Tuning the phase behavior of ionic liquids in organically functionalized silica ionogels

Abstract

We have synthesized mesoporous silica monoliths functionalized with 2-(4-pyridylethyl)triethoxysilane 1 and N,N-dimethyl-pyridine-4-yl-(3-triethoxysilyl-propyl)-ammonium iodide 2. The organically modified silica monoliths were characterized via IR spectroscopy, nitrogen sorption, small angle X-ray scattering (SAXS), thermogravimetric analysis-differential thermal analysis (TGA-DTA), and acid–base titration. The degree of functionalization can be changed by the ratio of the functional silane to the silica precursor tetramethyl orthosilicate (TMOS). The functionalized silica monoliths were filled with 1-ethyl-3-methyl imidazolium [Emim]-X (X = dicyanamide [N(CN)₂] or triflate [TfO]) ionic liquids (ILs) using an established methanol-IL exchange technique. The phase behavior of the resulting ionogels was investigated via differential scanning calorimetry (DSC). DSC curves show that the modification of the silica pore walls with organic groups strongly affects the phase behavior of the confined ILs. Modification with silane 1 completely suppresses the glassy state of [Emim][TfO] previously observed in unmodified silica monoliths (Göbel et al., Phys. Chem. Chem. Phys. 2009, 11, 3653). In contrast, modification with silane 2 leads to the appearance and disappearance, respectively, of a presumed additional phase in [Emim][TfO] and [Emim][N(CN)₂] with varying degree of monolith functionalization. The data thus show that organic modification of silica matrix materials could be a viable approach for the tuning of ionogel properties.