Micelle formation by N-alkyl-N-methylpyrrolidinium bromide in ethylammonium nitrate
2011
Shi, Lijuan | Zhao, Mingwei | Zheng, Liqiang
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.
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