Anion Enhancement at the Liquid Vacuum Interface of an Ionic Liquid Mixture

The liquid vacuum interface was investigated for a ionic liquid (IL) mixture containing 1-ethyl-3-methylimidazolium acetate, [C2MIM] [OAc], and 1ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C2MIM] [TFSI]. Herein, we detail a quantitative connection between molecular simulations and angle-resolved X-ray photoemission spectroscopy for an IL vacuum interface. Results show that for a mixture with a low concentration of [TFSI](-), the anion [OAc](-) is slightly depleted from the interface, whereas the [TFSI](-) anion is significantly enhanced relative to the bulk. Both experiments and simulations reveal that the mole fraction of [TFSI](-) increases significantly from the bulk value in the top 17 angstrom. Furthermore, simulations show that [TFSI](-) has a preferred orientation at the liquid-vacuum interface.