(390a) Silica Particles Dispersions in the Ionic Liquid [C4mim][BF4]

Shear thickening dispersions of colloidal particles in ionic liquids are being developed for 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, this can be challenging because the high ionic strength of ionic liquids screens the electrostatic stabilizing forces that are typically important for dispersing particles in polar solvents.  In our previous research (Gao et al. ACS Nano 2015), we created stable nanoparticle dispersion in the ionic liquid [C₄mim][BF₄] by inducing solvation layering, where  5 nm solvation layers form around the particle due to hydrogen bonding between anion [BF4]^- and the fluorinated alcohol functionalized particle surface.  However, the surface fluorinated alcohol functionalized silica particle dispersion did not exhibit strong shear thickening behavior due to the thick solvation layers preventing the formation of hydroclusters. 

To achieve stronger shear thickening, commercial silica particles with an organic functionalized particle surface were dispersed in the ionic liquid [C₄mim][BF₄].  It is expected that the organic coating will have weaker hydrogen bonds with anion [BF₄]^-  leading to a thinner solvation layer that is still sufficient for dispersion, but would enable  hydrocluster  formation at high shear rates.  Dynamic light scattering (DLS), small angle neutron scattering (SANS) and rheology were employed to determine the solvation layer thickness and microstructure of dispersions.  Analysis of SANS spectra across a broad range of particle concentrations was used to develop a quantitative model for the inter-particle interactions including the thickness of the solvation layer.  The effects of temperature and impurities (i.e. water) on microstructure and rheology are reported.   This work provides guidance on formulating colloidal dispersions in ionic liquids and may have implications for environmental and energy engineering as ionic liquids are candidates for remediation, separation, and recycling of nuclear waste.