Structural Effects of Anions and Cations on the Aggregation Behavior of Ionic Liquids in Aqueous Solutions

The formation of ionic liquids aggregates in aqueous solution is of great importance to the future applications of ionic liquids. In this work, aggregation behavior of 1-alkyl-3-methylimidazolium salts [C₈mim]X (X = Cl, Br, [NO₃], [CH₃COO], [CF₃COO], [CF₃SO₃], and [ClO₄]), 1-octyl-4-methylpyridinium bromide (4m-[C₈pyr]Br), and 1-methyl-1-octylpyrrolidinium ([C₈mpyrr]Br) has been investigated in aqueous solutions by conductivity, volume, fluorescence, dynamic light scattering, and transmission electron microscopy. The critical aggregation concentration (CAC), ionization degree of the aggregates α, the standard Gibbs energy of aggregation ΔG_m°, the average aggregation number N, the apparent molar volumes at critical aggregation concentration V_Φ,CAC, the apparent molar volumes in aggregation phase V_Φ^mic, and the change of the apparent molar volumes upon aggregation ΔV_Φ,m, have been derived from the experimental data for these ionic liquids. It is found that both nature of the anions and ring type of the cations significantly affect the aggregation in aqueous solution. The anionic effect basically follows the Hofmeister series, and the ability of anionic hydration is predominant for the aggregation behavior of the ionic liquids. Hydrophobicity and steric hindrance of the cations as well as binding strength of the cations with the anions are suggested to play important roles in the aggregation of [C₈mim]Br, 4m-[C₈pyr]Br, and [C₈mpyrr]Br. The investigated ILs were found to form spherical aggregates. Structures of anions and cations have very weak effects on the morphology, but they do affect the aggregate sizes.