Investigation of interactions between oxygen-containing groups and water molecules on coal surfaces using density functional theory

The oxygen-containing group is a crucial factor that affects the water wettability of a coal surface. An investigation of the interaction between the oxygen-containing group and water molecule is conducive for understanding the water wettability of the coal surface. In this study, water contact angle measurements and X-ray photoelectron spectroscopy were used to explore the surface wettability and oxygen-containing groups for different rank coals respectively. The adsorption behavior of water molecules on the coal surface was investigated using the density functional theory. The results indicated that the oxygen-containing group enhances the hydrophilicity of the coal surface. In oxygen-containing groups, oxygen atoms possess strong nucleophilicity, and enhance the electrophilicity of hydrogen atoms attached to them. The oxygen atom in the water molecule prefers to form strong hydrogen bonds with the hydrogen atom that is attached to the oxygen atom in the oxygen-containing group. The hydrogen atom in the water molecule prefers to face down, thereby generating a strong van der Waals force on the carbon rings. Based on the combined action of hydrogen bonds and van der Waals force, the effect of oxygen-containing groups on water adsorption capability follows the order -OOH > -OH > -CHO > -OCH3.