(604d) Particle Dispersion and Particle Interactions in Ionic Liquids

Shear thickening dispersions of colloidal particles in ionic liquids are being developed for possible use to improve the ballistic, puncture and abrasion resistance of space suits and micrometeorite and orbital debris (MMOD) shielding for spacecraft. Ionic liquids are proposed as the solvent phase of STFs formulation for space application because of their stability over the broad range of temperatures and low volatility. However, there is little known about dispersing particles into ionic liquids or the rheology of such novel dispersions. Suspensions of concentrated amorphous hydrophilic silica in ionic liquid 1-Butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF₄]) are reported to show some shear thickening behavior, but the successful dispersion of the particles is questionable. Simulations and experiments have been conducted to study the interparticle interactions in ionic liquids, showing that the high ionic concentration in ionic liquids effectively screens the electrostatic repulsion between silica particles, leading to significant particle aggregation. Therefore, to successfully disperse nanoparticles into the ionic liquid [Bmim][BF₄], we have modified the surface of silica nanoparticles with fluorocarbon 1H, 1H, 9H-hexadecafluoro-1-nonanol.  This steric layer is thought to both make the silica compatible with the ionic liquid and to provide a steric stabilization layer. 

A combination of rheology, dynamic light scattering, and small angle neutron scattering are applied to test an unproven hypothesis in literature, namely that stabilization of particles in this ionic liquid is a consequence of a solvation layer induced by hydrogen bonds between ionic liquid [Bmim][BF₄] and surface groups on the particles. Our rheological results indicate that the coated silica particles disperse in [Bmim][BF₄] at room temperature and aggregate and gel at high temperature, which may be due to the disruption of the solvation layer at higher temperatures by weakening the hydrogen bonding between the steric layer and the ionic liquid. SANS data enable determining the interparticle interactions and suspension microstructure in the ionic liquid under different temperatures and concentration conditions.  The SANS data (especially in the low q regimes) indicates that the suspension microstructure changes significantly with temperature and concentration. Modeling the structure factor using a core-shell sticky hard sphere model allows us to directly determine the stabilizing effect of the solvation layer in the ionic liquid.  This work also has potentially important implications for environmental and energy engineering as ionic liquids are leading candidates for remediation, separation, and recycling of nuclear wastes.