(150f) Harnessing Solvation Forces for Dispersing Colloids in Ionic Liquids with Application in Human Exploration of Space

Ionic liquids are proposed for many applications due to their remarkable properties. Of particular interest here are space-based applications such as for improving astronaut survivability through enhanced ballistic, puncture and abrasion resistance of space suits and micrometeorite and orbital debris (MMOD) shielding for spacecraft. Specifically, ionic liquids are proposed as the dispersing phase of shear thickening colloidal dispersions because of their stability over the broad range of temperatures and low volatility. However, dispersing particles in ionic liquids 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. Recently, (Gao et al. ACS Nano 2015), we created stable nanoparticle dispersions in the ionic liquid [C₄mim][BF₄] by inducing solvation layering, whereby a 5 nm layer or organized IL forms around the particle due to strong hydrogen bonding between anion [BF4]^- and the fluorinated alcohol functionalized particle surface. However, in agreement with theory, the presence of this steric solvation layer prevented shear thickening by suppressing hydrocluster formation. To achieve a stable dispersion that also exhibits shear thickening, commercial silica particles with an alcohol- functionalized particle surface were dispersed in the ionic liquid [C₄mim][BF₄]. It was expected that the alcohol coating will exhibit weaker hydrogen bonding with the anion [BF₄]^-, leading to a thinner solvation layer that is still sufficient for dispersion, but enables 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 on microstructure and shear thinning and shear thickening rheology are investigated. Dispersions lose their stability and transit from stable dispersion to “soft” gel due to the loss of solvation layers thickness with increasing temperature. 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.