Thermal rheology and microstructure of shear thickening suspensions of silica nanoparticles dispersed in the ionic liquid [C4mim][BF4]

A stable dispersion of silica nanoparticles in a common ionic liquid [C(4)mim][BF4] that exhibits significant shear thickening is formulated by controlling the strength of hydrogen bonding between the nanoparticle surface and the anion of [C4mim][BF4]. Colloidal stability is demonstrated to be due to the formation of solvation layers, the properties of which are determined by scattering measurements. Time-temperature superposition measurements and the onset of yielding behavior in steady shear indicate the increase of particle-particle attraction and loss of stability of colloidal dispersions above 30 degrees C. Small angle neutron scattering and dynamic light scattering measurements confirm the reduction in solvation layering at elevated temperatures, leading to a transition from stable dispersion to unstable, attractive gel with increasing temperature (i.e., an inverse melting transition). It is demonstrated how controlling specific surface-anion interactions through chemical functionality can be used to formulate nanoparticle dispersions in ionic liquids with specific rheological properties. (C) 2017 The Society of Rheology.