Studies on imidazolium-based ionic liquids having a large anion confined in a nanoporous silica gel matrix

The properties of large molecules confined in nanopores are expected to be different from those of the bulk. This study reports changes in the phase behavior and vibrational frequencies of an imidazolium-based ionic liquid (IL), namely, 1-butyl-3-methyl imidazolium octyl sulfate ([BMIM][OcSO(4)]) in a nanoporous silica gel matrix. Nanoporous silica gel matrixes have been synthesized by a one-step sol-gel technique using tetraethylorthosilicate (TEOS) as a starting precursor. The synthesized gel has been characterized by differential scanning calorimetry (DSC), BET, TEM and FTIR. The FTIR spectra show shifts in many vibrational bands; particularly, the vibrational bands related to the imidazolium ring, aliphatic chain, and SO(3) of the IL are found to shift significantly upon confinement. The DSC results show significant changes in the melting point (ΔT(m) ≈ 52 °C), crystallization temperature (ΔT(C) ≈ 14 °C), and glass transition temperature (ΔT(g) ≈ 2 °C). The IL used in the present study has a large anion ([OcSO(4)]), and ΔT(m) for this is much larger than those reported earlier for many other ILs with relatively smaller anions. A new approach, based on the liquid-drop model, has been suggested to explain this.