Water at the Ionic Liquid–Gas Interface Examined by Ambient Pressure X-ray Photoelectron Spectroscopy

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Ambient pressure X-ray photoelectron spectroscopy (APXPS) was used to quantitatively assess the chemical changes of the top few nanometers of the ionic liquid (IL)–gas interface of 1-butyl-3-methylimidazolium acetate, [BMIM][OAc], in the presence of water vapor at room temperature. Above 10 –3 Torr the uptake of water into the interfacial region was observed and increases up to a maximum water mole fraction ( x w ) of 0.85 at 5 Torr. Comparing APXPS to gravimetric analysis measurements, the kinetics of interfacial uptake are rapid compared to bulk water absorption. There is growing evidence from experiments and molecular dynamic simulations that water/IL mixtures undergo a phase transition from being homogeneously mixed to a system composed of nanometer sized, segregated polar and nonpolar regions near x w = 0.7 in the bulk. For x w > 0.6, APXPS C 1s spectra show a sudden change in shape. It is suggested that this observed spectral change in C 1s is due to a similar nanostructuring occurring near the IL–gas interface. Increasing interfacial water gives rise to relative binding energy shifts in O 1s, C 1s, and N 1s regions which increase with x w, thus suggesting that water significantly influences the electronic environment of both the anion and cation.