Ionic liquids as dynamic templating agents for sol–gel silica systems: synergistic anion and cation effect on the silica structured growth

Abstract

This work presents a detailed investigation about the influence of a complementary set of imidazolium ionic liquids ([NTf 2 ]) in the preparation of hydrolytic sol-gel silica wet gels and xerogels, providing insights into their hierarchical structures, morphologies of primary particles and aggregation domains. The reported IL decreased dramatically the gelation time (up to *500 times), evolving the systems to transparent or opalescent wet gels with distinct consistencies, or silica powders. Characterization of the obtained silica xerogels was performed by transmission and scanning electron microscopy, atomic force microscopy, X-ray diffraction, small-angle X-ray scattering and thermogravimetric analysis. In general, the IL allowed controlling the silica particle size, color, compactness, structures and morphology. The xerogels showed homogenous and ordered structures (spherical or triangular) of different sizes and with distinct microporosities, depending on the IL applied. These results highlight the key role of IL in the sol-gel silica synthesis with controlled properties, which can be further tuned for specific applications by changing small process parameters.Electronic supplementary material The online version of this article (

Recent Applications of Ionic Liquids in the Sol-Gel Process for Polymer–Silica Nanocomposites with Ionic Interfaces

“…The direct interaction of ILs with growing silica systems was confirmed by microporosity, fractal structuring and morphology changes [5]. To our knowledge, in all systems where ILs were applied from the beginning of the hydrolytic sol-gel reaction [3][4][5]60] they accelerated gelation process while presented an opposite behavior when applied after pre-hydrolysis step [80]. It shows an important role of ILs in catalyzing and organizing the sol-gel hydrolysis step, influencing the entire evolution process even when used in very small content.…”

“…This mechanism predicts the simultaneous existence of H-bonds between [BF 4 ] anions and silica matrix together with imidazolium rings π-π stacking, which resulted in mesoporous silica formation (Figure 4) [61]. Based on that, the geometry, size, polarity and Coulomb coupling forces between anions and cations contribute directly to the final particle size, compactness and morphology ( Figure 4) [4,5,[60][61][62][63]65,79,84,85]. [64,83].…”

“…Among those, the use of ionic liquids (ILs), due to their ionicity, anisotropy and multiple bond capacity, have shown to allow structure control. Perhaps most importantly, the ILs present the most dramatic effects when applied in the first sol-gel process steps, both in simple or complex gel growth [3][4][5]. Thus, the understanding of the sol-gel hydrolysis and condensation steps, and the basic parameters controlling them, is necessary for unveiling the IL's role.…”

“…However, it is known that the structural behavior of the original solution also determines the polycondensation and network formation in the sol-gel process [2]. Further, a series of studies were performed applying and investigating the influence of different ILs from the first moments of the reaction, allowing further particle structure control [3][4][5]8,60]. It was observed initially that the use of ILs with 1-ethylene glycol monomethyl ether-3-methylimidazolium [C3OMIm] cation associated to different anions ([BF4], methanesulfonate [MeSO3], and hexafluorophosphate [PF6]) resulted in highly distinct morphologies, depending on anion applied.…”

“…However, it is known that the structural behavior of the original solution also determines the polycondensation and network formation in the sol-gel process [2]. Further, a series of studies were performed applying and investigating the influence of different ILs from the first moments of the reaction, allowing further particle structure control [3][4][5]8,60] 6 ]) resulted in highly distinct morphologies, depending on anion applied. Moreover, further extracting the ILs exposed the ordered porous silica structures formed ( Figure 5) [60].…”