Study on dynamic wettability grading and wetting mechanism of coal by surfactants with different hydrophilic structures

The hydrophilic structure of surfactants significantly influences their dynamic wetting performance. Previous studies mainly used static methods and parameters to assess surfactant wetting on coal, with limited research on dynamic wetting, especially in quantifying and grading wettability based on maximum dimensionless wetting area. Therefore, this study employs a combined approach of impact wetting experiments and molecular dynamics simulations to investigate the effects of hydrophilic group type and chain length on the maximum dimensionless wetting area of droplets on coal surfaces. The dynamic wettability of the solutions was graded, and the microscopic wetting mechanisms of surfactants with different hydrophilic structures were analyzed. The results demonstrate that, at identical concentrations, the maximum dimensionless wetting area follows sodium alcohol ether sulfate (AES) > decaethylene glycol monododecyl ether (C₁₂E₁₀) > dodecyl trimethyl ammonium bromide (DTAB) (by hydrophilic group), and OP-10 > OP-40 > OP-7 (by chain length). According to the dynamic wettability grading criteria, AES was categorized as a strong wetting solution, DTAB was classified as a weak wetting solution, C₁₂E₁₀ exhibited a strong wetting solution when its concentration exceeded 0.03 %, and three octylphenol ethoxylate surfactants (OP-7, OP-10, OP-40) qualified as strong wetting solutions. The molecular dynamics simulation results revealed that the AES and OP-10 systems exhibited optimal relative concentration, interaction energy, and water molecule diffusion coefficients within their respective group, with the strongest wettability. Radial distribution function analysis demonstrated the following wettability trend for hydrophilic structures: sulfate group > hydroxyl group > ethoxy group > quaternary ammonium group.