Pressure-Dependent Confinement Effect of Ionic Liquids in Porous Silica

The effect of confining ionic liquids (ILs) such as 1-ethyl-3-methylimidazolium tetrafluoroborate [C₂C₁Im][BF₄] or 1-butyl-3-methylimidazolium tetrafluoroborate [C₄C₁Im][BF₄] in silica matrices was investigated by high-pressure IR spectroscopy. The samples were prepared via the sol-gel method, and the pressure-dependent changes in the C-H absorption bands were investigated. No appreciable changes were observed in the spectral features when the ILs were confined in silica matrices under ambient pressure. That is, the infrared measurements obtained under ambient pressure were not sufficient to detect the interfacial interactions between the ILs and the porous silica. However, dramatic differences were observed in the spectral features of [C₂C₁Im][BF₄] and [C₄C₁Im][BF₄] in silica matrices under the conditions of high pressures. The surfaces of porous silica appeared to weaken the cation-anion interactions caused by pressure-enhanced interfacial IL-silica interactions. This confinement effect under high pressures was less obvious for [C₄C₁Im][BF₄]. The size of the cations appeared to play a prominent role in the IL-silica systems.