Synergistic Regulation Mechanism of Coal Functional Groups on Surface Wettability Based on Electrostatic Potential and Density Functional Theory.

To reveal the synergistic regulatory mechanism governing the action of functional group combinations in coal with different degrees of metamorphism on the interfacial wettability between coal and clean fracturing fluid, multiple experimental techniques, including FTIR, 13C NMR, XPS, and contact angle measurements, together with molecular simulation, electrostatic potential (ESP) analysis, and density functional theory (DFT), were employed to reveal the regulatory mechanism of functional group combinations on coal-clean fracturing fluid interfacial wettability at molecular and electronic scales. The results indicate that the magnitude of the influence of functional groups on coal wettability decreases in the following order: aromatic structure, hydroxyl group, aliphatic structure, heteroatomic structure. Hydroxyl groups and heteroatoms exert positive effects on hydrophilicity, whereas aromatic and aliphatic structures exert negative effects. The π-electron clouds of aromatic structures form complementary ESP with hydrogen atoms of hydroxyl groups via the matching of ESP extreme regions, which weakens the hydrophilicity of the coal. The electron-donating effect of aliphatic chains attenuates the hydrogen bonding interaction between hydroxyl groups and water molecules, presenting a significant competitive relationship therein. In contrast, heteroatoms amplify the extreme values of ESP on the surface of hydroxyl-containing coal molecules and deliver a synergistic hydrophilic effect. The binding energy, van der Waals energy, and electrostatic energy at the interface between coal with varying degrees of metamorphism and clean fracturing fluid range from -1268.04 to -474.99 kJ/mol, from -597.57 to -268.22 kJ/mol, and from -662.45 to -178.94 kJ/mol, respectively. A higher proportion of hydroxyl groups in coal molecules strengthens the hydrogen bonding energy and electrostatic interaction of the interfacial system, enhances the total intermolecular interaction energy, reduces the contact angle, and thereby improves the wettability of the coal. As the proportion of hydroxyl groups decreases, van der Waals force gradually dominates the interfacial interaction, leading to the deterioration of coal wettability.