Interfacial chemistry of water at the ionic liquid-vapor interface probed by ambient pressure X-ray photoelectron spectroscopy

Date
2018
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University of Delaware
Abstract
Ionic liquids (ILs) are molten salts that are highly hygroscopic, so they absorb water over time. The sorption of water into IL has been heavily studied for bulk absorption while very little data has been collected of surface sorption. It is believed the sorption into these two different regions of the IL will have different water interactions. Using ambient pressure X-ray photoelectron spectroscopy (AP-XPS), the interaction of water with multiple ionic liquids was studied. After installation of a labbased AP-XPS and conducting experiments to show the capabilities of the system, studies of water with IL were conducted. For the first time, AP-XPS was used to quantitatively assess the chemical changes of the top few nm of the ionic liquid (IL)-gas interface of 1-butyl-3–methylimidazolum 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 AP-XPS to gravimetric analysis measurements, the kinetics of interfacial uptake are rapid compared to bulk water absorption. Increasing interfacial water gives rise to relative binding energy (ΔBE) shifts in O 1s, C 1s and N 1s regions which increase with xw. The binding energy shifts are due to a change in electronic environment for specific moiety. For xw > 0.6 there is a sudden increase in hydrophobic Calkyl signal at the IL-vapor interface. In light of recent small angle neutron scattering and molecular dynamic simulations, this observed increase in interfacial carbon is attributed to a water induced phase change that gives rise to nanometer sized, segregated polar and non-polar regions. This heterogeneous restructuring leads to an enhancement in hydrophobic nanostructures at the IL-vapor interface. Both AP-XPS and gravimetric analysis were used to quantitatively assess the effect of temperature on the water uptake into 1-butyl-3-methylimidazolium acetate, [C4mim][Ace], at the interfacial and bulk regions. It was found that water uptake into the interfacial and bulk increased with decreasing temperature, or increasing relative humidity. Additionally, there is more water present in the interfacial region compared to bulk within 0.4 < x w <0.75 at 0 - 42 °C. The thermodynamics [enthalpy (Δ H°), entropy (ΔS°), and Gibbs free energy (ΔG°)] of water sorption into both the interfacial and bulk region of [C4mim][Ace] were determined. The Δ H° of water sorption into the ionic liquid (IL) at both interfaces approached the enthalpy of vaporization for bulk water as more water was introduced to the system. The ΔS° was similar at both interfaces at xw < 0.6 and showed favorability within the interfacial region at xw > 0.6. At 20 °C, concurrently the ΔG° was more favorable in the interfacial region. This implies there is a difference in water sorption at the interface and bulk regions, where an energy barrier is present for the water to travel from the interface to the bulk. The deliquesce of a solid IL, [C4mim][Cl], was studied using AP-XPS at -5 °C. The deliquescence point is between 7.9 < %RH < 15.8. At the maximum water vapor of 1.1 Torr, [C4mim][Cl] has a maximum xw ∼ 0.77. Changes in the C 1s spectra as a function of water vapor pressure were also noted. The initial changes in the carbon spectra while the IL is a solid is believed to be due to carbon contamination desorbing from the surface. After the IL deliquesces, increases in the aliphatic carbon species may be due to restructuring of the interface where the hydrophilic carbon chain is at the surface and the hydrophilic species (anion, imidazolium ring, and water molecules) are clustered at the inner layer of the interfacial region. To confirm this restructuring, MD of the interface and/or depth profiling using a synchrotron AP-XPS system are needed.
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