Aggregation Behavior of Surface Active Imidazolium Ionic Liquids in Ethylammonium Nitrate: Effect of Alkyl Chain Length, Cations, and Counterions

The aggregation behavior of surface active imidazolium ionic liquids (ILs) with different alkyl chain length, cations, and counterions, namely, 1-alkyl-3-methylimidazolium bromide ([C n mim]Br (n = 8, 10, 12)), 1-dodecyl-2,3-dimethylimidazolium bromide ([C12bmim]­Br), 1-(2,4,6-trimethylphenyl)-3-dodecylimidazolium bromide ([C12pim]­Br), and 1-dodecyl-3-methylimidazolium tetrafluoroborate ([C12mim]­[BF4]) in a protic room temperature IL, ethylammonium nitrate (EAN), was investigated through surface tension measurements and 1H NMR spectroscopy. Surface tension results show that surface properties and micellization behavior of surface active ILs in EAN are significantly affected by the structure of the cations, the basicity of counterions, and the hydrophobicity of alkyl chains. A detailed analysis of chemical shifts of various protons of surface active ILs and EAN was employed to investigate the micelle formation mechanism. Hydrogen bonding interaction is found to occur between the protons at C-2 on the imidazolium ring and the oxygen atoms in [NO3]− anions, and the interaction varies as a function of the basicity of the counterions and the hydrophobicity of the side-chains bonded to the imidizolium ring. The micelle formation may be accompanied by a partial changeover from trans to gauche conformations in the alkyl chain. The solvophobic effect may exist between the hydrophobic portion of [CH3CH2NH3]+ and the hydrophobic chains of the surface active ILs.