Microscopic adsorption behaviors of ionic surfactants on lignite surface and its effect on the wettability of lignite: A simulation and experimental study

In this study, the microscopic adsorption behaviors of two ionic surfactants, sodium dodecyl sulfonate (SDS) and dodecyltrimethylammonium bromide (DTAB), on the lignite surface were investigated at the molecular level through molecular dynamics simulations. A series of properties of surfactant–lignite adsorption systems, including relative concentration profiles, radial distribution functions, surface areas (surface roughness), mean square displacements, and interaction energies were calculated to illustrate the microscopic adsorption behaviors in detail. Analysis of the equilibrated interfacial properties indicated that the adsorption of SDS on the lignite surface is dominated by van der Waals force and can decrease the surface area (roughness) of lignite. In addition, there is no preferential orientation for SDS adsorption on lignite surface. However, driven by electrostatic attraction, the head group of DTAB can be strongly adsorbed on the lignite surface. Furthermore, it was also found that the effect of surfactant adsorption on the surface wettability of lignite showed the same trends as the adsorption strength. The change in the wettability of lignite resulting from surfactant adsorption was discussed in depth, and the explanations were given based on the charge characteristics of head group of surfactant and adsorption orientation. The simulation evaluations of the adsorption behaviors and wettability alteration of lignite surface caused by these two ionic surfactants agreed well with the existing experimental data. This investigation present here visualized the microscopic characteristics of surfactant adsorption and facilitated a better understanding about the involved mechanism.