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 ([Cnpy-4-Cnpy][Br2], 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 (CnMPB), [Cnpy-4-Cnpy][Br2], have higher surface activity. The special structure of [Cnpy-4-Cnpy][Br2] that has a spacer in their hydrophilic head groups results in a lower surface excess concentration (Γmax) and a larger molecular cross-sectional area (Amin). Electrical conductivity studies show a lower degree of counter-ion binding to the aggregates. A smaller aggregation number (Nagg  ) is observed by the pyrene fluorescence quenching method. A series of thermodynamic parameters (ΔGagg0,ΔHagg0,-TΔSagg0) of aggregation derived from electrical conductivity indicate that the aggregation of [Cnpy-4-Cnpy][Br2] is enthalpy-driven, while aggregation of CnMPB is entropy-driven at low temperatures but enthalpy-driven at high temperatures.

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► Gemini pyrrolidine-based ionic liquids [Cnpy-4-Cnpy][Br2] are synthesized.
► [Cnpy-4-Cnpy][Br2] have higher surface activity compared with their monomers.
► The aggregation driving force of [Cnpy-4-Cnpy][Br2] is different from their monomers.