Interfacial mechanisms of benzene ring substituent position effects in low-rank coal flotation

Aromatic collectors have demonstrated excellent performance in enhancing the flotation of low-rank coal (LRC). However, the effect of the substituent position on their flotation efficiency is still not clear, stemming from a poor understanding of their molecular behavior at the LRC interface. Flotation tests, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations were conducted to systematically compare the ortho-, meta-, and para-substituted isomers: diethyl phthalate (DEP), diethyl isophthalate (DIP), and diethyl terephthalate (DTP). At 4000 g/t collector dosage, DIP delivers the highest combustible recovery (51.40 %) and the largest first-order rate constant ( k  = 0.90), compared with DEP (40.64 %, k  = 0.48) and DTP (45.64 %, k  = 0.56). DFT-derived molecular electrostatic potential calculations indicate that DIP has the lowest molecular polarity index (MPI) of the three collectors (DEP > DTP > DIP). This lower polarity, combined with a smaller exposed polar surface, promotes its durable adsorption on oxygen-rich LRC. Interaction Region Indicator ( IRI ) and Hirshfeld surface analyses show that interfacial contact strength correlates with the extent of hydrogen-bonding patches at the collector-coal interface. MD simulations further indicate that DIP has more stable adsorption configurations, with more favorable adsorption energies, larger collector-coal solvent-accessible surface areas ( SASA ), and lower mean-squared displacements ( MSD ). Collectively, these results establish a clear structure-performance relationship: the balanced polarity and low steric hindrance of the meta isomer yield the most robust adsorption on LRC, while the para isomer is intermediate and the ortho isomer is weakest. These insights provide a quantitative basis for rapid screening and rational design of high-efficiency collectors for enhanced coal recovery.