Water at the Ionic Liquid–Gas Interface Examined by Ambient

Mar 14, 2017 - ... University of Kansas, Lawrence, Kansas 66045, United States ...... McLean , B.; Li , H.; Stefanovic , R.; Wood , R. J.; Webber , G...
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Water at the Ionic Liquid−Gas Interface Examined by Ambient Pressure X‑ray Photoelectron Spectroscopy Alicia Broderick,† Yehia Khalifa,† Mark B. Shiflett,‡ and John T. Newberg*,† †

Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States Department of Chemical and Petroleum Engineering, University of Kansas, Lawrence, Kansas 66045, United States

‡

S Supporting Information *

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 (xw) 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 xw = 0.7 in the bulk. For xw > 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 xw, thus suggesting that water significantly influences the electronic environment of both the anion and cation.

1. INTRODUCTION Ionic liquids (ILs) are salts with a melting point below 100 °C. With tunable properties, ILs are sparking interest in a variety of applications including their use as electrolytes, catalysts for a large number of reactions, alternatives to traditional organic solvents, and as a CO2 absorbent from atmospheric flue gas.1−3 The interaction of water with ILs has been heavily studied, and bulk uptake experiments have emphasized that even hydrophobic ILs will absorb water over time.4 Water affects IL properties including density, viscosity, conductivity and electrochemical window, which have implications in various technologies including gas absorption, electrocatalysis, and tribology.5−7 Imidazolium-based ILs using acetate as the anion are hydrophilic and biodegradable and have low viscosities relative to other conventional ILs.8 Acetate-based ILs are widely used in the dissolution of biomass including cellulose and chitin.9 It has been shown that a small amount of water can be used to regenerate chitin and limit the dissolution of cellulose into ILs.10 Water has also been shown to affect CO2 absorption into acetate-based ILs and its mixtures with biopolymers.11 Molecular dynamic (MD) simulation studies of imidazolium acetate ILs suggest that the presence of water leads to a decrease in CO2 absorption as both water and CO2 compete for the same solvation sites of the acetate IL.12 Processes including gas absorption, gas separation, heterogeneous catalysis, and heterogeneous electrocatalysis involve a gas crossing the IL−vapor interface. A number of surface analysis tools have been utilized to understand the IL−vapor interface in the presence of water, including line-of-sight mass © 2017 American Chemical Society

spectrometry coupled with temperature-programmed desorption,13 X-ray reflectivity,14 sum frequency generation (SFG),15 neutral impact-collision ion scattering spectroscopy (NICISS),16 and ultrahigh vacuum (UHV) X-ray photoelectron spectroscopy (XPS).17 While surface science studies have provided valuable molecular level information for the interaction of water with IL−vapor and IL−vacuum interfaces, a quantitative assessment of water in the interfacial region and the impact of water on the electronic environment of the IL− water system as a function of water vapor pressure under near ambient conditions is still lacking. The vacuum−IL interface has been studied extensively using UHV XPS since 2005,18,19 although few UHV XPS studies have examined ILs with an acetate anion.20 Ambient pressure X-ray photoelectron spectroscopy (APXPS) allows for probing sample substrates up to Torr level pressures and has been used extensively to study solid−gas and liquid−gas interfaces.21−23 However, there is currently a dearth of APXPS studies for gas/IL systems. The interaction of CO2 with dihydroxyethyldimethylammonium taurinate has been measured at 0.7 Torr via APXPS.24 Herein we present results from a variable pressure APXPS study examining [BMIM][OAc] under adsorption−desorption conditions at room temperature from high vacuum up to a maximum of 5 Torr using APXPS to probe the top few nanometers of the IL−gas interface. Received: January 24, 2017 Revised: March 14, 2017 Published: March 14, 2017 7337

DOI: 10.1021/acs.jpcc.7b00775 J. Phys. Chem. C 2017, 121, 7337−7343

Article

The Journal of Physical Chemistry C

2. EXPERIMENTAL DETAILS APXPS experiments of the IL−water vapor interface were conducted using a lab-based system with a monochromatic Al anode X-ray source (Scienta MX650) operating at 20 mA and 10 kV and an electron energy analyzer with a differentially pumped electrostatic lens (Scienta, HiPP-2). The details for this system have been described previously.25 Experiments were performed using a 0.5 mm aperture and the swift acceleration electron capture lens mode.26 The collection angle of the electrons was 20° relative to the sample normal. Survey spectra were captured at 200 eV pass energy while C 1s, N 1s, and O 1s were captured at 100 eV pass energy. [BMIM][OAc] (Iolitec, 98%) has a melting point of −20 °C with a bulk density of 1.06 g cm−3.27 The commercial IL was used as received without further purification and stored in a vacuum desiccator at room temperature. Samples were prepared for entry into the APXPS vacuum chamber by placing a thin film ( 0.6) there an increase in the Calkyl signal. At ∼4 waters per IL pair the Calkyl/CTot ratio levels off to a value of ∼0.56 and remains constant up to nw/nIL = 5.5. This increase in Calkyl signal is fully reversible as evidenced by the C 1s spectrum returning to its initial shape after pumping out water (Figure S3a, top spectrum). While we cannot rule out the possibility that an increase in adventitious carbon with water vapor pressure may have contributed to these observations, there is reason to believe this is likely not the cause given the weak interactions with ILs and hydrocarbons. For example, the bulk uptake of hydrocarbons into ILs requires high pressures, where 1 bar often leads to a solubility (in mole fraction) of x < 0.01.35 Any adventitious hydrocarbon in the APXPS chamber under the experimental conditions herein would have a partial pressure many orders of magnitude lower than 1 bar, ergo an x ≪ 0.01 (below the XPS detection limit) even if the hydrocarbon were enhancement at the interface relative to the bulk. The observation of a sudden onset to Calkyl above an xw of 0.6 is consistent with growing evidence that there is a change in the structure of the IL/water system. Recently, the use of neutron scattering was used to directly probe changes in the bulk IL structure of [BMIM][BF4]/water mixtures as a function of increasing xw.36 Under low xw conditions water dissolved into the IL forming a homogeneous mixture that does not change the structure of the solution. At xw ≈ 0.7 water forms nanometer-sized clusters that grow with the addition of water. The results of these experiments were a confirmation of previous molecular dynamic (MD) simulations of the same [BMIM][BF4]/water system.37 Recently, Ghoshdastidar et al.38 used MD simulations and observed a similar phenomenon for imidazolium acetate−water mixtures. For long chain imidazolium cations (n = 4, 6) a sudden change in IL density was observed at xw ≈ 0.7. This change is due to a nanostructuring of the IL−water mixture that contains networks of polar regions (water, acetate, and imidazolium ring) and nonpolar regions (imidazolium alkyl chain). This unique onset in nanostructuring at xw ≈ 0.7 for [BMIM][OAc]/water mixtures is consistent with the aforementioned [BMIM][BF4]/water system.36,37 Based on these previous observations in the bulk,36−38 herein we suggest that the sudden onset of Calkyl intensity for xw > 0.6 (Figure 4) is due to this onset in nanostructuring which leads to an increase in hydrophobic nanometer size structures in the interfacial region. Note that this is distinct from previous observations of water induced restructuring of imidazoliumbased ILs by SFG39 and NICISS16,40 which probe the top 1−2 layers of the IL−vapor interface. As mentioned previously, the APXPS studies herein have a probing depth on the order of 8.5 nm (at 95% of the XPS signal) for these experiments, which is greater than the probing depth of SFG and NICISS. However, given the restructuring leads to nanometer size hydrophilic and hydrophobic regions, it is reasonable to suggested that if such structures were also present at the interface it would lead to observable changes in the APXPS C 1s spectra. MD simulations assessing the onset to nanostructuring at the IL−vacuum interface for [BMIM][OAc] as a function of increasing xw are needed. The results herein have significant implications for the uptake of other gas phase molecules such as CO2. The structural and chemical environment of the IL−gas interfacial regime is strongly influenced by ambient water vapor, which in 7340

DOI: 10.1021/acs.jpcc.7b00775 J. Phys. Chem. C 2017, 121, 7337−7343

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The Journal of Physical Chemistry C

water mole fraction increased up to a maximum of 0.85 at 5 Torr. Interfacial water uptake was found to be reversible with kinetics that are much more rapid than bulk absorption. A significant increase in the Calkyl signal at xw > 0.6 was observed and is attributed to a water induced structural change that leads to an enhanced concentration of hydrophobic nanostructures in the IL−gas interfacial region. The addition of water into the interfacial region leads to significant changes in the electronic structure of the anion and cation as evidenced from binding energy shifts. Density functional theory core level shift calculations are needed to help elucidate the nature of these shifts.

in the presence of water vapor and BEvac is the BE under vacuum (see Figure 2). As seen from Figure 5a, ΔBE values are

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b00775. Survey spectrum of IL; XPS peak constraints; timedependent assessment of XPS experiments; C 1s and N 1s peak pressure-dependent spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.T.N.).

Figure 5. Relative binding energy (ΔBE) shifts referenced to the Calkyl as a function of (a) water vapor pressure (log scale), (b) water mole fraction (xw), and (c) number of waters per IL pair (nw/nIL).

ORCID

Mark B. Shiflett: 0000-0002-8934-6192 John T. Newberg: 0000-0003-1576-4436

negligible below 0.01 Torr for all components due to the absence of water in the interfacial regime. From 0.1 to 5.0 Torr where interfacial water is present, an increase in ΔBE is seen for all components as a function of increasing pressure and therefore increasing water at the interface. Figure 4b shows that ΔBE increases roughly linearly with an increase in the water mole fraction, where the magnitude of the shifts increase in the order Ow > OAc ≈ Cox > Cring > Nring. Overall, these observed ΔBE changes show evidence of water significantly impacting the electronic environment of each moiety at the IL−vapor interface. Density functional theory (DFT) studies of water interacting with [BMIM][OAc] show that water significantly impacts the electronic environment of both the cation and anion depending on the optimized configuration of the water, anion, and cation relative to each other.47 The oxygen atoms of both water and acetate can act as electron donors during hydrogen bonding for water−[EMIM][OAc] interactions,48 which may be consistent with the large positive ΔBE for Ow and OAc. To help elucidate the magnitudes of these shifts for each moiety and to explain why they all shift in the positive direction relative to the alkyl chain on the cation (Calkyl), DFT core level shift (CLS) studies are strongly needed. Reinmöller et al.49 calculated CLS values of C 1s and N 1s for [EMIM][Tf2N] versus [OMIM][Tf2N], but these were on the pure ILs and do not consider any influence of additional molecules such as water. By converting xw to the number of waters per IL pair (nw/nIL; Figure 5c), the experiments herein provide a direct measure of CLS as a function of increasing waters up to a maximum of ∼5.5 waters per IL pair.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.T.N. acknowledges support from an ACS PRF New Doctoral Investigator award, a University of Delaware Research Foundation award, and a University of Delaware Energy Institute Innovative Energy Research grant. J.T.N. also acknowledges the gravimetric measurements conducted at the DuPont Experimental Station (Wilmington, DE).

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REFERENCES

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4. CONCLUSIONS The uptake of water into the top few nanometers of the IL− vapor interface of [BMIM][OAc] was examined using ambient pressure XPS. As the water vapor increased, the interfacial 7341

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DOI: 10.1021/acs.jpcc.7b00775 J. Phys. Chem. C 2017, 121, 7337−7343