Surface segregation in binary mixtures of imidazolium-based ionic liquids

Surface composition of binary mixtures of room-temperature ionic liquids has been investigated using time-of-flight secondary ion mass spectrometry at room temperature over a wide composition range. The imidazolium cations with longer aliphatic groups tend to segregate to the surface, and a bis(trifluoromethanesulfonyl)imide anion (Tf2N−) is enriched at the surface relative to hexafluorophosphate (PF6−). The surface of an equimolar mixture of Li[Tf2N] and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) has a nominal composition of [bmim][Tf2N] because of surface segregation and ligand exchange. The surface segregation of cations and anions is likely to result from alignment of specific ligand-exchanged molecules at the topmost surface layer to exclude more hydrophobic part of the molecules.