Microscopic mechanism governing the effect of the polarity of functional groups on the interfacial wettability between coal and clean fracturing fluids.

To investigate the effects of different types, polarity, and contents of functional groups in coal on the wetting properties at the coal-clean fracturing fluid interface, two composite wetting models were established based on graphene carbon sheets: "clean fracturing fluids with different functional group polarities (strongly polar hydroxyl -OH, weakly polar ether -C-O-C-, non-polar methyl -CH3)" and "clean fracturing fluid with different amounts of functional groups (4%, 2%, 1%)". Using molecular simulations along with Coulomb's law, the electrostatic potential, frontier molecular orbitals, MSD (mean square displament), RDF (radial distribution function), hydrogen bond, and surface free energy theory, the microscopic mechanisms by which the type and content of polar functional groups affect the wetting properties of the coal-clean fracturing fluid interface were revealed. The results indicate that the polarity of functional groups affects the contact angle of coal-clean fracturing fluids (in descending order) as follows: -CH3 > -H (pure graphene) > -C-O-C- > -OH. As the content of functional groups decreases, the contact angles of -OH and -C-O-C modified samples increase, while those of pristine graphene and -CH3 modified samples decrease. Differences in the HOMO-LUMO energy gaps (highest occupied molecular orbital-lowest unoccupied molecular orbital gap) of diverse functional groups govern the magnitude of interfacial electrostatic potential and hydrogen bonding interaction. Coupled with the combined effects of steric hindrance and van der Waals forces, the contact angle varies regularly with the type and loading of functional groups. The HOMO-LUMO energy gap containing -CH3, -C-O-C-, and -OH is 0.048202, 0.047961, and 0.04676 a.u., respectively. The smaller the HOMO-LUMO energy gap, the more significant the electron delocalization, the greater the electrostatic potential on the coal surface, and the more hydrogen bonds formed, the stronger their interactions, leading to a smaller contact angle and better wettability. As the amount of functional groups increases, the surface polarity of graphene containing -OH and -C-O-C- is enhanced, the interaction energy and van der Waals forces increase, the hydrophilicity improves, the contact angle decreases, and the wettability is increased; in contrast, for -H and -CH3, the interaction energy and van der Waals forces decrease, the contact angle increases, and the wettability deteriorates.