Effect and mixing of counter anions at the surface of aqueous solution of imidazolium-based ionic liquids

The purpose of this study was to find the primary factor determining the mixing of the counter anions of 1-alkyl-3-methylimidazolium-based ionic liquid (IL, CnmimX) at the air/water interface. First, the properties of the adsorbed films of the C10mimBr–C10mimBF4 mixtures were estimated by using the thermodynamic analysis of their surface tension data. The mole fraction of BF4− was lower in the adsorbed films than in the ideal state and the excess Gibbs energy of adsorption was negative, which was in marked contrast to those of the C6mimBr–C6mimBF4 mixtures. Meanwhile, the possibility of forming side-by-side ion pair of BF4− and imidazolium ring at the adsorbed films, as we found in the C6mimBr–C6mimBF4 system, was greatly reduced in the present case. These changes were mainly attributed to the reduction in the hydrogen-bonding interaction between BF4− and C10mim+, as the carbon number in the hydrophobic part of Cnmim+ was increased from 6 to 10. In addition, it was found that the values of excess Gibbs energy of adsorption were less negative in the C10mimBr–C10mimBF4 system than in the dodecyltrimethylammonium (C12TA)Br–C12TABF4 system, where the long-range Columbic interaction solely exists between the counter anions and surfactant cations. Thus, we concluded that the short-range hydrogen-bonding interaction played an essential role in the interfacial activity of ILs.