This work reports the study of the anion effect on the aggregation behavior of the long-chain spacers of dicationic imidazolium-based ionic liquids (ILs) in a 4.75 % (v/v) ethanol-water solution as well as in ethanol (95 %). The anions studied were Br⁻, NO₃ ⁻, BF₄ ⁻, SCN⁻ and NTf₂ ⁻. Aggregation behavior was investigated by differential scanning calorimetry (DSC), conductivity, surface tension, and fluorescence. In the ethanol-water solution, the critical aggregation concentration (CAC), free energy aggregation (∆G°_a), and the ionization degree (α) all significantly decreased with the increase in anion hydrophobicity. In ethanol, the CAC and ∆G°_a values also decreased with the increase in anion size and hydrophobicity. The free energy adsorption (∆G°_ads) data showed that the dicationic ILs have good surfactant activity, and this property improved with the decrease in the hydration radius of the anions. The anion volumes calculated also showed a good correlation with the CAC values for aggregation in ethanol-water solution and ethanol.

Graphical Abstract