Decrease in hydrophilicity and moisture readsorption of lignite: Effects of surfactant structure

The low moisture content of lignite is essential to its high utilization efficiency. There has been increasing attention given to the adsorption of surfactants to decrease the hydrophilicity and moisture readsorption of lignite. This has focused on findings surfactants that are effective, convenient, and inexpensive. To understand the effect of surfactant structure on decreased lignite hydrophilicity, eighteen surfactants including conventional types of surfactants were used for lignite treatment. Wetting heat results indicated that cationic and non-ionic surfactants were superior in their performance to decrease the hydrophilicity of lignite compared to anionic surfactants with the same hydrophobic alkyl chain. Ethanediyl-1,2-bis(dimethyl dodecyl ammonium bromide) (12-2-12) and lauryl polyoxyethylene ether (C12EO9) were found to be the most effective cationic and non-ionic surfactants in these samples, respectively. Linear shapes, long alkyl chains, and gemini structures were beneficial to decreasing the hydrophilicity of lignite. Structures to be avoided included anionic types, branches, aromatic rings, and glucoside. The adsorbed surfactant layer covered the oxygen containing functional groups on the surface of the lignite, and also blocked lignite pores to reduce the storage space of water, effectively reducing the hydrophilicity of the lignite. The contribution of the coverage of oxygen-containing functional groups was found to be more significant. In addition, we demonstrated the consistency between moisture readsorption and wetting heat in terms of presenting hydrophilicity of lignite and their differences were also discussed. Our results provide a reference for selecting an appropriate strategy and further developing the application of surfactants to decrease lignite hydrophilicity.