Wettability modification and restraint of moisture re-adsorption of lignite using cationic gemini surfactant

Abstract

Adsorption of surfactants onto lignite surface may result in wettability changes and slow the re-adsorption of moisture onto dried lignite. In this work, a cationic gemini surfactant, ethanediyl-1,2-bis(dimethyl dodecyl ammonium bromide) (12-2-12), was chosen, and its adsorption characteristics and the influence on the surface wettability of lignite were investigated. The results suggest that a strong attraction takes place between the negatively charged lignite surface and a gemini surfactant with positively charged head groups to form a monolayer adsorption in low surfactant concentration. Electrostatic forces play a dominant role in the adsorption of a cationic gemini surfactant on the lignite surface while hydrogen bonding does not. The adsorption density of a gemini surfactant on coal surfaces increased with an enhancement in the amount of polar oxygen containing functional groups and the addition of KCl. A decrease in wetting heat indicated that the lignite treated with the gemini surfactant led to a significant decrease in the hydrophilicity of lignite. The high concentration of oxygen-containing functional groups increased the negative charge of the coal surface, which is beneficial for adsorption of the cationic gemini surfactant adsorption onto the coal surface that decreased the hydrophilicity of lignite effectively. The lignite wettability surface modification results were compared with the traditional cationic surfactant cetyltrimethyl ammonium bromide (CTAB); these results suggested that the unique chemical structure of the gemini surfactant with two hydrophilic and two hydrophobic groups had a more prominent impact on lignite surface hydrophilicity. The gemini surfactant adsorbed on lignite surface also had a significant improvement in resisting moisture re-adsorption and could create pathways toward utilizing lignite industrially.