Counteranion-dependent sorption of imidazolium- and benzimidazolium-based ionic liquids by soot

Sorption of ionic liquids (ILs) to soil and porous materials as affected by anions was observed, but scarce effort has been focused on addressing the role of counteranions in sorption and the associated underlying mechanisms. In this work, two series of 1-butyl-3-methylimidazolium- (Bmim-) and N-butyl, methyl-benzimidazolium-based (Bmbim-based) ILs coupled with different counteranions were prepared to investigate the effect of anions on IL sorption by soot. The octanol-water partition coefficient (K-ow) and the ion-pair formation constant at infinite dilution in water (K-IP degrees) of ILs were independently measured to explore the contribution of counteranion-dependent hydrophobicity and ion-pair. A wide range of sorption coefficients (K-d) of ILs were achieved with values varying from 59.8 to 344.3 L.kg(-1) for Bmbim-based ILs and from 253.4 to 489.7 L kg(-1) for Bmbim-based ILs. Compared with other anions, bis(trifluoromethanesulphonyl)imide ([Tf2N](-)) and hexafluorophosphate ([PF6](-)) exhibit tighter association with IL cations in aqueous solution due to their larger K-ow and higher K-IP degrees Positive linear relationships between log K-IP degrees and K-d and between log K-ow and K-d evidenced that the counteranion-dependent sorption of Its relies on the association strengths of IL cations and counteranions, which further influence the hydrophobicity/hydrophilicity of ion pairs. Compared with that of strongly coordinating anions (such as [CH3SO3](-), [CF3COO](-), [BF4](-), [CF3SO3](-), and [Cl](-)), the addition of weakly coordinating anions (such as [Tf2N](-) and [PF6](-)) in solution contributes to markedly large sorption enhancement of ILs. Consequently, the contribution of different counteranions on IL sorption is essentially based on the formation of ion pair with different K-IP degrees and K-ow in aqueous solution. (C) 2018 Elsevier Ltd. All rights reserved.