Effect of Gemini surfactant on wettability of Lignite: An experimental and molecular dynamics simulation study

Abstract

In this study, the nonionic Gemini surfactant S465 was used for regulating the wettability of the lignite surface. The adsorption characteristics of the surfactant on the lignite surface are discussed, and the mechanism of adsorption is explained from a microscopic viewpoint. The adsorption experiments show that the adsorption of S465 on the lignite surface is consistent with predictions made by the Langmuir adsorption isotherm. Furthermore, it indicates that the adsorption process is spontaneous and exothermic. X-ray photoelectron spectroscopy (XPS) analysis shows that the oxygen-containing functional groups content (CO, C = O, and O = C-O) on the lignite surface decreases after the adsorption of the surfactant, which enhances the hydrophobicity of the lignite surface. It is found from molecular dynamics (MD) simulation that the polar ethoxy groups of the surfactant touch the lignite surface, and the nonpolar groups (alkyl chains) entwine closely with each other, forming a protective layer, which keeps water away from the surface of the lignite. The mobility of water molecules is higher in the lignite/surfactant/water system compared to the system without surfactants, and the adsorption of surfactants results in the lignite interaction with water being weaker, therefore, the hydrophobicity of lignite is improved after the adsorption of the surfactant.