Gelation of Ionic Liquids with Silica Nanoparticles: The Role of Cation Structure and Interactions
Abstract: This project investigates the gelation of ionic liquids using silica nanoparticles. Theoverall aim is to learn more about the ion-ion and ion-silica interactions in ionicliquid-silica dispersions and gels. Differences in the interactions between ionicliquid and silica are introduced by changing the structure of the cations. Fourdifferent ionic liquid based on imidazolium cation and [TFSI] anion have been usedto gel silica nanoparticles. The structure of the cations is varied by changing thelength and the end-group on the alkyl chain of the imidazolium ring, introducing e.g.the possibility of hydrogen bonding.Each ionic liquid was found to have a different gel concentration, 3.8wt% for[C2MIM][TFSI], 4wt% for [EtOHMIM][TFSI], 3.5wt% for [C5MIM][TFIS], and3.6wt% for [SMIM][TFSI]. At the gel concentration all samples were weak gels,showing a yield stress behaviour. Below 4wt% phase separation was observed forthe [EtOHMIM][TFSI] silica dispersions.The gelation of the dispersions does not have any dramatic influence on theproperties of the ionic liquid. The glass transition temperature does not change andthe conductivity decreases only slightly. Raman spectroscopy shows that there areno changes on the strong Raman band at 740 cm⁻¹ (expantion-contraction of theentire [TFSI] anion) with silica concentration. From the results obtained, it ispossible to conclude that in the gel, the silica matrix acts as a “container” for theionic liquid: immobilizing the ionic liquid without making important changes to theproperties.
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