Steric stabilization of inorganic suspensions in carbon dioxide

Nonionic and ionic surfactants with poly(fluoroacrylate), poly(fluoroether) or poly(dimethyl siloxane) tails stabilize hydrophilic and hydrophobic silica suspensions in carbon dioxide for hours as shown with turbidimetry. For hydrophilic silica, surfactant head groups with carboxylate, carboxylic acid, styrene and ethylene oxide functionalities adsorb to silica, in part due to hydrogen bonding to silanol groups. As density, and thus the solvent quality, are lowered an abrupt 'critical flocculation density' is observed for silica stabilized by poly(styrene-b-fluoroacrylate) (M_n values: 4.5 k-b-24.5 k). This loss in steric stabilization is observed just above the cloud-point density of poly(fluoroacrylate) in bulk CO₂, due to collapse of the stabilizer chains. In contrast, the loss in stabilization is continuous for perfluoropolyether COO^-NH₄/⁺ (2500 g/mol) due to its low molecular weight.