Miscibility of imidazolium and pyridinium ionic liquids including BF4− at the air/water interface

Abstract

We investigated miscibility of ionic liquids in the adsorbed film by surface tensiometry from the viewpoint of effect of the hydrogen bonding between cation and anion. The ion pair formation accompanied by two hydrogen-bond between cation and anion was indicated in 1-hexyl-3-methylimidazolium tetrafluoroborate (C6MIMBF4) system and also in hexylpyridinium tetrafluoroborate (C6PyBF4) system, the ion pairs of which structure is similar to that of C6MIMBF4 systems was suggested. The phase diagram of adsorption (PDA) of the ionic liquid–ionic liquid mixed system (C6PyBF4–C6MIMBF4) showed an ideal mixing. On the other hand, those of ionic liquid–alcohol mixed systems (C6MIMBF4–1-butanol, C6PyBF4–1-butanol) showed positive deviation in the adsorbed film. Judging from the values of excess Gibbs energy of adsorption of these systems, we concluded that the magnitude of the two hydrogen-bonds of C6PyBF4 and C6MIMBF4 is not significantly different, but the one between C6MIM+ and BF4− is slightly larger than that between C6Py+ and BF4−. The difference should be caused by the magnitude of difference of atomic charge distributions in the molecules owing to nitrogen atom in the ions, especially in the C(2)–H.