Effect of temperature and counterion on adsorption of imidazolium ionic liquids at air–water interface

The adsorption behavior of ionic liquids based on the 1-alkyl-3-methylimidazolium cation with different anions to the air–water interface was investigated through surface tension measurements. From the thermodynamic analysis of the surface tension data, a physical model for the structure of the adsorbed film is proposed: the HMIM+ and BF4− ions occupy the same level of the surface due to ion-pairing in the HMIMBF4 system, whereas the Br− ions are mainly distributed in the diffuse electrical double layer below the top surface in the DeMIMBr system. In the former case, the entropy change associated with adsorption increases with increasing concentration when the surface density is close to saturation because the hydrated water is released during the ion-pairing of HMIM+ and BF4−. Furthermore, the structure of the adsorbed film is almost the same as a neat ionic liquid surface; this is probably because both the anions and the cations are highly concentrated in the adsorbed film.