(643e) In Situ Grazing-Incidence Wide Angle X-Ray Scattering (GIWAXS) Investigation of Effects of Silica Nanopore Confinement on Crystallization of [BMIM] Ionic Liquids

Supported ionic liquids (ILs) in nanoporous matrices are being developed to overcome the cost and unknown toxicity of bulk ILs, and to accentuate the microenvironments they offer for separation and catalysis. However, it is not yet known to what extent confinement into nanometer-scale pores alters the surface-induced ordering of ILs. The present work gives insights into how silica mesopore confinement affects the crystallization behavior of two selected ILs based on in situ GIWAXS experiments performed at Argonne National Laboratory’s Advanced Photon Source.

1-Butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF₆) and [BMIM]Cl are among the most extensively studied ionic liquids (ILs), which makes bulk crystallization information available for comparison. Silica films with orthogonally oriented 10 nm mesopores are prepared by templating with Pluronic surfactant P123. Some films are modified by covalently tethering 1-(3-trimethoxysilylpropyl)3-methylimidazolium chloride [TMS-MIM]Cl onto the pore wall and XPS is used to verify uniform modification throughout the depth of the films. Both ILs underwent cold crystallization below their glass transition during heating from low temperatures. GIWAXS patterns were collected in situ during heating. The crystal phase transition temperatures and corresponding crystal phases were captured and compared for bulk ILs and confined ILs in silica nanoconfinement with and without tethering. Confinement of both ILs in unmodified silica mesopores causes changes in their crystal phases and phase transition temperatures compared with bulk ILs. The melting point, where the long range order completely disappears from the GIWAXS pattern, of [BMIM]PF₆ is about 17.5 °C higher with confinement compared with bulk [BMIM]PF₆ while it is about 190 °C lower for [BMIM]Cl. Confined [BMIM]PF₆ in silica mesopores tethered with [TMS-MIM][Cl] shows a stable and symmetric crystal structure which did not melt at room temperature even though the bulk IL as a film on a Si wafer melts at -11 °C. The melting point depression for [BMIM]Cl is consistent with the Gibbs-Thomson equation with a surface energy of 8.0 mN/m. However, the melting point increases for [BMIM]PF₆ cannot explained by capillary effects. The interaction between ILs and the silica surface (most likely [BMIM] interaction with the negatively charged surface) and molecular rearrangement due to nanoconfinement are believed to be the main reasons for the changes observed. These findings show that when ILs are confined, drastic changes in structure can occur that alter the liquid-state temperature operating range for applications such as ion conduction and supported catalysts.