Aggregation behavior of gemini pyrrolidine-based ionic liquids 1,1′-(butane-1,4-diyl)bis(1-alkylpyrrolidinium) bromide ([Cnpy-4-Cnpy][Br2]) in aqueous solution

Three gemini pyrrolidine-based ionic liquids, 1,1′-(butane-1,4-diyl)bis(1-alkylpyrrolidinium) bromide ([C n py-4-C n py][Br 2 ], n   =   10, 12, 14), were synthesized. Their aggregation behavior in aqueous solution was systematically investigated by surface tension, electrical conductivity, and steady-state fluorescence. Compared with their corresponding monomers, N -alkyl- N -methylpyrrolidinium bromide (C n MPB), [C n py-4-C n py][Br 2 ], have higher surface activity. The special structure of [C n py-4-C n py][Br 2 ] that has a spacer in their hydrophilic head groups results in a lower surface excess concentration ( Γ max ) and a larger molecular cross-sectional area ( A min ). Electrical conductivity studies show a lower degree of counter-ion binding to the aggregates. A smaller aggregation number ( N agg ) is observed by the pyrene fluorescence quenching method. A series of thermodynamic parameters ( Δ G agg 0 , Δ H agg 0 , - T Δ S agg 0 ) of aggregation derived from electrical conductivity indicate that the aggregation of [C n py-4-C n py][Br 2 ] is enthalpy-driven, while aggregation of C n MPB is entropy-driven at low temperatures but enthalpy-driven at high temperatures.