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 sil ica suspensions in carbon dioxide for hours as shown with turbidimetry. For hydrophilic silica, surfactant head groups with carboxylate, carboxylic ac id, styrene and ethylene oxide functionalities adsorb to silica, in part du e to hydrogen bonding to silanol groups. As density, and thus the solvent q uality, are lowered an abrupt 'critical flocculation density' is observed f or 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 clou d-point density of poly (fluoroacrylate) in bulk CO2, due to collapse of th e stabilizer chains. In contrast, the loss in stabilization is continuous f or perfluoropolyether COO (-) NH4+ (2500 g/mol) due to its low molecular we ight. (C) 2000 Elsevier Science B.V. All rights reserved.