(596c) Effects of Pore Size on Molecular Transport of Ionic Liquid ([BMIM][PF6]) Confined in Nanoporous Silica Films

Confinement of ionic liquids (ILs) in nanoporous materials is important for a variety of applications including separations, catalysis, and energy storage. There is evidence that near surfaces, ILs spontaneously order into alternating layers of cations and anions that propagate a significant distance due strong Coulombic forces, and this is likely to create well-defined microenvironments when they are confined in nanopores. Therefore, our research group has developed silica nanoporous thin films loaded with ILs to study the effects of confinement on transport of hydrophilic and hydrophobic redox probes.

Silica films with vertically oriented mesopores are used as model supports because of their chemical stability and tunable structure. Pore size is controlled by using as templating agents either Pluronic surfactant P123 (for 8 nm pores) or cetyltrimethylammonium bromide (CTAB, 3 nm pores). The presence of vertically aligned cylindrical pore channels is confirmed by grazing-incidence small angle X-ray scattering. The pores of some films are modified by covalently tethering 1-(3-trimethoxysilylpropyl)3-methylimidazolium chloride [TMS-MIM][Cl] onto the pore wall. A commonly used and hydrophobic IL, [BMIM][PF₆], was selected to be supported in the silica nanoporous films with and without tethering. Three-electrode electrochemical impedance spectroscopy was performed to compare the surface resistance of films on fluorine-doped tin oxide to transport of redox probes 1,1’-ferrocenedimethanol (FDM, hydrophilic) and 1,1′-dioctadecyl-4,4′-bipyridinium dibromide (DBD, hydrophobic) dissolved in aqueous KCl electrolyte. In 8 nm channels, [TMS-MIM][Cl] tethering greatly increases the surface resistance to transport of FDM but not to DBD. Compared with [TMS-MIM][Cl] tethering, simply confined [BMIM][PF₆] itself has less effect on surface resistance, although it is generally consistent with its hydrophobic character. In 3 nm channels, consistent effects are observed, but the increases in resistance is exaggerated due to [TMS-MIM][Cl] tethering and selective transport is observed for confined [BMIM][PF₆]. This work suggests that confinement of ILs, especially when covalently tethered, enhances their selectivity towards transport of solutes. Confinement of [BMIM][PF₆] in small nanopores can be used to improve their selective resistance to transport of hydrophilic solutes.