Effect of Lithium Salt on the Stability of Dispersions of Fumed Silica in the Ionic Liquid BMImBF₄

We have investigated the stability and interactions in dispersions of colloidal fumed silica, Aerosil 200, and the ionic liquid 1-butyl-3-methylimidazolium tetraflouroborate (BMImBF₄) as a function of the Li salt concentration (LiBF₄). Photon correlation spectroscopy was used to study the aggregation behavior at low silica concentrations, and Raman spectroscopy was used to investigate the interactions in the ionic liquid and with the silica surface. We find that the addition of LiBF₄ increases the stability of the dispersions, with smaller agglomerates of silica particles and higher gelation concentrations in the presence of Li salt. The increased stability with the addition of Li salt is explained by the formation of a more stable solvation layer, where Li ions accumulate on the surface. This leads to an increased interaction between lithium ions and the BF₄^– anions in the solvation layer, as seen by Raman spectroscopy. Upon gelation, the Li ions are expelled from the surface because hydrogen bonding between the silica particles are formed. For both neat BMImBF₄ and Li-salt-doped BMImBF₄/silica dispersions, a weak gel phase was found preceding the formation of a strong gel at slightly higher silica concentrations