molecules in the Up orientation have stronger adsorption capacity then Down orientation. The adsorption capacity order is as follows: aromatic structure > oxygen functional groups. In the aromatic structure, the adsorption capacity of adsorption sites is ranked as T > B > C. In the oxygen functional groups, the adsorption of methane by the coal molecule is caused by both the adsorption position and the adsorption direction. Under the influence of coal molecules, the structure and the Mulliken charge of methane molecules change significantly during the adsorption simulation. The adsorption forces between the coal molecules and methane molecules are mainly induced force, dispersion force, orientaion force and electrostatic force. This study enables us to better understand the adsorption mechanism of coal molecules at different ranks.