The strong hydrophilicity of the surface restricts the processing and utilization of low rank coal (LRC),and surfactant adsorption is one of the effective methods to control the wettability of LRC. Fourier transform infrared spectroscopy (FTIR),contact angle measurement and density functional theory (DFT) calculation were used to explore the regulation mechanism of anionic surfactants and cationic surfactants on the surface wettability of LRC. The anionic surfactant was sodium dodecyl benzene sulfonate (SDBS),and the cationic surfactants were decyl trimethyl ammonium bromide (C10TAB),dodecyl trimethyl ammonium bromide (DTAB) and cetyl trimethyl ammonium bromide (CTAB). Results showed that the control effect of surfactant on the wettability of LRC was mainly affected by the charge distribution of hydrophilic group,the length of hydrophobic group and the adsorption capacity of surfactant. The charge distribution of hydrophilic groups of surfactants affected the ability of adsorption. In the aqueous phase,the Mulliken charges of —COO- of LRC,—N＋(CH3)3 of DTAB and —SO-3 of SDBS after ionization were
-0.415e,0.483e and -0.69e,respectively. The interaction energy between DTAB and LRC was negative (-345.36 kJ/mol),while that between SDBS and LRC was positive (14.88 kJ/mol),indicating that cationic surfactants were easier to adsorb on the surface of LRC. The charge in the interaction region of —N＋(CH3)3 and —COO- was -0.341e lower than that of —COO- before adsorption,indicating that the hydrophilic group with positive charge neutralized with the negative charge on the surface of LRC,weakening the hydrophilicity of the hydrophilic point. The longer of the hydrophobic group of surfactants,the better coverage of hydrophilic points,and the better effect of wettability regulation. When the initial concentration of surfactant was too large,it would lead to excessive adsorption,and the hydrophilic group of the upper surfactant contacted with water,which was unfavorable to the weakening of hydrophilicity.