Emulsion stabilization and flocculation in CO2 .2. Dynamic light scattering

Dynamic light scattering (DLS) was used to investigate steric stabilization and flocculation of dilute poly(2-ethylhexyl acrylate) (PEHA, M-W = 92K) emulsions in liquid CO2 at 25 degrees C by monitoring changes in droplet size over time at various CO2 densities. Two steric stabilizers were investigated: a homopolymer, poly(1,1-dihydroperfluorooctyl acrylate) (PFOA, M-W = 1 x 10(6)), and a block copolymer, polystyrene (M-n = 4.5K g/mol)-b-PFOA (M-n = 25K). For a concentration of 0.03 wt %, aggregates of PS-b-PFOA were observed in CO2 with an average hydrodynamic diameter of approximately 80 nm and a polydispersity index of 0.2-0.3 from the method of cumulants. Critical flocculation densities (CFDs) of PEHA emulsions were determined by lowering the CO2 density at constant temperature to induce flocculation. The CFDs of the emulsions agree closely with the Theta point density of PFOA in CO2 for both the homopolymer and block copolymer stabilizers. Above the CFD, the emulsions have an average droplet size of 400-600 nn and are stable over several hours. With shear, the average droplet size of emulsions stabilized with PS-b-PFOA decreases with time because of shear-enhanced flocculation, and subsequent sedimentation of the flocculated droplets. The flocculation of the emulsion is attributed to a depletion mechanism induced by PS-b-PFOA aggregates in solution.