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

Abstract

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.

The Influence of Water Vapor on the Electrochemical Shift of an Ionic Liquid Measured by Ambient Pressure X‐ray Photoelectron Spectroscopy

“…The N 1s spectrum shows one symmetric peak at 403.1 eV associated with the nitrogen in the cation (N ring ). While these C 1s, O 1s and N 1s peak positions are referenced to the Au 4 f(7/2), if they are adjusted by using the common internal reference of C alkyl =285.0 eV, the aforementioned BEs are consistent with previously reported BEs for imidazolium acetate ILs [63] . The IL 4 : 4 : 2 stoichiometry observed for the C 1s spectrum indicates there was no adventitious carbon observed at the IL junction, including both the IL/vacuum and Au/vacuum interfaces.…”

“…The stoichiometric ratio of C alkyl : C ring : C ox in the C 1s spectra (Figure 6a) changes from 4 : 4 : 2 under vacuum, to an average of 5.9 : 4.8 : 2 in the presence of 0.5 Torr water vapor. This change is C 1s spectral shape is attributed to water induced nanostructuring of [C 4 mim][OAc] at the IL/water vapor interface, [63] and does not change significantly with bias. Compared to the vacuum spectra, there are two additional peaks in the O 1s spectra at BEs higher than O Ac (Figure 6b).…”

“…An Ag/AgCl is used as a QRE in order to appropriately control the WE. Recently, our group has utilized APXPS to examine IL/gas interfaces in the presence of water vapor [63–66] . Herein these studies are extended by examining the IL 1‐butyl‐3‐methyimidazolium acetate, [C 4 mim][OAc], as a function of water vapor pressure and external bias.…”

Water-induced reordering in ultrathin ionic liquid films

“…The peak at 535.9 eV is attributed to gas-phase water, i.e. water vapour surrounding the sample [34,35]. The peak at 533.3 eV (0.2 eV higher BE than that arising from C-OH), has been assigned to molecular water adsorbed on the IL film [36,37].…”

The solubility of gases in ionic liquids

“…Typical diffusion coefficients for CO 2 in imidazolium‐based ILs range from 1 × 10 −5 to 1 × 10 −6 cm 2 s −1 and heats of mixing from a few kJ mol −1 (physical sorption) to 30–40 kJ mol −1 (chemical sorption) . Gas–liquid interfaces are also being studied and experiments have shown interfacial water uptake is much more rapid at the surface than in the bulk . Heat of mixing calculations using EOS models are only semiquantitative; therefore, experimental measurements are needed to develop process simulations and accurate economic models.…”