Aggregation of ionic liquids with organic anions, driven by cation-anion interaction strength

The properties of ionic liquids (ILs) are related to the structure of their ions, which determines the nature of the cation-anion interaction. However, the direct effect that the structure has on properties such as aggregation and surface activity is not precise, especially when involving dicationic ILs with organic anions. In this study, the aggregation behaviour of imidazolium-based dicationic (1,8-bis(3-methylimidazolium-1-yl)octane - [C8(MIM)2]) ILs containing ibuprofenate [Ibu] and ascorbate [Asc] were evaluated (using conductivity and surface tension) and this was related to the cation-anion interaction strength determined using hydrogen nuclear magnetic resonance and surface plasmon resonance. The results showed that both ILs had a critical aggregation concentration value that was quite low compared to the analogue with a bromide anion. The [C8(MIM)2][2Asc] IL, which has the less hydrophobic anion and has more polar sites, had a greater cation-anion interaction strength and a lower critical aggregation concentration than the [C8(MIM)2][2Ibu] analogue. The cation-anion strength reflected the aggregation properties; for example, [C8(MIM)2][2Asc] had thermodynamically favoured aggregation and better packing of IL monomers at the air-water interface. These results are considered to be relevant for future studies related to the transport and drug loading associated with the use of these ILs.