Molecular Dynamics Simulations and Experimental Study of the Effects of an Ionic Surfactant on the Wettability of Low-Rank Coal

The surfactants sodium dodecyl benzene sulfonate, dodecyl dimethyl betaine, and dodecyl trimethyl ammonium bromide were used to study the effect of an ionic surfactant on coal wettability. A lignite-surfactant-water system was constructed using the Wender coal chemical structure model and Materials Studio molecular simulation software, and the static potential of a single molecule was determined by quantum chemical calculations. The initial and equilibrium configurations of the system, relative concentration distribution, and mean-square displacement (MSD) of water molecules were analyzed thoroughly. Additionally, the surface tension and contact angle of aqueous surfactant solutions with different mass concentrations were measured to analyze the relationship between concentration and the rate of reduction of surface tension and of contact angle. The analysis showed that when the surface static potential of the lignite molecule was larger than that of water molecules, the water molecules were easily attracted and exhibited a wetting phenomenon. The water-surfactant-coal system gradually tended from an initial unstable state to a stable state with low energy. Specifically, the hydrophilic groups in the surfactant were tilted toward the water phase and the hydrophobic chains were adsorbed on the surface of coal, which helped enhance the wettability of the coal. The relative concentration distribution range of the SDBS was 26–52 Å, which was wider than the other two surfactants. Additionally, the MSD curve showed that the diffusion coefficient of the water molecules in the water-SDBS-coal system reached a maximum value of 4.23 × 10-2, resulting in accelerated wetting of the coal body. When the SDBS mass concentration was 3%, the surface tension decreased to the minimum value (21.86 mN/m) and the minimum contact angle was 16.12°. Based on the simulation results and experimental test data, the wettability of the three surfactants to lignite was SDBS > BS-12 > DTAB