Studies on an ionic liquid confined in silica nanopores: Change in T g and evidence of organic-inorganic linkage at the pore wall surface

Abstract

Ionic liquid (IL) 1-ethyl-3-methylimidazolium ethyl sulfate [EMIM][EtSO4] has been immobilized in silica matrix by using nonhydrolytic one-pot sol-gel method, with tetraethyl orthosilicate (TEOS) used as precursor and formic acid as a reagent. The glass transition temperature (Tg) of the confined IL was ∼20 C higher than the bulk IL. The thermal stability of IL also increased upon confinement. These changes are explained on the basis of interactions between the inorganic SiO2 pore wall surface and the organic cation [EMIM]+ and anion [EtSO 4]- of the ionic liquid. Quantum mechanical calculation based on DFT has been used to show that the oxygen of SiO2 interacts with the C-H of the imidazolium ring of [EMIM]+ cation and the surface Si atoms interact with S-O of the [EtSO4]- anion. The latter has led to Si-O-S linkages, which have been experimentally confirmed by FTIR studies. The silanol OH group is also likely to interact but less dominantly as compared to the earlier discussed interaction. These interactions have also led to changes in the fluorescence spectra of the confined IL. © 2013 American Chemical Society.