Pressure-dependent cation-surface interactions of ionic liquids confined within nanoporous anodic aluminum oxide

Pressure-dependent infrared (IR) spectroscopy is used to clarify ionic liquid-pore wall interactions in a model anodized aluminum oxide (AAO) nanoporous host. Our high-pressure IR spectroscopic studies reveal confinement-induced modifications to the phase behavior of the ILs. Cation-wall interactions have a prominent role in directing pressure-induced solidification for confined ILs. For example, C-H stretching modes for 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C2MIM][TFSI]) are split under high pressure and undergo band shifting. Confinement eliminates band splitting and modu-lates the rate of band shifting due to pressure. Thus, the structural reorganization that accompanies pure ILs when subjected to pressure is impeded upon confinement. Confinement effects on the phase transition process are observed for both planar (imidazolium) and nonplanar (pyrrolidinium) cations as well as cations that contain short and long alkyl side chains. Thus, our observations appear to be broadly appli-cable to a variety of cation types. Interactions between both cations and anions with the Al-O and Al-OH surface groups are observed in the IR spectra. These likely play a crucial role in the phase transformations that transpire when the samples are pressurized. (C) 2022 Elsevier B.V. All rights reserved.