Micelle formation by N-alkyl-N-methylpyrrolidinium bromide in ethylammonium nitrate

The aggregation behavior of long-chain pyrrolidinium ionic liquids, N-alkyl-N-methylpyrrolidinium bromide (CnMPB, n = 12, 14, and 16) was investigated by surface tension measurements in a protic room temperature ionic liquid, ethylammonium nitrate (EAN), at various temperatures. A series of parameters, including critical micelle concentration (CMC), surface tension at the CMC (γCMC), effectiveness of surface tension reduction (ΠCMC), maximum surface excess concentration (Γmax), and the area occupied per surfactant molecule at the air/solution interface (Amin) were estimated. From these parameters, we demonstrated that the surface activity of CnMPB is much lower in EAN than that in water. Comparing CnMPB with alkylimidazolium bromides and alkylpyridinium bromides, the effect of the cationic group on micellization in EAN was also investigated. The thermodynamic analysis of micellization revealed that the micelle formation process for CnMPB (n = 12, 14, and 16) is entropy-driven at low temperature and enthalpy-driven at high temperature. The micelle aggregation number estimated from the 1H NMR data is about 21 for C12MPB in EAN, which is much less than that in water. The results of the surface tension measurements and 1H NMR spectra indicate that the [CH3CH2NH3]+ cations of EAN exist around the head groups of CnMPB when micelles are formed and the NO3− ions are adsorbed at the micelle surface.