Exploring the Microscopic Mechanism of the Effect of Nanofluid and Surfactant System on the Wettability of Coal

This study explores the challenges of poor wettability in deep coal seam mining by employing a combined approach of experimental methods and molecular dynamics simulations to investigate the mechanisms through which CDEA (coconut oil fatty acid diethanolamide)-modified SiO2 nanofluids improve coal wettability. In the wettability experiments, SiO2 nanofluids, CDEA solutions, and their mixtures with varying concentrations were prepared. Surface tension and contact angle tests revealed that the optimal ratio of 0.2 wt % CDEA to 0.2 wt % SiO2 resulted in the lowest contact angle and best wettability parameters on the coal surface. Microscopic results showed that compared to raw coal, CDEA and CDEA-SiO2 treatments significantly reduced the fatty carbon content while increasing hydrophilic groups (e.g., carboxyl and carbonyl), thus enhancing coal wettability. The treatments had minimal impact on the aromatic molecular backbone, as indicated by the nearly unchanged bridge carbon ratio across all samples. Molecular dynamics simulations were used to construct coal-water, coal-CDEA-water, and coal-CDEA-modified SiO2-water models. Electrostatic potential distribution, water molecule concentration, radial distribution function, and diffusion coefficients were analyzed to reveal the mechanisms by which CDEA-modified nanofluids promote water molecule adsorption and enhance coal hydrophilicity at the microscopic level. These findings offer theoretical insight and practical guidance for improving coal wettability in water-injection applications.