Probing the protic ionic liquid surface using X-ray reflectivity

Abstract

The structure of the free liquid surface of three protic ionic liquids, ethylammonium nitrate (EAN), propylammonium nitrate (PAN), and ethylammonium formate (EAF), has been elucidated using X-ray reflectivity. The results show all three liquids have an extended interfacial region, spanning at least five ion pairs, which can be divided into two parts. Adjacent to the gas phase are aggregates consisting of multiple cations and anions. Below this are layers oriented parallel to the macroscopic surface that are alternately enriched and depleted in cation alkyl chains and polar domains of cation ammonium groups and their anions, gradually decaying to the isotropic sponge-like bulk structure. The most pronounced layering is observed for PAN, driven by strong solvophobic interactions, while reduced hydrogen bonding in EAF results in the least structured and least extensive interfacial region.

Solid and Solid‐Like Composite Electrolyte for Lithium Ion Batteries: Engineering the Ion Conductivity at Interfaces

“…For example, for most aprotic ILs, clusters with a composition of [C] a [A] b (C = cation, A = anion with a and b values between 2 and 5) have been detected by electrospray ionization mass spectrometry . The existence of clusters has also been observed by X‐ray reflectivity (XRR) and NMR . At the mesoscopic level, ILs are believed to hold a similar segregation structure as their solid crystalline form, as supported by MD simulation and evidenced by small‐angle X‐ray scattering (SAXS), XRD, NMR results .…”

Formation of Surface Protic Ionic Liquid Nanodroplets for Nanofabrication

“…It is not expected that ethanol will have a large influence on the bulk structure of the IL. [40] Earlier studies have also revealed ordering of hydrocarbon liquids at solid interfaces, particularly for cyclohexane, which is the bulk solvent used here. [23] The dewetting may be linked to the competition of layers at the interface as ethanol is gradually exchanged by cyclohexane.…”

“…The conditions during the solvent exchange process are nominally dry, however ethanol, the cosolute could play a similar role on the surface properties. It is not expected that ethanol will have a large influence on the bulk structure of the IL . Earlier studies have also revealed ordering of hydrocarbon liquids at solid interfaces, particularly for cyclohexane, which is the bulk solvent used here .…”

“…Once exposed to air, it has been demonstrated that EAN exhibits ordering at the air interface such that the hydrophobic alkyl groups shield the more polar components of the IL . It is noted that the ILs are hygroscopic and will interact with atmospheric water.…”

“…Once exposed to air, it has been demonstrated that EAN exhibits ordering at the air interface such that the hydrophobic alkyl groups shield the more polar components of the IL. [40] It is noted that the ILs are hygroscopic and will interact with atmospheric water. For hydrophobic ILs water may partition to the air interface which is associated with reorientation of ILs as well as a reduction in the interfacial surface tension.…”