Molecular dynamics simulation and experimental characterization of anionic surfactant: Influence on wettability of low-rank coal

The influence of the anionic surfactant sodium fatty alcohol polyoxyethylene ether carboxylate (AEC) on the wettability of coal surfaces was studied by combining molecular dynamics simulations and experimental research. First, a molecular dynamics simulation of the AEC adsorption process on a low-rank coal surface was carried out. The simulation results show that AEC promotes the adsorption of water molecules on the coal surface, moves more water molecules to the coal surface, and improves the mobility of water molecules; AEC molecules were detected at the coal-water interface. After adsorption, the AEC molecules were connected to each other through alkyl chains to form a spherical-like structure, which covered the coal surface laterally. The calculated interaction energy between coal and water is negative, which shows that the adsorption process is spontaneous, and AEC enhances the interaction between coal and water. Second, the abovementioned simulation results are verified by experiments. The adsorption experiments show that AEC adsorbs on the lignite surface by monolayer adsorption, which can be described by the Langmuir isothermal adsorption equation; the contact angle first increases and then decreases, reaching a maximum near the critical micelle concentration (CMC). The XPS analysis shows that the change in the content of the oxygen-containing functional groups is the main reason for the wettability of the lignite surface. Among these oxygen-containing functional groups, the C-O group has the most significant effect on the wettability.