Steric stabilization of silica colloids in supercritical carbon dioxide

Silica colloids were sterically stabilized in supercritical CO2 by end-grafting poly(1H,1H-dihydroperfluorooctyl methacrylate) (PFOMA) onto the particle surfaces. Turbidity versus time measurements were used to determine the CO2 density below which the colloids flocculated, that is, the critical flocculation density (CFD). The CFD was determined as a function of stabilizer molecular weight and temperature as the solvent density was lowered. All of the CFDs occurred above the upper critical solution density for the corresponding finite-molecular-weight stabilizer in bulk CO2 and corresponded more closely with the estimated Theta density. The CFDs decreased (reflecting greater stability) when temperature was increased or the PFOMA molecular weight was decreased. The latter result suggests that, at lower solvent densities, the shorter chains experience better solvation and, hence, provide greater steric repulsion than the longer chains. For the stabilizers of highest molecular weight, the colloids become unstable slightly above the Theta density, possibly as a result of chain contraction from long-range van der Waals forces with the particle surface.