Nanoconfinement Effects on the Transport of Redox Probes in Ionic Liquid-Loaded Mesoporous Silica Thin Films

The transport of two redox probes, 1,1′-ferrocenedimethanol (FDM, hydrophilic) and 1,1′-dioctadecyl-4,4′-bipyridinium dibromide (DBD, hydrophobic), through ionic liquid-filled and/or functionalized silica nanoporous thin films is compared as a function of pore size (2.5 and 8 nm diameter pores). Electrochemical impedance spectroscopy (EIS) was used to measure the permeability of these aqueous probes through the bare silica films and silica films with pores containing a physically confined ionic liquid (IL, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆])), covalently tethered IL-like 3-methyl-1-[3-(trimethoxysilyl)propyl]-1H-imidazol-3-ium ([TMS-MIM]⁺), and [BMIM][PF₆] confined in pores with tethered [TMS-MIM]. Functionalization of the nanoporous thin films with an IL-like group provides a barrier to the hydrophilic redox probe (a 40-fold decrease in permeability relative to bare 2.5 nm porous silica films), with only a 50% corresponding decrease in the permeability of the hydrophobic probe. The selectivity to hydrophobic solutes suggested by the barrier properties of IL-functionalized silica is less pronounced in the films with the larger 8 nm pores. Physically confining an IL in the pores of both bare and IL-functionalized pores results in similar transport as the corresponding non-IL filled pores. The ability to tune solute transport through nanoconfined ILs using pore size and surface functionalization is critical to the design of composite thin films for IL-based separation and energy storage applications.