Temperature-Induced Molecular Rearrangement of an Ionic Liquid Confined in Nanospaces: An in Situ X-ray Absorption Fine Structure Study

Abstract

A temperature-dependent X-ray absorption fine structure (XAFS) study was performed to investigate structural changes in the 1-hexyl-3-methylimidazolium bromine (C(6)mimBr) ionic liquid (IL) confined within the channels of multiwalled carbon nanotubes (MWCNTs). The XAFS spectra at room temperature confirmed the charge transfer from the bromine anion of the encapsulated IL to the MWCNTs. R-space analysis at ambient temperature revealed the reduced distance between the anions and cations in the confined IL compared with that in the bulk state. Interfacial-induced solidification and nanoconfinement in MWCNTs induced the self-assembly of ions in the confined IL to form a layered arrangement near the inner surface of MWCNTs. In situ XAFS analysis revealed that with increasing temperature charge transferred from MWCNTs to Br anions because of the damage from the conjugative effect between Br and MWCNTs and the relatively strong electronegativity of Br atoms. R-space analysis also showed gradual reduction in distance between cations and anions with increasing temperature. This finding indicated that the anion moved toward the ring planar, and ions rearranged from the layered to the near-planar structure. Raman and XRD experiments confirmed the structural transformation of the confined IL at increased temperature.