Steric stabilization of colloids by poly(dimethylsiloxane) in carbon dioxide: Effect of cosolvents

Steric stabilization and flocculation of colloids with surface-grafted poly (dimethylsiloxane) (PDMS) chains are examined in liquid and supercritical c arbon dioxide with and without hexane as a cosolvent. Neither poly(methyl m ethacrylate) (PMMA) nor silica particles with grafted 10,000 g/mol PDMS cou ld be stabilized in pure CO2 at pressures up to 345 bar at 25 degrees C and 517 bar at 65 degrees C without stirring. The addition of 15 wt% hexane to CO2 led to stable dispersions with sedimentation velocities of 0.2 mm/min for 1-2 mu m PMMA particles. The critical flocculation pressure of the coll oids in the hexane/CO2 mixture, determined from turbidity versus time measu rements, was found to be the same for silica and PMMA particles and was wel l above the upper critical solution pressure for the PDMS-CO2 system. The a ddition of a nonreactive cosolvent, hexane, eliminates flocculation of PMMA particles synthesized through dispersion polymerization in CO2 with PDMS-b ased surfactants. (C) 2000 Academic Press.