Water structure at coal/water interface: Insights from SFG vibrational spectroscopy and MD simulation

The coal/water interface plays a significant role in coal processing and utilization, and hydrogen bonds (HBs) are the primary factors affecting the interfacial water structure. Sum frequency generation (SFG) vibrational spectroscopy was used in this study to analyze HBs at the coal/water interface, and molecular dynamics (MD) simulations were conducted to investigate the molecular dynamics properties of interfacial water. The results showed that water molecules formed strong HBs with the lignite surface using both oxygen and hydrogen atoms, whereas HBs at bituminous coal/water and anthracite/water interfaces mainly occurred through the hydrogen atoms of water molecules. Fewer HBs existed at the bituminous coal/water interface, resulting in more free hydroxyl groups of water molecules. HBs at the anthracite/water interface were weaker than those at the lignite/ water interface but stronger than those at the bituminous coal/water interface. Stronger HBs facilitated the adsorption of water molecules onto coal surfaces, reducing the number density and diffusion coefficient of water molecules within the water layer. For both properties, the MD simulation results showed the following order of decreasing magnitude: bituminous coal/water interface > anthracite/water interface > lignite/water interface.